Rozel Conglomerate Formation - 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 4 Rozel Conglomerate Formation
The Rozel Conglomerate Formation comprises coarse conglomerates with subordinate sandstones and mudstones. The main outcrop of the formation occupies the north-eastern corner of Jersey and forms the lowest of the cliffs along the northern coast of the island; the inland junction with the Bouley Rhyolite Formation is mainly concealed by superficial deposits. Two outliers are situated to the west, one in Les Hurets valley [665 548] to the west of Bouley Bay, and the other farther north, on the Pierre de la Fetelle heights [663 556] above Vicard Point.
The distinctive pebbly lithology of this formation (Plate 7) caught the observant eye of Philip Dumaresq as early as the 1680s (Dumaresq, 1685), though it was not until the second half of the 19th century that the formation was fully described and its main boundaries were defined with any precision (Ansted and Latham, 1862; Noury, 1886). Noury (1886, pp. 135–136) discussed whether the deposits were of marine or continental origin; he favoured the former, and it was not until Comes (1933) published his account that the continental nature of the formation was established to the extent that this remains the accepted origin (Squire, 1970; Renouf, 1974; Duff, 1979).
The Rozel Conglomerate is indisputably the youngest of the major hard rock formations in Jersey, but its age in terms of the geological column is uncertain. Recent work (e.g. Adams, 1976; Duff, 1979, 1980) has narrowed the field of choice from a Precambrian to Permo-Carboniferous range to one spanning Cambrian to Siluro-Devonian time, with the emphasis focusing on late Cambrian to early Ordovician (e.g. Squire, 1970; Renouf, 1974; Duff, 1979, 1980). Duff (1979, p. 359) suggested that the oldest stable natural remanent magnetisation (NRM) component of the mudstones in the sequence (p. 42) was acquired during authigenesis of hematite in Silurian or. early Devonian times, but that this was not incompatible with a Cambro-Ordovician age for the rocks themselves. A possible palaeontological indication of age has been given by B. H. Bland (1984), with his assignment of structures previously identified as raindrop and related prints in the mudstone below La Tete des Hougues (see p. 41) to the large impression fossil Arumberia Glaessner & Walter. This fossil has so far been found in rocks of latest Precambrian to Lower Cambrian age, but its full range has not been established. Adams (1976) gave a date of 427 ± 13 Ma (435 ± 13 Ma when recalculated) for a dyke of hornblende-lamprophyre [7110 5380] that has intruded the conglomerates just south of La Coupe Point; this Upper Ordovician/Lower Silurian date thus sets a minimum for the age of the conglomerates.
The base of the Rozel Conglomerate is exposed on the cliff slopes above Vicard Point and in the small bay below La Tete des Hougues. There was formerly an exposure of the base of the Les Hurets valley outlier on Bouley Bay hill [6638 5456] but this is now obscured. At all three localities the conglomerates rest on rhyolite; their bedding dips into or against the volcanic rock, indicating that the pre-conglomerate surface was eroded, the depressions being at least of the order of size of the Les Hurets valley itself.
Above Vicard Point the conglomerate [6645 5563] dips south-westward into the hillside at between 30° and 40°; the surface of the Bouley Rhyolite is visibly uneven, with northward-trending, smooth-sided channels of the order of several metres in length and up to 1 m deep (Thomas, 1977). Contours drawn on the base of the conglomerate indicate that the outlier fills a strongly defined hollow cut in the rhyolites. Where visible the basal beds are conglomerates, and lack the finer-grained beds such as are to be found in Les Hurets valley and in the main outcrop.
In the little bay [679 545] below La Tête des Hougues the conglomerates rest on both the Middle and the Lower Bouley Ignimbrite. The contact can be traced from the waterfall at the back of the bay, along its western side and then eastward for a short distance at low tide. There is a visible 2 to 3 m relief to the underlying surface of the volcanic rocks which has been revealed by recent erosion, and the presence of an isolated patch of conglomerate beside the path that descends to the modern beach emphasises the irregularity of the surface.
