Demise of an extensive biostromal microbialite in the Furongian (late Cambrian) Chaomidian Formation, Shandong Province, China

Abstract

This study focuses on an extensive biostromal microbialite (10–20 m in thickness and over 6,000 km2 in area) and the overlying grainstones in the Furongian Chaomidian Formation, Shandong Province, China in order to understand the demise of the microbialite. The microbialites are characterized by centimeter-to decimeter-scale maze-like maceria structures and/or chaotic mesostructures. According to the megastructures of microbialites and the proportion of non-microbial carbonate sediment, the biostromal microbialite is generally divided into the lower and the upper parts, separated by a distinct surface. The lower part is laterally continuous and generally flat-bedded, whereas the upper part shows variable domal megastructures and locally co-occurs with abundant non-microbial carbonate sediment. The entire microbialite bed is sharply overlain by grainstone-dominated deposits via an erosion surface which is either irregular with significant relief or flat with hematitic coating. The lower part of the biostromal microbialites with flat-bedded megastructures most likely deposited contemporaneously during sea-level highstand in the early middle Furongian, as evinced by well-correlated flat-bedded units separated by distinct bounding surfaces. The microbialites formed regional topographic variation, generally deepening toward southeast. The flat-bedded microbialites were drowned by subsequent rapid rise in sea level. In the topographic highs, the microbialites caught up with sea-level rise, forming large-scale domal megastructures. In the topographic lows, however, domal microbialites formed together with abundant non-microbial sediment, which were frequently reworked by storm-induced waves and currents. Subsequent deposition and migration of coarse-grained non-microbial sediment during sea-level rise terminated the entire microbialites.