The structure and tectonic evolution of the Jersey Shale Formation,St. Ouen's Bay, Jersey, Channel Islands

D. G. Helm

HELM, D. G. 1983. The structure and tectonic evolution of the Jersey Shale Formation, St.Ouen's Bay, Jersey, Channel Islands. Proc. Geol. Ass., 94 (3), 201-216. The Precambrian(Brioverian) flysch deposits of St. Ouen's Bay have been subjected to polyphase deformation.The first phase (D]) produced both periclinal and coaxial, singly plunging folds. The periclinesprobably represent early formed noncylindrical buckles initiated by irregularities in the beddingdue to channel infills. The periclines are characterised by very rounded profiles in contrast tothe more angular, singly plunging folds which succeeded them. A pressure solution, spacedcleavage (Sj) is axial planar to the singly plunging folds but cuts obliquely across the periclines.

The original approximately north-south axial trend of the D, structures was considerablymodified by the subsequent north-south Dz compression. D, deformation, which was much lesssevere than D], produced a prominent, open, easterly plunging major anticline together withparasitic folds, a system of conjugate faults and a non-penetrative axial planar fabric (Sz). Astriking array of radial fractures attributed to vertical stress generated by the uprise of basalticmagma which infills some of the fractures, overprint D, and Dz structures.

Department of Geology, Goldsmiths' College, University of London, Rachel McMillan Building,Creek Road, Deptford, London SES 3SU.

1. INTRODUCTION

The Precambrian supracrustal rocks of Jersey consti­tute two distinctly different formations, onemetasedimentary, the Jersey Shale Formation; theother, a younger, volcanic sequence, the Jersey Volca­nic Group. Both sequences have been assigned tothe Upper Proterozoic, Brioverian System (Graindor,1957) the main outcrop of which occupies much ofArmorica. The relationship between the Jersey ShaleFormation and Graindor's threefold scheme for theArmorican Brioverian is uncertain, though trace fos­sils recorded from the Jersey Shales by Squire (1973)indicate an Upper Brioverian age for themetasedimentary part of the sequence. Radiometricdating places Brioverian sedimentation and volcanismbetween 900 and 580 Ma. Renouf (1974) provides acomprehensive review of the geochronology of theJersey sequences.

Table 1 shows the stratigraphical sequence of theJersey supracrustal rocks. The paucity of fauna andlack of distinctive marker horizons within the JerseyShales precludes subdivision of the formation.

The Jersey Shales are deep water siliciclastics dis­playing' all the features that characterise depositionfrom turbidity flows. Well defined coarsening andthickening, but also, fining and thinning upwards sequ­ences have been recorded. Further sedimentologicalinvestigation is in progress (Helm & Pickering, Un­publ, data).

Despite the fact that the Brioverian metasedimentswere regionally metamorphosed to lower greenschistfacies and contain as much, if not more, psammiticthan pelitic material, it is proposed to retain the priorterm, Jersey Shales.

TABLE 1. Jersey supracrustal rocks

ApproximateStratigraphical succession thickness

L. Palaeozoic Rozel Conglomerate Formation 100-600 munconformity

emplacement of graniteand dioritised granite

Jersey Volcanic Group 2500mBrioverian ?unconformity

Jersey Shale Formation 8000 m(minimum)

Cleaved mudrocks, siltstones and greywacke sand­stones with locally developed greywacke conglomer­ates comprise the Jersey Shales. They crop out mainlyin St. Ouen's Bay on the west coast (Fig. 1), and in thesouth west corner of St. Aubin's Bay. Numerous smalland mainly isolated exposures occur inland. There aretwo small inliers, one is centred on Le Bourg and theother extends for about 0.75 km inland from Gorey onthe east coast. Exposures of similar extent occur inand to the east of Giffard Bay on the north coast. Theintertidal reefs of the Jersey Shales in St. Ouen's Bay(Fig. 1) provide nearly continuous exposure in a bandabout 1.0 km wide and 4.0 km long between Le Pina­cle, where they are in contact with the Northwestgranite, and L'Ouziere. Then there is an exposure gapuntil Le Braye is reached where the Shales reappear.500 m further south they are in contact with the South­west granite. The St. Ouen's Bay area was selected forthe present study because it is only here that the

201

202

St Ouen's Bay

~//

L'Ouziere

D. G. HELM

St.Aubin's Bay

+- ........ +- .. + ~ .. - .