The thickness of strata in the main outcrop is estimated at 500 to 800 m. Overall, the conglomerates are coarse-grained, poorly sorted rocks, which were deposited in units mostly upwards of 1 m thick. Boulders up to 2 m in diameter are found in some places, for example at and near the Tour de Rozel [6918 5520]; [6924 5508]. Bands of coarse grit occur here and there throughout the exposed succession [6792 5448]; [6924 5508]; [7114 5288], though none has been noted in either of the outliers. More localised are small thicknesses of much finer-grained beds containing maroon mudstone bands a few centimetres thick [6792 5448]; [6934 5440]; 704 520]; [7114 5288]. A lack of recognisable marker horizons has so far prevented the subdivision of the formation.
Below La Tête des Hougues the mudstones and associated finer-grained beds in the conglomerate succession show graded bedding (Plate 8) and mud cracks, and these indicate that the sediments were deposited in shallow pools, open to small influxes of muddy water and subject to drying out. Mourant (1933) noted 'impressions of raindrops, rill or ripple-marks' in these mudstones, but B. H. Bland (1984) has suggested that these structures represent the trace fossil Arumberia (see p. 40); Bland considered that though 'a braided stream or an intertidal sand and mudflat' was the most likely environment of deposition for Arumberia-bearing strata, 'shallow lakes in a flood-plain' were not ruled out. The mudstones below La Tête des Hougues were originally about horizontal but they now dip at about 36° to the northeast, as do the main conglomerate beds, thus showing that the dip has been structurally imposed. Unfortunately the other mudstones in the succession are too localised to do more than hint at the overall depositional development of the formation. The two principal mudstone outcrops, below La Tête des Hougues and opposite La Solitude Farm [7042 5202], are characterised by oxidation (maroon) and reduction (green) hues, though the full significance of this as regards the climate at the time of deposition has not been resolved.
The localised occurrence of the mudstones and the evidence they present of deposition in shallow surface pools agree with the overall picture of a continental deposit built up from floods of unsorted material brought down from a hilly or mountainous terrain by rivers. Imbrication of the flatter pebbles is common, and Squire (1970) made a preliminary study of the source direction, finding a predominantly northerly origin, though with one ESE result that has not been confirmed. Thomas (1977) noted imbricate structure at La Tête des Hougues [679 545] and confirmed the derivation from the north. However, Helm (1984) and Richardson (1984) have drawn attention to tectonic flattening and pebble rotation in the Rozel Conglomerate (see p. 76), which cast some doubt on the validity of inferences drawn from pebble imbrication as an indicator of palaeocurrent directions in this instance. Coarse cross-bedding is exposed at a number of localities and is well displayed between the Nez du Guet [6971 5479] and the foreshore outcrops immediately east of Rozel jetty.
The polygenetic nature of the pebbles in the conglomerates has long been recognised. The chief groups of rocks represented are Brioverian metasediments, volcanic formations, and various coarse-grained acid plutonic rocks. The acid plutonic rocks do not closely resemble those cropping out in Jersey, nor have foliated granitic rocks like those of Les Ecrehous been found, but the Brioverian sediments and volcanic rocks match perfectly. The pebbles of acid plutonic rocks are generally more rounded than the other constituents, suggesting that they may have been derived from farther afield, though probably from no more than 10 to 20 km away. It appears that the local Cadomian granites and the associated more basic rocks had not been unroofed at the time when the Rozel Conglomerate was deposited. Thomas (1977 and personal communication) recorded the occurrence of cobbles of conglomerate at La Tete des Hougues [679 545], which indicate that the deposit has been reworked; he also noted that volcanic material was common near the base, but rare or absent in the main body of the formation.
The matrix of the conglomerates is made of clay-size particles, but rarely exceeds 5 per cent of the total rock. There is little or no silt- or sand-sized fraction. The cement was originally hematitic, but this has been oxidised to limonite near the surface. The very localised occurrence of small reduction layers associated with the maroon mudstones has already been mentioned above.
Despite prolonged searches of the outcrops by many geologists, no trace of macrofossils other than the possible Arumberia (already mentioned) has been found, either as an original or as a derived constituent. The maroon mudstones and associated fine-grained beds have been exhaustively treated for the recovery of microfossils or organic traces, but with entirely negative results.