Sedimentation, geochemistry and palaeomagnetism of the West Runton Freshwater Bed, Norfolk, England

Abstract

The sedimentary succession of the West Runton Freshwater Bed (WRFB) is described and interpreted. The sediments represent a fluvial valley floor accumulation that trends west- to east, subparallel to the modern cliffline. Stabilisation of the stream channels was initially followed by deposition of fine-grained silt in standing water, punctuated by intermittent flood events. Decreased flood intensity resulted in vertical accretion of fine-grained, organic, fossiliferous sediments in carbonate-rich spring water.Increased organic deposition produced a transition to black detrital sediment. Periodic flooding continued, but flood frequency decreased in response to reduction in accommodation. Local erosion of channel banks and floors also occurred. The sediments were disturbed by water-release structures and bioturbation, the latter resulting from large mammals locally entering the channel and pools, churning the deposits during walking, bathing and so on. River-flow velocity diminished later as forest vegetation stabilised catchment and valley-floor ground surfaces. Stabilisation of the channel led to deposition from low-velocity flowing and standing water. The uppermost part of the succession indicates vertical accretion, sheet-like deposition. The organic silts were deposited in an anoxic environment, and organic sulphur from rotting organic matter allowed formation of early diagenetic sulphides.Deposition began during the late-glacial (Beestonian Stage) and continued into the first half of a temperate-climate, interglacial event (the Cromerian Stage s.s.). The infill comprises an overall fining-upward sequence, the stream adopted a stable meandering pattern. The WRFB stream showed typical chalk-stream behaviour.The magnetic polarity of the WRFB sediments is normal. The palaeomagnetic data indicates that the sediments are of Brunhes Chron age, and imply that they post-date mid-MIS 19.