Preservation of Echinoid Fossils, Paleocene and Eocene of Texas

Louis G. Zachos
Geological Sciences, The University of Texas at Austin, Austin, TX

Echinoid fossils are a minor yet widespread component of siliciclastic marine deposits of Paleocene and Eocene age in Texas. Echinoids are found in the marine siliciclastic units of the Wills Point Formation (lower Paleocene); the Sabinetown Formation (lower Eocene); the Reklaw, Weches, and Cook Mountain Formations (middle Eocene); and the Caddell Fm. (upper Eocene). Spatangoid echinoids, with a burrowing lifestyle, make up almost the entire fauna in clays, silts and fine sands, although the little clypeasteroid Fibularia is widespread in the Eocene section. Sand dollar clypeasteroid echinoids are characteristic of the coarser and calcareous facies of these deposits.

Spatangoid remains in the siliciclastic deposits are often very fragile and can be easily missed by collectors. The preservation, however, is often excellent. Scanning electron microscopy (SEM) and electron microprobe (EMP) analyses demonstrate that these fossils retain the original stereom microstructure and high-Mg calcite mineralogy (5-10 mole% MgC0₃) of the skeleton. This mineralogy is comparable to that of modern spatangoid and clypeasteroid echinoids and is evidence that the calcite has not undergone diagenetic transformation into low-Mg calcite. Preservation of echinoid calcite is best in clay-rich silts and fine siliceous sands, with progressively greater degrees of alteration in sandier sediment, suggesting that low permeability is the controlling factor in preservation. These sediments often contain significant amounts of pelletal “glauconite”, which may constitute more than 90% of the grains. Alteration of the echinoid calcite is indicated by FeCO₃ or MnCO₃ content greater than 0.5 mole%. Echinoids in the calcareous sediments or where calcite cementation is prevalent are usually completely altered to low-Mg calcite, and the stereom textures are filled with syntaxial calcite overgrowths.

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