A gravity survey of the Criffel Granodiorite

Abstract

The results are presented of a gravity survey of the Criffel - Dalbeattie granodiorite complex and the surroundingarea, carried out from the Geology Department of the Durham Colleges. Gravity stations are sufficiently closely spacedto allow accurate contouring by means of Bouguer anomaly isogals at one milligal intervals. A large number of densityspecimens collected during the course of the survey have been statistically analysed and the results used in theinterpretation. The surface geology of the area has already been mapped and described in some detail and this information has been used extensively in the interpretation of the gravity data. The opinion of previous authors is that the complex is of magmatic origin. The most striking feature of the gravity field is a pronounced negative anomaly over the main granodiorite with very steep gradients over the country rock to the north-west of the granodiorite but smoother gradients on the south-east Side. The suggested interpretation, based on three dimensional computer methods, is that the main granodiorite mass extends to a depth of eight miles and exhibits a marked gradational increase in density in its south-eastern portions. It is thought that this is the result of magmatic flow within the intrusion, the magma trailing up at a centre slightly offset from the present centre of the outcrop, becoming contaminated by the country rock and flowing down on the south-east side. The gravity field over the outlying parts of the complex in the Huehencairn region suggests that the granodiorites in this area extend to the same depth as the main intrusion and are an integral part of it. Regional modifications of the gravity field are also discussed and tentative interpretations put forward.