Fan deltas, their deposits and structural associations
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Fan deltas are like alluvial fans except they dip their toes in lakes and shallow seas. So, in addition to the alluvial component, there is subaqueous deposition down a relatively steep, angle-of-repose slope. Sedimentation along the delta front, or slope, commonly produces large, basinward-dipping foresets, one of the defining characteristics of fan deltas.
Fan delta deposits are generally coarse-grained; there is much sand and gravel. Distributary systems tend to be braided. Sediment is supplied to the delta front from where it avalanches down-slope or transforms to debris flows. Gravitational instability may also influence depositional mechanisms.
Fan deltas tend to accumulate where there is a decent supply of sediment; close to steep uplands, active faults, mountain fronts, thrust fronts, glacial lakes and fiords, and pull-apart basins. Deposition outboard of active extension faults can produce spectacular fan delta stacks on the hanging-wall block. Fan deltas associated with thrust faults may accumulate as basinward overlapping packages in the footwall, that are subsequently overthrust. In pull-apart basins, the locus of fan delta stacking parallels strike-slip displacement; often likened to a horizontal stack of dominoes – the Devonian Hornelen Basin (Norway) and Late Miocene Ridge Basin (California) are classic examples.
Here’s a paper on Bowser Basin fan deltas: Ricketts, B.D., and Evenchick, C.A. 2007. Evidence of different contractional styles along foredeep margins provided by Gilbert deltas; examples from Bowser Basin, British Columbia, Canada: Bulletin of the Canadian Petroleum Geologists, v. 55, p. 243-261.
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Typical Arctic fan deltas: Left Slidre Fiord. Braided stream supply to the delta front is clear, with the active channels regularly moving across the delta top. A gravel beach ridge formed along the inactive delta front, has become detached. Right: Small, steep sloped fan delta along Emma Fiord.
This small, very recent, dissected fan delta accumulated on the beach face at Kariotahi, south Auckland (Tasman Sea coast). Storm drainage through the weakly indurated Pleistocene dune-beach sands behind, deposited sand during high tide. The small delta built across the beach, and as the tide ebbed, the stream eroded into its delta. The overall concave (down) top surface is evident in both images.
Closer view of delta packages, shows foresets, and thin bedded interfan deposits. Icebox Canyon, Bowser Basin
Clear discordances between foreset conglomerate beds, and topset conglomerates in fan deltas at Mt. Cartmel (left), and Tsargoss Lake (right). Topset beds at Mt. Cartmel contain planar and trough crossbedded, clast-supported conglomerate that is interpreted as the briaded, alluvial portion of the fan delta. Bowser Basin.
Non-cohesive – greater degree of clast-support (left) and cohesive-muddy (right) debris flow conglomerate composing some fan delta foresets at the Mt Cartmel delta.
Reconstruction of fan delta-shelf and shelf-break gullies, outboard of active Late Jurassic thrusting, Bowser Basin, BC. For details, see: Ricketts, B.D., and Evenchick, C.A. 2007. Evidence of different contractional styles along foredeep margins provided by Gilbert deltas; examples from Bowser Basin, British Columbia, Canada: Bulletin of the Canadian Petroleum Geologists, v. 55, p. 243-261.