Geology topics
Kathryn M. Scanlon
Publications and source records attributed to Kathryn M. Scanlon.
Comment on the mapping of iron-manganese nodule fields using reconnaissance sonars such as GLORIA
.No abstract available.
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).
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 geologic map of the north insular shelf of Puerto Rico; Rio de Bayamon to Rio Grande de Loiza
No abstract available.
The neotectonic setting of Puerto Rico
The island of Puerto Rico, in the northeast Caribbean, lies within a broad deformation zone between the Caribbean and North American plates. The simplest model for the tectonic setting of Puerto Rico has major strike-slip movement on nearly east-west lines in the vicinity of the Puerto Rico Trench coupled to a small counterclockwise rotation of a Puerto Rico block within the broader plate boundary zone. This simple model is attractive because it predicts the tectonic regime south of Puerto Rico, and provides an explanation for a possible component of extension across the Puerto Rico Trench west of 65.5°W. GLORIA long-range sidescan sonar data and seismic reflection profiles have been used to test this model by mapping the major tectonic features across the plate boundary north and south of Puerto Rico. To the north, the new data help to resolve between conflicting models, of underthrusting or strike-slip motion at the Puerto Rico Trench. No direct evidence of compression is seen, although evidence for normal and strike-slip movement is abundant. This, combined with regional considerations, leads us to conclude that the main east-west-trending part of the Puerto Rico Trench between 65.5°W and 68°W lies within a strike-slip regime, although oblique convergence occurs both to the east and west where the plate boundary trends east-southeast. To the south of Puerto Rico, underthrusting of the Caribbean plate beneath the island decreases from west to east, and it is ultimately replaced by extension in the Virgin Islands Basin east of 65°W.
Geological interpretation of combined Seabeam, Gloria and seismic data from Anegada Passage (Virgin Islands, north Caribbean)
The Anegada Passage (sensu lato) includes several basins and ridges from Southeast of Puerto Rico to the corner of the Virgin Islands Platform. Seabeam (Seacarib I) and Gloria long-range sidescan sonar surveys were carried out in this area. These new data allow us to propose an interpretation of the Anegada Passage. Most of the features described are related to wrench faulting: (a) St Croix and Virgin Islands Basins are pull-apart basins created in a right-lateral strike-slip environment based on their rhomboidal shape and seismic data (e.g. the flower structure). These two pull-aparts are divided into two sub-basins by a curvilinear normal fault in the Virgin Islands Basin and a right-lateral strike-slip fault in the St Croix Basin. (b) Tortola Ridge and a 'dog's leg' shaped structure are inferred to be restraining bends between two right-lateral strike-slip faults. (c) We identified two ENE-WSW volcanic lineaments in the eastern area and one volcano lying between Virgin Islands and St Croix Basins. (d) As shown by the seismic activity main wrench motion occurs along the north slope of Virgin Islands Basin and through Anegada Passage. A branching of this main fault transmits the transtensional motion to St Croix Basin. A two-stage story is proposed for the creation of the basins. A first extensional event during Eocene(?)-Oligocene-lower Miocene time created Virgin Islands, St Croix Basins and the tilted blocks of St Croix Ridge. A second transtensional event from Pliocene to Recent gave the present day pattern to this area. However, the displacement along the strike-slip faults is no more than 15 km long. The proposed geodynamic model is based on the separation of the northeastern Caribbean boundary into two blocks. In the West, the indenter of Beata Ridge gives a northeastern motion to Hispaniola Block. In the East, as a result of Hispaniola Block's motion, the Puerto Rico-Virgin Islands Block could escape in an east-northeast direction.
Pockmarks in the floor of Penobscot Bay, Maine
Hundreds of depressions (pockmarks) were found within a 40 square kilometer area of the sea floor near the head of Penobscot Bay, Maine. These roughly circular depressions range in diameter from 10 to 300 meters and extend as much as 30 meters below the surrounding sea floor. The pockmarks have formed in marine mud of Holocene age, which unconformably overlies glaciomarine deposits. The presence of shallow interstitial gas in the mud suggests that the pockmarks are related to the excipe of gas from the sediments, although other factors must be involved. ?? 1989 Springer-Verlag New York Inc.
