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Abstract

Recent mapping efforts and hydrocarbon exploration in the South Park Basin have brought to light the magnitude in complexity of a structural basin already recognized for its unique sedimentary and tectonic setting. This field trip to one of Colorado’s scenic gems will examine how Paleozoic, Mesozoic, and Cenozoic strata record the tectonic signatures of at least three orogenic episodes. We will cross the Elkhorn–Williams Range thrust system into the structural block caught between Laramide uplifts, and preserving synorogenic sediments from the Pennsylvanian–Permian ancestral Rocky Mountain tectonic episode in juxtaposition with synorogenic sediments from the subsequent Laramide tectonic episode. Late Cretaceous marine sediments from the Western Interior Seaway caught up in complex fold-fault structures between Laramide uplifts create targets for petroleum exploration. Evidence of evaporitic tectonism originating from Pennsylvanian evaporite deposits and hinting at structural complexity dots the landscape. The trip will also explore a post-Laramide surface preserved in a graben developed in the hanging wall of the Elkhorn fault system and view post-Laramide volcanic features. Glacier-carved ranges held up by Precambrian crystalline basement and Paleozoic sediments hardened by contact metamorphism from Paleogene stocks and sills rim the basin. Pleistocene glaciofluvial deposits fan out from the high ranges to blanket the highly deformed basin, masking many of the primary structural features.

Introduction

The South Park Basin (Fig. 1) is an 80 χ 56 km (50 χ 35 mile [mi]) structural feature shaped by a long and varied history of geologic processes (Stark et al., 1949; De Voto, 1971; Scarbrough, 2001; Ruleman et al., 2011). It contains a wide variety of crystalline igneous and metamorphic rocks, sedimentary units, and volcanic rocks ranging in age from Precambrian through Cenozoic (Fig. 2). Figure 3 is a generalized geologic map compiled from recent mapping efforts by the Colorado Geological Survey and U.S. Geological Survey (Table 1). Figure 4 shows the major structural features identified in the basin. The basin is bound on the west by the Mosquito Range, on the east and north by the Front Range, and on the south by the Arkansas hills. Within the basin lie several significant north-south-trending topographic features controlled by underlying structure. From east to west, these include the rolling hills of the Elkhorn upland between the town of Jefferson and Eleven Mile Canyon reservoir, Reinecker ridge between the towns of Como and Hartsel, and the Red Hill hogback between Red Hill Pass and Hartsel. Major streams draining South Park include various forks of the South Platte River and Tarryall Creek, which have headwaters in the Mosquito Range and Boreas Pass area, respectively. These tributaries converge and exit to the east through the Front Range at the south end of the park near Lake George.

Figure 1.

Regional map of South Park. South Park Basin sits high in the Rocky Mountains in close proximity to the Denver metropolitan region and encompasses the watershed of the South Platte River. Main topographic features are labeled in italics and main tectonic features are in bold with a white outline. This structurally complex basin is caught between the Laramide Front Range and Sawatch uplifts. The Neogene Rio Grande rift passes by just to the west along the upper Arkansas River valley.

Figure 1.

Regional map of South Park. South Park Basin sits high in the Rocky Mountains in close proximity to the Denver metropolitan region and encompasses the watershed of the South Platte River. Main topographic features are labeled in italics and main tectonic features are in bold with a white outline. This structurally complex basin is caught between the Laramide Front Range and Sawatch uplifts. The Neogene Rio Grande rift passes by just to the west along the upper Arkansas River valley.

Figure 2.

Stratigraphic column for South Park. Strata in the South Park Basin span a nearly complete section of Paleozoic through Cenozoic strata that includes marine carbonate and clastic sediments as well as nonmarine clastic sediments and volcanic rocks. Silurian, Triassic, and Pliocene strata have not been identified in South Park and these ages are not shown in the stratigraphic column. This field trip focuses on the synorogenic sediments deposited during the Pennsylvanian-Permian ancestral Rocky Mountain orogeny, the Upper Cretaceous-Eocene Laramide orogeny, and Eocene-Oligocene sediments and volcanic rocks formed following the Laramide orogeny.

Figure 2.

Stratigraphic column for South Park. Strata in the South Park Basin span a nearly complete section of Paleozoic through Cenozoic strata that includes marine carbonate and clastic sediments as well as nonmarine clastic sediments and volcanic rocks. Silurian, Triassic, and Pliocene strata have not been identified in South Park and these ages are not shown in the stratigraphic column. This field trip focuses on the synorogenic sediments deposited during the Pennsylvanian-Permian ancestral Rocky Mountain orogeny, the Upper Cretaceous-Eocene Laramide orogeny, and Eocene-Oligocene sediments and volcanic rocks formed following the Laramide orogeny.

Figure 3.

Generalized geologic map of the South Park area. The South Park Basin preserves Phanerozoic sedimentary and volcanic rocks between uplifted Precambrian crystalline uplifts. Much of the topographic basin is mantled by Quaternary glacial outwash and other alluvial deposits (after Barkmann et al., 2015). Major structural features are shown in Figure 4.

Figure 3.

Generalized geologic map of the South Park area. The South Park Basin preserves Phanerozoic sedimentary and volcanic rocks between uplifted Precambrian crystalline uplifts. Much of the topographic basin is mantled by Quaternary glacial outwash and other alluvial deposits (after Barkmann et al., 2015). Major structural features are shown in Figure 4.

Figure 4.

Principal structural features of South Park. A series of north to northwest thrust and reverse faults and subsidiary folds, with histories of movement from Precambrian through Neogene, dominates the structural fabric of South Park. Other features include crosscut-ting faults that appear to compartmentalize the main features, as well as a number of normal faults and large downwarps. Only those surface features described in the text are labeled. The Hayden lineament, an inferred boundary separating the Pennsylvanian-Permian central Colorado trough from the ancestral Front Range uplift, underlies this complex fabric. Adapted from Scarbrough (2001) and Ruleman et al. (2011). ant—anticline; syn—syncline.

Figure 4.

Principal structural features of South Park. A series of north to northwest thrust and reverse faults and subsidiary folds, with histories of movement from Precambrian through Neogene, dominates the structural fabric of South Park. Other features include crosscut-ting faults that appear to compartmentalize the main features, as well as a number of normal faults and large downwarps. Only those surface features described in the text are labeled. The Hayden lineament, an inferred boundary separating the Pennsylvanian-Permian central Colorado trough from the ancestral Front Range uplift, underlies this complex fabric. Adapted from Scarbrough (2001) and Ruleman et al. (2011). ant—anticline; syn—syncline.

Reference List of Published Geologic Maps Covering the South Park Region (in alphabetical order by scale)

Table 1.
Reference List of Published Geologic Maps Covering the South Park Region (in alphabetical order by scale)
Map nameScaleAuthorsYear published
Alma1:24,000Widmann et al.2004
Antero Reservoir1:24,000Kirkham et al.2012
Breckenridge1:24,000Wallace et al.2002
Cameron Mountain1:24,000Wallace and Lawson2008
Castle Rock Gulch1:24,000Wallace and Keller2003
Climax1:24,000McCalpin et al.2012
Como1:24,000Widmann et al.2005
Elkhorn1:24,000Ruleman and Bohannon2008
Fairplay East1:24,000Kirkham et al.2006
Fairplay West1:24,000Widmann et al.2007
Garo1:24,000Kirkham et al.2007
Gribbles Park1:24,000Wallace et al.1999
Jefferson1:24,000Barker and Wyant1976
Jones Hill1:24,000Widmann et al.2011
Marmot Peak1:24,000Houck et al.2012
Milligan Lakes1:24,000Wyant and Barker1976
Sulphur Mountain1:24,000Bohannon and Ruleman2009
Guffey1:62,500Wobus and Scott1979
Bailey1:100,000Ruleman et al.2011
Denver West1:100,000Kellogg et al.2008
Park County1:100,000Scarbrough2001
Map nameScaleAuthorsYear published
Alma1:24,000Widmann et al.2004
Antero Reservoir1:24,000Kirkham et al.2012
Breckenridge1:24,000Wallace et al.2002
Cameron Mountain1:24,000Wallace and Lawson2008
Castle Rock Gulch1:24,000Wallace and Keller2003
Climax1:24,000McCalpin et al.2012
Como1:24,000Widmann et al.2005
Elkhorn1:24,000Ruleman and Bohannon2008
Fairplay East1:24,000Kirkham et al.2006
Fairplay West1:24,000Widmann et al.2007
Garo1:24,000Kirkham et al.2007
Gribbles Park1:24,000Wallace et al.1999
Jefferson1:24,000Barker and Wyant1976
Jones Hill1:24,000Widmann et al.2011
Marmot Peak1:24,000Houck et al.2012
Milligan Lakes1:24,000Wyant and Barker1976
Sulphur Mountain1:24,000Bohannon and Ruleman2009
Guffey1:62,500Wobus and Scott1979
Bailey1:100,000Ruleman et al.2011
Denver West1:100,000Kellogg et al.2008
Park County1:100,000Scarbrough2001

This field trip demonstrates the complex interrelations of tectonic events and stratigraphic responses across a suite of three superimposed events. The progression of tectonism starts with the development of the ancestral Rocky Mountains during the Pennsylvanian-Permian, moves through the Laramide orogeny during Late Cretaceous and early Tertiary, and culminates with late extensional deformation and evaporite dissolution that are sculpting the land today. The stops are designed to follow this temporal succession. Stop 1 provides an overview of the entire basin. Stops 2 through 7 illustrate the ancestral Rocky Mountains basin fill, including evaporite facies subject to later dissolution. Stops 8 and 11 illustrate the Laramide synorogenic basin fill, and Stops 9 and 10 illustrate evidence for Laramide compression and subsequent extension and graben formation.

In the broadest terms, the South Park Basin is an eastward-dipping intermontane basin filled with sedimentary rocks preserved between uplifts of Precambrian igneous and metamorphic rocks (Fig. 5). This simplistic view belies great complexity as this field trip will demonstrate. Locally, Cretaceous and Tertiary igneous stocks, sills, and dikes intrude these rocks. Remnants of formerly widespread Tertiary volcanic, lacustrine, and fluvial deposits overlie the older rocks in many parts of the basin. The high ranges bordering South Park were deeply sculpted by Quaternary glaciers, which produced debris deposited in extensive moraine complexes and outwash plains extending well into the basin.

Figure 5.

Schematic diagram of the structural setting of South Park. Only the major structural features defining the basin and surrounding tectonic features are shown in this schematic diagram. South Park is a structural basin caught between two Laramide uplifts that expose cores of Precambrian igneous and metamorphic rocks. The basin preserves Cambrian through Paleocene sedimentary strata. The younger Rio Grande rift system cuts through the Sawatch uplift to the west, forming the upper Arkansas River valley.

Figure 5.

Schematic diagram of the structural setting of South Park. Only the major structural features defining the basin and surrounding tectonic features are shown in this schematic diagram. South Park is a structural basin caught between two Laramide uplifts that expose cores of Precambrian igneous and metamorphic rocks. The basin preserves Cambrian through Paleocene sedimentary strata. The younger Rio Grande rift system cuts through the Sawatch uplift to the west, forming the upper Arkansas River valley.

The basin is structurally complex. The eastern half of the basin is underlain by Proterozoic igneous and metamorphic rocks, whereas the western part of the basin is underlain by thick Paleozoic strata. A series of faults, collectively known as the Hayden lineament (Maughan, 1988), separates the two sides of the basin. Late Cretaceous to Eocene folding, reverse faulting, and thrusting of the Laramide orogeny overprint the earlier structures across the basin (Stark et al., 1949; De Voto, 1971; Chapin and Cather, 1983; Scarbrough, 2001). Although the primary locus of faulting associated with development of the Rio Grande rift system lies to the west of the basin in the upper Arkansas River valley, examples of Neogene extension can be found throughout South Park, as described by Stark et al. (1949), De Voto (1971), and Ruleman et al. (2011). In addition, there is evidence of ongoing local deformation related to dissolution and possible collapse of Paleozoic evaporite deposits (Kirkham et al., 2012).

Structural Evolution of South Park

The primary geologic structures defining South Park consist of a downwarped basin bound by two generally northwest-trending Laramide uplifts (Fig. 5) and the faults that bound them, with the Elkhorn thrust on the east side dominating the basin architecture. Secondary to these primary regional structural features are the many faults, fault zones, and folds that bound the primary structural uplifts and basins and deform the rock units. Major structural features recognized in the literature are shown in Figure 4, which is adapted from Scarbrough (2001) and Ruleman et al. (2011) and incorporates findings from the many 1:24,000 quadrangle maps completed during the past two decades (Table 1).

Summary of Age Dates for Tertiary and Cretaceous Igneous Rocks of South Park*

Table 2.
Summary of Age Dates for Tertiary and Cretaceous Igneous Rocks of South Park*
UnitApproximate age
(Ma)
MethodQuadrangleReference
BasaltMioceneStrat.Gribbles ParkWallace et al. (1999)
Gribbles Peak tuff32-3340Ar/39ArGribbles ParkMcintosh and Chapin (1994)
Guffey Peak volcanicsOligoceneStrat.Guffey PeakEpis et al. (1976)
Thirtynine Mile volcanics34K-ArGuffey MountainEpis and Chapin (1974)
White rhyolite porphyry (later)33-35Fission-trackAlmaWidmann et al. (2004)
Antero Fm. Ash in Chase Gulch graben3440Ar/39ArStern (2016, personal commun.)
Antero Fm. ash flow tuff3440Ar/39ArAntero ReservoirKirkham et al. (2012)
Monzogranite porphyry35-40Fission-trackDenver WestKellogg et al. (2008)
Kenosha Pass andesite36Fission-trackBryant et al. (1981)
Wall Mountain Tuff3740Ar/39ArMcIntosh and Chapin (2004)
Eocene andesite3840Ar/39ArAntero ReservoirKirkham et al. (2012)
Buffalo Peaks volcanics3840Ar/39ArJones Hill, Marmot PeakWidmann et al. (2011), Houck et al. (2012)
Biotite quartz latite porphyry38Fission-trackJeffersonBryant et al. (1981)
Quartz monzonite porphyry Monzonite porphyry37-49, 65
42-44
Fission-track,
K-Ar Strat.
Alma AlmaWidmann et al. (2004) Widmann et al. (2004)
Monzodiorite porphyry42-43Strat.AlmaWidmann et al. (2004)
Quartz monzonite porphyry (early)37-49Strat.Fairplay WestWidmann et al. (2007)
South Park Fm. tuff in fine-grained arkosic member56K-ArBryant et al. (1981)
Link Spring Tuff60K-ArBryant et al. (1981)
Porphyritic intrusion6140Ar/39ArAntero ReservoirKirkham et al. (2012)
Granite porphyry of Tumble Hill6040Ar/39ArJones HillWidmann et al. (2011)
Granite porphyry of Black Mtn.6140Ar/39ArJones HillWidmann et al. (2011)
Diorite of Buckskin Gulch42, 67-72Strat.AlmaWidmann et al. (2004)
Conglomerate Member South Park Fm. tuff bed66K-ArBryant et al. (1981)
Reinecker Ridge Volcanic Member67-6940Ar/39ArComo and Fairplay
East
Widmann et al. (2005), Kirkham et al. (2006)
Granite porphyry64-7040Ar/39ArFairplay WestWidmann et al. (2007)
Whitehorn granodiorite69-70K-ArCameron MountainWallace and Lawson (2008)
UnitApproximate age
(Ma)
MethodQuadrangleReference
BasaltMioceneStrat.Gribbles ParkWallace et al. (1999)
Gribbles Peak tuff32-3340Ar/39ArGribbles ParkMcintosh and Chapin (1994)
Guffey Peak volcanicsOligoceneStrat.Guffey PeakEpis et al. (1976)
Thirtynine Mile volcanics34K-ArGuffey MountainEpis and Chapin (1974)
White rhyolite porphyry (later)33-35Fission-trackAlmaWidmann et al. (2004)
Antero Fm. Ash in Chase Gulch graben3440Ar/39ArStern (2016, personal commun.)
Antero Fm. ash flow tuff3440Ar/39ArAntero ReservoirKirkham et al. (2012)
Monzogranite porphyry35-40Fission-trackDenver WestKellogg et al. (2008)
Kenosha Pass andesite36Fission-trackBryant et al. (1981)
Wall Mountain Tuff3740Ar/39ArMcIntosh and Chapin (2004)
Eocene andesite3840Ar/39ArAntero ReservoirKirkham et al. (2012)
Buffalo Peaks volcanics3840Ar/39ArJones Hill, Marmot PeakWidmann et al. (2011), Houck et al. (2012)
Biotite quartz latite porphyry38Fission-trackJeffersonBryant et al. (1981)
Quartz monzonite porphyry Monzonite porphyry37-49, 65
42-44
Fission-track,
K-Ar Strat.
Alma AlmaWidmann et al. (2004) Widmann et al. (2004)
Monzodiorite porphyry42-43Strat.AlmaWidmann et al. (2004)
Quartz monzonite porphyry (early)37-49Strat.Fairplay WestWidmann et al. (2007)
South Park Fm. tuff in fine-grained arkosic member56K-ArBryant et al. (1981)
Link Spring Tuff60K-ArBryant et al. (1981)
Porphyritic intrusion6140Ar/39ArAntero ReservoirKirkham et al. (2012)
Granite porphyry of Tumble Hill6040Ar/39ArJones HillWidmann et al. (2011)
Granite porphyry of Black Mtn.6140Ar/39ArJones HillWidmann et al. (2011)
Diorite of Buckskin Gulch42, 67-72Strat.AlmaWidmann et al. (2004)
Conglomerate Member South Park Fm. tuff bed66K-ArBryant et al. (1981)
Reinecker Ridge Volcanic Member67-6940Ar/39ArComo and Fairplay
East
Widmann et al. (2005), Kirkham et al. (2006)
Granite porphyry64-7040Ar/39ArFairplay WestWidmann et al. (2007)
Whitehorn granodiorite69-70K-ArCameron MountainWallace and Lawson (2008)

Age dates compiled from mapping efforts throughout South Park Cretaceous and Tertiary igneous rocks fall into two episodes: (1) earlier intrusions between 70 and 56 Ma are coeval with the Laramide orogeny; and (2) a second, younger phase, between 49 and 33 Ma, coeval with post-Laramide transition from compressional to extensional tectonism for the region. *Date compilation after Barkmann et al. (2015).

