Evidence for Tectonic Re-Organization of Regional Sediment Dispersal Trends Within the Middle Turonian Frontier Formation (Vernal Delta Complex) of Colorado, Utah and Wyoming

Abstract

The mid-Turonian Frontier Formation forms the Vernal Delta Complex (VDC) exposed at the surface along the flanks of the Uinta Mountains in Utah, Colorado, and Wyoming. Current literature defines the VDC as a large shoreline bulge composed of deltaic facies that prograded into the Cretaceous Western Interior Seaway (KWIS). Ryer & Lovekin (1986, in AAPG Mem. 41, 497–510) argue that the VDC area is too great to represent a single delta, but instead is a prograding shoreline generated from differential subsidence. However, a detailed stratigraphic framework for the succession has to date been lacking, limiting interpretations and correlations between the north and south Uinta Mountain flanks. In this study, we address whether the VDC represents a single delta or multiple delta forms influenced by regional tectonic processes, specifically forebulge growth. We provide detailed facies reconstructions and regional correlations using 54 outcrop sections from the north, east, and south Uinta Mountain flanks and wireline logs from > 200 drillholes in the Uinta and Green River Basins. From this database, a detailed sequence stratigraphic framework is developed to facilitate understanding of regional depositional patterns. Data analysis shows two sediment dispersal trends. Exposures along the S and E Uinta MOuntain flanks contain distal delta front facies with SSE-oriented paleocurrent indicators. In this area, these facies overlie a Mowry Shale unconformity produced from a N-S elongate topographic high generated by forebulge flexure. The similarities between N-S paleocurrent and topographic trends suggest sediment dispersal influenced by an active forebulge. In thin, sharp-based proximal delta front (forced regressive) and coastal plain (lowstand) facies overlying the distal delta front in the Uinta and Green River Basins, ESE sediment dispersal is recorded. This indicates a sediment source separate to that responsible for the SSE trend. Forced regressive conditions were created from decreased accommodation during minimal forebulge flexure and sediment dispersal was not directly influenced by foreland basin tectonics. We interpret the VDC as the product of two deltas with trends controlled by the tectonic re-organization of sediment source and dispersal, rather than as a single prograding entity. These findings offer an alternative solution for the origin of the VDC, while providing insight on the formation of similar intervals within the KWIS stratigraphic record.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015