Abstract: Radiaxial Calcite in Space and Time: Clues to its Origin and to Global Change
BEALL, JENNIFER L. and BRENDA L. KIRKLAND, The University of Texas at Austin
To determine the distribution of radiaxial calcite through time, we surveyed 650 thin sections of carbonate rocks in the University of Texas. In Lower Paleozoic (Cambrian and Ordovician) and Cretaceous samples, radiaxial calcite is abundant and occurs in a variety of environments from deep to shallow. In Upper Paleozoic (Mississippian, Pennsylvanian, and Permian) samples, radiaxial calcite occurs preferentially in biolithite. Lower Paleozoic and Cretaceous radiaxial calcites probably formed as a result of a shifting of calcium carbonate lysoclines by the combined effects of high sea level, high PCO2, warm “greenhouse” conditions, and ocean chemistry enriched in Ca2+ and depleted in SO42-, Mg2+, and HCO3- which promoted formation of radiaxial calcite in shallower waters. The correlation of radiaxial calcite abundance and calcite seas suggests that radiaxial calcite is a primary, low Mg,marine calcite.
Radiaxial calcite is known to have variable Mg content. The association of Late Paleozoic radiaxial calcites and basin-margin biolithite suggests that they may have formed in association with upwelling or overturn of anoxic, alkaline, basinal waters.
We conclude that radiaxial calcite forms preferentially in waters with high PCO2 and possibly low oxygen. However, the chemical factors which control the formation of the unique set of features characteristic of radiaxial calcite are likely separate but related to the factors controlling Mg content. Although the Mg content of such calcites influences their stability, the information about the environment of precipitation (PCO2, for example) remains recorded by the radiaxial character.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah