Karst Development and Re-Inundation by Suboxic Fluids of the Ediacaran Basal Ara Carbonate, South Oman Salt Basin

Recognizing karsted ancient carbonate strata is critical to properly understand stratal geometries for basin analysis and hydrocarbon system development, correlate sedimentary units, and interpret geochemical data used for paleoenvironmental proxies and chemostratigraphic relationships. However, most karst models are based on long-lived, regional Phanerozoic supersequence boundaries and diagnostic criteria for shorter-lived Precambrian karsts are less well developed. We examine intraformational dolomite breccias in three cores of the Ediacaran Birba Formation, subsurface South Oman Salt Basin that we interpret as exposure and karst surfaces in the basal carbonate unit of the Ediacaran-Cambrian Ara Group (Basal Ara Carbonate, BAC). Karst breccias developed during BAC exposure are differentiated from depositional breccias in supratidal and subtidal facies by core and petrographic observations. Petrographic fabrics, C, S, and O stable isotopes, and chemical imaging of breccia indicate that karsting was followed by re-inundation of the BAC by suboxic fluids. Widespread Fe and Mn-poor calcite cementation by oxic meteoric fluids like those commonly seen in Phanerozoic karsts were not observed, suggesting that accommodation controls and oxygen content of water during Ediacaran karsting of the BAC differed from many younger karsts. The impact of exposure surfaces and karst-developing events on Precambrian stratas composition and texture, and the petrogenetic evolution of porosity and permeability, may therefore differ as well.