Sussex Geodiversity Partnership: marine aggregates off Sussex and their origins

Andrew Bellamy, Tarmac Marine Ltd
Nigel Griffiths, Hanson Aggregates Marine Ltd

Introduction

This article is written to highlight an aspect of Sussex geology now hidden from view by the sea but nevertheless closely associated with the evolution of the landscape in the county. The subjects of this article are the infilled and submerged river valleys of the eastern English Channel which lie off the Sussex coast between Selsey and Hastings, as depicted in the reconstruction provided with this paper. Much of our knowledge of these palaeovalleys off Sussex has come from prospecting surveys for marine aggregate (sand and gravel) extraction over the past thirty years as well as broader scale surveys by the British Geological Survey.

Geological origins

Marine aggregates are unconsolidated sands and gravels formed primarily during the Quaternary Period, like their land-based counterparts. Over the two million years of the Quaternary, the UK continental shelf has experienced not only marine conditions as at present, but also cold and temperate climate terrestrial environments arising from large scale global climatic and sea level changes. For most of the Quaternary, sea level was lower than at present as demonstrated by oxygen isotope records (James et al, 2011 [Figure 54]) and our status as an island nation is atypical of much of the Period. Instead, the British Isles formed an extension of the northwest European mainland for much of Quaternary time, with the present shallow seabed of the inner continental shelf exposed as land. This is illustrated in the hypothetical reconstruction in Figure 1, simulating the drainage of Sussex and the adjacent continental shelf towards the end of the last glacial stage about 12,000 years ago.

Beyond glacial ice limits lying further north and west in Britain, the extension of rivers onto the exposed continental shelf in cold stages of the Quaternary led to the accumulation of sands and gravels over braided channel floodplains in periglacial environments analogous to arctic Canada or Siberia. Marine aggregate prospecting off southern Britain has focussed on the preserved palaeovalley networks that these rivers eroded and infilled over prolonged and repeated episodes of lower than present sea level. Fluvially derived gravelly marine aggregates are especially well developed where Cretaceous Chalk occurs within former river catchments, as in Sussex and the wider Weald, with almost all of the floodplain gravel being composed of durable flint, derived from the easily eroded and water-soluble Chalk. For this reason the palaeovalleys and floodplains of the Sussex Arun as well as the “Solent River”, the Thames and its tributaries, the Norfolk Yare and the palaeovalleys of the eastern English Channel are all exploited for marine aggregates (Emu Ltd, 2009; James et al, 2011; Limpenny et al, 2011).

The erosion of the Weald anticline, the recession of the escarpments of the North and South Downs and the numerous dry valleys on the Chalk Downs is attributable to fluvial erosion, partly in periglacial climates, with much of the flint released from the Chalk now lying as rounded far travelled gravel within palaeovalley deposits on the seabed. The transition from glacial to interglacial stages throughout the Quaternary caused sea level to rise, rivers to cease gravel deposition and river valleys to become estuaries and tidal inlets where sands, silts, clays and peats accumulated. These deposits are found overlying flint gravels in many of the palaeovalleys indentified by the aggregates industry.

The palaeovalleys off Sussex and their importance for geo-conservation

The palaeovalley infills are typically 0.5 to 5 km wide on the seabed off Sussex and locally reach thicknesses of over 15 m. The palaeovalleys are commonly completely infilled with sediment, form a flat seabed and lack bathymetric expression, as illustrated in the multibeam bathymetric image provided with this paper. Only locally are there linear depressions on the seabed indicative of the former river courses, such as 25 km south of Littlehampton and 15 km SSW of Selsey Bill. The geo-diversity interest of the palaeovalleys lies in the complex fluvial cut and fill sequences and contrasting sediment types revealed from high resolution seismic profile and core sampling surveys. An example of an interpreted seismic profile is given with this paper. Bellamy (1995) describes the infill of part of the submerged course of the River Arun using these data sources and the wider context in the eastern English Channel is given in James et al (2011). The sediment infills in turn indicate a wide variety of past depositional environments and geomorphological features on the inner continental shelf, associated with sea level and climatic changes over conceivably several climatic cycles. The last marine transgression led to planation across the continental shelf and superficial erosion of pre-existing fluvial sediments. Sands and gravels were scattered to form sheets of veneer thickness over wide areas of the newly established seabed and locally sand banks and dunes formed from reworked sediment. With the record of environmental and geomorphological changes, the evolution of the palaeovalleys and wider seabed off Sussex has relevance not only to the Quaternary of Sussex but also to the UK and NW Europe.

