Seymour O. Schlanger

Geoacoustic modeling of deep-sea carbonate sediments

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 ...

Late Cretaceous and Eocene volcanism in the southern Line Islands and implications for hotspot theory

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.

Atoll stratigraphy as a record of sea level change: Problems and prospects

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.

Cretaceous volcanism and Jurassic magnetic anomalies in the Nauru Basin, western Pacific Ocean

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.

Elastic Properties Related to Depth of Burial, Strontium Content and Age, and Diagenetic Stage in Pelagic Carbonate Sediments

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.