In southwestern Montana (SWMT), Paleogene strata that are preserved in the Sage Creek basin were deposited during the transition from late-stage Sevier-Laramide compressional tectonism to extensional reactivation of the northern Rocky Mountain region. Similar to correlative deposits across the Cordillera, Sage Creek basin strata host diverse paleoclimate and paleoenvironmental proxies such as mammal fossil assemblages, paleobotanical fossil assemblages, and thick paleosol successions. As such, Sage Creek basin strata have been the focus of recent paleoclimate and paleoenvironmental studies, which have yielded results that indicate significant tectonic, climatic, and topographic evolution of the northern Cordillera during Paleogene time.
This study revisits a succession of Paleocene to Oligocene terrestrial deposits in the Sage Creek basin of SWMT that have recently been at the forefront of Cenozoic paleoclimate studies in the northern Rocky Mountains. We present detailed physical stratigraphic trends, cobble and detrital zircon provenance data, and detrital zircon–based age constraints for the Paleocene to Oligocene basin fill that reveal the evolution of depositional systems, sediment routing, and approximate rates of deposition in the basin. The new data are compared with regional trends in structural deformation and volcanism to interpret a detailed depositional and tectonic history of the SWMT region. Specifically, the Paleogene strata record (1) the final pulse of Sevier-Laramide deformation in the region; (2) widespread erosional exhumation of the Sevier fold-thrust belt and Laramide intraforeland uplifts during Paleocene and early Eocene time; (3) middle Eocene onset of localized, bimodal volcanism and volcaniclastic sedimentation associated with gravitational collapse of the Cordilleran complex; and (4) middle Eocene to Oligocene extensional reactivation of the Sevier fold-thrust belt and Laramide intraforeland province. Constraints on the timing, duration, and sedimentary response to such events provide a more complete understanding of the tectonic and topographic evolution of SWMT, which provides an essential framework for interpreting the suites of terrestrial paleoclimate and paleoenvironmental data in the northern Cordillera.
INTRODUCTION AND GEOLOGIC BACKGROUND
Paleogene strata that are preserved in the Sage Creek basin of southwestern Montana (SWMT) were deposited during the transition from Sevier-Laramide compression to extensional reactivation of the Cordillera at the latitude of southern Montana and central Idaho and comprise the earliest, most complete record of sedimentation directly following the end of Sevier-Laramide compression in the SWMT region (Hanneman and Wideman, 1991). The Paleogene succession has long been known to host diverse assemblages of mammal fossils and has drawn many paleontological researchers over the past century (e.g., Douglass, 1909; Fields et al., 1985; Tabrum et al., 1996). More recently, the stratigraphy has been of interest with regard to terrestrial paleoclimate and paleoaltimetry studies, as it hosts thick paleosol assemblages that bracket thermal events such as the early and middle Eocene climatic optimums (Kent-Corson et al., 2006; Chamberlain et al., 2012; Schwartz, 2015; Methner et al., 2016). Stable and clumped isotope records from the Sage Creek basin indicate overall climatic cooling and aridification of SWMT between Paleocene and Oligocene time, which was punctuated by brief hyperthermal events (Schwartz, 2015; Methner et al., 2016). Additionally, Kent-Corson et al. (2010) interpret a negative isotopic excursion in the Sage Creek basin δ18O record to represent multi-kilometer elevation gain in the westward-adjacent Sevier hinterland during early to middle Eocene time.
Despite the abundance of the paleontological and paleoenvironmental work that has been done in the region, the depositional history of the Sage Creek basin has been documented at a relatively coarse scale, and the absolute timing of depositional and tectonic events remains loosely constrained. The relatively poor age resolution of depositional sequences within the basin, as well as the incomplete understanding of the temporal relationship of sedimentation to tectonic events in SWMT, hinders the interpretation of paleoenvironmental proxies by obscuring potential correlations between tectonic activity, landscape evolution, and climatic variation. As such, the renewed use of Paleogene strata within the Sage Creek basin for paleoenvironmental studies warrants the development of a detailed, physical stratigraphic framework in the basin.
In addition to being recorded in suites of stable isotope data, climatic variation and kilometer-scale topographic development (e.g., Kent-Corson et al., 2010) should be chronicled in contemporaneous basin fill. A detailed understanding of the basin fill, in turn, has the capacity to greatly inform the interpretation of such geochemical proxies. This study synthesizes detailed stratigraphic and provenance trends from the Paleogene strata of the Sage Creek basin to interpret a depositional history in the context of the early Cenozoic tectonic and climatic evolution of the northern Cordillera. We correlate temporal variations in depositional systems and sediment provenance to regional variations in tectonic activity, and provide additional age constraints on the stratigraphy from ash and detrital zircon geochronology.
The Paleogene Sage Creek basin is located in a structurally complex region of SWMT where the Sevier and Laramide structural provinces overlap in the northern Rocky Mountain Cordillera (Fig. 1). In this area, the Cordillera comprises contractional tectonic terranes including (1) the Sevier fold-thrust belt, (2) Laramide intraforeland uplifts, and (3) Upper Cretaceous magmatic rocks associated with the Idaho, Boulder, Pioneer, and Tobacco Root batholiths (Fig. 1). In SWMT, east-directed, thin-skinned crustal shortening associated with the Sevier orogeny began in Late Jurassic time, and culminated with propagation of the Helena and McCartney thrust salients into SWMT during early Paleocene time (Schmidt and O’Neill, 1982; Harlan et al., 1988; Haley and Perry, 1991; Constenius, 1996; DeCelles, 2004). Initial emergence of basement-cored Laramide uplifts in the adjacent foreland region was partly contemporaneous with thrust sheet emplacement, but continued into early Eocene time (Schmidt and Garihan, 1983; Schwartz and DeCelles, 1988; Romero-Armenta et al., 2014). Emplacement of the Idaho and Boulder batholiths and their extrusive counterparts occurred between ca. 100–53 Ma, partly overprinting Sevier and Laramide structural features (Lund et al., 2002; Gaschnig et al., 2011).
Shortly following the cessation of regional contraction (ca. 60–50 Ma), the northern Cordillera underwent a phase of gravitational collapse (Constenius, 1996; Constenius et al., 2003) that reactivated preexisting contractional structures. In SWMT, middle Eocene extension was accompanied by extrusion of the Lowland Creek, Dillon, Melrose, and Virginia City volcanic fields (ca. 53–45 Ma; Rasmussen, 2003; Fritz et al., 2007; Dudás et al., 2010), as well as exhumation of the Anaconda and Bitterroot metamorphic core complexes (Coney and Harms, 1984; O’Neill et al., 2004; Foster et al., 2007). Adjacent to the study area, in Idaho and Wyoming, voluminous volcanism occurred in the Challis and Absaroka volcanic fields (Fig. 1) (Janecke, 1994; Hiza, 1999). Neogene extension associated with northern propagation of the Basin and Range extensional province and northeastward migration of the Yellowstone hotspot further modified the landscape, segmenting some of the preexisting Cenozoic topography (Fritz and Sears, 1993; Sears and Ryan, 2003; Fritz et al., 2007).
Regional Cenozoic Stratigraphy
As Sevier and Laramide deformation culminated in SWMT (ca. 65–50 Ma) and the final pulses of synorogenic sedimentation occurred (recorded by the Beaverhead Group; Fig. 2), warm and humid climate conditions in the northern Cordillera prompted deep fluvial exhumation of the Cordilleran orogenic wedge (Schwartz and Schwartz, 2013). Fluvial incision along zones of structural and stratigraphic weakness generated a topographically complex landscape with mountainous highlands separated by narrow intermontane basins (up to 3 km paleorelief; Dettman and Lohmann, 2000; Lielke, 2012; Schwartz and Schwartz, 2013) and generated a regional unconformity that variably spans Paleocene and early Eocene time across SWMT (Fig. 2).
