The tectonic evolution of Jersey, Channel Islands

D. G. Helm

HELM, D. G. 1984. The tectonic evolution of Jersey, Channel Islands. Proc. Geol. Ass., 95(1), 1-15. The structural sequence established (Helm, 1983) for the Brioverianmetasedimentary Jersey Shale Formation of St. Ouen's Bay, Jersey, is extended to the maininland and other coastal outcrops of the formation and is compared with the structure of theoverlying younger Brioverian, Volcanic Group. The structure of the succeedingCambro-Ordovician rocks, the Rozel Conglomerate is examined and for the first time a tectonicsynthesis for the entire supracrustal suite of Jersey rocks is formulated. In outline the sequenceof events is as follows: D] compression directed approximately E-W produced periclines andsingly plunging folds in both the metasedimentary and volcanic sequences. During Dz the main,N-S directed compression, refolded D, structures and produced the widespread major andminor E-W trending folds which modified the originally N-S, D, structural trend. Following theCadomian D, and Dz deformation and granite intrusion the area was uplifted and eroded toprovide the post-orogenic Rozel Conglomerate Formation. Renewed compression resulted inthe formation of the NW-SE trending Rozel Syncline of D3 age. The principal structures of D4age are the N-E trending Trinity and St. Helier Synclines which control the present outcrop ofthe Volcanic Group. The D3 and D4 structures are clearly post-Cadomian but lack ofstratigraphical control precludes more precise definition of their relative age.

Department of Geology, Goldsmiths' College, University of London. Rachel McMillan Building,Creek Road, Deptford, London SE83BU

1. INTRODUCTION

Geologically the Channel Islands are part of theArmorican Massif of NW France (Fig. 1). Thecrystalline basement is named the Pentevrian andthree principal orogenic events have been recognised(Roach et al., 1962) the youngest of which (Leutwein& Sonet, 1965; Leutwein, 1968) dates between 1100and 900 Ma (Table 1). The basement complex wassubjected to a protracted series of metamorphic anddeformational events.

In the St. Brieuc region of Brittany the Pentevrian,which according to Cogne (1959) possesses a NNEfoliation, is unconformably overlain by mainly lowgrade metasedimentary rocks belonging to the latePrecambrian, Brioverian succession. An E to ENEfoliation is said to be developed in both basement andcover rocks (op. cit.). The Brioverian occursextensively in northern France and is also found in theChannel Islands where it is represented by thePleinmont succession in Guernsey and the JerseyShale Formation and Volcanic Group in Jersey.

Although no radiometric dates have been obtainedanywhere from the lowermost Brioverian metasedi­mentary succession it is clearly younger than 1100­900 Ma, i.e., the minimum age of the Pentevrian at St.Brieuc and older than the oldest granites in Jersey(580 Ma-Adams, 1976) which cross-cut Brioverianstructures. The younger date of 533 Ma obtained fromthe overlying uppermost Brioverian volcanic rocks ofJersey, by Duff (1978), has been challenged by Bishopand Mourant (1979).

The Jersey volcanic sequence consists of andesitic

1

and rhyolitic pyroclastic rocks and lavas whereas atErquy in Brittany the sedimentary sequence is cappedby spilites although the Brioverian age of the latter isin some doubt (Bishop et al., 1975).

The Brioverian, and to some extent also thePentevrian basement, was deformed during the latePrecambrian Cadomian orogeny which producedmainly easterly structural trends in Armorica and anortherly trend in Guernsey (Roach, 1966). In Jersey(Helm, 1983) the Brioverian displays predominantlynortherly and easterly structural trends.

Syntectonic and post-tectonic intrusive rocks enablethe Cadomian orogeny to be dated with reasonablecertainty. In Finistere the 'gneiss de Brest' gives a dateof 690 ± 40 Ma (Adams, 1967) and the post­metamorphic Renards granite sets an upper age limitfor the Cadomian orogeny at 565 ± 40 Ma (Adams, inBishop et al., 1969). The syntectonic L'Ereeadamellite in Guernsey was emplaced around 660 Ma(Adams, 1976). Further confirmatory dates are givenin reviews by Renouf (1974) and Bishop et at. (1975).Post-Cadomian igneous activity also included theemplacement of penecontemporaneous major basicand later acidic intrusions in the Channel Islands andthe Mancellian and other granites in Normandy.

