Submerged Palaeo-Arun & Solent Rivers: Reconstruction of Prehistoric Landscapes Pt2

English Heritage ALSF summaries. 2002/2003

EH Project Number:	3277MAIN
Funded Unit:	Imperial College

Key Contacts: Dr. Sanjeev Gupta & Dr. Jenny Collier

Department of Earth Science & Engineering

Imperial College

Exhibition Road

London SW7 2AZ, UK

Tel: 020 7594 6527/020 7594 6443

Fax: 020 7594 7444

Email: s.gupta@ic.ac.uk

jenny.collier@ic.ac.uk

INTRODUCTION

This project is using high-resolution geophysical imaging sonars to investigate the geomorphology of submerged and buried landscapes on the northern English Channel shelf. Here submerged fluvial valley systems form important, but poorly understood, palaeo-landscapes on the shelf. Pleistocene sedimentary bodies associated with these valley systems hold considerable potential as aggregate resources, and also contain unique information on the palaeoenvironmental evolution of the region. Moreover, because onshore extensions of these valley systems contain some of the finest records of early human occupation in Britain, palaeo-land surfaces and sedimentary units in their offshore counterparts are likely to contain important archaeological data to help constrain the history of early human migration into Britain. Thus, there is a critical need to assess the archaeological significance of these offshore fluvial systems, prior to extensive aggregate extraction. The new data and technical developments provided by this study will contribute to the emerging field of prehistoric marine archaeology, in particular with regard to developing protocols to assess archaeological resource potential in offshore areas, and to the development of conservation strategies.

The aim of this project is to determine the morphology and internal stratigraphy of the relict fluvial valley systems of the offshore extension of the Arun river, off the south coast of Sussex. This is being achieved partly by the application of a new suite of high-resolution marine geophysical imaging sonars recently funded by JREI. We are working closely with Hanson Aggregates Marine Ltd. (principal contact: Dr. Ian Selby) and United Marine Dredging Ltd (principal contact: Dr. Andrew Bellamy), the two main aggregate industry companies operating in the region, who have provided extensive seismic and vibracore datasets to the project.

The insights gained from an integrated geophysical/ geomorphological/ sedimentological investigation of these fluvial systems will enable the development of concepts that will allow complex submerged landsurfaces and fluvial facies variations in the subsurface that have extensive potential to contain archaeological resources to be understood and predicted.

BACKGROUND

River terrace systems in northwest Europe have yielded striking evidence for the early human occupation, largely from the record of Palaeolithic artefacts recovered from their fluvial sedimentary deposits. Palaeoenvironmental interpretation of these deposits, in turn, has enabled the reconstruction of the landscapes inhabited by these early humans. Whilst much has been achieved in understanding the archaeological context of onshore fluvial systems, little is known about the offshore extension of these river systems - both in terms of late Pleistocene landscapes and evidence of human occupation. The submerged systems, developed as the rivers extended out across the exposed continental shelf during glacial sea-level lowstands, could potentially preserve a more pristine Palaeolithic/Mesolithic landscape than the emergent river valleys, which have a long history of modern anthropogenic disturbance. This project is using high-resolution marine geophysical technologies, sedimentology and seismic sequence stratigraphy to evaluate the landscape context and archaeological potential of submerged and buried fluvial systems in the English Channel.

The Solent and West Sussex rivers contain some of the world's most important early Palaeolithic archaeology. The dense concentration of palaeolithic artefacts within sediments of the Solent and its tributaries indicate that south coast river systems were corridors for human migration and favoured habitats for early (including pre-Anglian) human populations. The earliest hominid fossils from the British Isles were recovered from a Pleistocene raised beach at Boxgrove, West Sussex. The platform on which these deposits rest extends into the English Channel and would have been exposed during phases of low sea-level to form a land connection (or 'land bridge') with the European mainland. During cold stages, it is likely that this newly exposed land would have been colonised periodically by rich grasslands supporting a diverse mammalian megafauna including humans. The English Channel land bridge would have been a favoured route to the north and the river banks, preferred occupation sites. As well as preserving particularly rich archaeological information, shelf valley systems, such as the Solent, also record the interaction of sea-level fluctuations and climatically-controlled variation in sediment input from river catchments. Such factors provide the environmental and palaeogeographical context needed for models of early human dispersal, demography, adaptation and ecology.

AIMS AND OBJECTIVES

The aim of the project is to reconstruct the geomorphology of submerged and buried landscapes of the Arun and Solent valleys using regional high-resolution marine surveying, seismic sequence stratigraphy and sedimentological analysis. From this reconstruction we will develop process-driven models for valley evolution and sediment deposit distribution. The proposed work will enable us to identify seabed geomorphic elements and sedimentary architectures most likely to contain archaeological features. This will represent the first integrated, high-resolution analysis of this portion of the English Channel palaeovalley system and, as such, will provide a significant step toward understanding the evolution of fluvial systems of southern England.

