US Gulf deepwater seafloor dominated by canyons, ridges, and faults

Jia Y. Liu
William R. Bryant
Texas A&M University

TAMU, NOAA seek to improve seafloor maps

The continental slope off Texas and Louisiana has a regional slope gradient of less than 1°, but locally the seafloor is very rugged and the slope gradient can exceed 30°. The near-surface geology and topography of the continental slope of these areas are a function of the interplay between episodes of rapid shelf edge progradation and contemporaneous modification of the depositional sequence by diapirism and mass movement processes.

Many slope sediments have been uplifted, folded, fractured, and faulted by diapiric action. With increased interest in deepwater hydrocarbon discoveries in subsalt structures on the continental slope off Texas and Louisiana, understanding the deepwater environment becomes essential. In describing the deepwater environment, the morphology of the seafloor is the most elemental property.

Before 1992, the most detailed bathymetric map in the Gulf of Mexico (GOM) was published by Bryant et al. (1990). The authors used seismic data that had a grid space of 8 to 14 km. In 1992, National Oceanic and Atmospheric Administration (NOAA) released gridded multibeam data for the northwestern Gulf of Mexico continental slope with a grid of 250 meters.

In 1997, Liu used the released multibeam raw data from the National Geophysical Data Center (NGDC) and gridded the bathymetry of northwestern of GOM continental slope using 50 m spacing (Liu, 1997a). About the same year, Shell Oil Co. produced a similar bathymetric map that covers the same area. Earthfield Technology marketed the map and the software program.

Although Liu's and Shell's efforts revealed the fine detail and complexity of the seafloor offshore of a major portion of the continental slope of Texas and Louisiana, two problems remained:

• Seafloor morphology outside Liu's and Shell's work was still relatively unknown.
• Detailed bathymetric data was still difficult to access and/or too costly.

Gridding

The construction of the bathymetry data in the multibeam covered area includes georeferencing multibeam depth soundings and deleting erroneous soundings (Liu, 1997b). The depth soundings were interpolated using universal kriging with a linear model (Liu, 1997b) into the 50-meter grid south of 27.25°N and 25 m grid north of 27.25°N.

The difference in grid spacing is to accommodate higher resolution in the shallower water area while minimizing computer disk space needed for data storage. Outside the multibeam coverage, digitized contours were rasterized with a grid space of 300 meters (Arc/Infotrademark).

ArcView was used as a major map viewer and can be run on different computer platforms. The script language, Avenue, was used to:

• Customize the interface
• Select parameters for the user (e.g. shading direction, contouring interval, etc.)
• Query the data (based on feature names, Outer Continental Shelf/OCS area, and lease blocks).

Applications

Recent Seasat/Geosat/ERS satellite data compiled by Conoco and Scripps Institute offered a large picture of the land and seafloor morphology of the earth. The depth reading of the satellite data in the water was 10-20 km (Shirley, 1998). This is about 200 times the grid spacing of the multibeam data. The detail and resolution of the TGMNB CD-ROM set will improve our ability to explain the surficial slope geology.

For the past few years, we have successfully used the bathymetry data with side-scan sonar data and TAMU Deep Tow records to interpret the geological processes and to examine geohazards of the Gulf of Mexico continental slope. With the close relationship of bathymetry and underlying salt, the high-resolution bathymetry also allows us to fine-tune our interpretation.

The original interpretation was based on traditional seismic data. When the bathymetry is combined with shipboard Global Positioning System (GPS) during geophysical/geological surveys, we can observe the real-time ship position, estimate the time for each transit, and locate sampling locations.

An example

Bathymetric maps of the Alaminos Canyon are shown based on the seismic data (Figure 1 [27,361 bytes]) and multibeam data (Figure 2 [20,971 bytes]). The canyon is located in the lower continental slope of the northwestern of Gulf of Mexico and separates a dome-shaped Texas-Louisiana continental slope on the east from a relatively flat area on the west (Bryant et al., 1990).

Before the multibeam bathymetry data became available, the canyon was described as a broad, square-shaped feature, formed by complex faulting, folding, salt deformation, submarine erosion (Bouma et al., 1972), and by coalescing of salt canopies (Hardin, 1989).

