The Allostratigraphy and Sedimentology of lowstand and Transgressive Incised Shorefaces and Their Relationship to Incised Valleys in the Lower Cretaceous (Albian) Viking Formation, Alberta, Canada

Abstract

<p>The (Upper Albian) Viking Formation can be divided into highstand, lowstand, and transgressive systems tracts through the recognition of regionally extensive bounding discontinuities. Two complete stratigraphic sequences have been recognised, including lowstand incised valleys and associated lowstand shorefaces, and transgressive valley fill and related transgressive shorefaces.</p> <p>The oldest systems tract comprises a series of regional coarsening upward cycles. These consist of shelf-to-lower shoreface successions that form southeastward thinning progradational highstand systems tract. These successions merge with and possibly downlap onto the top of the underlying Joli Fou shale.</p> <p>Two approximately 30m deep incised valleys at Crystal field are interpreted to have formed in response to fluvial downcutting across an exposed shelf during two separate falls of relative sea level, each being at least 30m in magnitude. Backfilling of the valleys by brackish-to-marine sediments indicate that each relative 30m fall was succeeded by a rise of 30m or more. Several long, narrow, en-echelon, sand bodies encased in marine shales occur in the vicinity of the Crystal field and up to 100 kilometers to the northeast. The isolated, linear sandbodies rest within elongate, northwest-southeast trending asymmetric scours. The steep side of each scour faces northeast. Close to the steeper side of the scour, the sandbodies rest sharply on an erosion surface and comprise a coarsening upward succession of bioturbated, muddy sandstone to cross bedded sandstone. Four to five kilometers northeast of the steep edge of the scour the facies succession is more gradational; it begins with bioturbated silty shale, and then coarsens upward into cross bedded sandstone. The nature of the successions along with the morphology of the asymmetrical scours suggests deposition in an incised shoreface.</p> <p>The relationship between lowstand valley incisions at Crystal and the development of lowstand shorefaces has been determined from correlation of well logs and cores throughout the study area. The base of sequence 1 is characterised by a lowstand incised valley (Crystal incision #1) and the development of the Lindbrook (LBK) lowstand shoreface. A rise in relative sea level followed by stillstand moved the shoreline slightly to the west, where the Joarcam (J'CAM) sandbody was deposited as a transgressive incised shoreface succession. Renewed transgression brought the shoreline to Crystal, where sediments backfilled (fill #1) the previously incised valley from the Sunnybrook 'A' (SBK 'A') transgressive shoreface. Renewed transgression moved the shoreline to Chigwell (CHIG) where a third transgressive shoreface was incised.</p> <p>The base of sequence 2 was formed during a second fall in sea level. The Crystal valley was re-incised (Crystal incision #2) and the shoreface moved 90 kilometers to the northeast and incised at Beaverhill Lake (BHL). Rise in relative sea level moved the shoreface southwestward to Crystal, where sediment again backfilled the incision (fill #2) from the Sunnybrook 'B' (SBK 'B') transgressive shoreface. Resumed transgression cut a ravinement surface across the top of the Crystal and Sunnybrook 'B' sandbodies and moved the shoreline out of the study area and to the southwest.</p> <p>The incised shorefaces formed either at lowstand (LBK, BHL) or during pauses in an overall transgression (J'CAM, SBK 'A', CHIG, SBK 'B'). Lowstand shorefaces can be correlated with lowstand incised valleys, and transgressive shorefaces can be directly correlated with the transgressive systems tract deposits in the two incised valleys at Crystal. These internally consistent correlations recognise two sequences at Crystal rather than the one suggested in published illustrations.</p>