A seismic-reflection investigation of gas hydrates and sea-floor

features of the upper continental slope of the Garden Banks and

Green Canyon regions, northern Gulf of Mexico:

Report for cruise G1-99-GM (99002)

by Alan Cooper1, David Twichell2, Patrick Hart1

Open-File Report 99-570

1999

This report is preliminary and has not been reviewed for conformity with U.S. Geological

Survey editorial standards or with the North American Stratigraphic Code. Any use of trade,

firm, or product names is for descriptive purposes only and does not imply endorsement by

the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

1

Menlo Park, California

2

Woods Hole, Massachusetts
2

Introduction

During April 1999, the U.S. Geological Survey (USGS) conducted a 13-day cruise in the

Garden Banks and Green Canyon regions of the Gulf of Mexico (Figures 1 and 2). The R/V

Gyre, owned by Texas A&M University, was chartered for the cruise. The general objectives

were (1) to acquire very high resolution seismic-reflection data and side-scan sonar images of

the upper and middle continental slope (200-1200-m water depths), (2) to study the acoustic

character and features of the sea floor for evidence of sea-floor hazards, and (3) to look for

evidence of subsurface gas hydrates and their effects.

The Gulf of Mexico is well known for hydrocarbon resources, with emphasis now on frontier

deep-water areas. For water depths greater than about 250 m, the pressure-termperature

conditions are correct for the development of shallow-subsurface gas hydrate formation

(Anderson et al., 1992). Gas hydrates are ice-like mixtures of gas and water (Kvenvolden,

1993). They are known to be present from extensive previous sampling in sea-floor cores and

from mound-like features observed on the sea floor in many parts of the northern Gulf,

including the Green Canyon and Garden Banks areas (e.g., Roberts, 1995). Seismic-reflection

data are extensive in the Gulf of Mexico, but few very-high-resolution data like those needed

for gas-hydrate studies exist in the public domain. The occurrence and mechanisms of gas

hydrate formation and dissociation are important to understand, because of their perceived

economic potential for methane gas, their potential controls on local and regional sea-floor

stability, and their possible effects on earth climates due to massive release of methane

greenhouse gas into the atmosphere.

Three high-resolution seismic-reflection systems and one side-scan sonar system were used on

the cruise to map the surface reflectance and features of the sea floor and the acoustic

geometries and character of the shallow sub-surface (Table 1). The cruise was designed to

acquire regional and detailed local information. The regional survey covered an area about

3400 km2 in the Green Canyon and Garden Banks regions. Data recorded included 15 cu. in.

water gun multichannel seismic-reflection and Huntec boomer information. Detailed surveys

were planned in two parts of the study area, but due to a winch failure only one detailed

survey was done in the Green Canyon area. The detailed survey included collection of 15 cu.

in. water gun multichannel seismic-reflection, chirp seismic-reflection, and side-scan data.

The regions surveyed include basins and ridges formed by salt movements and characterized

by thick surficial, relatively young (Pleistocene and younger) sediments. Swath-bathymetry

data (Figure 2) show that the sea floor has extensive sea-floor faults, piercement features, and

slumps -- features whose development may be related partly to shallow gases and gas

hydrates.

The specific objectives of the cruise were (a) to investigate the distribution and character of

potential sea-floor failures (hazards) and their relationship to known and inferred gas hydrate

deposits; (b) to image known and suspected gas hydrate features with different seismicreflection systems to look at systematic variations in sub-surface structure in gas hydrate and

non-hydrate areas; (c) to outline the shallow structures of the gas hydrate stability zone

including shallow water-flow areas, to ascertain the potential impact of gas hydrate
Figure 1. Index maps showing (A) shaded-relief bathymetry in the northern Gulf of Mexico, 

with box around the Green Canyon and Garden Banks study area, and (B) bathymetric 

contours at 200 m intervals, and locations of seismic-reflection tracklines for cruise G1-99-

GM. Sidescan and chirp seismic-reflection data were collected only in detailed surveys in 

the eastern part of the area (i.e., Green Canyon area).
Figure 2. Shaded-relief map of NOAA swath bathymetry data in the study area. Locations of 

seismic-reflection tracklines for cruise G1-99-GM are also shown. The irregular bathymetry of 

basins, rises and sea-floor displacements result principally from movements of deep-seated 

salt.
3

formation/distribution on stability of the sea floor. Although seismic data alone cannot be

used to identify gas hydrate deposits unequivocally, they are useful in delineating the

anomalous subsurface features (e.g., structures, acoustic facies, seismic-attributes, etc.) that

are commonly associated with areas where gas hydrates have been recovered in cores.

