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Stratigraphy and sedimentology of shoreface and fluvial conglomerates: insights from the Cardium Formation in NW Alberta and adjacent British Columbia

Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, QC H3A 2A7

A. Guy Plint

Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7

In this paper we describe little-studied exposures of the Upper Cretaceous Cardium Formation in the northern Foothills and Peace River Plains, located north of Twp. 58. We correlate sixteen principal outcrop sections along the Foothills and across the northern escarpment of the Cardium as far east as the Smoky River. Correlations are based on tracing bounding erosion surfaces, initially observed in well logs and cores, and later extended into outcrop. The Kakwa Member is conglomeratic in many of these sections. The member displays a spectrum of conglomerate types, of both shallow marine and fluvial origin. Depositional environments are differentiated on the basis of stratigraphic context, conglomerate texture, sedimentary structures, stratification style and paleogeographic setting. We describe in detail four localities exemplifying the main paleoenvironments. At Bay Tree, a 12 m thick section consists primarily of clast-supported conglomerates representing a wave-dominated shoreface and beachface. This environment is dominated by broadly horizontal stratification with horizontal facies heterogeneity on a metre scale. At Cutbank Lake, swaley-stratified shoreface sandstones contain pebble-lined scours that show an upward increase in thickness and lateral continuity and represent rip current deposits. At Mount Niles, conglomerate forms a 2–6 m thick, >200 m wide, channelized body consisting of heterolithic matrix-supported conglomerates and pebbly sandstones capped by gravel wave ripples. Deposition may have been in a river-mouth setting. At ‘Horseshoe Mountain’, the upper part of the Kakwa Member consists of metre-scale crossbedded pebbly sandstone and matrix-supported conglomerate deposited in a pebbly fluvial system lacking marine influence. For the entire study area, paleocurrent observations indicate a strong NW to SE alongshore transport of gravel, with storm waves approaching from the NE. Fluvial crossbedding is directed towards the NE. Some of the outcrop facies can also be recognized in core from the adjacent subsurface. Our descriptions of conglomerate facies in outcrop facilitate interpretation of the depositional environment of comparable units in subsurface. Unfortunately, the limited extent of exposures does not allow us to determine with precision the geometry of conglomeratic units. However, a broad conglomerate-rich zone >90 km in dip extent and 10–20 km in strike extent can be traced across the northern outcrop belt and probably records the position of a major and long-lasting pebbly fluvial system. Subsurface analogs would be "exploration fairways". At the reservoir scale, conglomerate bodies showing greater influence of wave reworking would tend to be more elongate, and have greater facies continuity/permeability, in a shore-parallel direction. The outcrops display lateral textural heterogeneity at scales of metres to 10s of metres. This heterogeneity would affect reservoir performance, but could not be captured (for reservoir modeling) using typical development well spacing.

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