Single-channel seismic-reflection profiles collected aboard R/V POWELL, cruises P-2-85, P-3-85, P-4-85 in the nearshore waters around Puerto Rico and the Virgin Islands
Approximately 4,600 nmi (8,519 km) of single-channel seismic-reflection data were collected in the nearshore waters around Puerto Rico and the Virgin Islands simultaneously, but on a noninterference basis, with a gravity program funded by the Defense Mapping Agency. The survey was conducted on cruises P-2-85, P-3-85, and P-4-85 of the R/V POWELL between February 28 and April 8, 1985. Seismic instrumentation included two 40 in 3 (755 cm 3 ) airguns and a 200-ft (60-m) single-channel hydrophone streamer. In addition, a Uniboom sled and 8-ft (2.4-m) single-channel hydrophone streamer were used in water depths less than 660 ft (200 m). Navigation control was provided by a Magnavox Integrated Navigation System which integrates data from Global Positioning System (GPS) satellites, transit satellites, bottom-track sonar, Loran-C, Mini-Ranger stations, gyrocompass and speed log. GPS provided the most accurate navigational control, but was functional for only about 12 hours each day. The transit satellite system operated throughout the cruise and provided relatively accurate positions when GPS was not operational. The Mini-Ranger stations proved to be unreliable because of problems at the shore stations and Loran-C operated only on the north side of the islands, at times in only the range-range mode. Because the gravity program required accurate data and strict navigation control, some lines had to be run more than once or run with a following sea in a westerly direction only. Consequently, there is tight line spacing in some areas. The Uniboom high-resolution reflection data are good to poor in the shelf areas around Puerto Rico but are generally poor over the predominantly carbonate sediments around the Virgin Islands. The airgun data are fair to good, particularly considering the ship cruised at about 7 kn (13.0 km/hr) and at times reached 8 kn (14.8 km/hr) which is about the upper limit at which the system will function. Original seismic profile records can be seen at the U.S. Geological Survey offices, Woods Hole, MA 02543. Microfilm copies of the seismic profile records and trackchart can be purchased only from the National Geophysical Data Center, Code E64, 325 Broadway, Boulder, CO 80303 (303/497-6345).
Geology of the Caribbean
The Venezuelan and Colombian basins are located on the Caribbean Plate whilst the Yucatan basin is on the North American Plate. The processes occurring at the boundaries between the Caribbean Plate and the adjacent North American, South American and Cocos Plates, and the resulting surface features and patterns of volcanic and earthquake activity are described. Most of the Caribbean area is floored by atypical oceanic crust and its most valuable main geologic resources identified so far are petroleum, together with sand and gravel. Geological research is being carried out with techniques for broad-range swath imaging of the seafloor, such as GLORIA, and for directly measuring the movement between plates. -J.G.Harvey
Sedimentary framework of Penobscot Bay, Maine
Analyses of seismic-reflection profiles, along with previously collected sediment samples and geologic information from surrounding coastal areas, outline the characteristics, distribution, and history of the strata that accumulated within Penobscot Bay, Maine, during the complex period of glaciation, crustal movement, and sea-level change since late Wisconsinan time. Sediments that overlie the rugged, glacially eroded surface of Paleozoic bedrock range in thickness from near zero to more than 50 m and consist of four distinct units. Massive to partly stratified, coarse-grained drift forms thin (< 15 m) isolated patches along the walls and floors of bedrock troughs and constitutes a thick (up to 30 m), hummocky end moraine in the central part of the bay. The drift was deposited by the last ice sheet between 12,700 and 13,500 years ago during deglaciation and coastal submergence (due to crustal depression). Well-stratified, fine-grained glaciomarine deposits are concentrated in bedrock depressions beneath the main passages of the bay. During the period of ice retreat and marine submergence, these sediments settled to the sea floor, draped the irregular underlying surface of bedrock or drift, and accumulated without disturbance by physical or biologic processes. Heterogeneous fluvial deposits fill ancestral channels of the Penobscot River beneath the head of the bay. The channels were incised during a −40 m postglacial low stand of sea level (due to crustal rebound) and later were filled as base level was increased during Holocene time. Muddy marine sediments, which are homogeneous to weakly stratified and rich in organic matter, blanket older deposits within bathymetric depressions in the middle and lower reaches of the bay and cover a pronounced, gently dipping, erosional unconformity in the upper reach. These sediments were deposited during the Holocene transgression as sea level approached its present position and the bay became deeper. Late Wisconsinan and Holocene sedimentation in Penobscot Bay has smoothed the sea floor, but it has not completely obscured the ice-sculptured bedrock topography.
