Morphology of carbonate escarpments as an indicator of erosional processes
No abstract available.
Book chapter↗
Geology topics
William P. Dillon
Publications and source records attributed to William P. Dillon.
Maps showing gas-hydrate distribution off the east coast of the United States
These maps present the inferred distribution of natural-gas hydrate within the sediments of the eastern United States continental margin (Exclusive Economic Zone) in the offshore region from Georgia to New Jersey (fig. 1). The maps, which were created on the basis of seismic interpretations, represent the first attempt to map volume estimates for gas hydrate. Gas hydrate forms a large reservoir for methane in oceanic sediments. Therefore it potentially may represent a future source of energy and it may influence climate change because methane is a very effective greenhouse gas. Hydrate breakdown probably is a controlling factor for sea-floor landslides, and its presence has significant effect on the acoustic velocity of sea-floor sediments.
East Coast↗
Magnetic anomaly map of the central Cayman Trough, northwestern Caribbean Sea
This is the first large-scale published map of magnetic anomalies in the central Cayman Trough area. Two previously published very small scale maps based on much less data are a regional map (Gough and Heirtzler, 1969) and a map compiled from several tracklines running parallel to the axis of the Cayman Trough (MacDonald and Holcombe, 1978).
Miscellaneous Field Studies Map↗
Anomalous abundances of deep-sea fauna on a rocky bottom exposed to strong currents
Unusually high abundances of sponges and gorgonian corals, covering as much as 25% of the bottom, occur at depths greater than 3.5 km on the Blake Spur, a rocky cliff-dominated feature on the western Atlantic continental margin. This is the first report of such high abundances of megafauna from a non-hydrothermal or otherwise chemosynthetically enriched site in abyssal depths. Animal densities at other steep rocky sites at similar depths are usually lower by more than an order of magnitude. The deep slope of the Blake Spur is exposed to the vigorous Western Boundary Undercurrent, with local flow speeds that may exceed 100 cm s-1. Currents can control this anomalous animal abundance by removing sediments and by enhancing fluxes, rather than concentrations, of food particles to the dominant suspension feeders. ?? 1992.
Deep Sea Research Part A, Oceanographic Research P↗
Accretionary margin of north-western Hispaniola: morphology, structure and development of part of the northern Caribbean plate boundary
Broad-range side-scan sonar (GLORIA) images and single- and multi-channel seismic reflection profiles demonstrate that the margin of north-western Hispaniola has experienced compression as a consequence of oblique North American-Caribbean plate convergence. Two principal morphological or structural types of accretionary wedges are observed along this margin. The first type is characterized by a gently sloping (≈4°) sea floor and generally margin-parallel linear sets of sea-floor ridges that gradually deepen towards the flat Hispaniola Basin floor to the north. The ridges are caused by an internal structure consisting of broad anticlines bounded by thrust faults that dip southwards beneath Hispaniola. Anticlines form at the base of the slope and are eventually sheared and underthrust beneath the slope. In contrast, the second type of accretionary wedge exhibits a steeper (≈6–16°) sea-floor slope characterized by local slumping and a more abrupt morphological transition to the adjacent basin. The internal structure appears chaotic on seismic reflection profiles and probably consists of tight folds and closely spaced faults. We suggest that changes in sea-floor declivity and internal structure may result from variations in the dip or frictional resistance of the décollement, or possibly from changes in the cohesive strength of the wedge sediments. The observed pattern of thickening of Hispaniola Basin turbidites towards the insular margin suggests differential southwards tilting of the Hispaniola Basin strata, probably in response to North America-Caribbean plate interactions since the Early Tertiary. Based upon indirect age control from adjacent parts of the northern caribbean plate boundary, we infer a Late Eocene to Early Miocene episode of transcurrent motion (i.e. little or no tilting), an Early Miocene to Late Pliocene period of oblique convergence (i.e. increased tilt) during which the accretionary wedge began to be constructed, and a Late Pliocene to Recent episode of increased convergence (i.e. twice the Miocene to Pliocene tilt), which has led to rapid uplift and erosion of sediment sources on the margin and on Hispaniola, generating a submarine fan at the base of the insular slope.
Marine and Petroleum Geology↗
Comment and Reply on "SeaMARC II mapping of transform faults in the Cayman Trough, Caribbean Sea"
No abstract available.
Geology↗
A method of estimating the amount of in-situ gas hydrates in deep marine sediments
The bulk volume of gas hydrate in marine sediment can be estimated by measuring interval velocities and amplitude blanking of hydrated zones from true-amplitude processed multichannel seismic reflection data. In general, neither velocity nor amplitude information is adequate to independently estimate hydrate concentration. We propose a method that uses amplitude blanking calibrated by interval-velocity information to quantify hydrate concentrations in the Blake Ridge area of the U.S. Atlantic continental margin. On the Blake Ridge, blanking occurs in conjunction with relatively low interval velocities. The model that best explains this relation linearly mixes two end-member sediments, hydrated and unhydrated sediment. Hydrate concentration in the hydrate end-member can be calculated from a weighted equation that uses velocity estimated from the seismic data, known properties of pure hydrate, and porosity inferred from a velocity/porosity relationship. Amplitude blanking can be predicted as the proportions of hydrated and unhydrated sediment change across a reflection boundary. Our analysis of a small area near DSDP 533 indicates that the amount of gas hydrates is about 6% in total volume when interval velocity is used as a criterion and about 9.5% when amplitude information is used. This compares with a calculated value of about 8% derived from the only available measurement in DSDP 533.
