Dean A. Dunn

Correlations between sound velocity and related properties of glacio-marine sediments: Barents sea

Statistical analysis of laboratory-measured compressional wave (sound) velocity, porosity, wet bulk density, and selected textural parameters of surface sediments from the Barents Sea reveals that clay content and mean grain size are the best indices to sound velocity. These parameters are followed closely by porosity and wet bulk density, while sand content provides the weakest index. Although Barents Sea surface sediments are characterized by fairly high variability, the results of the present study are in general agreement with studies of similar sediment types in other continental shelf environments.

Sound velocity and related physical properties of fine‐grained abyssal sediments from the Brazil Basin (South Atlantic Ocean)

Empirical relationships between compressional wave (sound) velocity and selected physical properties have been determined for fine‐grained abyssal sediments from the east Brazilian continental margin and Pernambuco abyssal plain of the Brazil Basin (NW South Atlantic Ocean). Porosity and wet bulk density were found to be the best sedimentological indices to sound velocity. In agreement with numerous other studies, other sediment properties were lesser, but still very good, indices. In order of decreasing statistical significance, these properties include mean grain size and percent clay. Averaged values of sediment physical properties of the Brazil Basin were compared with the published results of Hamilton and Bachman [J. Acoust. Soc. Am. 72, 1891–1904 (1982)] and show good agreement. The observed differences between the two studies were insignificant and attributed to slight mineralogical differences that can be expected in light of the differing sedimentation processes of the Pacific and Atlantic Oceans...

Deep-sea drilling on the upper continental rise off New Jersey, DSDP Sites 604 and 605

Deep Sea Drilling Project Sites 604 and 60S on the upper continental rise are the first of a series of cored holes along the “New Jersey transect” which, when completed, will provide the first comprehensive dipwise suite of drill holes across a passive margin from the coastal plain to the abyssal plain. Our drilling results document the age of important seismic sequence boundaries and allow their correlation with wells on the continental shelf and slope as well as with the regional oceanic seismic stratigraphy. Hole 605,156 km (97 mi) southeast of Atlantic City, New Jersey, and drilled 816.7 m down to mid-Maestrichtian limestones, penetrated a near-complete Cretaceous/Tertiary boundary section overlain by a 200-m expanded Paleocene sequence. Unusually high amounts of terrigenous silts and glauconite are present at the boundary and immediately above. Among the several hypotheses discussed, we suggest that the terrigenous silts and glauconite may represent a high-energy event such as a tsunami caused by a Cretaceous/Tertiary impact. Site 604, 5 km (3 mi) seaward of Site 605, was terminated in upper Miocene glauconitic sands and debris flows at 294.5 m by unstable hole conditions. These sediments contain shelf-derived gravels and exotic blocks of Eocene chalk (up to 50 cm across) eroded from bedrock that is today widely exposed on the adjacent slope. Our drilling results show that denudation of the Eocene units was not limited to the Oligocene A u erosional event, but that major loss occurred during late Miocene and later glacial sea-level lowstands.

DSDP Site 603: First deep (>1000-m) penetration of the continental rise along the passive margin of eastern North America

Drilling at Deep Sea Drilling Project Site 603 has provided the first deep (>1000-m) penetration of strata beneath the continental rise off the Atlantic margin of North America. Nearly continuously cored through 1585 m of section down to Berriasian pelagic limestones, the site 435 km (270 mi) east of Cape Hatteras intersected an extensive Lower Cretaceous deep-sea fan complex, which provides new information on the petroleum potential of the continental rise. Hauterivian to early Aptian in age, this 208-m interval of interbedded limestones, sand, and black shale turbidites begs the existence of any post-Valanginian reefs along the Baltimore Canyon Trough. Less extensive terrigenous turbidites were encountered higher in the section up to the Cretaceous/Tertiary boundary, which is marked by a current-laminated sand rich in dark spherules. Pelagic early Paleogene clays are disconformably overlain by Miocene pelagic mud. Turbiditic silts and clays began to accumulate rapidly at this site during the middle Miocene, leading to deposition of muddy contourites that formed the Lower Continental Rise Hills of the Hatteras Outer Ridge as sand turbidites were ponded concurrently on its landward side. The section at Site 603 confirms the concept that eustatic and other large-scale events subdivide Earth history into distinct chapters allowing the correlation of deep-sea seismic sequence boundaries with continental shelf and margin unconformities.

Paleoenvironmental analysis of the lower Oligocene Mint Spring and Marianna Formations across Mississippi and southwestern Alabama

ABSTRACT Examination of calcareous nannoplankton floras from the Mint Spring Formation and the Marianna Limestone indicates that these formations are diachronous along their outcrop belt in Mississippi and southwest Alabama. In east-central Mississippi and Alabama, these strata correlate to the Ericsonia subdisticha Zone (NP21; Martini, 1971), while in west-central Mississippi they contain nannofossils of the younger Helicosphaera reticulata Zone (NP22). Quantitative species abundance data for samples from twelve outcrops were subjected to R-mode and Q-mode factor analysis, and to discriminant analysis. Factor analysis indicates that the Mint Spring Formation was deposited in a shallow nearshore inner-shelf environment, while normal pelagic sedimentation resulted in the Marianna Limestone Formation being deposited as a deeper-water carbonate shelf across east-central Mississippi and southwest Alabama. In western Mississippi, these strata were less affected by the early Oligocene transgression, as indicated by higher abundances of shallow-water species Based on discriminant analysis, the Mint Spring and Marianna formations reflect similar paleoenvironments in western Mississippi, with the uppermost Mint Spring samples near the formational contact containing nannofossil species assemblages that are indistinguishable from the nannofossil assemblages from the lowermost Marianna samples immediately above the contact. Both these units were deposited during the first half of sea-level cycle TA4.4 (Haq et al., 1987), and based on relatively constant species abundances throughout the extent of both formations, they appear to have been deposited as part of a single continuous transgression.

