The Chronology of Caledonian Folding in the British Isles by W

VOL. 48, 1962 GEOLOGY: W. S. McKERROW 1905 All of the writers wish to acknowledge the support of the Oak Ridge National Laboratorv under Subcontract 1491. Two of us (J. A. S. A. and J. J. W. R.) have received substantial funds under Grant C-009 from the Robert A. Welch Foundation for studies on the geochemistry of thorium. The Robert A. Welch Foundation under Grant K-054b also provided one of us (J. A. S. A.) with funds for both the development and construction of the laboratory and field radiometric equip- ment. The writers would like to thank numerous individuals for their many courtesies and stimulating discussions regarding the present work. In particular, the help of the following is ac- knowledged: A. P. Butler, Jr., D. Gottfried, E. S. Larsen III, and George Phair of the U.S. Geological Survey; K. B. Brown, D. Crouse, and F. Hurst of the Oak Ridge National Laboratory; J. B. Lyons of Dartmouth College; and M. P. Billings of Harvard University. 1 U.S. Atomic Energy Commission, Technical Information Service Report TID-8201 (1959). 2 McKelvey, V. E., Am. J. Sci., 258-A, 234-241 (1960). Adams, J. A. S., J. K. Osmond, and J. J. W. Rogers, Physics and Chemistry of the Earth, 3, 298-348 (1959). 4 Kline, M.-C., J. A. S. Adams, and J. J. W. Rogers, Geological Society of America Special Paper 68, 211 (1961). 5 Billings, M. P., and N. B. Keevil, Geol. Soc. Am. Bull., 57, 797-828 (1946). 6 Whitfield, J. M., Ph.D. Thesis, Rice University (1958). 7 Butler, A. P., Jr., U.S. Geological Survey Professional Paper 424-B, 67-69 (1961). 8 Lyons, J. B., U.S. Geological Survey Professional Paper 424-B, 69-71 (1961). 9 Billings, M. P., The Geology of New Hampshire; Part II-Bedrock Geology: New Hampshire Planning and Development Commission (1956). 10 Adams, J. A. S., Geological Society of America Special Paper 68, 125 (1961). Ahrens, L. H., Geochim. et Cosmochim. Acta, 11, 205-212 (1957). 2 Jizba, Z. V., Geochim. et Cosmochim. Acta, 16, 79-82 (1959). 13 Rogers, J. J. W., International Geological Congress, XXI Session, Part XXI, 275-280 (1960). 14 Ragland, P. C., Ph.D. Thesis, Rice University (1962). 15 Bean, R. J., Geol. Soc. Am. Bull., 64, 509-537 (1953). 16 Guilbert, J. M., and J. A. S. Adams, Nucleonics, 13, 43 (1955). 17 Anderson, A. L., Econ. Geol., 56, 177-197 (1961). THE CHRONOLOGY OF CALEDONIAN FOLDING IN THE BRITISH ISLES BY W. S. MCKERROW DEPARTMENT OF GEOLOGY AND MINERALOGY, OXFORD UNIVERSITY Communicated by James Gilluly, August 22, 1962 The Dalradian areas of Scotland and Ireland had their large major folding and metamorphism before the Arenig; strong pre-Silurian folding is known in the Welsh Borderland and western Ireland; at the end of the Silurian, we know there is folding in Berwickshire and Pembrokeshire; in Middle Old Red Sandstone time, the main movements are in South Wales, in the Welsh Borderland, and near fault zones in Scotland. The evidence for these events is provided by unconformities, radioactive age dating of granites and metamorphism, and the oncoming of Old Red Sandstone facies. It is suggested that the Caledonian orogeny is the sum total of all these events from the Upper Cambrian to the Middle Devonian which occur in, or adja- cent to, the Caledonian geosyncline. The stratigraphical evidence of the times of folding is reviewed with the following Downloaded by guest on September 25, 2021 1906 GEOLOGY: W. S. AIcKERROI R PROC. N. A. S. questions in mind: (1) Was there a single maximum folding episode? (2) If so, when was it? (3) If there were several events, were they local or widespread? Were they spasmodic or prolonged? Over what period of time did they take place? Spieker,34 in examining similar questions, stated that "the way in which we think about orogeny, the time scale, our stratigraphic sections, strongly controls the actual nature of the practical facts we gather in the field." For this reason, if for no other, it seems necessary to review recent work on the Caledonian orogeny. Major earth movements can be dated by unconformities, effects on sedimentation, igneous activity, and radioactive dating, which all reflect different aspects of orogeny; the same orogenic activity might result in different effects in different areas and at slightly different times. Of these, it is only the unconformity which is both direct proof of movements and capable of accurate stratigraphical control; some emphasis is therefore to be given to breaks in the stratigraphic column. The evidence is presented geographically, starting in the south of Scotland, then through England to Wales, westwards to Ireland, and finally back to Scotland. Southern Uplands and North of England.