Sedimentology of the Middle Jurassic Rock Creek Member in the subsurface of west-central Alberta

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Quartzarenites of the Rock Creek Member· (Middle Jurassic) are subtly distinct from overlying and adjacent quartzarenites and sublitharenites of the Ellerslie (Lower Cretaceous). Chert content of the Rock Creek is consistently lower than 1.5 percent, whereas the Ellerslie ranges from 3.5 to 10 percent. Rock Creek quartzarenites are characteristically planar laminated or centimetre thick cross bedded and associated with coquinoid sandstones. Ellerslie sandstones are typically decimetre thick cross bedded and have laterally equivalent facies of lenticular bedded sandstones and shales. Rock Creek sediments were deposited on a gently sloping, storm dominated shallow marine shelf and were derived from the east. Dark marine shales of the "Upper Fernie" (Passage Beds - Kootenay equivalent) overlie the Rock Creek in places. Exposure and partial erosion of the Jurassic units occurred during early Cretaceous time resulting in a gently sloping paleodrainage system which drained the central and eastern parts of the area to the northeast. Where erosion completely cut through resistant Rock Creek sandstones to the underlying soft black shales of the Poker Chip Shale, cuestas formed on the pre-Ellerslie surface with the paleotopographic highs capped by sandstones of the Rock Creek. Early Ellerslie sandstones were fluvial and confined to valleys incised into the Jurassic. Subsequently, these sediments were conformably overlain by transitional and marine sandstones and shales, deposited by the encroaching Ellerslie Sea, which filled and covered the· pre-Ellerslie topography.
Bibliography: p. 108-115.
Marion, D. J. (1982). Sedimentology of the Middle Jurassic Rock Creek Member in the subsurface of west-central Alberta (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/12204