Interpretations of individual structures have varied from author to author and over time. For example, earlier mapping (Stark et al., 1949; De Voto, 1971) interpreted many large-scale folds as traversing the basin. Recent maps revise the interpretation, and indicate that what may have appeared to be a fold is instead the juxtaposition of contrasting bedding orientation across faults or fault zones. Similarly, early efforts attempted to explain structural patterns using large-scale, through-going faults. Recent mapping suggests that strain was likely accommodated by movement along a number of smaller, more dispersed structures (Widmann et al., 2005). The variety in interpretations of structures is in part due to poor exposures that prevent following individual features across the landscape. Quaternary surficial deposits cover many of the critical areas. Identification of structural features and recognition of coherent patterns are also complicated where igneous intrusions are widespread, such as in the Mosquito Range. The same may hold for areas underlain by thick evaporitic facies of the Minturn Formation (Fig. 2), where dissolution and ductile deformation of gypsum and halite beds may impart a chaotic structural fabric.

Early Phases

South Park has undergone a long and complex structural evolution. This field trip focuses on the rock record beginning with Early Pennsylvanian time. Many episodes of deformation, from ductile to brittle, occurred between 1.7 and 0.54 b.y. ago, creating a complex structural fabric in the basement rocks. It is probable that many structural features originating from these early episodes have been reactivated by or were modified by younger events.

Precambrian crystalline igneous and metamorphic rocks are exposed at the surface, or can be found near the surface, in four areas in the South Park region. These old rocks are widely exposed in the Front Range to the east as well as to the west in the core of the Mosquito Range, which forms the eastern flank of the Sawatch uplift. Although these crystalline rocks tend to be very resistant to erosion and form high mountain ranges, in South Park they are also found in the subdued, rolling hills of the Elkhorn upland. Precambrian igneous rocks also appear as isolated outcrops beneath Tertiary volcanic and sedimentary cover between Hartsel and the Arkansas hills. Elsewhere in South Park, these rocks are covered by thick sections of younger units.

Cambrian through Mississippian sedimentary rocks underlie the western half of South Park. These units are present at, or near, the surface along a narrow band flanking the east side of the Mosquito Range. Prevailing dips of the strata and stratigraphic evidence indicate that the units extend in an eastward direction through the subsurface into the South Park Basin. In the subsurface, the units truncate against the reverse faults of the late Paleozoic uplifts to the east (Fig. 4; De Voto, 1971; Ruleman et al., 2011). These sediments record a series of marine flooding events across the continent’s interior and include the Cambrian Sawatch Sandstone and Dotsero Formation; Ordovician Manitou Formation, Harding Sandstone, and Fremont Dolomite; Devonian Chaffee Group; and Mississippian Leadville Limestone (Fig. 2).

Ancestral Rocky Mountain Orogeny

Today’s landscape, however, owes much of its character to tectonic phases commencing with Pennsylvanian-Permian uplift of the ancestral Rocky Mountains and development of the central Colorado trough (De Voto, 1972; Ruleman et al., 2011). Tectonic activity from the Early Pennsylvanian through the Permian resulted in the development of a series of mountain ranges and sedimentary basins across western North America. Figure 6 shows the mountain ranges and sedimentary basins that were present in Colorado during the middle part of the Pennsylvanian Period. As the ranges rose, erosion removed the older Paleozoic sedimentary cover, exposing the Precam-brian metamorphic and igneous cores of the uplifts. Concurrently, clastic sediments, carbonates, and evaporite deposits accumulated in the subsiding basins (De Voto, 1971; Kirkham et al., 2007; Ruleman et al., 2011; Houck et al., 2012; Kirkham et al., 2012).

Figure 6.

Middle Pennsylvanian paleogeographic map showing Colorado’s major uplifts and sedimentary basins during Middle Pennsylvanian time. Locations of the field-trip stops are also shown, as are locations of the Gore (G), Boreas Pass (B), Agate Creek (A), and Pleasant Valley (P) faults. The faults bounding the east side of the central Colorado trough are collectively known as the Hayden lineament (Maughan, 1988). The ancestral Front Range uplift, Woodland Park block, Ute Pass uplift, and Apishapa uplift are collectively known as Frontrangia (Mallory, 1958).

Figure 6.

Middle Pennsylvanian paleogeographic map showing Colorado’s major uplifts and sedimentary basins during Middle Pennsylvanian time. Locations of the field-trip stops are also shown, as are locations of the Gore (G), Boreas Pass (B), Agate Creek (A), and Pleasant Valley (P) faults. The faults bounding the east side of the central Colorado trough are collectively known as the Hayden lineament (Maughan, 1988). The ancestral Front Range uplift, Woodland Park block, Ute Pass uplift, and Apishapa uplift are collectively known as Frontrangia (Mallory, 1958).

During the Pennsylvanian and Permian Periods, Colorado was near the equator and on the western edge of Pangea (Stanley and Luczaj, 2015). The formation of Pangea caused the climate in North America to become gradually drier and more seasonal, and this was especially true in western North America (Rowley et al., 1985; DiMichele et al., 2013). These climate changes are evident in the changing lithologies and floras preserved in South Park. Coaly shale with fossils of spore-bearing plants in the older Pennsylvanian rocks gives way to evapo-rites, calcic paleosols, and fossils of seed-bearing plants in the younger Pennsylvanian and Permian (?) rocks. Late Paleozoic glaciations in the Southern Hemisphere are thought to have resulted in high-frequency sea-level changes, especially during the Middle Pennsylvanian Epoch (Heckel, 1986; Ross and Ross, 1987; Rygel et al., 2008). These are evident in the abrupt vertical facies changes from nonmarine to marginal marine to marine deposits in the late Paleozoic rocks of South Park. Pennsylvanian and Permian (?) sedimentary rocks occur in the western half of South Park and include the Belden, Minturn, and Maroon Formations. The Garo Sandstone is also included by some as part of this interval (DeVoto, 1965). To the south, the interval includes the Kerber and Sharpsdale Formations. Sediments deposited within the central Colorado trough tend to be easily eroded, forming the broad gentle landscape between the Red Hill hogback and the Mosquito Range.

North of Jefferson and south of Hartsel, the eastern edge of the central Colorado trough is relatively well constrained. However, between Jefferson and Hartsel, critical areas are covered by Mesozoic and Cenozoic rocks and surficial deposits. Unpublished subsurface data suggest that a concealed reverse fault with a trend and location similar to Reinecker Ridge (Fig. 6) formed the east edge of the trough. An alternate hypothesis, proposed by Kluth and McCreary (2006) and Sweet and Soreghan (2010), is that between Jefferson and Hartsel, the trough had no eastern edge and was instead connected to the Denver Basin through a trough, informally named the Woodland Park trough. However, the east-west faults that would have bounded such a trough have yet to be identified.

Post-Ancestral Rocky Mountain Orogeny Phase

Following the ancestral Rocky Mountain uplift phase, the region was sub-aerially exposed, subject to erosion combined with minor sediment deposition from Permian (?) through Jurassic Periods. Triassic strata have not been described in the South Park region (Stark et al., 1949; Scarbrough, 2001; Ruleman et al., 2011). Jurassic strata consist of the Morrison Formation and possibly the Garo Sandstone. Historically, the Garo Sandstone was considered to be Jurassic in age (Singewald, 1942; Stark et al., 1949) and possibly correlative with the Entrada Sandstone found farther to the west in the Colorado Plateau region (Scarbrough, 2001). Assignment of the Garo Sandstone to the Permian or Jurassic continues to be uncertain and poses a critical question about timing. It is found directly above both Precambrian igneous rocks of the Ute Pass uplift to the east and the central Colorado trough sediments to the west. This relation signals beveling of the landscape following the ancestral Rocky Mountain orogeny, and if the Garo Sandstone is found to be Permian in age, beveling would have been achieved much earlier than if it is found to be Jurassic.

The Western Interior Seaway

The next major phase of structural evolution began during the Cretaceous, a time marked by broad regional downwarp in Colorado, allowing advance of the Western Interior Seaway. Marine sediments blanketed the entire Rocky Mountain region for a period of over 30-40 Ma, marking the encroachment and eventual retreat of the Western Interior Seaway (Cobban, 1993; Cobban et al., 2006). Volumetrically, marine shale dominates sediments that accumulated during this period. Sedimentary units deposited during this marine incursion in South Park include the Dakota Sandstone, Benton Group, Niobrara Formation, Pierre Shale, Apache Creek Sandstone Member of the Pierre Shale, Fox Hills Sandstone, and Laramie Formation. These units are genetically linked by their direct association with the interior seaway, representing marine, shoreline, or coastal plain environments that are preserved in a broad arcuate belt extending across the middle of South Park from the Continental Divide north of Como to Hartsel in the south.

Laramide Orogeny

The next phase of tectonism began in the Late Cretaceous with initiation of the Laramide orogeny. Upper Cretaceous, Paleocene, and Eocene sediments record the evolution of this orogeny throughout the Rocky Mountain region (Chapin and Cather, 1983; Raynolds, 1997). During this period of uplift, which lasted from ca. 70–45 Ma (Sonnenberg and Bolyard, 1997; Dickinson et al., 1988, Dechesne et al., 2011), a series of Precam-brian basement-core blocks rose to the surface while basins subsided between and flanking the blocks. A precursor to this phase was retreat of the interior seaway and eastward migration of the Fox Hills shoreline. Eventually, basement-cored uplifts rose and were eroded of older Paleozoic and Mesozoic sedimentary cover, exposing their Precambrian basement cores (Raynolds, 1997). Clastic sediments shed off the rising blocks accumulated in the basins. Concurrent igneous activity contributed volcanic material to the initial sedimentary basin fill.

Although modified by later Neogene events (Ruleman et al., 2011), Laramide tectonism generated much of the present-day South Park Basin geologic framework. Progressive Laramide tectonism, dominated by westward-directed movement along the Elkhorn and other thrusts, deformed early synorogenic sediments. The geometry and style of uplift evolved during this prolonged period of uplift and details are still being unraveled through ongoing surface and subsurface mapping.

The South Park Formation represents the synorogenic sediments deposited in the South Park Basin during the Laramide orogeny. It has been subdivided into six formal and informal members based on compositional differences reflecting changes in source areas as Laramide tectonism evolved. Members consist of the Reinecker Ridge Volcanic Member, a conglomerate member, the Link Spring Tuff Member, a finegrained arkosic member, and an upper syntectonic conglomeratic unit (Sawatzky, 1967; Wyant and Barker, 1976; Widmann et al., 2005; Kirkham et al., 2006; Ruleman and Bohannon, 2008; Ruleman et al., 2011). A localized lower volcaniclastic member, not shown in Figure 2, is recognized east of Fairplay. Sedimentation continued as deformation progressed with accumulation of syntectonic conglomerates in the upper part of the South Park Formation as recognized by Ruleman et al. (2011) and Echo Park Alluvium, which appears to fill fault-bounded grabens along the Current Creek fault system south of the South Platte River (Chapin and Cather, 1983).

The Eocene Echo Park Alluvium is not always considered part of the South Park Formation. This unit is a boulder-rich, poorly stratified alluvium containing clasts of Precambrian crystalline rock and is limited to small areas in the southern part of South Park (Epis and Chapin, 1974; Scarbrough, 2001). Its relation to the younger members of the South Park Formation is not clear.

Outcrop patterns suggest that the South Park Formation was deposited on a landscape that was already active. Contact relations at the base of the South Park Formation suggest that there had been considerable deformation and differential uplift and erosion of the older sediments before the first South Park Formation deposition. Figure 7 is an interpretive map of pre-South Park Formation outcrop patterns of the Pierre Shale, Fox Hills Sandstone, and Laramie Formation based on outcrop relations at the base of the South Park Formation. The Laramie Formation is either very thin or absent in the southern part of the basin, where published maps indicate that the South Park Formation directly overlies Fox Hills Sandstone. Our Stop 8 will highlight a thrust-cored fold in the Fox Hills Sandstone that is beveled by an unconformity surface overlain by South Park Formation. Farther to the west, the South Park Formation has been mapped in direct contact with Pierre Shale with no evidence of Fox Hills Sandstone. This suggests that greater uplift had occurred in this direction prior to South Park Formation deposition. This uplift may represent early emergence of the Sawatch uplift, or early movement on the South Park fault zone. Clasts of Paleozoic formations found in the lower part of the South Park Formation suggest early emergence of the Sawatch uplift. Clast composition of the South Park Formation becomes dominated by Precambrian lithologies higher in the section, suggesting deeper incision of the Sawatch uplift, or later emergence of the Front Range uplift (Sawatzky, 1967; Ruleman and Bohannon, 2008). Later uplift of the Front Range block is also indicated by westward-directed deformation of the older South Park Formation by the Front Range uplift along the Elkhorn thrust.

Figure 7.

Pre-South Park Formation geologic map. Interpreted bedrock formations at the base of the South Park Formation. Distribution is based on outcrop relations where South Park Formation base is in direct contact with older formations, and has been projected beneath mapped extent of the South Park Formation.

Figure 7.

Pre-South Park Formation geologic map. Interpreted bedrock formations at the base of the South Park Formation. Distribution is based on outcrop relations where South Park Formation base is in direct contact with older formations, and has been projected beneath mapped extent of the South Park Formation.

Chapin and Cather (1983) proposed that the later stages of Laramide deformation reflect a change to a more northerly directed compressive stress and that the style of deformation changed from one dominated by east-west compression to one of right-lateral wrench faulting. This later phase may have been characterized by the growth of smaller fault-bound basins that accommodated the Echo Park Alluvium. Drilling-induced fractures showing a well-defined, east-west (095°) maximum principal stress in a deep well in South Park (Hunt Tarryall Federal 1-17,17-T10S-R75W) reflect relic Laramide stresses and align well with typical Laramide contraction vectors defined along the eastern flank of the Front Range (Erslev et al., 2004). Such fractures also occur in outcrop across the basin and argue against the late-stage wrench faulting proposed by Chapin and Cather (1983).

Cretaceous and Tertiary Igneous Activity

Igneous intrusions with Late Cretaceous through Tertiary ages are common in the South Park region (Table 2). The largest, and oldest, is the Whitehorn granodiorite laccolith that extends into the southwest corner of Park County (Wallace and Lawson, 2008). Other mapped intrusions include sills, dikes, and small stocks of felsic to intermediate composition concentrated in the Mosquito Range and Continental Divide (Barker and Wyant, 1976; Scarbrough, 2001; Widmann et al., 2004, 2005, 2007).

Generally, igneous intrusive activity in the area falls into two episodes correlative with volcanic activity in the region. Earlier intrusions between 70 and 56 Ma are coeval with Laramide deformation and the Reinecker Ridge Volcanic Member of the South Park Formation. Later, more felsic, intrusions between 49 and 33 Ma are coeval with post-Laramide volca-nism and sedimentation described below. The younger and more felsic intrusive bodies are more prevalent in the northern part of the region, where the greater resistance to erosion of the intrusive rocks and the adjacent altered host sediments hold up the high mountainous terrain.

Post-Laramide Phase

The region continued to be modified by tectonism following Laramide deformation, although the style changed. Rock units in South Park Basin record continued volcanism accompanied by fluvial and lacustrine sedimentation. A prolonged period of broad erosion led to beveling of the landscape following the Laramide tectonic event. The resulting surface upon which the post-Laramide volcanic and sedimentary units were deposited has been referred to as the late Eocene surface (Epis et al., 1976).

Post-Laramide volcanism in the area began ca. 38 Ma with eruptions near Buffalo Peaks and continued through ca. 33 Ma in the Thirtynine Mile volcanic center. During this period, much of the region was blanketed by volcanic rocks of varying composition. Sources include local eruptive centers such as the Thirtynine Mile and Guffey Mountain volcanic center at the southern end of South Park (Epis et al., 1976), and a postulated center just west of the Buffalo Peaks complex (Widmann et al., 2011; Kellogg et al., 2013). Many of the units, such as the Wall Mountain Tuff and Gribbles Peak Tuff, are believed to have been erupted from the caldera complexes associated with plutonism in the Sawatch Range and Bonanza caldera to the west, and southwest on the west side of the upper Arkansas River valley prior to development of the Rio Grande rift (McIntosh and Chapin, 2004).

Sediments accumulated in the region contemporaneously with active volcanism in the Eocene, Oligocene, and Miocene Epochs. Stratigraphic relations between sediments and volcanic rocks are complex and continue to be refined through ongoing mapping efforts. The Eocene Tallahassee Creek Conglomerate and Oligocene Antero Formation are interbedded with the volcanic flows and tuffs, while the Miocene Wagontongue Formation was deposited later after quiescence of the Oligocene volcanic event. Development of the Chase Gulch half-graben on the hanging wall of the Elkhorn thrust (Fig. 5) may have started as early as Oligocene as evidenced by a thick accumulation of Antero Formation sediments in its depocenter.

Cenozoic development of the Rio Grande rift system, which extends up through New Mexico into Colorado, represents the most recent phase of tectonism affecting the region. This transition represents a change from a compressional to an extensional stress regime. The later stress regime overprinted older structural features and, in places, reactivated them with opposite displacement. The main part of this rift system passes through the upper Arkansas Valley just to the west, where it divided the former Sawatch uplift into the modern Sawatch Range and the Mosquito Range. Extensional tectonism may have begun ca. 28 Ma (Landman and Flowers, 2013) accompanied by deposition of Dry Union Formation to the west in the upper Arkansas Valley part of the rift and Wagontongue Formation and/or Trump sandstone in the South Park area. The Wagontongue does not appear to have been deposited in a fault-bound graben and its relation to the Rio Grande rift is unclear.

Ruleman et al. (2011) and Houck et al. (2012) have identified many features and relations within South Park that suggest post-Laramide deformation was active in the area. Several faults in South Park display evidence of normal offset related to Neogene extension (Ruleman et al., 2011). The topographic expression of the Tarryall Mountains rising above the Elkhorn upland may be attributed to late Tertiary movement along faults parallel to Tarryall Creek (Sterne, 2006). The Elkhorn upland preserves a scattered veneer of post-Laramide volcanic and sedimentary cover that is absent on the top of the Tarryall Mountains, suggesting that the Tarryall Mountains have risen during Cenozoic extensional tectonism.

