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The sedimentological significance of concentrically laminated burrows from Lower Cretaceous Ca-bentonites, Oxfordshire

Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, PO Box 227, Reading RG6 6AB, UK

Concentrically laminated burrows are common at outcrop and in core in Mesozoic and younger shallow marine siliciclastic sedimentary rocks. They are generally referred to Cylindrichnus concentricus Howard. an imperfectly understood ichnotaxon here considered to be a nomen dubium. Concentric lamination can be formed in several ways: either actively contrived by the producer, or by passive filling (draught filling) of a burrow with constricted apertures. In the latter case the fill cannot be regarded as an ichnotaxobase. Passive infilling of concentrically laminated, bow-form, deep-tier burrows is described from Lower Cretaceous (Aptian) sedimentary rocks associated with fuller's earth (Ca-bentonite) at Baulking, Oxfordshire. The distribution and taphonomy provide information on the sedimentological setting of the tephra and associated sediments. The lower seam was already compacted and of clay grade before it was repeatedly eroded and colonized the latter has significantly reduced seam quality. Subsequent sedimentation was essentially aggradational, including the uniquely preserved upper bentonite which had a complex recovery history.

Among other ichnotaxa, concentrically laminated Thalassinoides (and possibly Diplocraterion) may be misidentified as ‘Cylindrichnus’. Since Thalassinoides and Diplocraterion are important indicators of bounding surfaces, information of significance in sequence stratigraphy may be lost as a consequence. Citations of Cylindrichnus in the literature are briefly discussed: many must be assigned to other ichnotaxa.

Keywords: Cretaceous, Aptian, bentonite, ichnofossils.

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