PALEOGENE LANDSCAPE EVOLUTION IN THE NORTHERN ROCKY MOUNTAIN CORDILLERA

Spatio-temporal compilations of tectonic, sedimentary, topographic, and climatic data provide a continental-scale view of landscape evolution and drainage reorganization in the Paleogene Cordillera. Late Cretaceous—Eocene Laramide crustal thickening caused widespread, diachronous surface uplift in the Western Interior region, largely inverting the Sevier foreland basin. Subsequent delamination of the Farallon slab was followed by diachronous gravitational collapse in the Cordillera, which modified pre-existing contractional topography. The tectonic events were coeval with a climatic shift from warm/wet during Late Cretaceous—Paleocene time to cooler/drier during late Eocene—Oligocene time. Despite this broad understanding of Paleogene landscape development and climate, the basin-scale interplay between tectonic, climatic, and topographic evolution remains poorly understood in many areas.

We present a detailed synthesis of sedimentary, structural, igneous, topographic, and climatic data and assess the timing, style, and causes of Paleogene landscape evolution in the southwestern Montana sector of the Cordillera. Results show: (1) Sevier-Laramide deformation caused crustal thickening and the development of a rugged plateau with surface elevations > 2 km. (2) Maximum elevations of > 4 km were attained in the Sevier hinterland ca. 50 Ma in response to slab removal and thermal uplift. (3) Concurrent with elevation gain, warm/wet climate conditions prompted deep fluvial exhumation, generating rugged topography (≥ 2 km relief). (4) Elevation gain in the hinterland caused a narrow rain shadow in the proximal foreland region. (5) Extension began ca. 43 Ma in the Sevier hinterland; it locally amplified topographic relief, but lowered mean surface elevations; negligible extension occurred in the adjacent foreland. (6) Extension beheaded paleodrainages that had previously transected the fold-thrust belt. (7) Progressive lowering of hinterland elevations allowed for the development of a regional rain shadow, emanating from higher topography in the Pacific Northwest.