Quaternary fluvial, pedogenic and mass-movement processes at St George's Down, Newport, Isle of Wight

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Abstract

Recent geological mapping on the Isle of Wight by the British Geological Survey has shown the ‘Plateau Gravel’ to be a mixture of fluvial, solifluction, pedogenic and marine deposits ranging from pre-Anglian to Holocene age. As part of the resurvey of the island, several new exposures of the ‘Plateau Gravel’ between Newport and Downend were examined. A working gravel pit on St George's Down, near Newport, revealed a succession of flint gravels with an inter-bedded sequence of laminated silts. An upper in situ succession of pre-Anglian fluvial gravels caps the plateau, but a second, probably younger suite of gravel-rich sediments is exposed in a quarry on a topographically lower spur. These overlie in situ Clay-with-flints resting on Upper Cretaceous Chalk. These lower sediments are well exposed and display a complex stratigraphy. They consist predominantly of flint gravel, but include a dipping succession of laminated silts and palaeosols preserved in a hollow or small channel feature, intercalated between two distinct soliflucted cold-stage gravel sheets. Palynological and pedological evidence analysis suggests that these laminated silts and sands were deposited under a temperate climate but with frequent episodes of disruption caused by mass-movement and possibly freeze–thaw. The age of these laminated sediments are not known with any certainty but are likely to date to a temperate interval within the Late Pleistocene. The top of the laminated unit is cut by a heavily cryoturbated horizon presumed to be of Devensian age.

Introduction

Quaternary superficial deposits are important repositories of palaeoclimatic and palaeoenvironmental information. The Hampshire – Isle of Wight region contains an extensive sequence of Quaternary river terraces and raised beaches associated with the development of the Solent River. During the Quaternary this was the largest river in southern England, but has now been truncated by sea-level rise (Everard, 1954). The terrace gravels on the northern side of the Solent River have been recognised for over a century and numerous lithostratigraphical schemes have been erected (see Allen and Gibbard (1993) for a comprehensive review). Many of these terrace and raised beach deposits are important archaeological sites; some, such as Boxgrove (Roberts and Parfitt, 1999) are of international significance. Although they have been extensively discussed in the literature the correlation and age of these deposits remain a subject of debate (Westaway et al., 2006, Briant et al., 2009). However, relatively little attention has been focused on the extensive Quaternary deposits that crop out on the on the southern side of the Solent river on the Isle of Wight. Most of these deposits were mapped as ‘Plateau Gravel’ during the primary geological surveys during the nineteenth century. However, the precise age, genesis and significance of these deposits as shown on the published geological maps (British Geological Survey, 1976) and memoirs (White, 1921) is poorly understood. It is unclear whether they represent shallow marine sediments, raised beaches, fluvial river terraces or solifluction sheets. Understanding the genesis and chronology of the ‘Plateau Gravel’ and placing its constituent parts into a robust chrono-stratigraphic framework will significantly improve the potential to interpret the archaeological record on the southern side of the Solent River. Recent remapping of the island by the British Geological Survey (BGS; in prep) demonstrates that the ‘Plateau Gravel’ are an amalgam of lithologically, temporally and genetically distinct deposits, and include Clay-with-flints, periglacial solifluction deposits (‘Head’), fluvial gravels, raised beaches and palaeosols. An understanding of the age and origin of these enigmatic deposits is critical to developing our knowledge of how the Isle of Wight fits into regional patterns of sea-level change, palaeoclimate and river system development within southern Britain, and contribute to the debate on the Quaternary uplift history of central southern England.

In this context, examining good sections through these deposits is critical. During the recent resurvey, a new section in the ‘Plateau Gravel’ was identified in the Bardon Vectis quarry on St George's Down near Newport (Fig. 1). The quarry, active at the time of the survey, is located 1.5 km southeast of Newport, adjacent to the Shide Chalk pit [SZ 506 881], but previous workings extended across much of the plateau between Shide and Arreton Cross [SZ 530 867]. This paper describes the deposits seen in the present working quarry (2009–2010) and provides evidence for a multiphase, polygenetic origin for the ‘Plateau Gravel’.