During the cruise, about 1400 km of high resolution multichannel and single-channel seismicreflection data and about 500 km of side-scan and chirp seismic-reflection data were recorded

(Figures 1 and 2). The following report describes the field operations and equipment systems

employed, gives some examples of ship-board seismic-reflection records, and outlines a few

preliminary results.

Cruise science staff

Alan Cooper Co-chief scientist, Geophysicist USGS-Menlo Park

David Twichell Co-chief scientist, Oceanographer USGS-Woods Hole

Chris Anton Geologist USGS-Woods Hole

Jeffrey Brody Computer specialist USGS-Menlo Park

Bill Danforth Geologist USGS-Woods Hole

Dave Foster Geologist USGS-Woods Hole

Patrick Hart Geophysicist USGS-Menlo Park

Barry Irwin Geophysicist USGS-Woods Hole

Larry Kooker Electronics Technician USGS-Menlo Park

David Nichols Electronics Technician USGS-Woods Hole

Thomas Obrien Electronics Technician USGS-Woods Hole

Walter Olsen Mechanical Technician USGS-Menlo Park

Graham Standen Huntec deep-tow technician GC

Harold Williams Mechanical Technician USGS-Menlo Park

Affiliations:

GC: Geoforce Consultants (under contract to USGS)

USGS: U. S. Geological Survey, Menlo Park, CA

Equipment Systems

The equipment systems used during the cruise are listed in Table 1. All equipment was

provided by USGS, with exception of a spare 15 in3 water gun that was furnished by

University of Mississippi. An operations log showing the amounts of data recorded with these

systems is outlined in Table 2. Appendix A gives a detailed list of trackline data collected.

Survey speeds ranged from 2.5 to 4.5 knots. Seismic-reflection systems were fired at different

energy levels and rates, as noted. The seismic-reflection data were sampled at fast rates (4

kHz to 33 kHz) at or near the capacity of the recording systems, to attain the greatest possible

resolution of waveforms for later data analysis. With the fast rates, nearly 60 gigabytes of

data were recorded during the 8 days of data collection.

Seas were mixed with highly variable wind condtions and seas generally less than 4-6 feet,

but operations were halted during a 2-day storm. Data quality is generally good, but is
4

affected in various places by noise from several sources: seismic-shooting ships, cross-talk

between boomer and water-gun seismic-reflection systems, and nearby drilling operations.

Additional information about the data recorded on the cruise, and its public availability can be

obtained at the website:

http://walrus.wr.usgs.gov/docs/infobank/tiger/g/g199gm/html/g-1-99-gm.meta.html

Table 1: Equipment systems used on cruise G1-99-GM (99002)

System Components Comments

Navigation A USGS real-time YONAV system was used for

lab/bridge display of positions and data recording.

Inputs were from a KVH Heading Indicator and a

SATPAK DGPS receiver.

Raster images of NOAA

swath bathymetry were

displayed for real-time

positioning of tracks, to cross

sea-floor features of interest.

Highresolution

multi-channel

seismicreflection

MCS data were recorded using a 24-channel ITI

streamer with 10 m groups and 3 phones per group. The

streamer was towed at an estimated 1-3 m depths (no

depth sensors on the streamer), and farthest offset

channel was about 270 m behind the source. A

Geometrics STRATAVIEW with marine controller and

2/4 gbyte DAT tape drive was used for the digital

recording system. The source was a 15 in3 water gun

fired by a SureShot gun-timing system. The navigation

system provided gun-shot-triggers on a constant-time

basis. A 50 scfm Bauer Compressor provided air at a

pressure of about 2000 psi.

