Palynofacies and sedimentology of some Late Jurassic sediments from the British Isles and northern North Sea

Abstract

The variation of physical, chemical and ecological conditions through stratified water bodies is shown to be a useful integrating tool in the palaeoenvironmental modelling of epeiric sea facies. Particular emphasis is placed on the implications of oxygen deficiency in meromictic water bodies for the deposition of 'black shales'. In epeiric settings preservation is probably a more important determinant of black shale deposition than primary productivity. Optimal preservation of marine carbon occurs in dysaerobic to anoxic bottom waters. Such facies can be identified by a synthesis of sedimentological, geochemical and palaeoecological data. In dysaerobic to anoxic environments the kerogen is dominated by amorphous material of marine, but indeterminate, origin. The proportion of terrestial organic matter correlates with grain size, proximity to fluvial sources and level of oxygenation. Palynomorph assemblages vary along gradients of hydrodynamic energy and proximity to fluvial sources. Dinocysts can be directly related to hydrographic stability; they are rare in basinal, bottom water, sediments except when redeposited. The Oxfordian Piper Formation was deposited in an aerobic, hydrographically unstable, shallow shelf regime. palynofacies trends suggest an approximately NW-SE onshore-offshore gradient. Apart from scale the Piper sequence is analogous to coeval onshore sediments at Brora. The eudoxus-pectinatus zones of the Type Kimmeridge Clay were deposited in a distal basin with an alternating mixed (aerobic-dysaerobic) and meromictic (dysaerobic-anoxic) watermass. The resulting cyclicity is analagous to Quaternary sapropel sequences. Coccolith limestone intercalations are interpreted as indicative of partial mixing of a meromictic watermass. A formal subdivision of the Type Kimmeridge Clay Formation is proposed. Kimmeridgian sediments of the Brora-Helmsdale outlier comprise dysaerobicanoxic basinal shales with interbedded redeposited sandstones. A similar, but more proximal, facies association is observed in the Toni-Thelma area. Late Jurassic distal, basinal, dysaerobic-anoxic shales occur in the Maureen field.