Mixed layer development and exceptional trace fossil preservation in Cambro-Ordovician siliciclastic strata

Instances of exceptional preservation-preservation of near-surface structures of high taphonomic fidelity, including bioglyphic detail-have been previously reported from Cambro-Ordovician trace fossil assemblages worldwide. However, it has hitherto been unknown to what extent this exceptional preservation characterises the Cambro-Ordovician record as a whole. Herein we present ichnological, sedimentological and taphonomic data collected from the Bell Island Group (Beach Formation) and Wabana Group (Powers Steps Formation, Scotia Formation, Grebes Nest Point Formation) of Canada and the Bynguano Formation of Australia, units selected as representative of Cambro-Ordovician marine shelfal settings. Additionally, data from outgroup (both lower and higher energy) facies were collected from the Barrancos Formation of Spain, the Juniata Formation of the eastern United States and the Pacoota Sandstone of Australia in order to assess the role of depositional environment in Cambro-Ordovician infaunal colonisation and exceptional preservation. The Bynguano, Beach, Powers Steps, Scotia and Grebes Nest Point formations are characterised by widespread exceptional preservation of shallow-tier ichnological assemblages and bioglyphic structures, implying formation and preservation in a cohesive substrate. In spite of the common density and complexity of trace fossil assemblages, ichnofabric is (and thus infaunal mixing was) only poorly to moderately well developed. Trace fossils are preserved as discrete structures both along bedding planes and in cross-section. Well mixed intervals are typically of limited spatial extent and composed of millimetre-scale structures resulting in limited disruption of macro-scale depositional fabrics. Bed-sole assemblages are cast onto the bases of thin (millimetre-to centimetre-scale) sandstone horizons or beds rather than penetrating through beds. Comparison with outgroup facies both confirms the limited extent of biogenic mixing across a range of Cambro-Ordovician marine settings and highlights the significance of heterolithic facies for exceptional preservation and recognition of shallow-tier assemblages.