MCS data were sampled at 4

kHz rate; and, one trace

displayed aboard ship with a

Printrex thermal plotter, via

the STRATAVIEW marine

controller. Water gun was

fired at 6 sec. intervals, and

towed about 1m depth.

Huntec A Huntec system owned by USGS was used, with its

boomer source fired at variable power settings of 500-

1000 joules. The systems two receivers were used (a

hydrophone inside of the tow fish and a 1-m-long, 3-

element streamer towed behind the fish). Fish tow

depth was about 150 m at a ship speed of 3.5 kt.

Seismic signals from the fish were fed through

shipboard amplifier/filters (set to OUT) and then to the

MudSeis recording systems and EPC graphic recorder.

The boomer was fired at

rates of 0.75 -1.25 sec.

depending on water depth.

Data were recorded at 32

kHz sampling rate. For the

last half of the cruise, a

second external streamer was

added in place of the internal

hydrophone. 60 hz noise is

common.

Side Scan A DataSonics SIS-1000 side-scan fish was used. Side scan fired at 1 sec. rate

giving a range of 750 m to

each side of the tow fish.

Chirp seismicreflectionAn internal chirp seismic-reflection system in the SIS1000 system was used.Fired at 1 sec. rate, with side

scan.
5

Table 2: Operations log for cruise G1-99-GM (99002)

Operation Time Spent Data Collected Comment

Multichannel seismicreflection: water gun~199 hours ~1400 km along 68

seismic-reflection

tracklines

15 in3 water gun; done on all

seismic-reflection and side-scan

tracklines.

Huntec deep-tow

seismic-reflection

system

~129 hours* ~900 km along 44

seismic-reflection

tracklines

Boomer source; not done when chirp

seismic-reflection was used.

Chirp seismic-reflection

system

~71 hours* ~500 km along 24

seismic-reflection

tracklines

4-10 kHz chirp swept source

Side-scan system ~71 hours ~500 km along 24 sidescan tracklines100 kHz side-scan

Transit ~49 hours* none To/from operational area

Equipment downtime ~10 hours* none Startup and repairs

Weather delay ~53 hours* none Transit and 23-hour layover

TOTAL 312 hours 13 days (total of * items)

Onboard Data Processing

Data from the SIS-1000 and multichannel seismic-reflection systems were partly processed

onboard ship during the cruise to facilitate data access and display.

MCS system: SEG-D digital data on magnetic tapes written by the field recording system

were reviewed, edited, compiled into complete profiles, and written to tape in SEG-Y format

for later processing at the Menlo Park(CA) processing facility.

SIS system: Raw side-scan sonar data from the SIS data were corrected for water travel path

and ship's position to created georeferenced images that were used to create a sea-floor mosaic

showing sea-floor backscatter. The chirp seismic-reflection profiles were cleaned up, merged

with navigation information and plotted in profile format. The navigation and chirp seismicreflection data were combined with other shipboard information (e.g., descriptive text,

shipboard photos, etc.) and written to a set of CD-ROMs for later data reference (Foster et al.,

1999).

Seismic-reflection Studies

The seismic-reflection studies (Table 3) were conducted in water depths of 250 to 1200 m, and

are in two general categories: 1) those focussing on the subsurface distribution and

characterization of gas hydrates and related features of the gas hydrate stability zone and 2)

those directed toward acoustic characterization of the sea-floor and the relation of gas hydrates

and related features to sea-floor stability and mass-wasting deposits. The three seismicreflection data sets are directed principally to the first objective, and the side-scan and chirp

seismic-reflection data sets are focused largely to the second.
6

Table 3: Seismic-reflection studies done on cruise G1-99-GM

Study Area Comments

Structure and stratigraphy of

the upper sedimentary section

and the geometry and extent of

the gas-hydrate stability zone

(GHSZ).

Upper- to middle-continental

slope from about 300 m water

depth (i.e, the approximate depth

of shallowest known gas hydrate

occurrence in the Gulf) to about

1200 m water depth.

Look for seismic-reflection

evidence of gas hydrates, trapped

gases, fluid-flow zones, and

local/regional GHSZ thickness

variations on long profiles across

the continental slope.

