Abstract
Sedimentary facies and stratigraphic architecture of non-marine carbonates are controlled by a range of environmental parameters, such as climate, hydrology and tectonic setting, but the few published facies models do not account for this variability. Outcrop and petrographic observations from the Mupe Member of the Purbeck Limestone Group (Upper Jurassic–Lower Cretaceous) in Dorset, southern England, are the basis for new depositional models of non-marine microbialites and associated carbonates in an extensional basin. Ten facies are defined, described and grouped into five facies associations. The Mupe Member is characterised by accumulation of in situ microbial mounds developed around tree remains preserved as moulds and silicified wood. Mounds occur within three stratigraphic units, separated by three palaeosoils, characterised by less-porous, bedded, inter-mound packstone–grainstone that commonly onlap mound margins. Mounds are developed mainly in the shallowest areas of the lake, as indicated by their shapes, facies relationships and association with palaeosoils. These microbial mounds are compared to modern (Laguna Bacalar, Mexico and Great Salt Lake, Utah, USA) and ancient (Eocene Green River Formation, Uinta Basin, Utah, USA) analogues to assess their value as palaeoenvironmental indicators. Facies transitions indicate an earlier, brackish-water lake and a later hypersaline lake for the Mupe Member, both within a semi-arid climate setting in an extensional basin. The fact that the microbialites are covered by evaporitic strata, together with sedimentological, palaeontological and stable isotope data, suggest that there was a sharp change from through-flowing brackish-water, to a closed hypersaline, lacustrine system.
Highlights
Non-marine carbonates accumulate in basins where the deposition and facies distribution are controlled by a wide range of external factors such as climate, tectonics, source area lithologies, drainage systems and sediment input
Jahnert and Collins (2011, 2012) studied the microbial deposits in modern-day hypersaline marine Shark Bay (Hamelin Pool, Western Australia) and Chidsey et al (2015) studied the ancient microbial deposits of the Green River Formation (Uinta Basin, Utah, USA) compared to its modern analogue, the Great Salt Lake (Utah, USA). Both studies showed an environmental control on microbialite development and demonstrated that a specific internal fabric reflects a specific water depth and a specific margin type, with stromatolites found in the shallowest part and in low gradient margins and thrombolites in deeper waters and in steeper gradient margins
This study demonstrates the importance of studying the details of facies and their transitions for the interpretation of shallow lacustrine deposits
Summary
Non-marine carbonates accumulate in basins where the deposition and facies distribution are controlled by a wide range of external factors such as climate, tectonics, source area lithologies, drainage systems and sediment input. Changes in lacustrine depositional environments and facies transitions can occur rapidly or over longer periods of time, both laterally and vertically, depending on the external controls involved (Platt and Wright 1991; Carroll and Bohacs 1999; Bohacs et al 2000). The nature of such transitions can be gradual and interbedded or abrupt, within a conformable vertical succession and closely linked to lake-level fluctuations and/or climate/drainage system changes (Carroll and Bohacs 1999). Of particular importance for this study, Bohacs et al (2000) proposed that transitions between lake types appear to be
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
