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.

摘要

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.