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Book Chapter

Organic Matter Characteristics Near the Shelf-Slope Boundary1

By
R. W. Jones
R. W. Jones
Chevron Oil Field Research Company
P.O. Box 446, La Habra, California 90631
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Published:
January 01, 1983

ABSTRACT

The organic facies of sediments deposited near the shelf-slope break depend upon the types and amounts of organic matter available at the depositional site and the early diagenetic history of the organic matter. On a global scale, the organic facies at the shelf-slope break record major tectonic events, eustatic sealevel changes, water circulation, and climate. These overall controls affect a specific sedimentation site through such factors as mean grain size, sedimentation rate, input of terrestrial organic matter, organic productivity in the photic zone, and the dissolved oxygen content of the water column, particularly the water associated with the water-sediment interface.

Oxygen deficient water masses in the oxygen minimum layer (OML) of the World's Ocean intersect the continental margins most often on the upper slope, and it is there that the best potential source rocks now being deposited preferentially exist. Oxygen deficient water locally reaches onto the shelf, most noticeably in areas of shallow upwelling of nutrient-rich water and resulting high productivity, as in offshore southwest Africa. In the past, due to such factors as climate change, different current patterns and eustatic sealevel changes, oxygen deficient water has transgressed well up onto the shelves on a regional basis. Such events have resulted in the deposition of source rocks for much of the world's oil in transgressive shelf deposits.

If the bottom water across the shelf-slope break is oxic, the primary controls on the organic matter distribution are the grain size, sedimentation rate, and the input of terrestrial organic matter. None of these factors can completely offset the negative effects of oxic bottom water on the deposition and preservation of an oil-prone organic facies.

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Contents

SEPM Special Publication

The Shelfbreak: Critical Interface on Continental Margins

Daniel Jean Stanley
Daniel Jean Stanley
Division of Sedimentology Smithsonian Institution
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George T. Moore
George T. Moore
Chevron Oil Field Research Company La Habra California
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SEPM Society for Sedimentary Geology
Volume
33
ISBN electronic:
9781565761636
Publication date:
January 01, 1983

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