Acoustic characterization of

the sea floor to identify

possible links between gas

hydrates, sea-floor failures,

mass-wasting deposits and

other sea-floor features (e.g.

authigenic deposits, mud

diapirs)

Upper- to middle-continental

slope where steep sea-floor

bathynetric gradients are known,

where fault scarps are common,

and where areas of high

backscatter occur.

Look for regional correlations of

sea-floor features in swath

bathymetry, backscatter, and

near-surface seismic-reflection

data to understand role of gas

hydrates.

Possible links between seafloor failures, shallow faulting,

intrusive structures and gas

hydrates of the uppermost

sedimentary section.

Areas of apparent sea-floor scarps

and slumps on the continental

slope, based on regional 100-mgridded swath bathymetry data.

Attempt to identify the role, if

any, of gas hydrates in surface

and sub-surface failures.

The following sections briefly describes the field studies (Table 3), and gives a few examples

of initial field observations.

Structure and stratigraphy

Extensive studies on the structure and stratigraphy of the Northern Gulf of Mexico and Green

Canyon and Garden Banks regions have been done (e.g., Goodwin and Prior (1989), Weimer

et al. (1998)). Yet, few long regional high-resolution multichannel seismic-reflection profiles

across the upper- and middle-slope have been published. We recorded many regional profiles

to establish the regional seismic character and variability of the shallow Pleistocene and

younger sedimentary section that is locally strongly deformed by deep-seated salt structures.

The three seismic-reflection systems imaged the sedimentary section at variable vertical and

lateral resolutions to a maximum depth of at least the sea-floor multiple reflection. Profile

locations were chosen using NOAA swath bathymetry maps, to extend from near the shelf

edge down the slope across typical basin and rise features that have locally faulted sea-floors,

sometimes with anomalous acoustic features on the sea floor (Figure 3). Some profiles were

down the axis of basins, and others crossed the basin flanks and salt diapirs. Profile locations

were also chosen to cross sites where shallow-water flows have been reported in drill holes

and where sea-floor gas hydrate deposits have been cored or seen from submersibles.
Figure 3. Example of side-scan data recorded on cruise G1-99-GM in the Green Canyon 

area, shown sea-floor backscatter information. Light colored areas are high backscatter 

(hard ground) and dark areas are low backscatter (soft ground). High backscatter is 

associated with sea-floor fault and diapir systems where gases, fluids, sediment rise to the 

seafloor resulting in likely diagenetic deposits (e.g., authigenic carbonates), gas hydrates 

(where stable), brines, and coarse or disrupted sediments. Blocky lines show approximate 

water depths. Solid white line is location of seismic-reflection Line 15 in Figure 4.
7

The extent of the gas hydrate stability zone (GHSZ) in the study area is not well known

because of potentially large variations in factors that affect hydrate-stability (e.g., water- and

subsurface-temperatures, gas concentrations and compositions, pore-water salinities, local

over-pressures, etc). A bottom simulating reflection (BSR), found elsewhere at the base of the

GHSZ was not observed; however, along many regional profiles the shallowest occurrence of

chaotic-seismic-facies zones with high-amplitude reflections (possible shallow gas?) in the

upper sedimentary section deepened with increasing water depths -- much like the

characteristic behavior of BSRs. The chaotic zones are stratigraphically controlled, and in

many cases where they lie close to the base of the estimated GHSZ, seismic amplitudes

increase (i.e., possible gas trapped beneath hydrate or stratigraphic horizons?). In areas of

complex shallow structure near piercement features, where gas hydrates have been sampled

by others, local high-amplitude reflections with cross-cutting geometries are sometimes

recorded. These reflections may be from trapped gases or deformation features near the base

of GHSZ.

Acoustic characterization

The acoustic character of the seafloor and upper-sedimentary sections is highly variable. The

sea floor ranges from low-backscatter (dark) to very-high back-scatter (light) tones (Figure 3).

