RATES AND MECHANISMS OF EROSION GENERATING A WAVE-CUT PLATFORM AT SARGENT BEACH, TEXAS, USA

Beaches with the highest rates of coastline retreat in Texas are characterized by a wave-cut platform and bluff morphology. At Sargent Beach, this shoreline topography is being cut into a substrate of weak, Holocene mudstone, composed of centimeter to decimeter thick horizontal beds and associated with the nearby coastal river. Its compressive strength ranges from immeasurably small when submerged and water saturated, to 206 kPa when moist, and 412 kPa when dry. Retreat rates for the face of the 1.5-m-high bluff are estimated using repeat aerial images collected from 2010 – 2014; these rates are 9.39 m/yr, 4.63 m/yr, and 3.73 m/yr. Retreat rates are also measured monthly using erosion pins. Monthly rates are 0.009 m/month and 0.053 m/month. Extrapolated over one year these rates equal 0.114 m/yr and 0.644 m/yr. The platform has a characteristic basinward dip between 1 and 1.5 degrees. Depending on the location, the platform may include centimeter – decimeter steps associated with discrete beds of varying strength in the mudstone or slope-parallel runnels with 0.05 – 0.10 m spacing and 0.03 – 0.05 m relief. All of these morphologies are produced by shell hash and concretion tools that abrade the mudstone within the zone of swash and backwash. Focused abrasion by shell and sediment tools leads to undercutting and ultimately failure of the bluff, produces the runnels, and grinds small potholes. These erosional processes are shut off when sections of the beach become covered with a layer of sand of sufficient thickness; its aerial coverage varies from month to month. We will examine how the widely variable rates of shoreline retreat and mudstone erosion are jointly controlled by changes in sand coverage and wave intensity associated with storms and cold fronts.