Petrography and Derivation of Jurassic-Cretaceous Clastic Rocks, Southern Rocky Mountains, Canada

Abstract

Late Jurassic and early Cretaceous sediments in the southern Rocky Mountain region consist of three lithologic divisions; in ascending order these are: (1) the Fernie Formation, consisting mainly of shale of Jurassic age, (2) the Kootenay Formation, consisting of shale and coal-bearing beds with subordinate sandstone, at least partly of Jurassic age, and (3) the Blairmore Group, consisting of a basal conglomeratic member of unknown age overlain by sandstones with subordinate shales of Early Cretaceous (Aptian) age. The precise position of the Jurassic-Cretaceous contact remains controversial. Toward the west, sections appear to be most complete, and a hiatus, or unconformity, increases in magnitude toward the east. Thus in the westerly sections coal-bearing Kootenay beds grade into the coarse-grained Blairmore clastics by increase in frequency, thickness, and grain-size of the sandstone and conglomeratic units. In the easterly sections there is no such gradation, and conglomeratic beds of the Blairmore cut into and overstep siltstones and coal-bearing beds of the Kootenay Formation. In the Mount Allan section (the source of most of the analyzed material), the cliff-forming conglomerate is composed of a series of interdigitating lenses of conglomerate with sandstone, and shale with sandstone. The majority of rock types analyzed are lithic or rock-fragment sandstones and conglomerates, the components of which consist of clastic mineral and rock fragments with clays, and authigenic minerals. The mineral and rock fragments are grouped into four suites which apparently originate from four distinct sources: (1) the Metamorphic Suite, from the Selkirks metamorphic province, (2) the Stable Detritals Suite, reworked from a Paleozoic and possibly pre-Paleozoic source, (3) the Pyroclastics Suite, from the Pennsylvanian-Permian volcanic province, and (4) the Phosphorite-Bitumen-Carbonate Suite from the uppermost Paleozoic and lowermost Mesozoic sedimentary sequence. Uplift and erosion of the distributive province in the west apparently began at the close of Permian time and continued intermittently into Jurassic time. The proportions of the components of the four suites are related to the grain-size of the sediments (an observation which is confirmed by statistical analysis) and thus to abrasion and sorting during the entire period of erosion of the distributive province. Sedimentary differentiation of the clastic material is related to sorting (or lack of sorting) by the transporting agent during a geologically short period of time. Rapid alternation of high- and low-velocity transporting media caused scouring of underlying beds and, with loss of transporting power, these media deposited poorly sorted coarse-grained material and ultimately moderately sorted fine-grained material. There was no sorting after deposition to complete sedimentary differentiation. Depositional features, such as festoon- and cross-bedding, and poorly sorted, bimodal sediments indicate that the clastics were laid down in a continental or alluvial environment. Late Jurassic uplift in the west resulted in increased erosion and rate of transport, culminating in the deposition of the basal Blairmore conglomerate and followed by the quieter estuarine and flood-plain conditions of middle Blairmore time. The course of the various rivers and their tributaries shifted laterally, so that the gross effect achieved is of one, homogeneous conglomeratic unit extending from northern Alberta and British Columbia to the United States. The Kootenay-Blairmore clastics are thus transitional between one type of sedimentary environment in the Jurassic and another in the Cretaceous and are indicative of one of a series of orogenies in the western hinterland which began at the close of the Paleozoic Era and culminated during the Tertiary Laramide orogeny.