Bentonites in the Chalk of central eastern England and their relation to the opening of the Northeast Atlantic

Abstract

Within the post-Cenomanian Chalk sequence of central eastern England about 20 thin persistent marl horizons occur. These marls contain from 30 to 65% non-carbonate material the bulk of which is composed of a well crystallised Mg-rich smectite with a notable paucity of illite and quartz. The Chalk matrix above and below the marls contains up to 2.0% of non-carbonate matter which again is dominated by the presence of an Mg-rich smectite but which contains a larger proportion of illite and quartz than occurs in the marls. For both sets of non-carbonate materials the excellent crystallinity and Mg-rich nature of the smectite strongly suggest that it was formed by the in situ alteration of an aeolian transported volcanic ash, i.e. it is a true bentonite. Quartz and illite in the Chalk matrix are probably detrital components. Relative to a standard shale, non-carbonate materials from the Chalk marl horizons exhibit a highly anomalous trace element pattern with accentuated levels of Zr, Nb and Th but depleted levels of Ba, Rb and K. This trace element chemistry corroborates a volcanogenic origin for the smectite insofar as it is not explicable in terms of any realistic sedimentary process. The chemistry is best interpreted in terms of the magnetic processes responsible for the formation of the original ash precursor of the marls. It appears that this precursor had a pantelleritic composition and therefore originated from a source overlying an area where incipient continental rifting was taking place. Micromorphological examinations prove that residues from the Chalk marls contain small amounts of pyroclastic debris, including pumice and fragments of sideromelane glass. The morphology of the fragments suggests that the volcanic eruptions responsible for the ejection of the original ash into the air were of the shallow-water phreatomagmatic (Surtseyan) type. The distribution of marls in the Chalk of eastern England suggests that the ashfalls responsible for their origin were of the tropospheric type and were drawn by westerly winds from a centre to the west or northwest of mainland Britain. Of the numerous volcanic centres that lie in offshore areas to the west, the Anton Dohrn Seamount in the Rockall Trough appears to a likely centre of origin for the ashfalls. However, ashfalls responsible for the formation of the traces of smectite in the Chalk matrix were probably of the general background (stratospheric) type and may have been drawn from more distant centres. Volcanism at the Anton Dohrn Seamount was related to tensional effects in the continental crust that immediately preceded the formation of the Rockall Trough and the separation of Greenland from northwestern Europe during Tertiary times. Activity at the centre, and Cretaceous bentonites derived therefrom, therefore represent the earliest stages in the development of the Thulean igneous province.