8: Progressive development of basement-involved foreland thrust belts: Results from the NSF/EarthScope Bighorn Project applied to the Laramide orogeny in the Colorado Front Range
Published:September 07, 2016
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Eric A. Erslev, Karen Aydinian, Laura Kennedy, 2016. "Progressive development of basement-involved foreland thrust belts: Results from the NSF/EarthScope Bighorn Project applied to the Laramide orogeny in the Colorado Front Range", Unfolding the Geology of the West, Stephen M. Keller, Matthew L. Morgan
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Lithosphere-scale seismic experiments, structural geometries, and minor structures in the Bighorn region of the northern Rockies show that Laramide deformation was controlled by horizontal shortening driven by detachment in the lower crust. This field trip visits three exposures in the Colorado Front Range that helped generate the detachment hypothesis, and uses observations from both areas to generate a thrust belt model for basement-involved foreland orogens.
Starting north of Boulder, a traverse following the trace of the classic Six Mile fold reveals minor structures showing early layer-parallel shortening overprinted by a progressively tightening, non-self-similar fault-propagation fold. The overlying petroliferous Niobrara marls show how sequential deformation predicts the unit’s natural fractures, which are critical to the success of individual wells in this major resource play.
South of Boulder in Eldorado Canyon (known worldwide for near-vertical climbing), the trip traverses excellent exposures of the Paleozoic strata in the hanging-wall above the Golden thrust fault. Out-of-the-basin and into-the-basin thrusts allow the restoration of the Rocky Flats 2D seismic line just to the south. From a revealing overlook, Front Range and Bighorn (using the seismic and structural results from the newly completed Bighorn Project) arch geometries can be generalized into a 4D (3D space + time) model for basement-involved foreland orogens. The final stop visits the enigmatic Boulder-Weld County fault system in the Denver Basin to the east to discuss the relative importance of regional tectonic setting (e.g., low-angle subduction), stress, lithospheric rheology, gravity, and fluids to the formation of basement-involved foreland thrust belts.
The geometries, kinematics, and dynamic controls of basement-involved arches in the foreland of Cordilleran orogens have been a source of long-standing debate. Are they fundamentally different from adjoining, highly shortened, and often thin-skinned Cordilleran thrust belts, as vertical tectonic and strike-slip hypotheses suggest, or are they just basement-involved versions of thrust belts? If they are just basement-involved thrust belts, why did they develop in the cratons of only certain sections of Cordilleran orogens? This field trip will combine new structural and seismic results from the NSF/ EarthScope-funded Bighorn Project with classic exposures along the eastern flank of the Colorado Front Range (Fig. 1) to generate a new thrust belt model for basement-involved foreland orogens.
Numerous hypotheses (Fig. 2) have been proposed to explain the Front Range’s well-exposed structures. Ziegler (1917) proposed motion on planar, ~50°-dipping reverse faults in the northeastern Front Range. Boos and Boos (1957; Fig. 2A) also showed planar reverse faults in analogous sections, but steepened the fault dips to ~80°. Matthews and Work (1978) continued the trend toward steeper fault dips by proposing that vertical or normal faults underlay the anticlines along the northeast margin of the Front Range. Jacob (1983) provided an explanation of contradictory older basement on younger sedimentary rock exposures by invoking local gravity sliding.
Prucha et al. (1965) proposed that fault dips steepened downward from ~30°-dipping thrusts in the sedimentary strata to vertical faults in basement. This “upthrust” geometry (Fig. 2B) allowed low-angle thrusts observed in the cover to be explained within a vertical tectonic framework. Braddock et al. (1970) and Le Masurier (1970) adopted this geometry for the Milner Mountain anticline west of Fort Collins (see Fig. 1), which they interpreted as being formed by a reverse fault dipping 60° at the surface that becomes vertical at depth. More recently, this geometry, which is reminiscent of strike-slip flower structures, and the local en echelon arraignment of anticlines were proposed to indicate primarily strike-slip deformation.
