Carbonate platform drowning in a foreland setting; the Mid-Carboniferous platform in western Urals (Russia)

Abstract

ABSTRACT The stratigraphic evolution through time of the mid-Carboniferous Russian Platform deposited on the distal margin of the West Uralian prograding foredeep basin can be observed in the approximately 3000-m-thick sedimentary cover that constitutes the upper Paleozoic foreland (mid-Carboniferous-Permian) of the Uralian fold belt. It is composed of wave-dominated, low-angle carbonate ramps of late Visean to early Moscovian age and carbonate platforms bordered by slope reef mounds from the Kasimovian to the late Artinskian-early Kungurian. Eastward, in the foredeep, the carbonate platforms pass into terrigenous deposits. A complete transgressive-progradational section at the transition between carbonate ramp and terrigenous deposits exhibits, from base to top, (1) deepening-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-upward section of turbidites ending in coarse-grained fan deltas, evaporites, and meandering-river redbeds by the earliest Triassic (the progradational section). This paper focuses on the uppermost part of the carbonate ramp deposits (Serpukhovian to early Moscovian). These ramp facies comprise peritidal channels and mud flats, wave- to storm-dominated shoals, and patch reefs that are overlain by channel-attached sand lobes of turbidites, black shales, and radiolarites of the condensed section. This large-scale deepening-upward trend from inner-ramp to outer-ramp environments reflects a change in the depositional profile, from 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 the ramp evolved from emergent in the north (Perm-Solikamsk area), as evidenced by karstification, and deepened to the south (Belaya area) with emplacement of carbonaceous foreslope debris flows and turbidites, capped by deep-marine black shales and radiolarites. The time gap increases northward and reaches a span including the Siuransky pro parte, the Akavassky, the Askynbashky, and the Asatausky pro parte biohorizons. The mapping of these facies belts across all the Urals shows that carbonate ramps wedge out in the middle Urals and give rise to widely distributed, diversified, and deeper marine facies belts not only to the South, towards the Precaspian depression, but also to the North, towards the polar Urals. The drowning of these carbonate ramps and the cutoff in carbonate production on the Russian Platform during the mid-Carboniferous is explained by a succession of low-frequency flexure-relaxation cycles in the foreland associated with dilution by siliciclastics, outpouring of volcaniclastics, and changes in the nature and direction of the sediment input and high-frequency cycles mainly controlled by eustasy.