NEOPROTEROZOIC CARBONATE SHRUBS: A BACTERIALLY PRODUCED CAP CARBONATE LITHOFACIES

The Noonday Dolomite, a Neoproterozoic cap carbonate overlying glacial deposits in Death Valley, California, contains an abundant shrub-like lithofacies, a previously undescribed cap carbonate texture. The Noonday carbonate shrubs range from 1 to 3 cm in height and 0.5 to 1 cm in width. They are composed of micrite microclots (averaging 100 micrometers in diameter) and are encased by early marine cements. The shrubs have a central region from which "branches" diverge. The "branches" are composed of "leaves", which are aggregates of micrite clumps. Shrubs are commonly oriented parallel to each other and perpendicular to the substrate with their branches radiating upwards. Using modern and ancient carbonate shrubs as analogues, we interpret the Noonday Dolomite carbonate shrubs to be biologically induced; the Noonday shrubs were once composed of bacteria that facilitated the precipitation of micrite in the immediately surrounding microenvironment. Analogues for the Noonday Dolomite shrubs include the microfossils Epiphyton, Renalcis, and Frutexites and bacterial carbonate shrubs found today in environments characterized by high alkalinity, high pH, and CaCO₃ supersaturation (hot springs and alkaline lakes). This is the first identification and documentation of a discrete, recognizable, bacterially constructed lithofacies in a cap carbonate, as most cap carbonate studies have highlighted inorganically precipitated phases.

The presence of carbonate shrubs in post-glacial oceans supports previously published hypotheses regarding paleoenvironmental conditions during and after low latitude glacial events. Oceanic alkalinity must have been very high in post-glacial oceans in order to accumulate cap carbonates globally. Thus, post-glacial conditions would have been ideal for carbonate shrub formation and we predict that bacterially produced carbonate shrubs may be more common in post-glacial cap carbonates than preciously recognized.