The West Runton Freshwater Bed and the West Runton Mammoth: Summary and conclusions

Abstract

1. IntroductionThis paper aims to synthesize the main conclusions from thevariouscontributionstothisissuecoveringthepalaeoenvironment,palaeoclimate and age of the West Runton Freshwater Bed (WRFB),as well as the life and death of the West Runton Mammoth (WRM)itself.Selectedpreviousresearchisalsobrieflydiscussed.Thewealthand breadth of the available information for the WRFB is almostunprecedented for any Pleistocene site. Although there are somerelativelyminordifferencesininterpretation,forthemostpartthereis verygood agreement between the various lines of evidence.For completeness, several mammalian taxa not otherwiseconsidered in this volume are included in Table 1, which lists thecomplete vertebrate fauna so far recorded from the WRFB.2. Sediments and depositional environmentAsdiscussedbyGibbardetal.(2010),FieldandPeglar(2010)andMaul and Parfitt (2010), West (1980) described a composite vege-tational sequence pieced together from pollen and plant macro-fossil assemblages at several localities along the WRFB exposure.He distinguished a Beestonian late glacial cold assemblage (l Be) atthe base of the sequence, succeeded by a series of Cromeriantemperate/interglacial assemblages (pollen subzones Cr Ia, Cr Ib, CrIIa and Cr IIb, of the Cromerian Interglacial). Although the entiresequence is not present at any one place, and the depositsundoubtedly include hiatuses and some reworking (see Gibbardet al., 2010), the various phases of vegetational development ofthe first part of the Cromerian are represented. However, siltshigherinthecliffwestofWestRuntonGap,whichWestassignedtothe second half of the Cromerian interglacial, almost certainlybelong to a distinct, younger episode (Zagwijn, 1996; Preece andParfitt, 2000).Gibbard et al. (2010) consider that the WRFB sediments weredeposited in a fluvial valley-floor accumulation, at least 100 mwide, which trended west to east, sub-parallel to the modern cliffline. The internal sequence of the WRFB is highly complex andshows considerable lateral and vertical discontinuity of individualunits, representing widely varying rates of deposition and hiatusesduetonon-depositionand/orsubsequenterosion. The depositionalbasin was subject to episodes of flooding on the one hand andperiodic drying out on the other. Nevertheless, Gibbard et al.consider that as a whole the WRFB represents the late part of theBeestonian and the first half of the Cromerian interglacial, which isconsistent with the pollen sequence described byWest (1980). Thisinterpretation implies that the composite WRFB sequence spansseveral thousand years, although there was never continuity ofdeposition at any one location.According to Gibbard et al. deposition of theWRFB began duringthelate-glacial partof theBeestonian cold stage, infilling an unevengravel surface that was inherited from a pre-existing braided cold-climate stream, and continued into the first half of a temperate-climate, interglacial event (the Cromerian Stage s.s.). Initial activestreamchannelswerefollowedbylowerenergyconditionsinwhichfine-grained silt was deposited in standing water, punctuated byshort intermittent flood events which washed in sand, while localerosion of channel banks and floors also occurred. Subsequentdecreased flood intensity resulted in vertical accretion of fine-grained, organic, fossiliferous sediments in carbonate-rich springwater, followed by increased organic deposition that producedatransitiontoblackdetritalsediment.Run-offandriver-flowvelocitydiminished later as forest vegetation stabilised catchment andvalley-floor ground surfaces. The lower part of the WRFB wasdisturbed syndepositionally by water-release structures and bio-turbation by large mammals. The mollusc fauna (Preece, 2010) alsoindicates faster-flowing conditions at the base of the sequence.