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Lookup NU author(s): Dr Richard Tyson
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There has been an active debate about the factors controlling the origin of organic-rich marine sediments since at least the 1920s. Most of this debate has focused on the relative roles of elevated primary productivity and enhanced preservation related to dysoxia-anoxia. In theoretical and empirical terms it is absolutely clear that the total organic carbon content (TOC) of thermally immature sediments is always a function of three, not two, main factors: organic matter input, organic matter preservation, and dilution by mineralic sediment components, any of which may be the dominant factor in different situations. These factors are inter-related strongly and may be obscured by variations in sediment granulometry. While modern sediment data indicating a limited oxygen effect are not generally disputed, the conclusions drawn are often flawed and inapplicable to many ancient rocks. The effect of dysoxic to anoxic conditions on organic matter preservation is only clear at slow sedimentation rates; it is therefore well expressed in ancient basinal facies, but expressed poorly in the modern rapidly deposited shelf and slope facies where oceanographers have predominantly studied it. Organic matter preservation is controlled strongly by the duration of exposure to oxygen; this can be modified by changes in the organic matter input, the sedimentation rate, or the bottom water oxygenation. An oxygen effect alone is unlikely to result in more than a three- to six-fold difference in TOC; the effect is non-linear and enhanced preservation mostly occurs below 1.0-0.5 ml/l oxygen. Minor differences in generally low oxygen values are never likely to be expressed in significant differences in TOC or HI. Very high TOC values (>10%) are unusual, and in ancient sediments commonly reflect a combination of higher preservation (dysoxia-anoxia) and low dilution. Geological models of source rock deposition have remained little changed during the productivity versus preservation debate, but the limitations of traditional modern analogues have been exposed. Studies of modern oxygen minimum zones (OMZ) demonstrate that modern dysoxic-suboxic slope waters do not typically result in the accumulation of very well preserved (oil-prone) organic matter, as was once assumed. The Black Sea is too extreme and unusual to be used as an actualistic analogue for most “black shales”: the geological euxinic basin model essentially refers to very large stratified basins, characterized by episodic to quasi-continuous bottom water anoxia, and the slow deposition of laminated and relatively organic-rich and oil-prone basinal sediments.
Author(s): Tyson RV
Editor(s): Harris NB
Publication type: Book Chapter
Publication status: Published
Book Title: The Deposition of Organic-Carbon-Rich Sediments: Models, Mechanisms, and Consequences
Year: 2005
Volume: 80
Pages: 17-33
Print publication date: 01/01/2003
Series Title: SEPM Special Publication
Publisher: Society for Sedimentary Geology
Place Published: Tulsa, Oklahoma, USA
Library holdings: Search Newcastle University Library for this item
ISBN: 1565761103
