How do thrombolites form? Multiphase construction of lacustrine microbialites, Purbeck Limestone Group, (Jurassic), Dorset, UK

This paper examines how non-marine thrombolites are formed through a complex, multiphase process of microbial framework construction, erosion, cementation, recrystallization and episodes of internal sedimentation. Recognition of such phases of thrombolite construction provides a framework for the interpretation of the fluctuating environmental conditions leading to their formation. Microbialite frameworks are examined in detail from the Purbeck Limestone Group and their affinities and palaeo-environmental significance assessed. Three types of thrombolite, one stromatolite and a leolite are described and interpreted. The thrombolite frameworks include: a peloidal mesoclotted type, a thrombolite constructed by the filamentous alga Cladophorites and a type with concentrically laminated micritic mesoclots. Physical and chemical erosion led to extensive early cavity formation within the frameworks. Early calcite rim cements with associated spherulites then developed over the microbial frameworks and these were reworked into cavities. Frameworks were also replaced by chalcedonic quartz and calcite spherulites. Internal sediments comprise peloids, intraclasts and brackish-water molluscs and ostracods, together with their debris. The thrombolites grew in moderate-energy to high-energy shallow, lacustrine, microbial mounds whereas stromatolites occurred in deeper-water settings. A brackish-water, lacustrine setting is indicated by the preserved macro-biota, microbes, absence of charophytes and syndepositional evaporites, and negative stable carbon and oxygen isotope ratios. Strontium isotopes suggest that the carbonate-rich waters were fed from erosion of Mid-Lower Jurassic limestones on the western basin margin with possible mixing with waters from nearby uplifted Upper Jurassic limestones and with Late Jurassic seawater. The research indicates that non-marine thrombolites have a complex, multiphase origin resulting in a diverse succession of textures and structures relating to microbially induced and influenced construction, dissolution, cementation, recrystallization and mineral replacement which have not been previously recorded and indicate the major differences between marine and non-marine thrombolites.

Automated summary

This research explores the complex, multiphase process of non-marine thrombolite formation, highlighting phases of microbial framework construction, erosion, cementation, recrystallization, and sedimentation. The study provides insights into the fluctuating environmental conditions that lead to the formation of thrombolites. Detailed examination of microbialite frameworks and description of different types of thrombolites contribute to understanding the diverse textures and structures of non-marine thrombolites.