As a consequence of the uplift and erosion of theCadomian fold belt, during the Cambro-Ordovicianthere was widespread deposition of coarse clasticdeposits such as the Serie de Montfort below the GresArmoricain at Rennes and the Rozel Conglomerate inJersey. Undeformed lamprophyre dykes, one of whichhas been tentatively dated at 427 Ma (Adams, 1976),indicates that by early Silurian times Jersey had

2

o Meso zoic

o Pa laeo zoic

~ Briove ria n

_ Pe ntevr ia n

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Lil Ca do mia n granite

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t

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TABLE 1. Stratigraphy and main tectonic events in J ersey

Era / eo n Su p e rg raup I I Fo rm a t io n /g roup ICambro - ~ 0

0 0 • 0 . ~ 1 Roze l ConglomerateO rdovician

0 0 00 ° 0 0

Formation00: 0 : 0 0 0

o " 0 ,,00 ° ;: 0

0° 0 °

00 0 0 0 0 0.; / /

(

rv---v--:-v-' v ~ v-T Volcan ic Group~:;-vv-v~v~v v v v V v

BRIOVERIAN/vvvv ~~

." v V v v "......... ... ........ ............... .

r.:-:-:..... ......... Jersey Shale FormationProterozo ic ... ........... ....

... .. . . . . .. ... ....

1//·~v/

VJ ? ~kPENTEVRIAN

~

O ro gen y

CADOMIAN70 0 - 50 0 Ma)w it h p lu t o nism

(110 0- 900 l

TECfONIC E VOL UTIO N O F JERSEY 3

become relatively stable. Auvrey et al. (1980) obtaineda 472 ± 5 Ma age for the Plouezec volcanics of N.Britt any. The Lower Palaeozoic age of the associat edred bed s succession is thus in accord with Adams (op.cit. ) inferred, somewhat younger dat e for the Jersey'Red Beds'.

In outline the geological history of Jer sey is asfollows. Following the 1100-900 event , a period offlysch sedimentation was followed by the depo sition ofvolcanic rocks showing an upward change incomposition from and esitic to rhyolit ic. This Briove­rian sequence was folded and metamorphosed duringthe Cadomian orogeny and intruded by a suite of basicand igneous rocks. Post-tectonic upl ift and erosiongave rise to the molasse-like Roz el ConglomerateFormation (Table 1).

Ap art from general comp arison s of the mainstructures affecting each of th e above formations therehave been no detailed attempts to correlate thevarious structural sequences or to explain the complexdeform ation al history of the rock s of the Island as awhole. Th e aims of this paper arc:(1) to describe and account for the major and minor

stru ctures displayed by each formation ,(2) to explain the apparently anomalous structural

sequences recorded in each format ion , and,(3) to elucida te the structural evolut ion of Je rsey.

Considerat ion of the global tectonic setting isbeyond the scope of this paper. For a relatively recentsumm ary of the structure and tectonic evolution of theArm orican Massif the reader is referred to the reportof the Te ctonic Studies Group conference by Roach(1980).

Mapping was carri ed out at a varie ty ot scalesdictated by the nature of exposure s and availability ofground contro l. The Huntings Aerosurveys 1: 25,000topographic map proved adeq uate for mapping therelat ively poor inland exposures but for some coastalarea s with up to 90 per cent exposure enlarged aerialphotographs at scales of bet ween 1 :3,000 and 1: 16were necessary.

2. STRATIGRAPHY AND LITHOLOGIES

The Pentevrian basem ent is not exposed in Jersey .The oldest rock s compri sing the Br ioverian , JerseyShale Formation (Table 1) outcrop in central andweste rn Jersey (Fig. 2). The y are bounded to the Nand S by Cadomian granites and to the E by theyounger Brioverian, Volcanic Group . The youngestrocks of Cambro-Ordovician age comprising the RozelConglomerate Formation , occupy a narrow NW-SEstrip across the NE corner of Jer sey.

(a) Jersey Shale Formation

The term Jersey Shale Formation is not ent irel y

satisfactory because the seque nce contains as much ifnot more psammit e than pelite and the rocks haveund ergone low grade regional metamorphism. Howev­er, the term is firmly entrenched in the literature andis therefore retained .

Th e paucity of faunal evidence and lack ofdistinctive marker horizons precludes subdivision ofthe formation. The discovery of Sabellarites-like wormburrows led Squire (1973) to suggest an age ofc.750 Ma but Downie (in Bishop et al., 1975) thoughtthey were more likely to be Vendian in age, i.e.between 680 and 570 Ma.

Cleaved mudstones, siltstones, greywacke sand­stones and occasional conglomerates comprise theJersey Shale Formation . The principal exposures (Fig .2) occur in S1. Ouen's Bay on the west coast (Helm,1983) and in S1. Aubins Bay on the south coast . Inlandthere are numerous small isolated exposures and threesmall inliers occur within the volcanic ou tcrop.

Prior to 1974 (Squire, unpub. Ph.D . the sis) work onthe Jersey Shale Formation had been very limited.Transon 's 1851 map of Jersey showed few structuraldetails and Casimir's (1934) maps of parts of theJersey Shale Form ation are only sketches. Casimir andHenson (1955) produ ced a more detailed map of theGiffard Bay area where the conta ct betwee n the JerseyShale Form ation and the Volcanic Group may beseen .

