CRYOGENIAN MICROBIAL FOSSILS FROM THE KINGSTON PEAK FORMATION

New microbial fossils expand the diversity and range of previously reported occurrences from the Cryogenian Kingston Peak Formation, Death Valley, eastern California. Fossil-bearing, black, microcrystalline chert in dolostone intervals is stratigraphically bracketed by diamictite. Striated clasts below and an apparent lodgment till in depositional contact with the Noonday Dolomite above indicate glaciation. There is little support for an age more precise than probably 750 to 650 Ma.

A microbial-microfacies succession is recorded in relict textures: (1) fragmented microbial mat forms oncoid nuclear bodies, (2) filamentous and coccoid microbes resembling cyanobactia occupy oncoid cortex laminations, (3) vase shaped microfossils, planktonic visitors, are sparsely captured in micritized oncoids, (4) late euendolithic fossils, probable eukaryotes, have modified oncoid grains, (5) filamentous and coccoid fossils populate the oncoid supporting matrix, and (6) microbes occupy fracture porosity associated with karsting.

Large oncoids occur near and at the top of a recognizable carbonate interval, a candidate cap carbonate based on C and O isotopes. Recently located Kingston Range spanning exposures, commonly comprised of oncolite unconformable over underlying silty dolostone, are up to 17.5 m thick compared to typical cap carbonates at <15 m.

Black chert replacing cm sized oncoids situated at the base of the oncolite bed contain the best preserved microbial fossils. Thinsections show that relict carbonate textures record a diagenetic sequence parallel to and consistent with alteration of limestones described by Zempolich (1988) for the Crystal Spring Formation and Corsetti et al. (2003) for the Johnnie Formation. Troxel and others have described a tectonic setting for the Kingston Peak Formation where normal faulting runs concurrently with glaciation and periodic quiet water carbonate shelf deposition. Meteoric water and marine pore water mixing in the subsurface provides a mechanism for early silicification. The preserved microbial detail provides a unique window on life in Cryogenian times.