Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 The Caledonides of the British Isles--reviewed. 1979. Geological Society of London. Caledonian correlations: Britain and Scandinavia Robin Nicholson SUMMARY: While there is widespread evidence of climactic late-Silurian thrusting in the southern Scandinavian Caledonides the scale of comparable early Ordovician (Grampian) movements is not yet established. Only in the lower, eastern, marginal nappes is there evidence of continuous sedimentation from late Precambrian up into lower Palaeozoic. In the metamorphic allochthon no fossils older than Tremodocian have been discovered. Barrovian metamorphism which seems to vary from nappe to nappe is widely developed: no genuine LT/HP metamorphism is recorded. The North Sea basin, largely founded on Old Red Sandstone (ORS) deposits of the terminal Caledonian stage, covers the junction between the two distinct Caledonian provinces of Britain and Scandinavia. The problem of what can be usefully meant by opposite to one another since Devonian times. the term Caledonian has been discussed at some There is faunal evidence, also, of the probable, length by Dewey (1974). Narrow definitions earlier, side-by-side co-existence of Anglo-Welsh work only for restricted parts of the grand and southern Norwegian-Swedish areas of lower assemblage; deformation belts evidently are Palaeozoic sedimentation (Rushton 1974). The diachronic structures. Thus the original definition presence of a mid-European arm of the of Suess (1906, If, 82) was that the Caledonian Caledonides, long speculated on (Bailey & Hol- mountains are 'Those pre-Devonian mountains tedahl 1938) and apparently confirmed by off- which proceed from Norway and form the whole shore evidence (Ziegler 1977)must complicate, of Scotland, together with the overthrust outer however, the simplest ideas of continuity bet- margin along the zone of Eriboll...'. For Scan- ween the two in the lower Palaeozoic. Of the rest dinavia, although not for Britain, an arbitrary of the Caledonides (cf. Dewey & Kidd 1974) it is upper age limit of the base of the Cambrian • more reasonable to suppose on evidence now would be easy to accept since there is little available that their present arrangement is not at evidence of the late Precambrian history of all like that which pertained during sedimenta- deformation so widespread in Britain. The lower tion. Not only is this so but there are so many pre-Devonian limit in turn is more difficult to differences between Britain and Scandinavia that accept in areas where the deformation of Devo- is is clear the North Sea basin developed along nian rocks is an obvious continuation of earlier the zone of union of two distinct Caledonian events and represents the end of Caledonian provinces. activity. As the Variscides are approached (Fig. 1) deformation of Devonian rocks, in contrast, An overview and speculations represents the beginning of a new cycle. The Scandinavian Caledonides is part of a The outer limit of the Caledonides in space major Siluro-Devonian collision belt--the Erian now may be placed at the line enclosing all Belt of Dewey & Kidd (1974). It is dominated by cleaved autochthonous Cambro-Silurian rocks thrust sheets and contrasts strongly with the (and in some parts Devonian) as well as the British Dalradian belt of fold nappes and related thrust nappes which usually reach beyond it so as thrusts with their early Ordovician metamorphic to superimpose the Caledonian allochthon on and tectonic climax. The Scandinavian belt, how- essentially underformed lower Palaeozoic rocks. ever, does contain elements at its northern The spatial limit is a deformation front (Fig. 1). extremity whose timing of deformation and metamorphism are similar to those of the Dalra- The present condition dian (Pringle 1971). Further, it has been argued Applying the spatial and temporal definition that such elements occur widely (e.g. Binns outlined above obviously places large areas of the 1978) so that the Scandinavian belt as a whole contiguous land areas of Britain and Scandinavia contains a western assemblage, resembling the together in the Caledonides, although there is no Dalradian at least in the timing of orogeny, but direct evidence of the course of the southeast combined during collision with tracts carrying the limit of the belt across the intervening North Sea. imprint of the Silurian tectonic climax. As Hol- It seems clear that they have been so placed tedahl (1939) has argued, the Moine thrust belt 3 Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 4 R. Nicholson of Scotland might be complementary to the east- Caledonides, between Britain and Scandinavia, erly directed Scandinavian thrusts of the collision apparently confluent with the Polish trough and belt (see also Dunning 1972). The Outer Isles perhaps resembling the slate belts of Britain, may thrust zone of the Outer Hebrides has provided be cut off and overlain at its northern end by the evidence of later Caledonian movement (Gibson, Scandinavian thrusts. However, the thrust front p.201, in discussion of Steel & Wilson 1975). may not be directly continuous across the North The southeast, or mid-European, arm of the Sea in this way, but may be replaced to the , ~.