Riccarton Group, Silurian, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The Riccarton Group, the southernmost exposed part of the Silurian succession in the Southern Uplands, is in fault contact with the Hawick Group to the north but is unconformably overlain by lower Carboniferous strata to the south. It is composed of a range of turbidite lithofacies interbedded with finely laminated carbonaceous siltstone, most probably a hemipelagic deposit. Wenlock graptolite faunas are common in the latter, ranging in age from the riccartonensis Biozone to the lundgreni Biozone in the Kirkcudbright coastal outcrop (Raeberry Castle Formation), but ranging no higher than the rigidus Biozone farther north-east between Langholm and Hawick (Caddroun Burn Formation). The component turbidite sandstone, variably fine- to coarse-grained, is quartz-rich (up to 67 per cent of the grains) but can also contain abundant carbonate, largely as an alteration product but also as rock fragments and bioclastic debris. Minor grain contributions include 5–10 per cent feldspar (predominantly K-feldspar), mica (particularly in fine-grained sandstone) and lithic debris. The latter forms a significant component of coarser-grained sandstone and includes quartzite, chert, limestone and acid and basic (spilitic) igneous material along with sedimentary material likely to be of intrabasinal origin.
In Kirkcudbrightshire, the Raeberry Castle Formation (about 1500 m) is divided into three, partly coeval structural tracts that are accorded member status (P912334). In contrast to the adjacent Ross Formation tracts, the Raeberry Castle Formation members are internally coherent, with very little disruption of the bedding. The Gipsy Point Member is composed of thin- to medium-bedded turbidites overlain by channel-fill accumulations that include slump deposits, and which are interspersed with overbank or levee deposits of thinly interbedded sandstone and mudstone with well-developed dewatering structures. The Raeberry Member consists of variable but well-bedded turbidites with isolated thick sandstone and hemipelagite beds. The Mullock Bay Member features channelised sandstone, some very coarse, alternating with thinly interbedded, fine-grained sandstone and mudstone; this assemblage is overlain by upward thickening and coarsening units that pass from hemipelagite through thin- to medium-bedded to thick-bedded sandstone.
The equivalent strata in the Langholm to Riccarton area make up the Caddroun Burn Formation (about 1500 m). This comprises thin-bedded fine-grained sandstone and mudstone with intercalations, tens of metres in thickness, of more thickly bedded, coarser sandstone with thin mudstone interbeds. Laminated carbonaceous siltstone occurs throughout, generally as relatively thin beds but locally much thicker (8 m at one locality near Saughtreegrain (NY 568 998)). Palaeocurrent data indicate that most sediment transport was towards the southeast quadrant, with a subsidiary flow towards the south-west. Some thin, fine-grained beds are rich in plant and crustacean remains.
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