Non-palimpsested crowded Skolithos ichnofabrics in a Carboniferous tidal rhythmite: Disentangling ecological signatures from the spatio-temporal bias of outcrop

Abstract

A heterolithic tidalite succession yielding spring–neap bundles is newly reported from a mid- Carboniferous (Serpukhovian) section of the Alston Formation of Northumberland, England. The rhythmite records deposition over an interval that can be confidently calibrated to at least 84 lunar days, and attests to a non-negligible tidal range in parts of the Northwest European Seaway in the late Mississippian. The tidalite is notable for the presence of a striking crowded Skolithos ichnofabric on both bedding planes and in vertical section. Bedding plane expressions of the ichnofabric reveal true substrates of sand piles excavated during burrow construction, in addition to an apparently remarkable equal spacing between individual burrows that is shown to be genuine through pair correlation function analysis. These characteristics show that the burrowed horizons were registered by contemporaneous ichnocoenoses, with no palimpsesting of burrows. The irregular vertical distribution of burrow horizons, despite a near-continuous semi-diurnal record of sedimentation, is suggested to be an artefact of spatial patchiness of burrowing communities in the depositional environment; imperfectly registered in a vertical profile with high-temporal, lowspatial resolution. The succession proves that burrow palimpsesting is not an inevitable ichnological conclusion of sedimentary stasis, and attests to intermittent palaeoecological fidelity of the stratigraphic record at the small spatio-temporal scales recorded at