Synopsis Marine studies in the Firth of Lorne have revealed the presence of heterogeneous basement and a constricted yet complex sedimentary basin. A total of forty three dive, dredge and borehole samples has allowed both the basement, which consists largely of Moinian, Torridonian and Old Red Sandstone, and the basin itself to be mapped. Using the criteria first employed by Kennedy to identify the line of the Great Glen fault, the course of the structure is extended along the Firth. Divergent fault splays with normal throw probably emanate from the principal Great Glen structure and in turn control the size and nature of the basin. The fill includes proven Triassic and Jurassic together with probable Cretaceous and Tertiary and a complex Quaternary sequence. Permo-Triassic syndepositional movements and late Kimmerian adjustments, together with mid-Tertiary activity are thought to have been the main periods of movement affecting the region.
Exploration wells drilled offshore west and north-west of Ireland penetrated a c . 3.7 m cumulative thickness of Permo-Carboniferous sediments overlying Dalradian metasediments intruded by Caledonian granitic plutons. In the north, late Tournaisian to early Visean clastics were deposited coeval with the Rheic transgression upon an emergent craton in a continental to marginal marine environment. They record an increasing marine influence upwards with the establishment of a carbonate shelf in the Asbian. Clastic/carbonate sedimentation, which became cyclical in late Visean to early Namurian, temporally correlates with the Yoredale cyclothems. Younger Namurian sediments are not preserved. Sedimentation in early Westphalian progressed from prodelta to coal-enriched delta-top and abandonment facies, and is conformably succeeded by brackish to freshwater sediments that contain evidence for local marine incursions of Stephanian age. An Autuno-Silesian section, that is preserved offshore, may also have been deposited onshore prior to erosion. Seismic data enable partial delineation of N-S and NE-SW early sub-basins marginal to the Main Porcupine Basin. These sub-basins which contain Silesian sediments, may have styled successive extensional episodes in the basin and augment support for rift models of Carboniferous basin development in the North Atlantic/NW European domain. BIRPS data provide supporting evidence for the offshore prolongation of pre-Mesozoic basement fault-zones.
Abstract Sediment delivery to the Gulf of Alaska occurs via four areally extensive deepwater fans, sourced from grounded tidewater glaciers. During periods of climatic cooling, glaciers cross a narrow shelf and discharge sediment down the continental slope. Because the coastal terrain is dominated by fjords and a narrow, high-relief Pacific watershed, deposition is dominated by channellized point-source fan accumulations, the volumes of which are primarily a function of climate. The sediment distribution is modified by a long-term tectonic translation of the Pacific plate to the north along the transform margin. As a result, the deep-water fans are gradually moved away from the climatically controlled point sources. Sets of abandoned channels record the effect of translation during the Plio-Pleistocene.
A gravity low immediately E of Dublin is interpreted as an Upper Palaeozoic to Cenozoic basin on the basis of seismic reflection profiles and direct sampling evidence. Maximum thickness of the sediment fill may exceed 4 km. It is considered to consist predominantlyof Carboniferous, undivided Permo-Triassic, and remnant proved Liassic. Thin Tertiary units, which unconformably overlie the main fill, are linked to sediments of similar character found in the Central Irish Sea Basin. Thick Quaternary deposits complete the sedimentary sequence. The basin is structurally a half graben located between two narrow, fault-bounded basement ridges, the Mid Irish Sea Uplift to the S and the Lambay Ridge to the N. The whole structure is bisected by a major NNW-SSE trending fault zone which with the margin faults has created two trapdoor-like structures. It is thought that the principal phases of movement which were instrumental in preserving the sediments occurred during the Triassic and late Jurassic period.
Over 100 samples from the submarine Black–stones Bank Igneous Centre have been collected by scuba divers. The samples include gabbros, dolerites and metamorphosed sediments but ultramafic rocks, expected in view of the high positive gravity anomaly on the bank, have not so far been sampled. The mode of occurrence, petrography and chemical composition of the samples show many similarities between the Blackstones Bank Centre and the Tertiary igneous centres of NW. Scotland. However, K-Ar age determinations on a basaltic dyke which cuts a gabbro suggest a minimum age for the Blackstones Centre of 70 Ma, while most plutonic rocks in the British Tertiary Province have ages of c . 59 and 53 Ma.
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