STRATIGRAPHY AND TECTONIC HISTORY OF THE UPPER DEVONIAN TO LOWER CARBONIFEROUS HORTON BLUFF FORMATION, NOVA-SCOTIA

The Devonian-Carboniferous Horton Group in the Maritimes Basin of Atla ntic Canada was deposited in local half grabens during an extensional phase that followed the Acadian Orogeny. In its type area, the southwe st part of the Mines Basin, the Horton Group is divided in ascending o rder into the Horton Bluff and Cheverie formations. Four members of th e Horton Bluff Formation are formally defined here. The basal Herding Brook Member, of latest Devonian to early Tournaisian age, is a braide d-fluvial sandstone unit developed above a paleosol on the underlying Meguma Group. The succeeding three members are shale/sandstone units o f Tournaisian age. The Curry Brook Member was deposited in deltaic cha nnels and interdistributary lakes, whereas the Blue Beach and Hurd Cre ek members were deposited in large, wave-dominated lakes and/or restri cted-marine bays, with evidence of deltaic input for the Hurd Creak Me mber. The overlying Cheverie Formation is a coarse grained alluvial de posit of Tournaisian age. Sediment style reflects an interaction betwe en sediment flux, subsidence controlled accommodation space, and clima te. 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 thickeni ng and facies change suggests that the Horton Bluff Formation was depo sited 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 rock s were rapidly exhumed prior to formation of extensional basins. The a lluvial-lacustrine-alluvial succession in the Horton Group type area c an be recognized in half grabens across Atlantic Canada, with a coeval (late Tournaisian) lacustrine phase. The widespread occurrence of thi s subsidence phase implies that the local, basin-bounding faults were components of a regional detachment at mid-crustal depth.