The Rise and Fall of Laramide Topography and the Sediment Evacuation From Wyoming

The modern topography within the Laramide region consists of high‐relief ranges and high‐elevation low‐relief (HELR) surfaces separated by intraforeland basins. However, the timing and development of this topography within the type‐locality of the Wyoming Laramide province is poorly understood. Previous models suggest that the modern topography is a young feature that was acquired after Laramide tectonism, post‐Laramide burial, and basin evacuation; however, evidence of such a progression is sparse. We present low‐temperature‐thermochronological data from two Laramide uplifts in Wyoming, the Wind River and Bighorn Ranges, which document an early record of Laramide exhumation, subsequent reheating, and significant cooling after 10 Ma. Our results indicate that the Laramide ranges were buried by post‐Laramide Cenozoic basin fill, creating a low‐relief topography by the early Miocene that was reduced due to late Miocene regional incision and basin evacuation. We suggest that HELR surfaces experienced further relief reduction from Pleistocene glaciation.

develop as basins were incised and evacuated after the late Miocene. This model also has implications for the formation of high elevation low relief surfaces (HELR) observed in many of the Laramide ranges. However, clear evidence of such basin filling, incision, and evacuation are lacking from existing thermochronological records.
This study presents new apatite fission track (AFT) results and thermal models from classic localities in the Laramide region of Wyoming including the Bighorn Mountains and Powder River Basin and the Wind River Range and Wind River Basin (Figure 1). Our results address the following questions: (a) When did Laramide erosion initiate in these ranges? and (b) How much basin burial and subsequent incision/erosion did Laramide basins and ranges experience? Understanding the thermal history of the Laramide region is important for understanding tectonomorphic processes associated with the development of the modern Laramide topography. Thus, our study  Love and Christiansen (1985) and Ranz (2000). 10.1029/2023GL103218 3 of 10 holds important implications for the tectonic and paleotopographic evolution of the Laramide region and other high relief regions on Earth affected by similar tectonic, erosional, and depositional histories.

Geologic Setting
The Laramide region is characterized by basement-cored uplifts and intervening intraforeland basins that developed by flexural and dynamic subsidence during the Late Cretaceous-early Eocene (Dickinson et al., 1988;Fan & Carrapa, 2014;Heller & Liu, 2016;Painter & Carrapa, 2013). Topography within these ranges is characterized by >4 km-high peaks and broad, low-relief surfaces exposed at high elevations (∼3.5 km). The development and age of HELR surfaces that are observed in many of the Laramide ranges remains uncertain. Some models interpret the HELR surfaces as low-elevation peneplains that were later uplifted during the Laramide (Blackwelder, 1915;Chamberlin, 1919;Davis, 1911). Other models infer that these surfaces are Late Cretaceous-Eocene and formed by erosion during Laramide uplift and subsequent basin filling (Steidtmann & Middleton, 1991). Post-Laramide development models suggest that HELR surfaces are remnants of Oligocene-Miocene erosional surfaces (Mckenna & Love, 1972) or relatively young features that attained high elevations during Laramide uplift and later experienced Pleistocene glacial erosion (e.g., Egholm et al., 2009).
The Bighorn and Wind River Ranges in Wyoming are prominent Laramide uplifts primarily composed of ∼2.9-2.55 Ga Archean basement ( Figure 1, Arth et al., 1980;Frost et al., 2000) that was uplifted during the Late Cretaceous-Eocene (∼75-40 Ma). The Bighorn Mountains are bound to the east by east-verging thrusts that placed Archean basement in contact with Paleocene-Eocene synorogenic strata filling the Powder River Basin (Hoy & Ridgway, 1997). The Wind River Range is bound by the southwest-verging Wind River thrust and Green River Basin on its western margin and the Wind River Basin along its northeastern margin (Fan & Carrapa, 2014;Fan et al., 2011;Steidtmann & Middleton, 1991). Thermochronological data from these ranges document rapid cooling at ∼65-50 Ma interpreted to be a minimum temporal estimate of Laramide deformation with the onset of exhumation occurring sometime before the late Paleocene (Peyton et al., 2012;Stevens et al., 2016). Early studies in the Wind River Range combining geologic evidence with AFT thermochronology interpreted cooling to have commenced as early as ∼100 Ma (Steidtmann & Middleton, 1991).
The Powder River and Wind River basins consist of >2 km of Maastrichtian-Eocene synorogenic sedimentary units (Fan & Carrapa, 2014;Hoy & Ridgway, 1997;Steidtmann & Middleton, 1991;Winterfeld & Conrad, 1983). Maastrichtian-early Paleocene deposits in both basins are composed of fine-grained facies that are interpreted to represent initial slow phases of uplift and unroofing of Paleozoic sedimentary cover (Shuster & Steidtmann, 1988). Accelerated Laramide deformation and erosion of Archean basement occurred during the late Paleocene-early Eocene and is recorded by up-section changes in synorogenic conglomerates from predominantly Paleozoic clasts to Archean basement clasts (e.g., the Wasatch Formation, Kingsbury and Moncrief conglomerate units, and the Indian Meadows and Wind River Formations (Fan et al., 2011;Hoy & Ridgway, 1997).
Regional sedimentation in the Laramide basins persisted until Oligocene-Miocene time as evidenced by tuffaceous fluvial and eolian deposits of the White River and Arikaree Groups preserved at high elevations (e.g., ∼2.8 km, Malone et al., 2022;Mckenna & Love, 1972). These deposits show evidence of basin filling and suggest that the Laramide basins and bounding ranges were partially buried by sedimentary deposits that have since been incised and evacuated along major drainage systems (Galloway et al., 2011;McMillan et al., 2006;Steidtmann et al., 1989). Thermochronological data recording such a history have been documented in the Beartooth Range (Carrapa et al., 2019;Omar et al., 1994;Ronemus et al., 2023), although this thermal signature has not been shown in other Laramide ranges and basins. Late Miocene incision of Laramide basins is supported by thermochronological data, albeit sparse, and reconstructions of post-Laramide basin fill surfaces suggesting that >1.5 km of incision has occurred in the Laramide region and western Great Plains since the late Miocene (Cerveny & Steidtmann, 1993;McMillan et al., 2006;Naeser, 1992;Omar et al., 1994;Steidtmann et al., 1989).

