Abstract

The Fernie Formation (Jurassic) in the southern Rocky Mountains is predominantly shaly and can be classified into depositional facies on the basis of grain size, sedimentary structures, organic carbon contents and macro-and micro-fauna. “Anaerobic” sediments accumulated in oxygen-poor bottom waters and are papery, organic-rich shales with a low diversity foraminiferal or radiolarian microfauna; “aerobic” sediments are poor in carbon and contain macrofauna, bioturbation and a diverse foraminiferal assemblage. Intermediate, “dysaerobic” facies have a mainly agglutinated foraminiferal fauna.

Organic-rich shales of the Poker Chip Shale (Lower Jurassic) accumulated on a shallow, gently-sloping shelf where energy was damped at the shelf edge. In the Highwood and Ribbon Creek Members (Middle Jurassic), similar sediments originated well below storm wave base, separated from shallow water deposits by a thick sequence showing evidence of intermediate oxygenation. Sandstones of the Rock Creek Member (lower Middle Jurassic) were deposited on a shallow shelf, where they were constantly reworked by tidal currents. More episodic storm and gravity flows were responsible for the deposition of the interbedded sandstones and shales of the Pigeon Creek Member and Corbula munda Beds (upper Middle Jurassic). Changes in sedimentary environment may reflect an evolving basin configuration as subsidence deepened the northwest of the area during the Late Bajocian. Uplift led to westerly sediment sources in the Middle Bathonian.

Sediments group into five shallowing-upward cycles in each of which “anaerobic” mudstones pass upward into “dysaerobic” and thence into “aerobic” mudstones and sandstones. Cycles are terminated by condensed transgressive units where lower sedimentation rate led to abundant faunal concentrations and early diagenetic calcite, phosphate and berthierine formation. Relative sea-level rises at the end of cycles occurred in earliest Bajocian, Early Bajocian, Middle-Late Bathonian and Early Oxfordian times. The fifth cycle is that of the Passage Beds, capped by the Kootenay Group. Sudden rises in sea-level or increments of subsidence appear to have controlled the cycles and there is some correlation with global patterns of eustasy.