DISTINCTION BETWEEN DETRITAL AND VOLCANOGENIC CLAY-RICH BEDS IN TURONIAN-CONIACIAN CHALKS OF EASTERN ENGLAND

The acid insoluble <2 mu m fraction of 36 clay-rich marls, each from a stratigraphically distinct bed, in the Turonian and Coniacian chalks of Lincolnshire and East Yorkshire has been analysed. All beds are dom inated by smectite, with traces of illite and sporadic trace amounts o f kaolinite. Major, trace and rare-earth element whole rock data show that the clay-rich beds fall into two groups. The majority have a rela tively constant K2O/Al2O3 ratio (mean 0.12), low Zr/Al2O3 ratio (mean 0.002) and variable TiO2/Al2O3 ratio (mean 0.07); shale normalized rar e-earth element (REE) data displays a negative Ce anomaly and a variab le, gradual depletion in heavy REE. By analogy with clays in the Germa n Chalk, these beds are considered to be detrital. Six beds (Barton 1, Melton Ross, Deepdale Lower, North Ormsby, Ulceby and Little Weighton 2) are characterized by a variable K2O/Al2O3 ratio (mean 0.05), relat ively high Zr/Al2O3 ratio (mean 0.007) and low TiO2/Al2O3 ratios (mean 0.03). Shale normalized REE data typically displays a slight negative Ce anomaly and a variable, but commonly marked, negative Eu anomaly. By analogy with German examples, these beds are interpreted as bentoni tes derived from the in situ alteration of volcanic tephra. Geochemica l data indicate that tephra had a rhyolitic to pantelleritic compositi on. The occurrence of mid Turonian to mid Coniacian bentonites in the Chalk of eastern England provides the first evidence that clay-rich be ds in the English Chalk have more than one origin.