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.
Figures & Tables
The Shelfbreak: Critical Interface on Continental Margins
The shelfbreak is that point where the first major change in gradient occurs on the outermost edge of the continental shelf. Although this environment delimits the boundary between two principal and well-defined provinces, the continental shelf and slope - and thus is of the first order of importance on continental margins - it has received surprisingly little specific attention in either modern oceans or in the rock record. This volume, the first compendium dedicated specifically to the shelfbreak, was derived from an SEPM Research Symposium convened at the joint Annual Meeting of SEPM and AAPG on June 2, 1981. The material is organized in a manner to illustrate examples of the shelfbreak in both modern oceans and the rock record.