Original paper

Physicochemical precipitation of fine-grained carbonate in seawater - an example of Triassic marine micrites from the Western Balkanides, Bulgaria

Chatalov, Athanas G.

Abstract

The obtained sedimentological data testify to differing mineralogical precursors of the Triassic marine micrites from the Western Balkanides. Thus, the "microsparites", which are largely predominant in the Upper Olenekian to Middle Anisian interval, were consolidated through transformation and cementation of former aragonite-dominated muds. In turn, the real "micrites" are found only sporadically in this part of the Triassic sequence, but occur alone in the Upper Anisian to Lower Carnian interval. They resulted from stabilization of former high-Mg calcite-dominated muds with good preservation of the primary microfabric. All muddy sediments were altered in a partly closed diagenetic system with participation of meteoric water. Several lines of evidence suggest that the major source of pelitomorphic carbonate grains was physicochemical precipitation with possibly variable supply of skeletal fine-grained material. The established irreversible transition in the original mud mineralogy during the Triassic period is ascribed to progressive decrease in the Mg/Ca ratio of ambient water and/or regional or local factors that controlled the degree of seawater carbonate saturation. This trend conforms well to the secular oscillations in the primary composition of Phanerozoic micrites and abiotic carbonates. The obtained results also support the hypothesis for extensive inorganic precipitation of lime mud in the global "aragonite sea" at least during the Early Triassic epoch. This study demonstrates that ancient micrites may provide valuable information on the abiotic primary mineralogy for some stratigraphic intervals that lack oolitic and winnowed carbonate sediments.