Geophysical field surveys - 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 9 Geophysical field surveys
A gravity survey of Jersey was carried out by Briden and others (1982) as part of a systematic coverage of the Channel Islands. A total of 206 observations was made using a La Coste-Romberg gravity meter, giving a station density greater than 1 per square kilometre and generally confining the standard error in field readings to ± 0.02 mGal (1 mGal = 10−5 m/s2). This compares with the original study by Day (1959) at a station density of less than 1 per 3 km2 and field reading accuracy of ± 0.1 mGal. Gravity measurements were referred to the National Gravity Reference Net 1973 datum at Jersey Airport (Masson Smith and others, 1974), where observed g was determined as 980 991.720 ± 0.009 mGal, because the original St Helier base station occupied by Day (1955) no longer existed. Data were reduced by means of the 1967 International Gravity Formula, and terrain corrections (out to 21.9 km about each station) and Bouguer corrections were made for a nominal density of 2.67 g/cm3. Elevations were referred to mean sea level, and wherever possible benchmarks or other levels for which an accuracy of ± 0.01 ft was claimed were utilised; elsewhere, spot heights quoted to the nearest 0.1 ft or 0.5 ft by Hunting Surveys Ltd were used. For the less accurate spot heights, the uncertainty in elevation resulted in an error of about ± 0.05 mGal in the Free Air and Bouguer anomalies, in addition to other measurement errors.
The Bouguer anomaly map (Figure 21) was computer contoured using the SACM routine refined by Z. K. Dabek of the Applied Geophysics Unit of the Institute of Geological Sciences (now the British Geological Survey); this map is available from BGS as an overlay to the 1:25 000 geological map, and the data have been incorporated into the Bouguer Gravity Anomaly Map, Guernsey Sheet (Institute of Geological Sciences, 1979). The Bouguer anomaly field (Figure 21) is dominated by a large, circular positive anomaly, which is centred several kilometres east of St Helier and affects the Bouguer contours over the eastern two-thirds of Jersey. A circular symmetrical positive anomaly has therefore been subtracted from the Bouguer data to facilitate recognition of smaller-scale 'residual' and longer-wavelength 'regional' anomalies. Because this idealised procedure is undoubtedly an oversimplification, using it to define the shape of the positive anomaly, and hence the geometry of its causative body, gives rise to residual anomalies that are spurious in the sense that they merely reflect departures by the real anomaly from the circular model. The existence of a regional gravity gradient across the Channel Islands was demonstrated by Briden and others (1982); however, across the island of Jersey the regional component of the gravity field can be considered uniform at 9 mGal, except possibly on the extreme west of the island. This constant has been removed from the Bouguer data before interpretation (Figure 23).
The centre of the model main circular anomaly was placed in the valley north of Bagot [6655 4875] by visual fit to the Bouguer contours (Figure 21). Bouguer anomalies were plotted at a function of radial distance from the chosen centre (Figure 22a), and a polynomial was fitted to those data which were unbiased by local features. The adoption of the background field as a circularly symmetrical positive anomaly as shown was justified by the good grouping of data in the plot and the lack of overall trend to the residuals.
The 9 mGal positive anomaly (Figure 22)b requires the existence of a subsurface body with density at least as great as that of the St Saviour's Andesite Formation, since the anomaly does not extend into the outcrop area of the andesites. The density contrasts required to produce realistic models of the causative body suggest a minimum density of 2.90 g/cm3, and hence imply dioritic or more basic rocks. Although the positive anomaly does not match the surface geology in detail, it does overlie the various dioritic bodies within the south-east granite. These are believed to be metasomatised gabbro (see the section on gabbro and diorite, Chapter 5) and thus the most likely cause of the gravity anomaly seems to be a major dioritic or gabbro body at depth.
A quantitative attempt has been made to interpret the anomaly, using the three-dimensional modelling program of Cordell (1970). Two possible (best-fit) model types are shown in (Figure 22)b, representing circularly symmetrical lenses with diameters of some 12 km. The upper model has a flat top constrained at a depth of 1 km which results in a maximum thickness for the causative body of 1 km for density contrast of 0.3 g/cm3 (or 1.5 km for 0.2 g/cm3). The lower model has the centre of the causative body fixed at 1.75 km depth and gives a similar thickness to that found for the upper model. Portions of the model extending beyond the eastern coastline are outside the survey area and have no data to support them. An alternative interpretation, though less plausible, is that the anomaly is wholly or partly due to a thick andesite sheet at small depth within the Jersey Shale Formation of south-east and central Jersey, similar in shape to the postulated gabbro body.
