Stratigraphy, fluviolacustrine sedimentology and cyclicity of the Late Devonian/Early Carboniferous Horton Bluff Formation, Nova Scotia, Canada

The Late Devonian/Early Carboniferous (Tournaisian) Horton Bluff Formation formed within the tectonically subsiding Windsor Subbasin with a major bounding fault (Cobequid Fault precursor) to the north and with onlapping relationships to the south. The formation is divided into four members, in ascending order the Harding Brook, Curry Brook, Blue Beach and Hurd Creek Members. The formation is at least 560 m thick in the type area (Avon River Valley), and has its greatest measured thickness (1,015 m) in the Soquip Noel ;Excellent exposure of the Blue Beach and Curry Brook Members shows that they thicken northward. Trends in thickness of cycles may be due to more rapid subsidence toward the fault-bounded, northern basin margin. Cycle thickness decreases upward, reflecting a decreasing rate of tectonic subsidence within the basin. A tripartite evolution of fluvial flow-through (Harding Brook Member) to ponding (Curry Brook, Blue Beach and Hurd Creek Members) to fluvial flow-through is proposed for the Windsor Subbasin, and possibly for the Horton Group of the Maritimes Basin. This stratigraphic sequence is explained by a change in tectonic subsidence rate alone, with a lacustrine interval reflecting a phase of rapid subsidence and underfilling of the basin.;The Harding Brook Member rests unconformably on basement and consists of fining-upward pebbly sandstones and trough cross-bedded sandstones deposited within a braided-stream environment. Planar-laminated units are possibly deltaic and transitional into the overlying member. The Curry Brook Member is dominated by fining-upward successions of mudstone and sandstone deposited in interdistributary lakes as crevasse splays and minor deltas in an overall upper delta plain setting. Thick planar bedded sandstones were laid down in the major distributary channels. The Blue Beach Member is dominated by clayshale and siltstone deposited in repeated shallowing-upward cycles of a hydrologically open lacustrine system. A shallowing succession of wave-dominated shoreline deposits shows hummocky cross-stratified (HCS) siltstone (with wave-produced V-shaped grooves), wave-rippled sandstone, and planar-bedded siltstone, the finer grain size of the latter resulting from shallow-water wave attenuation. A transition to marsh occurs at the shoreline. The Hurd Creek Member contains lacustrine cycles similar to those of the Blue Beach Member, but also coarsening-upward sandstone-dominated cycles that are interpreted as distributary channel and wave-reworked delta deposits. Downward-injected clastic dykes occurring beneath the HCS deposits were triggered by rapid deposition and wave cyclic loading. Dyke orientation was controlled by the prevailing wave orientation.