Thrust Tectonics of S Devon
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J. geol. Soc. London, Vol. 140, 1983, pp. 215-228, 11 figs, 1 table. Printed in Northern Ireland. Thrust tectonics of S Devon M. P. Coward & K. R. McClay SUMMARY: The tectonics of the area from Start Point to Chudleigh, SW England, are shown to be dominated by northward-verging thrust nappes. Two major thrusts are postulated. The 'Dartmouth antiform' is consideredto be a major thrust structure, nota fold structureas previously interpreted. The Torquay limestones are allochthonous and part of a klippe of a major thrust nappe with a minimum displacement of13 km northwards. A progressive and diachronous sequence of deformation has been established with deformation starting in the S and moving N. Many of the folds and cleavages are probably related to the strains developed locally in thrust tips and may have no regionalcorrelation. More intense and complex deformation is found in the S compared to the N. The diachroneity is also reflected in the K-Ar mineralages. In the S, at Start Point, southward-verging backfolding is the last significant Hercynian deformation phase folding the thrust nappes. In SW England, Hercynian orogenesis has affected the Devon we found thatthe structuralinterpretations Lower to Upper Palaeozoic rocks of Devon and Corn- indicated in previous publications and maps (e.g. wall. Much of this deformation post-dated the deposi- Lloyd 1933) were oversimplified. Further research has tion of Mid-Carboniferous Culm sediments but pre- revealed the presence of largerecumbent folds and dated the intrusion of the SW England granite bath- thrusts; it is the aim of this paperto describe the olith, which has yielded K-Ar mineral ages ranging evidence for these and discuss their significance in from 254 to 277 Ma (Dodson & Rex 1971). K-Ar terms of themajor tectonics. We do not aim to mineralages theon weakly metamorphosed provide a definitive synthesis of the tectonics of the Palaeozoic sediments range from Devonian to Carbo- region but rather to promote further discussion and niferous, the older ages occurring in the S (Dodson & research. Rex 1971). This suggests that deformation and accom- panying metamorphism may be either diachronous or polyphase, and that some deformation pre-dated the Stratigraphy deposition of Upper Devonianand Carboniferous sediments.Sanderson & Dearman (1973) noted The classical Devonianstratigraphy, as described by changes in style of structures from S to N in Cornwall the Geological Survey (Ussher 1903, 1904, 1913; and N Devon and they correlated these changes with Lloyd 1933) and later modified by Richter (1965), the variationin mineral ages. Theyrecorded north- House (1975) andHouse et al. (1977), isgiven in ward-verging recumbent folds and thrusts in S Corn- Table 1. This lists the local rock names as well as local wall but in N Cornwall and Devonthey recorded a stage names and both will be referred to in the text. later set of southward-verging folds and thrusts. These The lowermost rocks observed in this region are the later structures steepen in dipto the N(Sanderson monotonous red mudstones, siltstones (now slates and 1979) and in N Devon and S Wales they fan through phyllites) andsandstones, known collectively as the over 130" to becomeupright to northward-verging Dartmouth Slates. They have been interpreted as shal- structures. low water, fluviatile sediments (Shannon 1921, 1928; In SE Devon no major recumbent folds have been Dineley 1966; Richter 1965). They pass up into shales reported, thoughVachell (1963) reported athrust and thin sandstone beds and lenses, termed the Mead- structure, the Marldon Beacon Nappe, in the Torbay footBeds. The sandstones are often graded, cross- area. He noted that Lower Devonian rocks occupied bedded and contain storm deposits. They have been high topographic levels, higher than the adjacent Up- interpreted as estuarine-marinesediments (Richter per Devonianrocks. More recentlyScrutton (1979) 1967). The Dartmouth Slates and Meadfoot Beds con- describedinverted stratigraphy in the Babbacombe tain basic tointermediate volcanic horizons oftuff area and Hobson (1976, 1977) has recognized several breccia,agglomerate and lava. Intermediate dykes, phases of folding in theStart Point-Brixham area possibly feeders to thevolcanics, occur in the Meadfoot (Fig. 1).However, many of the stratigraphicand Beds. palaeontologicalreconstructions of the geology and In SE Devon, the Middle Devonian rocks are char- facies distributions do nottake these structures into acterized by the widespread development of lime- account (House & Selwood 1966; House 1975; Richter stones with a local development of stromatoporoid 1967). reefs andbanks (Scrutton 1977a, b). Detailed On taking undergraduate field classes around S palaeoenvironmental studies have been carried out by 0016-7649/83/03004215$02.00 0 1983 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/2/215/4887575/gsjgs.140.2.0215.pdf by guest on 01 October 2021 216 M. P. Coward & K. R. McClay Dartestuary Blackpool D '/Ci .d Cove //---7=T J Exeter S E\ Hallsands l *Start Paint Point FIG. 1. Map of SE Devon showing section lines of Fig. 2. Inset, location map and section line for Fig. 9. