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

Linda J. McCall¹, James Sprinkle², and Ann Molineux^3
1Paleontological Society of Austin, Austin, TX
²Department of Geological Sciences, Jackson School of Geosciences, Austin, TX
³Non-vertebrate Paleontology Laboratory, Texas Natural Science Center, Austin, TX

Access road construction on Interstate 35 in Georgetown, central Texas, exposed a rich and spectacularly preserved fauna within a thin karst zone in the Lower Cretaceous Edwards Formation, originally near the edge of the ancestral Gulf of Mexico. Altered calcified fossils occurred in red clay-filled cavities of a discrete 20 cm thick layer about 4 meters below the ground surface. More than 90.5 kg, including 15,000+ well preserved macrofossils and a large number of unpicked microfossils (<2 mm), were collected from this monopleurid zone by 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 partially cemented by spar. Current mineral composition of both fossil and matrix is altered calcite, breaking along rhombohedral cleavage planes with fracture surfaces cutting through both matrix and fossils. The slightly different mineral content of the fossils vs. the matrix allowed them to weather out intact, although many fossils found near the center of pockets have chemically etched surfaces. The 90.5 kg sample yields a mollusc-dominated fauna consisting of 56% bivalves (36% rudists, 3 species [mostly Monopleura], 20% other bivalves, 26+ species); 36.5% gastropods (49+ species), a scaphopod species; and 7% bryozoan/algae (2+ species). The remaining <1% of the sample is made up of scleractinian corals (3 species), 2-3 mm-sized foraminifera (5+ species), calcareous algae (2+ species), small crustacean claws, serpulid worms, elongate barnacle and clionid sponge borings, two pycnodont teeth, and several unidentified fossils. Some of the 100+ cavity taxa appear new. Significantly absent or rare are oysters, echinoderms, ammonoids, nautiloids, and brachiopods, indicating an unusual depositional environment, perhaps abnormal salinity and/or very shallow water conditions.

AAPG Search and Discover Article #90085 © 2008 GCAGS 58th Annual Meeting, Houston, Texas