The Chase Gulch half-graben east of Spinney Reservoir may also be a feature formed by Neogene extension. Evolution of this half-graben deserves further investigation as it may represent one of the best preserved rift-related structural features in the South Park Basin. It also may have hydrologic significance as it may be filled with relatively permeable coarse-grained sediments. Similarly, the Oligocene Fairplay paleovalley sediments just south of Fairplay, described by Kirkham et al. (2006), appear to be preserved in a fault-bounded half-graben.

Evaporitic Tectonism in South Park

Houck et al. (2012) and Kirkham et al. (2007, 2012) have identified features in the southwest part of South Park that may owe their origin to dissolution and plastic flow of beds of evaporite minerals within the Minturn Formation. Evaporitic tectonism has been recognized in other parts of the state with similar strata, where it has played a major role in shaping the geologic landscape (Kirkham et al., 2001). In other parts of the central Colorado trough (Freeman, 1971), as well as the Paradox Basin (Trudgill and Paz, 2009), evaporite tectonism started in the Permian, forming structural features before the onset of the Laramide orogeny. While evaporite deposits have long been known in the Pennsylvanian section in this region, their role in the structural evolution of South Park is not well understood. Evidence for dissolution has been well documented by recent 7.5 min quadrangle mapping in the Garo and Antero Reservoir areas (Fig. 3; Kirkham et al., 2007, 2012). Differential movement in the evaporitic facies, diapiric flow, and collapse may all modify the Laramide and post-Laramide features where these sediments are present at depth.

Quaternary Alluvium and Glacial Deposits

Quaternary deposits include extensive glacial drift and outwash deposits along with post-glacial alluvium along modern streams. Alpine glaciers during several stages carved the higher valleys of the Mosquito Range along the west side of South Park and the Continental Divide to the north. Glacial drift forms extensive moraine complexes at the mouths of valleys at the base of the ranges (Widmann et al., 2004, 2007). Out-wash deposits form series of terraces fanning out into the park below the moraine complexes. Mountains on the east and south sides of South Park, as well as interior ridges and highlands, do not display evidence of past glaciations, but emanating streams deposited extensive alluvium (Ruleman et al., 2011). Placer gold deposits have been exploited from many of the alluvial sediments, particularly those associated with streams originating in the mineralized mountains on the north and northwestern sides of the basin.

Logistics and Hazards of the Field Trip

This field trip covers 185 km (115 mi) along lightly traveled highways and dirt roads. The countryside is typically open and exposed with altitudes rising above 2590 m (8500 ft), so sun protection and adequate clothing for variable weather conditions are a must. There are opportunities for restrooms and food at convenience stores in Fairplay and Hartsel, otherwise the stops are fairly remote; water, snacks, and lunch should be packed. Stop 2 at Red Hill Pass and Stop 6 at Trout Creek pass have outcrops close to a busy high-speed highway and visitors must be mindful at all times. Several stops stay on public right-of-way or public lands unless described in the stop narrative; participants should not cross fences unless instructed by trip leaders where permission has been obtained in advance for the trip.

South Park Road Log

Listed mileage (in bold italics) includes both specific stops and points of geologic interest. Because automobile odometers are in miles, all distances are listed with miles first and kilometers in parentheses. Location information for each stop is listed as decimal latitude and longitude, WGS84. Where figures are included for a specific stop, numbering starts with the stop number followed by the number figure for that stop (i.e., S1-1).

This road log starts on Kenosha Pass in Park County, ~64 mi (103 km) south of Englewood, Colorado, on U.S. Highway 285. To get there from the Colorado Convention Center in Denver, follow Champa Street south to where it turns into Kalamath Street. Kalamath Street eventually becomes Santa Fe Drive. Continue on Santa Fe Drive south and exit on to Hampden Avenue/U.S. Highway 285 southbound. Distance from the convention center to the U.S. Highway 285 exit is ~6.4 mi (10.3 km).

Mile 0—STOP 1: Scenic Overlook of South Park

39.4035° N, 105.75457° W; elevation 3011 m (9880 ft). This stop, 0.8 mi (1.3 km) south of the trailhead parking area at Kenosha Pass, will serve as an introduction to the basin where general landmarks will be pointed out (Fig. S1-1). If permission has been obtained, participants will cross a fence onto private land on the east side of the highway just beyond the scenic overlook. Readers taking this trip on their own must obtain permission in advance.

Figure S1-1.

Photograph of South Park from Kenosha Pass. One of Colorado’s iconic views looking southwest from the scenic overlook on the south side of Kenosha Pass greets the viewer with the broad expanse of South Park and the Mosquito Range in the distance. If atmospheric conditions allow, several 4267+ m (14,000+ ft) peaks in the Sawatch Range may also be visible.

Figure S1-1.

Photograph of South Park from Kenosha Pass. One of Colorado’s iconic views looking southwest from the scenic overlook on the south side of Kenosha Pass greets the viewer with the broad expanse of South Park and the Mosquito Range in the distance. If atmospheric conditions allow, several 4267+ m (14,000+ ft) peaks in the Sawatch Range may also be visible.

We will use regional cross section A-A’ (Fig. S1-2) as a recurring guide during our circuit through the South Park Basin. The section shows that South Park encompasses three distinct types of structural basins of different ages. The first, which we will traverse in our morning stops, covers the western part of South Park and represents a portion of the central Colorado trough. The eastern edge of the trough has been called the Hayden lineament (Maughan, 1988) and trends north-south across the central portion of South Park. Seismic data (Fig. S1-3; line CSM 74-1, modified from Beggs, 1977) reveals upwards of 3048 m (10,000 ft) of synorogenic Minturn and Maroon section deposited on lower Paleozoic rocks west of a system of east-dipping faults that form the Hayden lineament. In places, the seismic and well (Amoco Reinecker Ridge #1, 34-T9S-R76W) data indicate intermediate structural blocks present beneath the thrust along the Hayden lineament and above the floor of the central Colorado trough. East of the lineament, Jurassic rocks rest directly on basement of the ancestral Front Range uplift (Mallory, 1958). The exception to this may occur within the postulated Woodland Park block (Fig. 6), where a relatively thin section of lower Paleozoic and Pennsylvanian-Permian strata may have been present across the axis of the ancestral Front Range uplift prior to Laramide uplift and erosion of the Front Range.

Figure S1-2.

Detailed cross section A-A’ through the South Park Basin. This balanced cross section through the South Park Basin is based on seismic data and oil and gas well subsurface data. It shows the complex structural framework of the basin. The partial annealing zone (PAZ), based on apatite fission-track (AFT) data, approximates the base of Phanerozoic sedimentary cover prior to the Laramide orogeny (Kelley and Chapin, 1997). S.L.—sea level. See Figure S1-4 for cross-section location.

Figure S1-2.

Detailed cross section A-A’ through the South Park Basin. This balanced cross section through the South Park Basin is based on seismic data and oil and gas well subsurface data. It shows the complex structural framework of the basin. The partial annealing zone (PAZ), based on apatite fission-track (AFT) data, approximates the base of Phanerozoic sedimentary cover prior to the Laramide orogeny (Kelley and Chapin, 1997). S.L.—sea level. See Figure S1-4 for cross-section location.

Figure S1-3. Time-migrated seismic lines across the eastern half of South Park. Time-migrated seismic lines, courtesy of Hunt Oil Company and Colorado School of Mines (modified from Beggs, 1977), show the principal thrusts and structures in the eastern part of the South Park Basin. The locations of the seismic lines are shown on Figure S1-4. Formation annotation is as follows: p-C—Precambrian basement; lPz—lower Paleozoic; IP-P—Pennsylvanian to Permian Belden, Minturn, and Maroon Formations; Jm-Kd—Jurassic Morrison Formation and Cretaceous Dakota Sandstone; Kn-Kpl—Cretaceous Benton Group, Niobrara Formation, lower Pierre Shale, and Apache Creek Sandstone; Kpu—Creta-ceous Pierre Shale; Kpu-Kfl—Cretaceous Pierre Shale, Fox Hills Sandstone, and Laramie Formation (?); KTsp—Cretaceous to Tertiary South Park Formation; Ta—Oligocene Antero Formation.

Figure S1-3. Time-migrated seismic lines across the eastern half of South Park. Time-migrated seismic lines, courtesy of Hunt Oil Company and Colorado School of Mines (modified from Beggs, 1977), show the principal thrusts and structures in the eastern part of the South Park Basin. The locations of the seismic lines are shown on Figure S1-4. Formation annotation is as follows: p-C—Precambrian basement; lPz—lower Paleozoic; IP-P—Pennsylvanian to Permian Belden, Minturn, and Maroon Formations; Jm-Kd—Jurassic Morrison Formation and Cretaceous Dakota Sandstone; Kn-Kpl—Cretaceous Benton Group, Niobrara Formation, lower Pierre Shale, and Apache Creek Sandstone; Kpu—Creta-ceous Pierre Shale; Kpu-Kfl—Cretaceous Pierre Shale, Fox Hills Sandstone, and Laramie Formation (?); KTsp—Cretaceous to Tertiary South Park Formation; Ta—Oligocene Antero Formation.

The second structural basin type, which will be the focus of some of our afternoon stops, largely shapes South Park as we see it today and is the product of uppermost Cretaceous to lower Tertiary Laramide contraction. On the east, the Elkhorn thrust is the southern segment of the Elkhorn-Williams Range thrust system that carries the Front Range westward over the Mesozoic sedimentary fill of the South Park and Middle-North Park basins. Structures within the sedimentary section below the Elkhorn thrust consist of bedding-parallel to low-angle fore- and backthrusts as shown on regional section A-A’ (Fig. S1-2) and in Figure S1-4, which depicts the time structure of the Cretaceous Dakota Sandstone. The time-structure map is based on ~322 km (250 mi) of seismic data and the 22 wells drilled for hydrocarbon exploration in the eastern part of South Park. Approximately 21 km (13 mi) of translation are required to retrodeform the Elk-horn thrust and the thrusts in its footwall (Sterne, 2006).

Figure S1-4. Time-structure map on the top of the Cretaceous Dakota Formation showing the principal structures in the eastern part of the South Park Basin. The map is based on surface geology, and subsurface well and seismic control.

Figure S1-4. Time-structure map on the top of the Cretaceous Dakota Formation showing the principal structures in the eastern part of the South Park Basin. The map is based on surface geology, and subsurface well and seismic control.

Control for the cross section across eastern South Park (Fig. S1-2) is based in part on a time migration of a seismic line (Fig. S1-3; line provided courtesy of Hunt Oil Co.), which passes through the Hunt/McMurry Tarryall Federal #1-17 well (sec 17, T10S-R75W). The well drilled 591 m (1940 ft) of basement carried in the hanging wall of the Elkhorn thrust before penetrating a subthrust section of Cretaceous-Tertiary South Park Formation and thrust-repeated Cretaceous Pierre Shale. The seismic image of the Elkhorn thrust and its footwall is excellent compared to most lines crossing the margins of basement uplifts. This appears to be a function of the lack of a topographic step at the thrust trace, and the lack of a significant velocity contrast between the subthrust sedimentary section and the hanging wall basement. The sonic log for the well shows an anomalously low basement interval velocity of 4265 m/s (14,000 ft/s) likely due to extensive fracturing at the thrust lip.

The third, and final, basin type observed in South Park consists of graben development during Tertiary extension in the hanging wall of the Elkhorn thrust (Fig. S1-2). As seen throughout the western Cordillera, Laramide contraction was followed by varying degrees of Tertiary extension (Sterne and Constenius, 1997). Such extension is largely responsible for the subdued hanging wall topography and has localized a series of grabens along the southern extents of the Elkhorn thrust, which we will visit at one of our afternoon stops. The regional section (Fig. S1-2) also shows a normal fault in the western part of the basin that is the continuation of Tertiary Rio Grande rift extension seen in the Blue River valley to the northwest near Breckenridge (Landman and Flowers, 2013).

From this vantage point, we can look west and discern many of the features we will see today as we traverse South Park Basin. On the western horizon rise the peaks of the Mosquito Range, which forms the eastern dip slope of the larger Laramide Sawatch uplift. Coming east in the gap at one o’clock we can make out the roadcut leading up the hogback to Red Hill Pass. This north-south ridge is held up by the Garo, Morrison, and Dakota members, which are underlain by thick fill of the central Colorado trough. The next prominent ridge to the east is Reinecker Ridge. This north-south topographic feature represents inverted topography held up by resistant South Park Formation caught in a syncline. This syncline is the western flank of the Reinecker Ridge anticlinorium, which formed during Laramide reactivation of the underlying splays of the Hayden lineament. The eastern flank of the Reinecker Ridge anticlinorium is cut by the low-angle, west-directed South Park and San Isabel thrusts. These Laramide thrusts are foot-wall imbricates to the Elkhorn thrust and have created structures across the valley floor below our vantage point. These include the Jefferson anticline, whose north-south axis passes through the town of Jefferson, and the Michigan Hill syncline, which underlies the ridge beyond Jefferson held up by resistant South Park Formation caught in its core. At our final stop, we will visit the axis of the Michigan Hill syncline as it plunges beneath the Elkhorn thrust. Finally, we are standing on Pre-cambrian gneiss carried by the Laramide Elkhorn thrust. The low ridge at 10 o’clock marks the bound between the basement and the sedimentary section, which continues to the south as the trace of the Elkhorn thrust. We will visit the fault plane at one of our afternoon spots.

From this stop, the field trip will follow U.S. Highway 285 ~16 mi (26 km) to Red Hill Pass.

0.4 mi (0.6 km): On the north side of the highway, a road-cut exposes blocks of highly fractured tan, fine- to medium-grained sandstone and variegated mudstone that resemble Morrison Formation. The hillside above is covered with colluvium made up of clasts of Pre-cambrian gneiss and pegmatite, but has been mapped as mixed metamorphic and igneous rock (Barker and Wyant, 1976). Low hills to the northwest rising above the basin floor are bodies of Eocene (37.6 Ma) quartz biotite porphyry (Bryant et al., 1981). The intrusive body appears to follow the contact between the Mesozoic sediments to the west and Precambrian gneiss and granitic rocks to the east. If the contact is a fault, the intrusion may have been controlled by that fault.

2.5 mi (4.0 km): The low hill on the north side of highway has been mapped as Fox Hills Sandstone, Laramie Formation, and South Park Formation (lower Reinecker Ridge Volcanic Member and conglomerate member) by Barker and Wyant (1976). The units dip almost due west but are poorly exposed. The quadrangle map shows that the Pierre Shale between here and Kenosha Pass has been attenuated by the Wahl strand of the Elkhorn-Williams Range thrust.

From here the highway crosses over broad outwash fans of alluvium deposited by Pleistocene courses of Jefferson, Michigan, and Tarryall Creeks.

Advance and Retreat of the Western Interior Seaway

The westward progression of the field-trip route descends stratigraphically through sediments that record the advance and retreat of the Western Interior Seaway. In this direction, we see the record starting with the youngest part of the record. The Upper Cretaceous Fox Hills Sandstone and Laramie Formation were deposited in near-shore and beach environments of the retreating Western Interior Seaway. The Fox Hills Sandstone is transitional with the underlying Pierre Shale and consists of yellow-brown to gray-brown fine- to medium-grained sandstone (Ruleman and Bohannon, 2008). Eastward retreat of the Western Interior Seaway caused individual overlapping sandstone bodies to climb up-section and become progressively younger to the east. The total thickness is estimated to be between 46 and 107 m (150 and 350 ft). The Laramie Formation is a nonmarine coastal plain deposit that is transitional to, and interfingers with, the Fox Hills Sandstone. It consists of overbank shale interbedded with lenticular beds of sandstone and coal (Wyant and Barker, 1976) deposited on a low-relief coastal plain following the retreat of the Western Interior Seaway. Near its base, this formation contains coal beds that were exploited in the nineteenth and early twentieth centuries. Thickness is estimated to be up to 91 m (300 ft), and the formation has not been identified at the surface south of Milligan Lakes ~8 km (5 mi) south of here. In the southern part of the South Park Basin, the Laramie Formation either cannot be differentiated from the Fox Hills Sandstone in the field, or it may have been removed by erosion prior to deposition of the younger South Park Formation.

The Pierre Shale consists of recessive gray calcareous marine shale with thin beds of fine- to very fine-grained sandstone; it conformably overlies the Niobrara Formation (Widmann et al., 2005; Kirkham et al., 2006, 2007). Thickness is difficult to estimate and the formation may be highly deformed in South Park, but estimates reach upwards of 1830 m (6000 ft, Barker and Wyant, 1976). The Upper Cretaceous Benton Group and Niobrara Formation have limited surface exposure in South Park, and are sometimes mapped together although they are separated by a disconformity. The Benton Group ranges in thickness between ~60 and 180 m (200 and 600 ft) and the Niobrara Formation ranges in thickness between ~105 and 150 m (350 and 500 ft). The Niobrara Formation consists of the dense gray Fort Hays Limestone and thinly bedded brown calcareous shale and limestone of the Smokey Hill Shale. The Niobrara Formation is the target of recent source bed petroleum exploration throughout the Rocky Mountain region. The Benton Group includes dark-gray shale and siltstone of the Graneros Shale; gray limestone, sandstone, and calcareous shale of the Greenhorn Limestone; and black shale and calcareous sandstone of the Carlile Shale (Widmann et al., 2005; Kirkham et al., 2006, 2007). It rests conformably on the Dakota Sandstone.

The Upper Cretaceous Dakota Sandstone consists of tan to light-gray sandstone, pebble conglomerate, and non-calcareous shale (Widmann et al., 2005; Kirkham et al., 2006, 2007) deposited along the advancing Western Interior Seaway shoreline. Estimated thickness ranges between ~55 and 140 m (175 and 450 ft).

4.8 mi (7.7 km): The low bluffs studded with homes northwest of the highway have been mapped as being underlain by the Reinecker Ridge Volcanic Member and conglomerate member of the South Park Formation. Michigan Hill rises exposing the arkosic member of the South Park Formation. Beds of boulder conglomerate within the arkosic member are folded in the south-plunging Michigan Hill syncline, forming a series of v-shaped ridges.

