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Lligwy Bay RIGS Site
NRW RIGS no. 380
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RIGS Statement of Interest:
Lligwy Bay RIGS Site comprises exposures in the lower Dinantian succession including the Lligwy Bay Sandstone and the Lligwy Bay Disturbance. Complex relationships between siliciclastic and carbonate units reflect deposition within deeply incised channels and the subsequent collapse of the karstified channel margin. This unique site is critical in providing evidence of the scale of, and the processes associated with, penecontemporaneous fluvial incision and karstification of the Anglesey Dinantian sequence. This sequence as a whole records the progressive growth of a carbonate platform during a pulsed Dinantian transgression. The Lligwy Bay RIGS significantly modifies the GCR description and interpretation proffered by Adams and Cossey (2004). Exposures at the western end of the site at Carreg Ddafad
Geological setting/context: The Dinantian succession of North Wales records the evolution and growth of a carbonate platform founded on the older Palaeozoic and Precambrian rocks of the region in response to pulsed, but sustained marine transgression (George, 1958, 1974; Somerville & Strank, 1989; Davies et al., 2004). The Dinantian sequence on Anglesey was deposited during the latter phases of this event, during the Asbian and Brigantian stages. Frequent falls in sea level (forced regressions) characterize this period of time and, as a consequence, the limestone successions on Anglesey, and elsewhere, are constructed from a series of shoaling-upwards sedimentary cycles. The tops of each cycle display features indicative of subaerial exposure, karstification and soil formation (Davies, 1991). However, the Anglesey succession accumulated at the landward margin of the platform and is unique in preserving features and deposits restricted to such a setting. Here, during periods of regression, fresh water streams flowed on to the emergent platform surface and incised deep channels (or palaeovalleys). Distinctive siliciclastic facies accumulated within these features and their margins display the effects of contemporaneous dissolution. The Lligwy Bay RIGS appears to provide unique evidence of the depths to which such channels were incised (over 50 m) and hence of the scale of contemporary sea level movements. It is unique in preserving features which testify to instability and collapse of the channel margins. The RIGS can be interpreted as casting doubt on the stratigraphical position of the Lligwy Sandstone of Greenly (1919), suggesting that it too may form part of the younger, channel-filling, siliciclastic sequence.
Network context of the site: The site forms one of series of 9 selected to illustrate the Anglesey Dinantian succession and the processes – erosional, depositional and diagenetic – which were active during and subsequent to its accumulation; these in turn from part of a broader network of Upper Palaeozoic RIGS in North Wales.
References:
ADAMS, A. E. and COSSEY, P. J. 2004. North Wales Shelf. In British Lower Carboniferous Stratigraphy (P. J. Cossey, A. E. Adams, M. A. Purnell, M. J. Whitely, M. A. Whyte and V. P. Wright, editors), Geological Conservation Review Series, No. 29, Joint Nature Review Committee, Peterborough, pp. 365 – 392.
Bates, D.E.B. and Davies, J.R. 1981. Geologists Association Guide No. 40: The Geology of Anglesey, 32 pp.
COPE, F. W. 1975. The age of the Lower Carboniferous conglomerate at Lligwy Bay, Anglesey. Geological Journal, 10, 17 – 22.
DAVIES, J. R. 1983. The stratigraphy, sedimentology and palaeontology of the Lower Carboniferous of Anglesey. Unpublished PhD thesis, University of Keele.
DAVIES, J.R. 1994. Palaeovalley fills in cyclical late Dinantian platform carbonates, Anglesey, North Wales. European Dinantian Environments II University College Dublin, 6th-8th Sept. 1994 Absracts, 8–9.
Davies, J. R. 1991. Karstification and pedogenesis on a late Dinantian carbonate platform, Anglesey, North Wales. Proceedings of the Yorkshire Geological Society, 48, 297–321.
GEORGE, T. N. 1958. Lower Carboniferous palaeogeography of the British Isles. Proceedings of the Yorkshire Geological Society, 31, 227–318.
GEORGE, T. N. 1974. Lower Carboniferous rocks in Wales. In: The Upper Palaeozoic and post-Palaeozoic rocks of Wales (Owen, T.R. ed.) University of Wales Press, Cardiff, 85–115.
GEORGE, T. N., Johnson, G. A. L., Mitchell, M., Prentice, J. E., Ramsbottom, W. H. C., Sevastopulo, G. D. & Wilson, R. B. 1976. A correlation of the Dinantian rocks in the British Isles. Special Report of the Geological Society of London, 7, 1–87.
GREENLY, E. 1919. Geology of Anglesey. Memoir Geological Survey, UK.
SOMERVILLE, I.D.& STRANK, A.R.E. 1989. Palaeogeographic reconstructions of the Dinantian in North Wales (U.K.). C.R. 4, 11th Congrès International de Stratigraphie et de Geologie du Carbonifère,(Beijing, China 1987), 313–318.
Site geometry: