Geological structure - Jersey: description of 1:25 000 Channel Islands Sheet 2
|From: Bishop. A. C. and Bisson. G.1989. Jersey: description of 1:25 000 Channel Islands Sheet 2. Classical areas of British geology, London: HMSO for British Geological Survey. © Crown copyright 1989.|
Chapter 7 Geological structure
Folds in the Jersey Shale Formation
The Jersey Shale Formation is best exposed at the northern and southern ends of St Ouen's Bay, and at St Aubin. In these and other areas the tectonic evolution of the formation has been studied recently by Squire (1974) and Helm (1983, 1984). Squire believed that the earliest folds in the Jersey Shale Formation, representing the main (Viducastian) phase of the Cadomian orogeny, were originally open in form, with horizontal E–W-trending axes, but that they were subsequently tilted to give steep plunges. He described a second set of folds with north-westerly axial trends and generally gentle plunges towards the south-east; these folds were open and asymmetrical, with average wavelength of about 700 m, axial planes inclined south-westward, and a poorly developed cleavage; he considered that this folding was also Cadomian and penecontemporaneous with the emplacement of the granite. Squire remarked that an E–W zone, dominated by gentle dips and folds with NW–SE trends, extends from Mont à la Brune [582 506] to Tesson Mill [617 508] and beyond.
Helm (1983) worked on the Jersey Shale Formation exposed in the intertidal reefs in St Ouen's Bay, and later (1984) extended his investigation to the remainder of Jersey. He recognised two main phases of deformation, D1 and D2 (Figure 18), but found no evidence for the early phase of N–S compression noted by Squire. The early (D1) folds are of two types, either singly plunging (Plate 15) or doubly plunging (periclinal). The axes of the periclinal folds trend from WNW–ESE, through N–S, to NE–SW; the periclines are generally asymmetrical, open to close, gently plunging, and mainly with a westerly vergence, but some are upright or have a slight easterly vergence. Helm concluded that the D1 periclines probably represent early-formed non-cylindrical buckles initiated by irregularities in the bedding owing to channel infills. The singly plunging folds are open to close, and some are isoclinal; they are commonly asymmetrical, generally with a Z-shaped profile and a mainly gentle to moderate south-easterly or south-westerly plunge; most have a dextral vergence. Many of the singly plunging D1 folds have a relatively strong, spaced, axial-planar cleavage, but no mesoscopic fabric is usually associated with the periclines. Helm suggested that the singly plunging D1 folds are parasitic on the eastern limb of an inferred major
D1 anticline, whose axial trace must lie some distance to the west of the outcrops in the intertidal reefs. He considered that his D1 folds might be equivalent to Squire's second set of folds. He also showed that in the 'domes and basins' area off La Crabière described by Casimir (1934) most of the structures are singly plunging D1 folds.
Although overall the dip of the bedding in the Jersey Shale Formation is eastward, there is a strip of westward-dipping beds running N–S through St Peter [596 516]. This local reversal is due to a major D1 fold pair, the St Peter Syncline and a complementary anticline about 1 km to the east.
The original approximately N–S axial trend of the D1 structures was modified by later N–S (D2) compression; this produced major folds, a non-penetrative axial-planar fabric (S2), and a system of conjugate shear faults. Helm's D2 folds are well exposed at the northern end of St Ouen's Bay. To the west of Le Pulec the strike of the bedding is NW–SE, but it changes to N–S and then NE–SW farther south, thus outlining a major anticline (the St Ouen Anticline), plunging eastwards and having a wavelength greater that 3 km and a minimum amplitude of 1.5 km. Helm noted that open to close parasitic folds, with wavelengths of 50 to 100 m, are present on each flank of the St Ouen Anticline; the associated cleavage is vertical to steeply inclined, and has a mean strike direction of 080°. The D2 folding has changed the orientation of the axial traces of the D1 folds, and on a smaller scale has resulted in cross-cutting cleavages and curvilinear Di fold hinges. Helm also recorded that the D1 and D2 folds have been overprinted by a system of late radial fractures attributed to vertical stress generated by the uprise of the basaltic magma which filled some of the fractures. In addition he suggested that the sporadic occurrence of closely spaced N–S joints might indicate that there had been a fourth deformation, but he detected no associated folds, although there was a similar fabric in the adjacent north-west granite.
Folds in the Jersey Volcanic Group
Benefiting from the lithological distinctions that he had made in the sequence of volcanic rocks, Thomas (1977) was able to recognise folds of several sizes and three orientations, namely E–W, N–S, and NE–SW. In the St Saviour's Andesite Formation at West Park, the St Helier Syncline plunges to the south-west (Figure 18) and has been intruded by granophyre with cross-cutting relationships. This fold is separated by an anticlinal area from the large northeastward-plunging Trinity Syncline in north-east Jersey. The Trinity Syncline affects both the Jersey Shale Formation and the volcanic rocks (Teilhard de Chardin, 1920; Mourant, 1933), but is modified by many smaller folds trending either E–W or N–S. The interference of the smaller folds has given rise to domes and basins, and some of the domes have brought andesite to the surface (commonly beneath loess) within the main Bonne Nuit Ignimbrite outcrop, for example north-west of Le Grès [669 524] and at intervals in the valley to the south.