.. .. .. .. .. -<- .

B Rozel Conglomerate Formation

Iv v vi Volcanic Group

o Jersey Shale Formationa Plutonic igneous

Fig. 1. Simplified geological map of Jersey.

Jersey Shales allow a complete tectonic sequence to beestablished.

Sporadic and limited observations on the structureof the Brioverian of Jersey have been made since 1851when Transon published a map which, however,showed few details of the Brioverian structure. Thefirst detailed map of parts of the Jersey Shales outcropwas produced by Casimir in 1934. In 1955 Casimir andHenson published a large scale map of Giffard Baywhere the contact between the two Brioverian forma­tions may be seen. Relatively recent and more detailedwork on the Jersey Shales is to be found in Squire'sunpublished Ph.D thesis (1974) on the Brioverian ofthe Channel Islands and Armorica.

The present paper presents the results of mappingcarried out between 1979 and 1982 using ground con­trolled aerial photographs at scales ranging from1:3000 to 1: 16.

The aims of this paper are:(1) To describe and account for the major and

minor tectonic structures of the Jersey Shalesexposed in St. Ouen's Bay, and;

(2) to elucidate the structural evolution of thismetasedimentary sequence.

2. TECTONIC SETTING

Tectonically Jersey belongs to Armorica where thePrecambrian succession consists of a gneissose base­ment complex, the Pentevrian, isotopically dated atbetween 2620 and 900 Ma (Adams, 1976) and over­lain by supracrustal rocks, the Brioverian. No isochrondates are available for the metasedimentary divisionthough an approximate age for the close of sedimenta­tion can be inferred from the fact that in Guernsey thePleinmont metasediments (of presumed Brioverianage) are intruded by the L'En5e adamellite from whichAdams (ibid.) obtained dates of -660 Ma. A wholerock Rb-Sr isochron age of 533 ± 16 Ma, determinedfor the Jersey andesite formation led Duff (1978) to

JERSEY SHALE FORMATION . CHANNEl. ISLANDS 203

suggest that Brioverian volcanism continued into theCambrian. Bishop and Mourant (1979) take the viewthat the 533 Ma isochron reflects the date of cooling ofthe andesite and that the weight of abundant isochrondates from other Channel Islands rocks, together withgeneral geological evidence, indicates a minimum ageof 580 Ma for the Jersey Shales.

In mainland France the main outcrops of Pentev­rian metamorphic rocks occur in the regions of theBaie de St. Brieuc and Coutance. Pentevrian meta­morphic rocks also occur in Guernsey, Alderney andpossibly in Sark but they are absent from Jersey. Theunconformity between the Pentevrian and Brioveriancan be demonstrated only near St. Brieuc (Cogne,1959). The structural trend in the metamorphic base­ment is N-S in Guernsey (Roach , 1957) and variesfrom NW/SE to NE/SW in Brittany (see e.g. Grain­dor, 1960).

The Brioverian of Normandy and Brittany displayspredominantly ENE/WSW to EIW structural trends,whereas in Guernsey and Jersey the general trend isN/S.

3. STRUCTURE OF THE BRIOVERIANMETASEDIMENTS OF JERSEY

The structure of the St. Ouen's Bay area is deceptivelysimple . It is dominated by an easterly plunging majorfold clearly outlined by the main north to southchanges in bedding strike (Figs. 2 and 3). If localcomplexities due to minor folding and refolding aredisregarded the general strike of the Jersey Shales isseen to swing from north west at the northern end ofthe outcrop through a broad curve, convex towardsthe east, to south west at the southern end of the bay(Fig. 1) The gentle to moderately steep, and some­times overturned, bedding dips and youngs mainlytowards the east . However, detailed analysis hasshown the area to be far more complicated than thesuperficially simple structure outlined above mightindicate . Two main phases of deformation , 0 1-02 ,

represented by folds and axial plane fabrics, have beenrecognised. They are overprinted by a late radialfracture system . The sporadic appearance of north­south oriented, closely spaced joints, may indicate afourth deformation (foliation of uncertain affinity , Fig.2). No obviously associated folds have been detectedand the age of the fabric has not been established. Asimilarl'y oriented , and possibly related fabric in theadjacent Northwest gran ite (Fig. 2) is currently beingfurther investigated. The structural sequence was de­termined using criteria such as fabric overprinting,refolding of axial surfaces and relative intensity ofdeformation.

