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David C. Twichell

Publications and source records attributed to David C. Twichell.

65 records · Page 4Linked to original sources
USGS1982

Morphology, distribution, and development of submarine canyons on the United States Atlantic continental slope between Hudson and Baltimore Canyons

The distribution and morphology of submarine canyons off the eastern United States between Hudson and Baltimore Canyons have been mapped by long-range sidescan sonar. In this area canyons are numerous, and their spacing correlates with overall slope gradient; they are absent where the gradient is less than 3°, are 2 to 10 km apart where the gradient is 3° to 5°, and are 1.5 to 4 km apart where the gradient exceeds 6°. Canyons range from straight to sinuous; those having sinuous axes indent the edge of the continental shelf and appear to be older than those that head on the upper slope and have straighter axes. A difference in canyon age would suggest that canyons are initiated on the continental slope and only with greater age erode headward to indent the shelf. Shallow gullies on the middle and upper slope parts of the canyon walls suggest that submarine erosion has been a major process in a recent phase of canyon development.

Baltimore Canyon, Hudson Canyon↗
USGS1981

Morphology and processes associated with the accumulation of the fine-grained sediment deposit on the southern New England shelf

A 13,000 km 2 area of the southern New England Continental Shelf which is covered by anomalously fine-grained sediment has been surveyed by means of high-resolution, seismic-reflection and side-scan sonar techniques to map its morphology and structure, and a near-bottom instrument system contributed to understanding present activity of the deposit. Seismic-reflection profiles show that the fine-grained deposit, which is as much as 13 m thick, has accumulated during the last transgression because it rests on a reflector that is geomorphically similar to and continuous with the Holocene transgressive sand sheet still exposed on the shelf to the west. The ridge and swale topography comprising the sand sheet on the shelf off New Jersey and Long Island are relict in origin as these same features are found buried under the fine sediment deposit. Southwestward migrating megaripples observed on the sonographs in the eastern part of the deposit are evidence that sediment is still actively accumulating in this area. In the western part of the deposit, where surface sediment is composed of silt plus clay, evidence of present sediment mobility consists of changes in the near-bottom, suspended-matter concentrations primarily associated with storms. Nantucket Shoals and Georges Bank are thought to be the sources for the fine-textured sediment. Storms and strong tidal currents in these shoal areas may still erode available fine-grained material, which then is transported westward by the mean drift to the southern New England Shelf, where a comparatively tranquil environment permits deposition of the fine material.

Journal of Sedimentary Petrology↗
USGS1981

Single-channel seismic-reflection profiles and sidescan-sonar records collected by the R/V Neecho, cruise NE 79-06, on the inner shelf east of Cape Cod, Massachusetts

Cruise NE 79-06 of the R/V NEECHO was conducted by the U.S. Geological Survey during September 27-0ctober 3, 1979, in the nearshore zone (3-30 m water depth) seaward of Coast Guard Beach and the northern part of Orleans Beach, east of Cape Cod, Massachusetts. The purpose of the study was to map the types and extent of nearshore bed forms and to define the late Pleistocene and Holocene history of the area. The equipment used on this cruise consisted of an EG&G Uniboom, Raytheon echo sounder, and Edo Western sidescan-sonar system. The Uniboom data were mostly filtered to 400-4000 Hz and were recorded at a 1/4-s sweep rate. The 60-kHz echo-sounding data were recorded on a 6-in strip chart on which the depth was calibrated in feet. The sidescan sonar had an operating frequency of 100 kHz and was set to scan 50 m or 100 m to each side of the towed fish. All three data types were collected along 153 km of trackline. Navigation during the survey was by Loran-C and Motorola miniranger systems. Two shore stations were set up for the miniranger system and fixes were collected at either 1- or 2-min intervals. This system malfunctioned during parts of the survey, and during these times navigation was by Loran-C using a Northstar system. The original records can be seen and studied at the U.S. Geological Survey Data Library at Woods Hole, MA 02543. Microfilm copies of the subbottom, echo-sounding, and sidescan-sonar records can be purchased only from the National Geophysical and Solar-Terrestrial Data Center, NOAA/EDIS/NGSDC, Code D621, 325 Broadway, Boulder, CO 80303, (303-497-6338).