Exposures of the South Park Formation form an arcuate belt that extends from just north of Jefferson ~45 km (28 mi) south of Hartsel (Fig. 3). This formation has been subdivided into six formal and informal members, starting at the base with the Upper Cretaceous lower volcaniclastic member. This basal member is derived from nearby volcanic sources and is localized to the west side of Reinecker Ridge. It consists of poorly sorted reddish-brown to greenish-brown medium- to coarse-grained sandstone with fragments of volcanic rocks believed derived from a local source (Kirkham et al., 2006). Above this is the Upper Cretaceous to Paleocene Reinecker Ridge Volcanic Member, which contains flows and breccias of trachyandesite, andesite, and dacite (Widmann et al., 2005; Kirkham et al., 2006). It varies in color from purple gray and brown to greenish-gray, to deep-red and contains thin discontinuous layers of gray tuffaceous sandstone and siltstone. 40Ar/39Ar dating of hornblende indicates an age of ca. 69-67 Ma for this unit (Widmann et al., 2005). Thickness relations indicate a possible source to the north (Kirkham et al., 2006). Thickness reaches up to 610 m (2000 ft).

The Paleocene conglomerate member overlies the Reinecker Ridge Member and consists of yellowish-brown, brown, greenish-brown, and greenish-gray conglomerate interbedded with sandstone and mudstone that are partially volcaniclastic or tuffa-ceous. Finer-grained sandstone dominates the unit to the south near Sulfur Mountain. It contains clasts of the older volcanic material mixed with clasts of limited Precambrian basement, Paleozoic rocks, and Cretaceous intrusives, indicating a source from the Sawatch uplift to the west (Widmann et al., 2005; Kirkham et al., 2006; Ruleman and Bohannon, 2008). K-Ar dating of biotite from a tuff bed indicates an age of ca. 66 Ma for this unit (Bryant et al., 1981). Thickness ranges between 366 and 1585 m (1200 and 5200 ft). This unit is overlain by the Paleocene Link Spring Tuff Member, consisting of yellowish-brown to gray laminated tuff with some volcaniclastic breccia and andesitic flows as well as minor cobble conglomerate (Wyant and Barker, 1976; Ruleman et al., 2011). K-Ar radiometric age dating indicates an age of ca. 60 Ma for this unit (Bryant et al., 1981). The unit is found south of Milligan Lakes and reaches a thickness of ~183 m (600 ft). It thins both to the north and south, where it eventually pinches out.

The Paleocene fine-grained arkosic member unconformably overlies the Link Spring Tuff Member in the north and the conglomerate member in the south; however, it is not present as far south as Sulfur Mountain near the town of Hartsel. It consists of pale-brown, greenish-gray, and gray calcareous mudstone, sandstone, siltstone, and conglomerate (Sawatzky, 1967; Wyant and Barker, 1976). The arkosic content and lack of clasts of Paleozoic sedimentary rocks indicates a possible source from the Front Range uplift to the east for this later unit. In the Milligan Lakes and Michigan Hill area, it contains lenses of a very large boulder conglomerate. K-Ar radiometric age dating of a tuff bed indicates an age of ca. 56 Ma for this unit (Bryant et al., 1981). Thickness is as much as 1067 m (3500 ft).

The syntectonic conglomeratic unit (Eocene?) is a poorly sorted, boulder-rich conglomerate believed to be derived from local Precambrian sources, and overlies the other South Park Formation members (Ruleman et al., 2011). This unit occurs along the perimeter of the Elkhorn upland and may have been deposited during uplift of Precambrian rock to the east.

On the left are coal prospects in the Laramie Formation. Laramie Formation coal was mined between 1870 and 1905 from the King (coal) mine ~5 km (3 mi) south of here along the outcrop belt. The mine was the greatest coal producer in the South Park Basin.

8.5 mi (13.7 km): Middle Pleistocene gravel deposits mantle the terraces above modern drainages. The deposits tend to be thin veneers lying on banks of exposed bedrock. Gold placer tailings are visible as bare hills on the right.

9.7 mi (15.6 km): The hills behind Como at 1 o’clock are underlain by the Reinecker Ridge Volcanic Member of the South Park Formation.

10.3 mi (16.6 km): The small closed depression in the Pierre Shale bedrock immediately southeast of the highway forms Como Lake. Several other closed depressions dot the landscape in this area. The origin of these depressions is open for discussion. Farther to the west, numerous sinks and depressions have been attributed to dissolution of evaporite minerals from the Pennsylvanian Minturn Formation. This depression, and a number of other closed depressions farther to the south, occur east of what is believed to be the eastern limit of the Minturn Formation within the central Colorado trough. Their formation is enigmatic, but could be related to eolian or Pleistocene periglacial processes if not to dissolution of deeper evaporitic sediments.

10.7 mi (17.2 km): Roadcut exposes Cretaceous Pierre Shale. The upper part of the Pierre Shale was locally removed by erosion prior to deposition of the younger South Park Formation in the southern part of South Park, where only between 610 and 760 m (2000 and 2500 ft) remain (Kirkham et al., 2007).

11.1-11.3 mi (17.6-18.2 km): In this stretch, roadcuts expose Pierre Shale intruded by Tertiary quartz monzonite porphyry sills. Similar quartz monzonite intrusions in the region range in age between 49 and 37 Ma (Widmann et al., 2005; Table 2).

12.3 mi (19.8 km): The highway crosses an abandoned valley of Trout Creek. Stream capture diverted the drainage to the south, leaving this broad valley 15-18 m (50-60 ft) above the modern stream course. The modern stream course will be crossed at mile 13.4 (21.6 km) ahead.

12.7 mi (20.4 km): Colluvium exposed in the roadcut contains clasts of Upper Cretaceous/Paleocene age Reinecker Ridge Volcanic Member of the South Park Formation.

13.2 mi (21.2 km): A large landslide on the east side of the highway (to the left) brings the Reinecker Ridge Volcanic Member of the South Park Formation down to the level of the highway. The Upper Cretaceous to Paleocene unit overlies Pierre Shale, which is highly prone to slope failure. The roadcut reveals blocks of andes-itic flow material in a disorganized matrix.

14.6 mi (23.5 km): Pierre Shale crops out in the hill on the west side of the highway (to the right). Below the obvious Pierre outcrop near the valley bottom are exposures in several gullies of the Apache Creek Sandstone Member of the Pierre Shale (Fig. 8). The Apache Creek Sandstone Member was the target of petroleum exploration in the 1990s. Straight ahead is the dip-slope of the Dakota hogback where beds dip 35-45° toward the east. Just in front of the Dakota hogback is a grassy bench demarking the outcrop belt of the Benton Group and Niobrara Formation. Other small closed depressions mark the valley bottom to the left.

Figure 8.

Photograph of Upper Cretaceous Apache Creek Sandstone. The Apache Creek Sandstone Member of the Pierre Shale is an offshore marine sandstone that has been the target for oil and gas exploration in South Park. It is exposed in gullies on the north side of U.S. Highway 285 east of Red Hill Pass.

Figure 8.

Photograph of Upper Cretaceous Apache Creek Sandstone. The Apache Creek Sandstone Member of the Pierre Shale is an offshore marine sandstone that has been the target for oil and gas exploration in South Park. It is exposed in gullies on the north side of U.S. Highway 285 east of Red Hill Pass.

15.1-16.0 mi (24.3-25.7 km): The highway crosses the outcrop belt on the east side of the Redhill hogback. As the road climbs the hill, it descends stratigraphically from the Pierre Shale through the Upper Cretaceous Niobrara and Benton Formations into the Dakota Sandstone and then the Jurassic Morrison Formation. There are glimpses of light-colored shaley limestone of the Niobrara Formation in gullies on the west side of the highway (to the right), whereas the Benton Group is mostly covered by colluvium. Dakota Sandstone stands out as buff sandstone and carbonaceous shale, and the recessive Morrison Formation is characterized by greenish- and reddish-gray shale.

15.6 mi (25.1 km): Morrison Formation is exposed in the road-cut. The Upper Jurassic Morrison Formation is made up of interbedded shale, sandstone, claystone, and basal limestone (Widmann et al., 2005; Kirkham et al., 2006, 2007). Thickness ranges from ~60-105 m (200-350 ft). The contact with the overlying Dakota Sandstone is disconformable, marked by evidence of erosion (Kirkham et al., 2007).

Mile 16.2 (26.1 km)—STOP 2: Garo Sandstone at Red Hill Pass

39.26221° N, 105.96134 °W; elevation 3059 m (10,035ft). Stay on the gravel of the highway shoulder to observe outcrops and views of South Park.

This stop is at a roadcut exposing the Garo Sandstone at the crest of the prominent Redhill hogback formed by Dakota Sandstone, Morrison Formation and Garo Sandstone. This conspicuous hogback separates the western part of South Park, which is dominated by Paleozoic rocks, and the eastern part of the basin, which is dominated by Mesozoic rocks and the Elkhorn upland. The Garo Sandstone overlies the Maroon Formation, and consists of calcareous sandstone and conglomerate (Widmann et al., 2005; Kirkham et al., 2006, 2007). Age of the Garo Sandstone is uncertain and has been controversial. Historically, it was considered to be Jurassic in age (Singewald, 1942; Stark et al., 1949) and possibly correlative with the Entrada Sandstone found farther to the west in the Colorado Plateau region (Scarbrough, 2001). De Voto (1965) described evidence of interfingering with the underlying Maroon Formation, and also Permian algae fossils, leading to his conclusion that the Garo Sandstone is Permian. Recent mapping by Widmann et al. (2005) and Kirkham et al. (2012) has adopted the Permian age for the unit. Thickness ranges between ~20 and 70 m (60 and 230 ft).

From this stop, continue ~3.2 mi (5.1 km) south on U.S. Highway 285 into Fairplay. The Sinclair gas station and convenience store on the west side of the highway (to the right) will provide an opportunity to use restrooms and purchase snacks and drinks. Another 0.3 mi after that, the trip continues to the north (right turn) on CO Highway 9 to a pull-off ~0.8 mi (1.3 km) north of Fairplay.

17.2 mi (27.7 km): The view to the west is of Horseshoe Basin, located high in the Mosquito Range at 1 o’clock. Cambrian through Mississippian strata overlie Precambrian in this cirque.

18.4 mi (29.6 km): Maroon Formation dips 20° to 35° to the east in the fields and roadcuts. The small lake to the north is yet another natural depression in bedrock.

19.4 mi (31.2 km): Glacial outwash mantles the terraces and alluvium underlies the modern course of the Middle Fork South Platte River. The modern course of the river is incised into the Pennsylvanian-Permian Maroon and Minturn Formations. The town of Fairplay straddles an Upper Pleistocene outwash terrace and Middle Pleistocene glacial till (Kirkham et al., 2006).

19.6 mi (31.5 km): Turn right onto CO Highway 9.

Mile 20.5 (33.0 km)—STOP 3: Upper Part of the Pennsylvanian Minturn Formation

39.22684° N, 106.00672° W; elevation 3570 m (10,030ft). Turn around and park on the gravel shoulder on the west side of the road (left, heading north). Stay on the wide gravel shoulder perched above the Middle Fork South Platte River.

The outcrop on the east side of the road is composed of both nonmarine (lower two-thirds of section) and shallow marine (upper one-third of section) deposits (Fig. S3-1). The marine deposits have been correlated with the White Quail Member of the Minturn Formation (Widmann et al., 2007), which is middle or late Desmoinesian in the type area (Tweto and Lovering, 1977). Conglomeratic fluvial channel deposits can be seen in the roadcut, as well as finer-grained overbank deposits with rooted horizons. The gravel clasts are Protero-zoic igneous and metamorphic rocks, and the sand is arkosic, indicating erosion of a basement uplift. Paleocurrent directions in this interval are directed southward (Houck, 2013). Fossil conifers have been found in the overbank deposits, indicating a seasonally arid climate. The shallow marine deposits include thin beds and nodules of micritic limestone, calcareous mud shale, and calcareous sandstone. The limestones are recrystal-lized, but they contain ghosts of marine fossils.

Figure S3-1. Stratigraphic column of the upper part of the Minturn Formation. The upper part of the Minturn Formation contains both marine and nonmarine sediments as shown in this measured section of the roadcut on CO Highway 9 just north of Fairplay.

Figure S3-1. Stratigraphic column of the upper part of the Minturn Formation. The upper part of the Minturn Formation contains both marine and nonmarine sediments as shown in this measured section of the roadcut on CO Highway 9 just north of Fairplay.

The Minturn Formation (Middle and Early Pennsylvanian) contains interbedded pebble to cobble conglomerate, sandstone, siltstone, carbonaceous shale, limestone, and beds of gypsum and halite (Wallace and Keller, 2003; Widmann et al., 2007, 2011; Houck et al., 2012). It includes a lower member, the Coffman Conglomerate Member, an evaporite facies, and an upper member. These sediments record a period of intensified tectonic activity of the ancestral Rocky Mountain uplifts. In the field-trip area, paleo-current measurements indicate that sediments came from both the ancestral Front Range uplift to the north and the Ute Pass uplift to the south (Fig. 6). The lower member and Coffman Member contain dark shales and limestones similar to those in the Belden Formation, as well as coarse sandstone, conglomerate, and coaly shale. The marine shales and limestones contain fossils indicating an early Atokan age (Houck et al., 2004). The conglomerate beds coarsen and thicken in the direction of the Ute Pass uplift (Fig. 6). They are interpreted to be fan-delta deposits shed from the Ute Pass uplift when it first began to rise. The lower member and Coffman Member also contain a variety of coal swamp fossils such as Lepi-dodendron and Calamites (Johnson, 1934; Gould, 1935), showing that wet conditions in eastern North America and western Europe extended into Colorado at this time.

The lower members of the Minturn Formation, including the Coffman Member which contains coal swamp fossils, are overlain abruptly by an evaporite facies in the middle part of the Minturn. It contains beds of gray shale, siltstone, gypsiferous shale, fine-grained sandstone, limestone, and gypsum. Salt water springs and wells in the area imply that halite and possibly sylvite are present in the subsurface (Kirkham et al., 2012). No age-diagnostic fossils were found in the evaporite facies in South Park. However, fossils in the Eagle Valley Evap-orite (north and west of South Park) indicate a Desmoinesian age and possibly a late Atokan age (Mallory, 1971). Fossils associated with the evaporites in the Paradox Basin indicate a Desmoinesian age (Mallory, 1972; Ritter et al., 2002). A new paleogeography analysis by Kluth and DuChene (2009) shows that the Paradox Basin and central Colorado trough were likely connected during deposition of the evaporites. Thus, it is a reasonable assumption that the evaporites in South Park are also Desmoinesian, and possibly late Atokan, in age. If this age assignment is correct, then the drier, more seasonal climate and high-frequency sea-level fluctuations that prevailed in the Desmoinesian probably contributed to the formation of the evaporite facies in the South Park Basin. The evaporite facies has particular relevance to the structural geology and water quality aspects of South Park. Detailed mapping in the vicinity of Antero Reservoir suggests that there may be a considerable thickness of this facies over a widespread area.

In South Park, the upper part of the Minturn Formation consists of sandstone, siltstone, shale, conglomerate, and limestone. These rocks are arranged in cyclic sequences of fluvial, deltaic, and marine rocks. The upper Minturn Formation is middle Des-moinesian and possibly younger in age (Tweto, 1949; Tweto and Lovering, 1977). The vertical stacking of nonmarine, marginal marine, and marine deposits seen in the upper Minturn Formation is probably at least partially attributable to the large sea-level fluctuations that occurred during deposition of the middle Des-moinesian and younger Pennsylvanian rocks. The climate was still seasonally arid; calcic paleosols and fossil conifers can be found in the upper Minturn Formation (Widmann et al., 2011; Kirkham et al., 2012). The Desmoinesian was also a time when tectonic activity associated with the ancestral Rocky Mountains was at its maximum (Dickinson and Lawton, 2003; Blakey, 2009). This is reflected in the many deposits of coarse conglomerate found in the upper Minturn Formation. Paleocurrent measurements show that the coarse sediments came from both the ancestral Front Range and the Ute Pass uplift. The Minturn Formation is exposed at or near the surface throughout much of the western portion of South Park, and total thickness ranges between 275 and 2135 m (900 and 7000 ft).

The Maroon Formation (Lower Permian (?) to Upper and Middle Pennsylvanian) overlies, and is gradational with, the Minturn Formation and is exposed in the central South Park Basin just west of the central hogback. The lower part of the Maroon Formation is similar to the upper part of the Minturn Formation, but the marine intervals and coarse conglomerates are fewer and less well developed. Paleocurrent measurements indicate that the conglomeratic sediments continued to come from both the ancestral Front Range and Ute Pass uplifts (Fig. 6). The trend of fewer marine intervals and fewer coarse sediments continues upward in the section, and the upper Maroon Formation is mostly siltstone and fine-grained sandstone. For these reasons, the upper Maroon Formation is relatively nonresistant in South Park and forms mostly valleys. Though its age in South Park is not known, it likely was deposited as ancestral Rocky Mountain tectonic activity waned. The uplifts surrounding the central Colorado trough were gradually eroded away, and the trough gradually filled with sediment. Total thickness reaches up to 1705 m (5600 ft).

From this stop, the field-trip route heads back south on CO Highway 9 through Fairplay to rejoin U.S. Highway 285for 1 mi (1.6 km) before heading to the southeast (to the right, heading south). The trip continues on CO Highway 9 ~10.5 mi (16.9 km) to the town site of Garo, where it heads west (right turn, heading south) on County Road 24 to the next stop.

21.3 mi (34.3 km): Junction with U.S. Highway 285. Turn right.

21.4 mi (34.4 km): Cross the Middle Fork South Platte River where it is incised into the Pennsylvanian Minturn Formation and flanked by Upper Pleistocene terrace deposits. Placer mine tailings line the river in the incised channel and cover the Upper Pleistocene outwash terrace to the southeast.

22.3 mi (35.9 km): Turn left onto CO Highway 9 where the road follows the Middle Fork South Platte River across an Upper Pleistocene outwash plain with bluffs to the west exposing the Pennsylvanian-Permian (?) Maroon Formation. The ridge to the east is the Redhill hogback of Maroon Formation, Garo Sandstone, Morrison Formation, and Dakota Sandstone, the same hogback as Stop 2.

24.2 mi (38.9 km): At 2:30 is a view of a Tertiary paleovalley cut into the Maroon Formation. White exposure at the top of the bluff is Oligocene tuff at the base of the paleovalley (Kirkham et al., 2006). Tertiary sediments fill the remainder of the paleovalley. It is interpreted to be a small half-graben bounded on the west by the concealed normal Fairplay fault.

29.6-30.5 mi (47.6-49.1 km): Roadcuts on the left (east) expose Garo Sandstone, and then the highway swings east to cross the Garo Sandstone outcrop belt before crossing the Middle Fork South Platte River.

