Abstract
Synopsis Development of a regional geological interpretation for the Southern Uplands Terrane has been driven historically by advances in graptolite biostratigraphy, sandstone petrography and turbidite sedimentology (particularly the recognition of reliable way-up criteria), then, more recently, by a growing appreciation of the nature of accretionary processes at active plate margins. The latter has led to the widely held view of the terrane as an archetypal fossil accretionary prism, built up by the accretion at the margin of Laurentia of the sedimentary cover of Iapetus oceanic crust, the cover being stripped from the oceanic crust during its subduction beneath Laurentia. The achievement of this broad consensus has not been without controversy, which still persists in respect of some aspects of the overall model. Suggestions that the northernmost (Ordovician) part of the terrane might have originated in a back-arc basin have been disproved by a combination of mineral dating and basin thermal history analysis utilizing clay mineralogy. Nevertheless, it remains possible for that part of the terrane to have a continental margin, forearc depositional history. The southernmost (mid-Silurian) part of the terrane most probably developed in a foreland basin setting, as a fold-and-thrust belt overriding Avalonia, following the closure of the Iapetus Ocean. Progress in understanding the origins of the Southern Uplands Terrane has been achieved only by the collaborative application of multidisciplinary techniques – biostratigraphic, sedimentary, structural, geochemical and geophysical. This review is a tribute to all those who have contributed. Supplementary material: Coloured versions of the maps comprising Figure 1, showing details of the regional lithostratigraphy, are available at http://www.geolsoc.org.uk/SUP18769.
Highlights
The Southern Uplands Terrane is arguably the World’s best-preserved ancient accretionary complex
Development of a regional geological interpretation for the Southern Uplands Terrane has been driven historically by advances in graptolite biostratigraphy, sandstone petrography and turbidite sedimentology, more recently, by a growing appreciation of the nature of accretionary processes at active plate margins. The latter has led to the widely held view of the terrane as an archetypal fossil accretionary prism, built up by the accretion at the margin of Laurentia of the sedimentary cover of Iapetus oceanic crust, the cover being stripped from the oceanic crust during its subduction beneath Laurentia
Within each tract the strata young towards the NW but over the entire terrane the strata in the SE are younger than those in the NW. This is the Southern Uplands paradox, summed up by Anderson & Cameron (1979, p. 263) from the Ards Peninsula, Northern Ireland, as follows: “Folding and stratigraphy combine in the classic Southern Uplands paradox: north-younging limbs are predominant, graptolites invariable indicate the presence of younger sediments to the south.”
Summary
The Southern Uplands Terrane is arguably the World’s best-preserved ancient accretionary complex. At the base of the succession in each tract an assemblage of chert and graptolitic mudstone (very rarely, and only in the Ordovician tracts, with subjacent basaltic lavas) lies immediately to the NW of a major strike-parallel fault; the mudstone is conformably overlain by a very much thicker sequence of turbidite siltstone and sandstone. This is the Southern Uplands paradox, summed up by Anderson & Cameron (1979, p. 263) from the Ards Peninsula, Northern Ireland, as follows: “Folding and stratigraphy combine in the classic Southern Uplands paradox: north-younging limbs are predominant, graptolites invariable indicate the presence of younger sediments to the south.”
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