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Geology of the Crawford Bay map-area. Livingstone, Kent Wayne

Abstract

The Crawford Bay map-area is fifty square miles on the east side of Kootenay Lake, southeastern British Columbia, and is part of a regional structural belt, the Kootenay Arc, (Hedley, 1955). It is underlain by strongly deformed and metamorphosed strata of Late Proterozoic to possible Middle Paleozoic age. Sill-like bodies and boudins of metamorphosed amphibolite, and subordinate post-kinematic granitic intrusions and basic dykes of probable Mesozoic age are present in the west part of the map-area. Lithologic succession was determined by use of primary structures in the east part of the map-area and by lithologic correlation in the more highly metamorphosed western part. Horsethief Creek Group, Lower Hamill Group, Upper Hamill Group, Mohican and Badshot Formations, and Lardeau Group comprise an apparent conformable succession. Rock-unit nomenclature is the same as used for similar strata in the Kootenay Arc of the Lardeau and Salmo areas. However, fault-bound Upper Hamill is composed of strata which do not have defined lithologic equivalents outside the map-area and more detailed study of this part of the succession is suggested. The sequence is unfossilifereous and and age of the strata is based on correlation of Badshot Formation with Badshot outside the map-area that has yielded Archeocyathids of late Lower Cambrian age. Three phases of deformation, of variable intensity, are recognized within the map-area. Phase one folds are the earliest tectonic event and delineate the main structural trend. Three major folds, outlined by stratigraphic mapping, the Plaid Lake syncline, the Washout Creek syncline, and the Crawford Bay synform, are believed to be phase one structures. Early phase folds are isoclinal "similar" with axial planes parallel to gross lithologic layering. Fold axes and related penetrative mineral lineation plunge up to 20° northward except where locally deformed by subsequent structures. Only small scale phase two folds are recognized. These are asymmetric "similar" folds with southeast trending axial planes which dip moderately to the southwest, and westward plunging fold axes. Major faults may have formed during either phase of deformation. Kinks and related quartz veins formed during phase three deformation. Folding of bedding by phase three kinks is considered to be flexural-slip. Regional metamorphism is synkinematic with phase one and two deformation. Culmination of metamorphism appears to be post-faulting and post-phase two deformation. Phase three deformation is post-metamorphic. Chlorite, biotite, garnet, diopside, and sillimanite zones delineate an increasing metamorphic gradient from east to west. Distribution coefficients, KD= Mg/Fe garnet / Mg/Fe biotite, of Mg and Fe in coexisting garnet and biotite pairs in Mohican Formation, are larger with increasing metamorphic grade from the east limb to the west limb of the Crawford Bay synform.

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