EXCEPTIONAL OUTCROP EXPOSURES OF ANTIDUNES IN THE WIDESPREAD CORBULA “MARKER” BED OF THE GLEN ROSE FORMATION, LOWER CRETACEOUS, CENTRAL TEXAS: PATTERNS INDICATIVE OF TSUNAMI DEPOSTION

The Corbula bed was deposited in a wide backreef lagoon. It is a multibed form only +/- 1 m thick and yet covers an area of 13,000 km². The lower half is <0.5 m thick and can be observed in various outcrops covering at least 4,000 km². In outcrops, it is this lower half that contains antidunes, related supercritical flow structures, and most of the Corbulid clam fossils. Altogether it is a packstone, or prior to lithification, a medium-very coarse carbonate sheet sand. Old cursory studies, decades prior to the knowledge gained from the geologic work of modern tsunamis, thought that the “ripples” of the Corbula bed might have a tidal origin. But the extent is too large.

The sedimentary structure of the “ripples” is the characteristic mound shape, slightly asymmetrical with a longer stoss side. Wavelengths of 30-60 cm and wave heights of 2-5 cm match field and flume studies of antidunes. Field examples along with polished samples show lenticular structures with concave-up erosional troughs with low angle cross-beds that dip up-current (i.e., backset beds), which give way to a central swelling vortex. Across the top of the mound are imbricated Corbulid steinkerns which dip up-current within down-current dipping lamina on the lee side. Their elongate shells, the size of granules to small pebbles up to 7 mm in length, align parallel with flow. To shear granules off the seafloor requires bottom velocities approaching 2 m/s. Tsunamis are turbulent flows that induce strong traction currents and much faster suspended loads.

The stratigraphy and flow reversals of the Corbula bed fit the pattern of tsunami deposits. The most prominent antidune bed indicates that the paleoflow moved towards the northeast (NE) from the southwest (SW) completely opposite of the flow in the lagoon prior to Corbula bed deposition. This current created an antidune bed at least 90 km long (SW to NE) and 40 km wide. As the tsunami waned a thinning and fining sequence was deposited above the antidunes, a pattern observed in other known tsunami deposits. A debris flow with poorly sorted Corbulid steinkerns occurs in the SW part of the study area. It is directly associated with the antidunes and has load features and sand volcanoes formed by liquefaction on a low angle slope of <0.05^o. Submarine earthquakes emanating from tectonic features towards the SW possibly triggered the event.