Paleoecologic and biostratigraphic interpretations of late Miocene foraminifera at DSDP site 265 (leg 28), southeast Indian Ocean
Engelhardt, Nancy L.
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The biostratigraphy, paleoecology and systematics of the Miocene benthic and planktonic foraminifera of Deep Sea Drilling Project Site 265 (cores 15 and 16), located in the southeast Indian Ocean, have been examined in detail. At this site the stratigraphic succession consists of a Miocene basalt overlain by 75 m of Miocene calcareous (dominantly nannoplankton) ooze which is in turn overlain by 370 m of Pliocene- Pleistocene siliceous (diatomaceous) ooze. A distinct disconformity marks the boundary between the calcareous and siliceous successions. The foraminiferal fauna obtained from an examination of 12 samples distributed through the 75 m succession of calcareous ooze totals 187 species. Of this total 14 are pelagic species (several taxa are reworked Cretaceous forms), 23 are agglutinated species and 150 are calcareous benthic species. The planktonic component includes a dominance transition up this calcareous sedimentary succession from the warm water Neoglobo- quadrina acostaensis (Blow) to the temperate Globigerina woodi Jenkins and warm water forms of the temperate Globigerina bulloides d'Orbigny. Near the top of the succession, tests similar to the cold water Neoglo- boguadrina pachyderma (Ehrenberg) appear. The benthic component is dominated by Epistominella exigua (Brady), and Globocassidulina subglo- bosa (Brady). Most of the benthic fauna is long-ranging and found in present-day deep-water environments. Benthic and pelagic taxa exhibit varying states of dissolution with the preservation of the benthos being inferior to that of the pel agios. Census studies suggest that most faunas are slightly modified biocoenoses (life assemblages) and that modification stems primarily from dissolution and minor reworking by bottom currents. Two distinctive and probably bi-thermal assemblages contribute to a combined biocoenosis: a planktonic population which lived in near surface waters in temperatures of 8-17°C and a contemporaneous bathyal cold water (0-2°C) benthic fauna. During the late Miocene (10.5-7m.y.) a thermally stratified water column circulated over the Site 265 area. At that time a cold water mass may have generated close to Antarctica in response to early glaciation on the Antarctic Continent. This would have affected the deep-water masses, but during that time there was no influence in the surface waters of the southeast Indian Ocean. The sudden appearance of Pliocene siliceous deposits above the late Miocene ooze points to a decrease in thermal stratification as cold water reached the surface and the Antarctic Convergence moved north of Site 265 in repponse to intensified glaciation of the Antarctic Continent. Up to 3 m.y. of the stratigraphic record is missing across the late Miocene- Pliocene disconformity. It is probable that either a significant amount of late Miocene calcareous sediments was lost to the solution effects of cold undersaturated (with respect to CaCC^) Pliocene waters or the sediments exist in the 28.5 m uncored interval between the siliceous and calcareous sediments. The latter is supported by planktonic biostratigraphy and faunal stability that indicates gradual climatic cooling occurred in the late Miocene.