Abstract: 

Conditions for deposition of tidally influenced facies are most commonly associated with transgression; however, tidal influence can also be observed in regressive deposits. Herein we present a regional study that correlates the Loyd and Sego sandstones across the Uinta–Piceance basins, an area rich in both outcrop and subsurface data. Variable energy conditions in these tidally modified deltas result in a complex arrangement of facies and environments, making it difficult to correlate sequence stratigraphic surfaces. We highlight the controls, including tectonics, eustasy, and sediment supply, on depositional environments and sand distribution in these regressive tidally modified deposits.

Five environments of deposition (EOD) were identified from a combination of vertical and lateral facies trends and stratal geometries found in outcrops and core. EODs were assigned unique well-log signatures and identified, correlated, and mapped in subsurface logs and outcrop from the Uinta to the Piceance Basin. From a proximal to distal position along shorelines these EODs are: 1) coastal-plain channels and floodplains, 2) tidally influenced valley fills, 3) tidally modified deltas, 4) delta front, and 5) open marine. The regional distribution and stacking of these environments were used to identify ten flooding surfaces and map nine genetic sequences: two in the Loyd Sandstone, four in the lower Sego Sandstone, and three in the upper Sego Sandstone.

The interplay of sediment supply, sea-level fluctuations, and active tectonics contribute to the distribution of environments of deposition and sequence stacking patterns. This study is the first to identify the Loyd Sandstone in outcrops along the Rangely Anticline. During deposition of the Loyd Sandstone, sediment supply from up-dip fluvial systems was high. The Loyd Sandstone is interpreted as being deposited in river-flood-dominated but tidally modified deltas that exhibit a generally lobate geometry. In contrast, the lower Sego Sandstone comprises tidally influenced deltas and valley fills, and forms regionally extensive shore-parallel sandstone belts. An increase in tidal energy prevalent in the lower Sego Sandstone is attributed to a combination of high sediment supply, slow relative base-level rise, and active faulting over the Laramide-age Douglas Creek Arch. This process formed complex coastline geometries, which likely amplified tides. The upper Sego Sandstone is interpreted to comprise aggradationally stacked sequences composed of tide- and wave-modified deltas. During the deposition of the upper Sego, sediment supply and accommodation were balanced, causing deltaic depocenters to remain stationary over time. Decreased tectonic activity in the upper Sego diminished the influence of tides on sedimentation. This study correlates the Loyd and Sego from the Uinta to the Piceance basins and sheds light on the applicability of this interval for use as analogs for tidally influenced systems in regressive sequences.

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