Comparison of Highstand, Lowstand, and Transgressive Fluvial Deltaic Deposits Offshore South Texas
The Quaternary Rio Grande delta offshore southern Texas provides an excellent opportunity to compare fluvial deltaic deposits from highstand, lowstand, and transgressive systems tracts. The interpretation of the Rio Grande system is based on approximately 2500 km of 2d, high-resolution seismic data (with a maximum vertical resolution of 1 meter) and 20 platform boring descriptions (of up to 90 meters in length). Oxygen isotope stratigraphy, biostratigraphy, and radiocarbon dates from one 90 meter core provide the chronologic framework for the project. The study area is also linked seismic stratigraphically to previous studies on the eastern and central Texas shelf.
In South Texas, fluvial deltaic deposits are interpreted as occurring in all three systems tracts, but varying in geometry, thickness, lithology, and areal distribution. Highstand fluvial deltaic deposits are characterized by large areal extents (up to 8500 km2) with thinknesses of up to 50 meters. The lithology of the deposits is primarily mud-prone with a small percentage of sand-prone deposits proximal to the sediment source. Lowstand fluvial deltaic deposits are confined to shore perpendicular incised valleys and the shelf margin. The deposits can be greater than 75 meters in thickness and are interpreted as being sandy. Transgressive fluvial deltaic deposits fill erosional channels and form a mid-shelf shoreline during a relative stillstand in the last sea level rise. The deposits tend to be thinner than those found in other systems tracts, with thicknesses of approximately 25 meters. The areal extent of these deposits is up to 1500 km2 with a significant sandy component. Due to their sandy lithology and large areal extent, lowstand and transgressive fluvial deltaic deposits are the best candidates for hydrocarbon reservoirs in the study area.