Abstract
The Whitby Mudstone Formation in northeast England contains three shale facies (normal, restricted and bituminous) which have previously been differentiated on the basis of sedimentary structures and faunal assemblages. This paper shows that the three facies can also be differentiated by mineralogical, geochemical and microtextural criteria determined using BSEM, EDXRA, XRD and XRF. The bituminous shale facies consists of finely laminated shales containing an early diagenetic mineral assemblage dominated by pyrite, calcite and dolomite. The weakly laminated restricted facies shales contain an early diagenetic assemblage dominated by pyrite. The normal facies shales are very weakly laminated and contain siderite in addition to pyrite. The geochemical conditions which left a dominant early diagenetic imprint on the three facies are interpreted to be carbon-rich sulphidic, carbon-poor sulphidic and very carbon-poor sulphidic respectively.

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