Sequence Stratigraphy of Late Mesozoic Rocks of the Colorado Plateau

Abstract

The Zuni sequence, as defined by Sloss, encompasses rocks ranging in age from the latest Early Jurassic to about the Cretaceous-Tertiary boundary. In current Exxon sequence stratigraphic charts this Sloss sequence is divided into three global supersequence sets, all of which can be readily recognized in the rocks of the Colorado Plateau: (1) the lower Zuni A (global) corresponds to the San Rafael Group, (2) the lower Zuni B is represented by the Morrison Formation, and (3) the upper Zuni is the Late Cretaceous deltaic and marine package (Mancos Shale and equivalents). The global Zuni sequence is divisible into twelve supersequences, the boundaries of which generally correspond to major regional unconformities in the Rocky Mountains. Not all unconformities correspond to Exxon's global sequence boundaries, however, because processes such as regional marine transgression (base Curtis SS; J3 unconformity), foredeep rebound (intraformational unconformity in the Sanpete Formation) and forebulge migration (Dakota SS truncation) produce local unconformities totally independent of regional sea-level change. The stratigraphy of the Zuni sequence of the Colorado Plateau clearly supports the concept that global sea-level change is the principal determinant of large-scale lithofacies architecture, including those of mostly nonmarine origin. At the scale of the supersequences (10 m.y.),more » the globally recognized episodes of sea-level fall have all produced major unconformities within Zuni rocks of the Colorado Plateau. This suggests that the large-scale packaging of rocks in the Phanerozoic reflects eustatic cycles, not tectonic ones as maintained by Sloss.« less