Mesozoic-Cenozoic clastic depositional environments revealed by DSDP Leg 93 drilling on the continental rise off the eastern United States

Summary Prior to Deep Sea Drilling Project (DSDP) Leg 93 (1983), drill data along the continental rise of the Atlantic margin of the United States were quite limited compared to those of the adjacent continental shelf and the deeper, more seaward expanses of the North American Basin. Interpretations of the geologic history and of processes that controlled sedimentation along the rise were strongly dependent on studies of seismic reflection profiles. DSDP Leg 93 drilled deep holes on both the lower and upper rise, allowing correlation with commercial wells on land and offshore (as well as with subsequent DSDP Leg 95 holes along the ‘New Jersey Transect’) and providing the first down dip suite of drill holes across a passive continental margin from the coastal plain to the abyssal plain. Site 603 on the lower rise 270 miles east of Cape Hatteras was cored nearly continuously over 1585 m to Berriasian pelagic limestones. It intersected an extensive Lower Cretaceous deep-sea fan complex which provides new information on the petroleum potential of the rise. Hauterivian to early Aptian in age, this 208 m interval of interbedded limestones, sand and blackshale turbidites calls into question the existence of any post Valanginian reefs along the Baltimore Canyon Trough. Less extensive terrigenous turbidites were encountered as far up in the section as the Cretaceous-Tertiary (K-T) boundary. The K-T boundary is marked by a current-laminated sand rich in dark, 1 mm diameter spherules which may denote an extraterrestrial impact event. DSDP Sites 604 and 605 on the upper rise, the first along the ‘New Jersey Transect’, are located some 100 miles south-east of Atlantic City, New Jersey. Hole 605, drilled 816.7 m down to mid-Maestrichtian limestones, penetrated a near complete Cretaceous-Tertiary boundary section, above which 20 m of lower Palaeocene are separated by a disconformity from an expanded 175 m Palaeocene sequence. Terrigenous silts and glauconite at the K-T boundary and immediately above suggest either significant sea-level change, increased current erosion along the adjacent shelf and slope, increased terrigenous input caused by decreased vegetation, a high energy event (tsunami?), or some combination of these possible factors. Site 604, 3 miles seaward of Site 605, was terminated by unstable hole conditions at 294.5 m within a unit of Miocene glauconitic sands and debris flows. Emplaced largely during the Tortonian (8.2–10.0 Ma; Vail cycle TM3.1), these upper Miocene sediments contain shelf-derived gravels, exotic blocks of Eocene chalk (up to 50 cm across) eroded from the adjacent slope, and clasts of middle and upper Miocene carbonates or silts derived from canyon walls or shallow water strata upslope. Study of closely spaced, high resolution seismic profiles suggests that large-scale regional erosion (canyon cutting), which is related to the debris flows, began during the late middle Miocene. On the lower rise, turbiditic silts and clays began to accumulate rapidly during the middle Miocene. Under the influence of a strengthening Western Boundary Undercurrent, these were deposited as muddy contourites in antidune-like sediment waves which, at site 603, grew rapidly with no appreciable break in sedimentation until at least early Pleistocene times to form the present Lower Continental Rise Hills of the Hatteras Outer Ridge (HOR). The somewhat elevated edge of the Lower Continental Rise Terrace formed as a natural levee behind which the coarser portions of the terrigenous turbidites were ponded to form the terrace. No coarse clastics that bypassed the pond were deposited with the clays of the HOR at this locality. Throughout the study, seismic sequence boundaries of the upper and lower continental rise were calibrated and correlated with continental margin unconformities as well as with deep sea reflection horizons.

Stable-Isotopic Composition of a Late Eocene Archaeocete Whale, Basilosaurus cetoides, from Wayne County, Mississippi

ABSTRACT An exposed Late Eocene Archaeocete whale near Matherville, Wayne County, Mississippi was discovered in Spring 1993. Although this specimen consisted mainly of marginally-preserved thoracic and cervical vertebra, with a few rib fragments, a pre-molar tooth with intact enamel was recovered from an indurated block of the Pachuta Marl Member, Yazoo Formation. Although paleotemperature analyses of Tertiary strata have been routinely made using calcareous microfossils, this specimen represents one of the few isotopic measurements of a homeothermic animal. Stable-isotopic analysis has been made of phosphate tooth enamel of this specimen, and when combined with isotopic analysis of carbonate and phosphate of bone (rib fragments and the cervical vertebra) from the same animal, provides the opportunity to assess previously-published Late Eocene paleotemperature estimates. Estimates of isotopic fractionation of this Late Eocene whale from local sea water are complicated by the fact that there is no modern analog of this fossil whale.

Elasticity of Fine-Grained Abyssal Sediments, Brazil Basin, South Atlantic Ocean

The interaction of sound with the sea floor is an important variable in underwater acoustics, and both propagation modeling and performance evaluations of sonar systems must consider the effect of this boundary on the incident sound wave. Realistic numerical definitions of the sea floor have been developed by acousticians and geophysicists, using “geoacoustic” models based on the acoustic and physical properties of seafloor sediment samples (Hamilton, 1974a, 1980). Most of these models also simulate depth gradients of the properties, which are critically dependent on the determination of accurate values at the sediment surface.