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“Oceanic Anoxic Events” are time envelopes of increased organic carbon burial, rich in carbon-12 by biogenic fractionation, accompanied by a strong positive shift of the del carbon-13 values of the carbonate in the late Cenomanian — early Turonian. The return to lower values reflects erosion and reoxidation of the carbon-12 rich organic material during times of tectonic activity. The evolution and diversity of planktonic foraminifera is closely bound to the event. The diversity is inversely correlated to del carbon-13 and the occurrence of distinct species in the stratigraphical record provides evidence of changes of the Mid-Water Oxygen Minimum Zone.

A systematic study has been made of physical and acoustic properties of 269 DSDP core samples representing a complete ooze‐chalk‐limestone sequence on the Ontong–Java Plateau (sites 288 and 289) and a sequence of clay‐rich carbonate sediments in the Coral Sea Basin (site 210). Gradational increases in density (ρ), compressional velocity (Vp), shear velocity (Vs), compressional and shear velocity anisotropies (Ap, As: horizontal velocities faster than vertical velocities), and shear velocity orientation anisotropy (Aso: horizontally propagated shear velocities are faster when the particle motion is horizontal rather than vertical) are directly related to increasing depth (subbottom) and diagenetic stage. Silica enrichment increases ρ, Vp, and Vs but does not significantly affect Ap and As. Clay enrichment, on the other hand, decreases ρ, Vp, and Vs and increases Ap and, to a greater degree, As. It is found that Ap≳As in carbonate sediments, whereas As≳Ap in clay‐rich sediments. Viable models are discussed ...

The Ontong Java Plateau, with its thick, capping sequence of Cretaceous and Cenozoic pure pelagic carbonate sediments, forms an ideal setting for the study of the acoustic properties of this lithology on an oceanic rise. Borehole logs, recorded on Deep Sea Drilling Project leg 89 at site 586, provided detailed data on in situ acoustic properties of Pleistocene to early Miocene sediments to a depth of 623 m below seafloor. Comparison of these logging results and the sonobuoy-based results of Johnson et al. (1978) with previous laboratory measurements from the Ontong Java Plateau shows that velocity/depth functions determined from the logging and sonobuoy methods are concordant but diverge significantly from functions derived from laboratory measurements. Log densities and compressional velocities exceed those measured by laboratory techniques; the density discrepancy is strongly influenced by laboratory method. The differences between log and laboratory compressional velocities are greater than and extend to greater depths than those between densities. These differences can be attributed to reductions in the frame bulk modulus and dynamic rigidity, caused by the removal of overburden pressure in the absence of significant porosity rebound. Agreement of site 586 log velocities with velocities derived from the earlier sonobuoy measurements across the plateau argues for the interpretation that both methods measure in situ values. The disagreement between the site 586 log results and the sonobuoy results with both the empirical velocity/depth function of Carlson et al. (1986) and the empirical velocity/porosity function of Raymer et al. (1980) supports the conclusion that pelagic carbonate sediments on oceanic plateaus and rises have unique acoustic properties, primarily arising from the presence of intraparticle porosity, and should not be grouped with other oceanic lithologies in acoustic modeling studies.

Laboratory measurements of ultrasonic velocity (VP, VS) and attenuation (Q−1P, Q−1S) in deep-sea carbonate sequences at DSDP Sites 288, 289 and 316 in the equatorial Pacific were made in conjunction with studies of sediment density, porosity and pore geometry in order to investigate the role of diagenesis in the development of physical properties. Bulk porosity decrease appears to be related more significantly to depth of burial than to age of strata. Both depth of burial and age, however, are important factors controlling the modal pore diameter. In deep-burial diagenesis the modification of pore geometry is influenced by the presence of silica during diagenesis. In carbonate sequences at the three DSDP sites studied, shear wave attenuation anisotropy ( Q −1 SHH Q −1 SHV ) correlates with the shear wave velocity anisotropy. Pore orientation, resulting from overburden pressure and other deep-burial diagenetic processes, is an important factor controlling the increase of VP anisotropy with age and depth of burial. On the basis of observed minor changes in anisotropy values with increasing pressure for some samples, other contributions to VP anisotropy such as grain orientation and bedding lamination cannot be ruled out.

Banks of living red calcareous algae and coral reefs contribute skeletal debris to form algal gravels and calcarenites in the shallow water around La Paz and Isla Espiritu Santo near the southern tip of Baja California, Mexico. A core taken in deep water, on the western slope of the La Paz Basin downslope from the algal banks, displays graded beds containing abundant shallow-water carbonate skeletal debris; downslope transport of this skeletal debris by tidal, storm, and turbidity currents is indicated. The calcareous skeletal debris in these graded beds are tecturally, faunally and florally similar to skeletal carbonate debris described from flysch deposits of Early Tertiary age in Switzerland and surrounding areas. Both present-day southern Baja California with its adjacent basins in the Gulf of California and the flysch cordilleras and basins of Early Tertiary Switzerland share the following characteristics: ( 1 ) a tropical marine climate prevails around; ( 2 ) in land areas, including islands chains, of high relief close by deep marine basins and troughs, the topography in large part is controlled by fault block structures; ( 3 ) the deep marine basins and the land areas are separated by narrow, shallow fringing shelves; ( 4 ) upon these shelves algal banks and coral reefs grow, contributing skeletal debris to form algal gravels and calcarenites containing coral, algal, molluscan, bryozoan, echinoid and foraminiferal debris which is subject to ( 5 ) resedimentation by tidal, storm and turbidity currents, thus contributing shallow-water skeletal debris to deeper-water sediments. The La Paz area is of interest to students of Alpine geology because around La Paz one may study the type, scale and intensity of sedimentational processes such as must have been active during the Early Tertiary filling of pre-Alpine flysch basins in Switzerland.

