EARTH SCIENCE LECTURE SERIES
Thursday, January 19th, 2017
1:00 pm, Room 104, Forestry and Geology Building, UNBF
A case study of the unusual alkaline pyroclastic rocks of the middle Cretaceous Crowsnest Formation, Southwestern Alberta.
Fantastic breccias and where to find them.
Robin Adair (P. Geo, géo)
Zorayda Consulting Ltd.
52 Lexington Lane, Fredericton, New Brunswick, E2A 5S6 firstname.lastname@example.org
The Middle Cretaceous Crowsnest Formation has been dated at 96 Ma (Folinsbee et al., 1957), and is comprised dominantly of alkaline pyroclastic rocks deposited under sub-areal conditions, as well as related epiclastic rocks, exposed in Southwestern Alberta, Canada (Adair and Burwash, 1996). The deposits contain abundant zoned crystal clasts and lithic fragments. The crystal clasts are sanidine, melanite garnet, aegirine-augite, titanite, and analcime with axillary magnetite/ilmenite, apatite, and sulphides. Rock fragments are heterolithic and comprised of cognate, juvenile, and accidental rock clasts. Juvenile and cognate rock fragments are distinctly porphyritic and exhibit sanidine, garnet and analcime phenocrysts. The crystal clasts in the pyroclastic deposits reflect these phenocryst assemblages. Accidental fragments exhibit a very wide variety of rock types from the Phanerozoic sedimentary rocks of the Alberta basin, to clasts likely carried from the crystalline basement based on the presence of igneous and metamorphic rock clasts that can locally be identified. Geochemistry from cognate/juvenile rock fragments supports a compositional range using field terms and phenocryst composition from “Blairmorite: (sodic: analcime, and lesser sanidine and melanite phenocrysts – can also be analcime cumulate) to analcime phonolite (intermediate: sanidine, analcime, and melanite phenocrysts) to trachyte (potassic: sanidine, and melanite phenocrysts). Trachyte is there dominant clast type. The rocks are silica-undersaturated, alkalic, and classify as metaluminous feldspathoid-bearing (analcime). There are few documented exposures of effusive or intrusive rocks and the vent or vents have not been identified in the field. The Crowsnest Formation is informally divided into a recessive, crystal-rich lower member dominated by well-bedded air-fall, pyroclastic flow, and pyroclastic surge deposits, and a resistant (ridge-forming) upper member composed of massive pyroclastic breccias dominated by lithic fragments as well as some minor air-fall deposits near its top. The lower member is light green to beige coloured, thin to thickly bedded, and depositional units exhibits a variety of internal stratification, cross stratification and complex grading of both crystal and lithic clasts, typical of emplacement by pyroclastic mechanisms (Adair, 1986; Adair and Burwash, 1996). The upper member is composed of dark green, massive, pyroclastic breccia deposits containing abundant heterolithic rock fragments and much fewer crystal clasts than observed in the lower member. Many lithic clasts in the upper member noticeably exhibit baked margins and/or exhibit a reaction halo in the matrix surrounding the fragment. These pyroclastic breccia units are thickly-bedded (10s of metres) west of Coleman and very poorly stratified other than normal, coarse-tail grading of the lithic fragments.
The formation is exposed in the front ranges of the Rocky Mountains in a series of en echelon thrust slices and related folds. The best and most complete exposure through the formation is along highway #3 approximately 3.2 km west of the town of Coleman, Alberta. This section is interpreted as vent proximal. The thrust slice exposing this section provides the thickest sequence of the formation (~400 m thick) with appreciable thinning in all directions. The formation can be traced laterally 37 km to the north and 50 km to the southeast of this section. Exposures in en echelon thrust slices are identified 17.9 km to the east (~+70 km reconstructed) and 8 km to the west (~ +20 km reconstructed). Crowsnest rocks conformably overlie the fluvial-deltaic Blairmore Group and are unconformably overlain by the marine Blackstone Formation. They were emplaced/ejected in a within plate setting to the west of the Western Interior Seaway that extended from the gulf of Mexico to the arctic during mid Cretaceous time and to the east of highlands created by on-going subduction of the Farallon and Kula plates beneath the North American plate. Immediately following eruption, the inland sea transgressed over the area of Crowsnest volcanism. Thrust and fold deformation occurred during the formation of the Rocky Mountains that were created some 1100 kilometres inland from the subduction zone during the Laramide Orogeny between 70-40 Ma.