VI.—The Petrology and Sedimentation of Upper Ordovician Rocks in the Rhinns of Galloway, South-west Scotland

Abstract

SynopsisThe petrology of a group of geosynclinal rudites and arenites of Upper Ordovician age from the Rhinns of Galloway, south-west Scotland, is described. The rudites are confined to the oldest formation, the Corsewall Group, and consist of pebbly microbreccias and “boulder-beds” interbedded with coarse greywackes and platy siltstones. The composition of the microbreccias and boulder-beds varies little with stratigraphic level.The arenites comprise lithic, coarse-grained greywackes with occasional quartzose or feldspathic varieties. They carry, in aggregate, a wide range of igneous, metamorphic and sedimentary rock-fragments, quartz of variable source, a variety of feldspars, ferromagnesian minerals, micas, chlorites and a common to conspicuous argillaceous matrix. The greywackes are well defined stratigraphically as regards both overall composition and the character of the large mineral grains and rock-fragments (“granules”). Moreover composition appears to be related to grain-size in a consistent manner. The size-distribution, sorting and skewness of the greywackes based on thin section measurements show little stratigraphic variation.The aggregate suite of heavy minerals is meagre and only minor differences exist in the individual formation-suites.The character of the rock-fragments and mineral grains suggests that the earlier formations were derived from a nearby source-area in which plutonic and hypabyssal acid and basic igneous rocks were extensively exposed, together with spilitic lavas and glaucophane schists. The petrology of the upper formations reveals that a metamorphic terrain of Southern Highlands aspect became an important additional source of material. However, the highest beds were derived from a region dominated by andesitic and spilitic tuffs and lavas.The principal directional sedimentary structures are described briefly and it is suggested that the lower part of the Corsewall Group, derived mainly from the north and north-east, was deposited in a sheltered pro-delta environment, but that the boulder-beds and associated rocks were formed within a more boisterous region. The Kirkcolm Group is a turbidite-sequence deposited in a marine trough of moderate depth by currents which at first flowed dominantly in opposing directions, towards either north-east or south-west. However, transport from the north-east became progressively established. The Portpatrick Group is another turbidite-sequence but this formation was deposited by currents flowing from the west or south-west.