The seismic signatures at several resolutions range from well-stratified to seismically opaque

(Figures 4, 5, and 6). Areas of structural complexity are commonly marked by seismic wipeout zones or by few coherent reflections that also have a high-backscatter signature in the

side-scan images. Areas underlying known sea-floor gas hydrates commonly exhibit similar

wipe-out zones with some high-amplitude chaotic seismic facies. Over broad basinal and

basin-flank areas, the upper sedimentary section is uniformly stratified but laterally deformed

in thin horizons (10-50 m thick) that can be traced over large distances, up to tens of

kilometers. In these areas, side-scan images show low back-scatter for the sea floor. In some

places on the upper slope, the thin deformed horizons may extend up to the seafloor where

they outcrop and are marked by areas of high backscatter (Figures 3 and 4). The high

backscatter here and across similar sea-floor features on the upper slope is likely hardground

possibly including coarse sediment, authigenic carbonates, hydrates, and other debris. On

basin floors, the shallow subsurface is marked in many places by chaotic or seismically

opaque zones that also have high sea-floor backscatter -- these are likely channel and masswasting deposits.

The profiles across the continental slope also exhibit strong local amplitude variations in the

upper sedimentary section. Many factors can cause high-amplitude reflections (e.g., largeimpedance contrasts with interbedded gas, sands, gas hydrates; geometric focusing; etc.). In

the shallow subsurface depths, the acoustic facies and geometries of high-amplitude

reflections are suggestive of gas accumulations (Figures 5 and 6); however, the polarity of the

reflections, expected to be negative from gas deposits, could not be determined in the field

data. Large-amplitude variations are also observed (a) in zones along underlying structures,

especially on the flanks and crests of ridges, in places where gases and fluids may be

advecting along faults and diffusing through the uppermost strata, and (b) where gas hydrates

and authigenic deposits are found above shallow diapiric structures of likely mud/sand
Figure 4. Examples of seismic-reflection Line 15 recorded across a fault zone during cruise G1-99-GM.

A. Chirp seismic-reflection, and B. Water gun seismic-reflection. Chirp seismic-reflection show a likely 

mud volcano adjacent to the fault zone where near-surface seismic-reflection amplitudes are high and the 

seafloor is disrupted. Water gun data illustrate that the high-amplitude reflections extend downward in two 

zones, one that projects steeply (i.e., the master fault with likely gas/fluid) and another that projects 

laterally along apparent stratigraphy (i.e., a potential gas/fluid charged stratal unit). The fault zone has 

high backscatter at the sea floor (Figure 3), and is an area of likely upward migrating gases, fluids, and 

mobilized sediment, with contributions from the two subsurface high-amplitude zones.
Figure 5. Example of water-gun seismic-reflection Line 12 recorded across the upper continental slope 

during cruise G1-99-GM. A well-stratified unit covers sediments with acoustic-wipeout and high-amplitude 

zones, broadly associated with deep-seated diapir and fault structures. Shallow water flows are reported 

from the area (0.6-0.9 sec water depths) where a chaotic high-amplitude zone (gases?) occurs and where 

shallow faults that displace uppermost strata sole out. Hydrates are stable near the sea-floor at these 

depths, depending on gas compositions and bottom-water temperatures. Here, as elsewhere, subsurface 

faulting and intrastratal deformation that displaces the sea floor seems enhanced by gas/fluid flows, which 

may in places be controlled at shallow depths partly by features of the gas hydrate stability zone.
Figure 6. Example of Huntec seismic-reflection Line 12 recorded across a large diapir structure 

(i.e., Figure 5) beneath the upper continental slope during cruise G1-99-GM. Numerous nearvertical faults are seen with some sea-floor scarps and mound-like features. Localized highamplitude chaotic reflections may be from gassy and/or disrupted strata. The area is marked by 

high sea-floor backscatter.
8

derived from adjacent buried beds. Where shallow strata are within the estimated gas hydrate

stability zone, large amplitude variability occurs locally within chaotic subsurface layers

sandwiched between well-stratified units. Some of these occurrences are in areas where

shallow-flows have been encountered in drill holes. In general, the large-amplitude variability

in the shallow subsurface appears stratigraphically and to a lesser extent water-depth

controlled, but the specific explanations require further analysis and acquisition of detailed

geologic core data.

