Mapping the Seafloor
Geology Matters 16

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Mapping the Seafloor
Dr Benjamin Thébaudeau

Recently, a team consisting of Dr
Benjamin Thébaudeau (contract Post Doc), Natalie Duncan (Geoscience Intern) and Clara Murcia-Castillo (Leonardo DaVinci Scholar), started working with Xavier Monteys on a project for INFOMAR, aiming to be the first detailed investigation of the Hatton-Rockall Basin area, the most westerly area surveyed as part of the Irish National Seabed Survey (INSS). The multibeam survey carried out in 2000 and 2001 produced bathymetric and backscatter datasets for an area of about 95,000 km2 (the whole of Ireland is 84,421 km2) and allowed the creation of a Digital Terrain Model (DTM) of about 60m in resolution. This DTM and backscatter data, combined with seismic datasets (representing shallow subbottom stratigraphy), magnetic susceptibility images and shallow sediment samples, are being used to identify geomorphological features and to classify the nature of the seabed substrate. Detailed maps of these findings are due for completion in the coming months. The initial part of the study has identified key geomorphological features, which we describe here.

Marine Mapping Team
INSS Marine mapping team at GSI, Natalie Duncan, Ben Thebaudeau and Clara Murcia-Castillo

The study area is located at the northwest reaches of the designated Irish Exclusive Economic Zone (EEZ) from 16 - 25o W and 54 - 58o N, with a depth range of between 459 - 3507m. It comprises the south-west of the Rockall Bank, the south of the Hatton-Rockall Basin, the south of the Hatton Bank, the Edoras Bank, the Fangorn Bank surrounded by Helm’s Deep, and parts of the Maury Channel. The Irish Petroleum Affairs Division (PAD) offshore geological database recognises the Edoras Bank as an igneous extrusive complex, and several mounds that appear in the bathymetry as igneous intrusive bodies. Similarly, the Hatton Bank corresponds closely to an expression of continental lava flows. The Banks and western half of the zone have been classified as lying over basement bedrock whereas the Hatton-Rockall Basin is composed of undistinguished Mesozoic formations (Makris et al., 1991; Hitchen, 2004). Example of geomorphological features identified:

1. Volcanic mounds: These mounds have a very distinct peaked morphology with dimensions on average 2km wide and 300m height above the seabed. These make them comparable in size to the Great Sugar Loaf Mountain in Co. Wicklow. They have been designated as intrusive bodies (due in part to magnetic surveys) and more than 600 have been identified in the study area, mainly concentrated on the two seamounts and beside the Maury Channel. 

2. Carbonate mounds: These smaller features are encountered on the slopes of the Rockall Bank. Some 290 individual mounds are found as a field on the south-west slope of the Rockall Bank at a depth of about 600 to 700m. They are roughly circular with a diameter of about 400m and height above seabed about 20m, making their dimensions roughly comparable to that of the Custom House in Dublin.

3. Seamounts: Two seamounts are present in the designated area; the Fangorn Bank and the Edoras Bank. They are both roughly round plateaus about 60km in diameter and with a height above the seabed of about 500m at their centre point. They are comparable in area with county Antrim and have a comparable diameter to the long axis of the Wicklow mountains. They are surrounded by sharp escarpments and long canyons on their northern and south eastern side.

4. Bedrock ridge: A high rugosity ridge has been recorded lying from the Hatton Bank and running south. The nature of this ridge is unknown but its morphology suggests a bedrock ridge resulting from tectonic forces perhaps linked with the opening of the Atlantic Ocean or the volcanic intrusion in the neighbouring seamounts. The ridge has its peak lying about 200m above the surrounding seabed, has a width of about 10km and runs along a distance of about 80km. This makes this ridge quite comparable to the Wicklow mountain range.

5. Maury Channel: The Maury channel is a recognised sea channel running in a north east to south west orientation at depth of about 3000m. It is at least 300 km long with 200km inside the Irish EEZ and has a rough U-shaped profile with a flat valley floor 4km in width and lying 100m deeper than the surrounding seabed. The channel has a meandering aspect comparable to some land flowing rivers.

6. Polygonal faulting: The seafloor of the Hatton-Rockall Basin is covered by a network of cracks identified as polygonal faults. These features have been recognised recently in literature as resulting from compaction of sediment and fluid expulsion (Berndt et al., 2012). They are analogous in shape to ones found on land on desiccated surfaces and, in other seabed areas, have been used as analogues for modelling polygonal features on Mars. They occur in our survey area at about 1400m depth with polygons about 2 to 3km in diameter. The cracks are about 500m wide and 5 to 7m deep.

7. Iceberg scours: More than 60 iceberg scours have been identified in the shallowest part of the study area on top of the Rockall Bank, limited to a water depth of less than 600m. They are eroded streaks on the seabed, created by passing icebergs. The longest found are about 15km long with an indentation of a few meters; no particular orientation is recognised.

8. Contourite drifts: Two well-known contourite drifts are recognised in the study area; the Hatton Drift to the north and west of the Edoras Bank continuing to the Endymion spur, and the Gardar Drift to the extreme western part of the Irish EEZ. These appear as long sediment ridges of up to 20km in length, 500m in width and height of up to 20m. The Hatton Drift presents ridges aligned to the bathymetric contours as representing a current flow from the south-west and contouring the Edoras Bank to the north. The Gardar Drift displays ridges with much less relief that are aligned roughly in an east-west direction.

Seafloor

This article details for the first time a number of geomorphological features of the Hatton-Rockall Basin area. These initial results are based on the interpretation of sub bottom, bathymetric and backscatter data acquired as part of the INSS. This seabed mapping project aims to further define these and other geomorphological features as well as to classify the seabed substrate in the Irish offshore with the creation of maps and the publication of corresponding articles in the Journal of Maps.


Berndt, C., Jacobs, C. L., Evans, A., Gay, A., Elliott, G. M., Long, D., & Hitchen, K. (2012). Kilometre-scale polygonal seabed depressions in the Hatton Basin, NE Atlantic Ocean: Constraints on the origin of polygonal faulting. Marine Geology, 332-334, 126–133. Hitchen, K. (2004). The geology of the UK Hatton-Rockall margin. Marine and Petroleum Geology, 21(8), 993– 1012. Makris, J., Ginzburg, A., Shannon, P. M., Jacob, A. W. B., Bean, C. J., & Vogt, U. (1991). A new look at the Rockall region, offshore Ireland. Marine and Petroleum Geology, 8(4), 410–41