Carbonate Chemostratigraphy Caveat: Micrite Undergoes Diagenesis

Abstract

Carbon isotope chemostratigraphy using calcite microcrystals (i.e., microcrystalline calcite or micrite) is a technique to correlate strata that otherwise lack more obvious features (e.g., erosion surfaces, microfossils). This technique, whether in pure or muddy carbonates, assumes that the stable carbon isotopic composition of calcite microcrystals retains a primary paleo-environmental signal. Indeed, it is recovery and interpretation of this paleo-environmental signal that is the chief concern of paleo-oceanographers and paleo-climatologists. However, numerous workers have observed from both modern and ancient deposits, from both shelfal and deep marine deposits, from both surface exposures and reservoirs that calcite microcrystal neither forms in the modern ocean nor occurs in a single crystal morphology. From these data, it has been suggested that calcite microcrystals form during diagenesis.

Some workers have correlated the different calcite microcrystal morphologies with diagenetic events (e.g., dissolution, precipitation). Even with the highest spatial resolution techniques (e.g., SIMS, EMPA), direct sub-sampling of calcite microcrystals is currently not possible due to their diminutive size (most are less than nine microns). Cathodoluminescence, a more qualitative imaging technique, reveals that the chemistry of calcite microcrystals can vary over small length scales. Bulk stable carbon and oxygen isotopes have shown evidence of both meteoric carbon and elevated temperatures consistent with deeper burial.

These observations highlight a gap in our understanding of carbonates: calcite microcrystal diagenesis. With permeabilities orders of magnitude smaller and surface areas orders of magnitude larger than coarse-grained carbonates, calcite microcrystals diagenesis should have a different tempo and style. The diagenetic growth of calcite microcrystals with increasing burial of deep marine deposits of coccoliths and foraminifera (chalks) present one important diagenetic model. However, these systems are monotonously low-Mg calcite at deposition and are likely to never experience meteoric water, removing two of the most significant drivers for diagenesis in shelfal carbonate accumulations. We need new models of micrite diagenesis. These models need to explain the observed variation in crystal morphology and chemistry at least in terms of their surface area and permeability.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018