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The AAPG/Datapages Combined Publications Database
CSPG Special Publications
Abstract
Sedimentology
The Middle Jurassic Rock Creek Member and Associated Units in the Subsurface of West-Central Alberta
Abstract
Quartz-arenites of the Rock Creek Member (Middle Jurassic) are subtly distinct from overlying and adjacent quartz-arenites and sublitharenites of the Ellerslie Member (Lower Cretaceous). Chert content of the Rock Creek is consistently lower than 1.5 percent; in the Ellerslie it ranges from 3.5 to 10 percent. Rock Creek quartz-arenites are characteristically planar laminated or centimetre thick crossbedded and associated with conquinoid sandstones. Ellerslie sandstones are typically decimetre thick, crossbedded, and have laterally equivalent facies of lenticular bedded sandstones and shales.
Rock Creek sediments were deposited from an easterly source on a gently sloping, storm dominated shallow marine shelf. 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 northeast trending paleodrainage system which drained the central and eastern parts of the area.
Where erosion completely cut through resistant Rock Creek sandstones into 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. Coarse to medium grained fluvially derived litharenites and thinly bedded coals of the earliest Ellerslie were confined to valleys incised into the Jurassic. Subsequently, these sediments were conformably overlain by tidally influenced transitional and marine quartz-arenites, sublitharenites and shales (upper part of the Ellerslie), deposited during the transgression of the Ellerslie Sea, which filled and covered the pre-existing topography.
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