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Uppermost Pleistocene Banks Along the South Texas Shelf Edge: A Clear Case of Drowning Based Upon Their Morphologies

Abstract

A series of ten drowned banks occur along the south Texas shelf edge. These banks have been studied for quite some time but it was not until the mid-1970's when their overall morphologies were roughly determined through 2D profiling. In September 2012, a research cruise on R/V Falkor, funded by the Schmidt Ocean Research Institute, collected high resolution multibeam and high frequency chirp data sets which gave us the opportunity to study these banks in exquisite details using bathymetric maps of less than 0.5 m resolution. Typical morphological features, such as terraces common to most of the banks displaying systematic backstepping, spurs and grooves, and even a micro atoll for Dream Bank, have been identified on the newly acquired bathymetric maps. These morphologies indicate that the drowned banks were classic coralgal keep-up reefs until they synchronously drowned because their crests lie into a narrow 2-3 m water depth range between 58-61 mbsl. Acquired chirp in addition to already existing seismic data sets demonstrate that the full thickness of the banks do not correspond to their exposed 25 m high edifice above the sea floor, because they are partially buried, as much as 20 m, by the Texas Mud Blanket. The 45 m thick coralgal reefs apparently were established at the beginning of last deglaciation and were capable to keep up with sea level rising at rates as high as 40 mm/y during Meltwater Pulse 1A. Surprisingly they ultimately drowned most likely during the onset of Meltwater Pulse 1B when the rate of sea level rise was lower than Meltwater Pulse 1A. The detailed survey of these banks illustrate well how coralgal reefs can be established on lowstand coastal siliciclastic deposits at the onset of the sea level transgression during early deglaciation. Apparently their growth rates were high enough, first to keep up with unusually high rates of sea level rise. Then the occurrence of systematic terraces, flat areas bounded by 2-3 m steep slopes, shrinking in area through time can be interpreted as a response to stress, resulting in typical backstepping morphologies, prior to their ultimate demise.