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David C. Twichell

Publications and source records attributed to David C. Twichell.

At least 37 records · Page 2Linked to original sources
USGS2002

High-Resolution Multichannel Seismic-Reflection Data Acquired in the Northern Gulf of Mexico, 1998-99

During June 1998 and April 1999, the U.S. Geological Survey (USGS) conducted two research cruises in the northern Gulf of Mexico to acquire high-resolution seismic reflection data across the upper and middle continental slope as part of an investigation of the seismic character, distribution, and potential effects of naturally-occurring marine gas hydrates and related free gas within the gas hydrate stability zone. Over 1600 km of two-dimensional multichannel seismic reflection profiles were acquired during these two cruises. The specific objectives of this investigation are (a) to produce high-resolution images of the gas hydrate stability zone; (b) to study the distribution and character of potential seafloor failures and their relationship to known and inferred gas hydrate deposits; (c) to look at systematic variations in subsurface structure in gas hydrate and non-hydrate areas; and (d) to estimate, if possible, the amounts of hydrates present within the gas hydrate stability zone. The multichannel profiles provide high-quality images with approximately 5 meters of vertical resolution and up to 2 km of penetration. This report gives an overview of the acquisition and data processing of the multichannel seismic reflection profiles and provides references and links to reports with more detailed information. Geologic interpretations of these seismic profiles regarding gas hydrate occurrence and distribution within the study areas of this investigation are given in Cooper and Hart (2002).

Open-File Report↗
USGS2001

Surficial geology and distribution of post-impoundment sediment in Las Vegas Bay, Lake Mead

Sidescan sonar imagery and seismic-reflection profiles were collected in the northwestern part of Las Vegas Bay to map the distribution and volume of sediment that has accumulated in this part of Lake Mead since impoundment. The mapping suggests that three ephemeral streams are the primary source of this sediment, and of these, Las Vegas Wash is the largest. Two deltas off the mouth of Las Vegas Wash formed at different lake elevations and account for 41% of the total volume of post-impoundment sediment within the study area. Deltas off the other two washes (Gypsum and Government) account for only 6% of the total volume. The sediment beyond the front of the deltas is primarily mud, and it only occurs in valley floors, where it forms a flat-lying blanket that is mostly less than 1.5 m thick. Although a thin layer, the fine-grained sediment accounts for approximately 53% of the total post-impoundment sediment volume of 5.7 x 106 m3 that has accumulated in the study area. This sediment appears to have been transported several kilometers from the river sources by density flows.

Nevada↗
USGS2000

Late-stage development of the Bryant Canyon turbidite pathway on the Louisiana continental slope

GLORIA sidescan imagery, multibeam bathymetry, seismic profiles, and piston cores (3–5 m penetration) reveal the near-surface geology of the Bryant Canyon turbidite pathway on the continental margin of Louisiana. This pathway extends from the continental shelf edge, across the continental slope, to a deep-sea fan on the continental rise. The pathway is narrow (<2 km) where it crosses shallow salt deposits. Turbidites have been sampled from these narrow segments, and radiocarbon dates indicate that some of them accumulated as recently as 10,150 yr B.P. The pathway broadens however, where it crosses mini-basins whose floors are covered largely by muddy mass-transport deposits and coarse silt turbidites. Mass-transport deposits in the upper 4.7 m of cores from the floors of mini-basins accumulated 18,140-3,400 yr. BP. Seismic profiles show that the mass-transport deposits in some of the mini-basins are as much as 225 milliseconds thick and that turbidites in the basin floor are buried by these deposits. Salt movement has had a major impact on this pathway, and its thalweg no longer has a continuous down-slope gradient. Some mini-basin floors along the pathway are now more than 500 m deeper than their basin’s spill point. We propose a 6-stage conceptual model to explain our observations for the evolution of a mini-basin along this turbidite pathway. In this model, an active channel feeds sand to a mini-basin (Stabe B). Once the mini-basin is filled, the sand deposit is entrenched by a bypass channel (Stage C). When the turbidite system shuts off, salt migration oversteepens the mini-basin walls (Stage D) which collapse and create a layer of mass-transport deposits on the mini-basin floor (Stage E). The depositional succession is capped by a layer of highstand hemipelagic drape (Stage F). The Bryant Canyon turbidite pathway provides a recent example of a large turbidite pathway in the Gulf of Mexico that crosses an area of active salt tectonics thus providing a conceptual model for older systems in similar settings. In Bryant Canyon, thick turbidite sands presumably are found in mini-basins however, they are sealed by thick, fine-grained, mass-transport deposits which terminate mini-basin turbidite deposition cycles. The importance of mass-transport deposits in basins along this turbidite pathway is in startling contrast to the Trinity-Brazos pathway whose shallow subsurface expression is virtually free of mass-transport deposits and has undergone minimal deformation by salt movement.

