Architecture of coastal and alluvial deposits in an extensional basin: the Carboniferous Joggins Formation of eastern Canada



Abstract The Joggins Formation was deposited in the Cumberland Basin, which experienced rapid mid-Carboniferous subsidence on bounding faults. A 600 m measured section of coastal and alluvial plain strata comprises cycles tens to hundreds of metres thick. The cycles commence with coal and fossiliferous limestone/siltstone intervals, interpreted as widespread flooding events. These intervals are overlain by coarsening-upward successions capped by planar-based sandstone mounds, up to 100 m in width that represent the progradation of small, river-generated delta lobes into a standing body of open water developed during transgression. The overlying strata contain sand-rich heterolithic packages, 1–8 m thick, that are associated with channel bodies 2–3 m thick and 10–50 m wide. Drifted plant debris, Calamites groves and erect lycopsid trees are preserved within these predominantly green-grey heterolithic sediments, which were deposited on a coastal wetland or deltaic plain traversed by channel systems. The cycles conclude with red siltstones, containing calcareous nodules, that are interbedded with thin sandstones and associated with both single-storey channel bodies (1–1·5 m thick and 2–3 m wide) and larger, multistorey channels (3–6 m thick) with incised margins. Numerous channel bodies at the same level suggest that multiple-channel, anastomosed river systems were developed on a well-drained floodplain. Many minor flooding surfaces divide the strata into parasequences with dominantly progradational and aggradational stacking patterns. Multistorey channel bodies are relatively thin, fine grained and modestly incised, and palaeosols are immature and cumulative. The abundance and prominence of flooding surfaces suggests that base-level rise was enhanced, whereas the lack of evidence for abrupt basinward stepping of facies belts, coupled with the absence of strong fluvial incision and mature palaeosols, suggests that base-level fall was suppressed. These architectural features are considered to reflect a tectonic architectural signature, in accordance with the high-subsidence basinal setting. Evidence for restricted marine influence and variation in floral assemblages suggests modulation by eustatic and climatic effects, although their relative importance is uncertain.