Slope failure of chalk channel margins: implications of a Campanian mass transport complex, southern England

The importance of mass transport and bottom currents is now widely recognized in the Upper Cretaceous Chalk Group of Northern Europe. The detailed dynamics and interaction of the two phenomena are difficult to study as most evidence is based on seismic data and drill core. Here, field observations provide evidence for recurring margin collapse of a long-lived Campanian channel. Compressionally deformed and thrust chalk hardgrounds are correlated to thicker, non-cemented chalk beds that form a broad, gentle anticline. These chalks represent a slump complex with a roll-over anticline of expanded, non-cemented chalk in the head region and a culmination of condensed hardgrounds in the toe region. Observations strongly suggest that the slumping represents collapse of a channel margin. Farther northwards, the contemporaneous succession shows evidence of small-scale penecontemporaneous normal faulting towards the south, here interpreted as gravitational settling of the chalk immediately adjacent to the channel margin. Detailed biostratigraphic studies and sedimentological observations provide evidence for at least two discrete collapse events and suggest the slumping to be the result of channel margin oversteepening rather than evidence for a regional tectonic phase. The described example thus serves as an analogue for processes commonly only inferred from subsurface data.