31.2 mi (50.2 km): Turn right onto County Road 24 at the town of Garo.

Mile 32.8 (53.1 km)—STOP 4: Lower Part of the Pennsylvanian-Permian (?) Maroon Formation

39.08659° N, 105.89858° W; elevation 2801 m (9170 ft). A short walk onto undeveloped private land provides views of the Maroon Formation.

The lower part of the Maroon Formation is similar to the upper part of the Minturn Formation, containing both marine and nonmarine deposits. This outcrop contains fluvial channel and overbank deposits. The channel deposits are conglomerate and conglomeratic sandstone composed mostly of quartz, feldspar, and Proterozoic igneous and metamorphic rocks. They contain trough cross-beds formed by the migration of megaripples. Paleocurrent measurements collected on the ranch to the southeast show that sediment transport was toward the north (Fig. 6). The outcrop also contains sandstone and siltstone overbank deposits, but they are poorly exposed. North and northeast of Stop 4, mineralization has occurred where small faults cut sandstone beds in the Maroon Formation. Uranium, vanadium, radium, and copper were mined here briefly in the 1950s (Wilmarth, 1959; Nelson-Moore et al., 1978). Older Pleistocene alluvium caps this ridge (Kirkham et al., 2007).

Continue west on County Road 24. From here, the field trip follows a series of roads through a small subdivision. Stay on the main roads and pay close attention to signs and mileage to navigate through the turns.

34.7 mi (55.8 km): Turn left on Parmalee Road.

35.2 mi (56.6 km): Turn left on Rogers Drive.

35.7 mi (57.4 km): Turn left on Bare Trail.

36.3 mi (58.4 km): Turn right on TRPA Road.

Mile 37.7 (60.7 km)—STOP 5: Outcrop of Pennsylvanian Minturn Formation Evaporite Facies

39.05735° N, 105.92162° W; elevation 2792 m (9155 ft). This stop can be viewed from the shoulder of the lightly used County Road.

Generally, the evaporite facies crops out poorly, but here a small outcrop of coarsely crystalline gypsum (Fig. S5-1) can be seen in a roadcut on the west side of TRPA Road. Behind the fence on the west side of the road, some small sinkholes can be seen. The land in this area has been subdivided, and the lots sold to private owners. Some lots are entirely underlain by the evapo-rite facies, so there is no place to build a structure without risk of an underlying ground collapse.

Figure S5-1. Photograph of crystalline gypsum in the Minturn Formation evaporite facies. Crystalline gypsum of the Minturn Formation evaporite facies in the drainage ditch on the west side of TRPA Road (39.05735° N, 105.92162° W).

Figure S5-1. Photograph of crystalline gypsum in the Minturn Formation evaporite facies. Crystalline gypsum of the Minturn Formation evaporite facies in the drainage ditch on the west side of TRPA Road (39.05735° N, 105.92162° W).

Resistant volcanic rocks hold up the Buffalo Peaks and Thunder Mountain highland to the west. The high ridge to the southeast, locally known as the Lone hills, is composed of Eocene volcanic rocks similar in age and composition to the rocks in the Buffalo Peaks (Kirkham et al., 2007). In the Lone hills, the sequence includes lahars, flow breccias, lava flows, and tuff exposed in the western flank of the High Creek syncline (Fig. 4). These rocks may have been deposited in a Tertiary paleovalley incised into the Minturn evaporite facies (Kirkham et al., 2007). The nonresistant evaporite facies has been eroded around the paleovalley, leaving the resistant rocks behind as a ridge.

The similarities in composition and age between the Buffalo Peaks volcanic sequence and the volcanic rocks exposed on the flanks of the High Creek syncline to the east within the South Park Basin suggest a genetic link. However, the base of the andesitic rocks in South Park sits at an elevation of ~2805 m (9200 ft mean sea level [MSL]) while the base of the Buffalo Peaks andesitic flows sit at over 3230 m (10,600 ft MSL). The lower tuff unit of the Buffalo Peaks sequence fills a paleovalley sloping to the northeast, and the elevation difference between the Buffalo Peak andesite flows and South Park flows could simply reflect original topography sloping away from the Buffalo Peaks eruptive center. However, Kirkham (2016, oral commun.) postulates that the High Creek syncline was formed by dissolution within the Minturn Formation evaporite facies, based on stratigraphic relations and the presence of numerous sinkholes in the area.

From this stop, the field-trip route turns around and heads back out through the subdivision to join U.S. Highway 285, where it heads south ~12.5 mi (20.1 km) to Trout Creek Pass.

37.7 mi (60.7 km): Turn around and go north on TRPA Road.

38.3 mi (61.6 km): Turn left on Bare Trail. The road crosses the southwest limb of the northwest-plunging Bare syncline. The rocks are those of the upper Minturn Formation. Pieces of chert along the side of the road are silicified marine limestone from the Robinson Limestone Member.

39.4 mi (63.4 km): The prominent color change in the road, from tan to red, marks the position of the Bare fault. It is a northwest-trending, down-to-west fault. It sets the Minturn evaporite facies (tan) on the east against the Maroon Formation (red) on the west, and has over 1220 m (4000 ft) of strati-graphic displacement.

41.0 mi (66.0 km): Turn left on Rogers Drive.

42.3 mi (68.1 km): Turn right at the T-intersection with County Road 24.

A Saline Groundwater System Just beneath the Surface

Halite within the Minturn evaporite facies is not found in outcrop in the South Park Basin, which is not surprising given its solubility in this relatively wet environment. Groundwater associated with modern streams and tributaries is shallow, and flow systems within the porous surficial deposits can be prolific. However, there is considerable evidence that beds of halite are present in the Minturn evaporite facies at depth. Salt Works Ranch, 1 mi (~1.6 km) to the northeast across U.S. Highway 285, was a homestead claimed around a natural saline spring in 1862. In 1864, Colorado Salt Works was organized with commercial production of salt using wood-fired evaporation kettles, but operated only until 1869 (Simmons, 2002). A water quality sample collected by the U.S. Geological Survey in 1974 contained reported concentrations of 28,200 mg per liter (mg/L) total dissolved solids, 9400 mg/L sodium, 2300 mg/L sulfate, and 335 mg/L bicarbonate. The chloride concentration was anomalously low, which may be a reporting error.

Evaporite minerals within the Minturn evaporite facies could potentially have a widespread influence on groundwater quality, wherever the unit is close to the surface. Freshwater quality in many shallow water wells in this area may simply reflect adequate flushing by prolific near-surface groundwater. Deeper wells, on the other hand, reflect a deeper, more saline, groundwater system in this area (Fig. S5-2).

Figure S5-2. Water quality pie diagrams from water wells in the Minturn Formation. Water quality in the Minturn Formation is locally influenced by evaporite minerals in the evaporite facies as shown by the pie diagrams for water quality analyses from wells and springs in the area. Diagram diameter reflects total dissolved solids and pie diagram shows major ion concentrations. Many wells in shallow water wells indicate a fresh shallow groundwater system, yet some wells and springs reflect a deeper more saline system. The largest diagram is from the Salt Works spring. (Data source: USGS National Water Quality Monitoring Monitoring Council Water Quality Data [http://waterqualitydata.us/portal/, all data through 2014].)

Figure S5-2. Water quality pie diagrams from water wells in the Minturn Formation. Water quality in the Minturn Formation is locally influenced by evaporite minerals in the evaporite facies as shown by the pie diagrams for water quality analyses from wells and springs in the area. Diagram diameter reflects total dissolved solids and pie diagram shows major ion concentrations. Many wells in shallow water wells indicate a fresh shallow groundwater system, yet some wells and springs reflect a deeper more saline system. The largest diagram is from the Salt Works spring. (Data source: USGS National Water Quality Monitoring Monitoring Council Water Quality Data [http://waterqualitydata.us/portal/, all data through 2014].)

43.5 mi (69.7 km): To the north, we can look northwest toward the High Creek Fen, marked by a grove of spruce trees. It is an area with numerous depressions and sinkholes of all sizes. The Minturn evaporite facies is near the surface, and the sinkholes are presumed to be created by dissolution of the evaporite minerals within the Minturn Formation (Kirkham et al., 2007). Turn around and go west on County Road 24.

45.0 mi (72.1 km): Turn left onto U.S. Highway 285.

45.4 mi (72.7 km): The highway crosses an upper middle Pleistocene paleovalley filled with outwash deposits of the South Fork South Platte River. Stream capture diverted the river to the south before deposition of younger upper Pleistocene outwash.

46.0 mi (73.5 km): Cross the South Fork South Platte River, which has exposures of the Minturn Formation along the banks.

48.6 mi (77.9 km): This is an outcrop of the Minturn Formation. To the left is the ridge of Eocene volcanic rocks that we saw at Stop 5.

53.6 mi (86.2 km): Entrance to Salt Works Ranch is on the east side of the highway (to the left). Hall butte, on the north side of the ranch, is held up by Eocene andesite, and Mount Hall, on the south side, is held up by the slightly younger Eocene Wall Mountain Tuff (Kirkham et al., 2012).

57.4 mi (92.4 km): Turn right onto Forest Road 311. Go 0.1 mi (0.3 km) and park in the parking area on the left side of the road.

Mile 57.6 (92.7 km)—STOP 6: Trout Creek Pass; Pennsylvanian Belden Formation, Lower Member of the Minturn Formation, and Coffman Conglomerate Member of the Minturn Formation

38.91282° N, 105.97148° W; elevation 2878 m (9480 ft). The field trip crosses by foot onto a Colorado State Land Board parcel, where the path follows an abandoned railroad grade. A temporary permit for access is required and must be obtained by people taking this trip on their own. This is a good lunch stop away from the busy highway.

The high ridge to the southwest contains Cambrian through Mississippian quartz sandstone, dolomite, and cherty limestone. As we walk northeast and through the fence, we traverse the dark, organic-rich shale of the Belden Formation. Though it is predominantly shale, it also contains minor limestone, siltstone, and fine sandstone (Fig. S6-1). The Belden Formation was deposited in and adjacent to coal swamps, under marginal marine conditions. In this area, these were the first sediments to have been deposited in the central Colorado trough. During deposition of the Belden Formation in the South Park area (late Morrowan; Musgrave, 2003), parts of the ancestral Front Range and San Luis uplifts were up and shedding coarse sediment. However, the lack of coarse sandstone and conglomerate here indicates that at that time there were no mountainous uplifts nearby to provide coarse sediment. In the northern part of the Mosquito Range, the Belden changes facies to coarse sandstones and conglomerates of the Minturn Formation (Widmann et al., 2004; Henry, 1998), and it may thin to the east. Total thickness reaches up to 365 m (1200 ft) near Jones Hill.

Figure S6-1. Photograph of Belden Formation and Coffman Member of the Minturn Formation. The roadcut on the west side of U.S. Highway 285 exposes the contact between carbonaceous shale and sandstone of the Pennsylvanian Belden Formation and pebble-rich sandstone of the Pennsylvanian Coffman Conglomerate Member of the Minturn Formation (38.91282° N, 105.97148° W).

Figure S6-1. Photograph of Belden Formation and Coffman Member of the Minturn Formation. The roadcut on the west side of U.S. Highway 285 exposes the contact between carbonaceous shale and sandstone of the Pennsylvanian Belden Formation and pebble-rich sandstone of the Pennsylvanian Coffman Conglomerate Member of the Minturn Formation (38.91282° N, 105.97148° W).

During deposition of the lower members of the Minturn Formation, including the Coffman Member (early Atokan), the Ute Pass uplift came up and started shedding coarse-grained sediment. Northeast along the railroad cut are sandstone beds that mark the base of the lower member of the Minturn Formation. The basal sandstone beds are quartz-rich and contain chert pebbles, indicating that their source was likely the local Lower and Middle Paleozoic rocks. Continuing up the section, conglomerate beds appear, marking the base of the Coffman Conglomerate Member of the Minturn Formation. Also, the sand becomes arkosic, and the gravel is composed mostly of Proterozoic igneous and meta-morphic rocks. Clast sizes in the Coffman Conglomerate Member decrease to the north and west, and the conglomerate beds pinch out in these directions, indicating that the sediment came from the south and east. The lower member and Coffman Conglomerate Member of the Minturn Formation are early Atokan in age (Houck et al., 2004).

The field trip returns to U.S. Highway 285, where it heads back north for less than a mile (1.6 km) before branching east on U.S. Highway 24 to Hartsel.

57.7 mi (92.8 km): Turn left on U.S. Highway 285.

An Outpouring of Eocene and Oligocene Volcanic Rocks, and Deposition of Clastic Units

The oldest volcanic rocks in the South Park Basin are those of the Buffalo Peaks sequence, which consists of ash fall deposits, lahar deposits, volcanic breccia, and andesite flows (Widmann et al., 2011; Houck et al., 2012). Field relations suggest a local source and Kellogg et al. (2013) have suggested that a small intrusive body of andesite ~1.5 km (0.93 mi) southwest of the main volcanic field may be a feeder. 40Ar/39Ar radiometric age dating on hornblende indicates an age of ca. 37.79 ± 3.5 Ma (Houck et al., 2012). The units are resistant to erosion and form cap rocks holding up Buffalo Peaks. Their thickness is quite variable, reaching up to 460 m (1500 ft). An unnamed andesitic and dacitic unit in the Lone Hills is Eocene in age, and has yielded an 40Ar/39Ar age from biotite of 38.59 ± 0.14 Ma (Kirkham et al., 2007).

The Eocene Wall Mountain Tuff came next and blanketed a large part of the region, reaching as far east as Castle Rock. Its likely source was from a postulated caldera that formed as the Mount Princeton pluton evolved in what is now the Sawatch Range northwest of Salida (Chapin and Lowell, 1979; McIntosh and Chapin, 2004). It is a rhyolite to trachytic ash-flow tuff that is moderately to densely welded (Wallace and Keller, 2003; Kirkham et al., 2012) and forms resistant cap rock. Small remnants have been preserved scattered across much of the southern part of South Park. Thickness is variable, possibly exceeding 60 m (200 ft) in the area. Radiomet-ric age dating indicates a 40Ar/39Ar age from sanadine of 36.69 ± 0.09 Ma (McIntosh and Chapin, 1994).

The Oligocene Thirtynine Mile volcanic field and Guffey volcanic center is a large composite volcanic field forming the hills bounding the south end of South Park, concealing older rocks and geologic structures below. It consists of flows of rhyolitic, andesitic, and basaltic composition; volcanic breccias; ash fall deposits; and lahar deposits (Epis and Chapin, 1968; Wobus and Scott, 1979; Scarbrough, 2001). K-Ar radiometric age dating indicates an age range of ca. 34-32 Ma for the volcanic field. Other volcanic units found within the same area are the Gribbles Park Tuff and Badger Creek Tuff (Wallace et al., 1999), which are interpreted to have been sourced from caldera complexes in the northeastern San Juan volcanic field and Sawatch Range, respectively (McIntosh and Chapin, 2004). Stratigraphic relations are complex, and thickness can be quite variable because of the rugged volcanic terrain at the time of eruption.

Other scattered occurrences of volcanic rocks have been described for the South Park area. These include small flows of andesite southeast of Kenosha Pass and northwest of Hartsel that are ca. 37 Ma in age based on fission-track dating of zircon (Bryant et al., 1981; Rule-man et al., 2011).

Tallahassee Creek Conglomerate consists of fluvial sediments containing clasts of varied composition, reflecting the variety of rocks exposed in South Park (Scarbrough, 2001; Wallace and Keller, 2003; Kirkham et al., 2012). It contains boulders up to6m (20 ft) across that can be conspicuous even at a distance. It is reported to have a tuffaceous matrix and boulders of Wall Mountain Tuff. This places the unit younger than the late Eocene Wall Mountain Tuff. It is in turn overlain by the Antero Formation and it may have predated the Thirtynine Mile volcanic field. It also contains silicified and opalized petrified wood. This unit fills paleovalleys scattered across much of the southern part of South Park and thickness is variable up to 245 m (800 ft).

The Antero Formation consists of fluvial sediments shed off of the eroding highland into a restricted basin north of the Thirtynine Mile volcanic field. It consists of a number of distinct facies, including sandstone and conglomeratic facies, limestone facies, and ash-flow tuff facies (Scarbrough, 2001; Kirkham et al., 2012). Radiometric age dating of an ash-flow tuff bed indicates an age of ca. 34 Ma based on 40Ar/39Ar dating of sanadine. Total thickness may reach ~610 m (2000 ft). The Florissant Formation near Lake George, famous for fossilized wood and insects, shares a similar age and depositional setting. The tuff and sediments filling a paleovalley farther north and viewed at mile 24.2 (38.9 km) may correlate with the main body of the Antero Formation.

The Miocene Wagontongue Formation is another conglomeratic unit of mixed lithologies that unconform-ably overlies the Antero Formation southwest of Hartsel. A conglomeratic facies of the Wagontongue was previously mapped as the Trump sandstone based on its prevalence in the southwest part of the region (Stark et al., 1949; Wallace and Keller, 2003). Recent mapping in the Hartsel area does not differentiate the two facies (Rule-man et al., 2011; Kirkham et al., 2012). This unit unconformably overlies earlier units and indicates a return to a higher-energy fluvial environment. The change may signal initiation of Cenozoic extensional tectonism. Total thickness is estimated to be ~425 m (1400 ft).

58.5 mi (94.1 km): Turn right onto U.S. Highway 24.

59.4-60.2 mi (95.6-96.9 km): We are passing by roadcuts exposing Penn-sylvanian Minturn Formation evaporite facies. Typically the Minturn Formation does not form good exposures, particularly so for the evaporite facies. There are good views of Eocene Wall Mountain Tuff capping the hills south of the highway.

62.0 mi (99.8 km): U.S. Highway 24 passes between small buttes held up by Wall Mountain Tuff, and then through the outcrop belt of the Tallahassee Creek Conglomerate (low hills to the right covered in large boulders), as it crosses into the northwest flank of the Antero syncline. The axis of the syncline trends approximately north to south just to the east of the highway.

63.0-64.0 mi (101.4-103.0 km): The Oligocene Antero Formation underlies the lowlands traversed by the highway. Miocene Wagontongue Formation fills the center and forms the low rolling hills east (to the right) of the highway. To the south and east, the higher hills and ridges are held up by flows in the Thirtynine Mile volcanic field and Guffey Mountain volcanic center (ca. 37-34 Ma).

