Controls on trace fossil diversity in an Early Cambrian epeiric sea: new perspectives from northwest Scotland

Abstract

The Lower Cambrian Eriboll Formation of northwest Scotland is renowned for the high density, low diversity trace fossils (Skolithos ichnofacies) found in its upper Pipe Rock Member. Ichnofabric analysis of the member indicates that relatively small examples of Skolithos terminating at the same foreset boundary were formed during a brief colonization window after a single depositional event, that particularly long Skolithos specimens are equilibrichnia, and that palimpsests of Skolithos represent the marginal, slightly deeper water fringes of the Pipe Rock Member depositional environment. Nearest neighbour analysis, however, suggests that such palimpsests were uncommon. A much more diverse trace fossil assemblage is present in the overlying Fucoid Member (An t-Sron Formation), comprising Cruziana barbata, Dactylophycus, Didymaulichnus, Halopoa imbricata, ?Margaritichnus, Monocraterion, Monomorphichnus, Palaeophycus striatus, P. tubularis, ?Phycodes, Planolites montanus, ?Polarichnus, Rusophycus ramellensis, ?Psammichnites, Skolithos and various unidentified traces, and represents the Cruziana ichnofacies. Above the Fucoid Member, the Salterella Grit Member ichnofauna is more impoverished, yielding only Cruziana, Monocraterion, Rusophycus, Skolithos and ?Spirophyton. The ichnological variations between the Pipe Rock, Fucoid and Salterella Grit members are interpreted as being driven by changes in sea level. The low trace fossil diversity in the Pipe Rock Member indicates opportunistic colonization of laterally extensive, shoreface sediments deposited by regular influxes of terrigenous material, which were overlain by more distal, ichnologically diverse sediments (Fucoid Member) as sea level rose. A minor regression then caused an increase in terrigenous sediment input, producing an impoverished, proximal Cruziana ichnofacies (Salterella Grit Member).