--> The Potential Temporal Significance of a High-Abundance, High-Diversity Ichnologic Assemblage in an Otherwise Apparently River-Dominated Deltaic Succession: The Cretaceous Loyd Delta, Rangely, Colorado

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

The Potential Temporal Significance of a High-Abundance, High-Diversity Ichnologic Assemblage in an Otherwise Apparently River-Dominated Deltaic Succession: The Cretaceous Loyd Delta, Rangely, Colorado

Abstract

The Loyd Delta (Loyd), found in the Buck Tongue of the Mancos Shale, outcrops near Rangely, CO and provides a potential analogue for hydrocarbon and groundwater reservoirs. The Loyd preserves sedimentary structures, facies, and sandbody/shale architectures characteristic of a river-flood dominated deltas including low angle planar laminated foresets, trough cross-stratified mouth bars and subaqueous terminal distributary channels, and hyperpycnites on a distal delta front to prodelta, and lacks wave-modified deposits. A low-diversity, low abundance ichnologic assemblage is commonly associated with river-flood dominated deltas. However, the Loyd contains a high abundance, high-diversity ichnologic assemblage that includes: Ophiomorpha, Thalassinoides, Planolites, Phycodes, Macaronichnus Schaubcylindrichnus, Palaeophycus, Piscichnus, Taenidium, Teichichnus, Diplocraterion, Helminthopsis, and Bergaueria indicative of a fully marine, non-stressed, environment. The bulk of these trace fossils are found at the base of low-angle planar laminated sandstone beds and within what appear to be tidally-modified muddy siltstones containing flaser-wavy-lenticular bedding found between sandy foresets. Ophiomorpha commonly penetrates deeply (∼1 meter) through multiple beds of this facies, interpreted to be sheet-like distal mouth bars on the delta-front. Despite strong evidence of sedimentary structures and sandbody geometries characteristic of a river-dominated delta, such as hyperpycnite deposits, mouth bars, and channelization on the delta-front, the high abundance high diversity trace fossil assemblage in the Loyd suggests that the bulk of time on the delta-front is recorded by tidally modified fine-grained deposition and extensive bioturbation. Extended periods of low-energy, low sedimentation rates, and relative quiescence were likely punctuated by deposition of sandy traction flows and channelization during river-flood conditions. Marine tracemakers are sensitive to salinity fluctuations; therefore the trace fossil assemblage suggests that fully marine conditions were common while brackish conditions were relatively rare. The Loyd Delta provides insight into the complexities of delta reservoir models and highlights the difficulty of modeling complex heterogeneous delta fronts from subsurface data alone. A laterally extensive outcrop analogue is invaluable for predicting the distribution of facies in this unique example of a river-dominated, tidally influenced delta.