ALBERT

Description: A taxonomic and biostratigraphic investigation has been carried out on Upper Triassic (Carnian-Rhaetian) nannofossils from Sites 759, 760, 761 and 764 drilled on the Wombat Plateau during ODP Leg 122. The recovery of continuous sequences containing well preserved nannofossils has enabled us to refine the previous taxonomy and biostratigraphy of this interval. Fossil assemblages are of two major types: (1) previously described calcareous taxa were recovered at Sites 761 and 764; and (2) sideritic forms, which may represent diagenetic replacement of calcareous nannofossils, were observed in material from Sites 759 and 760. The sideritic forms proved difficult to study taxonomically due to inadequate optical properties. Calcareous nannofossil assemblages in the Upper Triassic are dominated by Prinsiosphaera triassica. We show that the multitude of identities of this species in the light microscope are the result of selective etching on a layered structure. We propose an evolutionary lineage for the earliest known coccoliths, with Crucirhabdus primulus as the ancestor. This species gave rise to C. minutus and Archaeozygodiscus koessenensis. The Upper Triassic can be subdivided based on the sequential first occurrences of C. primulus and Eoconusphaera zlambachensis in the upper Norian. The late Norian and Rhaetian were times of slow evolution of calcareous nannofossils. However, we noted three morphometric changes in this time-interval which possess biostratigraphic utility: (1) P. triassica increases in diameter from an average of 6 µm to over 9 µm; (2) E. zlambachensis evolves from a stubby to an elongated shape; and (3) C. primulus increases in size. Upper Triassic assemblages from the Wombat Plateau are similar in composition and diversity to those which have been described in detail from the Alps. In both areas, nannofossiliferous sediments interfinger with massive limestones deposited in reef and peri-platform environments. Stable isotopic analyses of Wombat Plateau nannofossil assemblages indicate that they thrived in open ocean conditions. Biostratigraphy allows sequence chronostratigraphic interpretation of ODP Site 761 and supports the chronostratigraphic cycle charts of Haq et al. (1987).

Description: A summary of calcareous nannofossil biostratigraphy performed for Late Jurassic (Kimmeridgian) to Early Cretaceous (Hauterivian) cores of Site 765 (Cores 123-765C-58R to -55R) and Site 261 (Cores 27-261-33 to -27), Argo Abyssal Plain, off northwestern Australia is presented. Precise age determinations were limited by variable preservation and the exclusion of a number of marker species due to provincialism. However, the presence of species, such as, Stephanolithion bigotii bigotii, Watznaueria manivitae, Tubodiscus verenae, and Cruciellipsis cuvillieri results in a reasonably good degree of biostratigraphic control. Biogeographic interpretation of the nannofossil data suggests that the Argo Basin occupied a position transitional between the Tethyan and Austral nannofloral realms. A cooler water regime is suggested by the absence of thermophyllic Tethyan forms, such as Nannoconus, and the presence of taxa that display bipolar distribution, such as Crucibiscutum salebrosum. Two new species, Zeugrhabdotus cooperi and Cyclagelosphaera argoensis, and one new combination, Haqius ellipticus are described.

Description: Sediments from the Argo Abyssal Plain (AAP), northwest of Australia, are the oldest known from the Indian Ocean and were recovered from ODP Site 765 and DSDP Site 261. New biostratigraphic and sedimentologic data from these sites, as well as reinterpretations of earlier findings, indicate that basal sediments at both localities are of Late Jurassic age and delineate a history of starved sedimentation punctuated by periodic influx of calcareous pelagic turbidites. Biostratigraphy and correlation of Upper Jurassic-Lower Cretaceous sediments is based largely on calcareous nannofossils. Both sites yielded variably preserved nannofossil successions ranging from Tithonian to Hauterivian at Site 765 and Kimmeridgian to Hauterivian at Site 261. The nannofloras are comparable to those present in the European and Atlantic Boreal and Tethyan areas, but display important differences that reflect biogeographic differentiation. The Argo region is thought to have occupied a position at the southern limit of the Tethyan nannofloral realm, thus yielding both Tethyan and Austral biogeographic features. Sedimentary successions at the two sites are grossly similar, and differences largely reflect Site 765's greater proximity to the continental margin. Jurassic sediments were deposited at rates of about 2 m/m.y. near the carbonate compensation depth (CCD) and contain winnowed concentrations of inoceramid prisms and nannofossils, redeposited layers rich in calcispheres and calcisphere debris, manganese nodules, and volcanic detritus. Lower Cretaceous and all younger sediments accumulated below the CCD at rates that were highest (about 20 m/m.y.) during mid-Cretaceous and Neogene time. Background sediment in this interval is noncalcareous claystone; turbidites dominate the sequence and are thicker and coarser grained at Site 765. AAP turbidites consist mostly of calcareous and siliceous biogenic components and volcanogenic smectite clay; they were derived from relatively deep parts of the continental margin that lay below the photic zone, but above the CCD. The Jurassic-Lower Cretaceous section is about the same thickness across the AAP; turbidites in this interval appear to have had multiple sources along the Australian margin. The Upper Cretaceous-Cenozoic section, however, is three times thicker at Site 765 than at Site 261; turbidites in this interval were derived predominantly from the south. Patterns of sedimentation across the AAP have been influenced by shifts in sea level, the CCD, and configuration of the continental margin. Major pulses of calcareous turbidite deposition occurred during Valanginian, Aptian, and Neogene time-all periods of eustatic lowstands and depressed CCD levels. Sediment redeposited on the AAP has come largely from the Australian outer shelf, continental slope, or rise, rather than the continent itself. Most terrigenous detritus was trapped in epicontinental basins that have flanked northwestern Australia since the early Mesozoic.