The middle Eocene to lower Miocene Renova Formation is the first record of widespread sedimentation following the Sevier and Laramide orogenies in SWMT (Hanneman and Wideman, 1991) and infills the series of paleovalleys that were incised into the orogenic wedge during Paleocene–early Eocene time (Schwartz and Schwartz, 2013). Significant lithologic variability, poor outcrop continuity, and postdepositional extensional modification of the region make stratigraphic correlation difficult within and between basins (e.g., Kuenzi and Fields, 1971; Hanneman and Wideman, 1991). Age relationships of Cenozoic strata are primarily based on mammal biostratigraphy (e.g., Fields et al., 1985; Tabrum et al., 1996) and similarities between lithologic assemblages (e.g., Hanneman and Wideman, 1991; Schwartz and Schwartz, 2013), with few geochronometric constraints (Fig. 2). Because biostratigraphy has historically been one of the most robust age measures of Cenozoic nonmarine strata, informal members of the Renova Formation are commonly referred to by their North American Land Mammal Ages (NALMA; Fig. 2).
Regionally, the Renova Formation is subdivided into many informal members with variable nomenclature (see review in Rasmussen, 2003). More broadly, the Renova Formation can be split into three lithologic sequences defined by intra-formational unconformities and/or hiatal surfaces (after Hanneman and Wideman, 1991). Sequence 1 (middle Eocene; late Wasatchian to early Uintan NALMA) overlies the regional Paleocene–early Eocene erosional surface. In the Sage Creek basin study area, Sequence 1 rocks are represented by the lower Dillon Volcanics and the Sage Creek and the Dell members of the Renova Formation (Fig. 2). Sequence 2 (upper Eocene to lower Oligocene; late Uintan to late Orellan NALMA) is separated from Sequence 1 by a disconformity in the Sage Creek basin and is represented by the Cook Ranch member of the Renova Formation (Fig. 2). Sequence 3 (middle Oligocene to lower Miocene; Arikareean NALMA) is relatively conformable with Sequence 2 in the Sage Creek basin and is represented by the White Hills and Blacktail Creek members of the Renova Formation (Fig. 2).
The Renova Formation is overlain by the Miocene to Pliocene Sixmile Creek Formation (Fig. 2). The two formations are separated by an angular unconformity that ranges from very subtle (subhorizontal) to pronounced (up to 20°). The Sixmile Creek Formation consists of interbedded gravels, tuffs, and reworked volcanic deposits (Fig. 2) (Fritz and Sears, 1993).
The northwest-trending Sage Creek basin lies ∼50 km south of Dillon, Montana (Fig. 3). The basin and its surrounding highlands have a mixed ancestry related to Sevier and Laramide contractional deformation and subsequent Cenozoic extension. The current basin is topographically confined on the west by the Sevier fold-thrust belt culmination (Tendoy Mountains) and on the north and south by the Blacktail Mountains and Snowcrest Range (Laramide intraforeland uplifts; Fig. 1B). Although they are contraction-related features, the structural culminations are currently bound by high-angle normal faults that initiated during Cenozoic extensional reactivation of the area (Fritz and Sears, 1993; Fritz et al., 2007). The Paleogene to Neogene stratigraphy within the Sage Creek basin is similarly crosscut by predominantly northwest-trending extensional faults (Fig. 3) (Lonn et al., 2000).
Representative stratigraphic sections of the Sage Creek, Dell, and Cook Ranch members of the Renova Formation were measured at 10-cm resolution to document fine-scale trends in sedimentary architecture and provenance. Paleocurrent data are from trough, tangential, and tabular cross-stratification (after DeCelles et al., 1983), imbricated cobbles, channel margin orientations, and current-oriented logs. Cobble counts of at least 100 clasts are provided for conglomeratic deposits in order to identify sediment source regions. Conglomerate clasts were divided into the following age-lithologic categories: Archean to Proterozoic metamorphic rocks; Proterozoic siltite, sandstone, and conglomerate; Paleozoic limestone, sandstone, and chert; Mesozoic sandstone and conglomerate; Cenozoic volcanic rocks; and microcrystalline siliceous rocks (siltite) of unknown age (after Schwartz and Schwartz, 2013).
Detrital Zircon Geochronology and Provenance
In order to better constrain sediment provenance and depositional age, detrital zircon samples were analyzed for the Paleogene succession. Samples were preferentially collected from medium- to coarse-grained alluvial fan or fluvial sandstone deposits (sample locations are listed in Table 1; corresponding measured sections are displayed in Fig. 3). A rhyolite tuff from the lower Dillon Volcanics, which directly underlies the Sage Creek member, was also analyzed to provide an additional age constraint on the succession.
Analytical and Statistical Methods
Detrital zircon grains were isolated following standard density and magnetic separation techniques, summarized in Supplemental Item 11. Uranium-lead (U-Pb) geochronology of zircon grains was conducted by laser ablation–multicollector–inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the University of Arizona LaserChron Center using standard analytical techniques, also summarized in Supplemental Item 1 (see footnote 1) (after Gehrels et al., 2006, 2008). The best age of each grain was determined from the 206Pb/238U age for analyses with 206Pb/238U <1000 Ma and from the 206Pb/207Pb age for analyses with 206Pb/238U age >1000 Ma. Individual analyses with (1) >10% 1-σ uncertainty in 206Pb/238U age; (2) >10% 1-σ uncertainty in 206Pb/207Pb age for grains >500 Ma; (3) a 206Pb/238U age >500 Ma and with >20% discordance; and (4) a 206Pb/238U age >500 Ma and with >5% reverse discordance were not included in provenance analyses. All new analytical data and a detailed list of concordance filters are reported in Supplemental Item 22.
Probability density plots were generated using Isoplot 3.7 (Ludwig, 2008) in Microsoft Excel. Cumulative age probability plots were constructed using open-source Excel programs available from the University of Arizona LaserChron Center.
To help constrain the depositional age of each sample (and therefore each stratigraphic unit), we provide the weighted-mean age of the youngest detrital zircon age cluster present as a maximum depositional age (MDA) of the sample. In this study, the “youngest cluster” is defined as the youngest mode of grains in the sample distribution with overlapping ages (within 1-σ error), with n ≥ 2 (after Dickinson and Gehrels, 2009; Schwartz et al., 2016). As such, we report the calculated ages as YC1σ MDAs (after Dickinson and Gehrels, 2009). Within each cluster, we assume that all zircon grains are comagmatic (i.e., of the same source) and that scatter around that age can be explained by analytical uncertainty and/or subtle variation in crystallization age. For samples with peaks composed of compound, multi-modal clusters of analyses (such as sample TS12-SCS6 from the Dell member of the Renova Formation; Fig. 4), the AgePick function in Isoplot 3.7 (Ludwig, 2008) was used to identify the youngest subgroup of zircon grains within the cluster.
RESULTS: SAGE CREEK BASIN STRATIGRAPHY AND PROVENANCE
The Sage Creek basin contains a series of Paleogene depositional units that are separated by erosional and/or hiatal surfaces of varying magnitudes (Fig. 2). In SWMT, pre-Cenozoic rocks within Sevier and Laramide structural culminations are locally overlain by the synorogenic Maastrichtian–Paleocene Beaverhead Group. In the Sage Creek basin, the upper Beaverhead Group is represented by the Paleocene Red Butte Conglomerate (Fig. 2) (Perry et al., 1988; Haley and Perry, 1991). The Red Butte Conglomerate is in turn overlain by the middle Eocene to Oligocene Sage Creek, Dell, and Cook Ranch members of the Renova Formation (Fig. 2). The following section summarizes lithologic characteristics, detrital zircon provenance signatures, and brief interpretations of depositional environments for each of the Paleogene units.