65.6 mi (105.6 km): Alkali sinks mark the plain between U.S. Highway 24 and the South Fork of the South Platte River. Kirkham et al. (2012) propose that many of the irregular structures and closed depressions in this area have resulted from dissolution of the Minturn Formation evaporite facies.

67.1 mi (108.0 km): The highway exits the Antero syncline heading north-northeast with Wall Mountain Tuff forming the high butte on the east side (right) of the highway.

68.7 mi (110.5 km): The Red Hill hogback to the northeast exposes east-dipping Dakota Sandstone, Morrison Formation, and Garo Sandstone where the Garo Sandstone overlies Maroon Formation. The higher ridge just to the southeast has exposures of Precambrian igneous rocks.

70.3 mi (113.1 km): The tan sandstone on the south side of the highway has been mapped as Garo Sandstone lying on Minturn Formation (Beggs, 1976). Grain composition and bed forms resemble the Garo Sandstone at Red Hill Pass, but cementation is different and the typical nodular texture is not evident. The unit appears bleached and fractures are highly stained with hematite. This may be the result of hydrothermal alteration originating from thermal water sourced by a deep fault system passing nearby. There is a small hot spring just southeast of the outcrop.

Mile 71.0 (114.2 km)—STOP 7: Hartsel and Outcrop of Garo Sandstone in Contact with Precambrian Basement

39.02120° N, 105.80219° W; elevation 2706 m (8880 ft). The stop is viewed from the gravel shoulder of the highway near the banks of the South Fork South Platte River. Be mindful of fast traffic on the main road.

Here, the Garo Sandstone, as mapped, lies on 1.7-b.y.-old Precambrian monzogranite. It also has layers containing pebbles of quartz and feldspar, which contrast with the relatively uniform grain size and quartz dominant composition of the Garo Sandstone elsewhere. Recall that, less than a mile back to the west, the Garo Sandstone overlies Minturn Formation, hinting that we are near the edge of the central Colorado trough, as interpreted by De Voto (1972). Morrison Formation is exposed across the highway.

Continue on U.S. Highway 24 into the town of Hartsel, where there is opportunity for a restroom and snack break at the convenience store on the north side of the highway (left side). The Bayou Salado trading post offers a variety of mineral specimens and crafts, as well as ice cream and espresso drinks. From Harstel, the route continues east on U.S. Highway 24 ~4.8 mi (7.7 km) to the Elkhorn Road, where we head north to Stop 8. The Elkhorn Road is a well-maintained gravel road.

72.6 mi (116.8 km): U.S. Highway 24 crosses North Fork South Platte River. The low hills ahead, and to the right, are capped with Tallahassee Creek Conglomerate (Ruleman et al., 2011). The riverbank of the North Fork South Platte River at the base of the hill exposes deformed Pierre Shale.

75.9 mi (122.1 km): Turn left onto Elkhorn Road (County Road 15).

76.9 mi (123.7 km): Turn left onto Pike View Avenue.

Mile 77.7 (125.0 km)—STOP 8: The “Book Cliffs” of South Park and an Angular Unconformity between the Fox Hills Sandstone and the South Park Formation

39.05675° N, 105.72979° W; elevation 2774 m (9100 ft). This stop can be covered from the right-of-way of the public road. Climbing up to the cliffs involves crossing rugged ground onprivate propertyand isnot encouraged.

The Paleocene South Park Formation, consisting of inter-bedded fine-grained arkose with a conglomeratic member, is on the face of the scarp; Upper Cretaceous Laramie Formation and Fox Hills Sandstone are lower in the hills (Stark et al., 1949; Bohannon and Ruleman, 2009). The Elkhorn thrust is to the east, where slivers of Precambrian metamorphic and igneous rocks form high ridges.

The base of the South Park Formation rests on faulted and folded Fox Hills Sandstone in this location. The contact of the basal South Park Formation varies within the study area. Near Jefferson, the contact is nearly conformable between the Laramie Formation and South Park Formation. Southward, the contact gradually shows erosion down to the Fox Hills Sandstone, as at this stop. At Reinecker Ridge, however, erosion down to the Pierre Shale indicates that additional uplift has taken place east of this fault zone. This gives a snapshot of the basin geometry before the South Park Formation started to accumulate in the latest Cretaceous (Fig. 7).

The basal fill of the South Park Formation also differs throughout the study area. In the north, a thicker section is present, starting with volcanic and volcaniclastic deposits at the base of the section. The Reinecker Ridge Volcanic Member of the South Park Formation thins toward the south, almost equal with the nature of the increased erosion of underlying units, indicating gradual early uplift in this region. At Stop 8, the Reinecker Ridge Volcanic Member is absent from the section. The volcanic source area may have been too distant for the flows to advance this far. Alternatively, uplift in the area created a high that was not covered until the lows just to the north were filled. This interval contains large, meters-deep channels and associated large-scale bedforms of a braided fluvial system. Several pulses of coarser-grained sediment are interbedded within the finer units of the South Park Formation.

The regional cross section (Fig. S1-2) captures the recent El Paso CDOW #11-13-10-76 well and two older Amoco wells (the Reinecker Ridge #1 and the State AK #1). The Amoco wells reveal an unconformity beneath the South Park Formation, beneath which the Cretaceous Laramie Formation, Fox Hills Sandstone, and the upper part of the Pierre Shale have been removed. These relations reveal two phases of Laramide uplift within the basin. The first occurred prior to deposition of the South Park Formation. At this stop, we see evidence for this structural event well exposed.

As noted originally by Stark et al. (1949), the Cretaceous section beneath the South Park Formation is highly deformed at this locale. As shown in map view on Figure S8-1, two north-plunging anticlines lie beneath the relatively flat-lying South Park Formation. The core of the eastern anticline is cut by a west-dipping thrust and the anticline is east-vergent (Fig. S8-2). Further evidence for this structural style is provided by the Burton Hawks Hartsel Federal 1 well (swsw 15-T11S-R75W) located five kilometers northwest along strike of Stop 8. The well spud in South Park Formation dipping approximately 30 degrees northeast toward the axis of the San Isabel syn-cline. At 168 meters (550 feet), the well entered upright Fox Hills section that based on the dipmeter log was dipping approximately 30 degrees to the southwest. Over the next 457 meters (1500 feet) the wells traversed a faulted S-fold cored by an overturned section of Fox Hills before reaching total depth in upright, west-dipping Pierre Shale. This well is shown in its more regional context on Figure S8-3. The cross section captures several wells drilled for hydrocarbons on the Hartsel anticline (dipmeter information in the Shell Federal #1-4285 well is from Clement and Dolton, 1970). The cross section shows the overturned fold encountered in the Burton Hawks well carried on an east-vergent back-thrust rooted in the west-vergent San Isabel thrust. The San Isabel thrust is a low-angle detachment that daylights above the South Park thrust on the east flank of the Reinecker Ridge anticlinorium. Near its leading edge, the San Isabel thrust rides in a bedding plane detachment near the top of the Pierre Shale. The backthrust encountered in the Burton Hawks well soles into this detachment. This detachment also forms the roof thrust for the triangle zone depicted east of the Burton Hawks well in Figure S8-3. The triangle zone allows for significant translation across the trailing portion of the San Isabel thrust sheet and relatively minor amount of translation along its leading edge.

Figure S8-1. South Park “Book Cliffs” detail map. This annotated aerial photograph of Stop 8 (Fig. 3) shows a general picture of relatively flat-lying arkose and conglomerate of the South Park Formation resting unconformably on faulted and folded Fox Hills Formation and Pierre Shale. At the contact, the South Park and underlying Fox Hills formations are conformable and dipping 45° to the north, however, within 1.5 m above the contact, beds within the South Park Formation flatten and dip gently to the north.

Figure S8-1. South Park “Book Cliffs” detail map. This annotated aerial photograph of Stop 8 (Fig. 3) shows a general picture of relatively flat-lying arkose and conglomerate of the South Park Formation resting unconformably on faulted and folded Fox Hills Formation and Pierre Shale. At the contact, the South Park and underlying Fox Hills formations are conformable and dipping 45° to the north, however, within 1.5 m above the contact, beds within the South Park Formation flatten and dip gently to the north.

Figure S8-2. Photograph of faulted and folded Fox Hills Sandstone. This view to the north at Stop 8 (“Book Cliffs”) shows relatively flat-lying South Park Formation resting on highly deformed and faulted Fox Hills Formation and Pierre Shale (person for scale).

Figure S8-2. Photograph of faulted and folded Fox Hills Sandstone. This view to the north at Stop 8 (“Book Cliffs”) shows relatively flat-lying South Park Formation resting on highly deformed and faulted Fox Hills Formation and Pierre Shale (person for scale).

Figure S8-3. Regional cross section B-B’ from near the “Book Cliffs” shows the complex structural features that evolved prior to, during, and following deposition of the South Park Formation. S.L.—sea level. See Figure S1-4 for cross-section location.

Figure S8-3. Regional cross section B-B’ from near the “Book Cliffs” shows the complex structural features that evolved prior to, during, and following deposition of the South Park Formation. S.L.—sea level. See Figure S1-4 for cross-section location.

As we will see at Stop 9, the second phase of Laramide thrusting was more pronounced, with the Elkhorn thrust responsible for carrying Front Range basement rocks westward over the Mesozoic section and the synorogenic South Park Formation. Multiple stage Laramide structuring is well developed farther north in the Middle-North Park Basin (Wellborn, 1977; Dechesne et al., 2013). It is interesting to note that drilling-induced vertical fractures measured in the Hunt/McMurry Tar-ryall Federal #1-17 well indicate a present-day maximum principal stress (sigma 1) oriented almost due east-west (azimuth of 95°). This orientation aligns better with typical Laramide stress patterns (Erslev et al., 2004) rather than those associated with later extension. This shows that Laramide stresses are preserved in the footwall of the Elkhorn thrust even though, as we will discuss later, the hanging wall has experienced later Tertiary extension. In addition, the Fox Hills at Stop 8 is cut in places by steeply dipping fractures with an azimuth of 95°, indicating a match between surface and subsurface Laramide stress orientations.

The final basin type observed in South Park consists of graben development during Tertiary extension in the hanging wall of the Elkhorn thrust. We will discuss this basin type in more detail at Stop 10.

The route returns to the Elkhorn Road where it continues north ~2.4 mi (3.9 km) to Stop 9.

77.9 mi (125.3 km): Turn around at the T-intersection and retrace steps on Pike View Avenue.

78.9 mi (127.0 km): Turn left onto Elkhorn Road.

Mile 81.3 (130.8 km)—STOP 9: Exposures of Dakota Sandstone and Morrison Formation and the Elkhorn Thrust

39.09038° N, 105.71845° W; elevation 2803 m (9195ft). Remain in the shoulder of the county road.

The beds along the road are Dakota Sandstone (Bohannon and Ruleman, 2009) and the Morrison Formation, steeply dipping in front of the Elkhorn thrust. Road maintenance work during May 2016 exposed characteristic greenish-gray and reddish-gray shale of the Morrison Formation.

At this stop, we will examine what may be the first reported exposure of the Elkhorn thrust. The exposure is located in the drainage ditch on the east shoulder of the Elkhorn Road. The fault plane strikes 335° and dips 53° northeast, placing dark-brown schistose basement on the north over white rock flour, derived from the adjacent Morrison Formation, on the south (Fig. S9-1). Regionally, the Elkhorn thrust forms an arcuate trend as it bows west across the central South Park area. Based on subsurface data from the Hunt/McMurry Tarryall Federal #1-17 well, and the extensive seismic grid in the basin, the reason for the arcuate fault trace appears to be folding of the fault by subthrust structures. Figure S9-2 is a schematic time-structure strike section along the basin. It shows the fault dropping across the central part of the basin, rising north due to the appearance of the subthrust Jefferson anticline, and rising south due to the appearance of the subthrust Glentivar dome and its brethren. Similar embayments related to the appearance of subthrust structures can be seen north along the trace of the Williams Range thrust.

Figure S9-1. Photograph of Elkhorn thrust plane. View to the east showing the Elkhorn thrust putting Precam-brian basement over Morrison Formation. Fault plane is highly contorted and shows an overall moderate dip to the northeast. The fault strikes to 315° and dips 53° northeast at the top of the trench (8.5” χ 11” standard paper for scale, 39.09034° N, 105.71838° W, ± 4 m).

Figure S9-1. Photograph of Elkhorn thrust plane. View to the east showing the Elkhorn thrust putting Precam-brian basement over Morrison Formation. Fault plane is highly contorted and shows an overall moderate dip to the northeast. The fault strikes to 315° and dips 53° northeast at the top of the trench (8.5” χ 11” standard paper for scale, 39.09034° N, 105.71838° W, ± 4 m).

Figure S9-2. Schematic time-structure strike section C-C’. This north-south time-structure strike section shows the Elkhorn thrust diving structurally across the central part of the South Park Basin and rising to both the north and south due to folding by subthrust structures.

Figure S9-2. Schematic time-structure strike section C-C’. This north-south time-structure strike section shows the Elkhorn thrust diving structurally across the central part of the South Park Basin and rising to both the north and south due to folding by subthrust structures.

Continue north on the Elkhorn Road ~5.5 miles (8.8 km) to Stop 10.

81.6 mi (131.3 km): The view to the south is of Sulfur Mountain along the graben formed by the Chase Gulch fault. Shell drilled several wells in the 1950s in this area; all were in the fold-fault belt southwest of the Elkhorn thrust system.

82.6-88.0 mi (132.9-141.6 km): For the next several miles, the Elkhorn Road traverses the undulating post-Eocene erosion surface. In this area, scattered exposures of Oligocene Antero Formation can be viewed in gullies and on isolated knobs interspersed with outcrops of Precambrian igneous and metamorphic rocks. The Antero Formation includes beds of sandstone, tuff, lacustrine limestone, and occasional chalcedony that lap onto an irregular erosion surface on bedrock.

Mile 86.8 (139.7 km)—STOP 10: Oligocene Antero Formation in the Northern End of the Chase Gulch Graben

39.15789° N, 105.74674° W; elevation 2870 m (9410ft). A short walk up a moderate hill at this stop crosses private land that requires permission to access.

The small hill on the east side of Elkhorn Road (right side, heading north) is capped by indurated, cross-bedded arkose of the Antero Formation (Ruleman et al., 2011). In map view, this small butte has a distinct oval shape with the long axis oriented west-northwest. A few cross-beds indicate an eastward current direction. The shape, combined with the exposed bedforms, suggests that this may be the remnant of a paleochannel in the Antero Formation. An optional short walk to the top of the hill provides views of the arkose as well as the surrounding landscape.

As mentioned at Stop 1, the third and youngest basin type in South Park consists of extensional graben formation on the hanging wall of a Laramide uplift. The Chase Gulch half-gra-ben displays many of the characteristics of this type of post-Laramide deformation of the hanging wall of the Elkhorn thrust. Such basins are common features overprinting earlier Laramide foreland uplifts from Montana south to Arizona (Sterne and Constenius, 1997; Kellogg, 1999). The deepest part of the Chase Gulch graben (Fig. S10-1) system lies toward the southern part of the Elkhorn thrust near the old Glentivar town site. The depth of the graben in this area is based on a single uranium exploration borehole by Rocky Mountain Energy, reported by Shafer (1980). This borehole reportedly intercepted over 490 m (1600 ft) of the Antero Formation. It is not clear whether the sediments intercepted could have also included Miocene sediments. The Chase Gulch fault has been investigated for possible Quaternary movement (Shafer, 1980; Powell, 2003), and the feature was included in the statewide earthquake potential by Kirkham and Rogers (1981). Powell (2003) describes evidence for movement as recently as the last 13,000-30,000 yr. Figure S10-2, adapted from Shafer (1980), shows the interpretation of partial reversal of the Elkhorn thrust and the development of the Chase Creek graben in its hanging wall. It is interesting to note that Stark et al. (1949) showed several normal faults at the northern end of this feature, the significance of which seems to have been lost over the years. Sulphur Mountain and Spinney Mountain are on the trace of the fault where it heads southeast.

Figure S10-1. Chase Gulch graben structure map. The Chase Gulch fault drops a half-graben on the hanging wall of the Elkhorn thrust. A depth of almost 490 m (1600 ft) of what was thought to be Antero Formation was reported from a uranium exploration borehole near the graben depocenter. Using that depth, and a projection of exhumed surfaces at the graben edge, as much as 600 m of fill is indicated.

Figure S10-1. Chase Gulch graben structure map. The Chase Gulch fault drops a half-graben on the hanging wall of the Elkhorn thrust. A depth of almost 490 m (1600 ft) of what was thought to be Antero Formation was reported from a uranium exploration borehole near the graben depocenter. Using that depth, and a projection of exhumed surfaces at the graben edge, as much as 600 m of fill is indicated.

Figure S10-2. Cross section through the Chase Gulch graben. This cross section, modified from Shafer (1980), illustrates how the Chase Gulch normal fault may form a half-graben isolating slivers of the Elkhorn thrust hanging wall at Sulfur Mountain and Spinney Mountain.

Figure S10-2. Cross section through the Chase Gulch graben. This cross section, modified from Shafer (1980), illustrates how the Chase Gulch normal fault may form a half-graben isolating slivers of the Elkhorn thrust hanging wall at Sulfur Mountain and Spinney Mountain.

An ash layer sampled from the Antero section within the graben near Stop 10 gives an 40Ar/39Ar age from biotite of 33.8 ± 0.2 Ma, which coincides with the Eocene-Oligocene boundary. If the ash were deposited while the graben was forming, it indicates an earlier onset of extension than the 28 Ma onset recognized within the northern Rio Grande rift proper (Landman and Flowers, 2013). South of South Park, Chapin and Cather (1983) attribute lower Eocene fill within the Echo Park graben to late Laramide strike-slip faulting along the Currant Creek fault trend, which is the southeastern continuation of the Reinecker Ridge anticlinorium or South Park fault trend. Alternate explanations for the Echo Park graben include Eocene collapse of the early phase Laramide contractional structures seen along the Reinecker Ridge trend, or late Laramide reactivation of the thrusts along this trend, resulting in local development of synorogenic deposits akin to the upper syn-tectonic conglomerate member of the South Park Formation of Ruleman et al. (2011).