Open-File Report↗
E-4 Central Kentucky to the Carolina Trough
E-4 is one of eight Geodynamics transects that cross the Atlantic margin of North America between Georgia and Newfoundland. Five of the transects are in the United States and three are in Canada. Transect E-4, which is 110 km wide and more than 1,100 km long, extends from the stable North American craton just west of the Grenville front near Lexington, Kentucky southeastward across Cape Fear, North Carolina, on the Atlantic coast to oceanic crust east of the Blake Spur magnetic anomaly. Like all of the other U.S. Atlantic coast transects, it crosses Cambrian and Jurassic continental margins of North America as well as the Appalachian orogen. The display, based upon published information, portrays the geology, tectonic style and geophysical expression of this segment of the eastern North American continental margin and interprets its Phanerozoic history. The Decade of North American Geology 1983 geologic time scale (Palmer, 1983) is used throughout the display and text.
Book↗
Magnetic and bathymetric data from R/V FARNELLA cruises FRNL87-1, 87-2, 87-3, 87-4 and 87-5 in the U.S. Atlantic Margin EEZ, southern Blake Escarpment, and Nova Scotia continental rise
During the winter and spring of 1987, the U.S. Geological Survey, in cooperation with the Institute of Oceanographic Sciences of the United Kingdom, collected approximately 31,350 line kilometers (km) of magnetic data and approximately 32,280 line km of bathymetric data in the U.S. Atlantic Margin Exclusive Economic Zone (EEZ), Southern Blake Escarpment, and Nova Scotia Rise (Fig. 1). Collected simultaneously with the magnetic and bathymetric data was a suite of geophysical data. These geophysical datasets will be released at a later date and include GLORIA (Geological LOng-Range Inclined Asdic) sidescan-sonar digital image data, 3.5 kilohertz (kHz) high-resolution sub-bottom profiler records, 10 kHz bottom profiler records from which bathymetric data were derived, and two channel seismic-reflection profiles. Survey line spacing was about 25 km in water depths exceeding 3,000 meters (m) and gradually decreased to less than 5 km in 200 m water depths. No areas were surveyed with depths shallower than 200 m. Orientation of the tracklines varies with respect to the trend of the bathymetric contours for each of the five legs (Fig.l).
Open-File Report↗
Seismic interpretation of gas hydrates in the Blake Ridge Area: Natural gas research and development
No abstract available.
Conference Paper↗
The development of the continental margin of eastern North America-conjugate continental margin to West Africa
The continental margin of eastern North America was initiated when West Africa and North America were rifted apart in Triassic-Early Jurassic time. Cooling of the crust and its thinning by rifting and extension caused subsidence. Variation in amounts of subsidence led to formation of five basins. These are listed from south to north. (1) The Blake Plateau Basin, the southernmost, is the widest basin and the one in which the rift-stage basement took longest to form. Carbonate platform deposition was active and persisted until the end of Early Cretaceous. In Late Cretaceous, deposition slowed while subsidence persisted, so a deep water platform was formed. Since the Paleocene the region has undergone erosion. (2) The Carolina Trough is narrow and has relatively thin basement, on the basis of gravity modeling. The two basins with thin basement, the Carolina Trough and Scotian Basin, also show many salt diapirs indicating considerable deposition of salt during their early evolution. In the Carolina Trough, subsidence of a large block of strata above the flowing salt has resulted in a major, active normal fault on the landward side of the basin. (3) The Baltimore Canyon Trough has an extremely thick sedimentary section; synrift and postrift sediments exceed 18 km in thickness. A Jurassic reef is well developed on the basin's seaward side, but post-Jurassic deposition was mainly non-carbonate. In general the conversion from carbonate to terrigenous deposition, characteristics of North American Basins, occurred progressively earlier toward the north. (4) The Georges Bank Basin has a complicated deep structure of sub-basins filled with thick synrift deposits. This may have resulted from some shearing that occurred at this offset of the continental margin. Postrift sediments apparently are thin compared to other basins—only about 8 km. (5) The Scotian Basin, off Canada, contains Jurassic carbonate rocks, sandstone, shale and coal covered by deltaic deposits and Upper Cretaceous deeper water chalk and shale.
Journal of African Earth Sciences↗
The Blake Plateau Basin and Carolina Trough
Presently, the continental margin of the southeastern United States (Fig. 1) forms a zone of transition between the actively building, steep-fronted carbonate platform of the Bahamas and the typical eastern North American terrigenous clastic-dominated, drowned, shelf-slope-rise configuration. This region of the continental margin is underlain by two major sedimentary basins—the Blake Plateau Basin and the Carolina Trough (Fig. 2)—which are different in shape, basement structure, and history. Indeed, the two southern basins show some of the greatest contrasts of any basins of eastern North America, especially in their early response to rifting and in the change from rifting to drifting. The region has experienced abrupt major changes in geological conditions, most notably the onset of Gulf Stream flow in the early Tertiary. Morphologically, the area is dominated by the broad, flat Blake Plateau at about 800-1,000 m water depth (Fig. 1). The plateau is bounded to the east by the extremely steep Blake Escarpment, descending to 5,000 m water depths. To the west, a short continental slope rises to a continental shelf. This Blake Plateau morphology characterizes the margin east of Florida and north of the Bahamas. North of Florida the margin merges into the typical shelf-slope-rise morphology. Just north of the Blake Escarpment and its northern projection, the Blake Spur, the Blake Ridge extends away from the continental slope at water depths exceeding 2,000 m (Fig. 1). This broad ridge is a Cenozoic, sedimentary drift deposit controlled by bottom currents. (For the reader who is beginning to wonder why half of the features of this region seem to be named "Blake", the Blake was a Coast Survey steamer from which investigations off the southeastern U.S. were carried out in 1877 to 1880. Ferromanganese nodules were discovered on the Blake Plateau at that time [Murray, 1885].)
Blake Plateau basin; Carolina Trough↗