The Gault Clay–Folkestone Beds junction in West Sussex, Southeast England

Abstract

The Iron Grit, a limonitic sandstone developed at the base of the Gault between Midhurst and Washington, is suggested to be a primary deposit which accumulated in lagoonal conditions on the flanks of the newly emergent Portsdown Swell. The underlying Folkestone Beds have a coarsening-upwards profile combined with upward decrease in amplitude of cross-bedding and pass upwards into parallel laminated sands directly beneath the Iron Grit, a sequence interpreted as representing a beachface profile. The Iron Grit represents a drop in sustained depositional energy and removal from the beachface environment. SEM studies of quartz grain surface texture demonstrate an increase in impact textures upwards in the Folkestone Beds, but a decrease in the Iron Grit. Heavy mineral suites of the Folkestone Beds are characteristic for the Weald, but the suite in the Iron Grit is deficient in kyanite. The wide variety of mineral species confined to the Iron Grit illustrates protection from intrastratal solution by early cementation. The iron is suggested to have originated from weathering on the emergent land area. The Portsdown Swell uplift is related to a mid-Leymeriella tardefurcata phase of regional tectonism.