CALCIUM CARBONATE SEAFLOOR FANS FROM NEOPROTEROZOIC AND LOWER TRIASSIC STRATA, DEATH VALLEY REGION: A SNOWBALL'S CHANCE?

Seafloor-precipitated calcium carbonate fans (pseudomorphs after aragonite), exceedingly rare in post-Paleoproterozoic time, make a dramatic reappearance in the post-glacial cap carbonates associated with Neoproterozoic low latitude glaciation (snowball Earth) and in Lower Triassic strata around the world. Their presence is commonly interpreted to indicate elevated seawater alkalinity; the source of the anomalous alkalinity has been a critical, much-debated component in competing snowball Earth hypotheses and may also be important with regard to the prolonged recovery interval following the end Permian extinction events. Small seafloor fans are present in the Neoproterozoic Rainstorm Member of the Johnnie Formation, Death Valley region. The fans were deposited on the shelf during transgression and record negative d¹³C values as do all known cap carbonates, but they do not rest on a known glacial deposit. Similar seafloor fans occur in the Spathian (uppermost Lower Triassic) slope-basin facies of the Union Wash Formation in the Death Valley region and in other Lower Triassic formations around the world. The Union Wash Formation examples also record highly enriched d³⁴S values, as do many Neoproterozoic cap carbonates. While it is possible that the Neoproterozoic examples could be associated with glaciation where the glacial deposits are not preserved, the Triassic examples are clearly not related to glaciation. Therefore, processes independent of glaciation may form cap carbonate-like facies and isotopic anomalies, and caution must be exercised when interpreting the significance of seafloor precipitates in association with snowball Earth events.