Computer enhanced GLORIA sidescan sonar images of the surface of the Mississippi Fan R. Wen\ R. Sinding-Larsen1 and N.H. Kenyon2

1Department of Geology and Mineral Resource Engineering, Norwegian Institute of Technology, University of Trondheim, Hogskoleringen, N-7034, Trondheim, Norway

' Institute of Oceanographic Sciences, Deacon Laboratory, Brook Road, Wormley, Godalming, Surrey GUS 5UB, UK

lntroduction and method In 1985 the United Stares Geological Survey and the Institute of Oceanographic Seiences (UK) mapped the seafloor of the Gulf of Mexico, within the US Exclusive Economic Zone, using GLORIA sidescan sonar (Fig. 43.1). The GLORIA system operates at a frequency of 6.5kHz and the beam has a wavelength of c. 23cm. The digitally processed mosaic is published as black and white images at a scale of 1:500000 (EEZ-Scan 85 Scientific Staff 1987). The interpretation of the images and profiles from the Mississippi Fan is discussed in Twichell et al. 0991) . The GLORIA data are available on a CD-ROM optical disc. Further enhancement has been carried out for that part of the data that covers the Mississippi Fan, the Florida Escarpment and neighbouring regions (Sinding-Larsen and Wen 1990). This enhancement involved filtering, colour coding, feature extraction and classifications; the resulting image is shown as Fig. 43.2.

There have been a number of studies of the causes of the relative strength of the sidescan sonar signal from regions of terrigenaus sedimentation. Studies of the relationship between relative backscattering strength and various parameters of seafloor sediments suggest that the GLORIAsignal can represent the bulk backscattering from a volume of sediments some distance into the seafloor, but as yet there is no satisfactory physical explanation for backscatter variations (Sinding-Larsen and Wen 1990; Gardner et al. 1991; Kenyon 1992; Lee et al. 1993). Published analyses of 61 piston cores, tagether with bottarn photographs and dredge samples, have been used as

ground truth for the GLORIA data from this region (Kenyon 1992). As a general rule, sandy sediments correspond to areas of greater backscatter. This was confirmed by a detailed study of the distal part of one of the depositional Iobes of the Mississippi Fan (Twichell et al. 1992; Nelson et al. 1992; Lee et al. 1993; Twichell et al. Ch 41.) However, Gardner et al. 0991) found that an area with a thick sand unit (greater than 44 cm of sand in the available short cores) corresponded with very low GLORIA backscatter Ievels. Kenyon 0992) discussed other potential causes of the strong backscatter on the Mississippi Fan, such as slump structures and thin ironstone crusts. Thus caution is required in interpretation of acoustic facies in terms of sediments.

Discussion The principal fan elements that can be identified (Figs 43.2 and 43.3) are:

1. The main Mississippi Channel, which runs for c. 500 km from the shelf edge and has a moderate sinuosity (maximum of c. 1.7). There are supposed crevasse splays at meander bends seen as higher backscattering patches that in high resolution sonographs are arcuate lineations, bowed downchannel (Kastens and Shor 1985).

2. The very broad levees of the main channel. These have the lowest Ievels of backscatter on the image (ranging from blue to black to dark red). The right-hand levee has a pattern of streaks of medium backscatter (yellow) that rundown the gentle gradient from near the levee crest. These may be the tracks of partially channelized overbank flows (Twichell et al. 1991).

3. The slumped deposit on the middle fan, which was first reported by Walker and Massinghill 0970). It starts from the steepest and most proximal part of the left-hand levee and partly buries the main channel. The overall backscattering Ievel is moderate (yellow to

Fig. 43.1. Location diagram showing the tracks along which the GLORIA sonographs were obtained.

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, R.N. Hiscott, N.H. Kenyon, F. Ricci Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

88° 86° 84° r-------~------r-------~------.-------~------1

- GLORIA TRACK 1985

-GLORIA TRACK 1982

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Fig. 43.2. Mosaic of the GLORIA sidescan sonar images of the Mississippi Fan, in false colour.

orange) and there is a swirly pattern on its surface. There is little sand in the few cores from this slump and the higher backscattering is attributed to surface and internal roughness associated with the slump structures (Kenyon 1992).

4. The sandy depositional Iobes on the distal fan. These have an overall high Ievel of backscatter (yellow to red) and cover an extensive area. Eight Iobes were identified by Twichell et al. 0991) as bodies up to 50 m thick. One of these Iobes has been subdivided into at least ten sublobes following a more detailed survey (Twichell et al. Ch 41). They are supplied one at a time by flows that came down through the main channel. The highest backscatter is found some distance from the main channel, especially in the central zone of each lobe, which produces the appearance of detachment from the channel, as in some types of turbidite system classifications, e.g. Mutti (1985). However, the proximal part of the Iobes have feeder channels with a distributary pattern that originates near the main channel, rather than having a scour zone, thus they are best described as attached Iobes. In cores the high backscattering areas are seen as graded sandy turbidites and chaotic silt beds, suggestive of debris flows (Nelson et al. 1992; Twichell et al. Ch 41).

