The mid-Cretaceous Peninsular Ranges orogeny: a new slant on Cordilleran tectonics? III: The orogenic foredeep

Abstract

The Cretaceous Western Interior Basin reflects the interplay between the North American craton and allochthonous terranes to the west. We divide the basinal stratigraphy into three successions, Aptian-Albian, Cenomanian-Turonian, and Santonian-Maastrichtian, each related to periods of deformation in the adjacent fold-thrust belt. Here we focus on the Cenomanian-Turonian succession, where progressive west to east uplift and fluvial incision of older Aptian-Albian sedimentary rocks (Cedar Mtn-San Pitch-Thermopolis-Skull Creek-Mannville) are interpreted as a migrating forebulge. Uplift was underway at 103 Ma in the west (Paddy-Blackleaf-Muddy sandstones) and propagated eastward throughout the trough by 99.5 Ma (Viking-Bow Island-Newcastle sandstones). The incised fluvial valleys were subsequently filled by swampy and shallow marine facies, then overlain by dark, marine Neogastroplites-bearing shale and associated bentonites of the 100-97.5 Ma Shell Creek-Mowry-Slater River-Goodrich-Shaftesbury-Westgate shales. The shales are characterized by a distinctive condensed horizon with abundant fish scales, teeth, and bones. They are interpreted as outer-trench slope deposits, with the overlying anoxic horizon representing a starved isochronous unit formed atop the slope deposits. The starved horizon is overlain by prodeltaic muddy clinoforms of easterly migrating clastic wedges (Trevor-Dunvegan-Frontier-Cintura-Mexcala) that can be traced 800 km atop the fish-scale hash and contain hinterland-derived 99-90 Ma detrital zircons. Although the Western Interior Basin has long been considered a retro-arc trough, the overall succession instead suggests that the Cretaceous-Turonian part represents a collisional foredeep created during the ~100 Ma collision between the arc-bearing Peninsular Ranges composite terrane and North America. The accretion brought tyrannosaurids, pachycephalosaurs, snakes and marsupials to North America.