Abstract

The Gates Formation forms part of a northerly prograding wedge of marine and nonmarine sediments of Albian age which was deposited in the Rocky Mountains foreland basin. Details of the facies developed during a fourth-order transgressive/regressive cycle in the “upper Gates” were derived from outcrops in the Foothills and from coal company borehole data, including more than 4800 m of core.

Linear bodies of conglomerate and sandstone, up to 40 m thick and 0.4 to 2 km wide, occur at the base of the “upper Gates”, overlying the D coal seam. Evidence of a tidal influence is provided by landward dipping crossbedding, reactivation surfaces, and herringbone cross-stratification. In outcrop, this unit exhibits large-scale bedforms which include 1.5 to 3 m thick sets of crossbedding and low-angle (less than 17 degrees) lateral accretion surfaces up to 20 m apart. These deposits are interpreted as having formed in an estuarine setting during the transgressive part of the “upper Gates” cycle. Specific depositional environments in the estuary mouth included laterally migrating channels and shoals with chute bars, and tidal deltas. Other transgressive facies include a thin marine lag, and shelf sandstone storm deposits.

Stillstand sediments are represented by lagoon, coarse grained barrier bar, and channel-fill deposits preserved at the landward limit of the marine transgression. The regressive sequence includes shelf sandstones, distributary channel and shoal sandstones, tidal flat, and coastal plain deposits. The linear conglomerate body at the base of the “upper Gates” is an example of an estuarine deposit formed during a marine transgression and relative rise of sea level. Together with other linear conglomerate bodies (e.g., lowstand channel deposits of the Cardium Formation) it forms part of a variety of linear conglomerate bodies in the Cretaceous Interior Basin of Western Canada whose origin is best understood in terms of sequence stratigraphy and relative sea-level changes.