Methods
We analyzed 11 basement samples from three transects in the Bighorn Mountains and 6 basement samples from two transects in the northern Wind River Range (Figure 1). In the Wind River Range, we targeted vertical profiles that capture significant elevation change (>3 km) and low internal relief (<300 m) surfaces. In addition, we analyzed four orthogneiss clasts, one from the Kingsbury Conglomerate and two from the Moncrief Conglomerate in the Powder River Basin; and one from the Wind River Formation in the Wind River Basin (Figure 1). Samples were prepared for AFT thermochronology by separating, mounting, and etching in 5.5 M nitric acid for 20 s at 21°C and mica prints were etched in 40% HF for 45 min following protocols of Donelick (2005). For each sample, 20-30 apatite grains were dated using the external detector method and confined fission track lengths and Etch-pit diameter (D par ) measurements were collected for thermal modeling (Table S3, Gleadow et al., 1986;Wagner & Van den Haute, 1992).
We used the modeling software QTQt (v. 5.8.0) to generate inverse thermal models based on AFT age, length, and D par measurements (Gallagher, 2012). QTQt applies the Bayesian transdimensional Markov chain Monte Carlo inversion scheme to determine the time-temperature cooling pathway of a given sample. Fifteen basement and four cobble samples were suitable for thermal modeling. Models were initially run unconstrained to explore statistical space and allow the maximum degree of freedom; then model parameters were adjusted to consider geological constraints (Table S2). Modeling results presented in this study were generated based on time-temperature paths for ≥200,000 runs. Samples from the Squaretop Mountain transect were modeled together as a vertical profile to reduce uncertainties in the model results. Individual sample data and model parameters and outputs are available in Supporting Information S1.
Thermal modeling results for these samples show cooling between ∼130 and ∼50 Ma with most samples showing rapid cooling between ∼75 and 50 Ma (Figure 2a). Four basement samples show continued and slow cooling after 50 Ma. The remaining five samples show a consistent period of reheating between ∼50 and ∼10 Ma followed by cooling after 10 Ma. Models for cobble samples from the Kingsbury and Moncrief Conglomerates were run with and without deposition of the Wasatch Formation at ∼50-45 Ma (Hoy & Ridgway, 1997) and indicate relatively slow cooling between ∼800 and ∼50 Ma, followed by post-depositional reheating between ∼50 and ∼10 Ma and cooling after 10 Ma (Figure 2b). We note that inverse modeling results from the Kingsbury and Moncrief cobble samples document long thermal histories prior to 200 Ma, outside of the scope of this study, and are therefore not discussed in this paper (Figures S12, S13, and S14).
Samples from the Squaretop Mountain transect follow a near-vertical profile over a total relief of ∼600 m ( Figure 2). Results from this thermal model show cooling at ∼100-80 Ma. Samples from the Goat Flat transect were modeled individually as these samples were collected too far apart to be considered a true vertical profile (Figures 2b and 2d). Sample GF1 collected from the highest elevation shows cooling at ∼70 Ma and sample GF5 from the lowest elevation shows cooling at ∼55 Ma. Both transects show reheating between ∼30 and 10 Ma followed by cooling to surface temperatures after 5 Ma.
The thermal model for the Wind River Formation cobble sample (2DBI) was run with and without deposition of the Wind River Formation at ∼52 Ma (Clyde et al., 1997;Robinson et al., 2004;Smith et al., 2008). These models are similar to basement samples from the Goat Flat transect showing cooling between ∼70 and ∼50 Ma, followed by post-depositional reheating between ∼50 and ∼10 Ma, and cooling after 3 Ma (Figure 2d).

