Thrombolites, spherulites and fibrous crusts (Holkerian, Purbeckian, Aptian): Context, fabrics and origins

Abstract

Carbonates from the Holkerian (Lower Carboniferous) Concretionary Beds of the Bristol area, the Upper Jurassic-Lower Cretaceous Purbeck Limestone Group of southern England and the Aptian Pre-salt succession of the South Atlantic exhibit spherulites and a range of related crystal shrubs and fibrous crusts. Some of these fabrics are associated with framework microbialites, including thrombolites, that developed under varying degrees of non-marine influence. Brackish or fresh waters affected the Holkerian and Purbeckian microbialites, although the Holkerian also experienced marine conditions. The South Atlantic Pre-salt microbialites were deposited under saline lacustrine conditions. The fabrics are a topic of intense discussion and disagreement over the role (if any) of microbes in spherulite formation. Whilst the case for microbial influences on the Holkerian and Purbeckian spherulites is strong, the alternative influence of Mg-Si gel or Mg-clay (particularly stevensite) is appraised in detail for the Aptian spherulites, including reference to studies of modern microbial analogues. Results of published laboratory experiments concerning such gels and studies on contemporaneous Aptian facies in the same basins lacking Mg-clay, but including spherulites, are considered in an attempt to resolve the abiotic-versus-biotic spherulite origin. That spherulites may be either initiated in suspension or reworked and resedimented provides an added dimension for all three cases. The evidence is strongly in favour of spherulite nuclei being directly created by microbial metabolism but the fibro-radial cortices may be entirely inorganic or passively controlled by microbial extracellular polymeric substances (EPS) which apparently occur as relatively abundant inclusions within the spherulite crystals. The body of evidence suggests that the spherulites and their associated shrubs and crusts are at least partly microbially or biotically influenced.