ABSTRACT

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 32 m deep. The valley trends south-north, with a smaller arm trending east-west; the open sea lay to the north and east. The valley cuts into five regionally extensive sandier-upward successions of open marine bioturbated mudstones and sandstones. There were two separate episodes of valley filling. The first consists of (1) dominantly parallel-laminated 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 valley. The valley morphology, restricted trace fauna, and tripartite facies distribution suggest an estuarine environment. The sandstones in the south are interpreted as deposits of a bay-head delta, and the central basin mudstones represent the turbidity maximum. The bioturbated sandstones in the north were emplaced into the estuary from the marine end. 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 till. The second fill consists of (1) interbedded fluvial sandstones and conglomerates in the south that grade northward through (2) sandstones into (3) bioturbated sandstones. There are no central basin mudstones, and the filling appears to have been fluvially dominated.

The base of Valley 1 represents an original fluvially 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 valley-fill deposits are truncated by a transgressive surface of erosion. The valley morphology suggests a drop of relative sea level of at least 30 m to incise Valley 1, and an equivalent rise to fill the valley. There was then a second 30 m relative fall and a second 30 m rise to fill Valley 2.

The sandbodies within the valley are poorly connected as reservoirs. Their distribution is related to the two separate phases of valley cutting and filling, and to the different points of coarse sediment input; the bayhead delta, and the marine ends of the main valley and the east arm.