These three galleries show pictures of rock texture and pattern in strata of Caswell Bay Mudstone Formation (CBMF) at Caswell Bay on the Gower Peninsula in South Wales. This is the type location for the CBMF – the place from which the rocks were first described and named. The rocks are part of the Pembroke Limestone Group of the Tournasian/Visean epoch of the Mississipian subdivision of the Carboniferous Period. The Carboniferous Period lasted from 359 to 299 million years ago (mya) but the Tournasian/Visean part only lasted from 359 to 326 mya. The CBMF were deposited around the middle of that period. The total thickness of rocks deposited during the Tournasian/Visean epoch was around 750 metres but the CBMF is just a narrow band – with estimates of its thickness varying from 0-14 metres (George, 2008) to 3 – 7.5 metres thick (Barclay, 2011). The CBMF is sandwiched between the Gully Oolite Formation limestone below and the High Tor Limestone Formation above.
Barclay (2011) says that: the Caswell Bay Mudstone Formation is composed of thinly bedded calcitic and dolomitic mudstones and micritic limestones (George, 1978; Ramsay, 1987). The basal bed is a calcrete (Heatherslade Geosol of Wright, 1987b), with beds of algal laminate and oncoid limestone. The rocks are pale grey to greenish grey, buff, brown and yellow, locally with some red staining.
Barclay says there are some but not many fossils. Also that: the rocks formed in shallow water environments referred to as “lagoon phase” by Dixon and Vaughan (1912). They are interpreted as shallow-water, peritidal deposits formed in a tidal flat lagoon complex behind a beach barrier in a humid climate, with abundant evidence of sub-aerial exposure in the form of dessication cracks (Riding and Wright, 1981; Wright, 1986; Ramsay, 1987).
The Caswell Bay Mudstone Formation lies unconformably on top of of the sub-aerially weathered palaeo-karst surface of the Gully Oolite Formation limestone. It means that there was a time lag between the deposition of the limestone and the next phase of deposition of the mudstones. The palaeo-karst surface is full of dissolved pot-holes. These pot-holes are a common karstic feature on Gower south coast beaches – with Mewslade Bay and the Worms Head Causeway exhibiting some good examples.
The Gully Oolite Formation limestone was deposited in warm tropical seas at a time when sea-level was standing still or slowly falling. The extended period of sub-aerial weathering that created the palaeo-karst surface occurred during a significant relative fall in sea-level (George, 2008, 85). The Caswell Bay Mudstone Formation was formed during a subsequent phase of slow sea-level rise.
Barclay, W. J. (2011) Geology of the Swansea District: a brief explanation of the geological map Sheet 247 Swansea, British Geological Survey, Natural Environment Research Council, ISBN 978-085272581-8, pp 4-6.
Dixon, E. E. L., and Vaughan, A. (1912) The Carboniferous succession in Gower (Glamorgan) with notes on its fauna and conditions of deposition. Quarterly Journal of the Geological Society of London, Vol. 67, pp 477-571.
Geological Society Field Guide to Caswell Bay.
George, G.T. (2008) The Geology of South Wales: A Field Guide, published by firstname.lastname@example.org, ISBN 978-0-9559371-0-1, pp 82- 86.
George, T. N. ((1978) Mid Dinantian (Chadian) limestones in Gower. Philosophical Transactions of the Royal Society, Vol. B282, pp 411-462.
Ramsay, A. T. S. (1987) Depositional environments of Dinantian limestones in Gower, 265-308 in European Dinantian environments, Miller, J., Adams, A. E. and Wright, V. P. (editors). (Chichester: John Wiley & Sons Ltd.)
Riding, R. and Wright, V. P. (1981) Palaeosols and tidal flat/lagoon sequences on a Carboniferous carbonate shelf: sedimentary associations of triple disconformities. Journal of Sedimentary Petrology, Vol. 51, pp 275-293.
Wright, V. P. (1986) Facies sequences on a carbonate ramp: the carboniferous Limestone of south Wales. Sedimentology, Vol. 33, pp 221-241.
Wright, V. P. (1987b) The ecology of two early Carboniferous palaeosols, 345-358 in European Dinantian environments. Miller, J., Adams, A. E. and Wright, V. P. (editors). (Chichester: John Wiley & Son.)