Evolution of the Rio Grande System
Incision in the Rio Grande system forms primarily during sea-level fall. This dataset cannot determine the timing of incision relative to sea-level falling below the shelf break. Incisions are mostly filled with lowstand deposits. On the inner shelf the fill has a chaotic seismic reflection configuration, is composed of olive-gray fine sands and silts with some clay layers, and is interpreted as delta plain deposits. On the outer shelf, the reflectors demonstrate a mounded onlap to complex sigmoid oblique seismic facies pattern, while a lithologic description consists of olive-gray silty sand and sandy silt overlain by olive-gray fine sand. The deposits are interpreted as a prograding mouth bar.
Delta formation was active in the Rio Grande system during all three systems tracts. Highstand deltas are interpreted as wave- and lobate fluvially-dominated. Lowstand deltas are also interpreted as lobate fluvially-dominated. Transgressive deltas are interpreted as both elongate fluvially-dominated during early transgressive time and wave-dominated during late transgressive time. Wave-dominated deltas are predicted to have higher sand to shale ratios due to wave reworking of the delta front sediments.
Minimal transgressive reworking is indicated by this study. However, Eckles identified extensive sand-prone units to the north in central Texas that are thought to have been partially sourced form the Rio Grande system during the transgression. Also, Suter and Berryhill (1985) and Berryhill (1986) discuss a surficial layer of medium and coarse sands mixed with shells indicative of a modern shallow-shelf environment. They interpret the sand sheet as forming due to the transgressive reworking of beach, deltaic, fluvial and nearshore sands.
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