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Abstract

The Virgelle Member of the Milk River Formation, Alberta, Canada represents a sandy progradational depositional systems tract that contains linkages between offshore, estuarine, and coastal plain environments. Distinct upward-shoaling depositional successions include regional erosion surfaces that punctuate transitions from storm- and fair-weather-dominated deposition to tidal sand bars and estuarine channel complexes that developed as the systems tract prograded basinward.

The lower part of the Virgelle Member is characterized by hummocky and swaley cross-bedded sandstones depicting the transition from offshore to storm-dominated middle shoreface. A sharp, regionally flat erosion surface separates middle shoreface deposits below from two end-member upper shoreface/foreshore lithofacies associations above: (1) rare fair weather wave-reworked deposits or, (2) common tidally reworked deposits represented by outer estuarine tidal bars. The upper shoreface unconformity is thus dominantly a tide-cut source diastem (TSD), overlain by bathymetrically equivalent subtidal to intertidal sand bars typified by planar-bound, herringbone, cross-bedded sandstone.

The erosive base of extensive, laterally accreted estuarine channels (ECh) cuts into middle shoreface deposits and truncates the flat disconformity and the above-mentioned shoreface and esmarine successions. Tidal influence within the channels is recorded by carbonaceous bundles and couplets, reactivation surfaces, and subordinate, flood-directed, three-dimensional dunes. In addition, a restricted ichnofauna documents the influence of an estuarine environment.

The resulting depositional model depicts a west-northwest/east-southeast trending estuarine system, open to the east, that truncates the storm-dominated middle shoreface, and is itself cut by a belt of meandering estuarine channels merged into overlying supralidal coastal plain mudstones. The genera] distribution of palynomorphs is consistent with the progression of marine dinoflagellate-rich assemblages in outer estuarine tidal bars to mostly terrestrial assemblages in ebb-dominated estuarine channels.

A qualitative analysis of depositional regime variables Q, M, D and R within the supply-dominated depositional systems tract of the Virgelle Member highlights the critical importance of the sediment dispersal function D, in this case controlled by tides and storms. The corresponding relationship may be expressed as Q MD R. The proposed model for a progradational estuary contrasts with sequence stratigraphic models of transgressive estuaries because it is not restricted to specific relative sea-level stages, and because it arises from the linkage of depositional processes along the entire systems tract, from offshore to coastal plain.

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