Cannington Park, Lower Carboniferous, Bristol and Gloucester region

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Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)

Cannington Park[edit]

Generalised horizontal section to illustrate facies and thickness changes in the Carboniferous Limestone between the Forest of Dean and Cannington Park. For location of section see P948961. The Dinantian stages in the right-hand column refer only to Cannington Park; for the relationship of the stages to the successions elsewhere refer to P948962 and P948964. (P948963)

The Carboniferous Limestone inlier of Cannington Park, which lies 5 km north-west of Bridgwater, is the carbonate platform locality nearest to the Lower Carboniferous basinal sequences of west Somerset and Devon. Structurally, it forms part of the northern limb of the main Quantocks Anticline. Knowledge of the succession has been greatly increased by the BGS Knap Farm Borehole (Whittaker and Scrivener, 1982[1]). The borehole succession is as follows:

m
Approximate thickness
(corrected for dip)
m
Depth
CARBONIFEROUS LIMESTONE
Cannington Park Limestone 73 93.2
Knap Limestone 195 249.55
Cynwir Castle Limestone 278 351.21
Cynwir Cherty Limestone 446 557.68
Cannington Reef Limestone 626 778.41
Black Rock Limestone 776 966.35
Lower Limestone Shale 890 1105.91
DEVONIAN SANDSTONE seen to 1153.00

The borehole is situated on the southern edge of the inlier; thus at least 150 m must be added for the thickness of the Cannington Park Limestone at outcrop, to give a total thickness of the Carboniferous Limestone in excess of 1000 m. This is considerably greater than that of the corresponding strata in the Mendips, but is comparable with that known in south Pembrokeshire. The dips in the core range from 30° to 40°. The limits of the stages are given in Figure 8; the youngest out-cropping strata belong to the Holkerian Stage.

In terms of the Mendip sequences, the top three formations at Cannington Park are equivalent to the Burrington Oolite in the western and central Mendips, and the Vallis Limestone in the eastern Mendips (P948963). At Cannington Park, these formations comprise pale grey, bioclastic and oolitic, coral-bearing rocks, which extend up into the Holkerian; they correspond to the generally finer-grained, more variable Clifton Down Limestone and Burrington Oolite in the areas to the north. Grains described as ooliths during field-logging of the Knap Farm borehole cores were subsequently shown by detailed petrographic work to be mostly well-rounded, micritised, crinoidal and other debris. It is probable, like-wise, that the oolitic content of the Burrington Oolite of the Mendips has been over-estimated.

The Cynwir Cherty Limestone is similar to the upper part of the Black Rock Limestone of the Mendips, except that the chert development in the Mendips is thinner. The extension of the Black Rock Limestone facies up into post-Chadian strata is paralleled by a similar development in the eastern Mendips. At Cannington Park, it is noteworthy that the evidence for appreciable shallowing of the sea in post-Courceyan times is first seen in the early Arundian, and that there is no obvious sedimentary evidence for the intra-Chadian sea level lowering that so strongly affected sedimentation to the north. Thereafter, it appears that the rate of sedimentation was greater than the rate of subsidence and, in consequence, a shallowing of the sea took place.

References[edit]

  1. Whittaker, A, and Scrivener, R C. 1982. The Knap Farm Borehole at Cannington Park, Somerset, Report of the Institute of Geological Sciences, No.82/5, 1–7.