Sequence stratigraphy of nonmarine Jurassic–Cretaceous rocks, central Cordilleran foreland-basin system

Abstract

An analysis of the Upper Jurassic–Lower Cretaceous Morrison and Cedar Mountain Formations of Utah and Colorado has resulted in a general sequence-stratigraphic model for nonmarine rocks. In this model, nonmarine deposition is governed by changes in basin accommodation development and corresponding shifts in depositional base level. These fluctuations result in deposition of systematically varying facies and architectural elements that allow nonmarine depositional sequences to be recognized. Internally, nonmarine depositional sequences comprise three systems tracts—degradational, transitional, and aggradational—which are analogous to the lowstand, transgressive, and highstand systems tracts of marine depositional sequences. Degradational systems tracts overlie sequence-bounding unconformities and consist of relatively coarse-grained, low-sinuosity fluvial deposits that are either contained within incised valleys or deposited as widespread, thin sheets above shallow erosion surfaces. Transitional systems tracts represent an increase in basin accommodation development following degradational systems-tract deposition. They are marked by the transition from laterally continuous, low-sinuosity fluvial channel sandstones and conglomerates to lenticular and ribbon-like, meandering and anastomosing channel sandstones. Aggradational systems tracts are characterized by meandering-anastomosing channel sandstones and abundant fine-grained overbank and lacustrine deposits. The Upper Jurassic–Lower Cretaceous nonmarine rocks of the study area contain three depositional sequences. The first of these, the UJ-1 sequence, consists primarily of an aggradational systems tract overlain by a sequence-bounding unconformity. However, the lower parts of this sequence are transitional with older marine rocks and can be considered the late stages of a marine highstand systems tract. The upper Morrison sequence (UJ-2) consists of degradational, transitional, and aggradational systems tracts. Above the UJ-2 sequence are a sequence-bounding unconformity and degradational and transitional systems tracts of the LK-1 sequence represented by the Buckhorn Conglomerate. The Buckhorn is overlain by a sequence-bounding unconformity and transitional-aggradational systems tracts of the LK-2 sequence that is composed of the upper part of the Cedar Mountain Formation. The Upper Jurassic–Lower Cretaceous sequences in Utah and Colorado can be traced regionally and correlated with nonmarine depositional sequences in central Utah and sequences that contain nonmarine, transitional, and marine rocks in central Wyoming. These sequences were deposited in the back-bulge, forebulge, and distal foredeep depozones of the Late Jurassic–Early Cretaceous foreland-basin system.