In side-scan data, the sea-floor commonly has high backscatter over the crests of salt

structures and along fault zones where gas hydrates, authigenic carbonates, and biologic

communities have been observed. High backscatter is also found on steep slopes and in basin

floors associated with mass-wasting deposits. Backscatter is generally low over slope and

basin areas that are dominated by well stratified and undisrupted strata.

Possible links

The new side-scan and high-resolution seismic-reflection data provide additional evidence, to

the 1998 cruise data (Cooper et al., 1999), to suggest that gas hydrates are more widespread at

the sea floor, and possibly below, than currently known from other studies (e.g., Roberts,

1995). Features suggestive of the gas hydrates occur near many of the extensive sea-floor

failures evident in the NOAA swath bathymetry data across the continental slope (Figure 2) –

failures that are in turn related on a regional scale to underlying salt movements and

development of the numerous basins and rises. On a local scale, structures possibly

associated with sea-floor gas hydrate deposits are complex and varied (e.g. diapirs, circular

fault systems, en echelon headwall scarps, etc.), and in many cases are also associated with

nearby mass-wasting features. Tracklines were run across many such sea-floor and buried

failures (e.g., slumps, slides, flows) to determine the extent to which they may be affected or

caused by shallow gas-hydrate-related processes.

In swath bathymetry data, sea-floor failures range widely in displacement and style of failure.

In shipboard seismic-reflection profiles, the displacements commonly increase at depth

implying prior and continuing deformation to the present time. Lateral intrastratal

deformation within shallow and thin layers along which potentially large areas of the sea-floor

have moved (or be moving?) is also observed. These deformed layers can be traced in places

upslope to sea-floor failures (e.g., Figure 4). In other places, the widespread lateral

deformation is confined to a thin zone sandwiched between flat lying strata. The association

of such features, some of which may be actively moving and deforming the sea floor, with

possible shallow-subsurface gas deposits (e.g., Figure 5) at depths where gas hydrates are

stable, suggests that such failures and gas hydrates are linked. Further assessment of the

potential role of gases and gas hydrates in facilitating slope and sea-floor failures awaits data

processing and analysis.

Summary

High resolution seismic-reflection profiles and side-scan sonar images were recorded in April

1999 across the Green Canyon and Garden Banks regions of the Gulf of Mexico to study the
9

distribution of potential gas hydrates, and to determine the link, if any, between gas hydrates

and known slope and sea-floor failures. Shipboard records show evidence of widespread

shallow gas and subsurface deformation within or directly below the estimated gas hydrate

stability zone. These data, after futher processing, should more clearly illustrate the

relationship between migrating gases/fluids, gas hydrates and slope and sea-floor failure

features.

Acknowledgments

Funding for the cruise was from the USGS Coastal and Marine Geology Program and a grant

from the Department of Energy. We gratefully acknowlege the efforts of the ships's crew, the

USGS science-support staff, and the Geoforce technician to ensure a safe and productive

cruise. The Minerals Management Service, New Orleans, provided valued assistance

accessing data used in site selections. We thank the University of Mississippi for the use of

their 15 cu.in. water gun. We appreciate the help of Rebecca Cooper in compiling data and

preparing maps, and Michael Hamer and Carolyn Degnan in preparation of swath-bathymetry

maps.
10

References

Anderson, A.L., Sloan, E.D., and J.M. Brooks, 1992, Gas hydrate recoveries in the Gulf of

Mexico: What is the shallow water depth limit for hydrate occurrence?, Proceedings 24 th

Offshore Technology Conference (1992), Paper 6853, 381-385.

Cooper, A.K., Hart, P., and I. Pecher, 1999, Cruise Report for aa seismic investigation of gas

hydrates in the Mississippi Canyon region, northern Gulf of Mexico -- Cruise M1-98-GM,

U.S. Geological Survey Open-file report 98-506, 25 p.

Foster, D.S., Twichell, D.C., Danforth, W.W., Irwin, B.J., Nichols, D.R., and O'Brien, T.F.,

19XX, Archive of Datasonics SIS-1000 chirp subbottom data collected during USGS cruise

GYRE 99002, Gulf of Mexico, 9-22 April, 1999: USGS Open-File Report 99-XXX, 3 CDROMs.

