There is ongoing debate regarding the presence, timing, magnitude, and extent of high-elevation areas in the North American Cordillera during Late Cretaceous and Paleogene time. Large compilations of tectonic, sedimentary, and climatic data from the North American Cordillera and Western Interior region provide a continental-scale view of landscape evolution, but in spite of our broad understanding of Late Cretaceous−Paleogene events, the basin-scale relationships among tectonic activity, climatic change, and topographic evolution remain poorly understood in many parts of the Cordillera and Western Interior.
The southwestern Montana sector of the North American Cordillera spans the structural boundary between the Sevier fold-and-thrust belt and the Laramide foreland province, ostensibly spanning the edge of a Paleogene high-elevation orogenic plateau. In order to assess the paleotopographic evolution of this area, we provide a synthesis of sedimentary, structural, igneous, paleolandscape, and paleoclimate data that includes a compilation of new and existing stable isotope data (paleosol carbonate δ18O and δ13C) from across the southwestern Montana region. Using detailed stratigraphic information as the context for interpretation, we integrated the multiple data types to evaluate Late Cretaceous−Paleogene landscape evolution in southwestern Montana, with special emphasis on δ18O and δ13C data. We also highlight apparent relationships between sedimentary environments (i.e., soil-forming conditions) and paleosol carbonate δ13C values, wherein the relative dryness of the local soil-forming environment correlates to the magnitude of δ13C values from paleosol carbonate (i.e., environmental dryness is positively correlated to δ13C).
Results show that hinterland surface elevations progressively increased between Late Cretaceous and middle Eocene time, after which extensional reactivation across the fold-and-thrust belt region progressively lowered surface elevations and significantly modified the antecedent landscape: (1) Sevier-Laramide deformation resulted in the development of a rugged plateau with surface elevations >2 km between ca. 65 and 58 Ma. (2) Maximum peak elevations increased to ≥4 km in the Sevier hinterland ca. 50 Ma following thermal uplift of the lithosphere; δ18O data support this, suggesting 2.3 km to 3 km of relief between the Sevier hinterland and the adjacent Laramide foreland at ca. 47 Ma. (3) Concurrent with elevation gain between ca. 58 and 53 Ma, warm/wet climatic conditions prompted deep fluvial incision into contractional terranes, generating a rugged, high-relief (up to 2 km) topography and a network of fluvially connected intermontane basins. (4) Extensional reactivation of structures in the fold-and-thrust belt region began ca. 50 Ma and continued through ca. 25 Ma; faulting locally amplified predecessor topographic relief but simultaneously lowered basin-floor surface elevations (and thus mean surface elevation). Extension also beheaded paleodrainages that had previously crossed the fold-and-thrust belt, producing ponded, normal fault−bounded basins on top of the fold-and-thrust belt. (6) Progressive extension was concurrent with progressive aridification between late Eocene and early Miocene time, which is supported by both δ18O and δ13C trends from across the basin network.