Carbon and strontium isotope record on the vendian-tommotian transition in the NW Argentina

Abstract

Carbonates of the Las Tienditas Fm., Salta province, recorded the Vendian-Tommotian transition in the top of this carbonate sequence, while carbonates of La Laja Fm., San Juan, recorded this transition next to the base of the Formation. C, 87Sr/86Sr, Si, Mg/Ca, Mn, Fe and Si chemostratigraphic profiles in the uppermost portion of Las Tienditas are very similar to those for the lowermost portions of La Laja Formation, suggesting synchronous deposition. C values ( 0 º/ooPDB) in carbonate lenses of the Caucete Group, Pie-de-Palo complex, coupled with high 87Sr/86Sr (0.709 to 0.710) and presence of Vendian ichnofauna suggest that sediments of this Group were, perhaps, deposited synchronically with carbonates of the La Laja Fm. The high 87Sr/86Sr for all of the Formations studied resulted from the nature of the continental crust (high Rb/Sr, or very ancient crust, or both) being eroded during the time of deposition of these marine carbonates. The moderate shift of C from +1 º/ooPDB to -2 º/ooPDB in this Vendian-Tommotian transition contrasts with strong shifts observed in most localities in the world. Similar behavior has been observed in Siberia and a shift from +5 º/ooPDB to +2.7 º/ooPDB was recorded in Bhander and Sirbu limestones, Vindhyan basin in India. These observations demonstrate a nonuniform isotopic behavior of the Vendian-Tommotian transition, at least, for continents that once were part of Gondwanaland. — ( May 18, 2001 )