{"id":90283,"date":"2022-10-10T16:03:11","date_gmt":"2022-10-10T16:03:11","guid":{"rendered":"https:\/\/www.bgs.ac.uk\/?p=90283"},"modified":"2024-03-05T15:05:26","modified_gmt":"2024-03-05T15:05:26","slug":"new-seabed-maps-offer-a-window-into-the-distant-past","status":"publish","type":"post","link":"https:\/\/www.bgs.ac.uk\/news\/new-seabed-maps-offer-a-window-into-the-distant-past\/","title":{"rendered":"New seabed maps offer a window into the distant past"},"content":{"rendered":"\n
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BGS has begun publishing a new series of high-resolution offshore geological maps showing the distribution of bedrock and sediments that make up the seabed around our coasts. Maps of the Bristol Channel<\/a> and Anglesey<\/a> were the first to be published, with further regions such as Yorkshire and East Anglia to follow as our mapping programme progresses.<\/p>\n\n\n\n

This article takes a closer look at the seabed featured in the the first map of the series, which stretches along the central part of the Bristol Channel, from south of Swansea Bay in the west to Newport in the east. <\/p>\n\n\n\n

The Bristol Channel<\/h2>\n\n\n\n

The Bristol Channel is a large estuary and river system that extends from the Celtic Sea eastwards to the limit of tidal influence along the River Severn at Gloucester. The channel separates South Wales from Devon and Somerset and has the second-largest tidal range in the world at Avonmouth, with a 12.3 m mean spring range (SouthWalesPorts.co.uk<\/a>).<\/p>\n\n\n

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Figure 1:\u00a0 Extent of the Bristol Channel. Contains OS data \u00a9 Crown Copyright and database right 2020.<\/p>\n

The derived bathymetric layer was produced from Maritime and Coastguard Agency (MCA) data \u00a9 Crown Copyright. Not to be used for navigational purposes.<\/em><\/p>\n<\/div>\n\t\t\t\t\t

\"Expand<\/a><\/div>\n\t\t\t\t<\/figcaption><\/figure>\n\n\n

Bedrock geology<\/h2>\n\n\n\n

The seabed of the Bristol Channel is partially covered by soft sediments (superficial deposits) but much of the area is free of sediment cover, allowing for the detailed mapping and interpretation of bedrock units and features. The bedrock geology recognisable in the bathymetric data will be familiar to anyone who knows the coastline of south-east Wales, which is characterised by low, Carboniferous limestone cliffs surrounded by younger, Triassic- and Jurassic-aged rocks. <\/p>\n\n\n\t\t\t\t

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Figure 2a. Inliers of fractured Carboniferous limestone and surrounded and overlain by Mesozoic rocks – shows interpreted geology displayed on the bathymetric data.<\/p>\n

The derived bathymetric layer was produced from Maritime and Coastguard Agency (MCA) data \u00a9 Crown copyright. Not to be used for navigational purposes<\/em><\/p>\n<\/div>\n\t\t\t\t\t

\"Expand<\/a><\/div>\n\t\t\t\t<\/figcaption><\/figure><\/div>
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Figure 2b. Inliers of fractured Carboniferous limestone and surrounded and overlain by Mesozoic rocks – without background information.<\/p>\n<\/div>\n\t\t\t\t\t

\"Expand<\/a><\/div>\n\t\t\t\t<\/figcaption><\/figure><\/div>\t\t\t\t<\/div>\n\t\t\t\t\n\n\n

The bedrock geology presented on the new maps shows a pattern of small outcrops of highly fractured Carboniferous rocks of the Pembroke Limestone Group<\/a>, which would have formed a landscape of hills and valleys following uplift and erosion caused by the Variscan Orogeny. <\/p>\n\n\n\n

A window into the distant past<\/strong><\/h2>\n\n\n\n

During the Triassic, this part of Wales was hot and arid with wadis, flash flooding and scree slopes (Mercia Mudstone Marginal Facies<\/a>) surrounding the hills and cliffs. The low ground in between featured lakes that deposited mudstones and periodically evaporated to leave salt flats (the ‘red beds’ of the Mercia Mudstone Group<\/a>). These deposits sit unconformably on the Lower Palaeozoic landscape. <\/p>\n\n\n\n

During the late Triassic and Lower Jurassic, a marine transgression, or rise in sea level, flooded the landscape. A series of deposits recording this event can be seen in the cliffs of the Vale of Glamorgan and traced off shore (Penarth Group<\/a>). By the Jurassic, this part of Wales had become marine and units of the Lias Group<\/a> were deposited, including the, which is divided into the St Marys Well Bay, Lavernock Shales and Porthkerry members.\u00a0The youngest bedrock unit identified is the Inferior Oolite Group<\/a>, which records deposition in a shallow, sea-shelf environment.<\/p>\n\n\n

\"Cycles<\/a>
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Figure 3. Cycles of limestone and calcareous mudstone of the Porthkerry member showing striking resemblance to the cliffs at Dunraven Bay. Geological interpretation is displayed on the bathymetric data.<\/p>\n

The derived bathymetric layer was produced from Maritime and Coastguard Agency (MCA) data \u00a9 Crown copyright. Not to be used for navigational purposes.<\/em><\/p>\n<\/div>\n\t\t\t\t\t

\"Expand<\/a><\/div>\n\t\t\t\t<\/figcaption><\/figure>\n\n\n

All of these rocks were subsequently deformed, most likely by the Alpine Orogeny. This was a period of mountain building\u00a0in central and southern\u00a0Europe\u00a0and\u00a0west Asia. The orogeny resulted in a network of folds, fractures and faults that is especially apparent in the deformation and offsetting of the rocks in the well-bedded Porthkerry Member and Inferior Oolite Group. <\/p>\n\n\n

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Figure 4. View from Dunraven Castle, south east across Dunraven Bay, north coast of Bristol Channel, Vale of Glamorgan. BGS \u00a9 UKRI<\/p>\n<\/div>\n\t\t\t\t\t

\"Expand<\/a><\/div>\n\t\t\t\t<\/figcaption><\/figure>\n\n\n

Geological structure<\/h2>\n\n\n\n

The north-east to south-west orientation of the inner part of the Bristol Channel (which is included in the eastern part of our new dataset) and much of the Severn Estuary is influenced by the Severn Estuary fault zone. This is a dextral, strike-slip fault zone considered to have been active during the early Carboniferous. Comparable north-east to south-west trending faults have been recognised within the eastern part of the area covered by this dataset, where they dissect earlier-formed folds and offset the bedding within the Porthkerry Member. <\/p>\n\n\n\n

However, within the central and outer parts of the Bristol Channel, the faults are dominated by: <\/p>\n\n\n\n