Anachronistic and unusual carbonate facies in uppermost Lower Triassic rocks of the western Balkanides, Bulgaria

Abstract

Anachronistic and unusual carbonate facies (AUCFs) are identified in four localities with exposed Spathian strata in the western Balkanides. These include thin-bedded micritic limestone, flat-pebble breccia/conglomerate, mud-chip conglomerate, limestone-marl ribbon rock, vermicular limestone, and microbial oolite. Their depositional and/or early diagenetic origin is interpreted on the basis of petrographic characteristics, results from previous studies, and comparison with analogues from the geological record. Various controlling factors are distinguished in the context of their relative influence on global, regional, or local scale, i.e., environmental conditions (high degree of CaCO3 supersaturation, fluctuations in oxygen levels and salinity), biological controls (bioturbation, microbial blooms, scarcity or abundance of metazoans), and uniformitarian sedimentary processes (wave agitation, storm action, terrigenous input, seismic shocks). Most of the AUCFs are assigned to features associated with enhanced CaCO3 precipitation, while the vermicular limestones belong to fabrics that formed due to limited biologic activity. The thin-bedded micritic limestones, flat-pebble breccias/conglomerates, and limestone-marl ribbon rocks represent anachronistic facies, while the remaining AUCFs are regarded as unusual sedimentary features and fabrics. This study reports a new occurrence of diverse Spathian AUCFs formed in subtidal settings besides those described from the southwestern USA and south China. The results show that anomalous paleoceanographic conditions for carbonate sedimentation persisted locally in the shallow Western Tethys until late Early Triassic time.