Open-File Report↗
USGS1981

Single-channel seismic-reflection profiles and sidescan-sonar records collected on Georges Bank

Subbottom seismic-reflection, echo-sounding, and sidescan-sonar records were collected by the U.S. Geological Survey during two cruises over Georges Bank aboard the R/V OCEANUS (cruise 056, March 6-15, 1979 and cruise 067, August 6-13, 1979) (fig. 1). The data collected during these two cruises contributed to a study of bed-form types and distribution and of sediment transport paths on Georges Bank. The equipment used on these cruises consisted of an EG&G Uniboom 12-kHz echo-sounder, and Klein sidescan-sonar system. The Uniboom data were collected on the OCEANUS 067 cruise along 185 km of trackline (fig. 1), and the 12-kHz echo-sounder and sidescan-sonar data were collected during both cruises along 465 km of trackline. Navigation during both cruises consisted of Loran-C fixes logged at least every 15 minutes. After the cruise, the navigation data were digitized and stored on magnetic tape. The original records can be studied at the U.S. Geological Survey Data Library at Woods Hole, MA 02543. Microfilm copies of the subbottom, echo-sounding, and sidescan-sonar records can be purchased only from the National Geophysical and Solar-Terrestrial Data Center, NOAA/EDIS/NGSDC, Code D621, 325 Broadway, Boulder, CO 80303 (303-497-6338).

Georges Bank↗
USGS1981

Geology and potential hazards of the continental slope between Lindenkohl and South Toms Canyons, offshore mid-Atlantic United States

Because sediment instability, or slumping, has been identified as a potential hazard to petroleum development of the east-coast Continental Slope, the U.S. Geological Survey, in cooperation with the U.S. Bureau of Land Management, began a detailed study of a segment of the Continental Slope between Lindenkohl and South Toms Canyons off New Jersey. This 40-km x 35-km area was chosen for study because it lies within the area of high interest for petroleum development (Lease sales 49 and 59), and because it includes several wells which provide stratigraphic control. This report discusses the results of geologic mapping, using seismic-reflection data acquired in 1978 and 1979. Some initial results from more recently acquired data are included. The Continental Slope in the study area has a complex surface with ridges, canyons, and valleys. Three slump or slide features were observed in the heads and on the walls of canyons and valleys, and two slides were identified on an intercanyon area. The identified slumps or slides are found in Quaternary sediments and total about 1.3 percent of the Continental Slope area mapped. The slope is generally mantled by less than 2 m of Holocene sediments. Pleistocene sediment (primarily silty clay) is about 450 m thick at the top of the slope and thins to nearly zero or is absent on much of the mid and lower slope, where sediments of Miocene to Eocene age are exposed. Ridges on the midslope (water depths of 800-1,500 m) and parts of the lowr slope (1,500-2,150 m) result primarily from Pleistocene and older deposition. The intervening valleys show evidence of erosion along the deepest parts of their courses. Mid-range sidescan-sonar data show evidence that processes of bottom-current erosion and downcanyon transport of material may be active in the present day. However, major features of the sea floor appear to be unchanged since late-Pleistocene time.

Lindenkohl and South Toms Canyons↗
USGS1981

Bed form distribution and inferred sand transport on Georges Bank

Four bed-form provinces have been identified on Georges Bank using sidescan sonar and echo-sounding techniques: large sand waves, small sand waves, megaripples, and featureless sea floor. The large sand waves are found on the bank crest where the surface tidal currents are strongest, and are bordered, north and south, by areas of small sand waves and/or megaripples, formed where tidal currents are moderate in strength. Featureless sea floor is found farthest from the bank crest where surface tidal currents are weakest. Bed-form asymmetry and surface-sediment texture have been used to infer bedload transport paths on the bank. In the large sand-wave area, bed forms converge on linear northwest-striking ridges from both sides implying erosion from the troughs and growth of the ridges. The asymmetry of the small sand waves along the south side of the bank indicates that sand is also transported southward away from the bank. Though the bed-form asymmetry of megaripples could not be determined in this study, the occurrence of megaripples between the sand-wave provinces and areas of featureless sea floor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand transport pattern is further supported by surface-sediment texture which becomes progressively finer both to the north and southwest away from the crest of Georges Bank.

