GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 240-2
Presentation Time: 9:00 AM-6:30 PM


NELSON, Lyle L.1, SMITH, Emily F.2 and MACDONALD, Francis A.1, (1)Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, (2)Department of Paleobiology, Smithsonian Institution, PO Box 37012, MRC 121, Washington, DC 20013-7012,

In the southern Panamint Range in the Death Valley region of California, the South Park Member of the Kingston Peak Formation includes <1 km of mixed carbonate and siliciclastic strata deposited during the ca. 666-640 Ma Cryogenian non-glacial interlude between putative Sturtian and Marinoan snowball Earth glaciations. New detailed geologic mapping combined with stratigraphic and chemostratigraphic datasets demonstrate a laterally variable and discontinuous dolomitization front within the upper carbonate unit – known informally as the Thorndike submember – immediately below the Marinoan glacial diamictite. While limestone samples within this unit preserve highly enriched carbon isotope values (~ +4 to +9‰ δ13C) that are typical of the “Keele peak” in upper non-glacial interlude strata globally, dolomite samples within laterally equivalent strata preserve carbon isotope values that are significantly more depleted (~ -4 to +1‰ δ13C).

Three ways of explaining the extreme range in observed δ13C values within the Thorndike include: 1) early-stage diagenetic alteration due to exposure of the carbonate platform to meteoric waters, 2) late-stage orogenic diagenetic alteration due to fluid flow during Cordilleran burial and metamorphism, and 3) primary, marine differences in δ13C values reflecting diachronous deposition of the unit. The integrated mapping, sedimentological, and geochemical results of this study support the first explanation because the isotopic alteration pattern mirrors the mapped dolomitization front. A Cordilleran age for the dolomitization is rejected since other pre-Cordilleran carbonate units in the region are not similarly altered, and since both limestone and dolomite clasts of the Thorndike are preserved within the overlying Marinoan diamictite. Thus, this is the first study to document an early dolomitization front drastically shifting δ13C within carbonate-dominated strata, depleting values by up to 10‰ in comparison to those of stratigraphically equivalent limestone. These results emphasize the need for δ13C chemostratigraphic studies to be firmly grounded in a stratigraphic and geologic context before they are used for correlation to other sections globally and the construction of an age model.