Laboratory measurements of density (p), compressional (Vp) and shear (Vs) velocities, and velocity anisotropies (Ap) and (As) in a pelagic ooze-chalk-limestone sequence from DSDP Site 289 are viewed in light of its present depth of burial, sediment age and diagenetic stage.

The late Oligocene to early middle Miocene was a period of widespread reef and associated carbonate deposition over a wide latitudinal range in the offshore region of southeast Asia. A generally rising eustatic sea level during this period allowed the accumulation of thick reefal units. An expansion of the coral-algal regional facies belt north to Japan and south to New Zealand occurred during the early Miocene as the result of a reduced oceanic latitudinal temperature gradient. An episode of enhanced organic carbon burial is reflected in a positive ^dgr13C excursion from 17.5 to 13.5 Ma. Increasing paleo-oceanographic equability may have resulted in reduced ocean circulation with the resultant development of an expanded oxygen-minimum zone. Marine oil-prone or anic carbon was deposited in silled anoxic basins both bounded by, and within, island-arc systems. A combination of tectonic and paleo-oceanographic factors, therefore, created conditions that favored contemporaneous deposition of organic carbon-rich source beds and reef reservoirs during the latest early to early middle Miocene. Paleogeographic and facies reconstructions for an 18-m.y. interval indicate that deposition occurred in three tectonic settings: (1) passive continental margins, (2) convergent plate boundaries, and (3) obliquely convergent plate boundaries. In all these settings, the vertical-motion histories of fault-bounded blocks and of larger tectonic elements within which they are contained directly controlled the geographical distribution of carbonate buildups, their scal and morphology, duration, diagenesis, and relationship to siliciclastics.

Large amounts of organic carbon were de­ posited and preserved in marine sediments of late Bar- remian through middle Albian and late Cenomanian- earty Turonian age owing to the development of poorly oxygenated oceanic water masses and expanded oxy- § en minimum zones during "oceanic anoxic events." ediments rich in organic carbon which were depos­ ited during such events are thick sequences of basinal black shale or mudstone, thin black beds in shelf chalks, and thin beds and lenses in rudist reef and associated limestones. Analysis of the stratigraphic distribution of both known oil and giant oil-field reser­ voirs by many workers has indicated that a large part of the world's oil is of Mesozoic age and that most giant reservoirs are in rocks of Mesozoic age. We pro­ pose that the relative abundance of Mesozoic oil in the world oil picture is in part the result of maturation of organic carbon deposited during Cretaceous oceanic anoxic events. Many giant fields of Cretaceous age have reservoirs of shallow-water carbonate complexes such as rudist reef and associated fades buildups. We propose that the oil in such reservoirs originated as follows: (1) dur­ ing middle Cretaceous marine transgressions, equable world cHmate with high sea-surface temperatures pre­ vailed and led to carbonate reef buildups on shallow shelves and marginal platforms; (2) at the same time marine oxygen-nilnlmum zones expanded and deeper basinal oxygen deficits were Intensified owing to the lack of strong ocaanic mixing because of stable densi­ ty stratificatio n and possible low oceanic thermal gra­ dients; (3) these oxygen deficits enhanced the preser­ vation of organic carbon in basin, slope, and some shelf fades; (4) later Cretaceous transgression result­ ed in the drowning of the carbonate buildups which were then sealed under a cap of fine-grained hemipe- iagic sediment; interim regressions resulted In en­ hancement of porosity of reef fades and may have al­ lowed deposition of interbedded sand bodies in some sections; (6) many of these carbonate complexes and source beds were buried to suitable depths by Late Cretaceous and Tertiary clastic wedges and, depend­ ing on local geothermal gradients, hydrocarbon matu­ ration in the black stiale basinal fades occurred. These hydrocartions niigrated to shelf-edge reservoirs, such as platform cart)onate rocks, through intermediate fa­ des. This scenario with its combination of oceano- graphic and geokigic events led to development of fields such as those in the Golden Lane in Mexico. Thus, prime exploration targets are deeply buried ru­ dist buMuBs that are stratigraphically linked to basinal black shaw source tieds. Such buildups should be found on subsided passive margins of low to middle latitude regions; the source beds formed where nearly contemporaneous low-oxygen conditions obtained in adjacent basins.

Laboratory studies have been carried out to determine the causes of velocity and attenuation anisotropy in pelagic carbonate sediments from Deep Sea Drilling Project site 288 on the Ontong-Java Plateau. Compressional velocity Vp was measured under independently controlled pore pressure and confining pressure. Velocity anisotropy for compressional waves (horizontal velocity > vertical velocity) is found to decrease with increasing effective pressure, i.e., with closure of pores. Previous studies have suggested that velocity anisotropy in pelagic carbonate sequences is due largely to preferred vertical orientation of calcite c axes. The present results indicate that preferred horizontal orientation of grain contacts and flat pores is also a significant cause of the observed anisotropy for many samples. Significant anisotropy in ultrasonic attenuation for compressional (Qp−1) and shear (Qs−1) waves is observed such that the losses are greatest for propagation modes that can generate relative motion across horizontal grain contacts or flat horizontal pores. The results are discussed in terms of the possible mechanisms of dissipation.