The plant debris beds of the Early Cretaceous (Barremian) Wessex Formation of the Isle of Wight, southern England: their genesis and palaeontological significance

The Early Cretaceous (Barremian) Wessex Formation of the Isle of Wight primarily represents high sinuosity fluvial, but also lacustrine and terrestrial deposition on a low relief floodplain occupying half grabens, with active normal faults immediately to the north. Plant debris beds form a very small proportion of the succession but are the main source of vertebrate fossils, including dinosaurs. These conspicuous, but thin, grey units are distributed randomly throughout the Wessex Formation and are of limited lateral extent. The lower part of most plant debris beds comprises a poorly sorted, matrix-supported conglomerate. The upper part of most consists of structureless mudstone, which usually grades up into colour-mottled but predominantly red and purple floodplain mudstones. The plant debris beds contain an unusual mixture of freshwater molluscs, diverse aquatic and terrestrial vertebrates, and mainly terrestrial plants with a variable quality of preservation. We conclude that each plant debris bed represents a locally generated sheetflood, which was then transformed on the floodplain into a debris flow by the acquisition of surface material. The debris flows surged into, and were deposited in depressions on the alluvial plain, including abandoned chute and cut-off channels, oxbows and ponds. These units are therefore intrabasinal and interfluvial, and the fossils that they contain are locally derived. Similar deposits of palaeontological significance have not been reported elsewhere in the literature. In view of this, their origin and sedimentology, their palaeontological significance, and the taphonomy of the flora and fauna they contain are discussed.