Squire (1974) considered the Brioverian flysch seriesto have been deposited by turbidity curre nts and tohave formed a submarine fan in a eugeosynclinalsetting with the coarsest conglomerates, which for mpart of the northern margin of the outcro p, infilling acanyon or canyons. Th e Jersey Shale Formation iscer tainly a deep water siliciclastic succession anddisplays all the featu res that charac terize depositionfrom turbidity flows, but the relative uniformity of thelithology throu ghout much of the sequence, togetherwith the lateral constancy of bed thickne ss traced overwide areas , suggests that cauti on should be exercisedbefore assigning them to any part icular sedimentolo­gical regime and sedimentary subenvironm ent (Picker­ing,1 982).

(b) The Volcanic Group

The main outcrop of the volcanic rocks occupies anopen syncline (Trinity Syncline) in the northeast of theIsland and a much smaller syncline (St. HelierSyncline) on the west side of S1. Helier . Th e lower twothirds of the sequence consists of andesitic lavas andpyroclastic rocks whereas the remainder of thesuccessio n is compose d largely of rhyolitic tuffs andacidic lavas. Mourant 's (1933) threefold classificationof the volcanics into : Andesite (oldes t) , PorphyriticRh yolite and Non-porphyri tic Rh yolite , was confirmedand only slightly modified by Th omas (1977).

-l:>

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, / ---~ -~'"'~~ ~~...- < " ~~.... ~ .......;: , . • •• • •• • • • • . • • :- -.-.

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.... ... \ \..... \ \ \......... \ \ ,----- \ ... ...... \... \\ ... \ \" \ \

--,.... _....... ...... ....<.>. " ...._--, ... --.... .... " <,

" <, -', ~~~ '-- "< ,.................. ....._-- -~

,,-,\"

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Fig. 2. Simplified geological map of Jersey showing localities, orientation of bedding (Jersey Shale Formation and RozelConglomerate Formation) and accumulation surfaces (Volcanic Group).

TECTONIC EVOLUTION OF JERSEY 5

(c) The relationship between the Jersey ShaleFormation and the Volcanic Group

For the most part the shale/volcanic junction isfaulted. In one instance (64355052) an apparentlyconformable passage between the two formations wasrecorded (Thomas, 1977) and Mourant measured atemporary section in St. John's Road (64704955) inwhich 1.55 m of conglomerate was seen to separate theJersey Shale Formation from andesite. A much thickerrudite named the Breccio-conglomerate by Thomas(1977) crops out in Giffard Bay where the base is seento be composed largely of angular shale and andesiteclasts. It also occurs near Les Rouaux (657563) whereagain the base is characterised by angular blocksidentical to rock types in the underlying Jersey ShaleFormation and which passes up into a sediment withobvious volcanic affinities. On the basis of mappingand in the light of the probable tectonic history ofJersey it is argued below that a disconformityseparates the two Brioverian rock units.

(d) Plutonic Rocks

The northwest, southwest and southeast corners ofJersey are occupied by plutonic igneous rocks. Whilstthe NW and SW granite masses may have played arole in determining the local stress conditions duringthe deformation of the supracrustal rocks, theirpetrology is not directly relevant to the presentsynthesis. There is abundant evidence of syn- orpost-emplacement deformation in the form of closespaced joints and local shear zones with an associatedschistose fabric. Renouf (in Callow & Cornford, 1984)confirms the existence of strong local deformationgiving rise to both N-S and E-W fabrics at L'Ouaisne(593475).

(e) Rozel Conglomerate Formation

The Rozel Conglomerate is a reddish-brown and verypoorly sorted grain supported polymict unit containingfragments of the underlying metasediments andvolcanic rocks. However, granite clasts cannot readilybe matched with present outcrops of local intrusions.Evidence provided by pebble imbrication indicatescurrent flow from the north (Squire, 1970).

Shales with mudcracks and rainpits occur near thebase of the succession at the Tete des Hougues(679544). Some of the coarser beds are graded andparallel with contiguous conglomerate demonstratingthat the depositional dip of the latter was horizontal(J. Renouf, pers. comm.). The Rozel Conglomeraterests on the irregular exhumed landsurface of theunderlying volcanic rocks.

3. STRUCTURE

Hitherto, the structure of Jersey has been studied in apiecemeal fashion. The only detailed work on the

Brioverian metasediments was by Squire (1974).Thomas (1977) was more concerned with thepetrology and geochemistry of the volcanic rocks thantheir structures though he produced a sketch map (hisfig. 12.1) showing their main structural elements.

No structural or tectonic synthesis of the supracrus­tal rocks has previously been attempted. This sectionsummarizes the structure and tectonic sequence of theBrioverian metasediments established in St. Ouen'sBay (Helm 1983), extends it to the Brioverianmetasediments of the remainder of Jersey andexamines the structure of the younger supracrustals ofthe Volcanic Group and the Rozel ConglomerateFormation.