Goodwin, R.H., and D.B. Prior, 1989, Geometry and depositional sequences of the

Mississippi Canyon, Gulf of Mexico, Journal of Sedimentary Petrology, 59/2, 318-329.

Kvenvolden, K. A., 1993, Gas hydrates -- geological perspective and global change, Rev.

Geophys, 31(2), 173-187.

Roberts, H. H., 1995, High-resolution surficial geology of the Louisiana middle-to-upper

continental slope, Gulf Coast Association of Geological Societies Transactions, v. XLV, p.

503-508.

Weimer, P. et al., 1998, Sequence stratigraphy of Pliocene and Pleistocene turbidite systems,

northern Green Canyon and Ewing Bank (offshore Louisiana), northern Gulf of Mexico,

AAPG Bulletin, 82/5B, 918-960.
11

Figure Captions

Figure 1. Index maps showing (A) shaded-relief bathymetry in the northern Gulf of Mexico, with

box around the Green Canyon and Garden Banks study area, and (B) bathymetric contours at 200 m

intervals, and locations of seismic-reflection tracklines for cruise G1-99-GM. Sidescan and chirp

seismic-reflection data were collected only in detailed surveys in the eastern part of the area (i.e.,

Green Canyon area).

Figure 2. Shaded-relief map of NOAA swath bathymetry data in the study area. Locations of

seismic-reflection tracklines for cruise G1-99-GM are also shown. The irregular bathymetry of

basins, rises and sea-floor displacements result principally from movements of deep-seated salt.

Figure 3. Example of side-scan data recorded on cruise G1-99-GM in the Green Canyon area,

shown sea-floor backscatter information. Light colored areas are high backscatter (hard ground) and

dark areas are low backscatter (soft ground). High backscatter is associated with sea-floor fault and

diapir systems where gases, fluids, sediment rise to the seafloor resulting in likely diagenetic

deposits (e.g., authigenic carbonates), gas hydrates (where stable), brines, and coarse or disrupted

sediments. Blocky lines show approximate water depths. Solid white line is location of seismicreflection Line 15 in Figure 4.

Figure 4. Examples of seismic-reflection Line 15 recorded across a fault zone during cruise G1-99-

GM. A. Chirp seismic-reflection, and B. Water gun seismic-reflection. Chirp seismic-reflection

show a likely mud volcano adjacent to the fault zone where near-surface seismic-reflection

amplitudes are high and the seafloor is disrupted. Water gun data illustrate that the high-amplitude

reflections extend downward in two zones, one that projects steeply (i.e., the master fault with likely

gas/fluid) and another that projects laterally along apparent stratigraphy (i.e., a potential gas/fluid

charged stratal unit). The fault zone has high backscatter at the sea floor (Figure 3), and is an area of

likely upward migrating gases, fluids, and mobilized sediment, with contributions from the two

subsurface high-amplitude zones.

Figure 5. Example of water-gun seismic-reflection Line 12 recorded across the upper continental

slope during cruise G1-99-GM. A well-stratified unit covers sediments with acoustic-wipeout and

high-amplitude zones, broadly associated with deep-seated diapir and fault structures. Shallow water

flows are reported from the area (0.6-0.9 sec water depths) where a chaotic high-amplitude zone

(gases?) occurs and where shallow faults that displace uppermost strata sole out. Hydrates are stable

near the sea-floor at these depths, depending on gas compositions and bottom-water temperatures.

Here, as elsewhere, subsurface faulting and intrastratal deformation that displaces the sea floor

seems enhanced by gas/fluid flows, which may in places be controlled at shallow depths partly by

features of the gas hydrate stability zone.