(a) D.-FoldsThe area was divided into 8 domains, each of whichconstitutes a relatively homogeneous structural sub­area. Two types of early folds have been distinguished,

one set is singly, the other doubly, plunging (periclin­al) (Fig. 4). The periclinal folds are unevenly distri­buted (Fig. 2). There are none in domain 6, only onein domain 8 but they are the predominant fold type inthe northern half of domain 7. In plan the majority ofpericlines have an axial ratio of 1: 2 but may showgreater elongation with an axial ratio of 1:7. The longaxis of the largest pericline measured approximately125 m but most fall between 25 and 40 m. The curva­ture of their axial traces has been effected by laterdeformations, principally O 2, Axial trends range fromWNW, through N-S to SW reflecting the influence oflater movement phases. The periclines are generallyasymmetrical, open to close, gently plunging (10--25°)and have a mainly westerly, slight to moderate verg­ence (Fig. 5b & c). Less commonly they are upright orpossess a slight easterly vergence. Their profiles showvarying degrees of roundness. On a PQR diagram(Williams & Chapman, 1979) the periclines plot veryclose to the centre. They are primary periclines, simi­lar to those in the Cambrian of N. Wales attributed byHawkins and Jones (1981) to tectonic 'quilting', i.e.their culminations and depressions are original fea­tures and not the result of fold interference .

The singly plunging folds are open to close andoccasionally isoclinal (Fig. 6a) . They are commonlyasymmetrical, generally possess a Z-shaped profile ,and have a mainly gentle to moderate south-easterlyor south-westerly plunge. They characterise domains 2and 5 and are well developed on the southern marginof domain 7. In domain 4 they are closely associatedwith periclinal folds . The singly plunging folds areweakly disharmonic (Fig. 5a) and show a variableamount of thickening in the hinge. For example westand southwest of La Saline (Fig. 2) the fold amplitudeis seen to be progressively reduced from a maximumof about 100 m to zero through a thickness of 300 m.The folds have weakly convergent dip isogons, theorthogonal thickness varies but the bedding thicknessparallel with the axial plane is not truly constant, i.e .they exhibit features typical of flattened parallel folds(Ramsay, 1962). The wavelength of maximal foldsvaries from 50--100 m. The largest and most abundant0 1 folds are to be found in domain 2 where they havea south-easterly axial trend. Somewhat less conspi­cuous 0 1 folds in domain 3 have a mainly south­westerly orientation. This sharp change in orientationacross the boundary between domains 2 and 3 (Fig . 7)can be shown to be due largely to reflexing by O2, Atall scales the majority of the singly plunging 0 1 foldshave a dextral vergence. It is suggested that they areparasitic folds on the eastern limb of an inferred majorOJ anticline whose axial trace must lie some distancewestwards from the outcrops in the intertidal reefs.

(b) Dj-CleavagesWith one exception , the anticlinal pericline due westof Les Laveurs (Fig . 9a), the periclines lack any

204

55

100m'----'

87 62

~ :-= ~-~ : .;;t. ",'77;-;-, -

'/ ~~,:'''...,, " . ~ ...., • I 66, ,: :~...>.-

..

D . G . HELM

_.~-"';''::.;.- --. t c-:; ....::..

Fig. 2. Detailed structural map of the Jersey Shales of northern half of St. Ouen 's Bay. Stereograms show poles to bedding inrelatively homogeneous structural domains . The figure in large type refers to the number of the domain and the adjacentfigure in smaUtype gives the total number of points . Contour intervals in percentages are also shown against each stereogram.A-B, C-D and E-F are lines of section in Fig. 5.

obvious mesoscopic fabric . However, the D1 singlyplunging folds often possess a relatively strong axialplanar cleavage, Sb for example west of La Saline. Inthe case of the latter folds the development of cleav­age varies from weak in the more psammitic to mod­erately strong in the more pelitic units (Fig. 8a). It is a

pressure solution, spaced cleavage, the spacingapparently being controlled by the size of the largerclasts (Fig. 8c). The reason for the difference in degreeof cleavage development between the periclines andthe singly plunging folds is not entirely clear. Twostages of development may be indicated; the initial

