Geological Structure of the Malin Sea

Geological structure of the Malin Sea

MAXWELL R. DOBSON & DANIEL EVANS

SUMMARY Geophysical surveys in the Malin Sea have fault belts have also been identified, together revealed the presence of two asymmetric fault with a large number of igneous bodies, the bounded basins, here termed the Loch Indaal latter having been confirmed by dredge basin and the Rathlin trough. They extend in sampling. In addition the offshore extents of a NE-SW direction between Northern Ireland the Fanad and Thorr Plutons have been and Western Scotland, and are thought to be delineated. largely Permo-Triassic in age. Four major

G ~. o P a r s x c A L and geological surveys in the Malin Sea (up to 56°N) by the University College of Wales, Aberystwyth have included shallow seismic re- flection, magnetic, gravity and sonar with rock sampling using dredges and divers. Geophysical investigations and an initial programme of seabed sampling and rotary drilling have been carried out by I.G.S. to the east of 7°W. This account is a preliminary summary of the joint results of the Aberystwyth and I.G.S. surveys largely compiled in Aberystwyth. The geological map (Fig. i) is general- ized and includes evidence to the north of 56°N and west of Colonsay from McQpillin & Binns (1973), while the geology from the area to the east of 6°W is taken from Deegen (1974). Further work is in hand in I.G.S. The geology is controlled by 4 major fault belts--the Great Glen fault (McQuillin & Binns i973) , the Loch Gruinart fault (Dobson et al. 1974), the Lough Foyle fault and the Tow Valley fault. As in the structurally similar Sea of the Hebrides (McQ.uillin & Binns i973) , the faults form margins to Mesozoic filled troughs which are floored by downthrown Palaeozoic and Precambrian. Although there is no direct evidence as to the presence of Tertiary sediments, thickness of fill above the Mesozoic interface is calculated to exceed 200 m locally. The map specifically excludes all supposed Tertiary and Quaternary deposits. The best established basin, here termed the Rathlin trough, extends more than 50 km northeast from the Antrim coast and is controlled by the Tow Valley and Lough Foyle faults. Numerous crossfaults cut out the trough towards the northeast. The Mesozoic infill in this trough is probably similar in thickness to the sequence found in the Port More borehole which drilled over 1500 m of Mesozoic without reaching the base (Wilson 1972). To the west the Loch Indaal basin (Dobson et al. I974) is controlled by the Loch Gruinart fault which itself extends northeastwards to the Great Glen fault. A more restricted veneer of Mesozoic rocks to the east of Colonsay may also be controlled by this structure. Further west the Mesozoic troughs and basins associated with the Great Glen fault are imprecisely known at this time. Finally, in the extreme southeast a trough is located in the North Channel. The Rathlin trough, on the basis of interpreted gravity (Kenolty, unpublished) is thickest beneath Rathlin Island and close to the projected Tow Valley fault.

Jl geol. Soc. Lond. vol. x3o , i974, pp. 475-478, I fig. Printed in Northern Ireland.

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The Port More borehole has revealed the existence of at least I53 ° m of Mesozoic further west, but in the same trough the MagiUigan Point borehole cored only 86o m of Mesozoic (Wilson 1972 ). Although the seismic system employed failed to record the trough floor all the available evidence indicates that the trough tilts towards the southeast. To the north of Islay a restricted veneer of dipping rocks is known to contain red sandstone (Chesher et al. 1972 ) of probable Permo-Triassic age. South of Islay the Loch Indaal basin contains two Mesozoic facies, which are often separ- ated by a fault. A Triassic interpretation best fits the geological data. In the main part of this basin the minimum Mesozoic thickness, calculated using the gravity data ofRiddihough & Young (1971) and assuming a density contrast ofo'4 × IO: kg/m: with the basement is IOOO m. In places more than 2oo m of sediment have been recorded above this material, the lower part of which could be Neogene in one restricted locality in the southwest comer of the faulted structure. These thicknesses of superficial deposits are similar to those reported by Binns et al. (1974) in the Hebridean area.

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OR GE'NERa4. DP (

OT~R FAULTS

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G. ---- Colonsay B.H. = Borehole Dotted llne represents eastern and northern boundaries of surveys conducted by U.G.W., Aberystwyth.

