Potrmarnock, Ireland to Porth Dafarch, Anglesey Sub-Sea Cable
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HEADLAND ARCHAEOLOGY (UK) LTD. Ireland to Anglesey Sub-Sea Cable: Archaeological Assessment of Marine Geotechnical Data SCIS11 Potrmarnock, Ireland to Porth Dafarch, Anglesey Sub-Sea Cable Volume 4 Archaeological Assessment of Marine Geotechnical Data Prepared for Noordhoek Surveys B.V on behalf of Geo Networks Ltd. HEADLAND ARCHAEOLOGY (UK) LTD. Ireland to Anglesey Sub-Sea Cable: Archaeological Assessment of Marine Geotechnical Data CONTENTS Summary 1. INTRODUCTION 3 1.1. Aims and Objectives 3 1.2. Overview of the Area 3 2. METHODOLOGY 5 2.1. Assessment of the Offshore Geotechnical Data 5 2.2. Assessment of Onshore Geotechnical Data 5 3. RESULTS 5 3.1. Offshore Geotechnical Data Assessment 5 3.2. Onshore Geotechnical Data Assessment 7 4. DISCUSSION / PALAEOENVIRONMENTAL POTENTIAL OF THE MATERIAL 7 5. CONCLUSION 8 6. MITIGATION 8 7. REFERENCES 9 Figure 1. Location of CPTs and vibrocores - UK Figure 2. Location of CPTs and vibrocores - UK Figure 3. Location of CPTs and vibrocores - Ireland Figure 4. Location of CPTs and vibrocores - Ireland 2 HEADLAND ARCHAEOLOGY (UK) LTD. Ireland to Anglesey Sub-Sea Cable: Archaeological Assessment of Marine Geotechnical Data Archaeological Assessment of Marine Geotechnical Data for the proposed Dublin to Anglesey Subsea Cable Sarah-Jane Haston Headland Archaeology Ltd 1.0 INTRODUCTION This report is prepared for Noordhoek Surveys B.V on behalf of Geo Networks Ltd. and presents the results of an archaeological and palaeoenvironmental assessment of marine and onshore geotechnical survey data in connection with a proposed subsea interconnector cable between Ireland and the UK. This assessment covers the cable route within UK waters from Porth Dafarch Beach in Anglesey, North Wales to the Irish / UK Continental Shelf median line and in Irish waters from the Irish / UK Continental Shelf median line to Portmarnock, Co. Dublin. 1.1 Aims and Objectives The aim of this report is to provide an archaeological assessment of the palaeoenvironmental potential of sediments affected by the proposed route of the cable. This will be undertaken through the examination of the geotechnical data; namely piezocone penetration testing and vibrocore logs that have been taken. This assessment will provide specific site data that will aid in identifying potential impacts of the scheme on any sediments of palaeoenvironmental and archaeological interest. The specific objectives of the assessment are: • to review available data in respect of seabed and sub-seabed deposits likely to be of palaeoenvironmental and archaeological interest; • to identify any deposits of palaeoenvironmental potential within the off-shore 2 m depth of the sea-bed along the cable corridor; and • to present mitigation measures where appropriate to the findings of the assessment. 1.2 Overview of the Area In order to place the results of the geo-technical report into the wider context of the palaeoenvironment of the Irish Sea and Welsh and Irish landfalls, it is important to consider the previous work done in this area. A brief overview of such work is given below. 1.2.1 Sedimentology Within the study area the pre-Quaternary geology largely comprises of Carboniferous sedimentary rocks, principally limestone, siltstone and mudstone, largely derived from marine and tidal settings (Jackson et al, 1995). The solid geology beneath the seabed along the proposed cable route from Ireland through to Anglesey is both varied and complex as one would expect from such a large area. The Anglesey sea shelf is characterised by sub- cropping Precambrian and Lower Palaeozoic rock, the latter also present in Ireland, the Isle of Man and up the coast to Southern Scotland. Lower Jurassic and Cretaceous rocks occur in places along with some Tertiary sedimentary rocks. The Quaternary sediments are thin in the platform areas up to 300m thick in the Western trough. The substructure of the Irish Sea basin includes deep geological grabens, palaeovalleys and glaciofluvial scouring. Carboniferous-Devonian period rocks form the basement of the 3 HEADLAND ARCHAEOLOGY (UK) LTD. Ireland to Anglesey Sub-Sea Cable: Archaeological Assessment of Marine Geotechnical Data region with six main depositional basins with variable thickness sedimentary rocks within the Irish Sea basin; the Kish Basin being the main one lying within the cable route area. The shallow geology and sedimentology of the offshore cable route sediments are predominantly sedimentary many of which are glacial relict that are likely sourced from outside the region, consisting mostly of sands, muddy sands and unrelated to the underlying rock type. Further to this, there are a variety of sedimentary environments in the Irish Sea including coarse lag gravels, sand wave fields, sand banks and silt filled depressions. Their distribution is a function of several factors including glacial and post- glacial activity, sea level rise, tidal and current regimes (Global Marine Systems Ltd, 2009). The onshore areas of the proposed cable route are subject to ever-changing properties from tidal movement and current changes. Gravelly sediments are common and occupy a central belt extending from Scotland past the Isle of Man to Anglesey. They also dominate areas exposed to high currents. Sandy sediments are also common and found in narrow zones nearshore and offshore. Estuarine deposits including silts and organic peats have also been recorded within the intertidal areas of the cable route on both stretches of coastline, which are of Holocene date (e.g. Godwin, 1956; McQuade and O’Donnell, 2007). 