Lithology of the Chalk Group - Hardgrounds

Hardgrounds are intervals of indurated chalk, generally less than a metre thick, formed by enhanced sea-floor cementation associated with periods of non-deposition (Hancock, 1989). Cementation occurred just below the sea floor, initially producing nodules of harder chalk in softer sediment (Gale, 2000). Local exposure by sea-floor erosion facilitated further cementation, and allowed the hard chalk pavement to become bored into and encrusted by marine fauna, and sometimes there is replacement of the surface by minerals such as glauconite and phosphate. Mature Chalk Group hardgrounds are usually formed of buffish coloured chalk that is intensely hard, and rings when struck with a hammer. The name 'chalkstone' is sometimes applied to this distinctive lithofacies.

In the Chalk Group, hardgrounds are best developed over structural highs, and often represent intervals of stratigraphical condensation formed in response to major sea-level changes (Gale, 2000), or in response to local tectonic uplift. Hardgrounds are less distinct away from basin margins, and may eventually become almost imperceptible >omission surfaces=, perhaps only recognisable by weakly developed iron-staining.

References

HANCOCK, J. M. 1989. Sea-level changes in the British region during the Late Cretaceous. Proceedings of the Geologists' Association, 100 (4), 565-594.

GALE, A. S. 2000. Late Cretaceous to Early Tertiary pelagic deposits: deposition on greenhouse Earth. In WOODCOCK, N & STRACHAN, R. (eds), Geological History of Britain and Ireland (Blackwells).