Important questions that we aim to address include:

What are the palaeogeomorphic elements on the present seafloor and sub-surface?
What is the sequence of erosional and depositional events in the valley systems?
Which palaeogeomorphic elements of the submerged landscape hold potential for archaeological resources?
Which sedimentary architectural elements in the valley-fill stratigraphy hold potential for archaeological resources?
What techniques contribute most to the identification of potential prehistoric remains on the seabed?
SCIENTIFIC METHODS

During the 18-month programme we will determine the morphology and internal stratigraphy of the palaeo-Arun submerged valley system by combining high-resolution swath bathymetry data with sub-bottom profiling and core data. For clarity and to reflect the breadth of technical skills required to complete the proposed work, we have split the programme into two distinct sub-projects:

Project 1 is focussed on acquiring and interpreting new marine geophysical data utilising the new JREI-funded equipment pool. Specifically, we are collecting, processing and interpreting geophysical swath bathymetry data from the lower (unfilled) portion of the Arun system and sub-bottom profiling data from the filled upper portions.

Project 2 is focussed on analysis and interpretation of the existing dataset of analogue boomer seismic and vibracores collected for commercial exploring for aggregate resources the study area. These data are being provided by our aggregate industry collaborators, Hanson Aggregates Marine Limited and United Marine Dredging Ltd..

From these detailed observations of seafloor morphology, seismic sequence stratigraphy and sedimentary facies we will develop an environmental history for the eastern English Channel.

PROJECT 1 GEOPHYSICAL SURVEY RESEARCH PROGRAMME

Multibeam Swath Bathymetry Data Acquisition

Multibeam swath surveying was conducted in the downstream segments of the Arun river system during March 2003. The main aim was to survey the 3-dimensional surface of the present seafloor to determine the detailed morphology of the underfilled valley systems and their laterally adjacent interfluves.

Acquisition set-up and mobilisation

The 12 metre survey catamaran Xplorer was chartered for a period of 50 days to include seismic data acquisition and mobilisation, demobilisation and any downtime that may be incurred due to bad weather or equipment failure. The mobilisation of the survey vessel took place in Southampton and Portsmouth. The reliable conditions of Portsmouth Harbour were chosen for calibration and testing of the equipment following the complicated interfacing of the navigation, timing, motion, sound velocity and sonar sensors. The vessel then transited to Brighton Marina, where a tide gauge was installed to monitor water levels continuously. This enabled the sounding data to be reduced to a common datum. A Reson 8101 240 kHz swath sonar system was deployed in a pole mounted configuration. 10 hours work was achieved per day including the transit of 15 miles to the survey site. Due to favourable weather conditions, 13 days of multibeam data acquisition were achieved and an area 30% greater than planned was covered.

Post-cruise data processing and interpretation

Digital Terrain Models were produced using only raw data and predicted tides to give an impression of the geomorphology. Archiving of all survey data has now been completed and the reporting and initial analysis is underway. The raw swath data will be processed in detail using CARIS/HIPS software system. Processing will take 3-4 months to complete. After which it will be possible to construct a three dimensional reconstruction of the submerged landscape.

Sub-bottom Profile Data Acquisition

In order to undertake a comprehensive analysis of the stratigraphy of the Arun river system in the shallow subsurface, we collected extensive sub-bottom seismic profile data to complement the large dataset already available to this project from our commercial collaborators. Digital data will allow us to generate enhanced seismic images through digital data processing. This may be critical for the detailed imaging of buried landscapes and sedimentary elements, which is essential to determine the archaeological preservation potential.

We used two complementary sub-bottom profiling systems - a boomer and chirp. Between March 22nd and 29th, the chirp system was deployed during the latter part of the multibeam survey. Approximately 145km of data were recorded over the entire area, with numerous cross-profiles to examine the development of the valley system. After initial testing of the boomer system in the Solent with SOC, actual seismic surveying started on April 4th and was completed April 24th, with approximately 50% downtime due to bad weather conditions. Approximately 650 km of data was acquired, with some "overlays" to investigate the effects of weather conditions on data quality. Overall a considerable amount of data was acquired which should give a good overview of the seismic stratigraphy of Arun palaeo-valley system.The data will now undergo a lengthy period of processing using ProMAX toimprove both the vertical and lateral resolution and give the correct geometric relationships between reflections. The resulting high-quality images will be combined with the commercial seismic to undertake a combined interpretation - for details below.

PROJECT 2 INTERPRETATION OF EXISTING DATA RESEARCH PROGRAMME

Seismic interpretation of the existing dataset of analogue boomer seismic.

