Architecture of the Middle Devonian Kvamshesten Group, western Norway: sedimentary response to deformation above a ramp-flat extensional fault
Authors: Osmundsen P. T. , Bakke B. , Svendby A. K. , Andersen T. B. 
Published: April 2008

The Mid-Devonian Kvamshesten basin in western Norway formed during late- to post-Caledonian extensional and strike-slip tectonics. The basin is entirely continental in origin, and was probably a closed basin through most of its history. It was filled by alluvialfluvial deposits belonging to two main terminal fan systems, one sourced in the hanging wall, one sourced in the footwall of the basin-controlling fault(s). Footwall-directed migration of the hanging-wall-sourced fluvial system probably took place during phases of relatively rapid, fault-controlled subsidence. This gave rise to retrogradational stacking of marginal conglomerates and to the footwall-directed migration of the main depocentre(s). During periods of increased sediment influx from footwall catchments and/or decreased subsidence rates, coarse, footwall-sourced material prograded far into the basin. The intercalation of material derived from footwall- and hanging-wall source areas, respectively, gave rise to a pronounced, kilometre-scale rhythmicity that is inferred to reflect fault-block rotation and footwall erosion. The basin fill also displays a pronounced rhythmic organization on the 80–150 m scale. The Kvamshesten basin was deformed during deposition as a result of its position in the hanging wall of a ramp-flat extensional fault and because of extension-parallel shortening. Thus, the pattern of subsidence was non-uniform and the sediment preservation potential varied across the basin. Deposition and preservation of floodbasin fine sediments was enhanced in areas close to the breakaway of the basin-bounding fault and in the hangingwall syncline that developed above the ramp. The crest of the rollover anticline was mainly occupied by high-energy braided fluvial systems represented by amalgamated sandstone complexes with abundant channel scour surfaces. The migration of the rollover anticline towards the breakaway of the principal basin-bounding fault resulted in the footwall-directed migration of facies belts upwards in the stratigraphy. In our interpretation, a kilometresthick, footwall-skewed amalgamated sandstone complex was constructed that marks the trace of the migrating rollover anticline through the basin stratigraphy. On the basis of stacking patterns of marginal facies, large-scale grain-size variations, the geometry of stratigraphic units and the overall lithofacies distribution, we present a sequence stratigraphic model for the basin fill. Within individual systems tracts, lithofacies distribution as well as inferred A/S ratios vary laterally because of the effects of differential subsidence and multidirectional sediment transport. Inferences of A/S ratio from vertical sections in one particular part of the basin thus cannot be used to characterize a systems tract as such. Grain-size turnarounds may, however, be used in the definition of systems tracts and sequences if the above relationships are well constrained and if turnarounds occur in stratigraphic units that are easy to correlate across the basin.