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Marine strata deposited during late Cenomanian and early Turonian time display lithological, faunal, and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient. At, or very close to, the Cenomanian-Turonian boundary, between 90.5 and 91.5 million years ago, oxygen deficiencies were particularly marked over a period of less than 1 my. This short-lived episode of oceanic oxygen deficiency has been termed the Cenomanian-Turonian 'Oceanic Anoxic Event' (OAE). Marine sediments deposited during this event are, when compared with most of the Phanerozoic record, uncommonly rich in dark-grey to black, pyritic, laminated shales with total organic carbon contents that range from between 1 and 2% to greater than 20% which is largely of marine planktonic origin. The general lack of bioturbation in these beds is taken to indicate an absence of a burrowing fauna due to anoxic conditions. In coeval pelagic and shelf limestone sections the dark shales may be lacking; in such sections the Cenomanian-Turonian boundary is marked by δ 13C values of up to +4.0‰ or + 5.0‰ in contrast to δ 13C values of +2.0‰ to +3.0‰ in limestones directly above and below the boundary. The high δ 13C values are taken to indicate an enrichment of the global ocean in 13C values as a result of the preferential extraction of 12C by marine plankton, the organic components of which were not recycled back to the oceanic reservoir during this period of enhanced organic-carbon burial. In many basins benthonic foraminiferal faunas are lacking in strata at or near the Cenomanian-Turonian boundary or consist of depauperate agglutinate faunas whereas diverse planktonic foraminiferal faunas and radiolarian remains are locally abundant. These zones free of benthonic foraminifera have been previously interpreted as the result of bottom-water oxygen deficiencies. A correlation between high positive δ 13C values and manganese enrichment in shelf chalks has been pointed out by other workers; data presented here substantiates this correlation. Sediments that display one or more of the above characteristics have been studied and identified from diverse basinal settings such as Pacific Basin mid-ocean plateaus, North American cratonic interior seaways, European shelf and interior seaways, circum-African embayments and seaways, Tethyan margins and the Caribbean region. The oxygen-deficient water masses are proposed to have taken the form of an expanded and intensified oxygenminimum zone. Palaeobathymetric interpretation of strata from European and African shelf sequences and sections in the US Western Interior Basin show that shallow embayments, flooded by the rapid Cenomanian-Turonian transgression, were particularly favourable to deposition of anoxic sediments as were the neighbouring shelves and cratonic shallow seaways. The distribution of carbonaceous black shales and coeval light-coloured to red shallow-water limestones marked by a δ 13C 'spike' indicates that the upper surface of the widespread, intensified Cenomanian-Turonian oceanic oxygen-minimum zone was 100 to 200 metres below the surface of the sea in most areas; the lower surface was probably between 1.5 and 2.5 km below sea level. The main phase of the Cenomanian-Turonian OAE as exemplified by the Bonarelli Horizon in the Italian Apennines and the Black Band of Yorkshire and Humberside in England lasted less than 1 my. In some basins where coastal geometry and wind direction were effective in inducing strong upwelling conditions, the propensity for the deposition of carbon-rich facies increased and such facies were deposited in some predicted upwelling zones prior to and following the Cenomanian-Turonian OAE. However, the widespread distribution of anoxic sediments deposited synchronously during such a short-lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the Cenomanian-Turonian oxygen-minimum zone related to feedback between sea level rise and regional palaeoceanography. The palaeoceanography of the Cenomanian-Turonian OAE is discussed in detail in a companion paper by Arthur et al. 1987. © 1987 The Geological Society.