The first phase of Project 2 has focused on the interpretation of the boomer seismic dataset collected by the aggregate industry, which includes surveys obtained in 1990, 1991, 1995, 1998, 1999, and 2000. An assessment of the quality of the boomer data has determined the best surveys to be the 1995 and 2000 covering the most northern part of the licence block. These are better because of their degree of penetration and low level of acquisition artefacts and noise, and consequently have been the focus of initial interpretations.

The 2000 dataset was used to set up a preliminary interpretation of the geometry and internal architecture of the palaeo-Arun valley system. This analysis resulted in the mapping of the valley system and interfluve distribution throughout the area. The area is characterised by shallow and broad channel cuts in the west and east of the study area adjacent to the central palaeo-channel. The main palaeo-channel is complex in geometry and shows multiple infill sequences. The geometry of the main channel shows a gentle dipping eastern valley margin and a steep western margin consisting of up to 3 terraces. Our preliminary interpretation suggests that the channel geometry is clearly influenced by the underlying bedrock lithology, which varies in this area from Tertiary alternating sand and clay units. A catalogue of seismic facies was made and has been interpreted in terms of sedimentary environments. Seismic facies analysis is the interpretation of depositional environments and associated sedimentary processes from seismic reflection data, and involves the description and characterisation of seismic reflection parameters such as configuration, continuity and amplitude.

Data Integration

At the present the analogue data interpretation is being integrated with exciting geological data and vibrocore data, and converted into a digital format. This basemap will be used for the further interpretation of the seismic facies distribution and integration of newly acquired data.

Preliminary implications for marine archaeology

Our preliminary results indicate that there is likely to be some potential for discovering prehistoric archaeological resources on the English Channel shelf in the study area. We have identified a complex palaeo-topography on submerged shelf. Key geomorphic elements that have been identified include relict fluvial channel system, gorges, terraces elements, embayments, and estuarine systems. Embayment systems behind potential resistant bedrock barriers may have significant potential as they provide low energy environmental niches favourable to preservation. Analysis of sub-bottom profile and core data indicates the presence of well developed (a) fluvial macroforms eg. point bar deposits, and (b) fine-grained estuarine/floodplain deposits with potential to hold archaeological material.

WIDER BENEFITS

The reconstruction of the submerged and buried landscapes of the Arun will aid the management of submerged archaeological resources in both these and similar river systems e.g., Outer Thames estuary. In particular, the research project is anticipated to offer the following opportunities:

Provide a landscape context for Palaeolithic submerged landsurfaces which has potential application to understanding the pattern of human colonisation of Britain.
Improve predictive capability of archaeological resources within offshore sediments.
Develop strategies for archaeological resource management.
Extend professional archaeological capacity in the archaeological analysis of marine geophysics.
Project results will enhance public awareness of (a) the archaeological potential of the British continental shelf, and (b) the history of early human occupation of Britain.

PROJECT TEAM

Principal Investigators

Dr. S. Gupta Lecturer in Sedimentology, Department of Earth Science & Engineering, Imperial College.

Dr. J. Collier Senior Lecturer in Geophysics, Department of Earth Science & Engineering, Imperial College.

PDRAS

Andy Palmer-Felgate - marine geophysicist with expertise in swath sonar acquisition and processing.

Dr. Jonathan Pye - marine geophysicist with expertise in sub-bottom seismic acquisition and processing.

Dr. Julie Dickinson - sedimentologist and seismic stratigrapher

Dr. Ben de Mol - sedimentologist and seismic stratigrapher

External Collaborators

Dr. Ian Selby is Senior Resources Manager with Hanson Aggregates Marine Ltd and will advise on interpretation of industry boomer seismic data.

Dr. Andrew Bellamy is Resources Manager for United Marine Dredging Ltd. And will advise on interpretation of industry boomer seismic data.

Dr. Phil Gibbard is a Quaternary geologist at the Godwin Institute of Quaternary Research Department of Geography, University of Cambridge. He will advise on interpretation of the Quaternary stratigraphy.

Dr. Simon Parfitt is a Research Fellow, at the Department of Palaeontology, Natural History Museum, London. He will advise on the archaeological implications of our results with respect to human colonization of the UK.

Mr. Anthony Firth is an Archaeology Projects Manager at Wessex Archaeology. He will advise on the archaeological resource management implications of our research.

Dr. Bernard Coakley is an Associate Professor at the Geophysical Institute, University of Alaska, Fairbanks, USA. He will participate during the initial stages of swath surveying and assist us in processing of the data.

 

MEANS OF DISSEMINATION

Project results will be disseminated to English Heritage via written reports and oral presentations. A final report and presentation summarising the results of the research project will be prepared on completion.
Project results, in particular, academic results will be disseminated to a wider audience through conference presentations at archaeological and earth science conferences.
Research results will be published in international refereed archaeological and earth science journals.

This page was published 21/08/2003