Microstructure and chancelloriid sclerites within a microbial buildup of the Wirrealpa Limestone (Cambrian Stage 4), South Australia

Abstract

Well-preserved, distinctive columnar microbialites were discovered in the Wirrealpa Limestone (Stage 4, Series 2 of Cambrian) of South Australia. However, further studies are still required to unveil the microstructure and formation process of these microbialites. A microbial buildup from the Wirrealpa Limestone examined herein suggests that the microbialitic columns are composed of densely-distributed, polymorphic, and patched fabrics produced by microbes. These fabrics are inter-framed by packstone sediments that characterized by calcareous metazoan skeletons and cerebroid ooids. The patched fabrics can be categorized into three types: 1) laminated ‘patches’, 2) homogeneous microsparitic ‘patches’ (mesoclots), and 3) cemented ‘patches’ containing tightly-packed chancelloriid sclerites. There are two preservational conditions of chancelloriid sclerites. After death of the chancelloriid individuals (belonging to Chancelloria and rare Allonnia) that dwelled among or near the microbial reef, abundant sclerites were exfoliated, transported, and concentrated. Then, some of them were cemented by microbial communities, forming the third type ‘patches’, while others were accumulated in the packstones within reefal cavities and inter-framework space along with many skeletal fragments from trilobites and mollusks. Thus, this lower Cambrian thrombolite-stromatolite complex was formed within an ecosystem lacking archaeocyaths but dominated by microbes (reef builders) and chancelloriids (reef dwellers).