Dayton Dove

The seabed geomorphology and geological structure of the Firth of Lorn, western Scotland, UK, as revealed by multibeam echo-sounder survey

This paper presents recently collected swath bathymetry from the Firth of Lorn. 553 km2 of data were collected during 2012–2013 as part of the INIS Hydro (Ireland, Northern Ireland and Scotland Hydrographic Survey) programme. The area proves to consist of bedrock-dominated seabed, divided into narrow, stratigraphicallyconstrained and glacially-over-deepened basins. The bedrock is composed of late Proterozoic Dalradian metasediments overlain unconformably by Old Red Sandstone (ORS) sediments and lavas of ?Silurian-age. The central region of the Firth of Lorn is dominated by a vertical cliff, up to 150 m high and extending for approximately 24 km. This feature, here termed the Insh Fault, may have originated as a Dalradian extensional fault, been reactivated as an ORS feature and now forms a fault-line scarp with resistant ORS rocks on the downthrown side, flanking the more deeply eroded metasediments exposed in the basin. Tertiary intrusives are common, in particular, swarms of Paleocene dolerite dykes exposed on the sediment-free bedrock surfaces, and can be traced for many kilometres. Evidence for past glaciation is widespread, manifest in the extensive erosion of the bedrock platforms and the abundance of well-preserved moraines and over-deepened basins. The survey region includes the Corryvreckan Whirlpool and Great Race, beneath the tidal flows of which occur submarine dunes.

Benthos supported by the tunnel-valleys of the Southern North Sea

The tunnel-valleys of the southern North Sea are arcuate and linear seabed depressions. The origin of these features has been the subject of much discussion, but they are generally considered to have been formed by subglacial erosion and sediment backfill beneath the outer margins of a receding ice sheet. We present here a study of two tunnel-valleys, the Silver Pit and the Sole Pit. Extensive areas of Sabellaria spinulosa reefs have been identified on the western flanks of the Silver Pit that extend down to the valley floor, representing a resource of significant conservation interest. The eastern flanks were found to support a diverse faunal assemblage with widespread hydroid and bryozoans turfs and abundant ascidians. The seabed of the Sole Pit is characterized by much sandier deposits than that of the Silver Pit, and it supports an abundant bivalve community, dominated by Abra alba. This bivalve assemblage also contains Coarcuta obliquata, which has only been recorded once before in the UK.

National Programmes: Geomorphological Mapping at Multiple Scales for Multiple Purposes

A better understanding of marine geomorphology is a common goal for seabed mapping programmes, with various mapping approaches, methodologies and challenges associated with systematically describing geomorphological features. To address these issues, and highlight the overall value of geomorphological mapping, a group of representatives from the seabed mapping programmes of the geological surveys of Norway, Ireland and the United Kingdom have formed a partnership to share their knowledge, expertise and technologies. Here we describe the first year of collaboration by outlining the background to and motivation for the groups’ national seabed mapping programmes, and presenting several case studies as well as tests to potentially adopt a harmonised classification scheme.

Deglacial landform assemblage records fast ice-flow and retreat, Inner Hebrides, Scotland

High-resolution bathymetric data have been central to recent advances in the understanding of past dynamics of the former British–Irish Ice Sheet (BIIS). As approximately two-thirds of the former BIIS was probably marine-based during the Last Glacial Maximum (LGM) ( c. 29–23 ka), geomorphic observations of the seabed are required increasingly to understand the extent, pattern and timing of past glaciation. Until recently, glacial reconstructions for the Inner Hebrides, offshore of western Scotland, have been based primarily on terrestrial observations. Previous workers have proposed generalized reconstructions in which the Inner Hebrides are located within a significant former ice-sheet flow pathway that drained the western Scottish sector of the BIIS, feeding the Barra Fan during the LGM and earlier glaciations (Fig. 1). Results from numerical ice-sheet modelling suggest that former ice-flow velocities within the region were on the order of hundreds to thousands of metres per year, but yield further insight by demonstrating how dynamic binge/purge cycles may have affected ice-sheet mass balance over time (Hubbard et al. 2009). Following the LGM, ice-sheet retreat through the area is estimated to have been in the order of 20 m per year (Clark et al. 2012). Here we present swath-bathymetric data from the Inner Hebrides that provide in situ constraints on ice-sheet flow and subsequent retreat dynamics from within this important sector of the BIIS. Fig. 1. Swath-bathymetric images. ( a ) Inner Hebrides, Scotland (acquired with both multibeam (e.g. Kongsberg EM3002D) and interferometric systems for the Maritime and Coastguard Agencys UK Civil Hydrography Programme

Seabed Characterization - Developing Fit for Purpose Methodologies

We briefly describe three methods of seabed characterization which are ‘fit for purpose’, in that each approach is well suited to distinct objectives e.g. characterizing glacial geomorphology and shallow glacial geology vs. rapid prediction of seabed sediment distribution via geostatistics. The methods vary from manual ‘expert’ interpretation to increasingly automated and mathematically based models, each with their own attributes and limitations. We would note however that increasing automation and mathematical sophistication does not necessarily equate to improve map outputs, or reduce the time required to produce them. Judgements must be made to select methodologies which are most appropriate to the variables mapped, and according to the extent and presentation scale of final maps.