Depositional and geotectonic history of the Gala area, eastern Southern Uplands, Scotland

Abstract

The Ordovician and Silurian successions between Falahill and Galashiels encompass six flysch-dominated formations: the Upper Ordovician Portpatrick and Shinnel Formations representing the Leadhills Group, the Llandovery Mindork, Garheugh, and Buckholm Formations together comprising the Gala Group, and a formation indeterminate of age within the Hawick Group. Southward ensialic andesitic volcanic arc and northward low- to medium-grade sialic sources contributed sediment, whilst ophiolitic and subduction-related sources made minor contributions. Deposition took place firstly, in a SE-migrating back-arc basin bordering the northerly source, the Laurentian continent. Subsequent NW-directed underthrusting led to formation out of the back-arc basin of an imbricate thrust stack which migrated southeastwards. Ultimately a foreland successor basin formed ahead of the rising thrust stack.Flysch units are typically associated with linear outcrops of Moffat Shales which are the loci of major steep SE-translating reverse faults, two of which participate in a late-stage sinistral strike–slip duplex with large-scale imbrication. The faults divide the succession into a sequence of tectonostratigraphic blocks, successively younger to the SE. At least six of the ten blocks customarily recognised in the Southern Uplands, Blocks 3–8, are represented, some of which coincide with single or complete formations.

Biostratigraphical control of thrust models for the Southern Uplands of Scotland

“…7c). Kassi and Weir (1993), in interpreting the source of a sedgwickii Biozone fauna at Calfhope Burn [406 455] as Moffat Shale Group, discounted the possibility of continuous Gala 4 to Gala 7 greywacke deposition in this area. However, re-examination and re-collection of this, and other, critical localities shows the graptolitic host rocks to be siltstone interbeds within the greywacke sequence, and we have little doubt that continuous greywacke deposition is a valid option.…”

Silurian subduction-related assembly of fault-defined tracts at the Laurieston Fault, Southern Uplands accretionary terrane, Scotland, U.K.

“…(1) a combination of low Rb, K and Ba with high Sr over the Ordovician sequence N of the Orlock Bridge Fault, which suggests a plagioclase feldspar provenance within rocks of a dioritic or granodioritic composition; (2) the converse relationships S of the Moffat Valley Fault, encompassing the Hawick Group and the Buckholm Formation (Table 1; Kassi & Weir 1993) of the Gala Group, which indicate a dominantly potash feldspar provenance largely within moderately evolved, granitic continental crust; (3) an overlap in these trends across much of the Gala Group (up to Gala 7), between the Orlock Bridge and Moffat Valley faults, which suggests the interaction of different provenance areas rather than the geochemical evolution of a single source. Taken together, the eNd and G-Base data confirm an unroofing trend from the Ordovician sequence through the Llandovery Gala Group, with juvenile plutonic detritus of broadly granodioritic composition becoming progressively swamped by Proterozoic basement detritus (Fig.…”

Lithostratigraphy of the Ordovician rocks in the Southern Uplands: Crawford Group, Moffat Shale Group, Leadhills Supergroup

“…Within the last few years, the existing stratigraphical correlation in the Northern Belt has been extended eastwards into the Leadburn (Floyd 1994) and Gala Water (Kassi 1984;Kassi & Weir 1993) areas, E of Peebles, effectively completing the modern revision of the 180 km long belt begun by Kelling (1958) in the Rhins of Galloway. This has only served to emphasise the remarkable along-strike petrographical continuity and stratigraphical correlation in the Southern Uplands (Fig.…”