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The Confrontation of Structural Styles and the Evolution of a Foreland Basin in Central SW England

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The Confrontation of Structural Styles and the Evolution of a Foreland Basin in Central SW England Journal of rhe Geological Sociefy, London, Vol. 145, 1988, pp. 789-800, 18 figs. Printed in Northern Ireland The confrontation of structural styles and the evolution of a foreland basin in central SW England R. D. SEAGO & T. J. CHAPMAN' Department of Geological Sciences, Plymouth Polytechnic, Drake Circus, Plymouth PL4 &AA, UK Present address: Merlin Geophysical Ltd, Boundary Road, Woking GU21 5BX, UK Abstract: Detailed mapping of the rocks to the north and south of Plymouth reveals a sedimentary sequence deformed by a series of folds and thrusts. Two structural zones exist each with a different style of deformation. In the Lower to Upper Devonian rocks of the southern zone, slipvectors, vergenceand facing of D1 foldsindicate that the transport direction is tothe north-west. An associated first phase cleavage dips to the south-east. The northern zone, of Upper Devonian and Lower Carboniferous strata, extends into central Devon and east Cornwall and the geometry and facing of the early folds in these rocks indicate a transport direction to the south or south-east. D1 folds generally verge north but are downward facing towards the south. A flat-lying, penetrative slaty cleavage occurs in the mudrocks and also indicates southward facing. Generally the Carboniferous rocks in this area are inverted. The two structural zones confront each other at an E-W-trending line which passes through Cargreen, 8km north of Plymouth. Here the deformation is more intense as the cleavage,common to the southern zone, overprints that of thenorthern zone, resulting in the formation of D2 folds. The confrontationis interpreted as a northerly dipping backthrust producedby large-scale underthrusting. It is proposed that the northward-advancing Variscan thrust belt progres- sively underthrustthe Carboniferous foreland basin flysch depositswhich became inverted and backthrust towards the south. Forsome years it hasbeen accepted that zonea of importance of northwards prograding flysch sedimentation. south-facing D1 folds occurs in an E-W belt from Dartmoor Anotherimportant implication is the rejection of the to the north Cornish coast. These folds oppose the zone of existence of the zone of south-facing first phase folds, the northward facing first phase Variscan folds of south Devon facing confrontation and the idea of southwards transport andsouth Cornwall (Fig. 1). Onthe north Cornish coast for the nappes (Isaac et al. 1982; Selwood & Thomas 1984, these two contrasting zones of folding meetat what is 1985, 1986a; Selwood et al. 1985). It is argued that all the known as the Padstow or Polzeath facing confrontation first phase folds facenorthwards andthat the nappes of (Gauss 1967, 1973; Dearman 1970, 1971; Roberts & Upper Devonianand Carboniferous rocks (of centralSW Sanderson 1971; Freshney et al. 1972; Sanderson & England) are derivedfrom the south by gravity sliding Dearman 1973; Hobson & Sanderson 1975, 1983; Sanderson (Stewart 1981; Isaac et al. 1982; Selwood & Thomas 1984, 1979). Furthermore, Shackleton et al. (1982) and Coward & 1985, 1986b, 1988; Selwood et al. 1985; Isaac 1985; Turner Smallwood (1984) explained the confrontation in terms of 1985). southerlydirected backthrusting. The facing confrontation In this paper we describe the geology of a transect from was projectedeastwards tothe area north of Plymouth the SW Devon coast, northwards through Plymouth and up (Sanderson & Dearman 1973, fig. 1; Hobson & Sanderson theTamar Valley (Fig. 2).This transect crosses the 1983, fig. 6.5) where southerly facing folds were recognized postulated confrontation as projected ESE from the north in the Okehampton, Lydford and Tavistock areas. Figure 1 Cornish coast (Sanderson & Dearman 1973; Hobson & shows the distribution of fold facing directions in central SW Sanderson 1983). Inlandexposure in SW Devon is England observed by the authors and by those referred to extremely poorand work hasbeen confined mainly to above. coastalreaches. However, in Plymouth road and railway Subsequent to the work by the above authors a major cuttings,quarries andthe banks along the River Tamar geological mapping programme has been undertaken by a provideample exposure. Part of the work has involved group from Exeter University, in part on contract to BGS, re-mapping and detailed investigations of sheet dip, fold in centralSW England. This involved mapping and vergence, facing directions and sedimentary younging within associated structural,sedimentological and biostratigraphic the rocks of the Plymouth Group (see stratigraphy section). work in the areas between Dartmoor and Bodmin Moor and This has revealed a more complex