JERSEY SHALE FORMATION, C H A N NE L ISLANDS 205

Fo liat ion 0' uncert a inaff in ity

.......................... ........ ...... ~

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7 15~

o@,

;)

Bedd in g

Faull

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La Crabrer e

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periclinal buckling having taken place largely by flexu­ral slip whereas , with increasing compression, laterstages of deformation resulted in the production of thesingly plunging folds with an associated axial planarcleavage . Evidence to support this view may be seenjust west of Les Laveurs (Fig. 9a) where a nest ofpericlines is seen to be cut by a well developed spacedcleavage which, although superficially axial planar tothe folds , is actually slightly oblique to their axial

traces . The cleavage has the same general trend asmany of the singly plunging folds. It would appear thatthe periclinal form had been established prior to theimposition of the oblique spaced cleavage. There is afaint lineation on the bedding surfaces , often markedby very thin quartz veins parallel with the axes of thepericlinal folds.

This feature has been recognised in other periclinesin the St. Ouen's Bay exposures and occasionally may

206 D . G . H ELM

N

Fig. 3. Stereogram of total bedding poles. Contours drawn at 0.2; 0.6; 1.5; 3 and 5 per cent. 501 poles .

be clearly seen to be cut by the later spaced cleavage.Howe ver, the two events are considered to be endmembers of a deformation continuum although usuallyeach group bears an easily recognisable tectonic'stamp'. The relationship between SI and the peri­clines in St. Ouen's Bay may be compared with thesynchronously developed non-axial plane cleavagesand F, folds in the Southern Uplands of Scotland(Stringer & Treagus, 1980).

(c) Dz-FoldsAlthough locally the 0 1 folds are the most conspi­cuous structures, the feature which largely determinesthe topographic expression of the reefs in St. Ouen'sBay is a major O2 fold . The swing in strike of thebedding is clearly distinguishable even on the HuntingSurvey 1: 25,000 topographic map of the St. Ouen'sBay reefs. From Le Pinacle (Fig. 1) to a line extendingdue west from Le Pulec the overall strike is north­west/south-east. Traced furth er south the strike of thebedding is seen to have swung round towards thesouthwest thus forming a major , steepl y plunging D2anticline. With an estimated wavelength in excess of

3 km and a minimum amplitude of 1.5 km this is thelargest of the folds in the St. Ouen 's area. The changesin orientation of the axial traces of 0 1 folds are seen toparallel the variation in the general strike of thebedding, clearly demonstrating the effect of O2 onearlier formed structures (Fig. 10). On a smaller scalethe effect is equally striking, as shown for example bycross-cutting cleavages (Fig. 9b) and curvilinear foldhinges (Fig. 9c). In contrast to the 0 1 (singly plunging)folds, which are often rather angular in shape andhave a style approaching that of classical chevrons, theO2 folds have rounded hinges and curved limbs andgive rise to a sinusoidal pattern. The plans and aerialphotographs of O2 folds provide a good indication oftheir shape obviating the need to illustrate fold styleby means of cross-sections.

Open to close parasitic folds are developed on eachflank of the major anticline (Figs. 1 and 2). In thethickly bedded psammitic succession to the south ofthe seaward end of La Bouque, the parasitic foldshave wavelengths of between 50 and 100 m and ampli­tudes of 5-10 m. Folds with similar dimensions andshape occur in psammites near the shore between La

J ERSEY SH A LE FORMATION , C H A NNEL ISLANDS

~2'

fa ultBeddingA xia l t rece > 01

--- ... Ax ia l tr c ce - 0 2Dyke

207

Fig. 4. The so called 'domes and basins' area (Casimir, 1934) off Crabier e, showing that the majorit y of folds are in fact singlyplunging. The map also shows relationship between chevron folds and periclines of 0 1 age. Note refolding by O2 and radialfaults with associated dykes.

Saline and Les Laveurs. Whereas the wavelength offolds in contiguous pelites remains similar to those inthe psammites , in some instances their amplitudeattains almost half the wavelength .

