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IX,—A Gravity Survey of the Sanquhar Coalfield*

Published online by Cambridge University Press:  11 June 2012

Adam C. McLean
Adam C. McLean
Affiliation:
Department of Geology, University of Glasgow
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Synopsis

In the Sanquhar district a bsain of relatively light Upper Carboniferous rocks (relative density 2·54) overlies a denser basement of Ordovician greywackes (relative density 2·72). A gravity survey over the area with coverage of approximately one station per square kilometre, increasing to a station spacing of 100 yards along certain traverses in critical localities, shows a close relation between the Bouguer anomalies and the known geology. A local gravity low over the Carboniferous outcrop is superimposed on a steep regional gradient decreasing south-eastwards. The trend of this gradient is parallel to the strike of the Ordovician rocks, but its exact relationship to the basement structure is not clear. The residual Bouguer anomalies reflect the structure of the Carboniferous rocks; in the south-eastern part of the basin they agree closely with the calculated effect produced by the known thicknesses, structure, and density contrast. Westwards from Kirkconnel, however, there is an increasing discrepancy between the observed and the theoretical anomalies, a discrepancy which indicates the presence of an additional mass of light rock concealed under the known Coal Measures. It is inferred that this material consists of older Carboniferous strata, thickest under the present-day valley of the Nith but concealed by overstep of the overlying Coal Measures against the flanks of the basin. These hidden rocks thin gradually to the south-east across a shallow monocline with north-east trend and south-east of Kirkconnel are truncated by a large fault trending east-north-east. Both the monocline and the fault appear to affect the Westphalian strata to a lesser degree than the base of the Carboniferous rocks.

The stratigraphical significance of the concealed strata is that they evince the existence of a pre-Westphalian basin, trending north-west-south-east, which antecedes the later Hercynian fold, and also suggest that pre-Hercynian movements took place along a fault, near Kirkconnel which, trends north-east-south-west across the basin.

Type
Research Article
Information
Proceedings of the Royal Society of Edinburgh, Section B: Biological Sciences , Volume 68 , Issue 2 , 1961 , pp. 112 - 127
Copyright
Copyright © Royal Society of Edinburgh 1961

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Footnotes

*

This paper was assisted in publication by a grant from the Carnegie Trust for the Universities of Scotland.

References

References to Literature

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