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Open-File Report 2012–1112

Shipboard Surveys Track Magnetic Sources in Marine Sediments—Geophysical Studies of the Stono and North Edisto Inlets near Charleston, South Carolina

By Anjana K. Shah and M. Scott Harris

Thumbnail of and link to report PDF (105 MB)Abstract

Magnetic field data are traditionally used to analyze igneous and metamorphic rocks, but recent efforts have shown that magnetic sources within sediments may be detectable, suggesting new applications for high-resolution magnetic field surveys. Candidates for sedimentary sources include heavy mineral sand concentrations rich in magnetite or hematite, alteration-induced glauconite, or biogenic magnetite. Magnetic field surveys can be used to map the distributions of such sources with much denser and more widespread coverage than possible by sampling. These data can then provide constraints on the composition history of local sediments. Mapping such sediments requires the sensor to be relatively close to the source, and filtering approaches may be needed to distinguish signals from both system noise and deeper basement features.

Marine geophysical surveys conducted in July, 2010, over the Stono and North Edisto River inlets and their riverine inputs south of Charleston, South Carolina, showed 10- to 40-m-wide, 1- to 6-nT magnetic anomalies associated with shallow, sand-covered seabed. These anomalies are distinct from system noise but are too narrow to represent basement features. The anomalies are present mostly in shallow areas where river sediments originating from upland areas enter the inlets. Surface grab samples from the North Edisto River contain trace amounts of heavy mineral sediments including hematite, maghemite, ilmenite, and magnetite, as well as garnet, epidote, zircon, and rutile. Previous stream sediment analyses show enhanced titanium over much of the Atlantic Coastal Plain. The combined data suggest that the anomalies are generated by titanium- and iron-rich heavy mineral sands ultimately originating from the Piedmont and Blue Ridge provinces, which are then reworked and concentrated by tidal currents.

First posted July 3, 2012

For additional information contact:
Director, Crustal Geophysics and Geochemistry Science Center
U.S. Geological Survey
Box 25046, Mail Stop 964
Denver, CO 80225
http://crustal.usgs.gov/

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Suggested citation:

Shah, A.K., and Harris, M.S., 2012, Shipboard surveys track magnetic sources in marine sediments—Geophysical studies of the Stono and North Edisto Inlets near Charleston, South Carolina: U.S. Geological Survey Open-File Report 2012–1112.



Contents

Abstract

Titanium in stream sediments

Regional magnetic field

Shipboard geophysical surveys

Magnetics—short-wavelength spectral amplitudes over seafloor depth

Sidescan sonar—900 kHz

Anomalies and sediment type

Magnetic filtering via spectral amplitudes

Models

References


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