Red Butte Conglomerate of the Beaverhead Group (Torrejonian to Tiffanian)
Depositional Processes and Environment
The Red Butte Conglomerate was deposited as an assemblage of alluvial fans that flanked the frontal Sevier fold-thrust belt during its final stage of propagation into the SWMT foreland region (Haley and Perry, 1991). The unit is characterized by thick successions of boulder to cobble conglomerate, diamictite, and laminated sandstone (Haley and Perry, 1991). Conglomeratic units contain abundant first-cycle, angular limestone clasts derived from Paleozoic rocks in the Tendoy thrust system, as well as recycled limestone conglomerate and well-rounded quartzite clasts that were likely derived from underlying Beaverhead Group members during progressive thrust front advance (Haley and Perry, 1991). Southeast-directed paleocurrent indicators support sediment derivation from the Tendoy thrust system (Haley and Perry, 1991).
Detrital Zircon Provenance
A sandstone lens within the Red Butte Conglomerate at Spring Gulch (sample TS12-KBR-1) is dominated by Proterozoic (ca. 1900–950 Ma) and Archean (ca. 2900–2500 Ma) zircon grains with minor Paleozoic (ca. 500–300 Ma) grains (Fig. 4). All ages are consistent with sedimentary recycling of Proterozoic to Mesozoic sedimentary units located in the fold-thrust belt, with grains originally derived from terranes such as the Appalachian orogen and/or Grenville province, the southern North American craton, and the Canadian shield (Table 2) (after Fuentes et al., 2009, 2012; Gehrels et al., 2011). The youngest detrital zircon ages observed in the Red Butte Conglomerate sample are ca. 346 Ma (Mississippian), more than 280 m.y. older than Paleocene fossil ages reported for the unit (Nichols et al., 1985) and do not provide additional insight into the depositional age of the unit.
Sage Creek Member of the Renova Formation (Bridgerian to Early Uintan)
The Sage Creek member is locally split into lower and upper units and overlies the regional Paleocene–early Eocene unconformity (Fig. 2). The Sage Creek member is largely volcaniclastic, laterally restricted, and has a minimum thickness of 50 m. In areas where the contact is exposed, the Sage Creek member displays a buttress (onlap) relationship with erosional remnants of the Red Butte Conglomerate (Fig. 5) (this study; Fritz et al., 2007).
The early Bridgerian lower Sage Creek member is composed of tabular, meter-thick beds of very coarse-grained to granular sandstone with minor fine- to medium-grained sandstone and ashy mudstone (Fig. 6). Bedding surfaces and internal sedimentary structures are mostly obscured by pronounced pedogenesis, which has resulted in a massive, blocky, crudely bedded outcrop appearance (Fig. 7). Paleosol horizons are cemented by calcium carbonate and contain abundant rootlets and burrows. The sandstone is composed primarily of volcaniclastic material including volcanic rock fragments, plagioclase, hornblende, and volcanic quartz, as well as rare siliciclastic pebbles of heavily indurated, pink and purple quartzofeldspathic arenite. Individual beds typically fine upward and sometimes contain relict trough and/or tangential cross-stratification. Sparse paleocurrent indicators suggest a north to northeast paleodispersal direction (Fig. 6).
The late Bridgerian upper Sage Creek member is very similar to the lower member, but paleosols are not as abundant or well developed and do not obscure primary stratigraphic features as much as in the lower member. The upper Sage Creek member is composed primarily of tabular to broadly lenticular beds and bed sets of medium- to coarse-grained sandstone with minor mudstone and ash (Fig. 6). Individual beds are normally graded (Fig. 7) and variably contain planar lamination, trough and/or tangential cross-stratification, and ripple lamination. Tangential cross strata are commonly over-steepened and sometimes convolute due to soft-sediment deformation. In some areas, tabular beds are truncated by narrow (≤10-m wide) and shallow (≤2-m deep) erosional channels containing well-developed trough cross-stratification (Fig. 7). Channel fills are also mostly volcaniclastic but contain relatively abundant siliciclastic pebbles. Current-oriented logs (>1 m in length) are also common at these horizons, adjacent to channelized units. Paleocurrent measurements from trough cross-strata, log orientations, and channel orientations indicate an east-northeast paleodispersal direction (Fig. 6). Throughout the upper Sage Creek member, pedogenesis is marked by horizons of rootlets and burrows, petrified logs in growth position, decimeter-scale root casts, and bedding surfaces marked by heavily weathered feldspars (Fig. 7). The top of the Sage Creek member is locally marked by a well-developed caliche zone (Fig. 7).
Depositional Processes and Environment
The Sage Creek member is an assemblage of variably reworked volcaniclastic flows that, based on paleocurrent measurements, emanated from a volcanic center located to the west of the study area. In the lower member, normal grading of otherwise structureless, tabular units indicates deposition by sheet flow or surge processes (after Lajoie, 1984; Blair and McPherson, 1994a, 1994b). The presence of large pumiceous and rhyolitic clasts supports derivation from local sources with relatively short-distance transport. Pronounced bioturbation and soil formation within these units indicate that deposition was episodic, with significant time between events. Rare trough cross-stratification indicates intermittent reworking of sediment by small fluvial or alluvial channels that crossed the volcaniclastic alluvial plain. In the upper member, the presence of thin, normally graded sheet-flow deposits indicates that deposition of volcaniclastic sheet flows continued through late Bridgerian time. However, a decrease in grain size and bed thickness indicates that volcanic activity had waned. A greater abundance of sedimentary structures and the overall decrease in grain size within tabular units also indicate more pervasive reworking of volcaniclastic sheet flows by fluvial activity. The dimensions of preserved channel bodies and lack of organized channel stacking indicate that fluvial channels were likely small, shallow, and laterally mobile, forming a relatively low-relief braided to anastomosing channel belt (after Lielke, 2012; this study). Although individual tabular beds display distinct weathering horizons and some moderately developed soil profiles, soils are more poorly developed than in the lower member, further supporting more frequent reworking by laterally mobile channel systems. In addition, an increase in the abundance of siliciclastic pebbles and granules throughout the Sage Creek member indicates unroofing of non-volcanic source areas.
Detrital Zircon Geochronology: Rhyolite Tuff of the Lower Dillon Volcanics
A rhyolitic ash-fall tuff that underlies the lower Sage Creek member (sample TS12-RHY-1) contains two visually and texturally distinct zircon populations of substantially different ages (Fig. 4). Most zircon grains (n = 22/26) are light colored, subtly zoned, and yield ages that range from 47 to 46 Ma. These are likely first-generation volcanic zircon and provide a YC1σ MDA of 47.4 ± 0.4 Ma (Fig. 4), which is consistent with the 45.5 ± 0.8 Ma age of a nearby trachyandesite flow (Fritz et al., 2007).
The remaining zircon grains are dark colored, strongly zoned, and range in age from 97 to 95 Ma. The darker, older grains may have been (1) xenocrysts within the magma chamber that sourced the lower Dillon Volcanics; (2) derived from surrounding country rock during the eruption; or (3) detrital grains that were mixed into the tuff during deposition. We interpret that the older grains were likely xenocrysts inherited from the Challis-Idaho batholith province (e.g., Gaschnig et al., 2010, 2011, 2013). Not only are the grain ages consistent with crystallization ages in the Challis-Idaho batholith province (98–53 Ma; Table 2), similar zircon ages have not been established for any of the bedrock units in the vicinity of the Dillon volcanic field (Table 2). The paucity of 95–100 Ma grains within the study area make it unlikely that they were incorporated from local country rock during eruption, and similarly, unlikely that they were incorporated during ash deposition. The lack of additional age distributions supports that there was minimal mixing of the ash with detrital material during deposition.