Stratigraphic hiatuses and solution unconformities in the subsurface of Enewetak Atoll, northern Marshall Islands, record periods of atoll emergence during low stands of sea level. Changes in sea level are also recorded in the atoll subsurface by variations in the rate of sediment accumulation relative to the subsidence rate of the underlying volcanic edifice. Past sea levels can be derived from atoll stratigraphy by correcting the present depth of dated subsurface horizons for thermal subsidence and lithospheric flexure since the time of deposition. A correction for depositional paleodepth may also be necessary. As a result of erosion and nondeposition during periods of emergence, the history of sea level derived in this manner is discontinuous. Past sea levels derived from atoll stratigraphy can only be estimated to within ±50 m relative to present sea level owing to uncertainties in the corrections for subsidence and flexure; however, the minimum magnitude of sea level falls estimated from stratigraphic hiatuses can be estimated to within ±10 m. Owing to limited fossil-based age resolution, only long-term sea level trends can be deduced from sediments dated by means of biostratigraphy. Based on the biostratigraphic ages of subsurface horizons at Enewetak, we can discern very little long-term change in sea level from late Eocene through late Oligocene, a rise to ∼110 m above present sea level in the early Miocene, a long-term fall of ∼170 m through middle and late Miocene time, and a long-term rise of ∼60 m from the end of the Miocene to present. Resolution of the sea level history recorded beneath mid-ocean atolls may be improved by determining the age of shallow-marine carbonates by means of strontium isotope stratigraphy. Our interpretation of past sea levels based on 87Sr/86Sr chronostratigraphy from Enewetak confirms the long-term sea level trends inferred from biostratigraphic subsurface ages. In addition, we interpret three Oligocene sea level falls with minimum magnitudes of 30–50 m at ∼35, 33–30, and ∼25 Ma and four Miocene sea level falls greater than 30–95 m at 16–14, ∼12, ∼10, and ∼5 Ma. These estimates of the timing and magnitude of Oligocene and Miocene sea level changes, derived from Enewetak Atoll stratigraphy, are compared with sea level histories derived from continental margin stratigraphy and from ice volume changes inferred from deep-sea foraminifera δ18O records.

“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.

The Cretaceous is characterized by unusually widespread distribution of “black shales”-- sequences of variable lithology containing numerous beds with organic-carbon (OC) contents in excess of 1 percent by weight-- in both deep- and shallow-marine settings. General time envelopes of globally important organic-carbon burial during the Aptian-Albian, at the Cenomanian-Turonian boundary, and to a lesser extent in the Coniacian-Santonian, have been termed “Oceanic Anoxic Events (OAEs)”. The available stratigraphic and organic geochemical data and secular trends in the carbon isotopic composition of marine carbonates suggest that the timing of OC burial is broadly synchronous during these episodes and that the mass of OC buried during each is considerable. There may be other important episodes of more widespread OC burial, as yet poorly documented, such as the Valanginian- Hauterivian.

Normal marine deposition in the early Aptian was interrupted by an episode of ocean-wide dysoxia/anoxia. This event is recorded by the occurrence of organic carbon-rich sediments in land sections from Europe and Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) sites in the North and South Atlantic, Indian and Pacific Ocean Basins. To elucidate the origin, and spatial and temporal relationships of these carbonaceous sediments, we have conducted an integrated biostratigraphic, lithostratigraphic and geochemical investigation of fourteen sections from a range of geographic and oceanographic settings

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 ...

Correlation of the δ 13C spike with the well dated occurrences of strata rich in organic carbon detailed in Schlanger et al. (this volume), indicates that a global episode of intense organic carbon (orgC) burial took place during the latest Cenomanian-earliest Turonian 'Oceanic Anoxic Event' (OAE) (A. plenus through I. labiatus macrofossil zones and upper R. cushmani TRZ through W. archecretacea PRZ foraminiferal zones) over a period of no more than 1 million years (m.y.). The shape of the δ 13C curve indicates that rates of orgC burial gradually increased in the early part of the late Cenomanian, increased more rapidly in the later Cenomanian, and levelled off at peak values in latest Cenomanian-early Turonian time during the maximum rate of orgC burial. The δ 13C values decreased nearly to pre-late Cenomanian levels in the early to middle Turonian. The decrease in δ 13C reflects decreasing rates of orgC burial following the Cenomanian-Turonian 'oceanic anoxic event' as well as the probable oxidation and return of significant amounts of orgC to the oceans following regression and re-oxygenation of much of the deeper water masses in contact with the seafloor. The Cenomanian-Turonian OAE coincided with a maximum sea level highstand. We suggest that sea level, which may be responding to some volcano-tectonic event, is the common link and ultimately the driving force for orgC deposition in globally distributed basins under different climatic and ocean circulation regimes. The rate of production of warm, saline deep water may have been proportional to the area of shelf flooding such that the maximum occurred near the Cenomanian-Turonian boundary. As rates of deep-water formation increased, rates of upwelling of deeper oceanic water masses must also have increased thereby increasing sea-surface fertility and productivity. In somewhat restricted higher latitude basins, such as the Cretaceous Interior Seaway of North America, periodic high rates of freshwater runoff coupled with deepening seas during the transgression created periodic salinity stratification, oxygen depletion in bottom waters, and resultant enhanced orgC preservation. The disappearance of some types of keeled planktonic formainifers and ammonites at the Cenomanian-Turonian boundary is probably due to the rather sudden but short-term disappearance of suitable shallow midwater habitats because of widespread severe oxygen depletion in these levels. This interpretation is strengthened by the occurrence of benthic-free zones or depauperate benthic faunas near the Cenomanian-Turonian boundary in many localities. © 1987 The Geological Society.

