C - Lublin

LANDSCAPE AND GEOCHEMICAL CONDITIONS FOR GENESIS OF
DIFFERENT KINDS OF CARBONATE ACCUMULATIONS IN
CHERNOZEMS OF THE SOUTHERN PRE-URAL, RUSSIA
Khokhlova O.S., Oleynik S.A.
Carbonate accumulations (CAs) have been studied at the topographical
sequence (catena) of the Ordinary Chernozems developed on the redeposited
diluvium of the Permian clays and sandstones in the Southern Pre-Ural, Russia.
The study site is situated at the steppe area of this region in the floodplain of the
Salmysh River (the tributary of the Sakmara River). 4 pits studied have been
located on the different heights and distances from the waterlogged floodplain of
the local stream (Fig. 1). The uppermost soil I occupied the automorphic position
in the catena, the lowest one IV – hydromorphic position and two soils II and III in
the middle part of the sequence – transit positions in the catena. The length of the
catena was 200-250 m, the difference in the heights – 1,5 -2 m. CAs in the soils of
catena have presented by the white soft spots, hard nodules and some intermediate
forms. They have been studied morphologically (on macro-, micro and
submicroscopic levels) and using instrumental techniques (X-ray diffraction
pattern, thermal and isotopic analyses).
The Ordinary Chernozems of the uppermost part of catena are changed by the
Meadow Chernozems in the lowest part of it. In the profile of the Meadow
Chernozems there was water at the depth of 150-170 cm in summer. In the soil I of
the uppermost part in catena the CAs have been presented by large brightly white
soft sports, in the soils II and III of the transit positions in catena these spots
become less whiteness, smaller and had a hard core, in the soil IV of the lowest
part in catena there were small whity-gray hard nodules.
Internal micro- and submicrostructure of the CAs in the soils of catena were
different. The white soft spots in the soil I of the uppermost part of catena is made
of microcrystalline calcite, there are dissolving and recryslallizing calcite grains in
voids, all mineral grains of matrix are surrounded by calcite coating. The white soft
spots with hard core in the soils II and III of the middle transit part in catena have a
very characteristic microstructure: on the background of homogeneous clay and
carbonate plasma impregnating by microcrystalline calcite there are some packed
undifferentiated cryptocrystalline sections – cores or sections of ooidsepic fabric
when we can see not a single core between soft calcite accumulations but a lot of
hard cryptocrystalline concretions. The hard nodules in the soil IV of the lowest
part in catena have packed cryptocrystalline microstructure, the dissolving and
recryslallizing calcite grains are distinctly seen in voids.
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