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A deep-marine ichnofaunal assemblage from Llandovery strata of the Welsh Basin, west Wales, UK

Published online by Cambridge University Press:  01 May 2009

Patrick J. Orr
Patrick J. Orr
Affiliation:
Department of Geology, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK
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Abstract

In the Welsh Basin, turbidites of Llandovery age at Cwmerfyn, near Goginan (Devil’s Bridge Formation) and Neuadd Fawr, near Lampeter, contain similar, diverse, ichnofaunal assemblages, despite strong sedimentological differences between the two localities. The sediments at Cwmerfyn are a series of very thin- to thin-bedded silt/mud couplets. The silt-grade lower half of each couplet is ripple-cross- and/or planar-laminated; the mud-grade upper part is massive. At Neuadd Fawr, the strata are thin- to medium-bedded, and composed of fine- to medium-grained sand; individual beds are parallel-laminated or display pronounced convolute lamination. At both localities, the ichnofaunal assemblage is dominated by endostratal pascichnia observed on intrastratal, bedding-parallel, planes of splitting. All the ichnofauna is post-depositional. Both assemblages are dominated by examples of Nereites cambrensis and macleayii; examples of Neonereites uniserialis and biserialis, Protovirgularia dichotoma, Chondrites, Dictyodora scotica, Helminthoida crassa, two other ichnospecies of Nereites and examples of Nereites isp., Palaeophycus tubularis, Macaronichnus segregatus, Planolites beverleyensis and montanus and a ‘braided trace’ are also present. In contrast, the diverse ichnofaunal assemblage recorded from the Aberystwyth Grits Group by others is both pre-depositional and post-depositional in origin. The former include agrichnia (Paleodictyon and Squamodictyon) and cubichnia (Asteracites and Bergaueria), observed as secondary casts on the soles of turbidites. The differences in composition between this ichnofaunal assemblage and those at Cwmerfyn and Neuadd Fawr are interpreted to be a result of the degree of oxygenation of the sediment profile. In the Aberystwyth Grits, the producers of the agrichnia and cubichnia would have been able to tolerate reduced oxygen levels in the sediment profile, because they maintained a connection with an oxygenated water column. The post-depositional component of the Aberystwyth Grits assemblage is interpreted to represent opportunistic colonization of the sediment profile during short intervals of elevated oxygen conditions produced by episodic turbidite deposition. The producers of the endostratal pascichnia at Cwmerfyn and Neuadd Fawr did not maintain connections with the water column, implying the presence of oxygenated interstitial porewaters during their emplacement.

Type
Articles
Information
Geological Magazine , Volume 132 , Issue 3 , May 1995 , pp. 267 - 285
Copyright
Copyright © Cambridge University Press 1995

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