Detrital Zircon Provenance: Sage Creek Member
Detrital zircon samples were collected from the lower and upper Sage Creek members (samples TS12-SCS7 and TS12-SCS2, respectively; Fig. 6). However, the upper Sage Creek sample yielded few detrital zircon grains (n = 24; Supplemental Item 2 [footnote 2]). Because there is no significant change in the outcrop appearance and sedimentary architecture between the lower and upper members, we group the analyses from both members to assess detrital zircon provenance. Additionally, we include results from one previously published detrital zircon sample (sample 10-SC-01 from Rothfuss et al., 2012) to produce a composite probability density curve for the Sage Creek member of the Renova Formation (Fig. 4).
The Sage Creek member contains abundant Eocene zircon grains (55–45 Ma; Fig. 4), which are consistent with eruptive ages of the adjacent lower Dillon Volcanics (52–40 Ma; Fritz et al., 2007) and the Challis volcanic province, located ∼100 km to the northwest (51–43 Ma; Chadwick, 1981; Gaschnig et al., 2010, 2011). The Absaroka volcanic province, located ∼200 km to the east, also contains rocks within that age range (55–43 Ma; Feeley et al., 2002), but eastward-directed paleocurrent indicators within the Sage Creek member make it an unlikely source. The samples also contain abundant Proterozoic grains and scattered Paleozoic and Archean grains (Fig. 4) that indicate recycling of Proterozoic to Paleozoic strata in the fold-thrust belt and/or recycling of Cretaceous to Paleocene foreland basin strata that contain those lithologies (see Table 2). A weighted-mean age calculated from the youngest detrital zircon mode yields a YC1σ MDA of 46.6 ± 0.6 Ma (Fig. 4), which is consistent with the Bridgerian NALMA assigned to the unit (after Tabrum et al., 1996).
Sage Creek Breccia of the Renova Formation
The Sage Creek and Dell members are locally separated by a laterally discontinuous chaotic unit that is informally named the Sage Creek breccia (Tabrum et al., 1996). The breccia lies along an east-west–trending buttress unconformity between the upper Sage Creek member and the Dell member (Figs. 5 and 8) and is composed of angular clasts of gray volcaniclastic sandstone in a whitish-tan, sandy mudstone matrix (Fig. 8). Individual clasts range from <10 cm to rotated blocks that are >5 m in length that preserve original bedding trends (Fig. 8).
Depositional Processes and Environment
Stratigraphic relationships between the upper Sage Creek member, the Sage Creek breccia, and the overlying Dell member (e.g., Figs. 5 and 8) suggest that the Sage Creek breccia is an ancient landslide deposit that originated from a paleovalley wall that was incised into the upper Sage Creek member. The near-vertical contact between the Sage Creek member and the Sage Creek breccia is east-west oriented, which is consistent with other paleocurrent data collected in the Sage Creek basin (Fig. 6). The contact is also sub-orthogonal to regional structures, suggesting that the contact and breccia are sedimentary, rather than fault-related. The lateral discontinuity of the unit also supports a sedimentary origin. Although the breccia is largely chaotic, large blocks of volcaniclastic sandstone that display original bedding geometries further indicate slumping of Sage Creek strata into an adjacent area of accommodation. Finally, the onlap relationship between the Dell member and the Sage Creek breccia (Fig. 5) suggests that the Dell member filled a topographic low that was incised into the underlying Sage Creek member.
Dell Member of the Renova Formation (Uintan)
The Dell member disconformably overlies the Sage Creek breccia and upper Sage Creek member (Figs. 2 and 5) and is similar in color to the whitish-tan matrix of the underlying brecciated unit. The Dell member ranges from 100 to 150 m thick (Tabrum et al., 1996) and consists of lenticular conglomerates encased in tabular units of muddy sandstone and sandy mudstone that commonly contain floating pebbles and rare cobbles (Fig. 6). Conglomerate lenses are <5 m thick and <30 m wide and typically display a scour relationship with underlying sandy facies (Fig. 9). Conglomeratic units include mixed pebbles, cobbles, and boulders (up to 0.5 m). Boulders are typically subangular, whereas pebbles and cobbles are subangular to subrounded. Clast types include Proterozoic sandstone and conglomerate; Paleozoic sandstone, limestone, and chert; Mesozoic sandstone and conglomerate (including clasts of reworked Beaverhead Group rocks); and basalt that is likely Eocene in age (Fig. 6). Although all conglomerate lenses within the Dell member contain these clast types, proportions differ significantly from channel to channel (Fig. 6). Conglomeratic channel fills contain large-scale foreset units and commonly show strong clast imbrication that indicates eastward paleoflow (Figs. 6 and 9). Tabular units that encase the conglomeratic channels are generally structureless but sometimes display normal or inverse grading and are commonly capped by paleosols (Fig. 9). Paleosol horizons are often rooted and calcareous and become more common and better-developed upsection, displaying discrete zones of caliche (Fig. 9).
Depositional Processes and Environment
The Dell member is interpreted as an alluvial fan complex dominated by sandy mud flows. The structureless, tabular sandstone and mudstone units that contain floating pebbles and cobbles are consistent with deposition by cohesive debris- and/or mud-flow processes (Bull, 1972; Rodine and Johnson, 1976; Blair and McPherson, 1994a, 1994b). The presence of rooted horizons and paleosols at the tops of debris-flow deposits indicates that deposition was episodic. Lenses of conglomerate and current-structured sandstone are interpreted as alluvial fan distributary channel deposits that incised the alluvial plain. Imbrication and cross-stratification of gravels indicate reworking by fluvial currents.
East-directed paleocurrent indicators reveal sediment source regions to the west of the study area. The presence of boulders of Red Butte Conglomerate and Eocene basalt suggests a phase of local uplift and exhumation between Sage Creek and Dell time that would have exposed the underlying Paleogene units at the surface. Proterozoic and Paleozoic clasts may be derived from thrust sheets in the Sevier hinterland but may also be recycled from the underlying Beaverhead Group.
Detrital Zircon Provenance
An alluvial fan channel sandstone from the lower Dell member (sample TS12-SCS6) yields a detrital zircon distribution that is very similar to the underlying Sage Creek member (Fig. 4). Eocene zircon grains (52–45 Ma) are dominant, but Proterozoic grains are common (Fig. 4). The sample also contains scattered Paleozoic and Archean grains (Fig. 4). Whereas Eocene grains in the Sage Creek member are interpreted as first-generation grains, we interpret the middle Eocene grains in the Dell member to be recycled from older Cenozoic units (such as the Sage Creek member and the lower Dillon Volcanics). First, the erosional contact between the Sage Creek and Dell members (Fig. 5) indicates a period of erosion into Bridgerian (and older) sedimentary and volcanic units prior to deposition of the Dell member. Second, although the lower Dillon Volcanics remained active during Uintan time, the presence of basalt boulders (up to 50 cm) within alluvial fan channel facies (Fig. 6) indicates derivation of pre-Dell volcanic clasts from high-topography areas located to the west, where there is currently no record of basaltic lavas (Fig. 1). As such, the presence of basalt boulders in the Dell member conglomerate indicates that middle Eocene volcanic units had been structurally exhumed by Uintan time. Additionally, a YC1σ MDA calculated for the Dell member (48.1 ± 2.0 Ma; Fig. 4) is up to 2 m.y. older than the MDA established for the underlying Sage Creek member and is up to 3 m.y. older than associated fossil ages, further suggesting that the middle Eocene zircon grains are recycled from older units.
