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Stratigraphy of the Lower Rocky Mountain Supergroup in the Southern Canadian Rocky Mountains Scott, Darcy Lon

Abstract

The lower Rocky Mountain Supergroup of Pennsylvanian age contains five formations which in ascending order are: Todhunter, Tyrwhitt, Storelk, Tobermory and Kananaskis. All except the Kananaskis are new formations that are equivalent to the Tunnel Mountain Formation. The Todhunter, Tyrwhitt and Tobermory consist mainly of brown weathering, dolomitic and quartzitic, very fine- to fine-grained, pure, quartz-chert sandstones with some interbedded, locally f ossiliferous, sandy dolomite. The Storelk is quartzitic, very fine- to medium-grained, very massive, cross-bedded, white wheathering quartzi-chert sandstone. All these formations contain varying amounts of scattered medium and coarse sand. The Kananaskis consists of sandy and cherty, dense, microcrystalline, grey dolomite. The Todhunter, Tyrwhitt and Storelk Formations are interpreted as being Early Pennsylvanian (Morrowan) in age, and the Tobermory and Kananaskis Formation, which regionally may be facies equivalents, as early Middle Pennsylvanian (Atokan) in age. The contact between the lower Rocky Mountain and the underlying Etherington Formation (Late Mississippian Chesterian) is conformable and locally gradational. Contacts between the Todhunter and Tyrwhitt, and between the Storelk and Tobernory are unconformable, whereas that between the Tyrwhitt and Storelk may be conformable or unconformable. The lower Rocky Mountain is unconformably overlain by Permian, Triassic or Jurassic strata. The lower Rocky Mountain sediments were deposited in a structural basin which may have been partially isolated from adjacent basins to the north, west and south by low barrier arches. Individual formations in the succession thin in a northwesterly and southwesterly direction towards the basin flanks, where as little as 160 feet of the five formations was deposited and remains, whereas at least 1,175 feet of equivalent strata is preserved in the central part. The quartz sand was probably transported by rivers and longshore marine currents from a source lying to the east within the continental interior. Chert and phosphorite fragments ranging up to pebble size were probably locally derived. The strata of Chesterian and Morrowan age represent a regressive sequence. After Morrowan time, gentle warping, emergence, and erosion caused truncation of Lower PennsyIvanian and Upper Mississippian strata towards the east. Angular truncation, and local conglomerates composed of chert, phosphorite, sandstone and dolmite granules and pebbles mark this unconformity. Strata of Atokan age thicken westward and represent a transgressive, sequence which onlaps and truncates the underlying strata. The Pennsylvanian sandstones are a potential source of pure silica sand.

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