Abstract Recent work indicates that most modern continental sedimentary basins are filled primarily by distributive fluvial systems (DFS). In this article we use depositional environment interpretations observed on Landsat imagery of DFS to infer the vertical succession of channel and overbank facies, including paleosols, from a hypothetical prograding DFS. We also present rock record examples that display successions that are consistent with this progradational model. Distal DFS facies commonly consist of wetland and hydromorphic floodplain deposits that encase single channels. Medial deposits show larger channel belt size and relatively well-drained soils, indicating a deeper water table. Proximal deposits of DFS display larger channel belts that are amalgamated with limited or no soil development across the apex of the DFS. The resulting vertical sedimentary succession from progradation will display a general coarsening-upward succession of facies. Depending on climate in the sedimentary basin, wetland and seasonally wet distal deposits may be overlain by well-drained medial DFS deposits, which in turn are overlain by amalgamated channel belt deposits. Channel belt size may increase upward in the section as the DFS fills its accommodation. Because the entry point of rivers into the sedimentary basin is relatively fixed as long as the sedimentary basin remains at a stable position, the facies tracts do not shift basinward wholesale. Instead, we hypothesize that as the DFS fills its accommodation, the accommodation/sediment supply (A/S) ratio decreases, resulting in coarser sediment upward in the section and a greater degree of channel belt amalgamation upward as a result of reworking of older deposits on the DFS. An exception to this succession may occur if the river incises into its DFS, where partial sediment bypass occurs with more proximal facies deposited basinward below an intersection point for some period of time. Three rock record examples appear to be consistent with the hypothesized prograding DFS signal. The Blue Mesa and Sonsela members of the Chinle Formation at Petrified Forest National Park, Arizona; the Tidwell and Salt Wash members of the Morrison Formation in southeastern Utah; and the Pennsylvanian-Permian Lodéve Basin deposits in southern France all display gleyed paleosols and wetland deposits covered by better-drained paleosols, ultimately capped by amalgamated channel belt sandstones. In the Morrison Formation succession, sediments that represent the medial deposits appear to have been partially reworked and removed by the amalgamated channel belts that show proximal facies, indicating that incomplete progradational successions may result from local A/S conditions. The prograding DFS succession provides an alternative hypothesis to climate change for the interpretation of paleosol distributions that show a drying upward succession.

Abstract Sandstone units in the Middle to Late Devonian Lower Clair Group form the reservoir in the Clair Field, UKCS Block 206, West of Shetland. The sandstone facies vary from thick, well-sorted, medium sands deposited by aeolian processes, to amalgamated fluvial channel deposits of coarser sand, to thin sheets of fine sand deposited in floodplain or shallow lake settings. The distribution of these different facies within the basin-fill succession was controlled by the locations of sediment supply into the basin and the climatic controls on the processes of sand distribution and sedimentation. Six lithostratigraphical subdivisions (Units I to VI) are differentiated by changes in the predominance of fluvial, aeolian and lacustrine facies. During periods of high sediment supply and relatively humid climate (Units II, IV and V), fluvial conditions dominated in the form of sandy to pebbly fluvial distributary systems laid down as terminal fans. The water-lain sandstone facies characteristics vary from stacked, coarse channel fills deposited by braided rivers (Unit II) to decimetre sand sheets interpreted as the deposits of poorly channelized flow at the margins of the terminal fan (Unit V). At times of relative aridity, the fluvial system retreated and aeolian reworking resulted in extensive sheets of well-sorted sands deposited as dunes or on sand flats (Unit III). Periods of wetter climate and reduced clastic input resulted in widespread lacustrine conditions. Lakes were fed by rivers which terminated in lake deltas ranging from coarse, fan deltas (Unit I) to fine-grained deltas (Unit VI). The Devonian Clair Basin is an example of deposition in a basin of internal drainage which was predominantly controlled by climate and sediment supply variations. A predictive model for sandbody character and distribution can be developed using an understanding of these controls on the evolution of continental depositional systems in the basin.