Connecticut, Maine, Massachusetts, New Hampshire, ↗
USGS1980

Single-channel seismic-reflection profiles and sidescan sonar records collected during May 15-20, 1978, on the southern New England continental shelf

The U.S. Geological Survey completed a cruise aboard the R/V CAPE HENLOPEN during May 15-20, 1978, to map the surface character, thickness and extent of the fine-grained.sediment deposit that covers an area 100 x 200 km on the southern New England Continental Shelf. The study area lies between Great South Channel to the east and Black Channel to the west, and extends from the 50-m isobath to the shelf edge. Single-channel high-resolution seismic-reflection profiles and echo-sounding profiles were collected along 941 km of trackline, sidescan sonar records were collected along 673 km of trackline. The subbottom profiles were collected by using a Huntec*system that was towed at midwater depths. Filters were set at 1 to 7 kHz. Echo-sounding records were collected by using a 60 kHz EDO Western system. A Klein stdescan sonar, set to scan 100 m to either side of the towed fish, was used to collect the sonographs. Navigation during the survey was done by the scientific staff using Loran-C equipment. Fixes were recorded and logged at least every 15 minutes; after the cruise, they were digitized and stored on magnetic tape. The original records can be seen and studied at the U.S. Geological Survey Data Library at Woods Hole, MA 02543. Microfilm copies of the subbottom, echo­sounding, and sidescan sonar records collected during the cruise can be purchased from the National Geophysical and Solar-Terrestrial Data Center, NOAA (National Oceanic ancl Atmosphere Administration), Boulder, CO 80302.

Open-File Report↗
USGS1979

Summary report of the sediments, structural framework, petroleum potential, and environmental conditions of the United States Mid-Atlantic continental margin in area of proposed Oil & Gas Lease Sale No. 59

This report has been compiled to update and summarize the geological information concerning the area of the Atlantic continental margin off the U.S. Mid-Atlantic proposed for Oil and Gas Lease Sale No. 59 (fig. 1). The region of interest lies between 35° and 41° N and 70.5° and 76° W. The north-south dimension is about 660 km (356 nautical miles, 410 statute miles) and the maximum east-west dimension is approximately 250 km (135 nautical miles, 155 statute miles). The area is centered toward the outer Continental Shelf off Delaware and contains more than 154,000 km2 (38,000,000 acres) off the coasts of North Carolina, Virginia, Maryland, Delaware, New Jersey and New York. Water depths within the region range from 20 to slightly more than 4,000 m (fig. 1). This report begins with a discussion of the regional geology and geophysics, followed by a description of the oil and gas potential and potential environmental hazards.

Delaware, Maryland, New Jersey, New York, North Ca↗
USGS1979

Single-Channel Seismic-Reflection Profiles and Side-Scan Sonar Records Collected During June 8-14, 1977, in the Middle Atlantic Shelf Area

A cruise aboard the R/V OCEANUS (Cruise 027) was completed by the U.S. Geological Survey during June 8-14, 1977evaluate the presence, extent, and activity of potentially mobile bedforms on the Middle Atlantic Continental Shelf, eastern United States. Based on information collected by the U.S. Geological Survey during·a geophysical survey in 1975, seven sites showing possible bedforms were selected for detailed study. At each of the seven study areas, single-channel high-resolution seismic profiling and side-scan sonar techniques were used to identify and map the size, distribution, and orientation of the different types of bed­forms that were present.

Open-File Report↗
USGS1978

Heavy-mineral variability in the Baltimore Canyon trough area

Petrographic analyses of bottom sediments from 87 stations within a relatively large subarea (1700 square kilometers) define the local variability and the distributional processes of heavy minerals in the Baltimore Canyon Trough area (13500km 2 ). Of the 29 mineral groups that were identified, those most diagnostic of differences between stations were opaque minerals, garnet, hornblende, orthopyroxene, other amphiboles, and staurolite. Some of the common components cannot be used to define areal trends. The association of minerals by their specific gravities reflects the modern reworking and sorting of sediments in this area. The poor correlation between mineral abundance and the ridge-and trough topography suggests either that our analyses and samples did not resolve bathymetrically induced changes or that smaller bed forms (such as ripples) largely control the heavy-mineral distribution. Data from this study provide a baseline for evaluating manmade disturbances of the bottom sediments and show that relatively large changes in the heavy mineral composition can take place over small distances on this part of the Continental Shelf.

Baltimore Canyon Trough↗