But very high-angle reverse, normal, and strike-slip faults in the area are in conflict with the thrust geometries revealed by the trenching of major faults in the area, whose fault zones consistently showed older-over-younger relationships between cataclasized Precambrian and Phanerozoic rocks (Erslev and Rogers, 1993). These observations were supported by minor fault data (Fig. 3; Holdaway, 1998; Erslev and Koenig, 2009; Allen, 2010), geometric balancing constraints (Figs. 2D, 4; Erslev, 1993, 2005; Holdaway, 1998; Larson, 2009), regional seismic data, and well penetrations. All of these observations support horizontal shortening and compression in the Rocky Mountain foreland consistent with the interpretations of Ziegler (1917). Because larger thrusts with > 10 km of slip crop out on the western side of the Front Range, Erslev (1993) suggested that the northeastern Front Range is a backlimb-tightening zone (Figs. 2D, 4) in the back limb of a WSW-directed, basement-involved thrust sheet.
In 2010, the Bighorn Arch Seismic Experiment, the crustal-scale active and passive experiments of the NSF/EarthScope Bighorn Project (Yeck et al., 2014; Worthington et al., 2016), showed that the Moho under the Bighorn arch is not cut by major Laramide faults. This falsifies interpretations invoking planar high-angle Laramide faulting that should cut the Moho. In addition, Yeck et al. (2014) and Worthington et al. (2016) showed that Moho geometries are independent of Laramide arch geometries, necessitating detachment in the lower crust. This field trip will show that the Laramide basement-involved foreland orogen experienced the same sequence of layer-parallel shortening + detachment folding followed by fault propagation + fault-bend folding that is characteristic of thin-skinned thrust belts. We will then discuss how detachment-driven thrusting holds the key to understanding the controls on basement-involved orogens in cra-tonic forelands worldwide.
Stop 1: A Traverse of Six Mile Fold North of Boulder, Colorado (40.106° N, 105.283° W)
From the Colorado Convention Center (700 14th Street, Denver, Colorado 80202), get on Colfax Avenue going west, and then turn north on Speer Boulevard to join north I-25. Turn NW on U.S.-36 toward Boulder and stay on this road as it merges into 28th Street, which will eventually head NW. At the intersection with CO-93/Broadway Street, follow the road northward as it turns into N. Foothills Parkway. After ~5 km (3 miles), turn right toward Neva Road and park just beyond where the two entrances to the parkway intersect the E-W Neva Road. We will be walking for ~2 h, so bring appropriate clothing and supplies. Sturdy footwear, a water bottle, and sun protection (hat, sunglasses, and sunscreen) are strongly recommended.
This stop traverses the Six Mile fold (Fig. 5) whose outcrops reveal the classic thrust belt chronology of layer-parallel shortening followed by progressive fold tightening. We will start at a roadcut of the basal Fort Hayes member of the Niobrara Formation (see Fig. 6 for the complete stratigraphic column) containing slickensided fractures and solution cleavage. The Nio-brara Formation is a Cretaceous sequence of hydrocarbon-bearing marls and easily fractured chalks. This combination makes it an economic resource play for oil and gas, whose current production is driving oil production in NE Colorado and elsewhere in the eastern Rockies. Natural fractures are commonly cited as the factor that makes or breaks individual wells.
The traverse will follow the Fort Hayes member laterally as it beautifully outlines the progressively developed Six Mile fold (Fig. 5). An examination of the anticlinal crest reveals thrust faults, minor folds, and pencil cleavage, which confirm the fold’s origin by horizontal shortening during thrust tectonics. The origin of the fold will be discussed—is it a west-directed, basement-cored structure like many folds to the north, or is it related to triangle zones driven by the east-directed Golden fault system, which dominates structures to the south (Sterne, 2006)?
The traverse continues through the overlying marls and shales that are the main objectives of the Niobrara resource play. At a vantage point, we will discuss Niobrara core and outcrop interpretation. In outcrops, it is essential to decipher which fractures are present in the sub-surface and which fractures formed during exposure. In cores, determining which fractures are natural and which are induced, either from drilling or from handling, is similarly essential. In cores, the most obvious indicator of natural fracturing is mineralization. If mineralization is not present on the fracture surface, methods like Helical CT (computed tomography) scanning can detect mineralization on unbroken fractures, as well as quantify remnant natural fracture porosity and calculate angles between fractures.