-The oldest widespread breaks in sedimentation in this area are at the base of the Caradocian; they have been recorded in southwest Scotland, Cross Fell, and the Lake District. Williams38 has demon- strated that there have been subsequent sporadic movements throughout the Upper Ordovician in the Girvan area. He considers that recurrent impersistent con- glomerates are associated with repeated movements along faults bounding the basin of deposition. The Bala grades up into the Silurian without any big break in places where the contact has been described (Girvan, Moffat, Lake District). In fact, sedimentation seems to have been continuous over the whole area from late Ordo- vician times until the end of the Wenlock, or possibly well into the Ludlow. Then there was a big break. The Lower Old Red Sandstone rests unconformably on the Silurian in Berwick- shire (where the age of the overlying beds is proved by Pterygotus),13 near Jed- burgh, in the Girvan district, and possibly also in the Lake District where Capewell6 has suggested that the Mell Fell Conglomerate might be Lower Old Red Sandstone. This evidence of considerable folding at the end of the Silurian can be linked with the intrusion of some Galloway and Lake District granites. Lambert and Mills25 state that the post-tectonic Shap and Creetown granites have respective ages of 397 and 399 million years (see also ref. 10). Lambert and Mills also consider that it is "most probable that the Caledonian Shap, Creetown, and Leinster granites were intruded in the early part of the interval Upper Silurian to Upper Devonian." Both the major movements and these granites were thus probably either latest Silurian or Lower Devonian. These granites, and others nearby,4 12 were involved in further movements, possibly of Middle Old Red Sandstone age. Stratigraphical evidence of these later movements is provided by the unconformity between the Lower and the Upper Old Red Sandstone in the Midland Valley and in Berwickshire, but it is not clear how great an area was affected by these Middle Old Red movements. There is no certainty that they were continuous southwards to Wales, or even that they occurred everywhere between the Midland Valley and the Lake District. Later igneous activity is indicated by the ages of the Weardale and Skiddaw granites;"1 the 365-million-year date suggests a distinctly later (Upper Devonian) age for these two intrusions.24 Downloaded by guest on September 25, 2021 VOL. 48, 1962 GEOLOGY: T1'. S. McKERROW 1907 Wales.-In both North and South Wales, the Lower Cambrian rests unconformably on pre-Cambrian. The first break recorded in the Lower Paleozoic succession is the disconformity at the base of the Middle Cambrian Solva Series in South Wales. Evidence for stronger and more widespread movement is present at the base of the Arenig, which rests on folded and eroded Cambrian rocks in North Wales (e.g., St. Tudwal's Peninsula) and in South Wales (Pembroke). These pre-Arenig movements were, however, very slight in the Welsh Borderland. Later Ordovician movements have been recorded by Davies8 associated with the Cader Idris grano- phyre. The absence of Llandeilo rocks over much of North Wales suggests that these movements were quite widespread. But, while they may be related to the pre-Caradocian break in the north of England, there is no sign of these movements occurring in the Shelve inlier, where sedimentation is continuous throughout the Ordovician. Some movement did occur near Caer Caradoc, however, between the Tremadocian and the Caradocian. In South Wales, the main Upper Ordovician movements are later than in the north; local breaks occurring at the top of the Cara- docian indicate an emergence of relatively short duration.28 Thus, during the Ordovician, the main movements are mostly confined to the geosynclinal areas of Vales. At the end of the Ordovician, the geographical distribution of the main stratigraphical breaks is quite different; they occur in northeast Wales and the Welsh Border but not in the south, west, or northwest of the country. Strong pre- Silurian folding is evident in the M1alverns3" and the Shelve inlier, both situated to the southeast of the Lower Paleozoic geosyncline. By contrast, there are few con- current movements in the Welsh geosynclinal areas. Once Silurian sedimentation commenced (which was at different times throughout the Llandovery in different areas), it appears to have been continuous, except in southeast Wales, where local uplifts interrupted the Llandovery succession,2 and in south Pembrokeshire, where there is a big break at the base of the Upper Llandovery (Jones, ref. 40). The immediately post-Silurian breaks may extend throughout the geosynclinal parts of Wales: in the south, the small breaks seen at the base of the Old Red Sand- stone in Brecknockshire become larger to the west of Llandeilo, and there is no Upper Ludlow exposed in North Wales. It is possible that the whole geosyncline from Berwick to Pembroke might have suffered considerable movement at this time, but it is not possible to be definite about the age of the movements in North Wales or northern England.

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