Discussion
Combined AFT ages and thermal modeling results reveal a multi-phase thermal history of the Laramide region in Wyoming during the Mesozoic-Cenozoic. We focus on the Late Cretaceous-modern thermal record, pertinent to the development of the topographic relief between Laramide ranges and intervening Laramide basins, and the physiographic development of HELR surfaces within these ranges. Thermal models from cobble samples in the Powder River Basin document thermal histories prior to 200 Ma outside of the scope of this study (Figures S12, S13, and S14).

Thermal Record and Implications for Erosion and Basin Burial
The earliest record of rapid cooling is identified during the Late Cretaceous (∼100-80 Ma) by three basement samples from the Bighorn Mountains and samples from the Squaretop Mountain transect in the Wind River Range (Figure 2). Previous thermochronological studies in the Wind River Range document rapid cooling ∼66-50 Ma and suggest that these results provide only a minimum age of Laramide deformation-driven exhumation (Peyton et al., 2012;Steidtmann & Middleton, 1991;Stevens et al., 2016). Several additional lines of evidence support mid-Late Cretaceous cooling in the Wind River Range. First, Upper Cretaceous-lower Paleocene strata in the Green River Basin record a shift in paleocurrent direction from southeastward to southward-flowing systems accompanied by a change in sandstone composition from lithic-rich units derived from Paleozoic-Mesozoic cover strata to feldspar-rich units derived from Precambrian basement. This suggests that by early Paleocene time, Paleozoic-Mesozoic cover strata had been stripped away to expose Precambrian basement in the Wind River Range (Shuster & Steidtmann, 1988;Steidtmann & Middleton, 1991). Moreover, Late Cretaceous cooling is consistent with studies in southwestern Montana that document cooling at ∼100-90 Ma, interpreted as evidence for early Laramide exhumation (Carrapa et al., 2019;Ronemus et al., 2023). Our results from Squaretop Mountain combined with previous evidence support an early phase of cooling and erosion during the Late Cretaceous, although the exact mechanism is not well understood. We note the difference in cooling ages between our samples along the Squaretop Mountain transect (∼100-80 Ma) and samples from previous transects in the Wind River Range (∼65-50 Ma, Peyton et al., 2012;Stevens et al., 2016). We speculate that samples from the previous studies were collected in areas from which the rocks containing the earlier (Cretaceous) record of cooling and erosion have been erosionally removed.
Thermal models of basement samples from the Bighorn and Wind River Ranges and cobble samples from Eocene synorogenic deposits from the Powder River and Wind River basins show similar Cenozoic thermal histories indicating these regions experienced Eocene-Miocene (∼50-10 Ma) reheating to temperatures of ∼35-80°C (Figure 2). Reheating can be attributed to either: (a) an elevated regional geothermal gradient during late Eoceneearly Miocene magmatism, or (b) burial by late Eocene-early Miocene basin filling units, that is, the White River and Arikaree Groups and their correlative units. In the Bighorn Mountains, reheating via sediment burial is supported by remnant exposures of basin fill at high elevations (∼2.8 km) along the flanks of the Bighorn Mountains, approximately ∼1.2 km above the average Wyoming Laramide basin level of ∼1.5 km (Fan & Carrapa, 2014;Fan et al., 2011;Malone et al., 2022;Mckenna & Love, 1972;McMillan et al., 2006). We note that there are no exposures of Oligocene-Miocene basin fill units along the northern flanks of the Wind River Range where our samples were collected. However, these deposits are preserved along the southern flanks of the range (Steidtmann & Middleton, 1991). Although it is possible that Cenozoic reheating may be associated with Late Eocene-Oligocene magmatism in Wyoming, the reconstructed thickness and distribution of basin-fill remnants (McMillan et al., 2006) along with the magnitude of reheating indicated by our data can explain partial reheating caused by a period of sustained sedimentation within the Laramide basins, burying the flanks of the Laramide ranges. We note that a thermochronological signature of post-Laramide burial may not be preserved everywhere in the Laramide region due to different magnitudes of exhumation and burial; nevertheless, this signature has also been documented by studies in the Beartooth Range (Carrapa et al., 2019;Omar et al., 1994;Ronemus et al., 2023).
