2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 5
Presentation Time: 9:00 AM

Chemostratigraphic Correlation of Terminal Neoproterozoic Units from Caborca, Mexico

LOYD, Sean J.1, CORSETTI, Frank A.2, HAGADORN, James W.3, KAUFMAN, Alan J.4, LYONS, Timothy W.5 and MORALES, Ariel B.3, (1)Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089, (2)Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, (3)Department of Geology, Amherst College, Amherst, MA 01002, (4)Geology Department, University of Maryland, College Park, MD 20742, (5)Department of Earth Sciences, University of California, Riverside, CA 92521, loyd@usc.edu

At least two prominent negative carbon isotope excursions have been recognized globally in Ediacaran successions between ~580 Ma and the Precambrian-Cambrian boundary. Here, we present geochemical data from Neoproterozoic units near Caborca, Sonora, Mexico, to compare to the global record. Two negative carbon isotopic excursions occur in the Caborca succession; the lower excursion occurs in the Clemente Formation and reaches an isotopic nadir of –10‰ VPDB, whereas the upper excursion occurs in the La Cienega Formation and begins within strata containing Cloudina. The overlying members of the Puerto Blanco Formation contain the Precambrian-Cambrian boundary-marking trace fossil Treptichnus pedum. δ34S of carbonate-associated sulfate (CAS) from the lower excursion displays a negative shift from 20 to 12‰ VCDT and an increase in CAS concentration, concomitant with the carbon isotopic excursion. δ34SCAS values rise to over 30‰ in the strata between the two carbon isotopic excursions.

The lithologic, isotopic and CAS concentration trends are quite similar to the Death Valley Succession, located ~700 kilometers to the northwest, although the absolute value of δ34SCAS between Caborca and Death Valley is slightly different. Sulfur isotope data from Oman and China do not match the trends recognized in Death Valley and Caborca, nor do they agree with one another. The differences in δ34SCAS among these sites suggests that such proxies can be influenced by regional and/or diagenetic processes and do not necessarily reflect a global oceanic sulfate isotopic signal.

The cause(s) of these sulfur and carbon perturbations is unknown. One possibility is that an increase in terminal Neoproterozoic oceanic O2 (or alternate oxidizing compound) caused the oxygenation of isotopically-light, reduced phases of sulfur and carbon. An increase in oceanic oxygen was likely a necessary threshold to overcome in order to allow the proliferation of more complex life forms during the Cambrian Explosion.