CARBONATE PLATFORM DROWNING IN A FORELAND SETTING - THE MIDCARBONIFEROUS PLATFORM IN WESTERN URALS (RUSSIA)

The stratigraphic evolution through time of the mid-Carboniferous Russ ian Platform deposited on the distal margin of the West Uralian progra ding foredeep basin can be observed in the approximately 3000-m-thick sedimentary cover that constitutes the upper Paleozoic foreland (mid-C arboniferous-Permian) of the Uralian fold belt. It is composed of wave -dominated, low-angle carbonate ramps of late Visean to early Moscovia n age and carbonate platforms bordered by slope reef mounds from the K asimovian to the late Artinskian-early Kungurian. Eastward, in the for edeep, the carbonate platforms pass into terrigenous deposits, A compl ete transgressive-progradational section at the transition between car bonate ramp and terrigenous deposits exhibits, from base to top, (1) d eepening-upward carbonate ramp deposits, (2) a shaly condensed section with volcaniclastics, glauconite, and phosphorites of late Moscovian to early Asselian age (the transgressive part), and (3) a shallowing-u pward section of turbidites ending in coarse-grained fan deltas, evapo rites, and meandering-river redbeds by the earliest Triassic (the prog radational section). This paper focuses on the uppermost part of the c arbonate ramp deposits (Serpukhovian to early Moscovian), These ramp f acies comprise peritidal channels and mud hats, wave- to storm-dominat ed shoals, and patch reefs that are overlain by channel-attached sand lobes of turbidites, black shales, and radiolarites of the condensed s ection, This large-scale deepening-upward trend from inner-ramp to out er-ramp environments reflects a change in the depositional profile, fr om a tide- and wave-dominated flat ramp to a wave- and storm-dominated , steepened ramp margin and then to foreslope turbidites. Correlation of sections, dated by fusulinid faunas in southern Urals, shows that t he ramp evolved from emergent in the north (Perm-Solikamsk area), as e videnced by karstification, and deepened to the south (Belaya area) wi th emplacement of carbonaceous foreslope debris flows and turbidites, capped by deep-marine black shales and radiolarites, The time gap incr eases northward and reaches a span including the Siuransky pro parte, the Akavassky, the Askynbashky, and the Asatausky pro parte biohorizon s, The mapping of these facies belts across all the Urals shows that c arbonate ramps wedge out in the middle Urals and give rise to widely d istributed,diversified, and deeper marine facies belts not only to the South, towards the Precaspian depression, but also to the North, towa rds the polar Urals, The drowning of these carbonate ramps and the cut off in carbonate production on the Russian Platform during the mid-Car boniferous is explained by a succession of low-frequency flexure-relax ation cycles in the foreland associated with dilution by siliciclastic s, outpouring of volcaniclastics, and changes in the nature and direct ion of the sediment input and high-frequency cycles mainly controlled by eustasy.