Sedimentology and allostratigraphy of regional, valley-fill, shoreface and transgressive deposits of the Viking Formation (Lower Cretaceous), central Alberta

Abstract

<p>The Viking Formation (Upper Albian) is overlain by the marine mudstones of the Lower Colorado shales and is underlain by the marine mudstones of the Joli Fou Formation. The Viking sediments form a third-order regressive-transgressive cycle that can be correlated with the global sea level curve, specifically with the third-order eustatic fall and rise of sea level at 97-98 Ma. The absolute duration of the Viking is estimated at 1.2 million years. The sediments of the Viking are subdivided into four different packages; regional, valley-fill, shoreface and transgressive deposits. The packages are separated by bounding discontinuities labelled VE2, VE3 and VE4. The regional deposits consist of bioturbated mudstones, siltstones and sandstones, forming 12 coarsening-upward, shelf to shoreface successions. These deposits form a progradational parasequence set that off laps towards the east. Regional shoreline trends are oriented N-S or NNE-SSW. A large relative fall in sea level followed the deposition of the regional deposits. It was triggered by the 98 Ma eustatic fall in sea level, and was enhanced by uplift in southwestern Alberta. This sea level fall resulted in the cutting of the VE2 erosion surface, reorientation of the Viking shoreline trends to NW-SE and introduction of coarse-grained sediments into the study area. Deposits filling the lowstand incised valleys are located at Crystal, Cyn-Pem, Sundance and Edson. A first phase of incision took place during the "VE2" lowstand, and a second phase of incision and filling occurred during the "VE3" transgression. The valley-fill deposits consist of conglomerates sandstones and mudstones. Tripartite facies geometries (sand-mud-sand) exist in the Crystal, Sundance and Edson valley-fill deposits. Shoreface deposits related to the VE3 transgression are located along the Joarcam, Sunnybrook "B", Sunnybrook "A", Chigwell, Wolf Creek/Gilby-Joffre, Bickerdike and Caroline-Garrington trends. The shoreface sediments were deposited during stillstands and regressions that were superimposed onto the "VE3" transgression. They consist of bioturbated mudstones and sandstones, and are oriented NW-SE. The transgressive deposits occur in localized patches and are the uppermost sediments of the Viking. These sediments consist of thinly bedded mudstones, siltstones, sandstones and conglomerates, and were deposited during the latter stages of the "VE3" transgression. The Viking sediments consist of 19 parasequences; 12 regional cycles and 7 shoreface cycles. Each parasequence was deposited in approximately 63,000 years. This cyclicity may have been caused by glacio-eustatic mechanisms, differential subsidence or autocyclic processes; the exact mechanism or combination of mechanisms that produced this cyclicity is unknown.</p>