CHRONOSTRATIGRAPHIC DATING OF CARBONATES BASED ON 87Sr/86Sr RATIO: CASE STUDIES FROM BRAZILIAN SEDIMENTARY BASINS
RENÉ RODRIGUES
PETROBRAS, Centro de Pesquisas, Rio de Janeiro
Presented by ALCIDES N. SIAL
Strontium isotope stratigraphy is emerging as a powerful new chronostratigraphic tool that can be applied routinely to the correlation and dating marine sequences. The past variations of the seawater 87Sr/86Sr with time are related to changes in the relative contribution of mantle Sr input to the ocean and the Sr supply from continental weathering. The average 87Sr/86Sr ratio is 0.718 for Sr derived from weathering of continental crust and 0.703 for that derived from oceanic (mafic) crustal input. Therefore, the Sr isotope ratio in sea water is a balance between these two sources of Sr and is believed to be uniform at any given time, because the residence time of this element is much longer than oceanic mixing time.
Based on these variations of strontium isotope ratio with time, a composite strontium isotopic curve was constructed for marine carbonates of the whole Fanerozoic. The most promising high-resolution intervals of geological time correspond to those portions of the seawater curve that exhibit the greatest 87Sr/86Sr change rates as a function of time.
Two Brazilian case histories are presented, one from shallow water Eocene to Miocene marine carbonates of Cassiporé Basin, and the other from Pennsylvanian to Permian shallow water to restricted marine carbonates and anhydrites of the Amazon and Acre Basins. ( May 18, 2001 )
Publication Dates
-
Publication in this collection
05 Oct 2001 -
Date of issue
Sept 2001
