Transformation of aragonite-dominated lime muds to microcrystalline limestones

Abstract

Scanning electron microscopy of textures in several well-lithified Pleistocene “micrites” (mixed micrite and microspar) from south Florida and the Bahamas shows significant diagenetic alteration of the lime-mud precursors. The dominance of aragonite in those precursor muds is supported by the surprising presence of abundant aragonite relics in the neomorphic calcite crystals (confirmed by Feigl stain, X-ray and electron diffraction). Polished, very lightly etched surfaces of neomorphic calcite crystals, even in the microspar, show aragonite relics and pits, both absent in coexisting void-fill cements. Similarities between micrite and microspar crystals indicate their common origin through a one-step neomorphic process of calcitization. That is, the mud did not first calcitize to micrite and then alter to microspar by aggrading neomorphism. This conclusion has more general implications for the interpretation of the origins of other, older microsparitic carbonates. Both early calcitization and concurrent early meteoric cementation were significant in producing the Florida-Bahamas “micrites” studied, which have low porosities similar to those found in most ancient fine-grained limestones. These low porosities, despite the absence of significant overburden, indicate that major porosity reduction without compaction can occur in calcitized aragonite-dominated muds.