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The occurrence and stratigraphic distribution of source rock potential in distal fine-grained facies is strongly related to the prevailing palaeoxygenation regime, especially during the deposition of the condensed section (CS). The CS will tend to correspond to organic facies B-AB, with good to excellent potential, only when conditions are dysoxic-anoxic, and organic facies D-CD, with poor source rock potential, when conditions are oxic. The applicability of the Creaney and Passey organic facies model is limited by its assumption that palaeoxygenation remains constant within the basinal facies. The CS commonly shows decimetre-(Milankovitch)-scale redox cyclicity characterised by TOC and organic facies variations whose amplitude increases as sediment accumulation rates decrease. The greater frequency of dysoxic-anoxic conditions in the CS may be related to changes in accommodation space via the relationship between bottom water volume and the rate of deoxygenation, as observed in modern settings. The onshore Kimmeridge Clay diverges somewhat from the classic Exxon basin model as it was deposited in an intra-shelf setting where focusing and ponding of clayey sediment resulted in the basinal sections being both thicker and more complete. However, the best source rock intervals still coincide with the maximum flooding surfaces. The long term Late Jurassic (shallow to deep to shallow) sea level curve appears to influence the mean (oxic to dysoxic–anoxic to oxic) palaeoxygenation regime in the basin and the background trend in sediment accumulation rate; it is expressed in a symmetrical increase in TOC to 4-6 wt% from background values of about 1 wt%. The superimposed decimetre- to metre-scale redox cycles appears to be controlled by Milankovitch climatic controls on watermass stratification. The relative spacing of these cyclic organic-rich interbeds decreases during relatively condensed (MFS) intervals associated with the short term sea level curve, and the interval TOC values also increase and broaden. Intervals deposited during or shortly after lowstands are less organic-rich and have less and more widely spaced organic-rich beds. The individual organic-rich beds also appear to be condensed relative to adjacent mudstones; palynofacies and TOC vs. HI data suggest the origin of these beds is due more to cyclically decreased dilution and redox conditions rather than enhanced palaeoproductivity.
Author(s): Tyson RV
Editor(s): Hesselbo S; Parkinson N
Publication type: Book Chapter
Publication status: Published
Book Title: Sequence Stratigraphy in British Geology
Year: 1996
Volume: 103
Pages: 75-96
Print publication date: 01/02/1996
URL: http://dx.doi.org/10.1144/GSL.SP.1996.103.01.06
DOI: 10.1144/GSL.SP.1996.103.01.06