The final phase of cooling, recorded by basement and cobble samples in both study regions, began after 10 Ma and is consistent with the timing of widespread incision across the Rocky Mountain region (McMillan et al., 2006). The late Miocene regional shift from basin filling to basin incision is indicated by the reconstructed minimum basin fill surface based on the elevation and age of sedimentary and volcanic basin fill remnants (Mckenna & Love, 1972;McMillan et al., 2006 and reference therein;Pelletier, 2009), and thermochronological data from Laramide uplifts (Cerveny, 1990;Cerveny & Steidtmann, 1993;Omar et al., 1994;Peyton et al., 2012;this study). Basin incision and sediment evacuation are suggested to have occurred via major fluvial systems draining the Wyoming Laramide region, that is, the paleo-Mississippi, paleo-Red, and paleo-Platte river systems (Galloway et al., 2011). Since the Middle Miocene, these rivers served as major sediment routing systems to the Gulf of Mexico Basin as indicated by significant proportions of detrital zircon populations derived from western Cordilleran sources found in Early-Middle Miocene sedimentary units of the Gulf of Mexico Basin (Xu et al., 2022) and paleogeographic reconstructions from industry well and seismic databases (Galloway, 2008).

Regional Post-Laramide Topography
By the Eocene, relief between the Laramide ranges and basins was comparable to the modern relief in the Wyoming Laramide Province (∼2.3 km, Fan et al., 2011). Regional erosion of highlands and sedimentation within the Laramide basins persisted through the Oligocene-Early Miocene resulting in thick accumulations of sedimentary to volcanic basin fill (i.e., the White River and Arikaree Groups) and relief reduction between Laramide ranges and basins (Mckenna & Love, 1972;McMillan et al., 2006). The magnitude of basin filling remains contentious with some workers suggesting a minimum ∼1 km of burial (Stevens et al., 2016) while others suggest ∼4 km of burial, sufficient to obscure all but the highest peaks of the Laramide ranges (Steidtmann & Middleton, 1991). To estimate the depth of burial, we divided the maximum temperature during reheating indicated by our models by 60°C/km, a geothermal gradient appropriate for the volcanic setting at the time (Coney & Reynolds, 1977).
Our calculations indicate that reheated samples in the Bighorn Mountains were buried to depths of 0.4-1.1 km and suggest that the sedimentary fill in the Powder River Basin reached elevations of ∼3.4 km along the eastern flank of the Bighorn Mountains (Figure 3a). Samples from the Wind River Range and Wind River Basin show similar magnitudes of burial between 0.2 and 0.8 km and suggest that basin fill reached elevations of ∼4.2 km in some areas of the Wind River Range (Figure 3b). Our calculated basin fill elevations are comparable with the highest modern peaks in Wyoming that do not show partial reheating during the Late Eocene-Early Miocene, for example, Gannett Peak (∼4.21 km) and Freemont Peak (∼4.19 km), suggesting that by the Miocene, only the highest peaks were exposed (Peyton et al., 2012;Steidtmann & Middleton, 1991). Our results provide estimates of maximum burial and the establishment of a subdued low internal-relief topography in the Wyoming Laramide region during Early Miocene time and are comparable with minimum elevations reconstructed from basin-fill elevations predicted by McMillan et al. (2006).