The residual Bouguer anomaly map (Figure 23) has been derived by subtracting the background anomaly deduced in (Figure 22). The residual anomalies within the area that contained the circular positive anomaly are generally of low amplitude (less than 1 mGal) and many do not correlate with the known surface geology. The larger-amplitude anomalies are considered first.
Along the south coast of the island the gravity anomaly increases southwards by up to 7 mGal. The gravity gradient and the maximum anomaly attained on the edge of the survey area together imply the existence of a basic body at least 750 m thick, overlying the proposed gabbroic lens associated with the circular anomaly. This interpretation is consistent with submarine occurrences of gabbro south of Jersey (Lefort, 1975). In the Rozel area, a 3 mGal anomaly 'high' parallels the coast and has no closure on the seaward side. This anomaly is also visible on marine gravity maps (Bacon, 1975). Modelling by extrapolation from surface geology results in improbable thicknesses of several kilometres for the Rozel Conglomerate, even using increased density contrasts. The most likely cause of this anomaly is an offshore basic intrusion or possibly a thickening of the Brioverian supracrustal sequence comprising the Jersey Shale Formation and the Jersey Volcanic Group (see below). The remaining large-amplitude anomalies relate directly to the outcrop distribution within Jersey, and have been analysed using a two-dimensional modelling program.
The calculations assume bodies of infinite strike, and thus yield slight underestimates of true size if the assumed density contrasts are correct. Three N–S profiles across western Jersey (Figure 23) have been interpreted in (Figure 24) to suggest that the north-west and south-west granites are continuous beneath the Jersey Shale Formation, at depths ranging from 1800 m for profile A to 600 m for profile C. This compares with a depth of 300 m estimated by Day (1959) using a larger density contrast. All three profiles show the gravity 'high' associated with the south coast of Jersey, and profile C shows a positive anomaly ascribed to dioritised gabbro within the north-west granite at Sorel Point, where a thickness of at least 400 m of diorite is possible. The northern granite/ metasediment contact in profile C could not be fitted by any geologically viable model which is consistent with the adjacent profiles; this may be due to loss of two-dimensionality in the modelling, since the granite is flanked to its east and west by higher-density metasediments.
Elsewhere over the Brioverian sequence the residual anomaly field is almost uniform, despite significant density contrasts between the rhyolites, andesites, and metasedimerits (Table I). Gravity gradients across the contacts of the Brioverian with the three granites are low. It is therefore inferred that the Brioverian is a raft about 250 m thick, with little gravitationally resolvable structure, overlying rocks of granite density, implying that granites are continuous at no great depth beneath much of the island (Figure 25).
Those residual anomalies which do not relate to the known geology are short-wavelength features and therefore have a shallow origin. They are considered to reflect the topography of the granite/Brioverian interface, rather than any heterogeneities within the granite or small-scale structures within the underlying gabbroic lens. The depth of this boundary would be greater if a subsurface andesite body were evoked as contributing to the cause of the main positive anomaly (p. 97).
Apart from three unpublished German measurements of declination, magnetic field observations are not known to have been made on Jersey prior to a reconnaissance total field survey made in 1976 over the eastern third of the island by Leeds University workers (Figure 26). Field readings were taken to ± 1 nanotesla (nT), and were reduced to total magnetic field anomalies using the 1975 International Geomagnetic Reference Field (IGRF); no regional trends were removed from the data. Briden and others (1982) remarked that a large magnetic anomaly trending N–S across the island has its western side coincident with the westernmost outcrop of the St Saviour's Andesite; this anomaly is broader than the outcrop, suggesting that the andesites extend eastwards near the surface beneath the rhyolite sequence, consistent with the interpretation of the residual Bouguer anomaly field (Figure 25). Other significant features in (Figure 26) are the anomaly along the northern contact of the south-east granite, and the small magnetic signature of the diorite bodies within this granite. The former anomaly is most pronounced where the contact rocks are andesites, and these may be the source of the anomalous magnetisation. However, growth of magnetite and hematite is a common feature of metasomatic effects at granite contacts, and this would be an alternative explanation, although it has not been investigated specifically.