Petrography and diagenesis of the Thames Group at Whitecliff Bay, Isle of Wight, UK

Abstract

Previous studies of the Thames Group have emphasized stratigraphical or mineralogical aspects of the formation. We have used a combination of field observation, hand specimen description, optical and back-scattered electron petrography and X-ray diffraction to identify textures (grain size, bioturbation etc.), detrital, synsedimentary and diagenetic components. With these data lithologs have been constructed to describe the diverse lithologies which constitute the London Clay. Early diagenetic mineral assemblages are closely linked to depositional rate. Hiatus surfaces are associated with concentrations of glauconitic clay pellets. Bedded siderite concretionary carbonates were precipitated by diagenetic processes, below relatively brief hiatus surfaces, whilst ovoid ferroan calcite concretions were deposited during periods of very slow deposition. The glauconitic sandy fissures in the mudrock intervals of Divisions A and B are interpreted as storm deposits. The regular 1-1.5 m spacing of these sands is evidence of rhythmic sedimentation.