A.Thomas Martel
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Ph. D. Thesis
Stratigraphy, Fluviolacustrine Sedimentology and Cyclicity of the Late Devonian/Early Carboniferous Horton Bluff Formation, Nova Scotia, Canada
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The Horton Bluff Formation (Tournaisian age) was deposited in 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 Harding Brook Member consists of fining-upward pebbly sandstones and trough cross-bedded sandstones deposited within a braided-stream environment. Curry Brook Member fining-upward mudstone and sandstone sequences were deposited in interdistributary lakes as crevasse splays and minor deltas in an 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 sequence of wave-dominated shoreline deposits shows hummocky cross-stratified siltstone (with wave-produced V-shaped grooves), wave-rippled sandstone, and planar-bedded siltstone that resulted from shallow-water wave attenuation, overlain by marsh deposits. The Hurd Creek Member contains "Blue Beach-like" lacustrine cycles similar to those of 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.
The Blue Beach and Curry Brook Members thicken northward. Trends in cycle thickness can be explained by more rapid subsidence toward the fault-founded, 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. This stratigraphic sequence can be explained by a change in tectonic subsidence rate alone, with a lacustrine interval reflecting a phase of rapid subsidence and underfilling of the basin.
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