Channelling versus inversion: origin of condensed Upper Cretaceous chalks, eastern Isle of Wight, UK

Abstract

Evidence from regional stratigraphical patterns in Santonian−Campanian chalk is used to infer the presence of a very broad channel system (5 km across) with a depth of at least 50 m, running NNW−SSE across the eastern Isle of Wight; only the western part of the channel wall and fill is exposed. Within this channel were smaller erosional structures (<10 m deep) that truncate originally horizontal bedding, are floored by hardgrounds, and locally have a basal fill of granular phosphorite. The entire channel system was progressively infilled by chalk, as demonstrated by the expanded succession of the lower Campanian Culver Chalk Formation. The beds of the channel fill are cut by small step faults, resulting from gravitational collapse. Complete burial had taken place by the base of the upper Campanian Portsdown Chalk Formation, which is of even thickness across the region. The structures are interpreted with reference to high-resolution seismic profiles through chalk channel systems described from the German sector of the North Sea, and the Santonian−Campanian of the eastern Paris Basin, and were formed by persistent bottom currents. Previous interpretations of the condensed Santonian−Campanian chalks in the eastern Isle of Wight, involving penecontemporaneous tectonic inversion of the underlying basement structure, are rejected.