Cook Ranch Member of the Renova Formation (Chadronian to Orellan)
The Cook Ranch member of the Renova Formation disconformably overlies the upper Dell member (Fig. 2) and is up to 220 m thick (Tabrum et al., 1996). The base of the Cook Ranch member is marked by a regionally occurring, well-developed calcic paleosol (Tabrum et al., 1996).
The unit is dominated by tabular, white-colored tuffaceous mudstone and muddy fine-grained sandstone that display abundant evidence of pedogenesis (Fig. 6). Nodular calcium carbonate concretions are abundant throughout the unit (Fig. 10A) but are best developed in the middle and upper Cook Ranch member (after Tabrum et al., 1996). Paleosol horizons are also marked by densely burrowed and rooted horizons, with root casts ranging in length from a few mm up to 50 cm (Fig. 10B).
The Cook Ranch member intermittently contains small lenses of medium- to coarse-grained sandstone and conglomerate, which are commonly associated with relatively thick (up to 2–3 m), structureless and poorly sorted sandstone with floating pebbles (Fig. 6). Sandstone beds are typically thin (<50 cm) and are commonly pervasively bioturbated (Fig. 10C). Rare sandstone beds display climbing ripple cross lamination and gently undulating lamination (Fig. 10D). Conglomerate beds are also thin (<50 cm) and are generally matrix supported with small lenses of clast-supported pebbles. Inverse grading is common within the lenses. Clast types include Proterozoic sandstone, Paleozoic limestone and sandstone, and Eocene basalt (Fig. 6). In the upper (Orellan) part of the section, conglomeratic deposits are dominated by basalt (Tabrum et al., 1996). Ripple cross lamination and channel scour surfaces indicate an eastward paleocurrent direction (Fig. 6).
Depositional Processes and Environment
The Cook Ranch member is interpreted as an assemblage of distal alluvial fan deposits dominated by paleosols (Lielke, 2012; this study). Calcium carbonate–bearing nodular horizons with abundant burrows and rootlets define well-developed calcic paleosols. The structureless, tabular units of muddy sandstone and sandy mudstone that host paleosol horizons likely originated as sheet flows and/or mud flows that periodically reached the outer edge of an extensive alluvial fan complex. Sandstone- and conglomerate-bearing intervals represent the fill of distal distributary channels. Structureless sandstone and matrix-supported conglomerate represent mud flows and debris flows (cf. Blair and McPherson, 1994a, 1994b), whereas laminated sandstone represents tractive flow conditions during water-rich depositional events. Similar to the underlying Dell member, east-directed paleocurrent measurements and clast types indicate derivation from Proterozoic and Paleozoic source regions to the west, as well as nearby exposures of Eocene volcanic rocks.
Detrital Zircon Provenance
A fluvial sandstone from the type section of the Cook Ranch member (sample 10-CR-01 of Rothfuss et al., 2012) contains abundant Eocene to Oligocene grains (ca. 42–30 Ma) and Proterozoic grains, with minor Paleozoic and Archean grains (Fig. 4). Eocene to Oligocene grains were likely derived from the contemporaneous middle Dillon Volcanics (ca. 40–27 Ma; Fritz et al., 2007) that were erupted immediately northwest of the study area. The Oligocene grains yield a YC1σ MDA of 32.5 ± 1.0 Ma, which is consistent with two ash ages that were previously dated from the Cook Ranch type section (35.4 ± 0.5 Ma and 31.1 ± 0.7 Ma; Kent-Corson et al., 2006) and the Orellan fossil age assigned to the upper part of the unit. Interestingly, the Cook Ranch member lacks the 45–55 Ma peak that is characteristic of underlying volcaniclastic units (Fig. 4), further indicating that lower Dillon Volcanics had been replaced by the middle Dillon Volcanics as a sediment source during Orellan time. Similar to underlying units, Paleozoic and Archean grains indicate further recycling of Proterozoic to Paleozoic strata in the fold-thrust belt.
Sedimentary Evolution of the Sage Creek Basin Area
Cretaceous–Paleocene Synorogenic Strata
In the Sage Creek basin area, synorogenic cobble conglomerate and sandstone of the Red Butte Conglomerate were deposited on ephemeral alluvial fans during propagation of the Tendoy thrust system (Perry et al., 1988; Haley and Perry, 1991). Cobble types match source rocks that are currently exposed in the Tendoy Mountains area and the Blacktail-Snowcrest structural culmination. Paleocurrent orientations from incised channels support a northern Tendoy source, with drainage away from the thrust front toward the southeast (Figs. 11A and 12) (Perry et al., 1988; Haley and Perry, 1991). Detritus was also recycled from older members of the Beaverhead Group during progressive thrust-front advance (Haley and Perry, 1991). New detrital zircon data support this and suggest recycling of Proterozoic to Mesozoic strata that were uplifted and/or cannibalized by the Sevier fold-thrust belt (Table 2). Cumulatively, these facies and provenance data indicate that SWMT was topographically rugged during Latest Cretaceous and Paleocene time. Depositional systems emanated from high Sevier-Laramide contractional topography, filling adjacent intraforeland basins until late Paleocene time (Figs. 11A and 12) (after Haley and Perry, 1991).
Early Middle Eocene (Bridgerian) Volcanic and Volcaniclastic Deposits
Following a 7–8 m.y. depositional hiatus, the Dillon Volcanics of SWMT and associated volcaniclastic rocks of the Sage Creek member of the Renova Formation record the onset of postcontractional deposition in the Sage Creek basin area (Fig. 12). Basaltic to rhyolitic lava flows of the lower Dillon Volcanics were derived from local eruptive centers that are preserved today on the western to northern margins of the Sage Creek basin (Fig. 11B) (after Fritz et al., 2007).
The Sage Creek member is variably interbedded with lavas and tuffs of the lower Dillon Volcanics and displays an onlap relationship with the Red Butte Conglomerate (Fig. 5). The presence of a buttress unconformity between the two units indicates that the depositional hiatus that occurred between late Paleocene and early Eocene time represents a phase of erosion into the Laramide basin fill (Fig. 12), likely by fluvial networks that drained antecedent contractional uplifts. Fluvial incision carved an irregular topography (or, a series of shallow paleovalleys) across the synorogenic basin fill, which was locally filled by eastward-flowing alluvial plain and fluvial deposits of the Sage Creek member (Figs. 11B and 12). Abundant volcaniclastic detritus and Eocene detrital zircon ages (Fig. 4) indicate that much of the volcaniclastic material in the Sage Creek member was derived from contemporaneous, local volcanic complexes such as the lower Dillon Volcanics. It is also possible that volcanic sediment was derived from westward-located sources associated with the Challis volcanic field, which may have been transported eastward along a network of fluvial paleovalleys that had been incised into the extant Sevier fold-thrust belt (Fig. 11B) (after Janecke et al., 1999). It is during this time that Kent-Corson et al. (2010) interpret possible drainage capture of hinterland-derived fluvial systems by the Sage Creek depositional system based on a negative excursion in the Sage Creek basin δ18O record.
Although volcaniclastic sediment dominates the Sage Creek member, non-volcanic granules, pebbles, and cobbles indicate that pre-Cenozoic rocks were also exposed on basin margins during Bridgerian time. These include Proterozoic quartzofeldspathic arenite, Ordovician quartzite, and Mississippian limestone (Fig. 6). Recycling of Archean, Proterozoic, and Paleozoic sources is also reflected in detrital zircon data (Fig. 4). Based on northeast-directed paleocurrent indicators, potential source rocks exist in the Tendoy and Beaverhead Mountains located to the west of the study area. It is also possible that non-volcanic sediment in the lower Sage Creek member is recycled from the Red Butte Conglomerate (after Haley and Perry, 1991).
