chalkstone


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chalkstone

(chôk′stōn′)
n.
See tophus.
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In the zone VII-VII calcite content there is a substantial drop of its quality by several tenths of percent comparing to the upper part chalkstone position on the basis of the zone IXab (in Poohri Xb-"body").
Chalkstone complex in the uppermost parts of the boreholes, representing the eastern Poohri region and stratigraphically considered to be the zone Xd, does not correspond to the same zone, earmarked in the western Poohri region profile.
This way the eastern part is stratigraphically shifted up approximately by 30-40 m to the upper part and the chalkstone complex of the uppermost part of the Jizera formation is correlated with younger Rohatce formation (the Brezno formation, Coniacian) in the profile of the western Poohri region, represented by the Pd-1 borehole.
The morphology of the curve of carbonate content, representing here Soukup's zone IXcd, is identical with the course of the same curve of the chalkstone complex, creating the uppermost part of the Rohatce platform (Figs.
This correlation shows that most of the boreholes in their beginning part went through the chalkstone complex that is in this area mistakenly considered to be (with the exception of the Pd-1 borehole) the zone Xd.
The base chalkstone position Xb in the Poohri region reaches the thickness of 5-8 m and it is a typical chalkstone position in the whole Bohemian Cretaceous Basin also outside the Poohri region.
There is a mineral and chemical border D-4 over the base chalkstone position of the Soukup's zone IXab (in Poohri region Xb).
Here it is necessary to emphasize that already Krutsky (1975) draws the attention to a high lithological similarity of chalkstones of the zone Xb in the Poohri region with "similar chalkstone banks of a younger formation Xd" in the same region.
Its uppermost part is again ended by an outstanding mineral and chemical diskontinuity (operationally marked as D-3) that is a part of this complex and is correlated in the whole Bohemian Cretaceous Basin as an important minimum of carbonate content in the bed of chalkstone set that follows in its upper part.
From this point of view the idea of reduction of this thickness in well-known East Bohemian profiles is problematic as the zone Xd ("resounding inoceramus calcareous sandstones") is drawn near to the position of chalkstone complex (Soukup's zone IXcd, kalianas sandstones) to several meters.
The above mentioned chalkstone complex (Soukup's zone IXcd), easily correlated in the whole Cretaceous Basin, is in the eastern profile of the Poohri region mistakenly considered to be the zone Xd.