Biostratigraphy of selected taxa and radiolarian events in ODP Hole 183-1138A sediments

Well-preserved radiolarian assemblages of late middle Miocene to early Pliocene age are found in Ocean Drilling Program (ODP) Hole 1138A (Cores 183-1138A-12R to 20R), which was rotary drilled into the Central Kerguelen Plateau. The faunas are typical for Antarctic assemblages of this time interval, and the site appears to have been south of the Polar Front during the time period studied. Despite only moderate drilling recovery of the section, most late middle to early Pliocene radiolarian zones are present, although at the sample resolution used, subzones could not be identified. A significant discontinuity in the section is present at the boundary between lithologic Units I and II (between Cores 183-1138A-12R and 13R), corresponding to an interval from at least 4.6 to 6.1 Ma. Mixed late Miocene-early Pliocene assemblages are seen in the base of Core 183-1138A-12R (Sample 183-1138A-12R-3, 20 cm), and the overlying basal Pliocene Tau Zone appears to be absent. It cannot be determined if the discontinuity is due to incomplete recovery of the section and drilling disturbance or if it reflects a primary sedimentary structure - a hiatus or interval of condensed sedimentation.

Geochemistry of basaltic rocks from ODP Hole 183-1137A on the Kerguelen Plateau

Cretaceous basalts recovered during Ocean Drilling Program Leg 183 at Site 1137 on the Kerguelen Plateau show remarkable geochemical similarities to Cretaceous continental tholeiites located on the continental margins of eastern India (Rajmahal Traps) and southwestern Australia (Bunbury basalt). Major and trace element and Sr-Nd-Pb isotopic compositions of the Site 1137 basalts are consistent with assimilation of Gondwanan continental crust (from 5 to 7%) by Kerguelen plume-derived magmas. In light of the requirement for crustal contamination of the Kerguelen Plateau basalts, we re-examine the early tectonic environment of the initial Kerguelen plume head. Although a causal role of the Kerguelen plume in the breakup of Eastern Gondwana cannot be ascertained, we demonstrate the need for the presence of the Kerguelen plume early during continental rifting. Activity resulting from interactions by the newly formed Indian and Australian continental margins and the Kerguelen plume may have resulted in stranded fragments of continental crust, isolated at shallow levels in the Indian Ocean lithosphere.

Geochemistry of ODP Hole 183-1137A basalts

The Kerguelen Plateau and Broken Ridge in the southern Indian Ocean together represent one of the most voluminous large igneous provinces (LIPs) ever emplaced on Earth. A scientific objective of Ocean Drilling Program (ODP) Leg 183 was to constrain the post-melting magma evolution of Kerguelen Plateau magmas. In an effort to better understand this evolution, isotopic and trace element analysis of individual plagioclase crystals hosted within two Kerguelen Plateau basalts recovered from Elan Bank were undertaken. Previous whole-rock studies established that the two host basalts investigated in this study are samples of crustally contaminated (lower group) and relatively uncontaminated (upper group) basalt. Plagioclase phenocrysts from the uncontaminated basalt are dominantly normal zoned and exhibit a 87Sr/86SrI range of 0.704845-0.704985, which overlaps uncontaminated group whole-rock values previously reported. Plagioclase crystals from the contaminated basalt are dominantly reverse zoned and exhibit a 87Sr/86SrI range of 0.705510-0.705735, which all lie within contaminated group whole-rock values previously reported. There are no systematic within crystal core to rim variations in 87Sr/86SrI from either group, with the exception that contaminated group crystal rims have overall less radiogenic 87Sr/86SrI than other zones. These observations indicate that crustal assimilation occurred before the formation of Unit 10 plagioclase phenocrysts, which is supported by parent magma trace element abundance data inverted using carefully calculated partition coefficients. Trace element diffusion modeling indicates that the upper group basalt (Unit 4) experienced a more vigorous eruptive flux than the lower group basalt (Unit 10). We suggest that plagioclase phenocrysts in both the upper and lower group basalts originated from the shallowest section of what was likely a complex magma chamber system. We contend that the magmatic system contained regions of extensive plagioclase-dominated crystal mush. Crustal assimilation was not a significant ongoing process in this portion of the Elan Bank magmatic system. Both basalts exhibit compelling evidence for remobilization and partial resorption of crystalline debris (e.g., reverse zoned crystals, glomerocrysts). We suggest Unit 4 and 10 magmas ascended different sections of the Elan Bank magma system, where the Unit 10 magmas ascended a section of the magma system that penetrated a stranded fragment of continental crust.

Foraminifer, epifauna and infauna abundance of ODP Hole 183-1138A and core ELT54.007-PC, Kerguelen Plateau

Late Campanian and Maastrichtian benthic foraminifers are recorded from 12 samples from Ocean Drilling Program (ODP) Leg 183, Cores 183-1138A-52R through 63R (487.3-602.4 meters below seafloor), Kerguelen Plateau, Indian Ocean, and Danian benthics from one sample in the same section. The entire late Maastrichtian foraminifer fauna is noted from a dredge sample 220 km to the north. The structure of the fauna is compared with the Cenomanian-Turonian of the nearby Eltanin core E54-7. Faunas are reviewed in terms of planktonic percentage, composition, epifaunal/infaunal ratios, and dominance/diversity indices. The region was in the cool Austral Faunal Province through the Campanian-Maastrichtian and was probably warmer in the Cenomanian-Turonian. The ODP section is now 1600 meters below sea level and has subsided several hundred meters since deposition. Its fauna is dominated by epifaunal species suggesting little influence of upwelling. The dredge location has subsided little. Its fauna has a high infaunal content consistent with significant influence of upwelling near the plateau edge. The dominant benthic species remain constant through the ODP Cretaceous section, but subdominance changes, and the section is divided into three informal zones based on dominance/subdominance characteristics of the benthic fauna. Brief taxonomic comments are made on several species and some are figured.

Diatom biostratigraphy, Argon ages and chronology of ODP Hole 183-1138A

A thick Neogene section was recovered in the upper ~300 m of Ocean Drilling Program Hole 1138A, drilled on the Central Kerguelen Plateau in the Indian sector of the Southern Ocean. Sediment lithologies consist primarily of mixed carbonate and biosiliceous clays and oozes, with several thin (1-3 cm) tephra horizons. The tephras are glass rich, well sorted, and dominantly trachytic to rhyolitic in composition. Volcaniclastic material in these horizons is interpreted to have originated from Heard Island, 180 km northwest of Site 1138, and was likely emplaced through both primary ash fall and turbiditic, submarine flows. A Neogene age-depth model for Hole 1138A is constructed primarily from 36 diatom biostratigraphic datums. Nannofossil and planktonic foraminifer biostratigraphy provides supporting age information. Additionally, four high-precision 40Ar-39Ar ages are derived from ash and tephra horizons, and these radiometric ages are in close agreement with the biostratigraphic ages. The integrated age-depth model reveals a reasonably complete lower Miocene to upper Pleistocene section in Hole 1138A, with the exception of a ~1-m.y. hiatus at the Miocene/Pliocene boundary. Another possible hiatus is also identified at the Oligocene/Miocene boundary. High Neogene sedimentation rates and the presence of both calcareous and siliceous microfossils, combined with datable tephra horizons, establish Site 1138 as a suitable target for future drilling legs with paleoceanographic objectives. This report also proposes two new diatom species, Fragilariopsis heardensis and Azpeitia harwoodii, from Pliocene strata of Hole 1138A.