The Petrography of the Hunstanton Red Rock

Abstract

At Hunstanton Cliff and at Snettisham there is a gradual transition, without physical break, between the Carstone and the Red Rock. At Hunstanton there is a physical break, marked by a solution-surface, between the Red Rock and the Sponge Bed. At Snettisham the Red Rock is incomplete at the top.The Carstone shows a peculiar and characteristic assemblage of mineral constituents, with much glauconite and ferruginous ooliths. The heavy minerals are notable for the abundance of kyanite and staurolite and rarity of garnet.The Red Rock consists of three fractions: (a) calcium carbonate, (b) red mud, (c) sand. This sand is exactly like the Carstone, with the same heavy minerals, and it continues, though in diminishing amount, up to the very top of the Red Rock.The insoluble residue of the Sponge Bed is entirely different: it consists mainly of silicified foraminifera, with a few colourless quartz grains, and it contains no heavy minerals at all.There is an abrupt and complete lithological change at the base of the Sponge Bed.The red fraction of the Red Rock is rich in Fe2O3, and Al2O3, and it is suggested that it is of lateritic origin.The precise manner of formation of the Red Rock is not explained. It has not been found possible to visualize a set of conditions under which fairly large pebbles, ferruginous sand grains, ferruginous ooliths, red mud, and calcium carbonate were all deposited at the same time.It is suggested that the red mud is due to lateritic material derived from a neighbouring land area.The heavy mineral assemblage shows that the material of Carstone type did not come from the north, and probably not from the west. The only possible source is the Caledonian and Armorican plateau of London-Brabant or a continuation of it under the North Sea. The sudden change at the top of the Red Rock is due to the submergence of this land area by the Cenomanian transgression, which in Norfolk was accompanied by local erosion: this finally closed the source of supply of the heavy minerals: here and there a slight recurrence of transport of lateritic material gave rise to pink bands in the Chalk at higher horizons.The interpretation by Messrs. Kitchin and Pringle, on palaeontological grounds, of the relations of the Red Rock to the Carstone and to the Chalk is shown to be quite inconsistent with the results of this petrographical investigation.