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Microbialite Morphologies as Tools for Paleoenvironmental Analysis: Lessons From the Great Salt Lake, Utah


Recent studies have refined the morphological gradients found in marginal marine microbialites as criteria for interpreting ancient examples, but such data sets do not exist for non-marine forms. The Great Salt Lake (GSL) in Utah provides the largest natural laboratory for investigating the controls on microbialite morphology, potentially providing models for interpreting ancient examples. This large endorheic lake exhibits distinct gradients in microbialite morphology related to the effects of highly variable lake levels, where lake level fluctuations are the critical control on their preservation and morphology. In littoral or near-littoral zones, higher energy regimes and periodic subaerial exposure of microbialites result in truncation of domal forms, erosional channel incisions, and localized wave-worked debris. In sub-littoral areas, domal forms are prevalent, locally as large complex stratigraphic morphologies with low synoptic relief. Meter-scale forms cover an estimated 1,000 km2 throughout the sub-littoral areas of GSL and occur as intergrown laterally-connected domal forms as well as isolated individual microbialites. Morphologic gradients occur between sub-littoral domal forms and truncated littoral forms in spatially distant areas of GSL regardless of aspect. Similar differences between littoral to sub-littoral microbialites can be seen in other endorheic lakes such as Lake Thetis and Lake Clifton. Examination of changes in microbialite morphology along littoral to sub-littoral gradients in GSL and other endorheic lakes may provide insight into the location and orientation of ancient microbialite deposits in the sub-surface.

In exorheic hardwater lakes with stable lake levels, accommodation space becomes the critical control on vertical growth. Microbialites in the shallow sub-littoral zone accrete to the air-water interface after which the bioherms spread laterally producing mushroom and bench-like morphologies. In deeper sub-littoral waters, sufficient accommodation space allows the development of large pinnacle-like growths and larger coalesced biostromal buildups.