Condensation and channelling in Cenomanian chalks of the northern Anglo-Paris Basin; The Totternhoe Stone and related deposits

The Totternhoe Stone is a condensed, intraclastic unit developed in the Transitional and Northern Provinces of the English Cenomanian chalk, and rests on an erosional surface cut into chalk of the Mantelliceras dixoni Zone, locally downcutting into the underlying M. mantelli Zone in channels. The age of the chalk beneath this surface has been determined from the distribution of calcitic macrofossils with restricted ranges, known in detail from the basinal succession in southern England. The channels, up to 20 m deep, locally have a NNW-SSE orientation and are interpreted to have been formed by strengthened bottom currents which developed subsequent to a major sea-level fall of late M. dixoni age. The Totternhoe Stone is a complex condensed deposit of Cunningtoniceras inerme and Acanthoceras rhotomagense Zone age, composed of calcarenite formed largely of inoceramid prisms; it includes both remanie and indigenous faunas, and displays significant lateral variation in age. The Totternhoe Stone thickens into the channels (up to 10 m) and locally provided an important building stone. The Totternhoe Stone was deposited during the major transgression of the early Middle Cenomanian, and is correlative with the Basement Bed of the chalk of the eastern part of Dorset. A higher sequence, of Acanthoceras jukesbrownei Zone age, saw a smaller sea level fall, and also generated an erosional surface across the Transitional and Northern Provinces. Shallow channels were formed over the sites of those previously developed beneath the Totternhoe Stone, and these were infilled with calcisphere-rich calcarenites during the subsequent transgression, during which the Nettleton Stone was deposited across the region. This A. jukesbrownei Zone transgression onlapped the A. rhotomagense Zone in Dorset to rest on the Albian. The Totternhoe Stone and subjacent erosional surface are local expressions of a global regressive:transgressive cycle which probably had a glacioeustatic cause.