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 paleoenvironmenta l change is documented in an Early Carboniferous marginal marine deposition al 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 oppor tunistic paraparchitacean ostracodes (Shemonaella scotoburdigalensis, Shemo naella tatei, and Chamishaella sp.), Cavellina sp, and post-mortem Carbonit a scalpellus associated with hummocky cross-stratified siltstone and shale; (3) coastal pond dominated by C. scalpellus and Carbonita rankiniana assoc iated with pedoturbated calcareous mudstones; (4) distal lagoon dominated b y paraparchitaceans, Bairdia sp, Geisina sp, and Youngiella sp, all associa ted with silty glauconitic shale; and 5) coastal marsh dominated by the agg lutinated foraminifera Trochammina sp. associated with calcareous mudstones and in situ lycopsid trees. Low diversity, high dominance, and large popul ations characterize the microfossil assemblages that resemble modern ecosys tems where physical and chemical conditions are unstable. The Horton Bluff Formation records the brackish-freshwater transition at the landward termin us of an Early Carboniferous marine transgression in the Maritimes Basin.