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A Cambrian Tidal Sand Body: The Eriboll Sandstone of Northwest Scotland: An Ancient-Recent Analog

The origin of many mineralogically mature, marine sandstones can be related to paleotidal processes. The Cambrian Eriboll Sandstone of northwest Scotland is divisible into a prominently cross-stratified Lower Member and an Upper Pike Rock Member containing abundant Scolithus ichnofossils. The Lower Member consists of medium-grained, well-sorted, mature quartzarenite and subarkose. Sedimentary structures in this member include planar, herringbone cross-stratification, reactivation surfaces, interference ripples, and both current ripples and micro-cross-laminae superimposed at 90° on slip faces of cross-strata. The frequency distribution of set thicknesses are multimodal, whereas distributions of dip angles of cross-strata are bimodal. Cross-strata azimuth orientations are bimodal and multimodal. The Pipe Rock Member is massive-bedded with burrows of Scolithus and Monocraterion, the latter resembling "escape structures." Sedimentary structures in the Eriboll Sandstone are comparable bodies. Reactivation surfaces and bimodal distributions of both cross-strata set thickness and dip angles suggest a paleotidal flow mechanism characterized by time-velocity asymmetry. Escape structures of burrows combined with overlying intertidal and supratidal carbonate rocks also indicate a tidal depositional environment for the Eriboll Sandstone. The tidal sand body environment is characterized by intensive reworking, and by equilibrium sand transport patterns through alternate flood-dominated and ebb-dominated zones. Mineralogical and textural maturity is achieved by sediment transport over long distances combined with weathering at intermediate depositional sites. Other quartzarenites displaying similar structures were likely developed in tide-dominated depositional environments.