COMPARISON OF GLACIOMARINE SEISMIC STRATIGRAPHY OF THE NORTHWESTERN IRISH COAST WITH COASTAL MAINE, USA

Seismic reflection profiling in Lough Swilly, Ireland conducted in June, 2009 compares deglacial and postglacial stratigraphy and relative sea-level change with well-established records of the Maine inner shelf. Twenty-eight lines totaling 100 km trace cross-sections and axial profiles of Lough Swilly, a glacially sculpted embayment on the northern Donegal coast. Lough Swilly is an open estuarine embayment with minor tributary streams, such as River Swilly. It is ca. 150 square km in area, with shallow intertidal flats to the south and more open bay to the north, with a maximum depth of 26 m. Primary data were collected using an AAE boomer with peak power near 1.5 kHz, 20-element hydrophone, 0.125 s sweep, recorded digitally using a Geoacoustics GeoPro4 topside system, and GPS navigation georeferenced to the data. Boomer, hydrophone, and techniques were identical to those used to establish the Maine record, allowing direct comparisons. Bedrock is imaged in most cross-lines, reaching greater than 70 in mid bay, and below the 94 m total sweep depth in the outer bay. Till is of variable thickness and discontinuous. A prominent seismic facies with a well-stratified concentrically draping character is identical to glaciomarine (GM) units in Maine. This GM reaches over 60 m thickness near the estuary axis, and is capped everywhere by a distinct erosional unconformity. We informally refer to this unit as the Lough Swilly Formation. Unlike the more uniform GM Presumpscot Fm. formed at the grounding line of the Laurentide continental ice sheet in Maine, the Lough Swilly Fm. shows a distinct gradation from coarse, very well stratified reflectors to the SE (proximal) to a poorly stratified, uniform finer-grained unit at the mouth of the Lough (distal), consistent with a valley-confined deglaciation. A main paleochannel and several subsidiaries trace the lowstand valley of Swilly River, to a depth of at least 30 m at the outer headlands. This depth is consistent with our previously established local relative lowstand off Northern Ireland. The incised valley is filled with paleofluvial and estuarine deposits, including beachface sands off several modern strands. A distinctive tidal scour pit in the central estuary has excavated through earlier estuarine deposits and exposes GM at the bayfloor.