DEPOSITIONAL-ENVIRONMENTS OF THE MIDDLE CAMBRIAN ARCTOMYS FORMATION, SOUTHERN CANADIAN ROCKY-MOUNTAINS

The Middle Cambrian Arctomys Formation is a mixed dolomite-shale forma tion that varies from 20 to 160 m thick in the southernmost Rocky Moun tains of Alberta and British Columbia. The Arctomys Formation is the l ower shaly ''half cycle'' of the Arctomys-Waterfowl ''grand cycle''. I t contains no fossils, has a restricted easterly extent, but exhibits mud cracks, salt casts and metre-thick cycles. The Arctomys Formation comprises four facies assemblages: 1) thin-bedded to laminated carbona te mudstones; 2) shale-to-carbonate breccia cycles; 3) carbonate mudst one-to-shale cycles; and 4) interbedded grainstones and shales. The th in-bedded to laminated carbonate mudstone facies assemblage probably r ecords restricted peritidal to shallow lagoonal carbonate sedimentatio n. However, the metre-thick, upward-desiccating cycles that comprise t he bulk of the Arctomys Formation (shale-to-carbonate breccia cycles a nd carbonate mudstone-to-shale cycles) are most likely the deposits of inland shallow lakes and coastal seepage-fed lagoons that filled and developed dry mud flat soil caps. The most landward outcrops of the Ar ctomys Formation consist of interbedded grainstones and shales that re cord sheet-flood deposits on dry mud flats. The best overall modem ana log for the Arctomys Formation is the extensive coastal plain of South Australia and our proposed depositional model is based on this modem passive margin setting.