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Braarudosphaera-Rich Sediments on the Exmouth Plateau, Eastern Indian Ocean

SIESSER, WILLIAM G., Vanderbilt University, Nashville, TN, TIMOTHY J. BRALOWER, University of North Carolina, Chapel Hill, NC, and ERIC H. DE CARLO, University of Hawaii at Manoa, Honolulu, HI

Calcareous nannofossil assemblages enriched in Braarudosphaera occur in lower Oligocene to lower Miocene sediments at Ocean Drilling Program Sites 762 and 763 on the central Exmouth Plateau. Although beds of Braarudosphaera-rich sediments are well known in the Oligocene of the South Atlantic (the Braarudosphaera Chalk), this is the first published report of this phenomenon in the eastern Indian Ocean. Braarudosphaerids appear rather abruptly in the lower Oligocene (in Zone NP 21) on the Exmouth Plateau. They reach their greatest numbers in the lower Oligocene (in Zones NP 22 and NP 23), where they comprise up to 10% of some samples. Braarudosphaera bigelowii is the overwhelmingly dominant species, occurring together with rare specimens of B. discula and Micrantholithus pinguis. The ho ococcoliths Peritrachelina joidesa and Lanternithus minutus are also associated with the Braarudosphaera enrichment. There are two populations of B. bigelowii: one of normal size (10-14 micro m) and one of large specimens (20-22 micro m). The larger braarudosphaerids are more common than the smaller forms. Braarudosphaera-rich sediments are absent at the nearby Wombat Plateau sites during the same time interval. We attribute this to latitudinal control, since the Wombat sites are about 4 degrees north of the central Exmouth Plateau sites. The occurrence of braarudosphaerids on the Exmouth Plateau is related to an Oligocene to early Miocene oceanographic event. Stable-isotope and trace-element analyses suggest that upwelling of cool, low-salinity, nutrient-rich water along a Divergent Zon created the Braarudosphaera-rich sediments in the Indian Ocean.


AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)