THE OFFSHORE BAR REVISITED: A NEW DEPOSITIONAL MODEL FOR ISOLATED SHALLOW MARINE SANDSTONES IN THE CRETACEOUS FRONTIER FORMATION OF THE NORTHERN UINTA BASIN, UTAH, USA

摘要

Shallow marine sandstone bodies isolated within marine mudrock successions are still incompletely understood. A robust understanding of such bodies is of interest not only scientifically, but also in order to facilitate successful exploration for and production of petroleum and other natural resources. Such bodies are preserved in the Turonian Frontier Formation of the northern Uinta Basin in Utah and Colorado, U.S.A., where they have been previously interpreted as the products of "offshore bars." A reappraisal of these bodies in the uppermost Frontier Formation (herein defined as the Turonian Ashley Valley Member) reveals three northeast-southwest elongate, incised highstand, forced regressive, and lowstand offshore sandstone bodies partially to fully encased in offshore marine mudrock. These bodies are herein named the Buckskin Hills, Kane Hollow, and Raven Ridge sandstones (from oldest to youngest). Five lithofacies were identified from 47 measured outcrop sections located along the western and southern margins of Dinosaur National Monument. They are 1) marine shelf claystones and siltstones, 2) lower-upper offshore marine admixed siltstone and sandstone, 3) lower-upper offshore thin-bedded sandstones and siltstones, 4) distal lower shoreface heavily bioturbated very fine-grained sandstone, and 5) transgressive lag conglomerate. Wireline logs from 112 drillhole locations throughout the Uinta Basin were calibrated with nearby outcrop locations to construct cross sections and net-sandstone isochore maps. From these, regionally traceable horizons (flooding surfaces and internal erosion surfaces) facilitated an analysis of stacking patterns, development of a stratigraphic framework, and a depositional-environment interpretation. Overall, the Ashley Valley Member preserves a basinward-offlapping stacking pattern, indicative of shoreline progradation in response to the late Turonian Greenhorn global eustatic regression that was interrupted by two, high-frequency transgression-regression cycles. This nested cyclicity cannot be explained by global eustasy alone, and instead is suggestive of a secondary allogenic control, likely related to Western Cordilleran Foreland Basin (WCFB) geodynamic driving mechanisms. As a result, the Ashley Valley Member provides an example of tectonic forcing on stratal stacking patterns, highlighting the importance of intrabasinal tectonic processes on dispersal patterns and high-frequency relative base-level fluctuations within the WCFB stratigraphic record.