The Basal Quartz (Early Cretaceous) consists of a nested succession of mineralogically distinct, unconformity-bounded units. The second oldest of these, the Horsefly sandstone, occupies a broad (28–52 km wide), north-south trending, flat-floored valley (the Taber-Cutbank valley) in northern Montana and southern Alberta (south of Township 15) that has a maximum relief of 50 m. The valley-fill deposits are entirely fluvial in origin. Two internal sequences are recognized. Each begins with a regional erosion surface and sheet sandbody consisting of amalgamated channel sandstones. Contemporaneous overbank sediments (now represented by mudstone-pebble conglomerates) were completely eroded. These sandstones are overlain by mudstone-dominated deposits which consist mainly of red vertisols and contain isolated, ribbon channel sandstones and sheet sandstones formed by crevasse splays. Each sequence accumulated under conditions of continuously increasing accommodation space. Tectonic movements, perhaps in response to episodic thrust loading, are thought to be the major control on accommodation; eustatic fluctuations were probably not important because the study area lay far inland at the time of deposition.

Syndepositional block faulting partially overprinted the regional changes in accommodation, producing constrictions in the valley at the location of cross-valley uplifts; slightly greater uplift of the northern part of the valley caused an overall down-stream narrowing. Local areas of greater subsidence trapped the northward-moving, extrabasinal gravel, accelerating the proximal distal fining trend. Such areas also tend to display evidence of higher water tables, including the preservation of organic-rich deposits, and to contain a higher number of channel sandstones because the rivers preferentially avoided the subtly uplifted areas. This study shows that the influence of subtle faulting can be significant during times of limited accommodation because it is not masked by rapid sedimentation.

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