Abundance and preservation of radiolarian faunas of ODP Hole 122-761B (Table 1)

Sites 759 through 764 were drilled during Ocean Drilling Program Leg 122 on the Exmouth and Wombat plateaus off northwest Australia, eastern Indian Ocean. Radiolarian recovery was generally poor due to unsuitable lithofacies. A few Quaternary radiolarian faunas were recovered from most of the sites. Rare and poorly preserved Oligocene and Eocene radiolarian faunas were recovered from Holes 760A, 761B, 761C, and 762B. Poorly preserved Cretaceous radiolarians occur in samples from Holes 761B, 762C, 763B, and 763C. Chert intervals from Cores 122-761B-28X, 122-761C-5R, and 122-761C-6R contain moderately well-preserved Cretaceous radiolarian faunas (upper Albian, mid- to upper Cenomanian, and mid-Albian, respectively). Rare fragments of Upper Triassic radiolarians were recovered from sections in Holes 759B, 760B, and 764A. The only well-preserved pre-Quaternary radiolarians are in lower and upper Paleocene faunas (Bekoma campechensis Zone) recovered from Site 761, Sections 122-761B-16X-1 to 122-761C-19X-CC. The composition of these faunas differs somewhat from that of isolated coeval Paleocene faunas from Deep Sea Drilling Project sites in the Atlantic, Gulf of Mexico, tropical Pacific, eastern Indian Ocean, and near Spain and North Africa, as well as from several on-land sites in North America, Cuba, and the USSR.

Petrophysical measurement of ODP Leg 122 and 123 samples (Tables 1, 2)

The capillary-pressure characteristics of 22 samples of lithified post-Paleozoic Indian-Ocean carbonates were compared to published data from older carbonate rocks (lower Paleozoic Hunton Group of Texas and Oklahoma). The Indian-Ocean samples are considerably more porous than are the Paleozoic samples, yet all of the Indian-Ocean samples fit readily into a descriptive petrofacies scheme previously established for the Hunton Group. The Indian-Ocean samples may be assigned to four petrophysical facies (petrofacies) based on the shapes of their capillary-pressure curves, their pore-throat-size distributions, their estimated recovery efficiency values (for nonwetting fluids), and the visual characteristics of their pore systems, as observed with a scanning-electron microscope. Petrofacies assignments for the Indian-Ocean samples are as follows. Petrofacies I includes six samples collected from the coarse basal portions of event deposits (primarily turbidites). These samples have large throats, leptokurtic throat-size distributions, low- to moderate recovery efficiency values, concave cumulative-intrusion capillary-pressure curves, and high porosity values. Petrofacies II includes two sedimentologically dissimilar samples that have medium-size throats, platykurtic throat-size distributions, moderate- to-high recovery efficiency values, gently sloping cumulative-intrusion capillary-pressure curves, and high porosity values. Petrofacies III includes two polymictic sandstones and a skeletal packstone that have small throats, polymodal throat-size distributions, moderate recovery efficiency values, gently sloping cumulative-intrusion capillary-pressure curves, and high porosity values. Petrofacies IV includes 11 samples, mostly recrystallized neritic carbonates, that have small throats, leptokurtic throat-size distributions, high recovery efficiency values, convex cumulative-intrusion capillary-pressure curves, and low porosity values. Comparison of petrofacies assignment to core-, thin-section-, and smear-slide data, and to inferred depositional setting, suggests that pore systems in most samples from Holes 765C and 766A result from primary depositional features, whereas pore systems in samples from Hole 761C and one sample from Hole 765C have been strongly influenced by diagenetic processes. For Hole 761C, prediction of petrophysical parameters should be most successful if based on diagenetic facies patterns. By contrast, the distribution of favorable reservoir facies and of permeability barriers in less highly altered rocks collected from Holes 765C and 766A is related to depositional patterns. Recovery efficiency is inversely related to both porosity and median throat size for the present data set. This relationship is similar to that observed for carbonates of the lower Paleozoic Hunton Group and the Ordovician Ellenburger dolomite, but opposite of that observed for some other ancient carbonates. The coarse deposits of the massive basal units of turbidites are petrophysically distinct and form a coherent petrophysical group (Petrofacies I) with substantial reservoir potential. Two samples assigned to Petrofacies I have extremely large throats (median throat size at least 4 ?m, and at least six times that of any other sample) and therefore high permeability values. These two samples come from thin, coarse turbidites that lack or have poorly developed fine divisions and are interpreted to have been deposited on channeled suprafan lobes in a proximal mid-fan setting. The restriction of extremely high permeability values to a single depositional facies suggests that careful facies mapping of deep-sea fans in a deliberate search for such coarse turbidites could dramatically enhance the success of exploration for aquifers or hydrocarbon reservoirs. Such reservoirs should have substantial vertical heterogeneity. They should have high lateral permeability values but low vertical permeability values, and reservoir sections should include numerous thin units having widely differing petrophysical characteristics.

Stable isotope composition of carbonate components of Rhaetian shallow-water limestones of the Wombat Plateau

Stable oxygen- and carbon-isotope ratios of Rhaetian (upper Triassic) limestone samples from the Wombat Plateau, northwest Australia, were measured to explore possible diagenetic pathways that the material underwent after deposition in a shallow-water environment, before plateau submergence in the Early Cretaceous. Host sediment isotopic values cluster near typical marine carbonate values (d18O ranging from -2.57 per mil to +1.78 per mil and d13C, from +2.45 per mil to +4.01 per mil). Isotopic values of equant clear calcite lining or filling rock pores also plot in the field of marine cements (d18O = +1.59 per mil to -2.24 per mil and d13C = +4.25 per mil to +2.57 per mil), while isotopic values for neomorphic calcites replacing skeletal (megalodontid shell) carbonate material show a wider scatter of oxygen and carbon values, d18O ranging from +2.73 per milo to -6.2 per mil and d13C, from +5.04 per mil to +1.22 per mil. Selective dissolution of metastable carbonate phases (aragonite?) and neomorphic replacement of skeletal material probably occurred in a meteoric phreatic environment, although replacement products (inclusion-rich microspar, clear neomorphic spar, etc.) retained the original marine isotopic signature because transformation probably occurred in a closed system dominated by the composition of the dissolving phases (high rock/water ratio). The precipitation of late-stage equant (low-Mg?) calcite cement in the pores occurred in the presence of normal marine waters, probably in a deep-water environment, after plateau drowning. Covariance of d18O and d13C toward negative values indeed suggests influence of meteorically modified fluids. However, none of the samples shows negative carbon values, excluding the persistence of organic-rich soils on subaerial karstic surfaces (Caribbean-style diagenesis). Petrographical and geochemical data are consistent with the sedimentological evidence of plateau drowning in post-Rhaetian times and with a submarine origin of the >70-m.y.-long Jurassic hiatus.