Storm-dominated shallow marine deposits: the Fernie–Kootenay (Jurassic) transition, southern Rocky Mountains

Abstract

The transition from the Passage Beds of the Fernie into the lowermost part of the Kootenay Formation is exposed in 11 sections in the Crowsnest Pass to Banff area. Six distinct facies can be defined. Facies A (thin bedded turbidites) and facies B (thicker bedded turbidites) both have sole marks indicating north-northwestward flow. Facies C is the most important and consists of interbedded sandstones and shales, with sole marks indicating north-northwestward flow, and "hummocky cross stratification" as the characteristic internal sedimentary structure. It is interpreted to be formed by storm waves in depths below fair-weather wave base. Facies D contains low angle intersecting sets of parallel lamination (beach) and facies E is characterized by trough cross bedding (fluvial). Facies F consists of lenticular sandstones and shales, with coals. The facies occur in the sequence listed and indicate a northward prograding beach complex. Following intense storms, water driven toward the beach surges back seaward, entraining sand and developing into a density current. If deposition from the density current takes place in depths stirred by the storm waves, hummocky cross stratification is formed. In slightly deeper, quieter water the density current deposits a classical turbidite. Thus, the eastward-prograding Kootenay "delta" of previous interpretations now appears to be a northward-prograding beach complex fed by rivers, but with sand transported alongshore by waves. There is no evidence of tidal current activity, and shallow marine deposition is dominated by storms.