In these days of diminishing resources, any memoir from the BGS is to be welcomed, even if this one comes out 135 years after fieldwork started. This one is special because it covers a classic area of British geology that includes the Ballantrae Igneous Complex and the Northern Belt of the Southern Uplands. This present memoir is a compilation by J. D. Floyd (lithostratigraphy) with contributions from A. W. A. Rushton (Lower Palaeozoic palaeontology), I. B. Cameron (Devonian and Carboniferous), R. P. Barnes (structure), D. J. Fettes and P. J. Henney (intrusives) and G. S. Kimbell (geophysics). Floyd has partially resurveyed most of the area starting back in 1976 with his PhD mapping.
Chapter 1 contains an introduction to the geography and a useful review of the literature. Many of us are probably not aware of the efforts of the early Survey mappers and palaeontologists in the 1860's, 1870's and 1880's nor appreciated the effect Lapworth's work on the Moffat Shales had on delaying memoir publication: the work on memoirs from several Southern Uplands map sheets (including this one) being subsumed years later into Peach and Horne's magnificent memoir on the Silurian rocks of Britain, Volume 1: Scotland in 1899. I wonder how these early workers felt about this as the impression has been that Peach and Home did all the mapping.
Chapter 2 outlines the regional stratigraphic framework in terms of Iapetus Ocean terrane analysis and rather preempts the later discussion of their origin. Chapter 3 discusses the lithostratigraphy of basal rocks in the Northern Belt: i.e. Crawford Group basalts, mudstones and cherts and the Moffat Shale Group. The geochemistry of the basalts is critical for any interpretation for the Northern Belt especially if island arc environments could be discriminated, but this has not been determined. Chapter 4 describes the overlying turbidite successions formation by formation, the distinction of each being based on petrography, palaeontology and tectonic position within the various tracts. This illustrates the famous Southern Uplands paradox in which stratigraphic younging is towards the north whilst the tracts contain successively younger rocks towards the south: exactly what the accretionary prism model predicts. The lack of formally locating the type sections of these formations is a minus. I take issue with the confusion caused by allocating the Galdenoch rocks to a formation. These occur at different horizons within the Kirkcolm Formation and indicate that they must be members of the latter, as implied by the fact that the authors divide the Galdenoch into 3 geographically and stratigraphically separated members, albeit of their Galdenoch Formation. For the same reasons I look at the Glenwhargen Formation and conclude that it is actually a member of the Portpatrick Formation. I take personal pleasure in noting the southern Galdenoch member, full of pristine volcanic arc detritus, has palaeocurrent directions flowing from the north. Previously the BGS school has constantly pointed out that the Galdenoch Formation was derived from the south: evidence for a lost southern arc and that the Northern Belt was therefore a back arc basin. Southerly-derived volcanic-bearing Portpatrick Formation turbidites still support that contention. However, a survey of the bathymetry around present day New Zealand will show how volcanic arc and metamorphic basement detritus can be eroded from the mountain divides, can be transported eastwards and be swung through 180 degrees finally to enter the active Hikurangi Trench, travelling in a westerly direction.
Chapters 5 and 6 summarize the Ordovician and Silurian formations in the Ballantrae-Girvan area and the Midland Valley inliers. A chance to rationalize some of the nomenclature has been missed: e.g. the use of the term Penwhapple in the Ordovician Penwhapple Formation and again in the Silurian Penwhapple South Group should have been discontinued. The idea is well developed that the Ballantrae cover sequence represents the collapsed margin of the Midland Valley terrane formed at the same time as the nearby Southern Uplands accretionary prism/thrust stack. The importance of the Ballantrae faunas in the international sphere of faunal provincialism and regional correlation is rightly emphasized. Little new information about the Devonian and Carboniferous is presented in a short Chapter 7.
The structure chapter (Chapter 8) concentrates on post-Ashgill thrusting in the Ballantrae-Girvan area and the tract tectonics of the Southern Uplands terrane. That the early deformation in the latter might be syn-accretion and therefore diachronous as in the Central Belt is not discussed, presumably because there is no evidence. The definition and significance of the Southern Upland Fault terrane boundary along the Stinchar valley and the importance of the tract bounding faults in the Northern Belt is well presented. After assimilating the lithostratigraphic information in Chapters 2–4, I wonder why the Southern Upland Fault can be so important an Ordovician terrane boundary? The Tappins Group is clearly south of the boundary but its detritus is obviously proximal and derived from the Ballantrae Complex as proposed by the Durham/Edinburgh school. This chapter lacks a geological cross-section.
Chapter 9 is a description of the Cairnsmore of Carsphairn, the Loch Doon and Fleet concentrically zoned granite plutons together with various Caledonian minor intrusions that are mostly related to the plutons. The geochemical data have been collated from many unpublished sources and include REE data for the minor intrusives. Mechanisms of intrusion and petrogenetic evolution are comprehensively discussed Although the text refers to new zircon ages for the Doon and Fleet granites, the relevant Appendix has no data for the former. These 410–395 Ma granites are stated to be an expression of the collision between Laurentia and Baltica but no explanation or references are given. Following the published work, lamprophyric magmatism (which is decidedly unrelated to the granites) is presumed also to have originated in a collision setting: again this deserves more explanation.
Chapter 10 summarizes a surprising variety of geophysical data: magnetic susceptibility, Bouguer gravity, aeromagnetic, electromagnetic, radiometric, and seismic refraction data are all reviewed. Combinations of these techniques successfully discriminate between different tracts and locate the tract boundaries and later faults. The aeromagnetic surveys are particularly effective in picking up the Southern Upland Fault and defining formation boundaries in the Ballantrae district. The granite plutons show up very strongly on the Bouguer anomaly maps. Despite all this effort it is still open to question how deep the granites go and what is the nature of the upper crust below about 5 km from the surface: is it the fore-arc over which the Southern Uplands has been obducted, or is it simply the more deeply underthrusted, higher grade, better crystallized, portion of the accretionary prism? Although the latter view has been aired on more than one occasion it is ignored in this memoir. There is no indication from the geophysics that vertical fault structures at the surface become listric at depth and dip northwards, as predieted by the accretionary prism model.
The memoir concludes with short reviews of the Quaternary deposits (Chapter 11) and a summary of the economic geology (Chapter 12). The reference list is comprehensive up to 1996.
This memoir is long on details of Lower Palaeozoic lithostratigraphy and palaeontology and gives a good summary of structure, geophysics and igneous rocks but it lacks a synthesis. Regional metamorphism is never mentioned despite publications being available. The controversy over the origin of the Northern Belt is not really dealt with in depth i.e. is it an accreted back-arc? Is it a ‘normal’ part of an accretionary prism developed in a trench albeit with offscraped islands and subducted MORB? Or is it the collapsed marginal basin to the Laurentian continent formed after the accretion of the Midland Valley terrane and obduction of the Ballantrae ophiolite. This could have been discussed in a synthesis chapter: never mind if further work disproves all the current hypotheses, lets have a lively discussion now.
As already stated, any memoir on the Southern Uplands and Ballantrae is very welcome: specialists will enjoy sifting through the wealth of detail, but others will find this turgid. If this, and other BGS memoirs, were to include field excursion guides that illustrated the geology, they might appeal to the wider geological public, but probably not at this price (£37.50). The strength of this memoir is that it is full of good data.