Helm (1984) confirmed most of the structures in the volcanic rocks described by Thomas; he listed D1 folds, including a possible fold pair near Frémont Point and a syncline in Bonne Nuit Bay (both trending NNW–SSE), the E–W folds just north of Archirondel Tower (see p. 35), and a refolded and faulted syncline in Vallée des Vaux. D2 folds include anticlinal flexures that give rise to the inliers of the Jersey Shale Formation at Le Bourg and Gorey (the St Saviour Anticline; see p. 6). Helm also observed three previously unrecorded cleavages in the volcanic rocks, the general NW–SE, NE–SW and N–S trends of which suggest that they are axial-planar to similarly oriented folds mapped by him and Thomas, and described above.
Folds in the Rozel Conglomerate Formation
The palaeotopographical surface at the base of the Rozel Conglomerate has been folded into an open syncline (D3) with a sinuous axial trace trending WNW–ESE. Local departure from the NNW–SSE strike of bedding suggests that the main syncline may have been warped about NE–SW-trending axes (D4), though large-scale cross-bedding makes confirmation difficult; these flexures are associated with closely spaced joints and shear zones.
Helm (1984) and Richardson (1984) have shown that the Rozel Conglomerate has a strong, generally NW–SE-trending pressure-solution fabric associated with shearing and rotation of pebbles (Plate 16). Most of the pebbles show tectonic pitting, and the granitoid pebbles are extremely distorted and commonly interdigitate with adjacent metasedimentary clasts. In places, for example below La Tate des Hougues [679 546], mudrock layers have a slaty cleavage and contain reduction spots in the form of strain ellipsoids which indicate a flattening of about 38 per cent.
The main faults that are known to occur in Jersey are shown in( Figure 18). Faults can be traced most readily on the foreshore, especially where rocks of contrasting lithology are juxtaposed; elsewhere important faults may have gone undetected. The dykes are of particular value in demonstrating the effects of faulting. The distribution of the dykes in the intertidal reefs of south-west and south-east Jersey (Figure 17) has revealed a fault pattern that cannot be traced inland owing to poor exposure; the formational boundaries drawn on the map are the best approximation from the available outcrop evidence. In addition, many dykes occupy fault fissures.
The principal faults strike WNW–ESE, but many other trends are represented. Squire (1974) noted that most of the faults are nearly vertical, and that wrench faults predominate. Some of the faults predate the emplacement of the granites; for example, the E–W sinistral wrench fault north of Mont Mado Quarry is truncated by the Mont Mado granite [6366 5588]. Some faults are also earlier than the Rozel Conglomerate, as shown by the fact that the outlier of conglomerates at Pierre de la Fételle has not been affected by the Frémont Fault (below); however the Rozel Conglomerate is displaced by a later sinuous fault that follows the E–W valley to the south of Rozel Manor [698 533].
The La Bouque Fault trends WNW–ESE, roughly parallel to the nearby margin of the north-west (St Mary's) granite, and limits.the southward extent of thermal spotting in the shales. Folds and quartz veins in the rocks adjacent to the fault indicate a dextral movement that Squire (1974) put at more than 1 km; however, the amount of the displacement is uncertain, owing to the lack of marker horizons. Of the faults in the Jersey Shale Formation exposed in the intertidal reefs to the south, several share the strike of the La Bouque Fault, but other trends have been traced, many of them between NNE–SSW and NE–SW.
The fault in St Peter's Valley trends a little west of north at its northern end but is nearer SSE–NNW at its southern end. The fault dextrally displaces the margin of the northwest granite by about 1.3 km. It cannot be traced farther with certainty, but it is possible that the fault exposed at Crabbe [596 556] is its northward continuation. A fault of NNE–SSW trend, separating the north-west (Mont Mado) granite from the Jersey Shale Formation, was noted by Mourant (1933) in the trench for the Handois Reservoir dam [6322 5374]; its sinistral displacement was put at 400 m by Squire (1974).
The Frémont Fault (Figure 3) and (Figure 7), called the L'Homme Mort Fault by Mourant (1933) and other authors, is a broad zone of sheared rocks or cataclasite that separates the Jersey Shale Formation from the Frémont Ignimbrite at Frémont Point (Plate 17). Drag folds in the shales indicate dextral shear (Thomas, 1977). The fault is again found at L'Homme Mort, in the south-east corner of Giffard Bay (Plate 18), and it can be traced to the west side of Bouley Bay, south of Vicard Point. Dr A. E. Mourant has suggested (personal communication) that the fault continues beneath the Rozel Conglomerate to near Rozel Manor, where it may have brought up an inlier of basalt (see p. 23), although for this to be so the strike must change to NW–SE. The Frvmont Fault was thought by Mourant (1933) to be a normal fault with a downthrow of 6000 ft (1830 m) to the south, but Casimir and Henson (1955) reduced this estimate to 2000 ft (610 m), whereas Squire (1974) suggested a dextral displacement of 1 km and a downthrow to the north-east of a few hundred metres. Thomas (1977) disputed these figures and preferred a lateral movement of nearly 4 km, with little vertical throw.
