The combined effects of river-flood and tidal processes on the stratigraphy, ichnology, and stratal architecture of the Cretaceous (Campanian) Loyd Delta near Rangely, Colorado U.S.A.
Prather, Timothy John
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The Campanian Loyd Sandstone (Loyd) has been identified in outcrops in the Piceance Basin within strata above the Castlegate Sandstone near Rangely, CO. The Loyd preserves sedimentary structures, facies, sandbody/shale geometries, and stratal architectures characteristic of a mixed river-flood dominated tidally influenced delta. Characteristics include abundant low angle planar laminated foresets, flaser-wavy-lenticular bedding between foresets, trough cross-stratified mouth bars and subaqueous terminal distributary channels, and hyperpycnites on the distal delta front to prodelta. The Loyd contains a surprisingly high abundance, high-diversity fully marine ichnologic assemblage for a delta with river-dominated characteristics. The bulk of these trace fossils are found within and at the base of low-angle planar laminated sandstone beds, and in flaser-wavy-lenticular beds. Traces display a tiered relationship, with some burrows penetrating multiple beds on a meter scale. Despite strong evidence in sedimentary structures and sandbody geometries of river-dominance, the high abundance high diversity trace fossil assemblage and tidal indicators suggest that the bulk of time on the Loyd delta-front is recorded by tidally modified fine-grained sediment deposition and extensive bioturbation. Extended periods of low-energies, low sedimentation rates, and relative quiescence were likely punctuated by deposition of sandy traction flows and channelization during river-flood conditions. Although the Loyd is grossly similar to the river-flood dominated Panther Tongue and Ferron deltas, and contains structures similar to tide-dominated deltas of the Sego sandstone, it should not be classified as an endmember example of either type of delta. Integrating ichnological analysis to determine how processes fluctuate during an episode of deltaic deposition allows for a refinement of the common delta classification scheme.