Dz folds have convergent or weakly convergent dip

<, ,

Fig. 5. Profile sections of 0) folds. The lines of the sectionsare shown in Fig. 2.

isogons and hence a more or less parallel form with aconcomitant degree of disharmony . However , it isinteresting to note that the pattern (Fig. 10) of spacedcleavage and close joints, as strikingly revealed bystereoscopic examination of the air photographs, local­ly shows parallel form lines resembling the dip isogonsof typical similar folds.

(d) DrCleavage (Sz)

The persistent vertical to steeply inclined cleavage (Sz)has a mean strike of 0800 and may be represented by afracture cleavage, close jointing or kinks (Figs. 8b and9b) . The angle the fabric makes with the bedding isoften close to 90° and usually not less than 65°. Thinveins of quartz frequently emphasise the structure.Rocks of similar lithology and thickness with a closelycomparable sub-parallel fabric arra y have been de­scribed by Price and Hancock (1972) from S. Walesand the Southern Pyrenees . In these instances the fabric isthought to have been caused by hydraulic fracturing(Secor , 1968) and, in the case of beds with low dips, to

208 D. G. HEL~

a

Fig. 6. (a) Upright, near isoclinal, D1 fold south-west of Les Laveurs; (b) Dolerite dyke assoeiated with radial faults nearPetit Etaquerel; (e) Slightly oblique aerial photograph of the same area (6b). The width of the parallel sided radial fractures inthe centre of the top edge of the photograph is approximately three metres.

3Z':dyke

Jf younging

.<,/ ' cleavage

T bedding

JERSEY SHALE FORMATIO N . CHAN NE L ISLANDS 209

Fig. 7. Hinge region of major D2 fold between Slip de L'Ouest and La Saline (Fig. 2) showing re-orientation of D1 folds andcross-cutt ing S2 at junctions between domains 2 and 3.

have resulted from fluctuation in differential stressduring the formation of serial folds . S2 in the JerseyShale Formation appears to have a similar originforming after or towards the close of the formation ofthe folds in which it occurs.

(e) DrConjugate Faults

The almost bare surfaces of most of the rocky fore­shore afford near perfect conditions for recording themultitud e of large and small fractures that fragmentthe outcrop. Most faults are vertical or sub-verticaland their traces on the ground range in length from afew centimet res to tens of metres. Evidence fromhorizontal slickensides and local displacement of litho­logically distinctive beds , indicates that the movementon a considerable number of faults has a transcurrent

component. The absence of marker units of large scalemakes it difficult to determine the amount of horizon­tal movement but the continuity of structures acrosseven the faults with greatest topographic expression,e.g. La Bouqu e (Fig. 2), would suggest that the grossstructure of the St. Ouen's area has not been signi­ficantl y modified by wrench faulting. In some cases,fractures, separating adjacent blocks showing noappr eciable relative movement , can be traced overdistances of up to 50 m.

Collectively the faults virtually box the comp ass(Figs. 11 and 12). However , it is possible to filter outtwo discrete groups of faults within this otherwiseseemingly random patte rn. Of the 300 faults mea­sured , about 15 per cent occur as clusters radiatingfrom a number of centres (Fig. 12) whilst the remain­der show conjugate grouping with strongly pre ferred

210 D . G . H EL M

a

Fig. 8. (a) Mudrock opposite La Saline showing close spaced, S1> left to right ; cutting sub-vertical bedding sloping towards theleft of the photograph; (b) Photomicrogr aph (PPL) of D2 kink band developed in SI> left to right, scale bar 4 mm; (c)Phot omicrograph (PPL) showing S\ pressure solution cleavage, scale bar 100 /1, with spacing related to size of quartz andmuscovite/chlorite grains; (d) Photomicrograph of virtually undeformed dolerite dyke rock with amygdales of twinned calciteScale bar 0.75 mrn.

orientations ranging between 050°--095° (Fig. 11). Theradial faults are described in more detail in the nextsection.