(a) The Jersey Shale Formation-St. Ouen's Bay

In the St. Ouen's Bay area the Jersey Shale Formationis almost continuously exposed over an areaapproximately 4.0 km by 1.0 km. Two main phases ofdeformation are recognized:

The first phase (D 1) formed both periclinal andsingly plunging folds in response to an approximatelyeast-west directed maximum principal compressivestress. The periclines and singly plunging folds mayhave grown synchronously (cf. the experimental workof Dubey & Cobbold, 1977). The periclines have wellrounded hinges and curved limbs whereas the singlyplunging folds are chevrons. Only the latter have awell defined axial planar fabric (Figs. 5a & 6a). Thebedding dips and youngs mainly towards the east butthe westerly vergence of the D 1 folds indicates thepresence of a major anticlinal hinge some distanceseaward of the present coastal exposures. It can beshown that the presently variable axial orientation ofthe D 1 folds is due to the second deformation (D2).

During the second phase the maximum principalcompressive stress direction was approximately north­south. It produced the major easterly plunging St.Ouen Anticline which dominates the structure of thebay (Fig. 4). Parasitic folds of similarly open sinusoidalpattern modify each limb of the major anticline. Asystem of conjugate faults and an axial planarnon-penetrative cleavage (S2-Figs. 5b and 6b) areassociated with the development of the D2 folds.

Distinctive, serially arranged arrays of radialfractures which cut D 1 and D2 structures appear tohave been formed in response to a vertically orientedmaximum principal compressive stress (q. v. e.g. fig.12, Helm, 1983).

A consistently north-south striking fracture cleavageoccurs sporadically in the Jersey Shale Formation ofSt. Ouen's bay (Fig. 6d; see also fig. 2, Helm, 1983).A similarly oriented set of close spaced fractures isalso common in the granite outcrops. The relative ageand relationships of these fabrics to each other andother structures is uncertain though the cleavage in theJersey Shale Formation appears to be post-Dj.

6 D. G. HELM

(b) Jersey Shale Formation-remainder of Jersey

Other coastal exposures, which are good but not soextensive as those in St. Ouen's Bay, occur in thenorth at Fremont Point (641563, Fig. 2) and betweenthe eastern end of Giffard Bay (653560) and LesRouaux (657563); in the south the best exposuresoccur in the intertidal reefs west and southwest of St.Aubin's Fort (613485). Inland exposures in the mainoutcrop of the Jersey Shale Formation are principallyconfined to the incised north-south and east-westvalleys that dissect the otherwise largely drift co~ered

plain. Relatively poor exposures are to be found in theinliers centred on Le Bourg (687492) and Gorey(706503).

In spite of exposure gaps, the rectangulararrangement of fairly regularly spaced inland tran.sectsprovides good control and allows the regionalstructure to be deduced with some confidence. As Fig.2 shows, the beds follow a sinuous course from northto south both in the main outcrop and in the inliers.Dips vary from steep to overturned though locallyshallow dips are not uncommon. Except for a narrowstrip of westerly inclined bedding running north-souththrough St. Peters (596516) the majority of beds havean easterly dip (Fig. 3a, b and d). The local rev~rsal ofthe general trend defines the trace of a. major D Ifold-pair. Westerly dipping beds elsewhere m the mamoutcrop point to the possible occurrence of other D Ifolds. The general north-south structural grain (Fig. 2)is clearly the result of the first deformational .episodewhereas the sinuousity of the beds was largely mducedby the Dz north-south compressional phase.

The rather open Dz folds (Fig. 4) have axial traceswith a broadly east-west disposition but which maydepart by up to about 20° from this ~ene~al t:end. Thefold-pair with the north-easterly axial direction about1 km SW of St. Ouen's Church mayor may not be ofD z generation. Its position in the tectonic scheme isdiscussed below.

(c) Volcanic Group

The varied and distinctly different lithologies compris­ing the volcanic rocks facilitate the const~uction ofreasonably detailed and accurate maps which permitan appreciation of the structure more readily than inthe case of the lithologically monotonous underlyingmetasediments. This explains the similarity ofMourant's (1933) and Thomas' (1977) maps. Thomasdraws together all the structural information in anattempt to explain the diverse structural trendsrevealed by his and others' research. Thomasrecognized three fold trends namely, east-west;north-south and northeast-southwest.

Re-examination and re-mapping of much of thevolcanic formation confirms the existence of themajority of folds illustrated by Thomas (1977). Thereare two essential differences between Thomas'

structural map (his fig. 12.1) and that of the presentauthor (Fig. 4):

(i) Some additional folds are shown, principally aNNW trending fold-pair in the vicinity of FremontPoint (640563) with a similarly oriented syncline inBonne Buit Bay (647558); a series of approximatelyEW trending folds in the area immediately to the N ofArchirondel Tower (712517); the anticlinal flexureswhich produced the Jersey Shale Formation inliers ofLe Bourg and Gorey and; a refolded and faultedsyncline which follows the Vallee des Vaux (650510)and its offset E-W continuation approximately 500metres to the south. The presence of the refoldedsyncline is required to explain the anomalousrelationship between radially outward dipping bed­ding, suggesting the presence of an anticl~ne, and theoutcrop pattern which shows that the major structureis a NE plunging syncline, the Trinity Syncline.

