Sedimentology of the Cretaceous-Paleogene Boundary Interval in the Tornillo Group of West Texas
Cobb, Jacob D D
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ABSTRACT A recently discovered outcrop of the Tornillo Group at Rough Run Creek in Big Bend National Park provides the best exposure of the Cretaceous/Paleogene (K/Pg) boundary interval thus far known in the Big Bend region. An unusual conglomerate bed is exposed here, and vertebrate fossils found above and below the conglomerate, as well as charophyte oogonia found in the bed itself, indicate that it was deposited at or around the time of the K/Pg boundary. The unusual conglomerate appears to occupy the base of a broad paleo-valley incised into underlying strata. The conglomerate bed is generally less than a meter in thickness, but includes very large boulders, and is generally much coarser grained than conglomerates found in typical fluvial channel deposits in the Tornillo Group. Its composition is also unusual, and consists of rounded sandstone clasts, abraded pedogenic carbonate nodules, mudstone ‘rip-up’ clasts, and charcoal, all derived from local sources. The conglomerate was deposited during or shortly after motion on a small fault that was buried during sedimentation, and at least three successive episodes of sediment transport occurred under rapid (upper flow regime) conditions, with clasts transported only a short distance prior to deposition. Sand grains from the matrix of the conglomerate and from a sandstone layer at its top include quartz grains with planar deformation features, and spherules with ‘splash’ and ‘ablated’ morphologies. These unusual aspects of the conglomerate suggest that it may record the Chicxulub impact event. The very coarse conglomerate reflects a series of brief, high velocity flow events, perhaps recording the transit of tsunami waves upstream several hundred kilometers from the coastline at that time. If this interpretation is correct, then the conglomerate bed may provide the first documentation of this event in a proximal (within several thousand km from Chicxulub impact) non-marine setting. Other well studied proximal K/Pg boundary sites around the Gulf of Mexico and Caribbean are exclusively in marine deposits. Other known continental K/Pg boundary sites are in distal settings where only a thin atmospheric fall-out layer is recorded. Further detailed studies of the chemistry, mineralogy, and precise age of the conglomerate are necessary to determine whether it actually records the K/Pg boundary event.