ADVANCING CHRONOSTRATIGRAPHY AND STRATIGRAPHIC CORRELATIONS THROUGH RE-OS GEOCHEMISTRY

Rhenium-osmium geochronology provides radiometric ages for shales and argillaceous limestones with ~1% or more total organic carbon provided they have not experienced post-depositional oxidation. These volumetrically significant rocks in the sedimentary record are commonly fossiliferous, permitting correlation of biostratigraphy with radioisotope geochronology. The geochronology, in turn, permits correlations where fossil assemblages are lacking or do not overlap. Moreover, organic-rich sedimentary rocks contain geochemical signatures that reveal paleoenvironment. Thus, even where solid biological indicators are lacking, paleoenvironment can be constrained. Biozones may be time transgressive where habitats are controlled by water depth or temperature. In these cases, Re-Os chronology not only provides time lines within time-transgressive sequences, but also establishes correlations where biostratigraphy is ambiguous.

Importantly, Re-Os geochronology provides age control in Precambrian sections lacking fossils. In the Precambrian, correlation of sedimentary packages and numerical age control depend bracketing key horizons by radiometric dating of detrital or authigenic minerals, volcanic strata, or cross-cutting features. Re-Os geochemistry of organic matter and/or synsedimentary pyrite from black shales has provided us with time pins in sedimentary sections spanning as much as 200 m.y. with no internal age control.

A detailed study of Re-Os data from multiple localities across the Barents Sea and one in Sicily markedly reduces uncertainties on stage boundaries in the recently revised Triassic time scale. Bayesian statistics, applied to dated samples in a known stratigraphic succession, further increases precision. Geochemical proxies from the same samples document fluctuating climatic regimes through the Triassic. Further, Barents Sea Re-Os data provide correlation between the Boreal and Tethyan realms in the Middle Triassic, where ammonite biozones share no common species and age constraints for the Boreal realm were previously unavailable. Several additional examples will be presented to illustrate advances made possible by combining Re-Os geochronology with biostratigraphy and chemostratigraphy.