The tertiary stratigraphic column on the island of Guam, in the Mariana Island Arc of the western Pacific, is composed largely of volcanic rocks and limestones of shallow-water reef and bank facies. Small amounts of fine-grained limestones, of Eocene, Oligocene and Miocene age, occur interbedded with the volcanic rocks and reef limestones, and consist chiefly of planktonic foraminiferal tests embedded in a matrix of calcareous nannofossils. Although compositionally these pelagic carbonates are nearly identical to nannoplankton-foraminiferal oozes of the deep-sea floor, their deposition probably occurred at comparatively shallow depths — from a few hundred to perhaps 1000–2000 meters rather than several kilometers. The most favorable paleogeographic settings for pelagic deposition of this kind were probably intra-arc basins and shelf areas within the paleo-Mariana Island Arc during periods of volcanic inactivity. The spatial association of these pelagic limestones with shallow-water reef complexes, or with beds of redeposited, reef-derived coarse skeletal material, may serve to differentiate them from their deep-sea counterparts.

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.

Geological and geophysical studies along the entire length of the Line Islands were undertaken in order to test the hot spot model for the origin of this major linear island chain. Volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls. Volcanic rocks from the Line Islands are alkalic basalts and hawaiites. In addition, a tholeiitic basalt and a phonolite have been recovered from the central part of the Line chain. Microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high TiO2 (1.0–4.0 wt %) and Al2O3 (3.4–9.1 wt %) and are of alkaline to peralkaline affinities. Major element compositions of the Line Islands volcanic rocks are very comparable to Hawaiian volcanic rocks. Trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas; the Line Islands lavas are very much unlike typical mid-ocean ridge or fracture zone basalts. Dating of these rocks by 40Ar-39Ar, K-Ar, and paleontological methods, combined with Deep Sea Drilling Project data from sites 165, 315, and 316 and previously dated dredged rocks, provide ages of volcanic events at 20 localities along the chain from 18°N to 9°S, a distance of almost 4000 km. All of these dates define mid-Cretaceous to late Eocene edifice or ridge-building volcanic events. Eocene volcanic events took place from 15°N to 9°S, and Late Cretaceous events took place from 18°N to 9°S. In the southern Line Islands both Cretaceous and Eocene events took place on the same edifice or ridge, indicating recurrent volcanism at a single locality. The irregular distribution of atolls in the chain, the fact that Late Cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern Line Islands, and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1/2 related subsidence path. Magnetic surveys of 11 seamounts show that four of the seamounts, from the central Line chain, give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of Cretaceous seamounts. These four poles agree with other paleomagnetic data of middle-late Eocene-early Oligocene age from the Pacific. One of these four seamounts yielded a 40Ar-39Ar total fusion age of 39 Ma. Because the poles of all four seamounts fall into a tight group we infer that they are probably of middle-late Eocene age to early Oligocene age. The other three seamounts as well as one seamont from the Line Islands analyzed previously all give virtual geomagnetic poles which agree with Late Cretaceous paleomagnetic data from the Pacific. Of these four seamounts, three give Late Cretaceous 40Ar-39Ar ages ranging from 71 to 85 Ma; another, in the southern Line Islands, is interpreted, on paleontological evidence, to be of Late Cretaceous age. The origin of the Line Islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots. The single hot spot model cannot account for all of the volcanic edifices in the Line Islands, although it does explain a general age progression of 9.6±0.4 cm/yr from north to south along the chain derived from a number of dated edifices. The four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data. The petrologic data argue against a mid-ocean ridge or transform fault origin proposed by earlier workers. The complex volcanic province represented by the Line Islands remains a challenge to existing models for the origin of midplate volcanism in the Pacific. An atoll drilling program which could determine edifice building histories, paleolatitudes of seamount formation, and subsidence rates and patterns in the Line Islands is needed.