Section snippets

History of research

Although there has been a long history of geological investigation on the Isle of Wight, relatively little has been published on the extensive gravel and sand deposits (the ‘Plateau Gravel’) that occur widely across the central and northern parts of the island. These cap parts of the Chalk outcrop, form extensive spreads along the coast between Cowes and Ryde, where they are underlain by gently dipping Palaeogene strata, and also occur in association with the Medina River and Eastern Yar. The

St George's Down-site locality and context

The St George's Down deposit is one of the highest gravel spreads on the island and forms a prominent topographical feature between Newport and Arreton (Fig. 2) at the western end of the prominent Chalk ridge. Most of the deposit sits on a plateau at around 100–106 m OD, some 90 m above the floor of the present Medina valley, but slightly lower than the main Chalk ridge to the east. Smaller gravel-capped spurs occur at slightly lower elevations down to c. 80 m OD. The topographically highest

Stratigraphy of the St George's Down deposits

The St George's Down deposit can be divided into two major stratigraphic units, based largely on their topographical position. These are the main Upper Plateau gravels (UPG), now largely quarried away, and the slightly lower, more complex Shide Golf Course spur (SGC) deposits which were well exposed in the current Bardon Vectis quarry (Fig. 3 and Table 1). Gravel also caps several other spurs at similar elevations around Great East Standen Farm. Solifluction deposits mantle the slope down into

Gravel lithology and particle size

To characterise these deposits, 18 bulk sediment samples were taken for particle size analysis and grading (Table 2). These were collected from the Upper Plateau deposits (UPG-1), the gravels within the current working quarry (SGC-2 and 4), the laminated silt and sand ‘channel’ deposits (SGC-3), and the roadway section (SGC-4b). Sand and gravel fractions determined by a combination of wet and dry and sieving (Gale and Hoare, 1991). Gravel residue from the particle size procedure was then graded

Palynology

Seven samples were collected from the laminated silts and palaeosols of the main ‘channel’ succession for palynological analysis. These were collected from fresh faces to maximise the palynomorph yield, and to minimise the potential for contamination by solitary bees and other burrowing organisms. The results of the palynological analyses are given in Table 4, with the key taxa presented as a summary pollen diagram (Fig. 13). The pollen assemblages are well-preserved and abundant; the slides

Description of thin sections

Three thin section samples were also collected from the laminated silts and sands of the main channel succession for more detailed micro-morphological analysis (Fig. 9). One was collected from the distinctive ‘black band’ (SGC-D) and the other two from above and below this horizon (SGC-E and B, respectively). The samples were collected using kubiena tins (8 × 5 × 4 cm), air-dried before being impregnated with resin, and allowed to set prior to sectioning. The sections were described using standard

Discussion

The gravel deposits from St George's Down represent a complex history of environmental change.

Conclusions

The gravel deposits at St George's Down, formally mapped as ‘Plateau Gravel’, comprise a polygenetic sequence of fluvial, periglacial and temperate lacustrine sediments and palaeosols that formed over a long period of time. These deposits consist of topographically high-level in situ fluvial gravels flanked by lobes of later cold-stage soliflucted material that are locally inter-bedded with temperate laminated silts and sands. On the Chalk outcrop, the soliflucted gravels overlie relict

Acknowledgements

This work would not have been possible without the kind assistance of Stephen Burton, the manager of Bardon Vectis Quarry. Chris Holden is thanked for his assistance in the field in late 2009 and his work on the particle size analysis, clast lithological and morphometric studies that he carried out as a work experience student with BGS during 2009. This work formed part of his undergraduate dissertation at Derby University in 2010. AF, PH, JL and JBR publish with the permission of the Executive

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      Citation Excerpt :

      Indeed, these sheets of mass-flow deposits, where they spread over a significant height range, may well disguise topographically distinct terrace aggradations or platforms. This is demonstrated for example in the paper by Farrant et al. (2011) describing the high-level (c. 70–100 m OD) gravels at St George's Down. The descriptions of the deposits at Priory Bay (Wenban-Smith et al., 2009), at a much lower topographical level (c. 29 m OD) than those on St George's Down, also demonstrates polyphase fluvial deposition and in this case contemporary hominin occupation, as well as interbedded, poly-phase mass flow material.

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