Posted by: paulgarner | March 19, 2010

Notes from the field: bentonites in the English Chalk and a Welsh “Tuesday afternoon deposit”

I got back last night from a short field trip to a couple of well known geological sites. On Wednesday I was down on the south coast of England taking a look at the Upper Cretaceous Chalk. Of particular interest were the marl horizons within the Chalk which are well exposed in the cliffs at the foot of Friars Bay Steps in Peacehaven. Mineralogical and geochemical data suggests that at least some of these are bentonites – volcanic ash bands – demonstrating that explosive volcanic activity was occurring simultaneously with the deposition of the Chalk. It seems that the Chalk Sea wasn’t quite as placid as some have previously thought, a point that also comes across in Wray and Gale’s (2006) helpful review of the palaeoenvironment of Upper Cretaceous Chalks.

Then it was on to South Wales on Thursday where I visited the famous unconformity between the Carboniferous Limestone and the overlying ‘Littoral Lias’ (Lower Jurassic) at Ogmore-by-Sea. The Sutton Stone member at the base of the Jurassic sequence has generally been regarded as a transgressive shoreline-beach deposit laid down over a period of several million years. However, it was controversially reinterpreted by the late Derek Ager (1986a) as a mass-flow unit deposited rapidly during a single, major storm – one of his beloved “Tuesday afternoon deposits”. Although subsequent workers including Fletcher et al (1986) challenged Ager’s reinterpretation, Ager himself remained unbowed (1986b), and much can be said in favour of his more catastrophic hypothesis. After all, as Ager pointed out, the Sutton Stone is a matrix-supported conglomerate, which is characteristic of mass-flow deposits, and it appears to lack bedding planes. The bedding plane that Johnson and McKerrow (1995) claimed to be able to identify locally within the Sutton Stone may well be a stylolitic (pressure solution) feature.


Ager D. V. 1986a. A reinterpretation of the basal ‘Littoral Lias’ of the Vale of Glamorgan. Proceedings of the Geologists’ Association 97:29-35.

Ager D. V. 1986b. Reply. Proceedings of the Geologists’ Association 97:384.

Fletcher C. J. N., Davies J. R., Wilson D., Smith M. 1986. The depositional environment of the basal ‘Littoral Lias’ in the Vale of Glamorgan – a discussion of the reinterpretation by Ager (1986). Proceedings of the Geologists’ Association 97:383-384.

Johnson M. E., McKerrow W. S. 1995. The Sutton Stone: an early Jurassic rocky shore deposit in South Wales. Palaeontology 38:529-541.

Wray D. S., Gale A. S. 2006. The palaeoenvironment and stratigraphy of Late Cretaceous Chalks. Proceedings of the Geologists’ Association 117:145-162.



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