Late Middle Eocene (Uintan) Reworked Volcanic and Siliciclastic Deposits
Although it is partly contemporaneous with the lower Dillon Volcanics (Fig. 12) (after Fritz et al., 2007), the Dell member of the Renova Formation records a marked transition from volcanic-dominated environments to clastic sedimentary environments. The Dell member displays an unconformable onlap relationship with the Sage Creek member and Sage Creek breccia, forming a buttress to very slightly angular (<3°) unconformity (Fig. 5). Geochronology of bounding volcanic units and NALMAs from the upper Sage Creek and Dell members indicates that the unconformity is minor and represents ≤3 m.y. of time (Fig. 12).
Stratigraphic relationships between the Sage Creek member, Sage Creek breccia, and Dell member reveal a second phase of fluvial incision in the basin. Following deposition of the Sage Creek member, eastward-draining fluvial networks incised the volcanic landscape and generated an irregular topography characterized by steep-sided paleovalleys and interfluves. As interfluve areas were abandoned, they became hosts for well-developed calcic paleosols (e.g., Fig. 7). Paleovalleys, on the other hand, were areas of sediment accumulation.
The Sage Creek breccia is considered here as a sub-facies within the lowest Dell member, and represents mass failure of a steep paleovalley wall during earliest Dell deposition. The Dell member was deposited as an eastward-draining alluvial fan complex (Fig. 11C) that first filled erosional paleovalleys that were incised into the Sage Creek member and then occupied interfluve areas as paleovalley accommodation was filled (e.g., Fig. 5). Cobble and detrital zircon provenance data indicate local sediment sources including Proterozoic to Paleozoic strata, Mesozoic synorogenic strata, and Eocene volcanic units such as the lower Dillon Volcanics and the underlying Sage Creek member of the Renova Formation (Figs. 4 and 6). The presence of reworked boulders of Beaverhead Group conglomerate (up to 60 cm) and Eocene basalt (up to 50 cm) suggests very local exhumation of such units. Proterozoic to Paleozoic sandstone and limestone clasts were potentially derived from sources in the Tendoy Mountains but may also have been recycled from the Beaverhead Group. Although detrital zircon data are dominated by Eocene ages, they are similar to those in the underlying Sage Creek member (Fig. 4) and are likely recycled from older units.
Upper Eocene–Lower Oligocene Tuffaceous Deposits
The Chadronian to Orellan Cook Ranch member of the Renova Formation was deposited synchronously with the middle Dillon Volcanics (Fig. 12). The Cook Ranch member disconformably overlies the Dell member, and its base is marked by a very well-developed calcic paleosol (Tabrum et al., 1996). The disconformity represents a significant local hiatus in deposition of up to 5 m.y. based on ash geochronology and NALMAs (Figs. 2 and 12).
Similar to the underlying Dell member, the Cook Ranch member represents an assemblage of alluvial fan deposits that drained eastward across the Sage Creek basin area (Fig. 11D). However, the two units are markedly different. The Cook Ranch member is dominated by tuffaceous mudstone, rather than recycled siliciclastic sediment, and was deposited on the distal edges of an alluvial fan complex, rather than an area occupied by higher-energy channels and debris flows. The presence of tuffaceous sediment throughout the Cook Ranch member records voluminous input of ashy sediment by contemporaneous volcanic eruptions. Basaltic to rhyolitic lavas of the middle Dillon Volcanics have been identified to the north and east of the Sage Creek basin area (Fritz et al., 2007), indicating that eruptive centers were not directly adjacent to the basin at this time (Fig. 11D). Rather, tuffaceous sediment was likely delivered to the Sage Creek basin by air-fall, and low-energy environments dominated by pedogenesis allowed for preservation of the fine-grained ashy material. Rothfuss et al. (2012) also suggest Oligocene volcanic sources in the Great Basin, located 400–500 km to the south. However, the preservation of alluvial fan depositional facies indicates proximal sediment sources, and sparse paleocurrent measurements indicate sources to the west-northwest (Fig. 12). In addition, paleoatmospheric patterns for this time period have been interpreted to be from west to east across the Cordillera (e.g., Kent-Corson et al., 2006, 2010; Chamberlain et al., 2012), rather than north-south, indicating that voluminous input of ash from Great Basin eruptions was unlikely.
Cobble and detrital zircon provenance suggests continued recycling of Proterozoic, Paleozoic, and Paleogene units into Oligocene time (Figs. 4 and 6). Clasts of Proterozoic sandstone and Paleozoic limestone were derived from the adjacent Sevier hinterland or were possibly recycled from exhumed Beaverhead Group conglomerate (Fig. 12).
Temporal Provenance Variation in the Sage Creek Basin
Figure 4 displays that throughout the Paleogene depositional history of the Sage Creek basin, the most significant change in detrital zircon provenance is the appearance of Paleogene zircon in middle Eocene time. Beginning with the eruption of the lower Dillon Volcanics and deposition of the Sage Creek member of the Renova Formation, Paleogene zircon grains account for nearly 50% of the zircon grains analyzed from the Eocene and Oligocene samples (Fig. 4). Paleogene zircon grains are interpreted to be derived from coeval volcanic terranes proximal to the study area.
Pre-Cenozoic detrital zircon age groups remain consistent throughout the Paleogene basin history. In the synorogenic Red Butte Conglomerate and the clastic members of the Renova Formation, Proterozoic zircon grains are dominant, with lesser amounts of Archean and Paleozoic grains (Fig. 4). The repeated appearance of such age groups indicates (1) continued sediment derivation from Archean to Paleozoic source rocks in the Sevier hinterland and Laramide intraforeland uplifts during Paleogene time and/or (2) periodic recycling of zircon from Paleogene deposits as they were exposed by erosion or fault-related uplift within the Sage Creek basin. Deformation associated with the Sevier and Laramide orogenies exhumed Archean to Paleozoic basement rock in the Sevier fold-thrust belt and Laramide intraforeland uplifts proximal to the study area (Figs. 11A and 12). It is reasonable that the resulting topography persisted through Paleogene time and was augmented by extensional reactivation beginning in middle Eocene time (Figs. 11 and 12). As such, Archean to Paleozoic rocks that were exposed in the uplifted regions were likely sources of sediment throughout Paleogene time. In addition, the presence of erosional unconformities throughout the succession (e.g., Fig. 5) suggests periodic reworking of preexisting sedimentary units throughout the Paleogene basin history. Reworking of older sedimentary units is further supported by the presence of various cobble types that were reworked from underlying units (Haley and Perry, 1991; this study).
Interestingly, the Sage Creek basin provenance signatures differ from nearly age-equivalent samples of the Renova Formation in adjacent areas. To the north and northeast of Dillon, Paleogene sandstones commonly contain abundant Mesozoic zircon grains (e.g., ca. 100–120 Ma and ca. 68–80 Ma; Link et al., 2008; Stroup et al., 2008; Rothfuss et al., 2012) in addition to Archean, Proterozoic, and Paleozoic grains. The ca. 100–120 Ma grains may have been recycled from the Vaughn Member of the Blackleaf Formation (Stroup et al., 2008; Rothfuss et al., 2012), which is exposed in the Sevier fold-thrust belt north of the study area. The ca. 68–80 Ma grains are interpreted to be derived from Mesozoic magmatic sources including the Boulder, Pioneer, and Tobacco Root batholiths and the McCartney Mountain pluton (Table 2), which had been deeply exhumed by middle Eocene time (Schwartz and Schwartz, 2013). In addition, Stroup et al. (2008) suggest that zircon grains ca. 75 Ma that exist in Paleogene sandstones to the immediate west of the study area were derived from the eastern lobes of the Idaho batholith. Mesozoic zircon grains are rare in the Sage Creek basin units and account for <3% of the grains analyzed. The paucity of Mesozoic grains in the Sage Creek basin not only indicates that there was not a voluminous source of Mesozoic zircon grains for the basin, but also indicates that the basin was not interconnected upstream with basins that did have significant Mesozoic sources.
