Middle and Late Pleistocene environmental history of the Marsworth area, south-central England

Abstract

A B S T R A C TTo elucidate the Middle and Late Pleistocene environmental history of south-central England, we report the stratigraphy, sedimentology, palaeoecology and geochronology of some deposits near the foot of the Chiltern Hills scarp at Marsworth, Buckinghamshire. The Marsworth site is important because its sedimentary sequences contain a rich record of warm stages and cold stages, and it lies close to the Anglian glacial limit. Critical to its history are the origin and age of a brown pebbly silty clay (diamicton) previously interpreted as weathered till.The deposits described infill a river channel incised into chalk bedrock. They comprise clayey, silty and gravelly sediments, many containing locally derived chalk and some with molluscan, ostracod and vertebrate remains. Most of the deposits are readily attributed to periglacial and fluvial processes, and some are dated by optically stimulated luminescence to Marine Isotope Stage (MIS) 6. Although our sedimentological data do not discriminate between a glacial or periglacial interpretation of the diamicton, amino-acid dating of three molluscan taxa from beneath it indicates that it is younger than MIS 9 and older than MIS 5e. This makes a glacial interpretation unlikely, and we interpret the diamicton as a periglacial slope deposit.The Pleistocene history reconstructed for Marsworth identifies four key elements: (1) Anglian glaciation during MIS 12 closely approached Marsworth, introducing far-travelled pebbles such as Rhaxella chert and possibly some fine sand minerals into the area. (2) Interglacial environments inferred from fluvial sediments during MIS 7 varied from fully interglacial conditions during sub-stages 7e and 7c, cool temperate conditions during sub-stage 7b or 7a, temperate conditions similar to those today in central England towards the end of the interglacial, and cool temperate conditions during sub-stage 7a. (3) Periglacial activity during MIS 6 involved thermal contraction cracking, permafrost development, fracturing of chalk bedrock, fluvial activity, slopewash, mass movement and deposition of loess and coversand. (4) Fully interglacial conditions during sub-stage 5e led to renewed fluvial activity, soil formation and acidic weathering.

Distribution and characteristics of Pleistocene ground thermal contraction polygons in Europe from satellite images

“…Overall, the distribution of polygons coincides with that of ice‐wedge pseudomorphs and relict sand wedges described in the literature (e.g., 1,22–25,28,37,40,44,48–69 ), although in detail many areas where wedges were observed in cross‐section are not associated with cropmarks (Figure 13). This distribution helps delineate the area that was affected by permafrost or deep seasonal frost during the Pleistocene, in a context that was cold enough to allow repeated cracking of the ground by thermal contraction.…”

Progress in Understanding the Dynamics, Internal Structure and Palaeoenvironmental Potential of Ice Wedges and Sand Wedges

“…Improved local and regional palaeoenvironmental reconstructions based on studies in sections of ice‐wedge casts, sand wedges and composite‐wedge casts have recently appeared from Belgium (Buylaert et al ., ), France (Bertran et al ., ), Hungary (Fabian et al ., ), Poland and Ukraine (Ewertowski, ; Zielinski et al ., ), the UK (Bateman et al ., ; Hutchinson, ; Murton et al ., ), eastern Canada (Remillard et al ., ), China (Liu and Lai, ), Antarctica (Bockheim et al ., ), Russia (Arzhannikov et al ., ) and Argentina (Bockheim et al ., ). The regional overviews following the Late Permafrost Maximum mapping in China (Zhao et al ., ) and in North America (French and Millar, ) both identify ice‐wedge casts and sand wedges as the best proxies for delimiting past permafrost environments, but also emphasise that the conditions for development of these features may vary considerably.…”

Middle and Late Pleistocene environmental history of the Marsworth area, south-central England by Murton et al. (2015)

“…The present paper by Murton et al (2015) summarizes much of this research and provides a wealth of new information from several sections studied by various people over the years. The origin of a diamicton in the lower part of the sequence has been the subject of recent debate.…”

“…This is potentially interesting because it occurs to the south of the generally accepted limit of the Anglian glaciation (or indeed any glaciation in southern England). Murton et al (2015) have now dated the shells of three species of molluscs recovered from beneath the diamicton using AAR, which indicate that it is younger than MIS 9 and older than MIS 5e. The AAR dating is based on aragonitic shells and therefore is less tightly constrained than dates based on the calcitic opercula of Bithynia (cf.…”