Trace fossil assemblages and their occurrence in Silesian (MidCarboniferous) deltaic sediments of the Central Pennine Basin, England

In the Silesian rocks of the Central Pennine Basin, three types of ancient delta sequence are recognized. Each contributed to the progressive filling of the Basin and to the gradual development of fluvial/para lie conditions in Westphalian time.

The turbidite-fronted delta of the lowest Namurian, of the Pendleian Stage of the Skipton area in the north of the Basin, shows three depth-related sedimentary associations which correspond with overlapping but distinct trace fossil assemblages. The Turbidite Association contains a Rhizocorallium-Planolites-Bergaueria assemblage on the base or top of thin-bedded turbidites; the Slope Association consists of Lophoctenium and Curvolithus in laminated sandstones and siltstones; and the Delta Top Association is characterized only by Monocraterion-Skolithos and Pelecypodichnus in parallel-bedded and cross-bedded sandstones. The Turbidite and Delta Slope Associations appear to belong to the Zoophycos ichnofacies of Seilacher (1967), and the Delta Top to the Cruziana and Skolithos ichnofacies. Deltaic deposition thus advanced into water a few hundreds of meters deep, probably of nearly fully marine salinity. Trace fossils of the deeper-water Nereites ichnofacies are lacking. Sedimentological factors such as energy level, substrate and food supply, rather than bathymetry alone, may have influenced the distribution of trace fossils.

In the south of the Basin, the later Lower Kinderscoutian delta is similar sedimentologically to that of the Pendleian, but is devoid of trace fossils, except for Planolites and Pelecypodichnus assemblages in the upper part of the delta slope and on the delta top. The absence of trace fossils with obvious marine affinities is consistent with the inteipretation that, in intervals between marine inundations, basin water was less saline than it was in the Skipton area during Pendleian time.

During the Upper Kinderscoutian, Marsdenian, and Yeadonian stages, the Central Pennine Basin was filled, mainly from the north and east, by shallow-water sheet deltas, and by two shallow-water elongate deltas from the west. Trace fossils in the delta-plain sediments contain assemblages which can be assigned to the Cruziana, or rarely, Zoophycos ichnofacies, together with a variety of facies-crossing forms. They show progressive colonization of the delta-top paleoenvironments. They also suggest evolution of certain animal groups during this time. Thus bivalve escape shafts, attributed to cf. Sanguinolites, a marine genus in the Upper Kinderscoutian, show a steady increase in vertical extent, or height, throughout the period. By late Marsdenian time they were evidently formed by the non-marine genus Carbonicola. On independent evidence Carbonicola appears to have evolved from the bivalves which made the earliest escape shafts.

Lower Westphalian sediments indicate a gradual increase upwards in fluvial and swamp dominance of the extensive delta top. This is well substantiated by trace fossils, as far as they have been studied. They play a significant part in elucidating the general sedimentary environment; for instance, Pelecypodichnus escape shafts suggest seasonal flooding, as a result of monsoonal condition, in the west Lancashire coalfield. Xiphosurid traces {Kouphichnium and Limulicubichnus), which range from the Marsdenian upwards, provide insights into the more ephemeral aspects of sedimentation and paleoenvironments. Freshwater arthropod traces and vertebrate footprints (Scoyenia ichnofacies) are poorly known in the Westphalian of the Pennine area when compared with the roughly contemporary ichnofaunas of Nova Scotia, Canada, but such traces occur in the latest Silesian and early Permian rocks in other areas of Britain.