From the detailed multibeam bathymetric map, the mainly box-shaped canyon shows two re-entrants in the west of the canyon aligned NNW-SSE and NNE-SSW. A third re-entrant is in the northeast, aligned NE-SW. A 100 to 200-meter high ridge separates the third re-entrant from the main path of the canyon, which runs east to west then turns to the south.

There are numerous gullies along the main path of the canyon. The slope gradient varies from less than 5° to more than 40° with high angled slopes along the canyon rim and in the center along the main channel path (Figure 3 [13,568 bytes]). The topographic structure map highlights linear structures of the canyon (Figure 4 [28,022 bytes]).

In the topographic structure map, the main drainage path cuts through a series of terraces from east to west, then from north to south. Along the northern side of the main path are 2-km-wide gullies that are spaced 2 km apart. Along the western side of the main path, the gullies are approximately 1 km wide and 1 km apart. The faults in the southeast of the canyon are prominent in the topographic structure map. These faults seem to continue to the south-central portion of the canyon, but are interrupted by the canyon wall. The drainage network map indicates the potential routes for turbidity currents and debris flows from the upper slope to

the canyon (Figure 5 [18,737 bytes]).

The dendritic pattern is analogous to subaerial river systems. The heads of the streams start from the three re-entrants of the canyon and converge to the main drainage path that meanders from east, west, then to the south. Another separate drainage system occurs in the southeast of the canyon and tends to follow fault planes.

The project

The "TAMU Gulf of Mexico/NOAA Bathymetry (TGMNB) CD-ROM" project is a joint project of the National Sea Grant College Program and Texas A&M University. The purpose of this project is to utilize NOAA's and Texas A&M University's (TAMU) multibeam and seismic data to depict the up-to-date, most detailed seafloor morphology of the northern Gulf of Mexico continental slope.

The multibeam data include 3-year surveys by NOAA and a 1990 Alaminos Canyon survey (Sager, 1997). Outside the multibeam coverage, the bathymetry was based on Petty-Ray seismic data published by Bryant et al. (1990), and digitized by NGDC and the Mexican government.

The detailed bathymetry and bathymetry derived properties (slope gradient, topographic structures, and drainage patterns) are on a CD-ROM set that allows interactive zooming, color mapping, querying, printing, and data downloading. The CD-ROM set will be available in two versions: a standard version and an industrial version. The industrial version uses functions of ArcViewTM's spatial analysis extension, which lets the user have more controls on the contouring, shading, etc. The industrial version also puts more processing work on the computer.

The CD-ROM and posters are in the final stage of testing and are scheduled for release in the first quarter of 1999. Questions should be directed to Jia Y. Liu, Tel: 409-845-3528, Email: [email protected]; or to William R. Bryant, Tel: 409-845-8211, Email: [email protected].

References

• Bouma, A. H., O. Chancey, and O. Merkel, 1972, Alaminos Canyon area, in R. Rezak, ed., Contributions on the geological oceanography of the Gulf of Mexico: Gulf Publishing Co., Houston, Texas A&M University Oceanographic Studies, p.153-179.
• Bryant, W. R., J. R. Bryant, M. H. Feeley, and G. R. Simmons, 1990, Physiographic and bathymetric characteristics of the continental slope, northwest Gulf of Mexico: Geo-Marine Letters, v.10, p.182-199.
• Hardin, N. S., 1989, Allochthonous salt sheets in the Alaminos Canyon region, northwest Gulf of Mexico: present configuration and processes of emplacement, Texas A&M University, Ph.D. dissertation.
• Liu, J. Y., 1997a, Surficial geological characteristics of the Alaminos Canyon, Gulf of Mexico, Texas A&M University, Ph.D. dissertation.
• Liu, J. Y., 1997b, High resolution bathymetry in the Gulf of Mexico, covers 94-87.5 W longitude and 25.7-29.5 N latitude, AAPG Annual Convention (Abstract).
• NOAA, 1992, Atlas of NOAA's multibeam sounding data in the Gulf of Mexico Exclusive Economic Zone: NOAA/NOS/Coast & Geodetic Survey, Rockville, MD., v.1.
• Sager, W. W., 1997, Personal communication.
• Shirley, K., 1998, Ocean floor mapped from space: AAPG Explorer, v.19, no.10, p.20-23.

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