Tectonic Evolution of the Sage Creek Basin Area
The Paleogene sedimentary succession of the Sage Creek basin records a dynamic history of structural exhumation, topographic evolution, fluvial erosion, and sediment recycling associated with the transition between Sevier-Laramide compressional deformation and postcompressional extensional reactivation of structures in SWMT. Under the influence of east-west–oriented compression, Proterozoic to Mesozoic sedimentary sequences were exposed in high topography of the frontal Sevier fold-thrust belt and Laramide intraforeland uplifts (Figs. 11A and 12). Archean crystalline rocks were also exposed within Laramide culminations. Compression was accompanied by deposition of synorogenic deposits that were derived from the uplifted, pre-Cenozoic terranes (Haley and Perry, 1991). Proterozoic clasts were likely derived from the Medicine Lodge and Yellowjacket allochthons, whereas Paleozoic and Mesozoic clasts were likely derived from the frontal fold-thrust zone of the Sevier fold-thrust belt (Fig. 11A; after litho-tectonic thrust plate assemblages presented in Ruppel et al., 1981). Synorogenic sediments of the Beaverhead Group were progressively cannibalized as compression continued into late Paleocene time (Fig. 12) (Haley and Perry, 1991).
In latest Paleocene to Eocene time, structural exhumation of pre-Cenozoic basement terranes was amplified by erosional exhumation of the SWMT region (Fig. 12). As compression ceased, fluvial incision along zones of structural and stratigraphic weakness generated a topographically complex landscape of mountainous highlands (up to 3 km paleorelief; Dettman and Lohmann, 2000; Lielke, 2012; Schwartz and Schwartz, 2013) separated by intermontane basins of varying scales (Schwartz and Schwartz, 2013, and references within). Widespread erosion further exposed pre-Cenozoic basement terranes around the perimeter of the Sage Creek basin area and carved a rolling topography across synorogenic deposits of the Beaverhead Group (e.g., Fig. 5). During this time, the Sage Creek basin was an area of sedimentary bypass (Fig. 12).
Shortly following the end of Sevier-Laramide compression (within ∼7 m.y.), Eocene volcanic fields erupted across the northern Rocky Mountain province, extending from the Sevier hinterland in Idaho (Challis Volcanics; Chadwick, 1981; Gaschnig et al., 2010, 2011) across SWMT (Lowland Creek and Dillon volcanics; Fritz et al., 2007; Dudás et al., 2010), and into the Laramide foreland province (Absaroka Volcanics; Hiza, 1999; Feeley et al., 2002) (Figs. 11B and 12). Voluminous Eocene volcanism associated with the Challis and Absaroka volcanic fields was originally considered a part of late-stage arc volcanism (e.g., Armstrong, 1988; Armstrong and Ward, 1991) but has since been attributed to extensional reactivation of the Cordilleran orogenic wedge based on igneous geochemical trends and associations with extensional structures in those areas (Janecke, 1994; Hiza, 1999). However, much of the volcanism in the Challis and Absaroka volcanic fields largely predates peak extensional activity (Janecke, 1994; Hiza, 1999), which is consistent with temporal trends observed within the study area (Figs. 11 and 12) and in the Basin and Range Province to the south (Gans and Bohrson, 1998).
In the Challis volcanic province, northwest-southeast–oriented extension was partly coeval with middle Eocene volcanism (Fig. 12) (Vandenburg et al., 1998). Low-angle detachment faults may have initiated in the vicinity of Salmon, Idaho at this time (Fig. 11B), but temporal constraints on early extensional activity in the region are poor (Harrison, 1985). The Sage Creek basin area experienced no apparent, significant extension in early middle Eocene time (Figs. 11B and 12). Deposition of bimodal volcanic rocks of the lower Dillon Volcanics occurred locally in the Sage Creek basin, close to their eruptive centers (Fig. 11B). Associated volcaniclastic deposits, such as the Sage Creek member of the Renova Formation, locally filled the erosional topography that had been incised into underlying synorogenic deposits (e.g., Fig. 5). In the absence of significant local extension, sedimentation was predominantly local to source areas, and relative accommodation was low in the Sage Creek basin (Fig. 12). In addition, some sediment in the Sage Creek basin may have been derived from westward-located sources such as the easternmost part of the Challis volcanic province (Fig. 11B), which may have been transported eastward along a network of paleovalleys that transected the extant Sevier fold-thrust belt (after Janecke et al., 1999).
Into late middle Eocene time, extensional reactivation of the Cordillera migrated eastward, involving faults associated with the frontal portions of the Sevier fold-thrust belt that are immediately west of the Sage Creek basin (Figs. 11C and 12) (after Vandenburg et al., 1998). Normal reactivation of low-angle faults in the Tendoy-Beaverhead Mountains complex initiated a new phase of structural exhumation of the pre-Cenozoic basement terranes that were part of the Sevier fold-thrust belt (Figs. 11C and 12). The high-angle Red Rock fault, located on the eastern edge of the Tendoy Mountains, may also have initiated at this time (Fig. 11C). Uplift in the western part of the Sage Creek basin exhumed synorogenic deposits of the Beaverhead Group and basalt of the lower Dillon Volcanics (Figs. 11C and 12). Closely associated with this, uplift of the Tendoy Mountains increased local gradients in the Sage Creek basin, prompting fluvial incision into underlying deposits of the Sage Creek member. Similar to the Sage Creek member, the Dell member was deposited into an erosional landscape. However, sedimentation was more widespread during this time, and relative accommodation in the basin increased as extension continued (Fig. 12).
Extensional modification of the landscape continued in the Sevier hinterland (Vandenburg et al., 1998), and possibly in the adjacent foreland region (Ruppel, 1993), through the remainder of Paleogene time (Figs. 11D and 12). Extension further exhumed pre-Cenozoic basement terranes in extant Sevier and Laramide structures and was accompanied by extrusion of the middle Dillon Volcanics in SWMT (Figs. 11D and 12) (Fritz et al., 2007). Locally, latest Eocene time is recorded by a well-developed paleosol horizon (Tabrum et al., 1996), indicating a depositional hiatus in the basin. Based on the presence of nested erosional surfaces and buttress unconformities in underlying parts of the Paleogene succession, it is reasonable that the paleosol represents an interfluve area and is correlative to an erosional surface similar to those preceding it. In this scenario, the interfluve and its correlative erosional surface would indicate an additional phase of uplift, gradient increase, and sediment bypass (Fig. 12). Deposition of the Cook Ranch member into the erosional landscape occurred synchronous with continued normal faulting, into a relatively high-accommodation area (Fig. 12).
Implications for Paleoenvironmental and Paleotopographic Studies
The new detrital zircon–based age constraints presented here provide added insight into the age of fossil assemblages within the Sage Creek basin and broadly confirm the NALMA designations that were historically assigned to the Paleogene strata at their type sections (e.g., Fig. 2; after Tabrum et al., 1996). In addition, when viewed in context of detailed stratigraphic trends, the ages provide valuable temporal context for the tectonic and climatic events that affected the region during Paleogene time.
