Drilling at Site 534 in the Blake-Bahama Basin recovered 268 m of Lower Cretaceous, Berriasian to Hauterivian, pelagic carbonates, together with volumetrically minor intercalations of claystone, black shales, and terrigenous and calcareous elastics. Radiolarian nannofossil pelagic carbonates accumulated in water depths of about 3300 to 3650 m, below the ACD (aragonite compensation depth) but close to the CCD (calcite compensation depth). Radiolarian abundance points to a relatively fertile ocean. In the Hauterivian and Barremian, during times of warm, humid climate and rising sea level, turbiditic influxes of both terrigenous and calcareous sediments, and minor debris flows were derived from the adjacent Blake Plateau. The claystones and black shales accumulated on the continental rise, then were redeposited onto the abyssal plain by turbidity currents. Dark organic-rich and pale organic-poor couplets are attributed to climatic variations on land, which controlled the input of terrigenous organic matter. Highly persistent, fine, parallel lamination in the pelagic chalks is explained by repeated algal "blooms." During early diagenesis, organic-poor carbonates remained oxygenated and were cemented early, whereas organic-rich intervals, devoid of burrowing organisms, continued to compact later in diagenesis. Interstitial dissolved-oxygen levels fluctuated repeatedly, but bottom waters were never static nor anoxic. The central western Atlantic in the Lower Cretaceous was thus a relatively fertile and wellmixed ocean basin.

Leg 76 of the Deep Sea Drilling Project achieved two major scientific objectives. The first objective was met at Site 533, where on the Blake Outer Ridge, gas hydrates were identified by geophysical, geochemical, and geological studies. Gas-hydrate decomposition produced a volu-metric expansion of 20: I of gas volume to pore-fluid volume; this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions. The gas hydrate includes methane, ethane, propane, and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography. For the first time, marine gas hydrates were tested with a pressure core barrel. The second objective was achieved when coring at Site 534 in the Blake-Bahama Basin sampled the oldest oceanic sediments yet recovered. The sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the North Atlantic Ocean in detail. The oldest oceanic sediments are red claystones and laminated green and brown claystones of middle Cal-lovian age. This finding supports the interpretation that the beginning of the modern North Atlantic occurred in the early Callo-vian (~155 m.y. B.P.), as much as 20 m.y. later in time than often previously thought.

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The Miocene Great Abaco Member of the Blake Ridge Formation, cored at Sites 391 and 534 in the Blake-Bahama Basin, consists predominantly of redeposited deeper-water foraminifer-nannofossil chalk and minor shallow-water carbonate material, interbedded with radiolarian-diatom mudstone deposited below the calcite compensation depth. The mudstones comprise the background rain of pelagic sediments that may have been deposited and/or reworked by weak contour currents. The chalks, including the distinctive intraclastic chalks, and skeletal packstones were deposited by turbidity currents, and less so by debris flows. These gravity flows eroded and entrained large numbers of siliceous mud and clay intraclasts, as well as lithified chalk and limestone clasts, some of shallow-water origin. Possible sources of the carbonates include the Blake Plateau to the west and the Bahama Banks to the south. The Blake Plateau and Bahamian slopes provided the deeper-water foraminifer-nannofossil chalks, and the Bahama platform supplied the shallow-water carbonate grains and cemented limestone. Sea-level changes together with tectonism along the Great Abaco Fracture Zone and faults in the northern Bahamas may have provided the triggering mechanism that set large volumes of mostly unconsolidated carbonate material into motion. A submarine fan system and associated channels extending from the northern Bahamas into the Blake-Bahama Basin funneled debris flows and turbidites over 300 km into the basin, and subsequent turbidites filled in and leveled the topography, creating the prominant seismic reflector M. If similar deposits existed earlier in the Paleogene, they have been eroded, leaving only the Great Abaco Member as evidence of the tectonic and sedimentologic conditions existing in the western North Atlantic during the Paleogene and Miocene.

The hydrocarbon reservoir of the Hasdrubal field (offshore Tunisia) lies within the Eocene El Garia Formation. This formation was deposited on a shallow north‐ to NE‐facing ramp in the Early Eocene and is composed of a belt of nummulitic wackestones‐grainstones. The nummulitic fades occupies a range of depositional environments from outer to mid ramp. In addition to Hasdrubal, several other producing oil‐ and gasfields have been discovered in the variably dolomitised El Garia Formation offshore Tunisia. Cores from three Hasdrubal wells were examined. Reservoir quality shows a limited relationship to primary depositional fabric and has been influenced significantly by compaction and later diagenesis. The highest permeabilities are typically developed within a dolomitised zone which occurs near the middle of the reservoir interval across the entire field, and which may follow a primary wackestone lithofabric (typically 20–30% bulk volume dolomite, with porosities of 15–22% and permeabilities of l‐30mD). Fractures, particularly in zones surrounding faults, have resulted in enhanced permeabilities. Combined results of isotope (δ18 O ‐5.0 to ‐7.3%oPDB) and fluid inclusion (Th 80–90d̀C) analyses of dolomites from this dolomitised zone indicate that matrix dolomites are burial diagenesis cements. Dolomitisation of the reservoir was a “closed system ” event and was not the result of major fluid flow or mixing. Magnesium ions for dolomitisation were derived from the transformation of high‐Mg to low‐Mg calcite in nummulite tests within the reservoir fades. Our analyses indicate that calcite cements were precipitated at temperatures of up to almost 150d̀C in primary and secondary pores and in variably‐sealed fractures Fracture lining and filling cements show a range of δ18 O values, which suggests that the fractures acted as fluid conduits over a range of temperatures during burial diagenesis Fracture densities measured in core increase rapidly close to seismically‐resolvable faults in the reservoir facies Fracturing probably resulted in the leakage of hydrocarbons through the Compact Micrite Member seal which overlies the accumulation, as well as facilitating the ingress of hot fluids from stratigraphically deeper levels in the basin