ELECTRON MICROPROBE ANALYSIS OF PALEOCENE AND EOCENE ECHINOID CALCITE, WESTERN GULF OF MEXICO COASTAL PROVINCE

The tests of living echinoids are composed of a large number of individual plates. Each plate is composed of an intricate open meshwork structure of high-Mg calcite termed stereom. Fossil echinoid calcite is nearly always recrystallized to low-Mg calcite that is further encased in syntaxial overgrowths of low-Mg calcite. Only rarely are there reports of original stereom microstructure (with no overgrowths or recrystallization) in fossil echinoids. In Texas, Paleocene and Eocene clay-rich deposits of the Gulf Coast province contain fragile echinoid fossils that have the original stereom preserved. A series of these fossils from the Wills Point (Paleocene), Reklaw, Weches and Cook Mountain (Middle Eocene), and Caddell (Upper Eocene) formations have been studied using scanning electron microscopy (SEM) and electron microprobe (EMP). Evidence that the skeletal material is unaltered is provided by the composition the calcite, which contains approximately 10 mole % MgCO3 – comparable to concentrations found in modern echinoids from the same families. Sea surface temperatures can be estimated from Sr/Ca ratios preserved in the calcite. These can be used to model temperature trends in the western Gulf of Mexico during the Paleocene and Eocene, a period during which the global climate was changing from greenhouse to icehouse conditions.