In individual , structurally homogenous domainsthere is a close correlation between the orientation ofthe bisector of the acute angle (28) between thegroup s of conjugate pairs of faults and the generaltrend of 52 and also the axial traces of the major andminor D2 folds (cf. Fig. 10 with Fig. 11). Note alsohow, except in domains 3 and 4, the bisectors of theconjugate sets fan around the maximal D2 fold. Thestrong south easterly element of the plot of faults andfractures in Domain 4 clearly follows the trend of theaxial trace of D 1 folds which may therefore be coeval.In Domain 4 there is a strong bimodal tendency,probably the product of fracturing of both D] and D2

age. In most instances the conjugate sets with thepredominant mode have a 28 less than 45" and areoriented symmetrically with respect to the folds withwhich they are associated. The maximum principalstress direction would thus appear to be coincidentwith the 'c' axis of the folds . Following Han cock andAtiya (1979) it is suggested that the conjugate sets arepaired extension fractures. This explanation accordswith the suggestion above that relaxation of the D2compression was largely respons ible for producing 52'

The amount of displacement is usually much lessthan 10 m and often is negligible (Figs. 2 and 4).Mutually interfering pairs or sets of faults and faultswith no offsets are common and similar to thosedescribed by Horsfield (1980) which he ascribed tocontemporaneous movement along crossing norm al

211

Fig. 9. (a) One of the group of periclines due west of LesLaveurs (Fig. 2) illustrating the non-axial planar cleavage, SI>dipping steeply to the right (WSW). In this view fold plungeis towards the top of the photograph. 82 (striking 080°) formsthe left to right bevels in deep shadow and truncates theclosure of the periclinal anticline; (b) Small scale, Db parasi­tic periclines immediately to the north of pericline shown in9a. Steeply dipping, 81 , left to right (striking 120°) cut anddisplaced by S2 (060°) sloping to left of photograph; (c)Culminations and depressions (axial traces parallel with ham­mer shaft) in D 1 pericline off La Crabiere (Fig. 2) resultingfrom Dz compression. Strike of D, axial trace (parallel withhammer head) varies along the pericline between 340° and180°.

Lot Sa lin e_ -i

100m

~ a xi a l 1race . 0 2b ed dltlg

52

Fig. 10. General trend of bedding and relationship between Dz folds and form lines for S2 based on photolineaments and fieldobservations.

212 D. G. HELM

/'

Fig. 11.

'\ -, ! 100m,

D \ !

2 and later f --,1-racture p

atterns plott d -------e as rose diiagrams in relatively homogeneous structural d .omains.

/~Les Laveurs

___L_a_s_al_in:=,e~

La Crabiere i

~ ~

radial fractures

basic dykes

with detailed inset to show radial fault pattern (unornamented) and

".... ;'

Slip de 1.'0"//' uest

/

Fig. 12. Distrib .relationshi t uti~n of post-Dp 0 contiguous beddi 2

f(racture arraysmg dotted I'Illes).

213

F'Ig. 14, Photoco ' ,fold -pair d rnpilation sh .

h

evelop d owmz D .c anges la e at site f - I' sinelv I 'rgely controlled bv o. : ha nnel fi Ji ' p .ungmg

