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Geochemical discrimination of Silurian mudstones according to depositional process and provenance within the Southern Welsh Basin

Published online by Cambridge University Press:  01 May 2009

T. K. Ball
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
J. R. Davies
Affiliation:
British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY, U.K.
R. A. Waters
Affiliation:
British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY, U.K.
J. A. Zalasiewicz
Affiliation:
British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.

Abstract

A preliminary geochemical investigation of Silurian (Llandovery) basinal mudstones (turbidites and hemipelagites) from the Southern Welsh Basin is described. Turbidite mudstones show higher concentrations of Fe2O3, MgO, TiO2, MnO, LOI, Zn and Zr than laminated hemipelagites. This is consistent with the observed higher concentrations of chlorite and Ti-bearing minerals in turbidite mudstones. Laminated hemipelagites show higher values of REEs (Ce and La), concentrated within authigenic monazites, and Ni, As, Cu and Pb within sulphide minerals (pyrite and galena) reflecting the influence of primary organic carbon levels and anoxic bottom waters on early diagenesis. Deposition of hemipelagites under oxidizing conditions is reflected in lower concentrations of authigenic sulphide mineral hosted elements compared with laminated hemipelagic lithologies. There is a distinct geochemical difference between mudstones of easterly and southerly provenance in the Southern Welsh Basin. This is shown for both turbidites and hemipelagites. The differences are due to the increased input of illite and the chemical elements associated with this mineral (K2O, A12O3, Rb and Ba). Turbidite mudstones sourced from the south show increased levels of heavy minerals, especially those associated with Ti-rich minerals. There is also an increase in elements associated with detrital monazites: Th and Y. The hemipelagites show higher values of REE and chalcophile elements consistent with their more reduced nature.

Type
Articles
Copyright
Copyright © Cambridge University Press 1992

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References

Bridges, P. H. 1975. The transgression of a hard substrate shelf: the Llandovery (lower Silurian) of the Welsh borderland. Journal of Sedimentary Petrology 45, 7994.Google Scholar
British Geological Survey 1:50000 Sheet 179 (Rhayader). In press a.Google Scholar
British Geological Survey 1:50000 Sheet 178 (Llanilar). In press b.Google Scholar
Cave, R. 1979. Sedimentary environments of the basinal Llandovery of mid-Wales. In The Caledonides of the British Isles - reviewed (eds Harris, A. L., Holland, C. H. and Leake, B. E.), pp. 517–26. Special Publication, Geological Society of London, no. 8.Google Scholar
Cave, R. & Hains, B. A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoir of the British Geological Survey, Sheet 163 (England and Wales).Google Scholar
Davies, J. R., Fletcher, C. J. N., Waters, R. A., Wilson, D., Woodhall, D. G. & Zalasiewicz, J. A. In press. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey, Sheets 178 and 179 (England and Wales).Google Scholar
Kelling, G. & Woollands, M. A. 1969. The stratigraphy and sedimentation of the Llandoverian rocks of the Rhayader district. In The Precambrian and Lower Palaeozoic Rocks of Wales (ed. Wood, A.), pp. 255–82. Cardiff: University of Wales Press.Google Scholar
Milodowski, A. E. & Zalasiewicz, J. A. 1990. The origin and sedimentary, diagenetic and metamorphic evolution of chlorite-mica stacks in the Llandovery sediments of central Wales, U:K. Geological Magazine 128, 263–78.CrossRefGoogle Scholar
Milodowski, A. E. & Zalasiewicz, J. A. 1991. Redistribution of rare earth elements during diagenesis of turbidite/hemipelagite mudrock sequences of Llandovery age from central Wales. In Developments in Sedimentary Provenance Studies (eds Morton, A. C., Todd, S. P. and Houghton, P. D. W.), pp. 101–24. Special Publication, Geological Society of London no. 57.Google Scholar
Moroney, M. J. 1951. Facts from figures. Penguin Books.Google Scholar
Morton, A. C., Davies, J. R. & Waters, R. A. 1992. Heavy minerals as a guide to turbidite provenance in the Lower Palaeozoic Southern Welsh Basin. Geological Magazine 129, pp. 573–80.CrossRefGoogle Scholar
Smith, A. J. & Long, G. H. 1969. The Upper Llandovery sediments of Wales and the Welsh Borderlands. In The Precambrian and Lower Palaeozoic Rocks of Wales (ed. Wood, A.), pp. 239–54. Cardiff: University of Wales Press.Google Scholar
Smith, R. D. A. 1987. The griestoniensis Zone Turbidite System, Welsh Basin. In Marine Clastic Sedimentology: Concepts and Case Studies (eds Leggett, J. K. and Zuffa, G. G.), pp. 89107. London: Graham & Trotman.CrossRefGoogle Scholar
Smith, R. D. A., Waters, R. A. & Davies, J. R. 1991. Late Ordovician and early Silurian turbidite systems in the Welsh Basin. 13th International Geological Congress, Nottingham, Field Guide No. 20. Cambridge: British Sedimentological Research Group. 45 pp.Google Scholar
Solo Statistical System. LOS Angeles: BMDP Statistical Software Incorporated.Google Scholar
Soper, N. J. & Woodcock, N. H. 1990. Silurian collision and sediment dispersal patterns in southern Britain. Geological Magazine 127, 527–42.CrossRefGoogle Scholar
Stow, D. A. V. & Bowen, J. 1980. A physical model for the transport and sorting of fine-grained sediments by turbidity currents. Sedimentology 27, 3146.CrossRefGoogle Scholar
Waters, R. A., Davies, J. R., Fletcher, C. J. N., Wilson, D., Zalasiewicz, J. A. & Cave, R. 1992 a. Llandovery basinal and slope sequences of the Rhayader District. In Geological excursions in Powys, Wales (eds Woodcock, N. H. and Bassett, M. G.). Cardiff: University of Wales Press, National Museum of Wales and Geologists' Association (South Wales Group).Google Scholar
Waters, R. A., Davies, J. R., Fletcher, C. J. N. & Wilson, D. 1992 b. Discussion of ‘A late Ordovician/early Silurian non-depositional slope and perched basin along the Tywi Anticline, Mid Wales’ by D. M. D. James. Geological Journal 27.CrossRefGoogle Scholar
Wilson, D., Davies, J. R., Fletcher, C. J. N. & Waters, R. A. 1992 a. The Ordovician rocks of the Rhayader district. In Geological excursions in Powys, Wales (eds Woodcock, N. H. and Bassett, M. G.). Cardiff: University of Wales Press, National Museum of Wales and Geologists’ Association (South Wales Group).Google Scholar
Wilson, D., Davies, J. R., Waters, R. A. & Zalasiewicz, J. A. 1992 b. A fault-controlled depositional model for the Aberystwyth Grits turbidite system. Geological Magazine 129, pp. 595607.CrossRefGoogle Scholar
Wood, A. & Smith, A. J. 1959. The sedimentation and sedimentary history of the Aberystwyth Grits (Upper Llandoverian). Quarterly Journal of the Geological Society of London 114, 163–95.CrossRefGoogle Scholar
Ziegler, A. M., Cocks, L. R. M. & McKerrow, W. S. 1968. The Llandovery transgression of the Welsh Borderland. Palaeontology 11, 736–82.Google Scholar