Microbial mound origin for enigmatic, sea-floor, circular structures? Purbeck Limestone Group, offshore Dorset, U.K.

This paper describes and interprets diver-collected, offshore samples to establish the origin of enigmatic large (100–200 m across) circular, dome-shaped features imaged using Multibeam Echo-Sounding (MBES) on the sea-floor of Weymouth Bay, Dorset, U.K. The structures occur within the Durlston Formation of the Purbeck Limestone Group (Late Jurassic–Early Cretaceous) that accumulated in a lagoon of variable salinity. A previous morphological study using MBES images alone led to four possible hypotheses for their origin; as isolated erosional remnants, as evaporite-related diapiric structures, as periclinal folds, or as eroded carbonate mounds that grew within the Purbeck lagoon. The petrographic study of seafloor samples taken from the centre of these structures results in their classification into nine sedimentary facies; eight limestones and one chert. The most abundant of these facies are similar to the well-known Purbeck limestones outcropping in nearby cliff sections, however four out of the nine facies have previously unrecorded microbialite components (intraclasts of travertine, stromatolites, laminated filamentous mudstones, and post-depositional, cavity-lining endostromatolites). This petrographic analysis suggests a microbial carbonate mound origin for these structures that is also supported by their morphology, their restricted occurrence palaeogeographically and stratigraphically to within the Purbeck Limestone, and the occurrence of microbialites at this level in onshore outcrops. Carbonate mounds of this size, in a lagoonal setting, are previously unknown from the Wessex Basin but show some similarities with Early Cretaceous lacustrine build-ups in South Atlantic offshore basins. The work demonstrates how the interpretation of even an extremely well-known stratigraphy such as that of the Purbeck Group can be limited when only part of the marginal environment is exposed for study.