TERMINAL NEOPROTEROZOIC-LOWER CAMBRIAN TRANSITION II: WHITE-INYO MOUNTAINS, CA
Biostratigraphically useful Neoproterozoic-Lower Cambrian trace fossils typically occur in siliciclastic-dominated successions (e.g., the GSSP in Newfoundland), whereas small shelly fossils and chemostratigraphic data are most commonly preserved in carbonate-dominated successions (e.g., Siberia and China). The thick, mixed siliciclastic-carbonate succession in the White-Inyo region contains elements of both, and thus may promote correlation between these lithologic "end members". We applied an integrated stratigraphic approach to the White-Inyo succession, combining paleontological and carbon isotope chemostratigraphic data to address the temporal framework within the basin and to help facilitate worldwide correlation of the boundary.
High resolution sampling for carbon isotope chemostratigraphy was conducted though the Deep Spring Formation across the basin (providing ten times the stratigraphic resolution of previous studies). Samples for isotopic analyses were microdrilled from least-altered phases (usually microspar). Most of the Lower Deep Spring Formation, in association with Cloudina, records a positive isotopic excursion to ~+4 per mil PDB (the excursion is progressively truncated in the onshore direction, and reaches only ~ +2 per mil in more onshore sections). A negative excursion, commonly down to ~-5 per mil, is recorded from the top of the Lower Member through the middle of the Middle Member. This negative excursion is most pronounced in the offshore sections where the isotopic compositions plummet to ~-10 per mil. If these samples do not represent a diagenetic overprint, they may confirm the intensely negative excursions noted by others in the terminal Neoproterozoic of the Mackenzie Mts., Canada, and northern Siberia. Curiously, the isotopic nadir is associated with unusually diverse stromatolite development. The remaining middle Deep Spring and Upper Deep Spring record a positive excursion from ~-5 per mil to ~+2 per mil. T. pedum, the trace fossil used to correlate the Precambrian-Cambrian boundary, is found in association with negative isotopic compositions during this excursion, and thus provides a global tie-point to correlate siliciclastic dominated sections with carbonate dominated sections.