DETAILED BATHYMETRIC SURVEY OF UPPERMOST PLEISTOCENE DROWNED BANKS ALONG THE SOUTH TEXAS SHELF EDGE: A GLIMPSE AT THEIR GROWTH AND DEMISE DURING LAST DEGLACIATION

Short-lived drowned coral reefs along modern shelf edges, such as the drowned coralgal banks on the South Texas shelf, become opportune time windows to understand better sedimentary processes and environmental conditions necessary for reef establishment, rapid development, and early demise along mixed siliciclastic carbonate margins. Moreover, their detailed morphologies, once paired with future sampling, will encapsulate in great details unique information on the timing, amplitude, and rates of sea level transgression during last deglaciation.

Although the existence of these drowned banks has been known for quite some time, it was not until the mid-1970’s when their overall morphologies were determined via 2D profiling. Promising results of a 2006 high resolution bathymetric survey of only a few banks begged further for additional 3D mapping of all the South Texas banks. Extensive bathymetric data sets were collected in September 2012 during a two weeks-long research cruise, funded by the Schmidt Ocean Institute, on the R/V Falkor equipped with a Kongsberg EM 710 .5x1 degree multibeam 100 kHz sonar, yielding resolution of less than 0.5 m.

Initial results confirm that the crest of most drowned banks fall in a narrow depth range from 58 to 61 m and therefore point to their synchronous demise. As well established through seismic imaging, the depths of their exposed bases on the modern sea floor, ranging from 75 to 85 m, do not correspond to the bases of their initial growth, because the lower third to half of the drowned coralgal edifices has been buried by the Texas Mud Blanket since ~8,000 years BP. Moreover, the ultra-high resolution bathymetric maps display stacked 2-4 m high terraces shrinking with time, representing typical backstepping morphologies of coralgal banks struggling to keep up with fast sea level rise. Sets of elongated channels, a few meters wide and deep, separated by ridges, are found oriented ~90⁰ relative to the terrace edges of some banks. Those features would most likely correspond to spurs and grooves, common to many modern coral reef high energy margins. Detailed analyses and interpretation of these drowned bank high resolution bathymetric maps are expected to yield a wealth of information in terms of transgressive coralgal reef evolution and modes of ice sheet melting during last deglaciation.