(first published in the CBO Annual Report for 1980)
The Lower Palaeozoic rocks of the Southern Uplands of Scotland continue south-westwards along their strike across the North Channel to compose a large part of County Down. On both sides of the channel there are similar successions of sandstone, siltstone and shale. Lighthouse Island is part of this extension to the south-west (see fig 1).
The rocks are taken to be the same age as those on the coast immediately west, that is Ordovician, approximately 440-500 million years old. As no fossils have yet been found on the island to date the rock according to recognised graptolite zones, the age is inferred. The island lies to the north of the Orlock Bridge Fault Zone, a 50 metre zone of intensely brecciated rock at Orlock Bridge (Irish grid J 565831), trending north-east to south-west, which is continuous eastward to the Main Copeland Island. No Silurian strata have yet been proven to the north of this fault.
Lithologically, petrographically and structurally the rocks of Lighthouse Island are very similar to those present on the mainland in the area of Groomsport to Orlock (J 530840 to 565831). They are sedimentary rocks, no igneous rocks are recognised. Sandstones, medium to fine grained, grey, with quartz, feldspar and rock fragments, and 15-40% muddy matrix present are interbedded with grey or black siltstones, mudstones and shales.
The northern end of the island, north of J 59758606, following the coast anticlockwise to J 59408583, comprises sandstone beds 0.30 metres to 1.50 metres thick, graded from coarse to very fine sandstone, with rare sole structures on the base of the bed indicating drag of tools along a non-compacted sea-floor. These pass continuously into silty sandstones, plane laminated or with cross-laminae – infers a waning current to deposit these beds. No sedimentary structures occur in the few centimetres of mudstone or shale at the top of the unit, as the current strength was too low. Shale is common only in the thicker beds (> 10 cms thick). The erosional base of a coarse sandstone follows, and the unit is repeated, as shown in fig 2.
Each unit, up to a total of 1.75 metres in this area, may contain different proportions of the various beds; some beds may even be absent. This generalised succession is often termed the Bouma Sequence, and each bed is numbered a to e, representing a different horizon.
The sedimentary structures, including the graded nature of the units, indicate deposition from a submarine density current. This current, travelling below the surface at high velocities, depositing progressively finer material through time is given a special name – a turbidity current, and the deposits are called turbidites. As each turbidity current comes along the 'head' scours the mud surface deposited by the previous current, before deposition occurs from the 'body', hence the irregular base to the sandstone beds and the graded nature.
From time to time the turbidity currents cease and a period os slow deposition takes place. Mudstone and shale (rock splits easily along laminae due to a preferred orientation of minerals) are deposited in deep water. Occasionally fine siltstone beds, less than 1 cm thick may show parallel- or cross-laminae. Slatey Port in the south-west (J 59468569) has shales and siltstones. The rock may be termed slate only in localised areas where there is a later cleavage formed in the rock. Shales become more extensive to the south and east although many fine sandstones and siltstones occur; the extreme southern end of Gavney Gut has dominant sandstone. These shales form areas across strike between 5 and 150 metres.
The sediments were deposited, compacted and subsequently deformed during the Caledonian Orogeny. Post-depositional, pre-deformational, ellipsoidal calcareous concretions or nodules, maximum diameter 25 cm, can be used to estimate the strength of deformation (at the southern end of the island around J 59508605 and J 59758605).
As shown in figure 3 the beds dip dominantly north-west at 70-85 degrees, young north-west, hence the right way up (J 59688610). Local inversion occurs on south-east limbs of folds, inter-limb angle 30-70 degrees; the axial plane trend 050 degrees, diping steeply northwest; the plunge of the fold axis is steep to vertical to the east making the folds difficult to recognise. They are more easily seen on the north coast, where the sandstone beds show considerable hinge thickening (100%). Fig 3.
Numerous faults, trending 005 to 065 degrees, make it difficult to follow bedding any distance. Both horizontal and vertical movement has occurred; movement is post folding.
Veins of quartz – white and hard (cannot be scratched with a key) – occur throughout the island; size range 5 mm to 15 cm, for example at the Well, J 59808588. A few calcite veins are present – calcite is also white, but much softer and may be scratched with a coin.
During more recent geological time, the Triassic and Permian, red sandy beds were deposited over an area to the west, which are now only preserved at Newtownards (Scrabo J 477728). Run-off streams carried fine grained detritus, which through time, stained the existing rock reddish brown, particularly along joints and faults.
Some of the red clays on the island, such as those found at the pond, 75 metres south of the observatory, are boulder clays deposited by the retreat of the ice sheet to the north after the Pleistocene glaciation. Ver recent man (and woman) has made pots from these clays.
The island was mapped, but not in detail, by the Geological Survey of Northern Ireland in 1957, as part of Geological Sheet number 29. There are no previous records of geologists having visited the island. I am mapping the North Down area as my research project at Queen's University of Belfast. I would like to thank the members of the Copeland Bird Observatory for permitting my visit in September 1980. Thank you Ernie Donaldson and Raymond Russell who put up with a wandering geologist for four days – I learnt more about birds than they did rocks.
Fig 1 Geological setting of Lighthouse Island
Fig 2 Ideal sequence of structures in turbidite bed; the Bouma sequence
Fig 3 Typical fold from north coast of Lighthouse Island; note cleavage axial planar to fold hinge; bedding youngs north; concretions orientated in the cleavage plane.
