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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions Vol. 58 (2008), Pages 683-694

Spectacularly Preserved, Mollusc-Dominated Fauna from a Cavity Layer in the Lower Cretaceous Edwards Formation, Central Texas

Linda McCall1, James Sprinkle2, and Ann Molineux3

1Paleontological Society of Austin, P.O. Box 90791, Austin, Texas 78749

2Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712

3Non-vertebrate Paleontology Laboratory, Texas Natural Science Center, University of Texas at Austin, 10100 Burnet Rd., Austin, Texas 78758

ABSTRACT

Access road construction along Interstate Highway 35 in Georgetown, Central Texas, exposed a rich and spectacularly preserved fauna within a thin karst zone in the Lower Cretaceous Edwards Formation near the ancestral Gulf of Mexico margin. Altered calcified fossils occurred in red, clay-filled cavities of a discrete 20 cm (7.9 in) thick layer about 4 m (13 ft) below the ground surface. More than 90 kg (198 lb), including 15,000+ well-preserved macrofossils and a large number of unpicked microfossils (<2 mm [<0.01 in]), were collected from this monopleurid zone by the senior author (L. McCall) in mid to late 2006 before the area was covered over by concrete. Many specimens are complete and beautifully preserved, retaining growth lines and delicate ornament. Most of the original organisms were aragonitic and replaced by single crystals of sparry calcite soon after burial. The surrounding matrix is also partially cemented by the spar. Current mineral composition of both fossil and matrix is calcite with relic dolomite rhombs. The calcite monocrystals break along rhombohedral cleavage planes and fracture surfaces cut through both matrix and fossils. The slightly different mineral composition of the fossils versus the matrix allowed them to weather out intact, although many fossils found near the center of pockets have chemically etched surfaces.

The mollusc-dominated cavity fauna consists of 100+ taxa, some of which appear to be new. Significantly absent or rare are oysters, echinoderms, ammonoids, nautiloids, and brachiopods, indicating an unusual depositional environment, perhaps with abnormal salinity and/or very shallow-water conditions.

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