tectonic history than that farther west on the north Cornish coast. This work has gone previously described. Lithological observations onthe a long way towards unravelling the complex stratigraphical original 6" maps (Ussher 1907) have proved to be andstructural relationships in central ~SWEngland: in invaluable. particular the recognition of majornappe structures has helped to explain the existence of large areas of flat-lying, often inverted, Upper Devonianand Carboniferous strata Stratigraphy (Stewart 1981; Isaac et al. 1982; Selwood et al. 1985; ThePalaeozoic rocks of SW Devon(Fig. 3) range in agefrom Selwood & Thomas 1984, 1985, 1986a, 19866; Whiteley LowerDevonian (Siegenian) toCarboniferous (VisCan) and 1984; Isaac 1985; Turner 1985). These authors, hereafter generally young from south to north. To the south of Plymouth the referred to as the'Exeter group', also recognize the rocks are of Lower to Middle Devonian age, whilst in the city of 789 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/5/789/4889260/gsjgs.145.5.0789.pdf by guest on 28 September 2021 790 R. D. SEAGO & T. J. CHAPMAN Plymouththey range from Middle toUpper Devonian. Car- Key boniferous rocks are found farther north in the St Mellion area (Fig. 3 Northern zone-south facing 2). Southern zone-north facing Start Schists Lower Devonian Granite / Thrusts The oldest rocks are the Dartmouth Slates (Ussher1907), re-named theDartmouth Beds and whichyield Siegenian fossils (Dineley Facing directions 1966). Hendriks (1951) indicated that the Dartmouth Slates could be sub-divided into lithological units but she made no attempt at erectinga formal stratigraphy. Dineley (1966) showed that the DartmouthBeds were divisible intofour mappable units and so erectedlithostratigraphy.a His four units are;the Wembury Siltstones(youngest), the Yeah Formation,the Scobbiscombe Sandstonesand the Warren Sandstones (oldest). This waslater provedto be upside down (Hobson 1976a), however, Dineley’s lithostratigraphical units are still essentially correct, although some revision is necessary.We show that the Wembury Siltstones are divisible into twounits, the Renney Rocks Formation and the Wembury Formation. Secondly the term Scobbiscombe Sandstones is omittedfrom our lithostratigraphicsequence forreasons explainedbelow. This leaves four formations: the Renney Rocks Formation (oldest), the Wembury Formation, the Yealm Formation (cf.Dineley 1966) and the Warren Formation (youngest). We suggest thatthese should be included in theDartmouth Group extendingsouth-westwards along the coast from Andurn Point Fig. 1. Location maps showing the distributionof fold facing [SX49004980]to Wadham Rocks [SX58004680]. Faulting has directions in central SW England. Arrowswith long tails are repeated this succession between Wadham Rocks and Westcombe observations made by the authors. Short-tailed arrows are Beach [SX6350 45751 (Fig. 4). observations made by others (referred toin the text). B, Boscastle; BB, Bigbury Bay; BMG, Bodmin Moor Granite; FC,Facing Renney Rocks Formation. Thisformation extends along the Confrontation; L, Liskeard; MT, Mary Tavy; P, Plymouth; Pa, coastfrom Andurn Point in thenorth Heybrookto Bay Padstow; RFZ, Rusey Fault Zone;T, Tavistock; TV, Tamar [SX49704875] in the south, and contains the oldest rocks seen in Valley. SW Devon. The dominant lithology is red or pink sandstone with Permian and 00 H younger 0Meadfoot Group Start Complex Carboniferous c]..... Dartmouth Group Middle and Upper Devonian oa 5c 00 50 00 Fig. 2. Simplified regional geology of central SW England. A-B is the transect from which observations are used to construct the sections shown inFigs 6, 7 &c 15. BMG, Bodmin Moor Granite; DG, Dartmoor Granite; L,Lydford Gorge; p, Plymouth; mz, Rusey Fault Zone; StAG,St Austell Granite; SM,St Mellion; T, Torbay. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/5/789/4889260/gsjgs.145.5.0789.pdf by guest on 28 September 2021 CONFRONTATION OF STRUCTURAL STYLES, SW ENGLAND 791 PlymouthGroup Meadfoot Group Dartmouth Group 171 SaltramSlate Fm. m Staddon Grit Fm. Warren Fm. Compton Slate Fm. m Bovisand Fm. Yeah Fm. (with volcanics and limestones) Wembury Fm. Upper Limestone Mr. Plymouth Limestone Fm. L~~~~ ~i~~~t~~~Renney Rocks Fm. PlymstockVolcanic Mr. Jennycliff Slate Fm. U" oo 0 0 Ems.1 o O o o 0 Sieg. .... , .. Fig. 3. Schematic representation of the Devonian stratigraphyalong a NW-SE section through Plymouth (not to scale). Base of the Dartmouth Group not seen. Sieg., Siegenian; Ems., Emsian; Eif., Eifelian; Giv., Givetian; Fras., Frasnian; Fam., Famennian. 50 cross-sections it is determined that the stratigraphicthickness of this 55 formation is c. 600 m. Yealm Formation. Alongwith sandstone and slate this formationalso contains volcanics. It occurs in the vicinity of the Yealm estuary, the river Yealm having eroded along the outcrop of theeasily weathered
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