Chapter 12 Laramide Sedimentary Basins

Abstract

Sedimentary basins of the broadly defined Laramide province, which includes the Rocky Mountain region from Montana south to New Mexico, the Colorado Plateau, and the southwestern most US and northern Sonora, Mexico subsided adjacent to coeval basement-involved uplifts that began to rise as early as Turonian time and continued to develop into the Middle to Late Eocene. Basins include (1) perimeter basins, with external drainages to the Gulf of Mexico, (2) ponded basins, connected by an extensive internal drainage network for much of their histories, on the Colorado Plateau and in the central Rocky Mountains, (3) small axial basins directly east of the Colorado Plateau, and (4) inversion-flank basins adjacent to uplifted Jurassic–Early Cretaceous extensional basins south of the Colorado Plateau. Laramide basins are typically asymmetric, thickening and coarsening toward the active uplift. Deposition took place in continental settings, including alluvial fan, fluvial, lacustrine margin, and open lacustrine depositional environments. These environments shifted spatially in response to three changing factors: (1) rates of thrust faulting on the active basin margin, which determined the location of most rapid subsidence; (2) extent of the drainage networks that contributed water, solutes, and sediment to the basins, causing them to alternately spill into other basins or become isolated from one another across structural sills; and (3) climate, which also influenced water influx and freshness of lake systems. Detrital compositions in the basins generally record the rock types in nearby uplifts, with two exceptions: (1) the southern inversion-flank basins also received detritus from the Late Cretaceous magmatic arc and (2) the ponded basins of the central Rocky Mountains and northern Colorado Plateau were flooded with volcanic-lithic detritus from the Early–Middle Eocene Absaroka volcanic field. Laramide deformation above a buoyant, flat subducted slab is consistent with an observed northeastward advance of basin development during the Late Cretaceous and Paleogene.