Abstract

Leg 80 of the DSDP-IPOD program drilled a transect of four core holes (548–551) across the continent-ocean boundary at Goban Spur, a prominent southwest-trending structural high on the Irish continental slope. Multichannel seismic-reflection profiles show that, during rifting, continental basement rocks of Goban Spur were broken up by northwest-trending listric normal faults to form a series of half-graben basins. Two of these half-grabens were sampled during Leg 80 (Sites 548 and 549). Site 550 was located on the adjacent oceanic crust of Porcupine Abyssal Plain. Site 551 was located on transitional crust at the foot of Goban Spur. The objectives were lo analyze the structural, the depositional, and the paleoenvironmental development of this sediment-starved passive continental margin. At Sites 548 and 549, basement comprises continental Hercynian metasediments of Devonian age; at Sites 550 and 551, the basement is tholeiitic basalt. The oldest syn-rift sediments (Barremian age, or perhaps late Hauterivian) were penetrated at Site 549, lying uuconformably below Aptian? strata. Seismic sequence analysis reveals that Aptian? strata also overlie this unconformity farther northeastward in the basin. An unconformity above the Aptian? section marks the end of rifting and the beginning of sea-floor spreading. An Albian age for the initiation of sea-floor spreading was corroborated at Site 550 where abyssal late Albian chalks rest upon and are interbedded with oceanic basalts, indicating an initial water depth of ∼2,000 m.

As sea-floor spreading progressed, Goban Spur subsided rapidly, so that by Cenomanian time, bathyal to abyssal chalks were accumulating at deeper sites. After two periods of partial stagnation in the Aptian-Albian and in the Turonian, chalk deposition in well-oxygenated environments took place at all sites, modified chiefly by shifts in deep-circulation patterns and the calcite compensation depth (CCD), by periodic influx of terrigenous detritus during low stands of sea level (especially in the Cenozoic), and by frequent displacement of older carbonates from the slope to abyssal sites.

A number of major unconformities correspond to those most often reported from other widespread locations in the North Atlantic Basin and on surrounding continental shelves and coastal plains. Several unconformities are preserved undisturbed in our cores and can be correlated with sea-level fluctuations, with paleoceanographic events, and with tectonic movements.

A thick Quaternary section at Site 548 records prominent fluctuations of glacial-inter-glacial paleoclimates. An even thicker Paleogene section at Site 549 provides unusually well-preserved and uninterrupted sequences suitable for detailed sedimentological and stratigraphic studies.

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