The enhanced GLORIA image delineates the morphological features and the distribution of some of the sedimentary facies on the surface of the Mississippi Fan. It maps some features that are missed by the seismic sections that are conventionally used in facies analysis of fan sediments (e.g. Bouma et al. 1985; Weimer 1990). Sandy depositionallobes in the lower part of the fan indicate that the flows through the main Mississippi Fan Channel are efficient at transporring sands to the lower part of the fan and that the flows remain largely confined. This challenges some models for !arge, mud-rich fans in which the sands are believed to be present only as sheets deposited by unconfined flows (e.g. Shanmugam and Moiola 1991).

Heferences Bouma, A.H., Stelting, C.E. and Coleman, J.M. 1985.

Mississippi Fan, Gulf of Mexico. In: Bouma, A.H., Normark, W.R. and Barnes, N.E. (eds) Submarine Fans and Related Turbidite Systems. Springer-Verlag, New York, pp. 143- 150.

EEZ-Scan 85 Scientific Staff 1987. Atlas of the U.S. Exclusive Economic Zone, Gulf of Mexico and eastern Caribbean areas. United States Geological Suroey Miscellaneous Investigations Series I -1864-A.

Gardner, ].V., Field, M.E. , Lee, H.). , Edwards, B.D., Masson, D.G., Kenyon, N.H. and Kidd, R.B. 1991. Ground-truthing 6.5 kHz side scan sonographs: what are we really imaging? Journal of Geophysical Research, 96, 5955-5974.

Kastens, K.A. and Shor, A.N. 1985. Depositional processes of a meandering channel on Mississippi fan . American Association of Petroleum Geologists Bulletin, 69, 190-202.

Kenyon, N.H. 1992. Speculations on the geological causes of backscatter variation on GLORIA sonographs from the Mississippi and De Soto Fans, Gulf of Mexico. Geo­Marine Letters, 12, 24-32.

Lee, H.)., Schwab, W.C. , Kayen, R.E., Edwards, B.D., Twichell, D.C., Field, M.E. and Gardner, J.V. 1993. The relation between sidescan sonar imagery and sediment physical properties: A comparison of two deep-sea fan systems. Proceedings ofthe Institute of Acoustics, 15, 319-326.

Mutti, E. 1985. Turbidite systems and their relations to depositional sequences. In: Zuffa, G.G. (ed.) Provenance of Arenites. NATO-ASI Series, Reidel Publishing Co., Dordrecht, pp. 65-93.

Nelson, C.H., Twichell, D.C. , Schwab, W.C., Lee, H.). and Kenyon, N.H. 1992. Upper Pleistocene turbidite sand beds and chaotic silt beds in the channelised, distal, outer-fan Iobes of the Mississippi fan. Geology, 20, 693-696.

Shanmugam, G. and Moiola, R.J. 1991. Types of submarine fan Iobes: models and implications. American Association ofPetroleum Geologists Bulletin, 75, 156-179.

Sinding-Larsen, R. and Wen, R. 1990. Interpretation, characterization and simulation of GLORIA sonography using personal computer with a !arge database on CD­ROM. CODA TA Bulletin, 22, 79-96.

Twichell, D.C., Kenyon, N.H., Parson, L.M. and McGregor, B.A. 1991. Depositional patterns of the Mississippi fan surface: Evidence from the GLORIA 11 and high resolution seismic profiles. In: Weimer, P. and Link, M. (eds) Seismic Facies and Sedimentary Processes of Submarine Fansand Turbidite Systems. Springer-Verlag, New York, pp. 349-363.

Twichell, D.C. , Schwab, W.C., Nelson, C.H., Kenyon, N.H. and Lee, H.). 1992. Characteristics of a sandy depositional lobe on the outer Mississippi fan from SeaMARC 1A sidescan sonar images. Geology, 20, 689-692.

Walker, J.R. and Massingill, J.V. 1970. Slump features of the Mississippi fan, northeastern Gulf of Mexico. Geological Society of America Bulletin, 81, 3101-3108.

Weimer, P. 1990. Sequence stratigraphy, facies geometries and depositional history of the Mississippi fan, Gulf of Mexico. American Association of Petroleum Geologists Bulletin, 74, 425-453.

0

88

\ \ 100 km

0 84 ° 86

Sandy depositional lobesl?->'[~,j

Slumps fZZJ Major Channels ,.,..,.

Limit of GLORIA coverage .........-'\

Lineal ions on major levees--\

Fig. 43.3. Interpretation of the sidescan images showing the plan view relationships of some of the large-scale elements of the fan surface. The slumps have a swirly pattern whereas the supposed sandy Iobes have a branching pattern.

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