No abstract

Regional mapping at a 1 : 60,000 scale of a 30‐km strip from the Gulf of Oman (Muscat) across the Oman Mountains, 130 km to the south, provides the geologic setting for the (∼95 m.y.) Ibra section of the Samail ophiolite. Where best preserved, the Ibra ophiolite section is an ∼8 km‐thick section of oceanic consisting of ∼0.5 km of pillow lavas, 1.2–1.6 km of sheeted diabase dike complex, 0.2–1.0 km of high‐level noncumulate gabbro, and 3.0–5.0 km of cumulate gabbro that is underlain by tectonite peridotite 9–12 km thick. The Ibra section is found on the southward dipping limb of the Sayah Hatat antiform. The tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high‐temperature, low‐stress asthenospheric flow. Discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite. The structural base of the tectonite peridotite is overprinted by a high‐stress, low‐temperature deformation that can be related to its oceanic detachment. The layered gabbros are predominantly olivine‐clinopyroxene‐plagioclase cumulates, and orthopyroxene does not occur as a cumulus phase. Occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber. High‐level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes. Both the diabase dike complex and pillow lavas are hydrothermally altered, and alteration and metamorphism increase downward (zeolite (?) to epidote‐amphibolite facies). In spite of pervasive alteration, relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros. The present‐day Samail thrust surface truncates ophiolite stratigraphy and puts the Samail ophiolite on top of unmetamorphosed Hawasina melange. The last motion on this surface was probably no older than Maestrichtian (70–65 m.y.). Garnet amphibolites exposed as remnants of earlier thrusting (∼90 m.y.) record initial ophiolite detachment at a 14–20‐km depth within the Tethyan oceanic lithosphere. The Hawasina Group, which underlays the Samail ophiolite, is a block melange where exposed near Muscat in the north, and it grades into an imbricated broken formation at the southern limit of the map area. The Hawasina Group is thrust over Permian to Late Cretaceous shelf carbonates that represent autochthonous Arabian continental shelf deposits. Recent (post‐Miocene) collapse and dome structures, such as the Ibra dome, have complicated ophiolite stratigraphy south of Jabal Dimh. Our geologic studies strongly indicate the Samail ophiolite represents a large, coherent slab of transported oceanic lithosphere formed at a Late Cretaceous spreading center in the Tethyan Sea.

Red Sandstone plants, determined by Dr. Kidston as P~,,ilophyton and Pachytheca respectively, from a bed of black shale underlying the lavas on the northern face of Stob Dearg. Specimens of Psilophyton were also found in dark-grey shales and mudstones underlying the basic andesites on the north side of Glen Coe above Loch Achtriochtan. ~ Mr. Kynaston published brief summaries of his researches as they proceeded:; but his final, and in some instances modified, conclusions can only be found ill lhe manuscript notes left by him on his depart, ure for South Africa in 1903. In many directions little advance has been made from the position indicated in these notes. Thus Mr. Kynaston describes in detail the volcanic succession developed in Aonach Dubh and Bidean ham Bian, which, as will be seen later~ provides a typical section of the area. He a]so gives an account of the interesting boulders of granite, andesite, and quartz-porphyry in the basement conglomerate exposed on the hillside north of Loct~ Achtriochtan. Further, he accounted for the position of the rhyolites of Stob Dearg, or rather the absence of the Aonach Dubh andesites beneath them, by assuming an overlap of the rhyolites eastwards against an uneven floor of schists. And he even interpreted the vertical junction of the volcanic rocks with the schists in the Cam Glen as an extreme example of this unevenness. Here again we have followed Mr. Kynaston, after a careful consideration of all the evidence in the field. He also demonstrated that the Ben Cruachan granite is later than the Old Red volcanic rocks, since it invades and alters them ; and finally we may record that in An t-Sron, above Loch Aehtriochtan, he had begun mapping the boundary-fault of the Glen Coe cauldron. He had, in fact, realized, so far as was possible from a single section, the fundamental relation subsisting between this fault and the intrusive rock which so constantly accompanies it. We may illustrate this point by quoting his manuscript :-' On the south side of Glen Coe, south-southwest of Loeb Achtrioehtan, a well-marked line of fault, indicated by a deep cleft on the northeast slopes of An t-Sron, cuts off abruptly the basic andesites which are seen on the east side. On the west side occurs a mass of granite which shows a marginal facies along the line of the fault, so that it is possible that the fault may be older than the granite.' Our own connexion with the district dates from the years 1903 and 1904. a During 1903 Dr. Peach was still in charge, and did some mapping in the area himselL The time during which he was actually at work in the district was very brief, but the value of his influence will never be forgotten by those whom he introduced to the varied geological problems of Glen Coe.

Summary A great dislocation is associated with the Tertiary igneous complex of Rhum. In 1903 it was described by Harker, following closely upon Geikie, as an early Palaeozoic thrust. It is now claimed as a Tertiary ring-fault with central uplift. Harker was influenced by the curvature of its outcrop, a feature that at the beginning of the century was regarded as almost diagnostic of a low-angled thrust; today, of course, steep ring-faults * have become a commonplace. Harker was further misled by slump-bedding in the Torridonian, which he interpreted as tectonic; by a patch of Trias with basal cornstone, which he mistook for crushed Torridonian overlying transported Durness Limestone; and by superposition of Lower Torridonian shales on Torridonian sandstone, which latter he thought was post-shale Applecross Grit, instead of pre-shale Basal Grit. Harker misidentified these two grits, largely because he considered that certain associated gneisses were banded Tertiary intrusions, though Geikie had previously recognized their true nature as part of the Lewisian complex. Another correction may here be advanced. Harker took a set of Tertiary explosion-breccias which accompany the tilted Torridonian inside the ring-fault for crush-breccias developed by early Palaeozoic thrusting. Geikie had been hesitant; but Judd had been confident that these rocks were products of Tertiary explosions. The present account is in many respects an elaboration of some of Judd's ideas published in 1874. The outstanding tectonic feature of Rhum is a block uplift, with an elevation amounting locally to about 7000 feet. The uplift was accomplished at the start of Tertiary volcanicity, and was indirectly responsible for the famous inter-lava river gravels of the neighbouring island of Carina. The peridotites of Rhum were later than the block uplift, and they rose along ring-fissures about a centre well to the west of that surrounded by the great initial ring-fault.

Although it is unlikely that the east of Sutherland will ever rival the west in geological renown, its varied interests have already furnished material for a considerable literature. Attention has been specially focussed upon a down-faulted coastal strip of Mesozoic rocks, which, starting at Golspie, extends north-eastwards for twenty miles through Brora and Helmsdale to the county boundary at the Ord (fig. 1). Golspie itself stands on a narrow outcrop of Trias. Jurassic follows, with a generally ascending sequence that leads up to Kimmeridgian at Kintradwell, a couple of miles north of Brora (fig. 2). Beyond this Corallian reappears, but only for a short distance. Then Kimmeridgian returns and holds the coast-line continuously for nearly nine miles, until, at the Ord, it disappears beneath the sea. The width of the Kimmeridgian exposures is occasionally three-quarters of a mile, but generally much less.