K1-95-HW: Cruise Report 1995 - Preliminary results.

Phase III: Sediment Chemistry and Biological Sampling Survey

M.E. Torresan, M.A. Hampton, J.H. Barber, Jr., and F.L. Wong

U.S. Geological Survey Open-file Report 95-839

1995

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.

Summary
Introduction
Study Area
Previous Studies
  Designation
  Monitoring 1, 2
Methods
  Vessel
  Navigation
  Sampling
  Subsampling 1, 2
Results
Acknowledgments References 1, 2

Figures
1 Location
2 Box corer

Tables
1 Stations
2 Samples
3 Analyses
4 Bioassay

Appendices
1 Box Cores
2 Custody: Quanterra
3 Custody: Batelle

Cooperative Disposal Site Monitoring Studies (2)

The sidescan sonar mosaic displays three backscatter signatures that indicate different types of seafloor substrate. Each has been verified by seafloor sampling during the 1994 and 1995 sampling surveys. The dark-toned low backscatter regions that characterize most of the sonar mosaic are comprised of tan to beige colored carbonate sand (Figure 1; and Torresan and others, 1995- plate 1). The sonar mosaic also shows a light-toned high backscatter region that extends along the western and southwestern portion of the mosaic shown in Torresan (1995). This region is part of a drowned carbonate platform associated with reef growth during lower sea level (Kroenke and Woolard, 1966; Gregory and Kroenke 1982). The intermediate-toned, high-backscatter region centered over the disposal sites (Figure 1 and Torresan 1995) is the imprint of dredged-material deposits. Figure 1 shows that dredged materials form two major deposits that cover an area about three times the size of the designated disposal sites. The dredged material is characterized by high-backscatter, circular to subcircular footprints 25-150 m in diameter, spaced up to 300 m apart at the extremities of the deposits (Figure 1, and Torresan 1995). The footprints are apparently formed by individual disposal events; they coalesce to form a high-backscatter blanket over each disposal site. The blanket completely covers the natural, low-backscatter carbonate sediment that mantles the Mamala Bay seafloor. Subbottom seismic-reflection profiles show that the dredged material can form mounds up to 1 m high and spaced up to 300 m apart (Torresan and other, 1995).

Box cores collected during 1994 and 1995 show that the dredged material extends beyond what is visible on the sonar mosaic, comprising a heterogeneous deposit of cohesive gray mud that is admixed by the dredging process with coarse sand to cobble-size rubble. The rubble consists of limestone, shell, and coral fragments, and man-made detritus. The dredged material is easily distinguished from the native sediment, which is low backscatter, tan to beige colored, burrowed and bioturbated, clean to muddy carbonate sand. Bioturbation is ubiquitous in the native sediment, and some is present below the dredged material. Video and still photographs show that the seafloor is characterized by wavy bedforms, and is littered with carbonate rubble and man-made debris that includes tires, barrels, and military ordnance. Bedforms that range in size from ripples to mega-ripples to sand waves. There is a broad spectrum of ripple types, including symmetrical and asymmetrical forms, and fresh looking to degraded ripple types (Hampton and others, in prep). At times, sections of the 3.5- kHz profiles have closely spaced hyperbolic diffractions that correlate with and indicate the presence of rippled bedforms (Torresan and others 1995; Hampton and others, 1997). The natural roughness of the seafloor (in the form of bedforms as seen on video and still photography), the abundant rubble, and the coarse carbonate component of the dredged materials are likely responsible for the high-backscatter nature of the dredged material.

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