Sterne and Constenius (1997) show a pattern across the Western Cordillera of extensional overprint following earlier contraction in a given area, often with a time gap of as little as 5 m.y. Based on the published dates for structural events available at the time, Wyoming and Colorado experienced anomalously late extension relative to contraction. If the Echo Park strata are a signature of extension, they would provide evidence for an earlier onset to the extensional overprint of the Laramide uplifts, and bring this area into better agreement with the patterns seen in other parts of the Western Cordillera. Sterne and Constenius (1997) also noted a correlation between the slab rollback that followed flat-slab subduction beneath the Western Cordillera, Tertiary igneous activity, and the advent of extension in the form of graben and metamorphic core complexes.

In the South Park area, local changes in the thickness of the Eocene-Oligocene Tallahassee Creek Conglomerate and the Oligocene Antero Formation could provide the indications of evap-orite dissolution as suggested by Kirkham et al. (2007, 2012). With this model, sediments accumulate as infill of active dissolution centers. Alternatively, the large synclines cored by Tertiary section in the western part of the basin may be directly related to Tertiary extension (De Voto, 1971), rather than evaporite dissolution, which would again argue for less time lag between Laramide contraction and Neogene extensional overprint.

From here, the field trip continues north 11.1 mi (17.9 km) on the Elkhorn Road to County Road 32, where it heads northwest (left turn) to join U.S. Highway 285. The route follows U.S. Highway 285 to a right turn onto County Road 34 which it follows for 2 mi (3.2 km) to the final stop.

89.9 mi (144.6 km): Arriving at the intersection with County Road 17, we are near the site of the Hunt Tarryall Federal well, which was drilled on a pad at the left side of Elkhorn Road. The site has been reclaimed. This well penetrated 591 m (1940 ft) of Precambrian before hitting South Park Formation, reaching its total depth in the Pierre Shale.

90.6 mi (145.8 km): Heading to the northwest, the Elkhorn Road crosses the approximate trace of the concealed Elkhorn thrust. The low-lying hills in the foreground are underlain by the conglomeratic member and Link Spring Tuff Member of the South Park Formation. Beyond is Reinecker Ridge backed by the higher peaks of the Mosquito Range.

92.0 mi (148.0 km): The road crosses the outcrop belt of the Link Spring Tuff Member of the Paleocene South Park Formation, where it dips ~30° to the east in a highly deformed belt in front of the Elkhorn thrust. The Reinecker Ridge Volcanic Member underlies the higher hills on the flanks of the Mexican Ridge anticline, as well as the forested Reinecker Ridge ~3.5 mi (5.6 km) to the west.

94.7 mi (152.4 km): The Elkhorn Road re-crosses the outcrop belt of the Link Spring Tuff Member of the Paleocene South Park Formation.

95.5 mi (153.7 km): To the left is a large closed depression. The more resistant Link Spring Tuff Member forms a small ridge, closing the depression. The conglomerate member underlies the depression and the 1:24,000 scale Milligan Lakes geologic map delineates an eastern edge of cobble beds on the west side of the depression (Wyant and Barker, 1976).

97.9 mi (157.5 km): Turn left at the intersection with County Road 32.

99.7 mi (160.4 km): Here, the road crosses the outcrop belt of the Fox Hills Sandstone and Laramie Formation. The King coal mine is just over 0.8 km (0.5 mi) to the south (left).

101.5 mi (163.3 km): Turn right onto U.S. Highway 285.

103.9 mi (167.2 km): Turn right onto County Road 34.

105.9 mi (170.4 km): Turn around.

Mile 106.1 (170.7 km)—STOP 11: Boulder Conglomerate in Folded Paleocene South Park Formation

39.33474° N, 105.821914° W, elevation 2910 m (9550 ft). The walk up the hill to the north crosses U.S. Bureau of Land Management public land. Participants should be wary of numerous deep burrows.

A short walk up the hill on the north side of the road winds through very large boulders (up to 3 m [10 ft]) of mixed Precam-brian igneous and metamorphic rocks, and over ground covered with cobbles of quartzite as well as igneous and metamorphic lithologies. This stop discusses Laramide basin fill sediments of the Cretaceous to Paleocene South Park Formation and the timing of basin formation, fill, and deformation.

Clast compositions within the South Park Formation show distinctly different sources and clast sizes throughout the stratigraphy and study area. Reinecker Ridge rises due east of this location and is home to the type section of Reinecker Ridge Volcanic Member of the South Park Formation. It is composed of volcanic conglomerates and flows overlying, and intertonguing with, the lower volcaniclastic member of the South Park Formation (Rule-man et al., 2011). The Reinecker Ridge Volcanic Member is overlain by the conglomerate member of the South Park Formation. This unit consists of mixed composition conglomerates derived from Precambrian basement, Paleozoic rocks, and Cretaceous intrusive rocks (Fig. S11-1). Flow directions in the lower interval of the South Park Formation, combined with evidence that the unit thins to the east, plus the presence of intrusives of similar ages, suggest a source from the Mosquito Range west-northwest of this location (Kirkham et al., 2006). The conglomerate member is overlain by the biotite-rich and ashy Link Spring Tuff Member of the South Park Formation. It projects stratigraphi-cally below road level at our current stop. At road level and above are exposures of arkosic member of the South Park Formation and associated boulder beds (Wyant and Barker, 1976). These boulder beds should not be confused with the syntectonic conglomerate of Ruleman et al. (2011). The arkosic member (also fine-grained arkosic member) consists of mudstones, siltstones, sandstones, and conglomerate. Flow directions from cross-beds near this location indicate general flow toward the southwest. A provenance change, from a volcanically influenced hinterland west of the study area to a dominance of Proterozoic metamor-phic and igneous lithologies derived east of the basin higher up in the section, gives a striking contrast, and suggests the initiation of movement along the Elkhorn thrust (Fig. S11-1) between ca. 60 and 56 Ma.

Figure S11-1. Generalized stratigraphic column of the South Park Formation in the north end of South Park. Unit abbreviations match map descriptions. Boulder beds are interpreted to be within the fine-grained arkosic member of the South Park Formation because they are folded within the South Park Formation beds in the Michigan Hill syncline.

Figure S11-1. Generalized stratigraphic column of the South Park Formation in the north end of South Park. Unit abbreviations match map descriptions. Boulder beds are interpreted to be within the fine-grained arkosic member of the South Park Formation because they are folded within the South Park Formation beds in the Michigan Hill syncline.

Both the finer-grained South Park Formation and the boulder beds appear folded in the Michigan Hill syncline (Wyant and Barker, 1976; Fig. S11-2). The syncline consists of two parts, their axes offset by the Michigan Creek fault just north of U.S. Highway 285 (Wyant and Barker, 1976). The older boulder beds appear truncated by younger boulder beds within the arkosic member of the South Park Formation (at arrow on Fig. S11-2). This indicates likely movement during deposition of the South Park Formation at this location. Folding also occurred after deposition of the entire South Park Formation, which is at best constrained by a biotite K-Ar age in the uppermost part of the South Park Formation of 56.3 ± 1.3 Ma (Bryant et al., 1981).

Figure S11-2. Oblique aerial view of the South Park Formation in the Michigan Hill syncline. Perspective overview of Stop 11 (circle) looking east into the Michigan Hill syncline. Note how the Boulder beds (dashed lines) are folded with the South Park Formation (Tsp) strata. Arrow indicates apparent truncation of older beds. This would be a result of continuous folding in the syncline during deposition. Alignment of the Elkhorn thrust is shown in the background where the fault separates rolling hills of South Park from the Elkhorn upland.

Figure S11-2. Oblique aerial view of the South Park Formation in the Michigan Hill syncline. Perspective overview of Stop 11 (circle) looking east into the Michigan Hill syncline. Note how the Boulder beds (dashed lines) are folded with the South Park Formation (Tsp) strata. Arrow indicates apparent truncation of older beds. This would be a result of continuous folding in the syncline during deposition. Alignment of the Elkhorn thrust is shown in the background where the fault separates rolling hills of South Park from the Elkhorn upland.

This is the final stop of the field trip and the route returns to U.S. Highway 285 to return to Denver.

107.8 mi (173.4 km): Turn right onto U.S. Highway 285.

115.4 mi (185.7 km): Return to the Kenosha Pass overlook, back at the start of the field trip.

Acknowledgments

Sponsored by the Coalition for the Upper South Platte, Colorado Scientific Society, and the Colorado Geological Survey

Preparation of this field log has a long history, starting with water quality baseline studies studies for the Coalition for the Upper South Platte. This led to a field trip for the Rocky Mountain Association of Geologists, which transitioned to this trip for the Geological Society of America. Many have contributed along the way. First, we wish to acknowledge our sponsors: the Coalition for the Upper South Platte, the Colorado Scientific Society, and the Colorado Geological Survey. Next, we wish to thank all who have helped along the way, particularly Phyllis Scott, who inspired the original field trip through the Rocky Mountain Association of Geologists and then followed through with logistics and advice. We also wish to thank Jara Johnson and Carol Ekarius of the Coalition for the Upper South Platte for engaging the Colorado Geological Survey in taking a holistic view of the South Park Basin and its fascinating geologic history. Mark Hudson and Bob Raynolds provided critical reviews as the field log made its way to completion. We would like to thank Hunt Oil Company for generously allowing us to publish their seismic data.

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Figures & Tables

Figure 1.

Regional map of South Park. South Park Basin sits high in the Rocky Mountains in close proximity to the Denver metropolitan region and encompasses the watershed of the South Platte River. Main topographic features are labeled in italics and main tectonic features are in bold with a white outline. This structurally complex basin is caught between the Laramide Front Range and Sawatch uplifts. The Neogene Rio Grande rift passes by just to the west along the upper Arkansas River valley.

Figure 1.

Regional map of South Park. South Park Basin sits high in the Rocky Mountains in close proximity to the Denver metropolitan region and encompasses the watershed of the South Platte River. Main topographic features are labeled in italics and main tectonic features are in bold with a white outline. This structurally complex basin is caught between the Laramide Front Range and Sawatch uplifts. The Neogene Rio Grande rift passes by just to the west along the upper Arkansas River valley.

Figure 2.

Stratigraphic column for South Park. Strata in the South Park Basin span a nearly complete section of Paleozoic through Cenozoic strata that includes marine carbonate and clastic sediments as well as nonmarine clastic sediments and volcanic rocks. Silurian, Triassic, and Pliocene strata have not been identified in South Park and these ages are not shown in the stratigraphic column. This field trip focuses on the synorogenic sediments deposited during the Pennsylvanian-Permian ancestral Rocky Mountain orogeny, the Upper Cretaceous-Eocene Laramide orogeny, and Eocene-Oligocene sediments and volcanic rocks formed following the Laramide orogeny.

Figure 2.

Stratigraphic column for South Park. Strata in the South Park Basin span a nearly complete section of Paleozoic through Cenozoic strata that includes marine carbonate and clastic sediments as well as nonmarine clastic sediments and volcanic rocks. Silurian, Triassic, and Pliocene strata have not been identified in South Park and these ages are not shown in the stratigraphic column. This field trip focuses on the synorogenic sediments deposited during the Pennsylvanian-Permian ancestral Rocky Mountain orogeny, the Upper Cretaceous-Eocene Laramide orogeny, and Eocene-Oligocene sediments and volcanic rocks formed following the Laramide orogeny.

Figure 3.

Generalized geologic map of the South Park area. The South Park Basin preserves Phanerozoic sedimentary and volcanic rocks between uplifted Precambrian crystalline uplifts. Much of the topographic basin is mantled by Quaternary glacial outwash and other alluvial deposits (after Barkmann et al., 2015). Major structural features are shown in Figure 4.

Figure 3.

Generalized geologic map of the South Park area. The South Park Basin preserves Phanerozoic sedimentary and volcanic rocks between uplifted Precambrian crystalline uplifts. Much of the topographic basin is mantled by Quaternary glacial outwash and other alluvial deposits (after Barkmann et al., 2015). Major structural features are shown in Figure 4.

Figure 4.

Principal structural features of South Park. A series of north to northwest thrust and reverse faults and subsidiary folds, with histories of movement from Precambrian through Neogene, dominates the structural fabric of South Park. Other features include crosscut-ting faults that appear to compartmentalize the main features, as well as a number of normal faults and large downwarps. Only those surface features described in the text are labeled. The Hayden lineament, an inferred boundary separating the Pennsylvanian-Permian central Colorado trough from the ancestral Front Range uplift, underlies this complex fabric. Adapted from Scarbrough (2001) and Ruleman et al. (2011). ant—anticline; syn—syncline.

Figure 4.

Principal structural features of South Park. A series of north to northwest thrust and reverse faults and subsidiary folds, with histories of movement from Precambrian through Neogene, dominates the structural fabric of South Park. Other features include crosscut-ting faults that appear to compartmentalize the main features, as well as a number of normal faults and large downwarps. Only those surface features described in the text are labeled. The Hayden lineament, an inferred boundary separating the Pennsylvanian-Permian central Colorado trough from the ancestral Front Range uplift, underlies this complex fabric. Adapted from Scarbrough (2001) and Ruleman et al. (2011). ant—anticline; syn—syncline.

Figure 5.

Schematic diagram of the structural setting of South Park. Only the major structural features defining the basin and surrounding tectonic features are shown in this schematic diagram. South Park is a structural basin caught between two Laramide uplifts that expose cores of Precambrian igneous and metamorphic rocks. The basin preserves Cambrian through Paleocene sedimentary strata. The younger Rio Grande rift system cuts through the Sawatch uplift to the west, forming the upper Arkansas River valley.

Figure 5.

Schematic diagram of the structural setting of South Park. Only the major structural features defining the basin and surrounding tectonic features are shown in this schematic diagram. South Park is a structural basin caught between two Laramide uplifts that expose cores of Precambrian igneous and metamorphic rocks. The basin preserves Cambrian through Paleocene sedimentary strata. The younger Rio Grande rift system cuts through the Sawatch uplift to the west, forming the upper Arkansas River valley.

Figure 6.

Middle Pennsylvanian paleogeographic map showing Colorado’s major uplifts and sedimentary basins during Middle Pennsylvanian time. Locations of the field-trip stops are also shown, as are locations of the Gore (G), Boreas Pass (B), Agate Creek (A), and Pleasant Valley (P) faults. The faults bounding the east side of the central Colorado trough are collectively known as the Hayden lineament (Maughan, 1988). The ancestral Front Range uplift, Woodland Park block, Ute Pass uplift, and Apishapa uplift are collectively known as Frontrangia (Mallory, 1958).

Figure 6.

Middle Pennsylvanian paleogeographic map showing Colorado’s major uplifts and sedimentary basins during Middle Pennsylvanian time. Locations of the field-trip stops are also shown, as are locations of the Gore (G), Boreas Pass (B), Agate Creek (A), and Pleasant Valley (P) faults. The faults bounding the east side of the central Colorado trough are collectively known as the Hayden lineament (Maughan, 1988). The ancestral Front Range uplift, Woodland Park block, Ute Pass uplift, and Apishapa uplift are collectively known as Frontrangia (Mallory, 1958).

Figure 7.

Pre-South Park Formation geologic map. Interpreted bedrock formations at the base of the South Park Formation. Distribution is based on outcrop relations where South Park Formation base is in direct contact with older formations, and has been projected beneath mapped extent of the South Park Formation.

Figure 7.

Pre-South Park Formation geologic map. Interpreted bedrock formations at the base of the South Park Formation. Distribution is based on outcrop relations where South Park Formation base is in direct contact with older formations, and has been projected beneath mapped extent of the South Park Formation.

Figure S1-1.

Photograph of South Park from Kenosha Pass. One of Colorado’s iconic views looking southwest from the scenic overlook on the south side of Kenosha Pass greets the viewer with the broad expanse of South Park and the Mosquito Range in the distance. If atmospheric conditions allow, several 4267+ m (14,000+ ft) peaks in the Sawatch Range may also be visible.

Figure S1-1.

Photograph of South Park from Kenosha Pass. One of Colorado’s iconic views looking southwest from the scenic overlook on the south side of Kenosha Pass greets the viewer with the broad expanse of South Park and the Mosquito Range in the distance. If atmospheric conditions allow, several 4267+ m (14,000+ ft) peaks in the Sawatch Range may also be visible.

Figure S1-2.

Detailed cross section A-A’ through the South Park Basin. This balanced cross section through the South Park Basin is based on seismic data and oil and gas well subsurface data. It shows the complex structural framework of the basin. The partial annealing zone (PAZ), based on apatite fission-track (AFT) data, approximates the base of Phanerozoic sedimentary cover prior to the Laramide orogeny (Kelley and Chapin, 1997). S.L.—sea level. See Figure S1-4 for cross-section location.

Figure S1-2.

Detailed cross section A-A’ through the South Park Basin. This balanced cross section through the South Park Basin is based on seismic data and oil and gas well subsurface data. It shows the complex structural framework of the basin. The partial annealing zone (PAZ), based on apatite fission-track (AFT) data, approximates the base of Phanerozoic sedimentary cover prior to the Laramide orogeny (Kelley and Chapin, 1997). S.L.—sea level. See Figure S1-4 for cross-section location.

Figure S1-3. Time-migrated seismic lines across the eastern half of South Park. Time-migrated seismic lines, courtesy of Hunt Oil Company and Colorado School of Mines (modified from Beggs, 1977), show the principal thrusts and structures in the eastern part of the South Park Basin. The locations of the seismic lines are shown on Figure S1-4. Formation annotation is as follows: p-C—Precambrian basement; lPz—lower Paleozoic; IP-P—Pennsylvanian to Permian Belden, Minturn, and Maroon Formations; Jm-Kd—Jurassic Morrison Formation and Cretaceous Dakota Sandstone; Kn-Kpl—Cretaceous Benton Group, Niobrara Formation, lower Pierre Shale, and Apache Creek Sandstone; Kpu—Creta-ceous Pierre Shale; Kpu-Kfl—Cretaceous Pierre Shale, Fox Hills Sandstone, and Laramie Formation (?); KTsp—Cretaceous to Tertiary South Park Formation; Ta—Oligocene Antero Formation.

Figure S1-3. Time-migrated seismic lines across the eastern half of South Park. Time-migrated seismic lines, courtesy of Hunt Oil Company and Colorado School of Mines (modified from Beggs, 1977), show the principal thrusts and structures in the eastern part of the South Park Basin. The locations of the seismic lines are shown on Figure S1-4. Formation annotation is as follows: p-C—Precambrian basement; lPz—lower Paleozoic; IP-P—Pennsylvanian to Permian Belden, Minturn, and Maroon Formations; Jm-Kd—Jurassic Morrison Formation and Cretaceous Dakota Sandstone; Kn-Kpl—Cretaceous Benton Group, Niobrara Formation, lower Pierre Shale, and Apache Creek Sandstone; Kpu—Creta-ceous Pierre Shale; Kpu-Kfl—Cretaceous Pierre Shale, Fox Hills Sandstone, and Laramie Formation (?); KTsp—Cretaceous to Tertiary South Park Formation; Ta—Oligocene Antero Formation.

