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ABSTRACT

The Kimmeridge Clay of northern England is part of a highstand system tract, rich in organic material, whose offshore correlatives sourced much of the North Sea oil province. Spatial and temporal variation over a 35-km-long transect (extended to 100 km by correlation offshore), some 200 m thick ranging from the Cymodoce to the Pallassioides zones, and representing some 6.5 m.y. of marine sedimentation, was evaluated by study of four continuously cored boreholes sited to sample both basin and shelf facies.

Thin-bed stratigraphy, established by geophysical log signatures calibrated by the ammonite succession, enables organic-rich beds to be traced throughout the transect and into other English basins. These results show that total organic carbon may be computed reliably from a combination of resistivity, density, and sonic logs and increases by over 50% as each level is traced from shelf to basin where deeper waters are thought to have favored its entrapment and preservation in an environment more depleted in dissolved oxygen and with a more rapid burial. Associated, coccolith-rich, marker bands of the shelf pass into dolomites in the basin owing to the precipitation of early dolomite through bacterial decay of the organic-rich material.

Vertical distribution of the organic content of each section identifies a hierarchy of sedimentary cycles with periodicities of about 25,000 and 280,000 years. The short period cycles, less than 1 m thick, comprise alternations of more or less organic-rich beds. The analysis of the extracts suggests unity of origin of the organic matter of type II origin (zoo- and phytoplankton) throughout the period of sedimentation. The differences observed in the kerogens can be interpreted in terms of a fluctuation of oxygen deficiency whose variations (dysaerobic, anaerobic, or anoxic) are recorded in time and in space. Some of the variation, near coccolith-rich beds, can be related to a very high organic productivity (up to 40 wt.% total organic carbon). The second order cycles show maximum kerogen enrichment in the middle of the transgressive tract intervals, or at the base of high level, or platform edge prisms. These relationships allow the distribution of the organic matter to be deduced from sequence stratigraphic studies.

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