(ii) A second important difference is that thepresent author has attempted to present a unifiedmodel of the structural elements. Fig. 4 shows thepossible relationships between folds of widely differingorientations.

A number of previously unrecorded cleavages havebeen observed in most of the volcanic outcrops (Figs.5c & d). Because of the relatively competent nature ofthe volcanic rocks, tectonic fabrics are not so well oras frequently developed as in the un?erlying Je:seyShale Formation. The number of obtamable readmgswas insufficient to warrant construction of stereogramsfor individual localities. Instead, a composite stereo­gram is used to show all the recorded fa~rics (Fig. 6c).The following trends (mean of readmgs m eachparticular field) are apparently 140, 048 and 005.

It is difficult to relate the folds of different trends interms of their relative age because of the paucity ofsmall-scale tectonic features. The cleavages with fairlyconstant orientation recognized in the course of thiswork provide evidence of overprinting which stronglysuggests that the cleavage with the 140 trend wasproduced first and followed successively by the 048and 005 cleavages. These trends have their. counter­parts in the axial directions displayed by vanous foldsin the volcanics and to which the cleavages wouldseem to be axial planar. The relationship between thestructures displayed by the volcanic sequence and theolder metasedimentary sequence is discussed insection 4.

(d) Rozel Conglomerate Formation

Bedding in the Rozel Conglomerate is often indistinctand even where detectable may incorporate anelement of original sedimentary dip. Nevertheless,there are indications such as below the Tete desHougues (see above) that this was not high and mayeven have been near horizontal. Assuming that thedips are mainly tectonic, then the Rozel Conglomerate

c .

N

N

TECTO NIC E VOLUTI O N O F J E R SE Y

b.

d .

N

N

7

Fig. 3. Stereograms of poles to beddi ng (Je rsey Shale Formation) and accumulation surfaces (Volcanic Group), a) bedding(501 poles) Jersey Shales in St. Ouen 's Bay (contours at 0.2, 0.6, 1.5, 3 and 5% ; b) bedding (114) poles main outcrop of JerseyShale Formation (contours at 0.9, 3 and 5%) ; c) accumulation surfaces (281 poles), Volcanic group (contours at 1, 2 and 3%;d) total bedding (615 poles) Jersey Shale Format ion (contours at 1,2 and 4%).

can be shown to be disposed in a WNW trendingsyncline, the Rozel Syncline , with a sinuous axial tracewhich runs just south of the coastline between theTour de Rozel and Fliquet Bay (692552-712538, Figs.2 & 4) .

Below the Tete des Hougues the purpl e laminatedmudrock near the base of the succession has a slatycleavage striking 138° and dipping moderately steepl y

NE (Figs. 5e and 6e). Elsewhere . in the conglomerate,a SE trend ing sub-vertical fabric shows considerablerot ation and flattening of pebbles and also tectonicpitting (Fig. 5f) . The fabric is parti cularly welldeveloped near St. Catherines breakwater (713531)where it strikes 120°.

Ver y open flexure s with a NE axial trend have beensuperimposed on the main NW trend ing syncline and

00

N

o xic ! tr cce s....._ O.

_._._ ._ .- 03----- 02--- 0 1- - - bedd ing form lines_._-- foult s

- ---- -

~

~: ...=.: Rozel Conglomera te Format ionIvolco nic fo , mo' ;on.

Jersey Shal e for mat ion• •• Plutonic Igneous

Fig. 4. Map of main structural features of Jersey showing the orientation of fold axial traces of various ages.

a

c

e

b

d

Fig. 5. Photomicrographs of tectonic fabrics, a) pressure solution (SIl in laminated silstone Jersey Shale Formation, St.Ouen's Bay (scale bar-s-l um): b) dilatational quartz bands representing spaced cleavage of D2 age in siltstone, Jersey ShaleFormation, St. Ouen's Bay, scale bar-250 /lm; c) pressure solution cleavage (SI) in crystal-lithic tuft, Giffard Bay, scalebar-c-l um; d) microfolded fiamme (vertical in photograph) with associated pressure solution cleavage (SI) ignimbrite, AnnePort (713511), scale bar l um; e) slaty cleavage-i-S, (sloping towards right of photograph) in pelite from near the base of theRozel Conglomerate Formation, La Tete des Hougucs, scale bar-300 /lm; f) tectonic pitting of Brioverian pebbles in RozelConglomerate Formation, quarry (713531) west of St. Catherines Breakwater, scale bar-500 /lm.

