Carbonates within bituminous shales of the British jurassic - their petrography and diagenesis

Abstract

Bituminous shales containing enigmatic carbonate lithologies, such as calcite nodules, beef lenses and dolomite beds, form a significant portion of the southern England Jurassic succession. The highly kerogenous Kimmeridge Clay is the thickest such shale. Samples of Its carbonates collected from Dorset and elsewhere in the UK were studied using thin and ultra-thin sections, XRD, SEM and AA. The bituminous Oxford Clay and Lies were also sampled. Primary calcite was largely supplied by calcareous plankton and molluscs. During early diagenests, as a result of bacterial activity, magnesian calcite precipitated within a metre or so of the sediment surface as nodules and scattered minute crystals, forming in abundance during periods of reduced sedimentation. Calcium and magnesium ions were supplied by diffusion from the sediment surface. Ferrous iron, scarce In the calcite, was scavenged from pore waters to form pyrite. Processes occurring within basinal sediments of the Santa Barbara Basin, California, where pore waters display high Mg/Ca ratios and alkalinities, are suggested as a model for the early diagenesis in theJurassic bituminous shales. During deeper burial diagenesis other carbonate lithologies evolved. Each of these possess characteristic crystal fabrics that developed in response to overburden pressure as the carbonates formed or were altered. These fabrics can be evaluated using a simple XRD technique. Ferroan and non-ferroan elongate-calcite ('beef') formed at depths of up to hundreds of metres by a process of vertically displacive crystal growth. Both the long axes and c-axes of its crystals tend to be oriented perpendicular to bedding. Ferroan dolomite with the same fabric formed in a similar fashion. minor laterally displacive crystal growth giving rise to distinctive, large-scale polygonal thrust patterns in some beds. Both derived their carbonate largely from the breakdown of organic matter. Other dolomite formed by replacing magnesian calcite and as a cavity-filling cement. Magnesium in this carbonate could have been derived from the diagentle alteration of unstable magnesian calcite and dolomite, algal organic matter and clays. The diagenetic history of the carbonates within the Jurassic bituminous shales provides a model applicable to other marine bituminous shales.

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