Brown, A G and Basell, L S and Toms, Phillip (2015) A stacked Late Quaternary fluvio periglacial sequence from the Axe valley, southern England with implications for landscape evolution and Palaeolithic archaeology. Quaternary Science
15 The current model of mid latitudes late Quaternary gravel terrace sequences is that they are 16 uplift driven but climatically controlled terrace staircases relate to both regional scale crustal and 17 tectonic factors, and palaeohydrological variations forced by quasi cyclic climatic catchment 18 conditions in the 100 K world (post Mid Pleistocene Transition) This model appears to hold for 19 the majority of the river valleys draining into the English Channel which exhibit 8 15 terrace 20 levels over approximately 60 100m of altitudinal elevation. However, one valley, the Axe, has 21 only one major morphological terrace and has long been regarded as anomalous. This paper uses 22 both conventional and novel stratigraphical methods (digital granulometry and terrestrial laser 23 scanning) to show that this terrace is a stacked sedimentary sequence of 20 30m thickness with 24 a quasi continuous (i.e. with hiatuses) pulsed, record of fluvial and periglacial sedimentation over 25 at least the last 300 400 K yrs as determined principally by OSL dating of the upper two thirds 26 2 of the sequence. Since uplift has been regional, there is no evidence of anomalous neotectonics, 27 and climatic history must be comparable to the adjacent catchments (both of which have staircase 28 sequences) a catchment specific mechanism is required. The Axe is the only valley in North West 29 Europe incised entirely into the near horizontally bedded chert (crypto crystalline quartz) and 30 sand rich Lower Cretaceous rocks creating a buried valley. Mapping of the valley slopes has 31 identified many large landslide scars associated with past and present springs. It is proposed that 32 these are thaw slump scars and represent large hill slope failures caused by Vauclausian water 33 pressures and hydraulic fracturing of the chert during rapid permafrost melting. A simple 1D 34 model of this thermokarstic process is used to explore this mechanism and it is proposed that the 35 resultant anomalously high input of chert and sand into the valley during terminations caused 36 pulsed aggradation until the last termination. It is also proposed that interglacial and interstadial 37 incision may have been prevented by the over sized and interlocking nature of the sub angular 38 chert clasts until the lateglacial when confinement of the river overcame this immobility 39 threshold. One result of this hydrogeologically mediated valley evolution was to provide a 40 sequence of proximal Palaeolothic archaeology over two MIS cycles. This study demonstrates 41 that uplift tectonics and climate alone do not fully determine Quaternary valley evolution and 42 that lithological and hydrogeological conditions are a fundamental cause of variation in terrestrial 43 Quaternary records and landform evolution. 44 45