Finally, we will examine an intriguing roadcut that indicates multi-stage deformation, with strike-slip faulting (Laramide?) being overprinted by later normal faulting. Currently, a vital question with large implications for Niobrara resource play success is whether the unit’s open fractures and productive fracture networks were formed during the Laramide or are, at least in part, due to post-Laramide, mid-Tertiary, and Rio Grande rift extensional deformation. Because normal faults and joints from the latter events are commonly the dominant open fractures on the west flank of the Front Range, it is important to determine the impact of these extensional events on the eastern side of the Front Range; the latest stages of fracturing typically create open fractures that can enhance the fracture networks that dictate fluid flow and production success.
Stop 2: Structural Geology of Eldorado Canyon State Park, Colorado (39.930° N, 105.384° W)
Return south on N. Foothills Parkway, turning right on to CO-93 (aka Broadway Street through Boulder and S. Foothills Highway south of Boulder). Turn right on Eldorado Springs Drive in Marshall just south of Boulder and drive through Eldorado Springs, parking just beyond the Eldorado Canyon State Park entrance station. After a brief restroom stop, we will park ~1 km (0.6 mi) from the entrance station at the Fowler Trail trailhead.
This stop traverses the forelimb of the eastern Front Range in the hanging wall of the Golden thrust system at Eldorado State Park, whose excellent exposures are known worldwide for their high-angle climbing routes. We will start by examining the crystalline Proterozoic basement rocks of the Front Range and viewing their discordance with the overlying Pennsylvanian-Permian Fountain Formation, which is the basal sedimentary rock unit in this area. After lunch, we will follow the relatively flat Fowler Trail through the Paleozoic units exposed on the south side of the canyon.
The synorogenic arkoses and shales of the Fountain Formation were laid down during the Ancestral Rocky Mountain (ARM) orogeny. They are highly indurated in this area due to adularia (low temperature K-feldspar) cements, which give puzzling ages of 135 and 94 Ma (Warnock and van de Kamp, 2000). The ARM basement-cored structural geometries are quite analogous to those of the Laramide orogeny, although arch geometries trend more NW whereas Laramide arches trend more NNW. But these orogens are dissimilar in regional relationships as no strong evidence for low-angle subduction has been shown for the Ancestral Rocky Mountain orogen. Still, the similarities between these two basement-involved orogens suggest a common mechanical mechanism that seems to occur in different tectonic settings.
A spectacular spot overlooking the town of Eldorado Springs provides an excellent view of the basement exposures to the west, the Boulder Flatirons that bound the western side of the Denver Basin, and the Denver Basin to the east. We will discuss regional relationships revealed by a local seismic section (Fig. 7A) just south of this point, and the implications of 3D seismic data collected during the recently completed Bighorn Project (Yeck et al., 2014; Worthington et al., 2016), indicating lower-crustal detachment (Fig. 8).
Stop 3: Overview of the Range Front and the Boulder-Weld County Fault System South of Boulder, Colorado (30.960° N, 105.222° W)
Return to Eldorado Springs and head NE on Eldorado Springs Drive. Cross S. Foothills Highway, and turn right on to Marshall Road. Just past Marshall, turn left (north) on Cherryvale Road. Park on the outside (left) of the first sweeping right turn, carefully cross the road, and enter the Boulder County Open Space through a gate a little farther up the road.
Our final stop is just north of Marshall where we will look at the Boulder-Weld County fault system, an anomalous zone of NE-striking faulting in the latest Cretaceous Fox Hill Sandstone and overlying Laramie Formation that ends at the N-S mountain front (Fig. 1). The area has seen a large variety of hypotheses, including high-angle reverse and normal faults (Spencer, 1961), syndepositional faulting (Weimer, 1973), and detachment thrust faulting and associated folding (Kittleson, 1992). The latter hypothesis was generated by Ken Kittleson, who noted that multiple well penetrations to the northeast show clear repetitions of units that do not demonstratively vary in thickness across the faults. This helped motivate Bjorn Selvig (1994) to do a detailed structural analysis of the Marshall area.
