Surface sediment facies and physiography of a recent tidal delta, Brown Cedar Cut, central Texas coast

Coastal erosion and subsidence on Matagorda Peninsula, Texas, has resulted in retreat of the coast by 883 yards (approximately 800 meters) between 1858 and 1972, an average of 23 feet (7 meters) per year. This thinning facilitated breaching of the barrier and the initial formation of Brown Cedar Cut tidal delta in 1929. Processes resulting in the construction of this subaerially exposed flood delta were documented utilizing a sequence of more than 30 different aerial photographs and navigational charts made between 1929 and 1972. The effects of inlet spit migration, up to 2,100 yards (1,920 meters) as a result of southwestward littoral drift, and the effects of periodic hurricanes were found to be most influential in the formation and morphology of the delta. The delta became progressively asymmetric in the direction of littoral drift, while older isolated islands were stabilized by vegetation. Construction of a topographic map enabled detailed mapping of eleven physiographic features and six facies totaling 12 feet (3.6 meters) in thickness. Elevation and inlet proximity are controlling factors in the distribution of organisms, sediments. and sedimentary structures. From statistical analyses of 100 samples, trenches, and cores, trends of the delta facies are (1) a decrease in mean sediment size away from the inlet, (2) a decrease in mean size of the sediment with increasing age of the islands, and (3) a vertical sequence consisting of a basal unit of homogeneous, fine sand that was deposited during the initial stages of delta growth, a central unit of bioturbated, interbedded, highly variable sediments deposited during small tidal fluctuations, and an upper unit of thick, cross-stratified sands deposited during major storms.