9

10

a. N c .

D. G. HELM

N e . N

. '0 /0";"/

//

//

//

"..•••J

b. N d. N f. N

-, '0'."

I. \.

~(:"'j

.j

..

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Fig. 6. Stereograms of poles to tectonic surfaces (data for a) and b) mostly obtained from St. Ouen's Bay), a) S1 (315 poles)Jersey Shale Formation reoriented by D2 (contours at 0.3,3.0,6 and 13%); b) S2 (242 poles) Jersey Shale Formation (contoursat 0.4, 3, 7 and 10%); c) poles to all tectonic fabrics, Volcanic Group with 140, 048 and 005 trends shown as dashed lines; d)cleavage of uncertain affinity, Jersey Shale Formation; e) poles to tectonic fabrics, Rozel Conglomerate Formation; f) poles(202) to cleavages and close spaced joints, mainly from the north-west granite.

are responsible for its sinuous axial trace and thecurvature of the boundary which marks the base of theconglomerate.

4. TECTONIC SYNTHESIS

Occasional mesoscopic, refolded folds occur within theJersey Shale Formation which are probably slopegenerated slumps. They sometimes possess an axialplane cleavage but this appears to be the result ofearly compaction of the sedimentary pile (Maltman,1981).

(a) The D1 Phase

The earliest folds of undoubted tectonic origin are thepericlines and singly plunging folds which occur atintervals over most of the area covered by theintertidal reefs in St. Ouen's Bay (Fig. 2). Areas oflow dipping beds with variable strike possibly point tothe existence of periclines inland (Fig. 2). Theabundance of singly plunging folds in the St. Ouen's

intertidal reefs suggests that they are also likely to bepresent in the main outcrop though discontinuousexposure precludes positive recognition.

The singly plunging folds are thought to have beenproduced by the same compression that gave rise tothe earlier formed periclines. When the effects of Dzare ignored, the resultant structural grain of theBrioverian is seen to be approximately north-south,thus the maximum local principal compressive stressduring D[ must have been more or less east-west. Theoccurrence of a thick conglomerate with a highproportion of volcanic clasts, at the base of thevolcanic sequence, which also appears to overstepvarious units of the underlying metasediments (Figs. 2& 4), suggests that there is a considerable time breakbetween the two sequences. Moreover, the metasedi­ments are almost exclusively marine whereas thesucceeding volcanics would appear to be largelysub-aerial. However, the strong possibility of asub-volcanic group unconformity does not necessarilyimply any major change in the stress field which

TECTONIC EVOLUTION OF JERSEY 11

produced the 0] folds in the metasediments. Thevolcanics also display similarly oriented periclinal andsingly plunging folds. It is suggested, therefore, thatalthough there is a considerable hiatus separating thetwo sequences an east-west directed maximumprincipal compressive stress was active during andafter flysch sedimentation and well into the period ofvolcaniclastic deposition (Fig. 7a & b).

(b) O 2 Phase

During O2 the maximum principal compressive stressdirection was north-south. It was responsible forrefolding 0] folds, produced the major easterlyplunging St. Ouen anticline and imposed a markedsinuousity on the OJ, originally north-south structuralgrain of both the metasedimentary and volcanicsequences (Fig. 6c). Several major O2 folds in theJersey Shale Formation can be traced directly into theoverlying volcanics. Compare (Fig. 4), for example,the St. Saviour Anticline and adjacent syncline in thevolcanics with the folds outlined by the beddingform-lines in the Jersey Shale Formation just belowthe nearby volcanic rock/metasediment junction. It istempting to consider the cause of the north-southcompression to be in some way connected with theintrusion of at least part of the granite complexeswhich comprise the northwestern, southwestern andsoutheastern corners of the Island. If the graniteoutcrop around Belle Hougue Point (655564) repre­sents the eastern continuation of the northwest graniteand the southwest and southeast granites areconnected as shown in Fig. 2-4, then the Jersey ShaleFormation and Volcanic Group are seen to occupy anarrow eastwest elongated zone bounded by intrusiverocks. In fact, Briden et al. (1981) suggest that thewhole of the Island is underlain at shallow depth byCadomian granite.

It is interesting to speculate that the O2 structuresmight owe their eastwest axial pattern to the presenceof the bounding, relatively more competent intrusiverocks acting as rigid blocks against which theBrioverian sequences were compressed. Alternatively,O2 compression and intrusion may have been coeval.Indeed, the granites frequently display several sets ofstrong foliations (Fig. 6f) some of which may berelated to fabrics in the Brioverian succession.

(c) 0 3 Phase

Following the emplacement of the plutonic rocks andpost-D; the area was uplifted and eroded. Some of theerosion products accumulated in erosional hollowsto form the post-orogenic Rozel ConglomerateFormation which, as a result of renewed NE-SWcompression, now forms a north-westerly trendingopen flexure, the Rozel Syncline (Fig. 4) of 0 3 age.