Selvig (1994; see also Erslev et al., 2004) supported Kit-tleson’s (1992) hypothesis of thin-skinned detachment due to southeasterly down-dip slip into the Denver Basin. Selvig (1994) interpreted the NE-striking faults in the Marshall area as low-angle thrusts dominated by SE-NW slip. But recent observations, including near vertical (not low-angle), NE-striking faults in outcrop, N80W-trending thrust fault slickenlines, and N10E conjugate fault intersection, suggest that slip in the minor thrust faults in the Marshall area averaged 100°, which is closer to the ENE-WSW to E-W Laramide slip directions than the SE-directed detachment slip postulated by Kittleson (1992).
These observations suggest that this NE-trending swarm of faults may be analogous to the NE-trending Greyback monocline to the north near the Wyoming border, where fault fabrics and clockwise rotations of paleomagnetic poles indicate right-lateral transpressional movement during the Laramide orogeny (Holdaway, 1998; Tetreault et al., 2008). The reasons for (1) the slightly anomalous 100° slip direction and (2) the overall right-lateral shear indicated by the high-angle, NE-striking shear planes remain uncertain. However, late-stage Laramide slip directions can be variable, possibly due to gravitational inputs, and slip directions are similarly rotated clockwise toward the SE in the Greyback monocline. An overall right-lateral sense of shear suggests more Laramide shortening in the Denver Basin north of the Boulder-Weld County fault system than to the south of this fault system.
On a regional scale in the northeastern Front Range, minor faults form discrete domains, with zones of thrust faulting alternating between areas of 3D strike-slip accommodation, where (1) folds plunge out or (2) NE-striking Precambrian shear zones appear to have been reactivated by Laramide strike slip (Fig. 9; Erslev and Kennedy, 2013). The delineation of transpressive zones could have important implications for resource play petroleum exploration and production. If transpressional areas are characterized by subvertical fractures in the subsurface, these fractures could aid fracture stimulation attempts far more than the subhorizontal thrust faults and their associated nearly horizontal joints.
We will conclude the trip with a discussion of the controls on basement-involved thrust belts like the Rockies in cra-tons adjacent to Cordilleran orogens. These have importance to petroleum exploration and tectonics in general, as the distributed deformation in Cordilleran orogens only approximately follows the constraints of plate tectonics.
Directions for the Return to the Colorado Convention Center
Return to Marshall Drive and head east to the U.S.-36 frontage road going SE. Follow this until you intersect McCaslin Boulevard, turning left to head north to an entrance ramp for U.S.-36 heading SE toward Denver. Merge on to I-25 southbound, and turn southeast on Speer Boulevard to reach the Colorado Convention Center.
This trip is based on the long hours and hard work represented by the CSU graduate theses of Bjorn Selvig, Steve Holdaway, Scott Larson, and Cody Allen. Ned Sterne and Frank Ethridge are gratefully acknowledged for sharing their knowledge of the Front Range and providing informal reviews of this manuscript. We appreciate Matt Morgan’s careful and helpful editing of this paper.
Figures & Tables
Unfolding the Geology of the West
Prepared in conjunction with the 2016 GSA Annual Meeting in Denver, Colorado, this volume contains sixteen guides to field trips in this rich geologic region. The four “Great Surveys” of the late 1800s ventured west to explore and document the region’s unknown natural resources and collect valuable geologic information. Many of the field guides in this volume, aptly titled Unfolding the Geology of the West, will cover the same hallowed ground as the early geologic expeditions. Organized into four sections, this volume spans some of the major subdisciplines of geology: (1) stratigraphy, sedimentology, and paleontology; (2) structure and metamorphism; (3) Quaternary landscape evolution; and (4) engineering and environmental geology.