Abstract: Temporal Trends in the Mineralogy of Phanerozoic Micrite Precursors

Zakaria Lasemi, Philip A. Sandberg

Samples from over 250 micritic limestone units worldwide representing all Phanerozoic systems were subjected to a detailed scanning electron microscopy and elemental-isotopic analysis. Two major types of micrites are recognized: (1) an aragonite-dominated precursor type characterized by high-Sr microsparites (usually 5-18 microns) with pitted crystal surfaces that may contain aragonite relics, and (2) a calcite-dominated precursor type with low-Sr, unpitted, inclusion-free, finely crystalline calcite (usually <4 microns). Sr/Mg plots and stable isotope values (C, O) suggest that calcites were not usually low-Mg calcite-dominated.

These micrite types are not uniformly distributed throughout the Phanerozoic. Rather, they show an oscillatory trend similar to that suggested for the Phanerozoic inorganic carbonates. Micrites with aragonite-dominated precursor mineralogy occur mainly during emergent plate tectonic episodes (Cambrian, Pennsylvanian-Triassic, Neogene). Micrites with calcite-dominated original mineralogy, on the other hand, occur mainly during submergent plate tectonic episodes (Jurassic-Paleogene, Ordovician-Mississippian). Silurian micrites, however, diverge from this general trend for reasons as yet unclear. The results of this study show that micritic limestones can provide useful information concerning the physical-chemical conditions of Phanerozoic oceans.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994