~. A ~ &vvvvvvv A A & ~ A Zvvvvvv v vvvvvvvvvvvvvvvvvvvvvvvv% ,A~. ^ ^ A ^ '~v vvvvvv ~p~ ........ N vvvvvv~, ~vvvvvv, vvvv v orway vvvvvvvvvvvvvvvvvvvvv\ vvvvvvvvvvvvvvvvvvvvv', vvvvvvvvvvvvvvvvvvvvvvvvvv\ Laurentian vvv vvvvv vvvvvvvvv\ v-, Shield., vvv Late vvV~,~ ' ...... tcy v Caledanid~ vvy,,,,~vvvvvv~ v ~ v vvvvvvvv,, ~v ............ ~w~v vv ...... .........vvvvvvvvvvvv W~vvvVV~Vv~Vvvvvvvvvvvv~ ...... v v v v v v v v v v~,~ v v v v vv v ~ v v v.~ vvvvv u ~o~o~ Central Trau h v v v ,~.Xo^o,.,o, ...............g v v v ,,#o~o^o~o,"v v v , Ivvvvvvvvvvvvvv O.RS ~vvvvvvvvvvvvvi vvvvvvvvvvvvv Ivvvvvvvvvvvvv G.GF vvvvvvv vvvvvvvv vvvvvvvv vvvvvvvvv vvvvvvvvv vvvvvvvvv vvvv v~i vv ~v ~vv vvvvvvvvvvvvvv vvvvvvvvvvvvvvv vvvvvvvvvvvvvvv H.BF Undeformed Early Caledanides __ Cale dan ides - ...... _-_--_-_-. C-Sil ors ;arboniferaus S.U.F Carboniferous ~ O.R. S England ~ Limit of Caledonian 2 ~ ~ / Deformation ~'~ Variscan Fare Deep J ....... Fro. 1. An outline tectonic map of the British and southern Scandinavian Caledonides (after Ziegler 1977, fig. 1). HBF Highland Boundary fault C-Sil Cambro-Silurian MT Moine thrust zone ORS Old Red Sandstone SUF Southern Uplands fault Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Britain and Scandinavia--correlations 5 southeast by a zone of transform motion (Phillips the Vendian tillite of the Dalradian of Scotland et al. 1976). and Ireland (Table 1). They comprise the Hede- mark Group of Bj6rlykke et al. (1976) (the A brief tectonic-stratigraphic survey of the so-called sparagmite of earlier authors, a term southern Scandinavian Caledonides not used here) topped by a thin lower Palaeozoic Only in the Oslo graben do there occur thick cover. These authors, in part following earlier deposits of deformed and autochthonous accounts, explained the accumulation of this Cambro-Silurian rocks. Even here, the most non-marine Precambrian sequence by having it northerly and most strongly deformed rocks may form in a rift-bounded depression or aulacogen, be parautochthonous; deformation is greatest thus attempting to allow for the sporadic where the thrust nappes impinge on them and development, along the thrust front, of nappes dies out southwards. Elsewhere, the autochthon- containing this and comparably thick sequences. ous sequences of the Scandinavian Caledonides It is their view also that the Hedemark Group of are only thinly developed under thrust nappes. the southern Caledonides is parautochthonous, The only other lower Palaeozoic sequences of its lower Palaeozoic rocks being linked with those comparable thickness and stratigraphical range of the nearby Oslo graben where the Hedemark to those of Oslo occur in the lowermost nappes of Group itself is absent. Hossack (1978), however, Jamtland, west of Ostersund in Sweden (Fig. 2). interprets it as having been transported at least The allochthon is divisible (Fig. 2) into a number 100 km during thrusting. The Valdres nappe of units distinguished by both lithology-stratig- above, containing similar sequences, he regards raphy and deformation-metamorphism (Nichol- as being derived from still further west (Fig. 2). son 1974). The south Caledonides, however, has The late Precambrian sequence of this higher an especially complex tectonic pattern and, even Valdres nappe lies unconformably on igneous within it, tectonic correlations are controversial. rocks, often regarded as at one time continuous The lowermost tectonic-stratigraphical units of with the rocks of the overlying Jotun nappe, and the southern Scandinavian Caledonides are providing a link between the two structures. notable for a thick sequence of late Precambrian However, this view is not without its opponents sediments containing a tillite comparable with (Battey & McRitchie 1973). The granulite-facies rocks of the Jotun nappe now mainly lie in the so-called Central Trough or Faltungsgraben, run- TABLE l(a): The stratigraphy of selected areas in ning between the Trondheim depression to the the frontal nappes of the southern northeast and the curving trough of the Bergen Scandinavian Caledonides arcs. It has been suggested, following Vemdal Nappe Valdres Nappe Vemda Nappe Autochthon Goldschmidt (1912), that the displacement evi- Fleinsendin Bitihorn Mj6sa Oslo dent at the edges of the Jotun nappe, as it is Down Lud Ringerike S described here, is of secondary importance and Wen Sh L Sh that it roots in the trough itself. A discussion of Llan S this view is available in Hossack (1978). Sh Ash S Car L Sh TAaLE l(b): Key to abbreviations employed in Lland Sh Tables 1, 2 and 3 L Llan S A Andesite Ar Arkose Aren Beitostolen L B Black Ba Basalt(ic) Trem Ph Mellsenn QS C Conglomerate Ca Calcareous U. Cam Fleinsendin F Felspathic M, Cam Q Alum Shale G Greywacke Gn Gneiss Gs Greenschist or Greenstone L.