Sedimentology of the Viking Formation (Lower Cretaceous), Gilby A and B Fields, Alberta

Abstract

<p>Examination of 80 cores from the Viking Formation in the Gilby A and B Field area allows sub-division into eleven facies: 2 of which are predominantly mudstone, 2 fine to medium grained sandstone facies, 2 laminated-burrowed mudstone-sandstone facies, 2 pebbly sandstone facies, 2 conglomeratic facies and 1 facies (sideritized muddy siltstone with Skolithos) which represents an erosional or scoured surface. Trough cross-bedding is the dominant sedimentary structure preserved in the sandy facies. Low angle trough cross-bedding is observed in the pebbly sandstone facies. The conglomeratic facies lack stratification. Sandstone and conglomeratic facies are referred to as the Viking Formation "coarse sediment package" which is enveloped with sharp contacts by underlying siltstones and overlying mudstones.</p> <p>Both lateral and vertical variation of the facies sequence occur over the Gilby A and B fields. The Gilby A field may be delineated into 3 facies assemblages and the Gilby B field as 1 facies assemblage. The vertical variation in facies sequence of the coarse sediment package is observed across the width of the Gilby A and B fields, where 1 to 3 coarsening-upward sequences are observed.</p> <p>Detailed study of log markers (using 450 logs) and facies variations throughout the study region has revealed a scour beginning along the southwestern margin of the Gilby A and B fields. This scour surface is coincident with the lower surface of the coarse sediment package - represented by a chert pebble veneer to the southeast of the fields. The surface is also coincident with the sideritized muddy siltstone (with Skolithos) facies (immediately underlying the pebble veneer) found to the south and along the southwest margins of the fields, although the scour has removed this facies to the northwest. The scour surface drops stratigraphically towards the northeast margin of the fields and gently rises stratigraphically approximately 4.8-6.4 km beyond the northeastern margin of the fields.</p> <p>The development of this scoured surface is problematic. A transgressive period or relative rise in sea level could have eroded the shoreface (presuming the area was near the shoreface) leaving the scoured topography. This topography being subsequently infilled by the Viking coarse sediment package during a regressive period or relative drop in sea level. Conversely, a rapid drop in sea level could have resulted in the development of the scoured topography. This was followed by scour topography infilling during a continued, though slower regression possibly under influence of a longshore current system.</p>