Our correlation of tectonic and sedimentary events that affected the Sage Creek basin provides a necessary framework for interpreting paleoclimate and paleoenvironmental data sets collected from the region. Understanding the evolution of Paleogene depositional environments and paleodispersal pathways inherently suggests an evolution of regional paleogeography and paleotopography (e.g., Fig. 11), which is closely tied to paleoclimate patterns. A thorough assessment of the interactions between and relative influence of Paleogene tectonic events and climatic variation on sedimentary patterns is beyond the scope of this study. However, the tectonostratigraphic context summarized above has important implications for previous paleoclimate and paleoaltimetry studies done in the Sage Creek basin.
Paleocene–Oligocene cooling and aridification in SWMT (e.g., Schwartz, 2015; Methner et al., 2016) are broadly supported by the evolution of depositional systems interpreted for the Sage Creek basin. The progression from Paleocene alluvial fan and braided fluvial depositional systems (Red Butte Conglomerate of the Beaverhead Group) to Eocene–Oligocene calcic paleosol-bearing alluvial fan depositional systems (Dell and Cook Ranch members of the Renova Formation) supports net aridification throughout Paleogene time. Within the Paleocene to Oligocene succession, calcic paleosols first appear in the Sage Creek member of the Renova Formation (ca. 47–45 Ma) and become increasingly common in the overlying Dell and Cook Ranch members (Fig. 6). Likewise, some of the more temporally significant unconformities within the Sage Creek basin may be partial products of globally recognized hyperthermal events that occurred during Paleogene time. For example, the erosional unconformity that separates the Paleocene Red Butte Conglomerate from the middle Eocene Dillon Volcanics and Sage Creek member of the Renova Formation formed between ca. 56–52 Ma (Fig. 12), which is correlative with the late Paleocene thermal maximum and early Eocene climatic optimum (after Zachos et al., 2001). In addition, the unconformity that separates the Dell and Cook Ranch members of the Renova Formation formed beginning ca. 40 Ma (Fig. 12), which is broadly consistent with the timing of the middle Eocene climatic optimum (after Methner et al., 2016). It is reasonable that other erosional unconformities within the basin fill are related to similar, although possibly local, climatic phenomena.
Kent-Corson et al. (2010) report a negative isotopic excursion in a suite of δ18O data from calcic paleosols in the Sage Creek member of the Renova Formation. The magnitude of the excursion is interpreted to represent multi-kilometer elevation gain in the Sevier hinterland during middle Eocene time, and the abruptness of the excursion is interpreted to represent drainage capture of higher-elevation streams from the Sevier hinterland (Kent-Corson et al., 2010). However, this interpretation is complicated by the time-series paleogeographic interpretations depicted in Figure 11. Prior to deposition of the Sage Creek member, the compositions of conglomerate clasts in the Beaverhead Group indicate that Paleocene drainages reached at least 30 km westward into the Sevier hinterland (Fig. 11A; after Haley and Perry, 1991). Similarly, early middle Eocene volcanic paleovalleys have been mapped for similar distances across the Sevier hinterland (Janecke et al., 1999) and likely emptied into the foreland region during deposition of the Sage Creek member (Fig. 11B; this study). The paleogeographic interpretations demonstrate fluvial connectivity between the Sevier hinterland and Laramide foreland (Figs. 11A and 11B) until at least late middle Eocene time when extensional reactivation of the region segmented the drainage network (Figs. 11A–11C). In addition, if large-scale fluvial drainage capture occurred during deposition of the Sage Creek member, changes in depositional architecture, sedimentary provenance, and/or paleocurrent patterns would be likely as catchments were integrated and fluvial gradients were adjusted.
We suggest lateral mixing of depositional systems as an alternate interpretation to large-scale, topographically driven drainage capture. Based on the δ18O records reported by Kent-Corson et al. (2010), the lower Sage Creek member of the Renova Formation hosts paleosols that formed in the presence of meteoric water that fell at relatively lower elevations, whereas the upper Sage Creek member hosts paleosols that formed in the presence of meteoric water that fell at significantly higher elevations. Although this indicates an early middle Eocene modification of surface hydrology, the consistency in sedimentary architecture (Fig. 6), sediment provenance (Fig. 4), and average paleocurrent directions between the lower and upper Sage Creek members suggests that the style of sedimentation did not undergo a significant, contemporaneous change, as might be expected with large-scale drainage capture. However, the Sage Creek member does display a subtle transition from abundant, well-developed paleosols in its lower part to fluvially reworked units with fewer paleosols in its upper part (Fig. 6), suggesting a transition from overbank- to channel belt–type environments through time. It is reasonable that the isotopic excursion presented by Kent-Corson et al. (2010), which does correspond to the subtle architectural changes, more likely reflects lateral migration of hinterland-derived fluvial systems into the study area, resulting in fluvial reworking of locally derived volcaniclastic sediments.
In light of recent studies that utilize Paleogene terrestrial deposits within the Cordillera to interpret the climatic and topographic evolution of western North America, the Paleogene stratigraphy that is preserved in the Sage Creek basin of SWMT provides a unique opportunity to assess the sedimentary response to such tectonic and/or climatic events. The Paleogene succession that is preserved in the Sage Creek basin encompasses the tectonic transition from contractional to extensional deformation within the northern Cordillera and brackets significant climatic events that occurred during Paleogene time.
Mixed fluvial and alluvial, clastic and volcaniclastic deposits that are preserved in the Sage Creek basin record a dynamic history of structural exhumation, fluvial erosion, topographic evolution, and sediment recycling associated with the transition from Sevier-Laramide compressional deformation and postcompressional extensional reactivation. A time-series of events includes (1) deposition of synorogenic sediments associated with late-stage Sevier and Laramide tectonism; (2) fluvial exhumation of the Sevier and Laramide structural culminations; (3) local deposition of bimodal volcanic and volcaniclastic deposits that was contemporaneous with extension to the west of the SWMT study area in the Challis volcanic province; and (4) eastward propagation of the reactivated extensional domain, which resulted in multiple episodes of structural exhumation, fluvial incision, and basin-wide deposition on large alluvial fans (Figs. 11 and 12).
New age constraints on the basin fill and a more detailed understanding of temporal correlations between regional tectonic events and sedimentation in the Sage Creek basin allow for a more complete analysis of existing (and future) paleoenvironmental data sets. Detrital zircon ages substantiate the NALMA designations for mammal fossil assemblages in the Sage Creek basin and provide fundamental age constraints on regional tectonic events. In the context of those ages, the evolution of depositional systems interpreted for the Sage Creek basin supports paleoclimatic interpretations for Paleogene time, including progressive cooling and aridification punctuated by brief hyperthermal events. The tectonostratigraphic correlations also support isotopic records that indicate a higher-elevation Sevier hinterland relative to the study area (within the Laramide foreland) during Paleocene(?) to middle Eocene time, but illustrate that large-scale, topographically driven drainage capture is not necessary to explain such an isotopic excursion.
This work was supported by National Science Foundation (NSF) grant EAR-1019648. Author TMS also gratefully acknowledges financial support from the NSF Graduate Research Fellowship. Special thanks to Alan Tabrum for a personal introduction to the Sage Creek field area. Author TMS thanks Tess Menotti, Lauren Shumaker, and Matt Thomas for field assistance, and the team at the Arizona LaserChron center (supported by NSF grant EAR-1032156) for their analytical expertise and guidance. This manuscript benefitted from thoughtful reviews by Glenn Sharman, Richard Gaschnig, and Associate Editor Todd LaMaskin.