INCISION AND FILLING OF A LOWSTAND VALLEY - LATE ALBIAN VIKING FORMATION AT CRYSTAL, ALBERTA, CANADA

Crystal field in the Albian Viking Formation represents the fill of a lowstand incised valley at least 55 km long, 7-12 km wide, and up to 3 2 m deep. The valley trends south-north, with a smaller arm trending e ast-west; the open sea lay to the north and east. The valley cuts into five regionally extensive sandier-upward successions of open marine b ioturbated mudstones and sandstones. There were two separate episodes of valley filling. The first consists of (1) dominantly parallel-lamin ated sandstones in the south that grade northward into (2) bioturbated mudstones in the central part of the valley. These grade in turn into (3) bioturbated sandstones at the northern (seaward) end of the valle y. The valley morphology, restricted trace fauna, and tripartite facie s distribution suggest an estuarine environment. The sandstones in the south are interpreted as deposits of a bay-head delta, and the centra l basin mudstones represent the turbidity maximum. The bioturbated san dstones in the north were emplaced into the estuary from the marine en d. There is little evidence of tidal deposits, and the estuary appears to have been wave-dominated. The second valley at Crystal is narrower and cuts into the first fill. The second fill consists of (1) interbe dded fluvial sandstones and conglomerates in the south that grade nort hward through (2) sandstones into (3) bioturbated sandstones. There ar e no central basin mudstones, and the filling appears to have been flu vially dominated. The base of Valley 1 represents an original fluviall y formed sequence boundary completely modified by marine transgression , whereas the base of Valley 2 is a fluvially cut sequence boundary in the south and a marine-modified sequence boundary in the north. The v alley-fill deposits are truncated by a transgressive surface of erosio n. The valley morphology suggests a drop of relative sea level of at l east 30 m to incise Valley 1, and an equivalent rise to fill the valle y. There was then a second 30 m relative fall and a second 30 m rise t o fill Valley 2. The sandbodies within the valley are poorly connected as reservoirs. Their distribution is related to the two separate phas es of valley cutting and filling, and to the different points of coars e sediment input; the bay-head delta, and the marine ends of the main valley and the east arm.