Recent work on the dolomite problem has led several workers to suggest that dolomite found on, or in the subsurface of, islands and atolls in the Pacific Ocean is genetically linked to the diagenetic effect of Mg-rich residual brines produced by evaporation of sea water. This paper is intended to serve as an additional link between the suggestion and the geologic evidence for dolomitization through evaporation. A number of small, low islands such as Jarvis, Malden, Starbuck, McKeans, Sydney, and Enderbury in the Equatorial Dry Zone of the south central Pacific Ocean are characterized by small to ephemeral, hypersaline lagoons and salt and gypsum deposits. Development of evaporite deposits on these islands is a function not only of climate but of island size and topography which determines the degree of sea water inflow. Dolomite has not yet been reported from these islands but conditions on them are so similar to conditions described on Bonaire, a site of modern dolomite formation, that its apparent absence may only be due to a lack of detailed mineralogic work. A study of pre-Recent dolomite in the Pacific was made of localities where some information on the lateral as well as the vertical distribution of dolomite is known or could be inferred. Such localities include Eniwetok Atoll, Kita-Daito-Jima, Nauru, Ocean, Atiu, Niue, and Man-gaia Islands and several islands in the Lau Group of eastern Fiji. These ancient sites of dolomitization have in common with their probable modern analogues in the Equatorial Dry Zone small size, they are generally less than 6 miles in diameter, and basin-like topography or paleotopography. This topography was for some islands inherited from an original atoll-like form; for others it was produced by post-emergence subaerial solution. Thus emergence, or cyclic emergence and submergence, may have been necessary for the development of restricted lagoons within which significant evaporation of sea water could have taken place. Closely timed conjunctions of tectonic, eustatic, and climatic effects then may be preconditions for dolomite formation.

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.

Kikai, Amami, Toku, Okierabu and Yoron Islands in the Ryukyu island arc north of Okinawa show differing rates of vertical displacement during the past 130,000 years. Kikai has been rising at a rate of 1–2 m/103 years over this period whereas the other islands show either recent submergence at a rate of 0.7 m/103 years or little movement during the last 6,000 sidereal years. A structural axis probably exists between Kikai, the outermost island in the central segment of the Ryukyu Arc, and the four inner islands. Both the coralliferous nature of the limestone units on Kikai and 230Th growth method ages of individual corals indicate that the limestone units were deposited during interglacial stages. Dated limestones from Kikai are the: 1. (1) Raised Coral Reef Limestone; average age of 3 samples is 5,600 years. 2. (2) Araki Limestone; average age of 5 samples is 42,000 years. 3. (3) Riukiu Limestone (younger member); average age of 3 samples is 67,000 years. The age of the older member of the Riukiu Limestone is geologically inferred to be approximately 130,000 years, which correlates with the Barbados III terrace of Broecker et al. (1968). The rates of vertical displacement of Kikai compare closely to other rates determined for post-Miocene movement in the main islands of Japan.

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 dominance of volcanic processes and the importance of vertical tectonics in the geological evolution of the Pacific Basin has been recognised since the time of Charles Darwin. Data gathered on several legs of the Deep Sea Drilling Project (DSDP) and numerous marine expeditions in the past decade have confirmed Menard's postulate that the Pacific Basin was the scene of volcanism on an enormous scale in Mesozoic time. Widespread mid-plate volcanism between ∼110 and 70 m.y. B.P. characterised the area bounded by the Line Islands, the Mid-Pacific Mountains and the Nauru Basin-Marshall Islands. Heating of the Pacific lithospheric plate during this period of volcanism resulted in regional uplift and the bathymetric evolution of the area diverged significantly from a “normal” Parsons-Sclater subsidence curve. The Farallon plate, now almost entirely subducted, was also the scene of mid-plate volcanism that produced such features as the Nicoya Plateau now found as an allochthonous ophiolitic terrain landward of the middle America trench. Large, benthonic, reef-associated foraminifera comprising a pseudorbitoid fauna, hitherto considered to be largely restricted to Central America, have now been additionally recorded from DSDP Sites 165, 315 and 316 in the Line Islands, Site 462 in the Nauru Basin, and in New Guinea. The distribution of this fauna, of Campanian/Maastrichtian age, is interpreted as indicating “stepping stone” connections (aseismic ridges, plateaus and seamounts) between the Caribbean, Farallon, and Pacific plates 70–80 m.y. B.P. Similarities between the geology of the Nauru Basin and the Caribbean Ocean crust reinforce the interpretation of the latter as a former part of the Farallon plate. Estimates of the sea-level and continental freeboard change caused by the thermally induced uplift of the Pacific and Farallon plates, as well as substantial areas in the Atlantic and Indian Ocean Basins, indicate that such shallowing of the sea floor could have been the major factor in causing global Cretaceous transgressions.

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.

