The stratigraphy and structure of the Devonian Rocks in the Quantock Hills, West Somerset

B. D. Webby*

*Corresponding author for this work

Research output: Contribution to journalArticle

13 Citations (Scopus)

Abstract

The Devonian succession of the Quantock Hills has been differentiated into five major units, the Little Quantock Beds, Hangman Grits, Ilfracombe Beds, Morte Slates and Rodway Beds; two of which, the Hangman Grits and Ilfracombe Beds, being further subdivided. The Little Quantock Beds contain a marine fauna and underlie the non-marine, Old Red Sandstone succession of the Hangman Grits. They are succeeded by the shallow-water marine Ilfracombe Beds. The facies changes within the Ilfracombe Beds of the Quantock and Brendon Hills are described. Coral faunas from the Roadwater Limestone suggest a Middle or Upper Givetian age, and those from the stratigraphically higher Holwell Limestone indicate a possible uppermost Givetian age. The Givetian-Frasnian boundary lies above the Holwell Limestone perhaps near the top of the succession of Ilfracombe Beds (viz. in the upper part of the Leighland Beds), or between the Ilfracombe Beds and the Morte Slates. A comparison of the coral faunas in the Roadwater Limestone of the Quantocks and Brendons is presented. Two distinctive biotopes are recognised: the Brendon biotope dominated by rugose corals, and the Quantock biotope, by tabulates and stromatoporoids. The Brendon biotope possibly developed in a comparatively shallow-water, sub-turbulent situation, and the Quantock biotope formed in shallower, more-turbulent conditions, perhaps in a nearer-shore situation. Strata have been affected by cleavage folding, flexure folding (Webby, 1965) and faulting. Asymmetrical to overturned minor cleavage folds have been recognised. Quartz-diorite dykes oblique to the east-west cleavage folds are seen at the Coombe and probably post-date the cleavage folding. The dominant fold structure in the Quantocks is the Courtway Anticline, a broad, open, gently east-plunging flexure. The Cannington Park Limestone (Lower Viséan) is apparently faulted against Rodway Beds (Upper Devonian) beneath the covering of New Red Sandstone. Absence of all the Tournaisian and probably part of the Upper Devonian implies a fault with a displacement of only a few thousand feet, possibly a small thrust. It may be linked with a larger thrust lying north of Cannington Park. The Courtway Anticline may have been produced as a broad flexure on the crest of a major thrust system during the northward overthrusting of the Devonian strata. The north-west alignment of the Quantock Hills is controlled by the Cothelstone Fault (at present largely mantled by New Red Sandstone), having approximately three miles of dextral horizontal displacement and 6500 feet of vertical downthrow to the south-west. A narrow zone of crumpled slates is developed adjacent to this fault near Cothelstone.

Original languageEnglish
Pages (from-to)IN1-343
JournalProceedings of the Geologists' Association
Volume76
Issue number4
DOIs
Publication statusPublished - 1965
Externally publishedYes

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