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Ancient Fluvial Systems
Alluvial Sand Deposition in a Rapidly Subsiding Basin (Devonian, Norway)
A small (=< 2000 km2), late-orogenic basin with a 25 km stratigraphic thickness of alluvium has been examined in an effort to define how the alluvial facies vary laterally between the basin walls and along its axis, and to determine the extent, if any, to which the rapid subsidence of the basin floor influenced the vertical organisation of the sedimentary pile.
The longitudinal infilling of the basin is the result of an alluvial plain or sandy fan delta, dominated by ephemeral, low sinuosity streams, prograding into floodbasin/lacustrine areas. There is a lateral facies change, either proximal/distal or axial/lateral involving coarse sandstones and conglomerates dominated by trough cross-strata, passing through finer sandstones characterised by planar cross-strata, into alternating fine sandstones, siltstones and mudstones dominated by ripple lamination.
The basin-fill is remarkably well organised into some 200 basinwide, upward coarsening cyclothems (100–200 m thick). These represent prograding sandstone bodies and are probably a response to major episodes of vertical (as a consequence of lateral) movement of the basin floor with respect to source areas. Each of these bodies is itself subdivided into laterally extensive upward coarsening sequences (10–20 m thick), dominant in the proximal reaches and more symmetrical sequences (2–10 m) in the distal reaches. Rapid subsidence and abundant sediment availability, leading to strongly prograding tendencies in the stream systems, is reflected even at this level of organisation in the alluvium.
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