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The stratigraphy and setting of the Precambrian rocks of the Llangynog Inlier, Dyfed, South Wales

Published online by Cambridge University Press:  01 May 2009

J. C. W. Cope  and
R. E. Bevins
J. C. W. Cope
Affiliation:
Department of Geology, University of Wales, P.O. Box 914, Cardiff CF1 3YE, U.K.
R. E. Bevins
Affiliation:
Department of Geology, National Museum of Wales, Cardiff CF1 3NP, U.K.
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Abstract

Detailed mapping of the Precambrian rocks of the Llangynog Inlier, made possible by a series of new exposures, has allowed the establishment for the first time of the 1100 m thick succession within the Precambrian rocks of the area. All boundaries of the Precambrian rocks with younger strata in the area are either faults or unconformities. Some 350–400 m of rhyolitic lavas form the oldest rocks seen in the area; they are extensively silicified and devitrified and show little of their original texture, although flow-banding is visible in some outcrops and the presence of perlitic and snowflake textures imply that originally they were glassy in part. These lavas are succeeded by a varied succession dominated by volcaniclastic siltstones showing evidence of shallow water deposition and containing medusoids and trace fossils of Ediacaran age. Interbedded in the sediments are basaltic lavas, autobreccias and rare hyaloclastites displaying textural features characteristic of submarine eruption, and rhyolitic pumiceous and shardic ash-flow tuffs. The name Coomb Volcanic Formation is proposed for this succession, within which are recognized two members, a lower Castell Cogan Rhyolite Member succeeded by the Coed Cochion Volcaniclastic Member. In association are varied intrusions, comprising both dolerites and dacites. A palaeoenvironmental model, built on evidence of petrography, palaeontology and sedimentology is that of shallow water to emergent volcanic islands.

Type
Articles
Information
Geological Magazine , Volume 130 , Issue 1 , January 1993 , pp. 101 - 111
Copyright
Copyright © Cambridge University Press 1993

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