Am. Kassi et Ja. Weir, DEPOSITIONAL AND GEOTECTONIC HISTORY OF THE GALA AREA, EASTERN SOUTHERN UPLANDS, SCOTLAND, Transactions of the Royal Society of Edinburgh. Earth sciences, 84, 1993, pp. 161-173
The Ordovician and Silurian successions between Falahill and Galashiel
s encompass six flysch-dominated formations: the Upper Ordovician Port
patrick and Shinnel Formations representing the Leadhills Group, the L
landovery Mindork, Garheugh, and Buckholm Formations together comprisi
ng the Gala Group, and a formation indeterminate of age within the Haw
ick Group. Southward ensialic andesitic volcanic arc and northward low
- to medium-grade sialic sources contributed sediment, whilst ophiolit
ic and subduction-related sources made minor contributions. Deposition
took place firstly, in a SE-migrating back-arc basin bordering the no
rtherly source, the Laurentian continent. Subsequent NW-directed under
thrusting led to formation out of the back-arc basin of an imbricate t
hrust stack which migrated southeastwards. Ultimately a foreland succe
ssor basin formed ahead of the rising thrust stack. Flysch units are t
ypically associated with linear outcrops of Moffat Shales which are th
e loci of major steep SE-translating reverse faults, two of which part
icipate in a late-stage sinistral strike-slip duplex with large-scale
imbrication. The faults divide the succession into a sequence of tecto
nostratigraphic blocks, successively younger to the SE. At least six o
f the ten blocks customarily recognised in the Southern Uplands, Block
s 3-8, are represented, some of which coincide with single or complete
formations.