The Outer Thames Estuary Regional Environmental Characterisation

Abstract

The Outer Thames Estuary Regional Environmental Characterisation (REC) provides an environmental reference statement defining marine and seabed conditions within the study area. Prior to this study, regional environmental assessment of the Outer Thames Estuary was based upon dispersed data acquired over several decades. The Marine Aggregate Levy Sustainability Fund (MALSF) has provided the opportunity to acquire and interpret an integrated physical and biological dataset for the first time in this region. The dataset was acquired along geophysical survey transects spaced 10 – 20 km apart and grab samples were recovered at the intersection of survey lines. The Outer Thames Estuary REC provides a unique, robust scientific basis to define the regional marine environment, outlining the character of seabed conditions in the study area. This will permit informed, confident and consistent decision-making and consequently the Outer Thames Estuary REC will be of value to all stakeholders including government, marine industry, planners and environmentalists. The knowledge will contribute to the protection of the marine environment, promote the sustainable management of the seabed and focus future development investment. The Outer Thames Estuary REC develops the approach initially adopted in previous marine aggregate regional characterisation studies but also makes a comprehensive assessment of the heritage issues. The characterisation process begins by reviewing the physical conditions in the area, for example tides, currents, seabed geology and seabed sediment transport. The heritage assessment has been combined with the interpretation of the Quaternary geology to produce an integrated assessment, whilst the wreck and other modern archaeology are treated separately. Analysis of the benthic infauna and epifauna communities is combined with an evaluation of their associated physical conditions to produce a habitat and biotope assessment. The principle characterisation is supplemented with high-level summaries of marine mammals, birds and anthropogenic activities for context. The characterisation process also highlights regional environmental sensitivities, for example sites of potential conservation, fisheries or heritage significance, as well as informing marine spatial planning. Geology and Heritage The seabed consists of three distinct morphological zones. The Western Zone is dominated by a large coast-parallel sandbank system. The sandbanks are composed of well sorted fine-medium grained sand whilst sandy gravels lie on the seabed in the troughs between the banks. The Central Zone consists of a bedrock platform typically overlain by a discontinuous, thin, gravelly lag deposit, dispersed sandy bedforms and isolated sandbanks. Two parallel, north-south trending troughs, the Inner Gabbard Deeps, have been eroded into the bedrock in this zone. The seabed in the Eastern Zone consists of an extensive sand dune field. In general there is a net seabed sediment transport to the south across the area. The Outer Thames Estuary REC survey has revealed a geological and archaeological record of previously unquantified significance. The combined interpretation of bathymetry of varying resolutions, sub-bottom data, a limited amount of core and grab material and a full review of the extant geological and archaeological record, has achieved a significantly enhanced interpretation of the Outer Thames Estuary. Of central importance is the identification of c. 1,500 km2 of submerged palaeo-landscape dating from c. 720,000 BP. This represents a significant feature of both geological and archaeological interest. The seabed in the Outer Thames Estuary mainly comprises a thin, superficial layer of mobile and immobile sediments overlying exposures of Quaternary, Cenozoic and Cretaceous sediments. Ten enclosed deeps are identified and interpreted as being formed at the margin of the Elsterian-Anglian glacial maximum ice limit. The stratigraphic relationship of these features with the major eastwest river system (the Thames-Medway) that cuts across the Outer Thames Estuary REC area suggests this channel must have been incised prior to this glaciation (Oxygen Isotope Stage (OIS) 12: c. 450 kaBP). Further, the spatial correlation of this channel system with terrestrial river terrace gravels on the Essex coast suggests it may have been formed as early as Cromerian Complex II (OIS 18: c. 720 kaBP), but at least by OIS 15 (c. 600 kaBP). The southern edge of the Outer Thames Estuary REC area does cross the northern margin of the post-Elsterian/Anglian Thames-Medway river courses as they migrate southwards and separate to become the modern Thames and Medway systems. Again, through spatial correlation with the terrestrial record, c. 40 m thick sections of river channel infill sediments have been identified, which may hold a record of 450,000 years of sedimentation. The Outer Thames Estuary REC study reveals a potentially highly significant record from an archaeological perspective. There are two broad categories of submerged archaeological material that may potentially exist within the Outer Thames Estuary REC area: shipwreck sites and terrestrially deposited archaeological material subsequently inundated by rising sea levels. In terms of the wreck record 1,576 individual incidents are documented for the area, with an additional seven anomalies identified as a part of the characterisation process. The wrecks for which details were available date from between AD 1320 – 2005, and range in type from fishing vessels, through submarines, to second world war aircraft. Available survey data only covered a small proportion of the total area considered, and as such the location of only c. 6% of wrecks recorded in the UKHO could be verified. With regard to submerged terrestrially deposited archaeology, the Outer Thames Estuary REC area parallels one of the most important stretches of coastline for Palaeolithic archaeology in the British Isles. Bracketed by the key internationally significant Lower Palaeolithic sites of Clacton (to the south) and Pakefield (to the north), the coastline in this region has produced evidence for the earliest occupation of the British Isles at c. 600 – 700,000 BP. As such, the determination of a large area of submerged landscape dating from 720,000 BP offshore of these sites is deemed to be highly significant and worthy of future investigation to evaluate its potential. This determination is based on the value of this landscape as a resource to inform about the broader changes which occurred through the Pleistocene and into the Holocene, as much as the possibility for preservation of primary and secondary context archaeological material. In addition, there is a third class of archaeological material found within the survey area; the modern terrestrial feature of Walton-on-the Naze Pier which extends c. 17 m into the Outer Thames Estuary REC. This is a significant local feature and forms part of a conservation area.

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Identifiers

ORCID for Fraser Sturt: orcid.org/0000-0002-3010-990X

ORCID for Justin K. Dix: orcid.org/0000-0003-2905-5403

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