Sr stratigraphy and biostratigraphy for samples from DSDP Hole 32-305 and ODP Hole 123-766A

The strontium isotope ratios (87Sr/86Sr) of marine barite microcrystals separated from Cretaceous sedimentary deposits from Ocean Drilling Program and Deep Sea Drilling Project sites from the Pacific and Indian Oceans have been compared to the composite Sr isotope curve of McArthur et al. The barite in these cores accurately recorded the seawater 87Sr/86Sr ratio, thereby reaffirming the composite Cretaceous strontium curve. Moreover, marine barite is a more reliable recorder of 87Sr/86Sr than is carbonate in sedimentary deposits with high clay content, thereby providing an opportunity for Sr isotope stratigraphy and dating in carbonate-poor or diagenetically altered sections. We have used the barite-derived Sr isotope record to refine the biostratigraphic age models of the sites investigated.

Magnetostratigraphic synthesis of ODP Leg 123 sites

During ODP Leg 123, Sites 765 and 766 were drilled to examine the tectonic evolution, sedimentary history, and paleoceanography of the Argo Abyssal Plain and lower Exmouth Plateau. At each site, the quality of magnetostratigraphic and biostratigraphic records varies because of complicating factors, such as the predominance of turbidites, the presence of condensed horizons, or deposition beneath the CCD. Based primarily on the presence of nannofossils, the base of the sedimentary section at Site 765 was dated as Tithonian. A complete Cretaceous sequence was recovered at this site, although the sedimentation rate varies markedly through the section. The Cretaceous/Tertiary boundary is represented by a condensed horizon. The condensed Cenozoic sequence at Site 765 extends from the upper Paleocene to the lower Miocene. A dramatic increase in sedimentation rate was observed in the lower Miocene, and a 480-m-thick Neogene section is present. The Neogene section is continuous, except for a minor hiatus in the lower Pliocene. The base of the sedimentary section at Site 766 is Valanginian, in agreement with the site's position on marine magnetic anomaly Mil. Valanginian to Barremian sediments are terrigenous, with variable preservation of microfossils, and younger sediments are pelagic, with abundant well-preserved microfossils. Sedimentation rate is highest in the Lower Cretaceous and decreases continually upsection. Upper Cenozoic sediments are condensed, with several hiatuses.