Figure S1-4. Time-structure map on the top of the Cretaceous Dakota Formation showing the principal structures in the eastern part of the South Park Basin. The map is based on surface geology, and subsurface well and seismic control.

Figure S1-4. Time-structure map on the top of the Cretaceous Dakota Formation showing the principal structures in the eastern part of the South Park Basin. The map is based on surface geology, and subsurface well and seismic control.

Figure 8.

Photograph of Upper Cretaceous Apache Creek Sandstone. The Apache Creek Sandstone Member of the Pierre Shale is an offshore marine sandstone that has been the target for oil and gas exploration in South Park. It is exposed in gullies on the north side of U.S. Highway 285 east of Red Hill Pass.

Figure 8.

Photograph of Upper Cretaceous Apache Creek Sandstone. The Apache Creek Sandstone Member of the Pierre Shale is an offshore marine sandstone that has been the target for oil and gas exploration in South Park. It is exposed in gullies on the north side of U.S. Highway 285 east of Red Hill Pass.

Figure S3-1. Stratigraphic column of the upper part of the Minturn Formation. The upper part of the Minturn Formation contains both marine and nonmarine sediments as shown in this measured section of the roadcut on CO Highway 9 just north of Fairplay.

Figure S3-1. Stratigraphic column of the upper part of the Minturn Formation. The upper part of the Minturn Formation contains both marine and nonmarine sediments as shown in this measured section of the roadcut on CO Highway 9 just north of Fairplay.

Figure S5-1. Photograph of crystalline gypsum in the Minturn Formation evaporite facies. Crystalline gypsum of the Minturn Formation evaporite facies in the drainage ditch on the west side of TRPA Road (39.05735° N, 105.92162° W).

Figure S5-1. Photograph of crystalline gypsum in the Minturn Formation evaporite facies. Crystalline gypsum of the Minturn Formation evaporite facies in the drainage ditch on the west side of TRPA Road (39.05735° N, 105.92162° W).

Figure S5-2. Water quality pie diagrams from water wells in the Minturn Formation. Water quality in the Minturn Formation is locally influenced by evaporite minerals in the evaporite facies as shown by the pie diagrams for water quality analyses from wells and springs in the area. Diagram diameter reflects total dissolved solids and pie diagram shows major ion concentrations. Many wells in shallow water wells indicate a fresh shallow groundwater system, yet some wells and springs reflect a deeper more saline system. The largest diagram is from the Salt Works spring. (Data source: USGS National Water Quality Monitoring Monitoring Council Water Quality Data [http://waterqualitydata.us/portal/, all data through 2014].)

Figure S5-2. Water quality pie diagrams from water wells in the Minturn Formation. Water quality in the Minturn Formation is locally influenced by evaporite minerals in the evaporite facies as shown by the pie diagrams for water quality analyses from wells and springs in the area. Diagram diameter reflects total dissolved solids and pie diagram shows major ion concentrations. Many wells in shallow water wells indicate a fresh shallow groundwater system, yet some wells and springs reflect a deeper more saline system. The largest diagram is from the Salt Works spring. (Data source: USGS National Water Quality Monitoring Monitoring Council Water Quality Data [http://waterqualitydata.us/portal/, all data through 2014].)

Figure S6-1. Photograph of Belden Formation and Coffman Member of the Minturn Formation. The roadcut on the west side of U.S. Highway 285 exposes the contact between carbonaceous shale and sandstone of the Pennsylvanian Belden Formation and pebble-rich sandstone of the Pennsylvanian Coffman Conglomerate Member of the Minturn Formation (38.91282° N, 105.97148° W).

Figure S6-1. Photograph of Belden Formation and Coffman Member of the Minturn Formation. The roadcut on the west side of U.S. Highway 285 exposes the contact between carbonaceous shale and sandstone of the Pennsylvanian Belden Formation and pebble-rich sandstone of the Pennsylvanian Coffman Conglomerate Member of the Minturn Formation (38.91282° N, 105.97148° W).

Figure S8-1. South Park “Book Cliffs” detail map. This annotated aerial photograph of Stop 8 (Fig. 3) shows a general picture of relatively flat-lying arkose and conglomerate of the South Park Formation resting unconformably on faulted and folded Fox Hills Formation and Pierre Shale. At the contact, the South Park and underlying Fox Hills formations are conformable and dipping 45° to the north, however, within 1.5 m above the contact, beds within the South Park Formation flatten and dip gently to the north.

Figure S8-1. South Park “Book Cliffs” detail map. This annotated aerial photograph of Stop 8 (Fig. 3) shows a general picture of relatively flat-lying arkose and conglomerate of the South Park Formation resting unconformably on faulted and folded Fox Hills Formation and Pierre Shale. At the contact, the South Park and underlying Fox Hills formations are conformable and dipping 45° to the north, however, within 1.5 m above the contact, beds within the South Park Formation flatten and dip gently to the north.

Figure S8-2. Photograph of faulted and folded Fox Hills Sandstone. This view to the north at Stop 8 (“Book Cliffs”) shows relatively flat-lying South Park Formation resting on highly deformed and faulted Fox Hills Formation and Pierre Shale (person for scale).

Figure S8-2. Photograph of faulted and folded Fox Hills Sandstone. This view to the north at Stop 8 (“Book Cliffs”) shows relatively flat-lying South Park Formation resting on highly deformed and faulted Fox Hills Formation and Pierre Shale (person for scale).

Figure S8-3. Regional cross section B-B’ from near the “Book Cliffs” shows the complex structural features that evolved prior to, during, and following deposition of the South Park Formation. S.L.—sea level. See Figure S1-4 for cross-section location.

Figure S8-3. Regional cross section B-B’ from near the “Book Cliffs” shows the complex structural features that evolved prior to, during, and following deposition of the South Park Formation. S.L.—sea level. See Figure S1-4 for cross-section location.

Figure S9-1. Photograph of Elkhorn thrust plane. View to the east showing the Elkhorn thrust putting Precam-brian basement over Morrison Formation. Fault plane is highly contorted and shows an overall moderate dip to the northeast. The fault strikes to 315° and dips 53° northeast at the top of the trench (8.5” χ 11” standard paper for scale, 39.09034° N, 105.71838° W, ± 4 m).

Figure S9-1. Photograph of Elkhorn thrust plane. View to the east showing the Elkhorn thrust putting Precam-brian basement over Morrison Formation. Fault plane is highly contorted and shows an overall moderate dip to the northeast. The fault strikes to 315° and dips 53° northeast at the top of the trench (8.5” χ 11” standard paper for scale, 39.09034° N, 105.71838° W, ± 4 m).

Figure S9-2. Schematic time-structure strike section C-C’. This north-south time-structure strike section shows the Elkhorn thrust diving structurally across the central part of the South Park Basin and rising to both the north and south due to folding by subthrust structures.

Figure S9-2. Schematic time-structure strike section C-C’. This north-south time-structure strike section shows the Elkhorn thrust diving structurally across the central part of the South Park Basin and rising to both the north and south due to folding by subthrust structures.

Figure S10-1. Chase Gulch graben structure map. The Chase Gulch fault drops a half-graben on the hanging wall of the Elkhorn thrust. A depth of almost 490 m (1600 ft) of what was thought to be Antero Formation was reported from a uranium exploration borehole near the graben depocenter. Using that depth, and a projection of exhumed surfaces at the graben edge, as much as 600 m of fill is indicated.

Figure S10-1. Chase Gulch graben structure map. The Chase Gulch fault drops a half-graben on the hanging wall of the Elkhorn thrust. A depth of almost 490 m (1600 ft) of what was thought to be Antero Formation was reported from a uranium exploration borehole near the graben depocenter. Using that depth, and a projection of exhumed surfaces at the graben edge, as much as 600 m of fill is indicated.

Figure S10-2. Cross section through the Chase Gulch graben. This cross section, modified from Shafer (1980), illustrates how the Chase Gulch normal fault may form a half-graben isolating slivers of the Elkhorn thrust hanging wall at Sulfur Mountain and Spinney Mountain.

Figure S10-2. Cross section through the Chase Gulch graben. This cross section, modified from Shafer (1980), illustrates how the Chase Gulch normal fault may form a half-graben isolating slivers of the Elkhorn thrust hanging wall at Sulfur Mountain and Spinney Mountain.

Figure S11-1. Generalized stratigraphic column of the South Park Formation in the north end of South Park. Unit abbreviations match map descriptions. Boulder beds are interpreted to be within the fine-grained arkosic member of the South Park Formation because they are folded within the South Park Formation beds in the Michigan Hill syncline.

Figure S11-1. Generalized stratigraphic column of the South Park Formation in the north end of South Park. Unit abbreviations match map descriptions. Boulder beds are interpreted to be within the fine-grained arkosic member of the South Park Formation because they are folded within the South Park Formation beds in the Michigan Hill syncline.

Figure S11-2. Oblique aerial view of the South Park Formation in the Michigan Hill syncline. Perspective overview of Stop 11 (circle) looking east into the Michigan Hill syncline. Note how the Boulder beds (dashed lines) are folded with the South Park Formation (Tsp) strata. Arrow indicates apparent truncation of older beds. This would be a result of continuous folding in the syncline during deposition. Alignment of the Elkhorn thrust is shown in the background where the fault separates rolling hills of South Park from the Elkhorn upland.

Figure S11-2. Oblique aerial view of the South Park Formation in the Michigan Hill syncline. Perspective overview of Stop 11 (circle) looking east into the Michigan Hill syncline. Note how the Boulder beds (dashed lines) are folded with the South Park Formation (Tsp) strata. Arrow indicates apparent truncation of older beds. This would be a result of continuous folding in the syncline during deposition. Alignment of the Elkhorn thrust is shown in the background where the fault separates rolling hills of South Park from the Elkhorn upland.

Reference List of Published Geologic Maps Covering the South Park Region (in alphabetical order by scale)

Table 1.
Reference List of Published Geologic Maps Covering the South Park Region (in alphabetical order by scale)
Map nameScaleAuthorsYear published
Alma1:24,000Widmann et al.2004
Antero Reservoir1:24,000Kirkham et al.2012
Breckenridge1:24,000Wallace et al.2002
Cameron Mountain1:24,000Wallace and Lawson2008
Castle Rock Gulch1:24,000Wallace and Keller2003
Climax1:24,000McCalpin et al.2012
Como1:24,000Widmann et al.2005
Elkhorn1:24,000Ruleman and Bohannon2008
Fairplay East1:24,000Kirkham et al.2006
Fairplay West1:24,000Widmann et al.2007
Garo1:24,000Kirkham et al.2007
Gribbles Park1:24,000Wallace et al.1999
Jefferson1:24,000Barker and Wyant1976
Jones Hill1:24,000Widmann et al.2011
Marmot Peak1:24,000Houck et al.2012
Milligan Lakes1:24,000Wyant and Barker1976
Sulphur Mountain1:24,000Bohannon and Ruleman2009
Guffey1:62,500Wobus and Scott1979
Bailey1:100,000Ruleman et al.2011
Denver West1:100,000Kellogg et al.2008
Park County1:100,000Scarbrough2001
Map nameScaleAuthorsYear published
Alma1:24,000Widmann et al.2004
Antero Reservoir1:24,000Kirkham et al.2012
Breckenridge1:24,000Wallace et al.2002
Cameron Mountain1:24,000Wallace and Lawson2008
Castle Rock Gulch1:24,000Wallace and Keller2003
Climax1:24,000McCalpin et al.2012
Como1:24,000Widmann et al.2005
Elkhorn1:24,000Ruleman and Bohannon2008
Fairplay East1:24,000Kirkham et al.2006
Fairplay West1:24,000Widmann et al.2007
Garo1:24,000Kirkham et al.2007
Gribbles Park1:24,000Wallace et al.1999
Jefferson1:24,000Barker and Wyant1976
Jones Hill1:24,000Widmann et al.2011
Marmot Peak1:24,000Houck et al.2012
Milligan Lakes1:24,000Wyant and Barker1976
Sulphur Mountain1:24,000Bohannon and Ruleman2009
Guffey1:62,500Wobus and Scott1979
Bailey1:100,000Ruleman et al.2011
Denver West1:100,000Kellogg et al.2008
Park County1:100,000Scarbrough2001

Summary of Age Dates for Tertiary and Cretaceous Igneous Rocks of South Park*

Table 2.
Summary of Age Dates for Tertiary and Cretaceous Igneous Rocks of South Park*
UnitApproximate age
(Ma)
MethodQuadrangleReference
BasaltMioceneStrat.Gribbles ParkWallace et al. (1999)
Gribbles Peak tuff32-3340Ar/39ArGribbles ParkMcintosh and Chapin (1994)
Guffey Peak volcanicsOligoceneStrat.Guffey PeakEpis et al. (1976)
Thirtynine Mile volcanics34K-ArGuffey MountainEpis and Chapin (1974)
White rhyolite porphyry (later)33-35Fission-trackAlmaWidmann et al. (2004)
Antero Fm. Ash in Chase Gulch graben3440Ar/39ArStern (2016, personal commun.)
Antero Fm. ash flow tuff3440Ar/39ArAntero ReservoirKirkham et al. (2012)
Monzogranite porphyry35-40Fission-trackDenver WestKellogg et al. (2008)
Kenosha Pass andesite36Fission-trackBryant et al. (1981)
Wall Mountain Tuff3740Ar/39ArMcIntosh and Chapin (2004)
Eocene andesite3840Ar/39ArAntero ReservoirKirkham et al. (2012)
Buffalo Peaks volcanics3840Ar/39ArJones Hill, Marmot PeakWidmann et al. (2011), Houck et al. (2012)
Biotite quartz latite porphyry38Fission-trackJeffersonBryant et al. (1981)
Quartz monzonite porphyry Monzonite porphyry37-49, 65
42-44
Fission-track,
K-Ar Strat.
Alma AlmaWidmann et al. (2004) Widmann et al. (2004)
Monzodiorite porphyry42-43Strat.AlmaWidmann et al. (2004)
Quartz monzonite porphyry (early)37-49Strat.Fairplay WestWidmann et al. (2007)
South Park Fm. tuff in fine-grained arkosic member56K-ArBryant et al. (1981)
Link Spring Tuff60K-ArBryant et al. (1981)
Porphyritic intrusion6140Ar/39ArAntero ReservoirKirkham et al. (2012)
Granite porphyry of Tumble Hill6040Ar/39ArJones HillWidmann et al. (2011)
Granite porphyry of Black Mtn.6140Ar/39ArJones HillWidmann et al. (2011)
Diorite of Buckskin Gulch42, 67-72Strat.AlmaWidmann et al. (2004)
Conglomerate Member South Park Fm. tuff bed66K-ArBryant et al. (1981)
Reinecker Ridge Volcanic Member67-6940Ar/39ArComo and Fairplay
East
Widmann et al. (2005), Kirkham et al. (2006)
Granite porphyry64-7040Ar/39ArFairplay WestWidmann et al. (2007)
Whitehorn granodiorite69-70K-ArCameron MountainWallace and Lawson (2008)
UnitApproximate age
(Ma)
MethodQuadrangleReference
BasaltMioceneStrat.Gribbles ParkWallace et al. (1999)
Gribbles Peak tuff32-3340Ar/39ArGribbles ParkMcintosh and Chapin (1994)
Guffey Peak volcanicsOligoceneStrat.Guffey PeakEpis et al. (1976)
Thirtynine Mile volcanics34K-ArGuffey MountainEpis and Chapin (1974)
White rhyolite porphyry (later)33-35Fission-trackAlmaWidmann et al. (2004)
Antero Fm. Ash in Chase Gulch graben3440Ar/39ArStern (2016, personal commun.)
Antero Fm. ash flow tuff3440Ar/39ArAntero ReservoirKirkham et al. (2012)
Monzogranite porphyry35-40Fission-trackDenver WestKellogg et al. (2008)
Kenosha Pass andesite36Fission-trackBryant et al. (1981)
Wall Mountain Tuff3740Ar/39ArMcIntosh and Chapin (2004)
Eocene andesite3840Ar/39ArAntero ReservoirKirkham et al. (2012)
Buffalo Peaks volcanics3840Ar/39ArJones Hill, Marmot PeakWidmann et al. (2011), Houck et al. (2012)
Biotite quartz latite porphyry38Fission-trackJeffersonBryant et al. (1981)
Quartz monzonite porphyry Monzonite porphyry37-49, 65
42-44
Fission-track,
K-Ar Strat.
Alma AlmaWidmann et al. (2004) Widmann et al. (2004)
Monzodiorite porphyry42-43Strat.AlmaWidmann et al. (2004)
Quartz monzonite porphyry (early)37-49Strat.Fairplay WestWidmann et al. (2007)
South Park Fm. tuff in fine-grained arkosic member56K-ArBryant et al. (1981)
Link Spring Tuff60K-ArBryant et al. (1981)
Porphyritic intrusion6140Ar/39ArAntero ReservoirKirkham et al. (2012)
Granite porphyry of Tumble Hill6040Ar/39ArJones HillWidmann et al. (2011)
Granite porphyry of Black Mtn.6140Ar/39ArJones HillWidmann et al. (2011)
Diorite of Buckskin Gulch42, 67-72Strat.AlmaWidmann et al. (2004)
Conglomerate Member South Park Fm. tuff bed66K-ArBryant et al. (1981)
Reinecker Ridge Volcanic Member67-6940Ar/39ArComo and Fairplay
East
Widmann et al. (2005), Kirkham et al. (2006)
Granite porphyry64-7040Ar/39ArFairplay WestWidmann et al. (2007)
Whitehorn granodiorite69-70K-ArCameron MountainWallace and Lawson (2008)

Age dates compiled from mapping efforts throughout South Park Cretaceous and Tertiary igneous rocks fall into two episodes: (1) earlier intrusions between 70 and 56 Ma are coeval with the Laramide orogeny; and (2) a second, younger phase, between 49 and 33 Ma, coeval with post-Laramide transition from compressional to extensional tectonism for the region. *Date compilation after Barkmann et al. (2015).

Contents

GeoRef

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