Bedding and pseudo-bedding in the Early Jurassic of Glamorgan: deposition and diagenesis of the Blue Lias in South Wales

Sedimentary bedding planes in a succession of Lower Jurassic (Bucklandi Biozone) limestone-shale alternations at Nash Point, South Wales are represented by omission surfaces intermittently developed on the upper surfaces (limestone-shale contacts) of limestone beds. Other lithological contacts (shale-limestone contacts and the majority of limestone-shale contacts) are devoid of positive indications of an associated break in sedimentation. Statistical analysis of (a) limestone-shale contacts and (b) recorded omission surfaces suggests that limestone-shale contacts are regularly spaced and omission surfaces randomly distributed. Limestone-shale contacts and omission surfaces are interpreted as proxies for sedimentary bedding and shale-limestone contacts interpreted as pseudo-bedding planes, with the alternation of limestones and shales arising from the diagenetic differentiation of beds of lime mud. No short- or long-term cyclicities at a scale greater than that of an individual couplet can be detected by the statistical methods employed, and it is therefore unlikely that a Milankovitch-type cyclicity is present. Evidence of strati-graphical environmental succession, exhibited both by ichnotaxa and body fossil assemblages in the biofacies of omission surfaces, suggests that the succession represents part of a third-order shallowing event. It is proposed that beds of lime mud were deposited as a consequence of episodic storm action on a hemipelagic shelf, and diagenetic differentiation was ‘steered’ by this episodicity and not by any orbital control.