Conference Paper↗
USGS2000

Relationships among sea-floor structure and benthic communities in Long Island Sound at regional and benthoscape scales

Long Island Sound is comprised of a rich and spatially heterogeneous mix of sea-floor environments which provide habitat for an equally diverse set of assemblages of soft-sediment communities. Information from recent research on the geomorphological and chemical attributes of these environments, as well as from studies of the hydrodynamics of the Sound, provide the opportunity to develop a landscape, or "benthoscape" framework for understanding the soft-sediment ecology of this estuary and for guiding future research focusing on structure and function at multiple spatial scales. This contribution reviews past research on benthic communities in Long Island Sound and addresses how they may be shaped by sea-floor characteristics at regional and benthoscape scales. At the regional scale (i.e. the entire Sound), differences in benthic community composition correspond to the distribution of general sedimentary environments. However, significant variation in community structure also occurs at the benthoscape scale (within regions) related to local variations in sediment properties, and physical and biogenic topographic features. Several topical areas in particular need further research in Long Island Sound, including temporal dynamics of benthic communities relative to sea-floor structure and the interaction between the dynamics of benthoscapes and hydrologic seascapes.

Connecticut, New York↗
USGS1999

Surficial geology and distribution of post-impoundment sediment of the western part of Lake Mead based on a sidescan sonar and high-resolution seismic-reflection survey

Sidescan sonar imagery and high-resolution seismic-reflection profiles were collected in Las Vegas Bay and Boulder Basin of Lake Mead to determine the surficial geology as well as the distribution and thickness of sediment that has accumulated in these areas of the lake since the completion of Hoover Dam in 1935 (Gould, 1951). Results indicate that the accumulation of post-impoundment sediment is restricted to the original Colorado River bed which runs down the axis of Boulder Basin from Boulder Canyon to Hoover Dam, and the old Las Vegas Creek bed that bisects Las Vegas Bay. The sediment cover along the original Colorado River bed is continuous and is typically greater than 10-m thick throughout much of its length with the thickness in some areas exceeding 35 meters. The flat-lying nature of the deposits suggests that they are the result of turbidity currents that flow the length of the lake. The sediment cover in Las Vegas Bay is much thinner (rarely exceeding 2 m in thickness) and more discontinuous. The source for these sediments presumably is Las Vegas Wash and a series of other ephemeral washes that empty into this part of the lake. The presence of sediments along the entire length of the Las Vegas Creek bed suggests that turbidity currents probably are active here as well, and that sediment has been transported from these streams at least 10 km down the axis of this valley to where it enters Boulder Basin. Alluvial deposits and rock outcrops are still exposed on large parts of the lake floor.

Open-File Report↗
USGS1998

GLORIA sidescan sonar field data and navigation data collected off Puerto Rico in 1985 and the eastern United States in 1987

This CD-ROM contains copies of the navigation and field sidescan sonar data collected within the Exclusive Economic Zone (EEZ) surrounding Puerto Rico and off the eastern United States during cruises aboard the R/V FARNELLA. The survey of the Puerto Rico EEZ was completed during one cruise between 4 November and 3 December, 1985. The survey of the EEZ off the eastern United States was completed during five cruises between 2 February and 30 May, 1987. The data were collected as part of the USGS EEZ-SCAN program which was a cooperative mapping program between the US Geological Survey and the Institute of Oceanographic Sciences of the UK. The survey areas included the entire US EEZ of Puerto Rico and the US Virgin Islands as well as of the eastern United States from the Canadian international boundary at Georges Bank to the Bahamian international boundary east of Florida. GLORIA imagery was collected seaward of approximately 400 m water depth.

Open-File Report↗
USGS1996

Integration of orthophotographic and sidescan sonar imagery: an example from Lake Garda, Italy

Digital orthophotos of Lake Garda basin area are available at the scale of up to 1:10,000 from a 1994 high altitude (average scale of 1:75,000) air photo coverage of Italy collected with an RC30 camera and Panatomic film. In October 1994 the lake bed was surveyed by USGS and CISIG personnel using a SIS 1000 Sea-Floor Mapping System. Subsystems of the SIS-1000 include high resolution sidescan sonar and sub-bottom profiler. The sidescan imagery was collected in ranges up to 1500m, while preserving a 50cm pixel resolution. The system was navigated using differential GPS. The extended operational range of the sidescan sonar permitted surveying the 370km lake area in 11 days. Data were compiled into a digital image with a pixel resolution of about 2m and stored as 12 gigabytes in exabyte 8mm tape and converted from WGS84 coordinate system to the European Datum (ED50) and integrated with bathymetric data digitized from maps.The digital bathymetric model was generated by interpolation using commercial software and was merged with the land elevation model to obtain a digital elevation model of the Lake Garda basin.The sidescan image data was also projected in the same coordinate system and seamed with the digital orthophoto of the land to produce a continuous image of the basin as if the water were removed. Some perspective scenes were generated by combining elevation and bathymetric data with basin and lake floor images. In deep water the lake's thermal structure created problems with the imagery indicating that winter or spring is best survey period. In shallow waters, ≤ 10 m, where data are missing, the bottom data gap can be filled with available images from the first few channels of the Daedalus built MIVIS, a 102 channel hyperspectral scanner with 20 channel bands of 0.020 μm width, operating in the visible part of the spectrum. By integrating orthophotos with sidescan imagery we can see how the basin morphology extends across the lake, the paths taken by the lake inlet along the lake bed and the areal distribution of sediments. An extensive exposure of debris aprons were noted on the western side of the lake. Various anthropogenic objects were recognized: pipelines, sites of waste disposal on the lake's bed, and relicts of Venitian and Austrian(?) boats.

Lake Garda↗