Laramide High-Elevation Low-Relief Surfaces
Thermal models for basement samples collected along HELR surfaces in the Wind River Range (i.e., Squaretop Mountain and Goat Flat transects) provide thermal histories for these surfaces and test proposed models of their origin. Thermal models from both transects show cooling during Late Cretaceous-Paleocene time (∼100-80 and 75-55 Ma), minor reheating via basin burial heating (∼50-10 Ma), and subsequent cooling to surface temperatures during the Late Miocene (<10 Ma). These results are not consistent with models suggesting that HELR surfaces are remnants of low-relief surfaces that initially formed at low elevations before being uplifted during the Laramide tectonic event, which would require cooling ages before Laramide deformation commenced. Whether HELR surfaces were beveled during or after Laramide uplift remains unresolved by the AFT system alone due to the temperature sensitivity of this system. However, the abundance of Pleistocene glacial features in the Laramide ranges (i.e., glacial lakes, moraines, glacial valleys; Gosse et al., 1995;Leopold, 1980), suggests that glaciation may have played an important role in creating low relief surfaces at high elevations in these ranges ( Figure 4). Additionally, paleoaltimetry records suggest that by the end of the Laramide orogenic event, the Wind River Range had already been uplifted to 3-4 km near the Pleistocene equilibrium-line altitude of >3.05 km (Davies, 2011;Fan et al., 2011). We therefore suggest that HELR surfaces in the Laramide ranges are remnants of a more extensive high-elevation paleolandscape that was shallowly buried during the Late Eocene-Early Miocene and then incised by fluvial systems after the Late Miocene, producing isolated surfaces that experienced local relief reduction by glacial erosion. Similar models that support glaciation as a mechanism for production and preservation of HELR surfaces have been proposed in other high-elevation regions such as the Himalaya (Sherpa et al., 2022).

Conclusions
Our study indicates that the Bighorn Mountains and Wind River Range experienced multi-stage Laramide cooling and erosion, as early as ∼100-80 Ma with rapid cooling at ∼75-50 Ma, followed by significant sediment burial and reheating between ∼50 and 10 Ma. We estimate that basin fill in the Powder River and Wind River Basins reached elevations of ∼3.4 km along the eastern flank of the Bighorn Mountains and ∼4.2 km in the northern Wind River Range. Isolated remnants of Oligocene-Early Miocene (∼34-15 Ma) post-Laramide basin fill at high elevations throughout Wyoming provide further evidence of post-Laramide regional relief reduction and the establishment of low internal-relief topography by the Early Miocene. Our study supports Late Miocene incision of the Powder River Basin and regional evacuation of basin fill that resulted in the modern topography characterized by >2.5 km of local relief. HELR surfaces within the Wind River Range (Squaretop Mountain and Goat Flat) are consistent with this history and record cooling and erosion during Laramide deformation and reheating-burial by Late Eocene-Early Miocene. This suggests that the Laramide basins were completely filled, partially covering their bounding ranges, possibly across a more extensive high elevation paleolandscape that was later dissected during Late Miocene regional incision and sediment evacuation. Our thermochronological data, Panels from left to right show high topographic relief by the Eocene (∼50 Ma), relief reduction and maximum basin filling during the Early Miocene (∼20 Ma), and the modern high relief configuration including Oligocene-Miocene basin fill remnants exposed at high elevations. Modified after Blackwelder (1915). combined with Pleistocene glacial features in the Wind River Range, suggest that the Squaretop Mountain and Goat Flat surfaces are isolated remnants of this dissected paleolandscape that experienced further relief reduction due to glacial erosion.

Data Availability Statement
Data archiving of the results and figures supporting the conclusions presented in this study are available through the University of Arizona's Research Data Repository: https://doi.org/10.25422/azu.data.22068719.