Ballography: A Billion Nanosecond History of the Bee Bluff Impact Crater of South Texas

Study of an area of disturbed geology south of Uvalde, Texas in Zavala County has resulted in evidence for a meteorite impact site.* The local sedimentary geology, with an approximate two-to-three m thick hard cap of sandstone and siltstone, a two cm layer of hydrous iron-oxide, and a deep, soft-calcareous silt, dominates both residual site characteristics and impactite samples. A crater of about 1.6 km in diameter and about 10 m in depth is evident from 1942 aerial photographs. Loose silt debris flow and deposit of the prompt venting of the silt is evident from a wake deposit behind local uplifted rocks and throughout the region. A suite of samples obtained in the area shows evidence for high-pressure shock modification. Impact modification of the hydrous iron-oxide provides expression for complex shock-processes in these materials. Especially important to the scientific documentation is discovery of a 300 kg rock `The Uvalde Crater Rosetta Stone' that contains numerous features of the shock modification in a configuration that can be directly related to the target configuration and the impactite collection. Calculations show that the impact of an approximate 60 m diameter iron meteorite at a velocity of 14.2 km/sec with an impact pressure of 350 GPa produces a crater diameter of about 2 km. In the present thin-target configuration the meteorite quickly penetrates the hard rock with release waves from the bottom resulting in a prompt bottom-up rather than the usual top-down pressure release and melt. Such upward high pressure-temperature flow results in prompt melting and radial flow along the iron meteorite interface as evident in residual impactite instability patterns and the presence of Uvalde suevite. * R. A. Graham and W. F. Wilson, Lunar Planet. Sci., 2005, 1086.pdf