Abstract

Two cliff exposures, 1 km and 0.7 km long and 10 m high, of the Middle-Upper Cambrian Waterfowl Formation were mapped in detail on the Costigan Thrust in southwestern Alberta. The formation here comprises 5 rock types: intraformational conglomerate; cross-stratified grainstone; wavy-bedded grainstone-mudstone; massive dolomite mudstone; and prism-cracked, cryptmicrobial-laminated mudstone. These rock types form a grainstone body in the upper 7 m of the outcrops and 4 fining-upwards sequences in the lower 3 m. The grainstone body is divided into 5 storeys, each of which has a basal erosion surface overlain by an intraformational conglomerate with interbedded thrombolites. Storeys contain lateral accretion bedding that is composed of decimeter-thick bedsets which are inclined up to 12 degrees relative to the basal erosion surfaces. The fining-upwards sequences comprise basal cross-stratified grainstone overlain by wavy-bedded grainstone-mudstone that is capped by prism-cracked, cryptmicrobial-laminated mudstones. Three fining-upwards sequences are laterally continuous. However, one fining-upward sequence pinches out along the outcrop face. We interpret the grainstone body as the deposits of laterally migrating, sinuous tidal channels. It is not clear whether the stacking of channel deposits into storeys represents superimposed channel-bar and channel-fill deposits from a single channel belt or superimposed channel-belts. The fining-upwards sequences may be shallowing-upwards sequences in the sense of James (1984) and represent aggrading-prograding tidal flat deposits. Alternatively, these sequences may be crevasse-splay and levee deposits analogous to fluvial overbank flood sequences. In this case, these sequences may not be related to eustatic sea level changes, although they resemble classic shallowing-upwards sequences and "punctuated aggradational cycles."

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