From River to Delta: Down-Dip Changes in Amalgamated Sheet Sandstones Along an Exhumed Transect
Sediment storage within the fluvial segment of a depositional system impacts the volume of sediment reaching the contemporary shoreline zone, and thus decreases significantly the amount of sediment available for developing shallow-marine reservoirs. Exhumed examples of low-accommodation systems with a well preserved, and consistent sandy nature throughout the full transect are rare. In this study, we utilize the ~450 km transect of the Cretaceous Mesa Rica Sandstone, which is exhumed along its NNW-SSE depositional profile, from south-east Colorado to central-east New Mexico. These excellent conditions allowed conducting both seismic-scale and higher resolution studies of key-areas. Analysis of facies distribution and architectural geometries led to the recognition of multiple depositional environments, regional key stratigraphic surfaces, and sub-regional flooding surfaces. The upstream fluvial strata record deposition of an extensive sandstone unit that changes its channel architecture downdip, from vertically stacked channel belts forming multi-valley sheets, into a >80 km-wide single-storey channel sheet. Downstream, the coeval fluvial-marine transition zone represents deposition in a shallow mixed-energy setting. The most distal expression that can be studied is characterized by coalesced mouth bars, consistently overlain by sand-filled amalgamated distributary channels. This sheet-like geometry is interpreted to result from the combined effects of high sediment supply and low-accommodation, and bioturbation as a post-depositional homogenization process. This demonstrates that these sand sheets can be formed without necessary dominance of wave redistribution processes and therefore cautions against the classical interpretation of these geometries as the product of wave-dominated coastlines. Additionally, this study provides an example of sand-prone shallow-marine reservoir deposition despite significant fluvial sediment storage. The work is important to understand other systems elsewhere, like the Lower Cretaceous on Svalbard. Here, outcropping fluvial sandstones form the upstream equivalent of undrilled but seismically-imaged sediment lobes that form a potential play model in the adjacent Barents Sea Shelf. Studying exhumed systems like the Mesa Rica Sandstone improve our understanding of facies changes and sediment partitioning and distribution along different source to sink segments of low-gradient depositional systems, with important implications on sand prediction and reservoir quality.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019