(d) 0 4 Phase

Apart from the fold-pair about 1 km southwest of St.Ouen's Church, described in section 3b which, purelyon grounds of orientation, is ascribed to the 0 4 phase,the only other certain folds of this generation are themain northeasterly plunging Trinity Syncline in thevolcanics and the southwesterly plunging St. HelierSyncline. The sinuous outcrop of the Rozel Con­glomerate and the curved trace of the 0 4 syncline mayalso be the result of 0 4 flexuring (Fig. 6d).

(e) Radial Faults

The age of the arrays of radial fractures (Helm, 1983)is uncertain. They clearly post-date the O2 structureswhich they cut on the St. Ouen's Bay reefs but havenot been recognized elsewhere where late tectonicfractures occur.

This section has attempted to formulate a coherentmodel for the tectonic history of Jersey. It is based onthe structural sequence established in the Jersey ShaleFormation of the St. Ouen's Bay area (Helm, 1983),its application to the main outcrop, and comparisonwith the structure of the overlying volcanic rocks andconglomerate. Because of the absence of observablefolded contacts, the relationship between the foldsaffecting the metavolcanics and those of the underlyingJersey Shale Formation presents a problem. Moreov­er, within the volcanic rocks there is no convincingevidence for the sequence of deformation. Neverthe­less, comparison of fold geometry, orientation andgeneral disposition of fold associations allows theconstruction of a reasonably unified tectonic schemewhich also incorporates the structure of the RozelConglomerate Formation (Figs. 4 & 7).

5. DISCUSSION

Having recognized the polyphase nature of thedeformation of the Jersey Shale Formation, Squire(1974) distinguished a number of folds and associatedminor structures of ages ranging from Cadomian toVariscan (his table 33). Unfortunately none of themaps or diagrams feature structures of specified agesso that at times it is difficult to relate the structuralsequence to the mapped features.

Squire believed the earliest phase of deformation,Cn FrViducastian, i.e. main phase of the Cadomianorogeny, (q. v. Table 2, this paper), to have producedoriginally very open east-west trending upright foldswhich were later given a steep plunge by refolding.However, no structures of Cn F2 age, were specified.

The second phase of folds (Cn F3) , of late Cadomianage, were described as being open, gently plunging,asymmetrical, with a northeasterly vergence and. apoorly developed axial planar cleavage. Squiresuggested that the development of these structures was

12 D . G . H ELM

ROl e l Conglome ra te Formation

Vo lcan ic G roup

J er sey Sha le For m ation

Ba sem ent

Gr an ite

Fig. 7. Simplified block diagrams to show relationship between stress field, rock units and major structures at various periodsduring the tectonic evolution of Jersey, a) 0, ; b) late 0 1 ; c) O2 and d) 0 4 ,

contemporaneous with the empl acement of thegranites .

Variscan structures (VF1) were thought to havebeen produced mainly in the brittle field producingfaulting and kink bands. He ascribed the weak ,northeasterly trending flexuring of the Rozel Con­glomerate to this same period of deformation.

Under the heading of superimposed folding Squiredescribed three areas in the intertidal reefs of St .Ouen's Bay: west of La Crabiere , west of Les Laveursand southwest of La Bouque fault. The so called'Domes and Basins' area (Casimir , 1934) off LaCrabiere was used to illustrate the essentiall y similar

features of each of the areas regarded by him as beingthe result of superimposed folding . The periclineswere attributed to two sets of progressive foldingduring a single progressive deformation . The area wassaid to exhibit an open symmetrical structure with asoutheasterly plunge.

Having rejected a variety of possible mechanisms offormation , ranging from soft sediment deformation toigneous doming , Squire opted for a tectonic origin onthe basis of the north-south and west-southwestoriented cleavages that he observed. It was suggestedthat the folds were produced by the NE-SWcompression generated by the intrusion of the

TECTONIC EVOLUTION OF JERSEY

TABLE 2. Comparison of tectonic schemes

13

Squire (1974) Helm (this paper)

Deformation phase Structural trend

NE-SW to ENE-WSW(Rozel Conglomerate only)

NW-SEE-W

Deformation phase

D4}post-Cadomian

D3

D2} CadomianDr

Structural trend

NE-SWFlexures in RozelConglomerates;Trinity and St. HelierSynclinesNW-SE(Rozel Conglomerate only)E-W

N-S

adjacent granites. His sketch plan of the La Crabiereexposure shows a system of sub-circular and ellipticalfolds that differs from Casimir's map only in detail.Squire argued that the domes and basins were aby-product of the major NW-SE phase.

