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Organic-Carbon-Rich Rocks: Fast or Slow Organic-Carbon Accumulation?

Previous HitCarolineTop M. Isaacs, David Z. Piper, and Margaret A. Keller

Organic-carbon-rich rocks and sediments are generally attributed in the marine geologic literature to high rates of organic carbon accumulation, resulting either from rapid input and/or excellent preservation. An alternate interpretation suggested by evidence from both oil-source rocks and modern sediments is that many organic-carbon-rich strata result from comparatively slow accumulation of organic carbon that is little diluted.

The idea that organic-carbon-rich rocks represent rapid organic-carbon accumulation derives partly from the enhanced organic-carbon preservation associated with faster burial. Re-evaluation of published sediment trap and accumulation rate data in modern oceans shows, however, that sedimentation rate has been highly over-rated as a cause of high organic carbon abundance. As sedimentation rate increases, increased dilution outpaces increased preservation such that, other things being equal, more abundant organic carbon is associated with slower (not faster) sedimentation rates.

Compared to an equal thickness of rapidly accumulated organic-carbon-lean sediment in the geologic record, slowly accumulated organic-carbon-rich sediment can represent 10-20 times more time--but be misinterpreted as reflecting rapid organic carbon accumulation by the common practice of interpolating age linearly with strata thickness. This relation explains the "enigma" of transgressive black shales, including numerous oil source-rocks worldwide associated with early phases of sea level rise. In offshore locations (20-200 km from the coast), rising sea level may sharply reduce terrigenous supply without significantly affecting productivity. The result is an organic-carbon-rich condensed zone reflecting neither high productivity nor low bottom-water oxygen nor rapid sedimentation, but simply lack of dilution.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California