Abstract

AbstractA 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, low‐spatial 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 outcrop.

Highlights

  • The diurnal or semi-diurnal tidal deformation of marine waters manifests in coastal regions as a dominant incoming flood tidal current and subordinate outgoing ebb tidal current, each punctuated by a tidal stillstand at high and low tide (e.g. De Boer et al, 1989; Dalrymple et al, 1991; Archer & Johnson, 1997; Mazumder & Arima, 2005; Kvale, 2012)

  • The possibility that these may have been under-recognized in the past has implications for future ichnofabric studies because: (i) equifinal ichnofabric signatures can be generated through several mechanisms (Fig. 10) that may be impossible to differentiate without understanding the spatio-temporal scale of the studied package; (ii) non-palimpsested ichnofabrics have preservation potential within underfilled accommodation space, so prima facie assumption of time-averaging or overprinting should be avoided without direct evidence (e.g. McIlroy & Garton, 2010); and (iii) as individual outcrops may sample only very small spatiotemporal scales, extreme caution should be exercised when interpreting their ichnofabrics in a macroevolutionary context (e.g. Tarhan et al, 2015)

  • Heterolithic tidal rhythmite facies are newly reported from the Brigantian Alston Formation of the Northumberland Basin

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Summary

INTRODUCTION

The diurnal or semi-diurnal tidal deformation of marine waters manifests in coastal regions as a dominant incoming flood tidal current and subordinate outgoing ebb tidal current, each punctuated by a tidal stillstand at high and low tide (e.g. De Boer et al, 1989; Dalrymple et al, 1991; Archer & Johnson, 1997; Mazumder & Arima, 2005; Kvale, 2012). The bundles from the Alston Formation are imperfectly registered in a naturally-weathered rock outcrop of limited spatial extent (for example, Fig. 4), where the propensity for lateral discontinuity of individual laminae and bundles is more apparent than in a core sample They exhibit four regular and repetitive characteristics that strongly indicate that they are deposits arising from cyclic neap–spring variability in current strength: (i) the bundle laminae are always evident as couplets of sandstone and siltstone, analogous to modern. While the ichnofauna of the Alston Formation as a whole is reasonably diverse, and dominated by abundant deposit feeding traces (for example, Teichichnus, Zoophycos), these traces typically occur in low diversity or monospecific trace fossil associations (Booth et al, 2020), similar to the Skolithos assemblage discussed here As such it is likely that different parts of the Alston Formation cover the full range of deltaic subenvironments typified by the Phycosiphon and Rosselia ichnofacies (MacEachern & Bann, 2020). Elsewhere in the depositional environment, where local sedimentation rates were lower, tracemakers could achieve their full one year lifespans, but this scenario would rarely (or cryptically) be archived in the rock record due to the necessarily limited sediment accrual

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