Geological Appraisal of the Kish, Burford, Bray and Fraser Banks, Outer Dublin Bay Area

Abstract

Geological mapping of the seabed and sub-seabed strata in an area comprising offshore banks and intervening sediments in outer Dublin Bay is presented. Bathymetric comparisons suggest that the offshore banks are quasi-stable over time probably maintaining their position due to the interaction between wave and current regimes. Seven acoustic seabed facies are defined on the basis of side-scan sonar characteristics reflecting differences in bedforms and bottom types. Sediment waves indicative of a mobile substrate are common both on and between banks. Maximum sandwave development occurs on bank flanks and outer limits. The effects of wave action on seabed morphology are clearly discernible in the structure and appearance of the bank crests. Grain-size data and bedform interpretations suggest a northerly sediment transport system with gravel dominant in the south of the area (Bray Bank) grading to sands in the north (Kish and Burford Banks). Sub-bottom profiling reveals a consistent upper unit overlying a hard reflector allowing unit thickness (isopachs) to be defined. No internal structures or “hard” cores were revealed within banks. Two shipwrecks were also imaged. This report forms part of a larger project "Reconnaissance Assessment of Coastal Seabed Sand and Gravel Resources" whose objective is to provide a comprehensive national survey, for the island of Ireland, of near-shore sand and gravel resources, to a water-depth of 50m, pertinent to all end-users e.g. aggregates industry, fisheries, local authorities, etc. The aims of this larger project are to: 1. collate all known information (digital, documentary, archival and other sources) regarding the location and extent of the resources; 2. deliver this data as a national resource inventory using a GIS database in line with national standards; 3. ground-truth and extend the existing data coverage through additional surveys, where perceived data gaps or data ambiguities exist pertinent to national needs. The resulting database (items 1 and 2) is comprised of three elements: •A MS Access database hosting comprehensive records of 63 datasets. Accessible from the metadata record for each dataset are: •An archive of digital thematic data in ArcView shapefile format with associated legends, tables, and imagery; •A bibliographic database containing 406 bibliographic entries. This report addresses Aim 3 of the larger project (above): “ground-truth and extend the existing data coverage through additional surveys, where perceived data gaps or data ambiguities exist pertinent to national needs”.