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The North Channel trough is, in terms of fill, considered to be an extension of the Rathlin trough. NW-SE faults have been recorded in this sequence while gravity coring in this tidally scoured region has provided samples of red marls. One sample has been palynologically dated as Middle Triassic (Deegan pers. comm.). Igneous rocks, considered to be of Tertiary age, are a striking feature of the area: they take three forms. The first consists of the offshore extensions of the Antrim basalts along the Northern Ireland coast around Rathlin Island, and includes the Portrush sill. These rocks are prominent on the seismic records and are a readily mappable rock type. It is considered that the basalts once covered a wider area than at present, their present termination being largely fault controlled. The second category includes igneous masses located principally in the basins and also along the major faults. These are more varied in character than the first group. Those in the Rathlin trough for example are vertical sided and have locally disturbed the surrounding sediments. Many have been directly sampled and all proved to be of basic composition. The possible presence of the large intrusions along the Lough Foyle fault has yet to be substantiated. Three igneous masses occur to the northwest of Malin Head. These were recognized on the basis of magnetic data (Riddihough 1968 ) and one, the Malin Complex, may be a Tertiary centre, although a Caledonian age is not unlikely. The magnetic data show it to be a ring structure of acidic composition (Riddihough 1968 ). Finally, much of the area to the south of Islay has been invaded by a dyke swarm too extensive to be included on the map. Part of this swarm is assumed to be associated with the Blackstones Bank centre, possibly those within the limits set by Walker (196o) for the Islay-Jura swarm. The precise orientation and composition of the many dykes is not, however, known at present. In addition to Tertiary igneous rocks two areas of granodiorite have been mapped off the Donegal coast. The first relates to the offshore extension of the Fanad Pluton which is poorly developed onshore (Pitcher & Berger 1972 ). The second concerns the Thorr Pluton (Pitcher & Berger 1972 ) where the side scan sonar was particularly valuable in mapping its extent, which was principally delineated from magnetic data. The basement rocks of Lewisian, Dalradian, Torridonian and Lower Palaeozoic age have been subdivided on the basis of seismic character, nature of the sea- floor outcrop and the adjacent land geology. No samples, with the exception of the Otter Rock southeast of Islay, have been collected. However, where the Lewisian outcrops, the sonar records reveal a distinctive and readily recognizable picture, which in the case of the southwest extension of the Rhinns of Islay is supported by the land geology. In many localities the subdivision of the basement is made on purely seismic evidence. In the area immediately west of Kintyre are found Carboniferous rocks which extend seawards from the Machrihanish coalfield. As well as land geology, the age of these rocks has been determined from seismic character and the evidence of an I.G.S. borehole, while other adjacent boreholes have drilled red bed deposits in the Mesozoic sequence (I.G.S. borehole Nos. 73/3, 73/4: and 73/2, I.G.S. Annual Report 1973). In addition, faulted blocks of Carboniferous rocks are considered to occur south and east of Rathlin Island, from seismic character.

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The Mesozoic basins in this area, like the region to the north (McQuiUin & Binns I973) , have developed by extensive normal faulting during a pronounced tensional phase, probably during the Permo-Trias. The existence of deep marine basins of Permo-Triassic age close to only thin covers of Triassic rock in the Hebridean land area, both of which are locally covered by Rhaetic deposits, is taken as indicating syndepositional movements along the marginal faults. The possibility of later vertical movements is also recognized, although only Tertiary movement on the Tow Valley fault can be clearly demonstrated. Doubt also exists concerning the age of the crossfaults, some of which may be associated with the original late Devonian movements. The geology and geomorphology of the Malin Sea is related to: (i) The initiation of 4 major (?) transcurrent faults probably in late-Devonian or earlier times. (2) Syndepositional movements along these and other (North Channel) faults during the Permo-Trias (redbed deposits) followed by more gentle marine deposition later in the Mesozoic, notably during the Lias and Upper Cretaceous. (3) Igneous activity during the early Tertiary, followed by structural ad- justments which influenced the alignment of the Antrim coastline. ACKNOWLEDGEMENTS. The authors thank the officers and crew of the R.R.S. John Murray, M. V. Whitethorn and M. V. Profiler. The practical assistance of colleagues both at the Geology Department, U.C.W., Aberystwyth, and I.G.S., Edinburgh (Continental Shelf Unit II and Marine Geophysics Unit) is also gratefully acknowledged. This account is published with the permission of the Director of the Institute of Geological Sciences.

References ANON. x973. Institute of Geological Sciences Annual Report. B~s, P. E., HARLAm~,R. and HuonEs, M. J. x974. Glacial and post-glacial sedimentation in the Sea of Hebrides. Nature Phys. Sci. Lond. a45, 75 x. CHESHER, J. A., DE~GA.'% C. E., Am~us, D. A., BINNS, P. E. & F~Nm, N. G. T. x972. I.G.S. marine drilling with M. V. Whitethorn in Scottish waters x97o-7x. Inst. Geol. Sd. Report No. 72/Io. D~*EG~a% C. x974. The geology of the Clyde area. LG.S. Report Series. (in press). DoBsoN, M. R., Ew~a~s, D. & WHrrTmGTo~r, R. 1974- The offshore extensions of the Loch Gruinart Fault. Scott. J. Geol. (in press). McQmLLm, R. & Br~s, P. E. I973. Geological structure in the Sea of Hebrides. Nature Phys. Sci. Lond. 24 I, 2. PITCHER, W. S. & BERGER, A. R. t972. The geology of Donegal. Wiley-Interscience. RmDmOImH, R. P. I968. Magnetic surveys off'the north coast of Ireland. Proc. R. L Acad. ~B, 27. & YOVNG, D. G. G. r97r. Gravity and magnetic surveys of Inishowen and adjoining sea areas off the north coast of Ireland. Proe. geol. Soc. Lond. No. x664, 215. WALKER, F., x96o. The Islay-Jura dyke swarm. Trans. Geol. Soc. Glasg. 24, I2I. WILSON, H. E. x972. Regional Geology of Northern Ireland. H.M.S.O. Belfast. Received x8 December 1973; revised typescript received x4 March 1974- Maxwell Richard Dobson, Geology Department, University College of Wales, Aberystwyth. Daniel Evans, Institute of Geological Sciences, Continental Shelf Unit II, ~rest Granton Road, Edinburgh, EHI 5 IJA.

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