Jersey Geology Trail

Geophysical Surveys.

There are several different types of geophysical survey used to determine underground structures and rock types; the following have been used in Jersey.
Gravity surveys map the density changes of different rock bodies and their effect on the acceleration of gravity, and are used to detect and identify subsurface rock bodies and anomalies, eg. intrusions & faults.
Magnetic surveys are used to image anomalies in the earth's magnetic field caused by differences in rock body magnetism and dislocations underground, eg. igneous intrusions & faults.

Other types of survey used in geological and archaeological exploration are;
Resistivity surveys map the distribution of electrical potential in the ground around a current-carrying electrode which depend on the electrical resistivities and distribution of the surrounding soils and rocks.
Seismic or acoustic surveys map the seismic wave propagation which depends on the elastic properties of the subsurface rocks. They measure the travel times of the reflected or refracted waves and are able to estimate the location and depth of the different rock bodies. Since seismic wave propagation depends on the elastic properties of the subsurface, seismic techniques can detect both lateral and depth variations of the stratigraphy based on the different estimates of the seismic velocities.

In Jersey, results of the Gravity Survey by Briden & Clark, 1989, p. 95 - 103) along the south and north coasts are discussed in detail (BGS. 1989, p. 95 - 103).

Profiles A - C were drawn N - S (Fig. 1) eastwards from St. Ouën's Bay - Noirmont), across the Jersey Shale Formation deposits which indicate the 'Brioverian rocks form a raft c. 250m thick overlying rocks of granite density' (op. cit. p. 101).

Profiles D, E were also drawn from NW - SE & SW - NE respectively (Fig. 1) across the central part of the island. From these, it was concluded that a 'positive anomaly does not match the surface geology in detail' (1989, op. cit. p. 96 - 7), but the anomaly is above various diorites in the SE granite even south of the coast and seems to be caused by a major diorite or gabbro body at depth.

The results in the north east, around Rozel, result in 'improbable thicknesses' of the Rozel Conglomerate (loc. cit. p. 97) and may be a result of an 'offshore basic intrusion or a thickening of the Jersey Shale and Volcanic Group'. The residual anomalies which do not relate to 'known geology' have a short wavelength and are thought to reflect the topography of the granite Brioverian surface.

In Jersey, a Magnetic survey of the eastern third of the island was done by Leeds University workers (Duff, 1989, p. 103) and a Gravity survey was performed by Briden et al (1989, p.95 - 102) (Fig. 2).

A large Magnetic anomaly, noticed by Briden (BGS, Fig. 26. & p. 103 - 4), was found trending N - S. Its western edge coincided with the western edge of the St. Saviour's Andesite but a more distant eastern edge suggested the Andesite extended eastwards under the overlying Rhyolite sequence. Other features occurred along the northern edge of the SE granite and smaller ones within seemed coincident with the diorite (loc. cit.).

Were the Brioverian formations deposited on an eroded diorite/granite 'basement' surface, ie. our granites are older than published and their dates are 'unreliable' as has been noted?
Was there remobilisation and later intrusion to produce the post - Brioverian younger granites?
What is the 3D (block diagram) nature of the granite types in the SE, SW & NW areas?


Fig. 1.
Profiles A, B, C are N - S eastwards from St. Ouën's Bay - Noirmont across the Jersey Shale Formation deposits.
Profiles D, E are diagonally NW - SE & SW - NE are across the Andesites, Rhyolites and Conglomerate. (BGS, 1989, p. 101 - 102).


Fig. 2. Total field magnetic anomaly map of eastern Jersey (BGS op. cit.).


The following simplified block diagrams (a - d) show the stress field, rock units and major structures at various periods of the tectonic evolution of Jersey; D1, late D1, D2, D4. (Helm, D.G. 1984. PGA Vol. 95, p. 1 - 15). Compare the relative thicknesses (qualitative) with those from the geophysical surveys.



Bishop, A. C. & Bisson, G. 1989. Classical areas of British Geology: Jersey: description of 1:25,000 Channel Islands Sheet 2. pp. 95 - 103. (London HMSO for British Geological Survey) with references to Briden, Duff, 1989.