In common with most volcanic sequences correla­tion of units within the Jersey Volcanic Group isdifficult. However, the establishment of the tripartitesuccession by Mourant at an early stage provided theessential framework on which Thomas (1977) was ableto base his work. Mourant's perceptive paper (1933)also laid the foundation for a fuller understanding ofthe structure of the volcanic rocks. For example, heinferred the existence of a synclinal axis runningthrough Bouley Bay (673545) and suggested that themain volcanic outcrop was disposed in a northeastplunging synclinorium separated by an east-westanticline from the southwesterly plunging St. HelierSyncline (644488).

Thomas (1977) confirmed Mourant's structuralpropositions and in addition recognized the three setsof folds mentioned above. He attributed the formationof several small domes and basins to interferencebetween north-south and east-west folds. FollowingSquire's structural scheme for the Jersey ShaleFormation, Thomas accepted that an early east-westphase was also present in the volcanics, pre-dating hisown north-south phase. Thomas also described foldsplunging gently NE, in the Rozel ConglomerateFormation, which he tentatively correlated with thebroad upwarp in the vicinity of Bequet Vincent (6552).Determination of the relative ages of the N-S and E-Wfolds was precluded by the lack of evidence ofoverprinting, but he assumed that Squire's early E-Wtrending folds in the Jersey Shale Formation might beequated with E-W folds in the volcanics, thuspre-dating the N-S trending folds. Attention wasdrawn to the fact that no equivalents of Squire'sNW-SE (Cn F3) folds were present in the volcanics,

thus supporting Squire's contention that such folds inthe metasediments were caused by the nearby graniteintrusions.

According to Squire the main tectonic featuresmanifested in the Brioverian metasediments of Jerseymay be summarised as follows: Firstly, MainCadomian, E-W very open folds; secondly, NW-SEtrending folds associated with emplacement of theJersey granites and with tightening of the first phasefolds. After tilting and early brittle fracturing ofsupposed Caledonian age, Variscan compression gaverise to northeast-southwest very open folds, withassociated shears and joints. Thomas (1977) could findno equivalent of Squire's NW-SE, second phase foldsin the Volcanic Group but in essence accepted Squire'stectonic scheme.

The present writer has been unable to find anyevidence for an early north-south phase of compres­sion: rather the earliest movements appear to havebeen the result of an east-west compression whichproduced the general north-south tectonic grain. D 1folds (this paper) almost certainly equate with thesecond phase of deformation in Squire's chronology. Itis possible that his earliest, east-west folds might beequivalent to D2 in the present author's scheme. Thetwo structural sequences are summarized andcompared in Table 2.

6. CONCLUDING REMARKS

Whilse it is accepted that correlation of deformationphases on the basis of orientation alone may besuspect, it may prove fruitful to make the followingobservations.

Firstly, it is interesting to note that folds producedduring the first phase of deformation in Jersey have anortherly trend as does the general strike of bedding inthe Pleinmont area of Guernsey. Indeed the generalstructural grain of much of Jersey is northerly. Also,

14 D. G. HELM

Sutton and Watson (1957) mapped a N-S trending foldsystem in Sark. Secondly, in western BrittanyBradshaw et al. (1967) strongly suspected theBrioverian was affected by an early Cadomian('pre-Fe'), possibly N-S, phase of isoclinal, possiblyrecumbent folding. Further, Cogne (1959) noted aNNE foliation in the Pentevrian of the St. Brieucregion.

It would seem reasonable, therefore, to postulatethat one of the earliest effects of pre-Cambriandeformation in Armorica was to produce N-Sstructures. In St. Brieuc they seem to be pre­Brioverian because they are apparently (Cogne, 1959)overprinted by an approximately E-W foliation whichalso affects the overlying Brioverian rocks. InGuernsey the relationship between the Pentevrian andBrioverian rocks is not clear. The N-S structural trendproduced by the first phase of deformation D] inJersey may in fact have been influenced by earlierlineaments in an unexposed basement complex.

Secondly, the D2 structures in Jersey share the samegeneral E-W trend as the major Cadomian (Viducas­tian) folds in mainland France with which they may be

tentatively correlated. In western Brittany Bradshawet at. (1967) recognized an E-W phase of folding andattributed it to the main Cadomian movementsalthough subsequently there have been attempts toequate the E-W phase with the Variscan orogeny.Because structures of D2 age appear to be absentfrom the Cambro-Ordovician Rozel Conglomerate inJersey this phase is thought to be more likelyCadomian than Variscan in age.

ACKNOWLEDGEMENTS

The author is grateful to professors J. Watson and A.W. B. Siddans and Drs. A. C. Bishop, K. R. McClay,J. Renouf and B. Roberts for suggesting usefulimprovements to the final draft of the paper. Miss P.Hurndall is thanked for her assistance in obtainingfield data and for very helpful discussion. Mrs. D.Norman is thanked for typing the paper and Mr. T.Easter for his photographic assistance. Thanks are alsoextended to Goldsmiths' College Research Committeefor generous financial assistance.

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