Ostracodes and agglutinated Foraminifera as indicators of paleoenvironmental change in an Early Carboniferous brackish bay, Atlantic Canada

Ostracodes and agglutinated foraminifera characterize lagoonal, estuarine, marsh, and nonmarine depositional systems that span the land/ sea transition. Using microfossil and sedimentary data, high-resolution paleoenvironmental change is documented in an Early Carboniferous marginal marine depositional system that was previously interpreted as nonmarine. There are five main paleoenvironmental phases: (1) low-energy bay dominated by the euryhaline marine ostracode Copelandella novascotica associated with glauconitic shale and primary framboidal pyrite; (2) restricted nearshore dominated by opportunistic paraparchitacean ostracodes (Shemonaella scotoburdigalensis, Shemonaella tatei, and Chamishaella sp.), Cavellina sp., and post-mortem Carbonita scalpellus associated with hummocky cross-stratified siltstone and shale; (3) coastal pond dominated by C. scalpellus and Carbonita rankiniana associated with pedoturbated calcareous mudstones; (4) distal lagoon dominated by paraparchitaceans, Bairdia sp., Geisina sp., and Youngiella sp., all associated with silty glauconitic shale; and 5) coastal marsh dominated by the agglutinated foraminifera Trochammina sp. associated with calcareous mudstones and in situ Iycopsid trees. Low diversity, high dominance, and large populations characterize the microfossil assemblages that resemble modern ecosystems where physical and chemical conditions are unstable. The Horton Bluff Formation records the brackishfreshwater transition at the landward terminus of an Early Carboniferous marine transgression in the Maritimes Basin.