Abstract

Units C and D of the Falher Member, Spirit River Formation, have been studied in the area of Townships 66-73, Ranges 7-13W6. In both Units, the traditional interpretation of one northward-prograding shoreface is shown to be a major oversimplification. In Unit D, detailed cross-sections using well logs and all available cores show that the main sandbody can be subdivided into four separate sandbodies D1 through D4. The sandbodies are all characterized by flat lamination or swaley cross-stratification and are interpreted as wave-and storm-dominated prograding shorefaces. Conglomerates in the upper shoreface are present in D2, D3 and D4. Sandbodies D1 through D4 are separated by mappable mudstone partings (some with a pebble lag at the base) 10 cm or more in thickness. It is very unlikely that such thick and continuous mudstones could be deposited within a storm-dominated shoreface. They are therefore interpreted as the remnants of transgressive mudstones. It is suggested that after progradation of shoreface D1, a rise of relative sea level (RSL) terminated progradation and deposited a mudstone drape on the northern edge of the sandbody. A subsequent fall of RSL led to the progradation of shoreface D2. Continued fluctuations of RSL led to the progradation and draping of shorefaces D3 and D4. All of the shorefaces are overlain by floodplain mudstones, sandstones and coals. Deposition of Unit C began with a complex series of small rises and falls of RSL that deposited thin sandbodies C1 and C2. The major rise of RSL was followed by progradation of the first major swaley cross-stratified sandbody, C3. Conglomerate is locally present in the upper shoreface. Progradation of C3 was followed by a major RSL drop. The subsequent rise was accompanied by transgressive barrier islands and accretion of lagoonal deposits (unit C4 - lagoon) immediately north of the termination of the C3 shoreface. When RSL stabilized, the barrier shoreface prograded to form unit C4 - another swaley cross-stratified shoreface. This shoreface was fed by a south-north oriented channel at least 50 km long and 5-8 km wide. After avulsion, the channel was backfilled as a muddy estuary. The Unit D sandbody, although complex, extends almost continuously throughout the study area. The Unit C deposits are much more heterogeneous. being interrupted by east-west-trending mudstones (the C4 lagoon) and a south-north-trending mudstone-filled channel. The detailed high-resolution subdivision of Units D and C indicates RSL fluctuations of 10-30 m on a time scale estimated in the order of 40 000 years. The detailed subdivision also permits the exact stratigraphic and geographic placing of the productive shoreface conglomerates in units D2, D3, D4 and C3.

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