Differentiation of incised valley systems from mobile streams: some examples from the oseberg field, Norwegian North Sea

Abstract

The Statfjord Formation (Rhaetian-Sinemurian) and the Ness Formation (Bajocian) in the Oseberg Field (Viking Graben) feature interstratification of fluvial sandstone bodies, and packages of mudrock-dominated floodplain deposits. Together, these two formations contain a set of different continental depositional environments, including braided stream (Statfjord Formation) and humid-climate delta plain deposits (Ness Formation). A comparative study offers a potential of testing sequence stratigraphic concepts in a broad range of continental environments, including the problem of distinguishing incised valley fill from mobile stream deposits. Thick sandstone bodies (tens of metres) of fluvial origin may represent incised valleys cut during base-level falls and filled during subsequent periods of base-level rise. Consequently, their basal bounding surfaces attract attention as likely sequence boundaries within alluvial units. This simple allocyclic model contrasts with current autocyclic models of alluvial stratigraphy, which are based on rivers being erosive and mobile and able to sweep across entire alluvial plains. Using the autocyclic models, thick fluvial sandstone bodies are likely to form due to vertical aggradation of fixed channels during periods of rapid subsidence, or by vertical stacking of mobile channel belts during periods of slower subsidence. The latter models also relate channel deposit abundance and sandstone body geometry to such factors as avulsion period, floodplain width and deposition rate, in addition to subsidence. This study investigates fluvial sandstone bodies of variable thickness on a local scale, as a contribution to establishing sedimentological criteria for differentiating between deposits that can be related to valley incision and those that are the product of deposition within mobile streams that erosively sweep across depositional plains. Sandstone body thickness and width, character of the encasing deposits, and the relationship between erosional surfaces and scours at sandstone bases and palaeosol units in the surrounding sediments form the basis for interpretations regarding the origin of these sandstone bodies. In both formations, the existence of incised fluvial valleys can be disputed, and the observed alluvial architectures indicates that deposition was governed by generally declining long-term rates of accommodation space development.