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Ancient Fan-Delta Systems
Resedimented Conglomerates of a Miocene Fan-Delta Complex, Southern Alps, Italy
An Upper Serravallian - Tortonian regressive sequence, representing the submarine part of a fan-delta complex, developed in the rapidly subsiding Vittorio Veneto area concurrently with uplift of the Southern Alps. It consists of four members, the first three of which include channelized and trough-filling sandstones and conglomerates deposited in a neritic environment, dominantly by highly concentrated turbidity currents. The importance of subaqueous mass-gravity processes suggests that the fan-delta complex prograded directly onto a submarine slope. The first member consists of nested channel-fills enclosed in outer-neritic mudstones. The second member consists of a series of large composite bodies thought to represent the fills of large slide scars and/or fault-controlled troughs trapping gravity-displaced sediments at high levels on the slope. The third member is made up of closely spaced ribbon-like small channel-fills probably representing a system of gullies generated by retrogressive slumping; this system headed in a sub-littoral environment, as judged by abundance of wave ripples and hummocky cross-bedding in the associated sandstones. Finally the fourth member consists of upward-coarsening littoral sequences and is bounded at the base by an angular unconformity.
A modification of Lowe’s (1982) model for deposition from highly concentrated turbidity flows is proposed: each sedimentation wave is thought to evolve from an initial inertia-flow carpet, through suspension sedimentation, to a final stage of ‘traction’ sedimentation. Proximal-to-distal changes include increasing sand matrix content and development of inverse and normal grading in pebble-cobble conglomerates from essentially ungraded units, the latter commonly showing steep imbrication. In addition to ungraded beds, proximal resedimented conglomerates show abundance of two characteristic units: 1) stratified layers of small-pebble conglomerate showing repetitive inverse grading and strongly anisotropic fabric thought to be deposited by basal inertia-flow carpets, and 2) antidune-like bedforms.
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