Sedimentology of a Prograding Shallow Marine Slope and Shelf Sequence, Upper Jurassic Fernie-Kootenay Transition, Southern Front Ranges

Abstract

<p>The Fernie-Kootenay progradational transition consists of 10 interpretable facies described in ascending order. A marine greensand (2-7 m thick) lies at the base. It is overlain by 4 turbidite facies (totalling about 50 m) deposited on the prograding slope. Thin (1-50 cm) Bouma B and BC sandstones with solemarks (NNW flow) are interbedded with bioturbate siltstone. Next is a shallow marine facies of interbedded thick (12-485 cm) sandstones and bioturbate siltstones. Sandstone beds have sharp, toolmarked (NNW) bases and rippled, bioturbated tops. Internal structures are hummocky cross-stratification, and low-angle divergent and broadly trough-shaped sets of parallel lamination which probably represent primary hummocky cross-stratification. These beds result from reworking by variably-oriented currents during deposition from a seaward storm surge density current over fairweather platform sediments. A thick (7-34 m) sharp-based sandstone unit lies on siltstone of the previous facies. Long, low-angle divergent sets of parallel lamination dip in some preferred direction at each locality (especially to the E and SSE). The facies can be interpreted as shoreline beach-acolian deposits amalgamated onto a storm sand bed. A thick unit (average 7-8 M) of trough crossbedded sandstone (N and E flow) follows and represents the deposits of braided streams. Coastal plain coal, siltstone and crevasse splay sandstone cap the sequence. Facies can be recognized in core and develop characteristic nuclear log patterns. At the southern end of the shallow, elongate (N/S) basin offshore storm surge currents deposited shoreline sand on the platform (reworked into hummocky cross-stratification) and prograding slope (turbidites). As the platform built up to shallow depths, rapid progradation of small, marginal, river- and wave-dominated systems superimposed shoreline facies on hummocky sandstone beds, producing northward progradation of the coast. Sediment was derived from the Cordillera and supplied by large north-flowing and smaller east-flowing braided rivers.</p>