Stratigraphy and tectonic history of the Upper Devonian to Lower Carboniferous Horton Bluff Formation, Nova Scotia

Abstract

The Devonian-Carboniferous Horton Group in the Maritimes Basin of Atlantic Canada was deposited in local half grabens during an extensional phase that followed the Acadian Orogeny. In its type area, the southwest part of the Minas Basin, the Horton Group is divided in ascending order into the Horton Bluff and Cheverie formations. Four members of the Horton Bluff Formation are formally defined here. The basal Harding Brook Member, of latest Devonian to early Tournaisian age, is a braided-fluvial sandstone unit developed above a paleosol on the underlying Meguma Group. The succeeding three members are shale/sandstone units of Tournaisian age. The Curry Brook Member was deposited in deltaic channels and interdistributary lakes, whereas the Blue Beach and Hurd Creek members were deposited in large, wave-dominated lakes and/or restricted-marine bays, with evidence of deltaic input for the Hurd Creek Member. The overlying Cheverie Formation is a coarse grained alluvial deposit of Tournaisian age. Sediment style reflects an interaction between sediment flux, subsidence controlled accommodation space, and climate. Fluvial systems extended across the basin during periods of basin overfilling and fluvial/lacustrine/ estuarine deposits were laid down during periods of basin underfilling. Evidence of a northward thickening and facies change suggests that the Horton Bluff Formation was deposited in a half-graben bounded to the north by the Cobequid Fault or a precursor fault. Paleoflow was northward, across the Meguma Terrane, during deposition of the alluvial deposits. The late Devonian age for the basal Horton Group places an upper age limit on "docking" of the Meguma and Avalon terranes in the area, and suggests that Meguma rocks were rapidly exhumed prior to formation of extensional basins. The alluvial-lacustrine-alluvial succession in the Horton Group type area can be recognized in half grabens across Atlantic Canada, with a coeval (late Tournaisian) lacustrine phase. The widespread occurrence of this subsidence phase implies that the local, basin-bounding faults were components of a regional detachment at mid-crustal depth.