Figure 6. Example of Huntec seismic-reflection Line 12 recorded across a large diapir structure

(i.e., Figure 5) beneath the upper continental slope during cruise G1-99-GM. Numerous near-vertical

faults are seen with some sea-floor scarps and mound-like features. Localized high-amplitude

chaotic reflections may be from gassy and/or disrupted strata. The area is marked by high sea-floor

backscatter.
12

Appendix A: Trackline specifications for cruise G1-99-GM

Line # Julian

Day

Start

Time

End

Time

Line

Direction

Data

Types

test1 100 1751 1825 SW-NE mcs,ss,c

test2 100 1828 2236 SW-NE mcs,ss,c

1 100 2254 0203 N-S mcs,ss,c

2 101 0228 0447 SE-NW mcs,ss,c

3 101 0451 0700 N-S mcs,ss,c

4 101 0701 0808 S-N mcs,ss,c

5 101 0809 1131 E-W mcs,ss,c

6 101 1218 1619 NE-SW mcs, h

7 101 1630 1731 E-W mcs, h

8 101 1732 2050 SE-NW mcs, h

9 101 2103 2231 W-E mcs, h

10 101 2231 2338 NE-SW mcs, h

11 101 2346 0013 SE-NW mcs, h

12 102 0137 0451 N-S mcs, h

13 102 0458 0634 SE-NW mcs, h

14 102 0635 0832 S-N mcs, h

15 102 0902 1343 W-E mcs,ss,c

16 102 1355 1733 E-W mcs,ss,c

17 102 1740 2309 W-E mcs,ss,c

18 102 2312 0228 E-W mcs,ss,c

19 103 0308 0703 W-E mcs,ss,c

20 103 0713 1114 E-W mcs,ss,c

21 103 1123 1522 W-E mcs,ss,c

22 103 1534 1905 E-W mcs,ss,c

23 103 1921 2328 W-E mcs,ss,c

24 104 0021 0105 E-W mcs,ss,c

25 104 0325 0754 W-E mcs,ss,c

26 104 0810 1238 E-W mcs,ss,c

27 104 1252 1540 W-E mcs,ss,c

28 106 2112 2344 S-N mcs, h

29 107 0003 0024 E-W mcs, h

30 no line - - - -

31 107 0241 0502 N-S mcs, h

32 107 0508 0819 SE-NW mcs, h

33 no line - - - -

34 107 0827 1545 NE-SW mcs, h

35 107 1548 1653 E-W mcs, h

36 107 1654 2037 NW-SE-NW mcs, h

37 107 2050 2333 SE-NW mcs, h

38 107 2348 0018 NE-SW mcs, h
13

39 108 0110 0146 NW-SE mcs, h

40 108 0158 0423 NW-SE mcs, h

141 108 0456 0914 NE-SW mcs, h

143 108 0914 1010 SW-NE mcs, h

144 108 1317 1359 SW-NE mcs, h

41 108 1408 2143 SW-NE mcs, h

42 108 2143 0247 NW-SE mcs, h

43 109 0251 0743 NE-SW mcs, h

44 109 0753 1058 SE-NW mcs, h

45 109 1058 1351 SW-NE mcs, h

46 109 1352 1450 SE-NW mcs, h

47 109 1457 1741 NE-SW mcs, h

48 109 1744 2026 NW-SE mcs, h

49 109 2037 2109 SW-NE mcs, h

50 109 2109 2222 SE-NW mcs, h

51 109 2223 0544 NE-SW mcs, h

151 110 0611 0649 NE-SW mcs, h

152 110 0649 0730 SW-NE mcs, h

52 110 0733 1126 SW-NE mcs, h

53 110 1136 1520 SE-NW mcs, h

153 110 1558 1944 SE-NW mcs, h

54 110 1945 2143 NE-SW mcs, h

55 110 2147 2341 W-E mcs, h

56 110 2342 0030 S-N mcs, h

156 111 0121 0236 S-N mcs, h

57 111 0238 0502 W-E mcs, h

58 111 0503 0551 NW-SE mcs, h

59 111 0559 0944 SE-NW mcs, h

60 111 0950 1318 NW-SE mcs, h

61 111 1319 1425 SW-NE mcs, h

c = chirp seismic-reflection (4-10 kHz deep tow system)

h = Huntec boomer (ddp-tow high resolution seismic-reflection data)

mcs = multichannel seismic-reflection with 15 cu.in. water gun

ss = side-scan sonar data (100 kHz deep tow system)