1.2.2 Relative sea-level change Published relative sea level change curves for both the Irish and British sides of the Irish Sea have been consulted (Brooks et al,. 2008 and Shennan and Horton, 2002). The relevant sea level curves (Dublin and North Wales) closely match each other, once the different Chart Datum values used have been taken into account. The curves demonstrate that there has been little major change in RSL for the last 6000 years (i.e. from the early Neolithic) on either side of the Irish Sea. Given the rapid rise in RSL prior to this it is theoretically possible that there may be early Neolithic sites in the intertidal or immediately sub-tidal zones, particularly on the UK side. The rapid change in RSL means that Mesolithic remains might be found on the coastal shelf on both sides of the Irish Sea to a depth of approximately 10 m below Lowest Astronomical Tide (LAT). Prior to this rapid rise in relative sea-level, which began at approximately 8000 BP the UK and Ireland were connected and thus the proposed cable route would have been on an area of dryland (Carter et al, 1989). Therefore there is the potential for the presence of archaeological finds, such as flint scatters across the entire route of the cable. With respect to longer term RSL changes it should be noted that sea level change in the Irish Sea during the Pleistocene is relatively poorly understood. Although some models do extend back from the Holocene into the Late Pleistocene these parts of the models are generally backed by little or no direct evidence in the form of in situ palaeoenvironmental deposits that can be radiocarbon dated (Carter et al, 1989; Brooks et al, 2007). As such the probable depths at which Palaeolithic remains might have been originally deposited are difficult to model accurately, though it is possible that they exist (Brooks et al, 2007). 1.2.3 Palaeoenvironmental work and potential The proposed cable route will impact on the intertidal areas on both the Irish and UK shorelines. Previous work in the intertidal areas on both stretches of coastline have showed high potential for the presence of sediments of palaeoenvironmental interest such as intercalated sequences of organic sediments (e.g. peats and organic muds) and minerogenic sediments (e.g. estuarine silts). This is highlighted by the number of palaeoenvironmental studies have taken place around the Welsh coastline on such sequences in order to investigate changes in relative sea-level (e.g. Allen, 2001; Hill et al, 4 HEADLAND ARCHAEOLOGY (UK) LTD. Ireland to Anglesey Sub-Sea Cable: Archaeological Assessment of Marine Geotechnical Data 2007) and palaeoenvironments (e.g. Smith and Morgan, 1989; Walker et al, 1998), including around Anglesey itself (e.g. Godwin, 1956). There is great potential for similar studies to be undertaken on the Irish coastline, with suitable sediments noted around Dublin (e.g. McQuade and O’Donnell, 2007); however to date such studies have been largely absent leading authors (e.g. O’Sullivan, 1998; Van de Noort and O’Sullivan, 2006) to call for the development of such work across Irish coastal wetlands. In areas such as Anglesey, large tree remains have also been recorded in the intertidal area (Lyell, 1875), which are very important for palaeoenvironmental study as they provide an opportunity of direct investigation of former coastal woodlands (e.g. Timpany, 2007). Thus there is good potential for the occurrence of these sediments along the cable route, which could provide further palaeoenvironmental information to compliment and further previous work in these areas. Together with the potential for sediments of palaeoenvironmental interest there is also the potential for the presence of archaeological materials within these intertidal areas. Previous intertidal archaeological investigations (e.g. Bell, 2000, 2007; Bell et al, 2000) have shown these areas provide excellent (waterlogged) conditions for the preservation of archaeological organic remains, such as wooden objects and structures and are believed to have been important locations for people in prehistory (Bell, 2000; O’Sullivan, 2001). This has been seen recently in Co. Dublin from archaeological work in the Liffey Estuary where a wooden Mesolithic fish trap has been discovered (McQuade and O’Donnell, 2007). Similarly archaeological materials have been found in the Anglesey area, including flint scatters at Porth Rufydd (Kelly, 1982). Therefore together with sediments of palaeoenvironmental interest, there is also good potential for archaeological remains to be encountered within the intertidal areas.