Estuarine deposition, valley incision and soil development controlled by tectonic evolution of a growth-faulted coastal graben embayment, Stoddart Group - Belloy Formation, Carboniferous-Permian, Western Canada Basin

Subsurface stratigraphy analysis of the Stoddart Group and overlying Belloy Formation in the Western Canada Basin reveals the stratigraphy, sedimentary and regional structural evolution of an east-west Carboniferous-Permian graben complex developed on an apparently stable cratonic platform. The graben forms an intensely block-faulted central core of a broad craton-marginal Carboniferous-Triassic NW-trending downwarp (the Peace River Embayment). The graben is unusual in that it was very long-lived (100 million years), overlies anomalously-uplifted Precambrian granitic basement (Peace River Arch), and was downwarped along two sets of NW- and NE-trending high-angle, normal growth faults. Deposition of shallow marine fine-grained siliciclastics kept pace with syn-sedimentary fault movements such that the graben sea-floor topography was relatively smooth. Episodic eastward migration of subsidence and changes in subsidence patterns with time in the craton-marginal graben suggest that episodic orogenic activity occurred to the west perhaps analogous to the Devonian-Carboniferous Antler Orogeny or similar Carboniferous-Permian events in western U.S.A.; Although now intensely segmented into numerous km-wide horst and graben blocks, graben-central Stoddart Group strata can be resolved into originally flat-lying units. Stoddart Group strata (mainly late Late Viséan) can be divided into four unconformity-bound “Depositional Sequences” representing, from oldest to youngest, Golata Formation quiet offshore marine mudstones with an incised soil-imprinted upper surface, a basal Kiskatinaw Formation sandstone-dominated estuarine valley-fill complex, a lower to upper Kiskatinaw Formation shallow marine shelf with interbedded mudstones, sandstones and lime mudstones with abundant paleosols and tidal deposits and a Taylor Flat Formation (Serpukhovian) carbonate-dominated open marine shelf.; The graben complex controlled sedimentation patterns that characterise the Stoddart Group providing a rapidly subsiding sediment sink that filled continually with siliciclastics that were persistently shallow marine and, during and following deposition, heavily soil-imprinted. The graben geometry focused tidal energy, suppressed wave and storm energy resulting in tide-dominance and developing a large mesotidal to macrotidal estuarine complex during Late Viséan time (late Late Carboniferous). The graben also localised an incised valley system during a large relative sea-level drop and localised a mega-estuary during a subsequent relative sea-level rise. Graben casting stages controlled development of stratigraphic and sequence stratigraphic units.