The continental slope off New England: A long-range sidescan-sonar perspective
The first continuous overview of a large segment of the continental slope and rise off the northeastern United States has been obtained using the GLORIA II long-range sidescan-sonar system. Extensive dissection by canyon and gully systems and evidence of possible large-scale sediment sliding are seen on the slope. The style and degree of incision, as well as the numbers and locations of canyons, have been found to differ significantly from previously published maps. It is suggested that the slope is a significant source of the sediment that has been deposited on the rise, and that some abrupt changes in the courses of canyons may be the result of local structural control.
GLORIA sidescan and seismic data collected by the DESV STARELLA along the continental slope and upper continental rise of the Eastern United States in 1979
A cruise on the- DESV STARELLA was conducted between October 23. 1979, and November 8, 1979, by the U.S. Geologic.al Survey (USGS) in cooperation with the United Kingdom's Institute of Oceanographic Sciences (IOS). The purpose of the study was to gain an overview of the morphology of large areas of the Continental Slope and upper Continental Rise off the eastern ·united States. Sidescan-sonar and single-channel seismic-reflection data were collected along the tracklines. The ship's position. was monitored using LORAN_C and satellite navigation techniques. The sidescan data were collected using the !OS-developed Geologic Long-Range Inclined Asdic, Mark II (GLORIA II) system. GLORIA was operated at either a 20-second or 40-second sweep, giving maximum slant-range distances of 15 or 30 kilometers, respectively, to each side of the towed fish. The sidescan data were collected on graphic recorders and analog tapes. The tapes were played onto photographic paper in a facsimile machine; then these prints were photographically stretched to produce approximately isometric images .. The images were not corrected for slant-range distortion. Mosaics at a scale of 1:250,000 (Mercator projection) were made from the photographic images. The seismic· data consist of 40-in 3 airgun and 10-kHz echo-sounder profiles. The airgun data were recorded on EPC recorders at both 8- and 4- second sweep rates. The 10-kHz echo-sounder data were recorded on a wet-paper recorder at a 2-second sweep rate. Approximately 4,820 km of seismic and sonar-data wera collected from four areas of interest and along the lines connecting those areas. Area 1 includes the Continental Slope and upper Rise seaward of Georges Bank, between Northeast Channel and Alvin Canyon. Numerous lines were run here, resulting in approximately 2,500 km of seismic data and complete and overlapping GLORIA coverage of the area. A similar survey, consisting of approximately 1 2 200 km of trackline, was run in area 2, which includes the Continental Slope and upper Rise seaward of the middle Atlantic states between Hudson Canyon and Baltimore Canyon. A single line, 250 km long, was run through area 3 along the base of the Continental Slope from Cape Hatteras to latitude 33°N. off North and South Carolina. Two parallel lines, totaling 870 km, were run through area 4 along the Blake Escarpment, from the Blake Spur south to latitude 27°40'N. One line was run along the top of the escarpment and the other was run along the base of the escarpment. All the data can be studied at the U.S. Geological Survey offices in Woods Hole, MA 02543. Microfilm copies of the sidescan-monitor record as well as the airgun and 10-kHz profiles can be purchased only from the National Geophysical Data Center, NOAA, Code E64, 325 Broadway, Boulder, CO 80303 (303/497-6338). Photographic copies of the GLORIA mosaics can also be obtained only from this address.
Geomorphic features of the western North Atlantic continental slope between Northeast Channel and Alvin Canyon as interpreted from Gloria II long-range sidescan-sonar data
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