--> Abstract: Pan-European Correlation of Ash-Falls (Bentonites) in Upper Cretaceous Chalks, by David S. Wray; #90039 (2005)

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Pan-European Correlation of Ash-Falls (Bentonites) in Upper Cretaceous Chalks

David S. Wray
University of Greenwich, Chatham Maritime, United Kingdom

Thin (c. 2-20cm) clay-rich beds form a key component of regional lithostratigraphic schemes within the various Chalk provinces of Western Europe. Geochemical and mineralogical provenance studies have established that individual beds are derived either from a detrital or volcanogenic source.

The volcanogenic / detrital sub-division is best achieved on geochemical criteria, especially shale-normalised rare-earth element (SNREE) profiles. Detrital beds have a SNREE profile which is horizontal whilst volcanogenic beds have an enriched SNREE profile typified by a negative europium (Eu) anomaly. Volcanogenic beds are dominantly smectitic; whilst detrital beds contain a variable mix of smectite and illite, the proportions probably controlled by the composition of the hinterland. Because of their mineralogy, the volcanogenic beds are best classified as bentonites.

This sub-division of clay-rich beds has been successfully applied to beds of Turonian and Coniacian age in England, France and Germany, resulting in the creation of a pan-European tephrostratigraphic framework, essentially giving a series of chronostratigraphic time lines that can be observed in the field. This framework is independently supported by carbon isotope data. Preliminary work in northern Germany (Lagerdorf) has also demonstrated that a similar sub-division can be applied to clay-rich beds in younger chalks.

Recent work in Germany indicates that it is possible to uniquely fingerprint individual bentonites using a number of major- and trace-element ratios and on-going work seeks to apply this strategy to the condensed successions in England containing the Spurious Chalk Rock and Chalk Rock hardgrounds of mid to late Turonian age.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005