The Les Rouaux Fault trends generally E–W just south of Belle Hougue Point. It brings diorite (containing an enclave of Jersey Shale Formation) and granite of Belle Hougue type against Jersey Shale Formation and St Saviour's Andesite Formation ((Plate 19); p. 53). Squire (1974) recorded that it is a vertical fault with a downthrow to the south-east and an apparent dextral strike-slip component of about 300 m. Between this fault and the Frémont Fault, and also along the coast westward to La Saline [630 561], many smaller faults have been recorded. Similarly west and south of Bouley Bay, and along the coast between Archirondel and Anne Port, many minor faults of varied trends have been traced by distinguishing offsets of the members that make up the Bouley Rhyolite Formation. A fault largely concealed by loess is believed to course WNW–ESE to the south of Ville à l'Evêque [654 540] and Les Câteaux [670 533].
On the northern side of St Helier the Clos de Paradis Fault is exposed in Wellington Road [6618 4927], where the Jersey Shale Formation is in contact with andesite; the fault zone is about 3 m wide and is inclined at about 84° north-eastward. Mourant (1933) noted a further exposure of the fault in foundation trenches at Le Clos de Paradis housing estate, and traced the fault towards the top of Queen's Road. Squire (1974) considered this fault had a dextral displacement of about 1 km and an apparent downthrow to the north-east, but Thomas (1977) commented that a downthrow of a few hundred metres on a normal fault would also account for the observed disposition of outcrops. The fault has itself been displaced by lesser faults with trends between N–S and NE–SW. Again, to the north, several faults trending between NNW–SSE and NNE–SSW have affected the boundary between St Saviour's Andesite and Bonne Nuit Ignimbrite.
At the south-west side of the Victoria Marine Lake [642 488] a fault aligned NNW–SSE has dextrally displaced the axial trace of the St Helier Syncline. North and south of Elizabeth Castle, faults trending respectively about E–W and ENE–WSW have similarly moved the granophyre/diorite contact.
The hornblende-mica-lamprophyres at South Hill have been displaced by mylonite-filled faults which dip gently westwards. These faults have been cut by basic dykes of the main swarm, indicating that the movements took place before the major period of dyke emplacement.
Many of the Jersey dykes are subvertical—those on the east coast dip steeply south-east—and so it follows that the displacement of their outcrops must have been due to transcurrent faults. Most of the faults cut the dykes at about right angles, and though both dextral and sinistral displacements have been noted, the general movement was in a dextral sense, as can be judged by the displacement of the camptonitic hornblende-lamprophyre dyke and the 10 m-wide dioritic dyke south-west of Le Croc (Figure 17); the horizontal displacement of the dioritic dyke amounts to 300 m over a strike length of. 1500 m. These faults also displaced the later N–S basic dykes, and the maximum stress was directed slightly west of north.
Near Le Hocq, however, the granite boundary is assumed to have been displaced sinistrally for a considerable distance, though this movement could have occurred before the emplacement of the dykes which seem not to be offset in this way. As noted above (p. 61), however, the dyke swarm as a whole has apparently been displaced to the north between St Clement's Bay and its reappearance on the east coast northward from Gorey, although this need not necessarily have been the result of faulting.
Helm (1984) provided a synthesis of the tectonic history of Jersey, which briefly is as follows. During the Cadomian orogeny E–W compression gave rise to the early D1 folds, which had an approximately N–S orientation in both the Jersey Shale Formation and the volcanic rocks. D2 folding resulted from N–S compression, which refolded the D1 folds about E–W axes and produced a sinuosity in the N–S structural trend. (Helm suggested that the D2 compression might be related in some way to the intrusion of at least part of the granite complexes.) After the Cadomian folding and granite intrusion the area was uplifted and eroded, and the products of erosion accumulated in hollows to form the Rozel Conglomerate. D3 compression, directed NE–SW, gave rise to the NW–SE-trending Rozel Syncline. A further phase (D4) yielded NE–SW-trending folds, in particular the major Trinity and St Helier synclines.
Thomas (1977, citing Key, 1974) speculated that the association of a thick sequence of acid volcanic rocks with granites might be indicative of a caldera collapse. He demonstrated the concept by turning the geological map of eastern Jersey anticlockwise through 90°, when the distribution of outcrops might represent an oblique section through the supposed caldera. However, having taken all the evidence into account, he concluded (personal communication) that, while it is possible that the ignimbrites may have been associated with a caldera, no trace of it can now be identified, and the present arrangement of volcanic and granitic rocks is fortuitous.