TYING SEDIMENTATION IN THE WESTERN INTERIOR BASIN TO THE MID-CRETACEOUS PENINSULAR RANGES OROGENY USING STARVED OUTER-SLOPE TRENCH DEPOSITS AND POST-COLLISIONAL MOLASSE

Abstract

The Peninsular Ranges orogeny occurred at ~100 Ma, and affected rocks from Mexico to Alaska when the western edge of North America was subducted westward beneath the arc-bearing Peninsular Ranges composite terrane. Characteristic zones of the orogen over its length include, from west to east: a back-arc trough filled with post-collisional debris, a 140-100 Ma magmatic arc and seaway, post-collisional 99–84 Ma granitoid plutons emplaced into the hinterland, an east-vergent thrust belt, and a broad foredeep known as the Western Interior Basin. Except those within the thrust belt, the rocks of the Western Interior Basin were deposited east of the cratonic hingeline on stiff cratonic lithosphere with an effective elastic thickness of 100-200 km. This led to a broad and shallow flexural outer slope to the trench, limited foredeep bulge, and a much narrower isostatic basin beneath the thrusts of the accretionary prism sitting on the edge of the craton. Within the Western Interior Basin, the ~100-99 Ma Shell Creek Shale and Newcastle Sandstone were deposited on the broad outer slope of the trench, although rocks of the 103 Ma Blackleaf Fm. and the 101 Ma Thermopolis Shale and Muddy Sandstone, as well as the Westgate, lower Shaftesbury, and Goodrich formations in Canada, may have all been deposited there. Slab failure appears to have occurred just prior to the deposition of the condensed and essentially isochronous 99-97.5 Ma Fish Scales and Mowry formations, which contain abundant fish hash, extend the length and width of the trough, and mark a period of anoxia and abundant ash-falls, which we interpret to represent slab failure and start of exhumation in the hinterland with consequent restriction and shallowing of the foredeep. Zircon (U-Th)/He thermochronology in the thrust belt supports initiation of exhumation at ~100 Ma. The Fish Scales-Mowry unit is directly overlain by the Dunvegan and Frontier clastic wedges, which contain abundant <100 Ma detrital zircons diagnostic of the post-collisional plutons. Thus, much of the sedimentation within the early Late Cretaceous Western Interior Basin was post-thrusting molasse. The smectitic and highly fossiliferous non-marine Mussentuchit Member of the Cedar Mountain Formation of central Utah is 98.4 Ma on the basis of bentonite ages and is coeval with the marine Mowry–Fish Scales unit. Its abundant vertebrate fauna includes tyrannosaurids, pachycephalosaurs, snakes and marsupials with Asian affinities, which suggest that the vertebrates arrived with the Peninsular Ranges composite terrane at ~100 Ma.