(SE). sed Ime nta tion ' Bcd thickn ess

--=-= u x ea l t r ace 0 ,beddmq

Fig. 13. General beddi .ing trend with D fI olds show '

~~~~~~~~ mg systern ati '

"} r _. IC variation . ." < m axial traces oro ' rI lolds.

214

I

D . G . H ELM

La Sa line

SOm

Fig. 15. Stereo pair of the intertidal reefs between Les Laveur s and La Saline (Fig. 2) illustrating major 0 1 and O2 folds.

faults . This is considered to be further evidence insupport of the extensional nature of the final phase ofD2 ·

<0 Radial Faults

Aerial photographs, at 1: 3000 and larger scales, re­veal a pattern of radial faults (Figs. 6c, 11 and 12)distributed about discrete centres on a line running

southeastwards from Le Pulec to the reefs opposite theKempt Tower (south of La Crabiere). Man y of theradially disposed faults occur in the form of narrowand deep (up to 2 m), steep-sided miniature graben(Fig. 6b) with walls and floor that are composed ofmetasediment. In a number of other cases the slotshave resulted from the erosion of dolerite dykes.Tho se parts of the dykes which are still exposed arerelatively unaltered , essentially undeformed and are

J ERSE Y SHALE FOR M A T ION . C H ANNEL ISLANDS 215

amygdaloidal (Fig. 8d). It is possible that other slotswhich are partly sand and gravel filled or perm anentlyunder water also contain dolerite.

The association of rad ially arr anged vesiculardolerite dykes and small graben would seem to requ irethe generation of hydrost atic pressure exerted frombelow. A similar configuration of regularly arrangedradial dykes in the Spanish Peaks area of Coloradowas att ributed by Ode (1957) to pressure in the sur­rounding country rock induced by magma risingthrough a vertical cylindrical conduit. It is postulatedthat comparable conditions obtained in the St. Ouen 'sarea sometime after the formati on of D2 structureswhich are clearly overprinted by the radial fracturesystem. As might be expected the attitude of thebedding is locally disturbed. For example near PetitEtaquerel and in the area about 400 m due west of LesLaveurs (Fig. 12) the bedding shows pinching andswelling.

4. SUMMARY

The first tectonic event, Db involved the formation ofboth the singly plunging and the doubl y plunging folds

c ]

(Fig. 13) without a significant change in the overallstress field; they may even have grown synchronouslyin the manner suggested by the experim ents of Dubeyand Cobb old (1977) , i.e. the initiation of periclineswith markedly round ed hinges which then becameelongated and assumed a chevron style in the region ofculminations. In the case of the Jersey Shale Forma­tion , the required sites of inhomogeneity in the multi­layered sequence that seem to be a pre-requisite forthe initiation of periclines (ibid .) , appear to have beenprovided by bed thickness variation as a result ofsedimentary channel infilling. (Fig. 14). Although thepresent orientation of their axial traces shows con­siderable variation, it can be shown to be systematicand due to flexuring by D2 folding (Figs. 10 and 15).Remo ving the effect of D2 by unfolding the folds (Fig.16), restores the original co-axiality of the two sets ofD 1 structures and indicates that 0 1 (D I ) was approx­imately east-west. The failure to appreciate the in­fluence of D2 on earlier structures in par t explains thedifficulty earlier workers have experien ced in estab­lishing the correct structural sequence in the JerseyShale Formation .

- - Be dd ing in 0 , f o lds- - - Ge nera l t re nd of bedding

.... Axial tr a c e ·01_ ._. - A xia l tr a ce -02

bl

Fig. 16. (a) Present patte rn of D , and D2 folds; (b) Pre-D, disposition of D] folds.

216 D. G . HELM

During D2, 01 was oriented more or less north­south except for the closing stages of deformationwhen the least principal stress occupied this position.

Finally. the area was subjected to a verticallyoriented maximum principal stress which gave rise tothe radial fault and dyke array.

Previous workers' attempts to assign the variousperiods of deformation to specific stratigraphical levelshave been unconvincing. All that can be said with anydegree of confidence is that the deformations arepost-Brioverian and probably mainl y Cadomian,although Variscan and Alpine mo vements may havemodified earlier formed structures .

Detailed structural studies of the remainder of theBrioverian outcrop, including the VOlcanic sequence,and its relationship to the youngest formation, theRozel Conglomerate, are in progress and will be pub­lished subsequently.

ACKNOWLEDGEMENTSProfessors J. Watson and A. W. B. Siddans and Drs.

A. C. Bishop, J . Renouf, K. R . McClay and B.Roberts are thanked for critically appraising themanuscript. The author benefited from collaborationwith two of his students; Mr. M. Glover , whose under­graduate project was concerned with the area aroundL 'Etaque and Miss P. Hurndall, who mapped the areabetween Les Laveurs and La Crabiere and whoseassistance in the gathering and processing of structur a ldata from the Jersey Shales in general is greatlyappreciated . Thanks are also extended to Mrs. D _Norman who typed the paper and Mr. T . Easter whoassisted with the photographs. The author is alsograteful to B.K.S. Surveys Limited, Co . Derry, forpermission to reproduce parts of their excellent, highresolution aerial photographic coverage of the Islandand to Mr. M. Balston who piloted the light aircraftwhich enabled larger-scale photographs to beobtained. Goldsmiths' College Research Committee isalso thanked for providing generous financial support.

References

ADAMS, C. J. D. 1976 Geochronology of the ChannelIslands and adjacent French Mainland. J. Geol. Soc.Lond., 132, 233-50.

BISHOP , A. C. & A. E. MOURANT. 1979. Discussion onthe Rb-Sr whole rock age determination of the JerseyAndesite Formation, Jersey, c.r. J . Geol. Soc. Lond, 136,121-2.

CASIMIR , M. 1934 Studies in folding of the Jersey Shales.Proc. Geol. Ass ., 45, 162--6.

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