FLUVIAL CHANNEL SANDSTONES AND COASTAL PLAIN FACIES OF THE TURONIAN FRONTIER FORMATION, NORTHERN UINTA BASIN, UTAH: DELTA PLATFORM TOPSET OF A LOW ACCOMMODATION DELTAIC COMPLEX

The Frontier Formation of the northern Uinta Basin, Utah contains an interval of coastal plain and fluvial deposits that remain relatively understudied and inadequately placed within a stratigraphic context. In this study, coastal plain facies were analyzed from 23 measured vertical outcrop sections of the mid to late Turonian Frontier Formation, near Vernal, UT and Dinosaur National Monument (DNM). Grain-size, sedimentary structure, fossil (body and trace), and paleocurrent data were collected from measured sections, facilitating a facies analysis. Cross-sections and maps were constructed to observe the regional distribution of facies. The interval varies in thickness, from about 20 m near Vernal, UT to 0 m at the western boundary of DNM (paleoshoreline location). It overlies subaqueous mouth bar sands and is abruptly overlain by offshore marine mudstones. Facies comprise brown, organic-rich mudstones and siltstones, coal seams of variable thickness and lateral extent, and fine- to coarse-grained, sharp- to erosionally-based sandstones of variable lateral extent and thickness. Thick (3-7 m), cross-stratified (trough and flat-low angle), abruptly-based sandstone bodies with broad lensoid cross-sectional geometries are interpreted as fluvial channel sandstones within the lower coastal plain of a prograding delta. Fluvial processes dominated, although rare bioturbation and bidirectional cross-bedding 10 - 15 km inland of the paleoshoreline suggest a degree of tidal influence. Here, channel bodies often alternate with subaqueous mouth bar deposits, suggestive of terminal distributaries. 20 – 30 km west (landward), channel sandstones show less evidence of marine influence, as bioturbation and bidirectional cross-bedding are uncommon. Development of non-marine deposits above shallow marine deltaic facies records a complete progradation cycle. However, a lack of channel body amalgamation, a thin (< 20 m) veneer of non-marine facies, and an abrupt transition to offshore marine mudstones suggest coastal plain establishment during relative sea-level lowstand under low accommodation conditions. Marked by the abrupt transition to offshore marine mudstones (maximum flooding surface), rising sea-levels terminated coastal plain development in the region.