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

Abstract

AbstractIn 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 ofNereites cambrensisandmacleayii; examples ofNeonereites uniserialisandbiserialis, Protovirgularia dichotoma, Chondrites, Dictyodora scotica, Helminthoida crassa, two other ichnospecies ofNereitesand examples ofNereitesisp.,Palaeophycus tubularis, Macaronichnus segregatus, Planolites beverleyensisandmontanusand 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 (PaleodictyonandSquamodictyon) and cubichnia (AsteracitesandBergaueria), 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.