Identification and long-range correlation of bentonites in Turonian-Coniacian (Upper Cretaceous) chalks of northwest Europe

A geochemical and mineralogical study of thin (2-20 cm) clay-rich beds foun d in Turonian-Coniacian chalks from the Anglo-Paris Basin has been undertak en. On the basis of their rare-earth element (REE) and mineralogical compos ition it is possible to subdivide beds into bentonites, characterized by a negative Eu anomaly and elevated smectite content, and detrital beds (chara cterized by a minimal or absent Eu anomaly and a greater proportion of illi te). Six beds are considered to be bentonites (Glynde 1, Southerham 1, Cabu rn, Bridgewick 1, Lewes and Shoreham 2). Trace-element discriminant diagram s add support, as does comparison with similar studies undertaken in easter n England and northern Germany. Geochemical evidence demonstrates that the tephra was variably acidic in composition (rhyodacite-rhyolite-comendite/pa ntellerite), and was probably derived from crustal melting associated with initial rifting of the North Atlantic. Correlation of bentonites between th e Anglo-Paris Basin, eastern England and northern Germany can be demonstrat ed and independently tested, establishing a tephro-event stratigraphy acros s northwest Europe for this period.