THE SEDIMENTOLOGICAL SIGNIFICANCE OF CONCENTRICALLY LAMINATED BURROWSFROM LOWER CRETACEOUS CA-BENTONITES, OXFORDSHIRE

Concentrically laminated burrows are common at outcrop and in core in Mesozoic and younger shallow marine siliciclastic sedimentary rocks. T hey are generally referred to Cylindrichnus concentricus Howard, an im perfectly understood ichnotaxon here considered to be a nomen dubium. Concentric lamination can be formed in several ways: either actively c ontrived 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 Cretace ous (Aptian) sedimentary rocks associated with fuller's earth (Ca-bent onite) at Baulking, Oxfordshire. The distribution and taphonomy provid e information on the sedimentological setting of the tephra and associ ated sediments. The lower seam was already compacted and of clay grade before it was repeatedly eroded and colonized; the latter has signifi cantly 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 misident ified 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 C ylindrichnus in the literature are briefly discussed: many must be ass igned to other ichnotaxa.