Archaeological aspects

Occasional finds are made at aggregate wharves of cold climate mammal remains, including bones, tusks and teeth of mammoth, reindeer and other fauna dredged from the gravelly deposits within the palaeovalleys. Finds such as these provide further evidence of the terrestrial environments that existed off the present coast during the Quaternary Period. To date no flint tools have been recorded from the deposits offshore of Sussex, but there remains the potential, especially with the hominid finds in the early 1990’s at Boxgrove. Flint tools have been recovered from marine aggregate cargoes dredged from the outer Thames Estuary and off Great Yarmouth - these areas were also terrestrially exposed for long periods over the past two million years or so.

Economic geology

Extraction of marine aggregates currently takes place in ten licensed areas between the Isle of Wight and Hastings. Active dredge zones are defined by the operating marine aggregates companies over the extractable deposits, largely located in the palaeovalleys of the former Solent River, River Arun and “Channel River.” In order to assess the potential for aggregate extraction, high resolution shallow seismic surveys are undertaken followed by core sampling. This data allows mineral reserve volumes to be calculated and dredging locations to be defined with a high degree of accuracy. Dredging is monitored with an electronic monitoring system which records the time and location of dredging each time it takes place to ensure that all extraction takes place in the permitted locations. In addition, monitoring surveys are undertaken either annually or biennially using multibeam echo sounders to measure the depths of extraction below pre-dredge seabed levels. The bathymetric data example provided with this paper shows dredged depressions on the seabed within licensed extraction sites.

Concluding remarks

The seabed off Sussex has some of the best preserved palaeovalleys off southern Britain. The data acquired by the marine aggregate industry, in combination with regional datasets from the British Geological Survey, has enabled locally detailed reconstructions of the palaeogeography of the inner continental shelf during sea level lowstands and during the early stages of marine transgression. The link between the landscape evolution of the Weald, the Sussex coast and the palaeovalleys and their infilling sediments represents an important element in the diverse geology of Sussex. It is suggested that datasets collected from the continental shelf by the aggregates industry add to the conservation value of geological diversity in Sussex and record an aspect of Sussex geology and geomorphology that would otherwise be inaccessible to the public.

References

Bellamy, A.G. 1995: Extension of the British Landmass: evidence from shelf sediment bodies in the English Channel. In Preece, R., Island Britain: a Quaternary perspective. Geological Society Special Publication 96, pp47-62.
James et al, 2011: The Marine Aggregate Levy Sustainability Fund synthesis study in the central and eastern English Channel: British Geological Survey Open Report OR/11/01, 158pp.
Emu Ltd, 2009: Thames Estuary Regional Environmental Characterisation, for the Marine Aggregate Levy Sustainability Fund, 129pp.
Limpenny et al, 2011: The East Coast Regional Environmental Characterisation. Cefas Open Report 08/04, 287pp.

Hypothetical reconstruction of rivers within mapped palaeovalleys on the inner continental shelf off Sussex during the cold stages of the Pleistocene

 
Marine Site of Nature Conservation Importance (MSNCI)

Marine Sites of Nature Conservation Importance are non‐statutory sites identified on account of the special interest of their marine habitats, the fauna and flora, or for unusual geological and geomorphological features. The identification of these sites is to highlight their importance for marine wildlife and to emphasise the risks of certain operations damaging their interest. There are 23 MSNCIs off the Sussex coast.

Click on the points on the map to link to a printable version of the full details of each site.