SLUMPING OF LARGE CARBONATE MOUNDS INTO DEEP WATER: A RECORD OF RAPID CARBONATE PRECIPITATION FOLLOWING A NEOPROTEROZOIC DEGLACIATION

Glacially derived diamictites and associated enigmatic carbonates referred to as cap carbonates have been documented on all major continents around the world. Cap carbonates worldwide display unusual facies, strong negative carbon isotopic signals, and sharp stratigraphic contacts suggesting oceanographic conditions unparalleled in the Phanerozoic. Here we support the hypothesis of a rapid icehouse to greenhouse transition as evidenced by rapid carbonate precipitation and subsequent slumping following the deposition of glacial diamictite along a paleo-continental shelf-slope break.

Extensive sequence stratigraphic mapping of the pre-Cambrian lithofacies in the Death Valley region of California has shown distinct unconformities, which are regionally persistent. These unconformities act as timelines and are helpful in evaluating the timing relationships associated with Neoproterozoic glaciations. Peculiar fabrics such as traction lamination, tube structures, sheet cracks, and microbial mounds are observed in the cap carbonate unit and suggest a variety of depositional environments. The basinal facies of the cap carbonate is characterized by ~3 m thick, primarily abiotic, mechanically laminated silty dolomite indicative of an offshore sub-wave base depositional setting. The platform facies consists of a ~3 m thick mechanically laminated dolomite unit at the base of the cap, which grades into large 100+ m thick microbial mounds containing sheet cracks and tube structures in the upper section of the formation. These mounds show no evidence of storm wave bedding and are thus inferred to be below storm wave base as well. In most occurrences the carbonates directly overlie glacial diamictites. The lateral transition between the platform and basin is characterized by a carbonate unit containing (1) clasts of the tube bearing platform facies as well as (2) clasts derived from the glacial diamictite deposited within the laminated silty dolomite of the basin facies. This transition thus represents a zone of instability where slumping of both the large microbial mounds and underlying unconsolidated diamictite occurs at the shelf-slope break. These depositional relationships suggest a rapid build-up of carbonate on the platform immediately following deglaciation in the Neoproterozoic.