The Nauru Basin contains a well-documented pattern of Late Jurassic magnetic anomalies and a Cretaceous volcanic complex that has not disturbed the older magnetic structure. The following hypotheses are offered to explain this paradox. The Cretaceous basalts are normally and uniformly magnetized, making them invisible to a surface-towed magnetometer. Tension cracks and fractures associated with thermal uplift and/or magma wedging provided pathways for the Cretaceous magma through the Jurassic basement without disrupting the Jurassic magnetic structure. The same tension cracks admitted sea water that convectively removed heat from the magnetic layer and thus maintained the Jurassic basement below its magnetic blocking temperature for extended periods of time during intrusive thermal episodes. Finally, the maximum uplift associated with Cretaceous volcanism in the Nauru Basin could have been equal to, but was probably something less than, that associated with nearby seamount and atoll chains.

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.

Rocks dredged from a seamount 100 km northwest of Caroline Island, at the southern end of the Line Islands chain, contain Late Cretaceous fossils associated with volcanic debris. This association is evidence for the existence of a reef-bearing volcanic edifice with a minimum age of Late Cretaceous, 70 to 75 m.y., near Caroline Island. With the discovery of this seamount, the known occurrences of Late Cretaceous, reef-capped, volcanic edifices now extend a distance of 2,500 km, from Deep Sea Drilling Project Site 165 to 100 km northwest of Caroline Island. The apparent synchroneity of Late Cretaceous volcanism over this distance argues against the proposition that a single hotspot of the Hawaiian-Emperor type produced the Line Islands chain. Biochronologic data from the Line Islands indicate that the chain is not the temporal equivalent of the Emperor chain. Volcanic edifices of Cretaceous age are now known to extend from the Line Islands through the Mid-Pacific Mountains to the Marshall Islands and the western margin of the Pacific plate from Japan to the Marianas. A volcanic event occurred in the southern Line Islands during middle Eocene time; Eocene sediments were engulfed and altered by a volcanic eruption. The occurrence of both Cretaceous and Eocene volcanism in the southern Line Islands indicates that the history of the Line Islands is similar to that of the Marshall Islands.

Drilling results from mid-Pacific atoll and archipelagic apron sites in the Line Islands and Marshall Islands provinces lead to the conclusion that Oligocene sea-level falls detected in Atlantic passive margin sequences are also recorded in a mid-plate-tectonic setting in the Pacific Basin. The mid-Pacific sea-level falls are recorded by (a) the presence of distinct, coarse-grained, graded beds of turbidite origin, rich in reef-derived skeletal debris of Oligocene, Eocene, and Cretaceous age, that were redeposited in deep-water archipelagic apron carbonate sequences of middle and late Oligocene age now flanking the atolls and (b) a marked stratigraphic hiatus and solution unconformity in the subsurface of Enewetak atoll which dates an Oligocene period of atoll emergence correlative with both the deposition of the turbidites and the coastal offlap events discerned in Atlantic passive margins. Correlation of the subsidence path of Enewetak atoll with the development of the Oligocene solution unconformity shows that ca. 30 Ma sea level was as much as 100 m lower than at present.

This volume brings together research carried out within the Cenozoic Paleoceanography Project (CENOP), a goal of which was to generate paleoceanographic reconstructions of the Miocene Epoch, 24 to 5 Ma. On the basis of stable isotope and quantitative micropaleontological studies, the authors have related these data bases to parameters and processes of the Miocene Ocean, including paleogeography, stratigraphy, circulation, water mass distributions, temperature variability, vertical water mass structure, biogeography, and biotic evolution. That these authors attained their goal is attested to by the quality and scope of the 14 chapters presented in this valuable treatise. The Miocene Epoch was chosen because it was in this interval of time that major ice sheets became a permanent feature of Earth's surface. The presentation of this stunning amount of data and interpretation is very well organized for the researcher as well as for the students and professors who will surely use the book as a basis for seminars.

A period of atoll emergence results in the development of a karst surface on the atoll. Upon resubmergence, this surface is preserved as a subsurface solution unconformity that can be identified by petrographic characteristics and a distinctive stable isotope signature imprinted by subaerial or phreatic diagenesis. Determination of the magnitude of Miocene sea-level falls by considering subsidence rates and depths to solution unconformities must take into account the lowering of the emergent atoll surface by dissolution. Previously, the depth to a Miocene solution unconformity below Midway Atoll was assumed to record a maximum highstand of sea level that preceded a period of island emergence. However, because of surface dissolution, the solution unconformity must represent some elevation between a past highstand and a subsequent lowstand. The subsurface stratigraphy of Midway Atoll shows that a late Miocene sea-level fall of 75–125 m below present sea level took place. The magnitude of this fall is greater than that previously determined, on the basis of stable isotope data, from Midway Atoll but approximates Miocene sea-level falls determined by seismic stratigraphic methods.