Comparison of sedimentation on passive continental margins versus island arcs (by the example of the Ulutau Formation in the South Urals)

Fazliakhmetov A.M.

Institute of Geology usc ras, Ufa, Russia

famrb@mail.ru

The Magnitogorsk Megazone of the South Urals that corresponds to the Devonian volcanic arc terranes is typified by widespread volcanoclastic deposits. They have a variegated, mainly volcanimictic composition and belong to different genetic types. The deposits of the Ulutau Formation (Givetian to Lower Frasnian) can be taken as an example [7]. They are represented by granulometrically different clastoliths and siliceous rocks. Clastic material is dominated by andesites, andesi-basalts and plagioclases, with less common rhyolites, dacites, quartz and limestones.

It has been determined that the accumulation of the deposits in the Ulutau Formation took place by force of gravity currents under deep-sea conditions [5]. Their sources were the volcanic terranes of the Magnitogorsk Island Arc System [8]. There were several deep-sea fans replacing each other in both vertical and horizontal directions [3]. Such an intricate structure is the result of varied and in many respects unclear settings not only as regards sedimentation, but also as regards mobilization of volcaniclastic material. Their reconstruction needs a comprehensive analysis of all available factual data, including the investigation results on recent sediment genesis within the sea areas adjacent to island arcs. Equally important are the data for sedimentation processes on passive continental margins where the mechanism of gravitational sedimentary movements (turbidity currents, etc.) has been most thoroughly investigated. In all probability, gravitites of exactly these zones are now understood in the greatest detail. They can be used as the standard when reconstructing mobilization and sedimentation in other geodynamic settings, but one should take into account the specific features of the terranes under investigation (front/inner island-arc slope, ensymatic/ensialic arc, etc.)

A correlation between conditions and processes of clastic mobilization, transport and accumulation on passive continental margins and island arcs (Table) made it possible to reveal the following general features of the volcanisedimentary genesis relevant in reconstructing the geologic history of the South Urals during the Devonian.

Table. Comparison of sedimentation on passive continental margins versus island arcs.

Mobilization of the main volcaniclastic part within the island-arc terranes occurs through explosions and coastal abrasion (if the volcanoes protrude above the water level) [1,9]. Active tectonic and volcanic processes cause the formation of rugged and rapidly changing topography. This is the reason for the occurrence of multiple sedimentary systems (in Lisitsin’s terminology [6]) replacing each other along the strike of the island-arc slope. The sedimentary cycle, including mobilization to deposition of volcaniclastic material, is of short duration, sometimes at an avalanche speed [6,3]. Combined with the juvenile character of the sediments, it presents problems in recognizing rock genetic types both during the field description and when lithochemical methods are applied.

References:

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2. Fazliakhmetov, A.M. (2011₁) On the causes for avalanche sedimentation of the Ulutau Formation in the West Magnitogorsk Zone of the South Urals. Herald Inst. Geol. Komi Sci. Centre 193:19-21. (In Russian).

3. Fazliakhmetov, A.M. (2011₂) Sedimentation conditions of the Ulutau Formation in the West Magnitogorsk Zone of the South Urals. Lithosphere 2:42-52. (In Russian).

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9. Murdmaa, I.O. (1961) Recent marine sediments in the Kuril’s volcanic zone. In: Strakhov, N.M. (ed) Recent Sediments in Seas and Oceans. Moscow. USSR AS Publ. pp. 403-418. (In Russian).

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