STRUCTURAL MAPPING AND GEOMORPHOLOGY OF IRELAND’S SOUTHWEST CONTINENTAL SHELF USING HIGH RESOLUTION SONAR

Bathymetric surveys were conducted on the continental shelf off the southwest coast of County Cork, Ireland by the Marine Institute of Ireland, the Geological Survey of Ireland, and the INFOMAR project. Data were collected from July 2006 through September 2014 using a Kongsberg EM2040 multibeam echosounder aboard the R/Vs Celtic Voyager and Keary, and a Kongsberg EM1002 on the R/V Celtic Explorer. Sonar data were post-processed with CARIS HIPS and SIPS 9.0 to create 2D and 3D bathymetric and backscatter intensity surfaces. The offshore study site is part of the 286 Ma western Variscan orogenic front and has several massive outcrops, exhibiting 5 to 20 m of near-vertical relief. These outcrops were structurally mapped and relatively aged, and exhibit significant folding, rotation, tilting, and fracturing. Google Earth, ArcGIS, and previous terrestrial studies were used to further analyze how geomorphology is controlled by seafloor composition and structural features. Rock type and age were interpreted by comparing fracture analysis and backscatter classification of the joints and fold trends to similar onshore outcrops documented previously, to determine an age of 416-299 Ma for the shelf’s outcropping strata and associated structural features. The oldest features observed are first order anticlines and synclines containing Upper Devonian and Lower Carboniferous strata. Within the well preserved Devonian layers are NE-SW plunging second and third order folds. Jointing is observed in Devonian strata and is superimposed on folding, with some joints appearing to be pre-Variscan. Rotation of the first order folds is the youngest structural feature, as both the second and third order folds and rock fractures are warped. Our study shows that high resolution sonar is an effective tool for offshore structural mapping, and is an important resource for understanding the geomorphology and geologic history of submerged outcrops on continental shelf systems.