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The AAPG/Datapages Combined Publications Database

CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 29 (1981), No. 4. (December), Pages 583-621

The Middle Proterozoic Purcell Supergroup in the Southwestern Rocky and Southeastern Purcell Mountains, British Columbia and the Initiation of the Cordilleran Miogeocline, Southern Canada and Adjacent United States

Margaret E. McMechan

ABSTRACT

Purcell Supergroup strata exposed in the southeastern Purcell and southwestern Rocky Mountains comprise a turbidite sequence (Aldridge Formation) gradationally overlain by shallow-water deposits. The lower member of the Kitchener Formation represents the transition from intertidal and shallow-subtidal clastic sedimentation of the Creston Formation to the shallow-subtidal shelf facies of the upper member. Varicoloured siltite and argillite of the Van Creek Formation mark the return to shallow-water clastic sedimentation. They are abruptly overlain by volcanic rocks and shallow-water clastic sediments of the Nicol Creek Formation. Stromatolitic dolomite and quartzite of the Sheppard Formation comformably to disconformably overlie the Nicol Creek Formation. The siltite and argillite comprising the Gateway Formation are interpreted as lagoonal deposits. The overlying subaerial quartzites of the Phillips Formation form a northward and westward thinning marker unit. The Roosville Formation, which forms the top of the Purcell succession, has been removed by pre-Devonian erosion over much of the area.

The shallow-water deposits comprising the Creston through Phillips Formations can be subdivided into three composite stratigraphic units that are recognized throughout the northern Belt-Purcell basin. Thickness variations in the lower two units outline a north-trending basin margin that is deflected more than 200 km westward near 49°N latitude. The rectilinear shape can be ascribed to deeply rooted block faults developed during continental rifting. This shape governed the later development of a major structural re-entrant in the Rocky Mountain thrust and fold belt. Thickness variations in the upper unit record the evolution of epeirogenic structures that apparently controlled the distribution of volcanic rocks within the Belt-Purcell basin. A re-evaluation of middle and upper Belt-Purcell correlations indicates that the hypothesis of a 'dome' supposed to have formed during deposition of the lower Missoula Group was based on incorrect correlation.


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