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Pedolau to Eglwys Siglen RIGS Site
NRW RIGS no. 381
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RIGS Statement of Interest:
Pedolau to Eglwys Siglen exposes late Asbian transgressive-regressive cycles with subaerial exposure, karstic dissolution and soil formation.
Within the Anglesey Dinantian succession, the Pedolau to Eglwys Siglen RIGS provides excellent exposures of typical late Asbian transgressive-regressive carbonate cycles including evidence, at the tops of each cycle, of penecontemporaneous subaerial exposure, karstic dissolution and soil formation. It also includes a unique section in the Helaeth Sandstone, a sequence of fluvial conglomerates, estuarine mudstone and barrier beach sandstones. The Anglesey succession as a whole records the progressive growth of a carbonate platform during a pulsed Dinantian transgression. The late Asbian sequence on Anglesey, as elsewhere in North Wales and beyond, comprises a series of shoaling-upwards carbonate cycles each capped by a palaeokarstic surface. The latter record repeated falls in sea level (forced regressions) and the resulting subaerial exposure of the platform. The intervening rises in sea level (marine transgressions) flooded these surfaces, re-submerged the platform and initiated the next cycle of carbonate sedimentation. During the emergent phases, the sediment exposed at the platform surface underwent complex diagenetic changes associated with carbonate cementation, but also with soil formation (pedogenesis) and plant colonization. The resulting structures and fabrics are typical of modern day calcretes formed under a semi-arid tropical climate. Wind-blown volcanic ash was also trapped and accumulated on the platform surface and, beneath this derived soil cover, the newly lithified limestones were prone to dissolution by downward percolating rain waters. The resulting complex array of dissolution, diagenetic and pedogenetic features displayed by the Anglesey palaeokarstic surfaces and their immediately underlying strata, have been described, illustrated and interpreted in detail by Davies (1991). Many of these phenomena are displayed by the Pedolau to Eglwys Siglen RIGS. Key amongst these are hummocky surfaces formed by dissolution below a still preserved volcanic (bentonitic) clay fossil soil (or palaeosol); calcified root structures termed rhizocretions; laminated calcrete crusts and ooids; limestones altered to dark grey micrite; and growths of replacive microspar. In thin-sections, additional microscopic features such as early vadose cement fabrics and a range of brecciation and neomorphic replacement effects are also evident (Davies, 1991). In addition, the Anglesey Dinantian succession provides exclusive evidence that, during periods of subaerial exposure, freshwater streams flowed on to the Dinantian platform surface and incised channels into the underlying limestones. Sequences of fluvial, estuarine and marine siliciclastic facies were deposited, and are now preserved, within the confines of these features. The Helaeth Sandstone, which has its type section within the Pedolau to Eglwys Siglen RIGS, provides one of the best and most easily accessible sections through such a sequence and was important in the development of a depositional model for these channel-fills (Davies, 1983; 1994). Strata exposed at, and to the east of Pedolau
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, 1954; Somerville et al., 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. Distinctive siliciclastic facies accumulated within these channels and their margins display the effects of contemporaneous dissolution. The Pedolau to Eglwys Siglen RIGS provides exposures in typical late Asbian cycles and the range of dissolution and pedogenic phenomena developed at their boundaries. The lowest units in the section relate to, and were essential to the interpretation of features described in RIGS JRD5. Exposures in the Helaeth Sandstone were critical in developing a model for the infill of the fluvially incised channels which are a feature of the local Dinantian sequence, and unique within North Wales.
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:
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. 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.
WALKDEN, G.M. & DAVIES, J.R. 1983. Polyphase erosion of subaerial omission surfaces in the Late Dinantian of Anglesey, North Wales. Sedimentology, 30, 861–878.
Site geometry: