Sedimentologic and Geometric Characterization of Turbidites of Brazos-Trinity Basin IV in the Gulf of Mexico: Preliminary Results of IODP Expedition 308

Brazos Trinity Basin IV is located about 200 km offshore Texas, and belongs to a linked system of four intra slope mini basins. Basin IV provides a type section to characterize turbidites in salt withdrawal mini-basins of the Gulf of Mexico. IODP Expedition 308 has cored and logged complete pre-fan and fan sequences that are clearly distinguished with high-resolution seismic profiles at Brazos Trinity Basin IV at Sites U1319, U1320 and U1321. Seismically imaged pre-fan and fan units also can be distinguished and correlated with the sedimentological and logging data. Turbidite facies display distinct properties in terms of grain size, bed thickness, color, organic matter content, vertical organization of beds and lateral distribution in all the units of the fan through the basin. The pre-fan sequence is composed of terrigenous laminated clay with color banding and it is interpreted to result from deposition from fluvial plumes and/or muddy turbidity currents overspilling from basins upstream of Basin IV. The lower fan is characterized by laminated and bioturbated muds with thin beds of silt and sand, and represent the initial infill of the basin by mostly muddy turbidity currents, although an exceptionally sand-rich unit occurs at the base of the lower fan. The middle and upper fan represent the main pulses of turbidity current influx into Basin IV, and contain fine to medium sand turbidite beds organized in packets ranging in thickness from 5 to 25 m. The middle fan displays an overall upward increase in sand content at Site U1320, suggesting increased flow by-pass from the updip basins through time. Key examples of turbidites from each fan unit are analysed in detail to infer the depositional processes and infilling history of Brazos-Trinity Basin IV. The study of turbidites in a calibrated basin such as Basin IV provides ground truth for the sedimentological processes and resultant seismic facies, which can be used to interpret the infill history of other intraslope basins with similar seismic facies in the Gulf of Mexico where well calibration is not available.