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/2/215/4887575/gsjgs.140.2.0215.pdf by guest on 01 October 2021 Thrust tectonicsThrust of S Devon 217 TABLE1: Stratigraphy of the Torbay district, after intercalated with red mudstones and calcareous shales House et al. 1977. and thence intodeep water pelagic red shales and mudstones dated as Upper Devonian by goniatite Culrn Greywackes faunas (cf. House 1963). Thismarked upward facies rNarnurian (Slates of Anstey’s change is attributed to subsidence and fragmentation of the carbonateplatform with the development of rise Upper DevonianUpper Slates of Saltern and basin (‘Schwellen’ and‘Becken’) features as 1 i :::I observed over much of the Rheno-Hercynian zone in I Saltern Cove Beds NW Europe (see review in Anderton 1979). FrasnianBabbacornbe Slates et al. Givetian Torquay Limestone Middle Devonain Calcareous shales Structure Meadfoot Beds Lower Devonian Dartmouth Slates A simplified geological map of the area is shown in Fig. 1 and detailed cross-sections in Fig. 2. These Richter (1967) and Scrutton (1977a, b) who inter- sections will first be described, in areas from N to S, preted the facies as originating in shallow shelf seas and then the structure will be collated into an overall with carbonate banks. The Middle Devonian lime- schematic cross-section. The structure is complicated stones passupwards into thinly-beddedlimestones by the presence of Permiansediments and later MEADFOOT8AY U DEV SHALES WATER,VOE COVE BmAO SANDS SALTEHN COVE : ELBERRY COVE - _---_ START FQINT SCHISTS , *c+ S’ qv? $1 m .P-. F3 STRVCTURES2 F3 CRENULATICM yms LWMINANT l i\ l e YOUNGINGDIRECTION CLEAV-AGE SYNFQ9M\. f LATE FAULT FIG. 2. Cross sections along the Devon coast, section lines given on Fig. 1. m = Meadfoot Beds. See text and appendix for discussion. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/2/215/4887575/gsjgs.140.2.0215.pdf by guest on 01 October 2021 218 M. P. Coward & K. R. McClay normal block faults which mask many of the contacts. 1965). However, most of the limestones and the adja- A list of critical locations is given in the Appendix. centMeadfoot Beds onthe S side of theTorquay promontory show only one phase of northward- Petit Tor-Torquay Headland verging, gently inclined folds with associated thrusts. The relationship between the rocks which show two or This section comprises mainly flat-lying, right way morephases of deformationand those which show up Middle Devonian Limestones (Fig. 2, Section M). only one phase is obscured by late faults. A possible Locally, the limestones are inverted on the short limbs explanation, which involves the progressive develop- of major northward-vergingstructures (Castle 1979; ment and northward propagation of folds and thrusts, Scrutton 1979). Deformation is intense and the rocks will be discussed later. havea strong cleavage which is axial-planar tothe Minor, probably imbricate thrusts have been recog- folds and generally dips southwards more steeply than nized at Petit Tor, where Middle Devonian limestones the bedding. The limestones are cut by several small are thrust over Upper Devonian shales. They are also thrusts (Fig. 2), which cut the short inverted limbs of found within the limestonesat Hope’s Nose. How- the folds. ever, these thrusts are difficult to correlate because of Between Babbacombe and Anstey’s Cove (see Figs the complication of post-Permian block faulting; a 1 and 2for locations) Middle Devonian limestones major fault, of uncertain throw, strikes E-W across overlie Upper Devonian red shales, dated as Frasnian the Torquay Headland (Figs 1 and 2). S of this fault, (House 1966; House et al. 1977). AtBabbacombe, Lloyd (1933) and Shannon (1928) recorded several re- Scrutton (1979) considered this inversion to be due to petitions of Lower Devonian strata which, if correct, alarge recumbent fold of cleavage age. However, must indicate at least two major thrusts. However, this many of the beds show cleavage dipping more steeply stratigraphic repetition is based on the interpretation of to the S than bedding, a relationship which elsewhere Devonian shelly faunaand this needs further study in the Torquay Headland shows that the rocks lie on before the presence of these faults can be confirmed. the right way up limb of a N-facing, inclined to recumbentfold. The way up criteria in the locally Torquay to Brixham(Berry Head) intensely deformed limestones and underlying Upper (Cross-sections N, 0, P & Q) Devonianred shales are ambiguous-some indicate right way up, someinversion. From a study of the Thenorthern part of this section is obscured by stromatoporoid and other reef organisms at Long Permian deposits but according to Ussher (1903) and Quarry Point, much of the limestone appears tobe the Lloyd (1933) these deposits are underlain by Meadfoot right way up (Kershaw & Riding 1980).