See more from this Division: Gulf Coast Association of Geological Societies
See more from this Session: Gulf of Mexico Coastal Plain Paleontology
Abstract:
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.
See more from this Division: Gulf Coast Association of Geological Societies
See more from this Session: Gulf of Mexico Coastal Plain Paleontology