A Fluvially-Dominated Deltaic System from the Turonian Frontier Formation of the "Vernal Delta" Complex, Dinosaur National Monument, Utah and Colorado

Hutsky, Andrew; Fielding, Christopher

The "Vernal Delta" is one of three named depocenters of the Cenomanian-Turonian Ferron Sandstone/Frontier Formation along the western margin of the Cretaceous Western Interior Seaway Basin of Utah and Wyoming. Unlike the other two (Last Chance and Notom Deltas) however, the origin of the Vernal Delta is disputed. Ryer & Lovekin (1986, in AAPG Mem. 41, 497-510) stated, that while the complex consists mostly of deltaic deposits, the lack of a deltaic depocenter associated with the shoreline bulge suggests that the Vernal Delta "was probably not an actual delta". Rather, they attributed its preservation to differential subsidence. In this study, we provide a sedimentologic and stratigraphic analysis of the mid-late Turonian Frontier Formation of the "Vernal Delta" in the northern Uinta Basin that aims to evaluate whether a deltaic system is fully preserved. A total of 32 sections (90 - 130 m thick) were measured along a ~ 90 km depositional dip and strike-oriented outcrop belt flanking the western portions of Dinosaur National Monument (DNM). Sandstone body geometry, sedimentary structure, and ichnologic data were collected, serving as the basis for a detailed facies analysis. The addition of subsurface datasets (wireline logs, cores) allow for the generation of net sand isolith maps, contributing to the establishment of regional-scale sandstone body geometry dimensions. Together, both datasets provide for an interpretation of the processes controlling sediment accumulation and preservation.

The main body of the Frontier Formation preserves an entire progradational suite representative of a fluvio-deltaic system. Proximal delta front/coastal plain facies are present along the northern and western margins of DNM (NE of Vernal, UT), and pass down depositional dip (ESE) into their distal delta front/prodelta equivalents (Dinosaur, CO). Sedimentary structures (hyperpycnal flow deposits, syneresis cracks, meter-scale gutter casts) and stressed ichnofacies suggest that fluvial processes influenced delta progradation. Paleocurrent data from delta front, mouth bar, and channel facies highlight two delta lobe progradation trends (eastward and southeastward), reflecting channel avulsion and delta lobe switching. As such, mapping of lateral thickness, facies, and paleocurrent trends do appear to coincide with the planform of the Vernal Delta in that area, suggesting that along its SE margin at least, the Vernal Delta protuberance does reflect original topography.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013