RIO GRANDE SHELF-EDGE DELTA AND SUBMARINE SLOPE SYSTEM, OFFSHORE TEXAS, USA: TARANTIAN – EARLY HOLOCENE SEDIMENTATION OVERWHELMS TECTONICS (Invited Presentation)

The shelf-edge delta and submarine-slope system of the Rio Grande River provide a unique record of continental erosion during the last glacial episode for the southern Rocky Mountains and Sierra Madre mountains of Mexico. The linked depositional systems also serve as a case study for sedimentation overfilling sea-floor topography generated by active extensional faults and salt tectonics. Both systems have been mapped using ˜24,000 km of 2D seismic lines covering the continental shelf and slope outboard of the modern Rio Grande River delta. The seismic data consists of industry surveys from The National Archive of Marine Seismic Surveys and a ˜500 km survey of high resolution seismic data collected by the Jackson School of Geosciences. Age control was provided by previously identified nannofossil biostratigraphic markers from core located near the distal end of the submarine slope system. Regional mapping of the linked systems yields a conservative estimate of 1100 cubic km for the deposited volume of Tarantian (Wisconsin Glacial Episode) to early Holocene sediment. Accumulation of this very large sediment volume indicates a transport efficiency connected to the last glacial episode which cannot be predicted by analysis of the modern river. While most of this sediment is stored in the shelf-edge deltaic complex, a considerable amount also accumulated on the upper-to-mid continental slope. We focused our attention on the first 140 km of slope, positioned immediately down-dip from the shelf-edge; over this distance, water depths systematically increased by 1500 m. The submarine system was composed of a network of aggrading submarine channels and overbank surfaces. Individual channels had characteristic depths and widths of 10-30 m and 600-800 m. Bed slopes for these channels decreased from ˜0.05 to ˜0.005 over the 140 km, defining a concave-up profile. The associated column of accumulated strata also systematically decreased with distance from ˜450 m to ˜60 m. On average, reduction in deposit thickness was proportional to the reduction in bed slope, with ˜85% of the change in both occurring in the first 80 km. Properties of this linked transport system overwhelmed a province of active salt structures and affiliated growth faulting. We will examine it within the context of competing sedimentation and tectonic processes.