Abstract

During the Triassic, the supercontinent Pangea was at its greatest size—a single landmass symmetrically disposed about the paleoequator. This configuration maximized monsoonal circulation, a global climatic pattern that had a strong influence on the distribution and facies of continental deposits. Triassic paleogeography and deposystems of the Western Interior should be examined in a global context because at that time the region lay near the western equatorial coast of Pangea, where the effects of the tropical monsoonal climate were pronounced. In addition to the climatic controls on sedimentation, sea-level fluctuations and plate tectonics, both to the west in the miogeocline and to the east on the craton, affected Early Triassic marine and Late Triassic continental deposition. Concomitant with the end of Early Triassic miogeoclinal sedimentation, an evolving magmatic arc along the western margin of Pangea produced a Late Triassic continental basin, contributed volcanic detritus, and recycled uplifted, upper Paleozoic clastic detritus to the region.

The Early Triassic of the Western Interior was marked by marine-basin, marine-shelf, marginal-marine, sabkha, and deltaic systems on the west that graded eastward into coastal plain and continental red bed systems sourced from the east on the craton and from local uplifted highlands. Three Early Triassic transgressive and regressive sequences allow comparison of relative sea-level curves to Triassic marine systems throughout Pangea. Middle Triassic rocks in the Western Interior are rarely preserved because nondeposition and/or extensive erosion formed a regional unconformity. Rocks of this age are documented in only a few scattered localities on the craton, but extensive Middle Triassic marine strata are exposed farther west.

Late Triassic deposystems in the southern part of the Western Interior were dominated by continental facies that included abundant stream, floodplain, lake, and marsh deposits, and at the close of the Triassic by playa-lake, eolian sand-sheet, and eolian dune deposits. Farther north in the Rocky Mountain region, Late Triassic depofacies included stream, floodplain, arid-region lakes, and play a deposits. The prominence of arid continental systems in the northern Western Interior reflects the slightly higher latitudes of this region on Pangea during the Late Triassic, a location just north of the greatest influx of monsoonal precipitation. Triassic continental systems dominated by streams, lakes, and marshes indicate abundant precipitation, in contrast to preceding Permian and subsequent Early Jurassic deposystems that were dominated by eolian ergs, arid-region lakes, and arid-climate coastal-marine systems. This larger scale climatic change reflects two major controls: (1) the norm ward migration of the Western Interior near the west coast of Pangea from near the paleoequator in the Permian to a position farther north of the paleoequator in the Jurassic and (2) the demise of the monsoonal climate, resulting from the breakup of Pangea in the latest Triassic and earliest Jurassic. Interpretations of Triassic stratigraphy, sedimentology, paleontology, and paleoecology, and comparisons with sequences worldwide must be placed in the context of Pangean paleogeography, paleoclimate, and sea-level fluctuations.