The north border of the North Caspian depression consists of a series of flexural steps composed of Sargayev-Tournaisian, Oka-Serpukhovian, and Asselian-Artinskian carbonate rocks. Thick organic beds with biohermal buildups predominate in these border steps. To the north of these are fine-grained organo-clastic limestones, and to the south are dark, clayey thin-bedded limestones.
These carbonate steps between the North Caspian depression and the East European craton began to form in Late Devonian time during the Sargayev Stage. Thickness of the carbonate rocks increases from north to south from a few meters to 350-700 m in the region of the border step. Then it is reduced sharply to 100-30 m.
The carbonate steps separated areas of shallow-water shelf from a relatively deep-water basin. The development of these carbonate steps was uneven, thickness in the Yershov sector on the east being 300 m less than in the Krasnokut area on the west.
In the Early Carboniferous the area of uncompendated downwarping shifted toward the central regions of the North Caspian depression with the result that a new flexural downwarp formed to the south of the Sargayev-Tournaisian carbonate step. It developed as a result of abrupt thinning of the Oka-Serpukhovian complex.
In late Bashkirian-Vereiskian time the uncompensated downwarp that had formed in the earlier stages was filled by clastic material. Some of these clastic sections are a thousand meters thick.
A thick carbonate complex was deposited during Kashir-Artinskian time. It is subdivided into two parts: Kashir-Upper Carboniferous and Asselian-Artinskian.
Thickness of the Kashir-Upper Carboniferous sediments within the Lebyazhin structural terrace increases gradually toward the depression from 600 m to 800 m and then thins abruptly toward the south. This reduction ia due in part to erosion and in part to non-deposition.
The overlying Asselian-Artinskian rocks form the Permian border step, which is a high-amplitude flexural warp that joins two terraces of different hypsometric level. The upper is called the Lebyazhin, and the lower the Pigarev. Areas favorable for exploration are plotted on a map.
Taken from Kaleda and others, 1986; digested in
Petroleum Geology, Vol. 23, No. 5/6, three maps and
one cross section.
Copyright 2003 James Clarke. You are encouraged to
print out this News Letter and to forward it to others.
Earlier News Letters are available at:
http://geocities.com/internetgeology/
This News Letter is distributed without charge in the
interest of our science of petroleum geology. To be
added to the mailing list please send your e-mail address
to: jamesclarke@erols.com
For information on the journal Petroleum Geology please
telephone 703 759-4487 or FAX 703 759-3754. Volume 35
for the year 2003 has been published and is available
for $89.00.
Worldly fame is but a breath of wind that blows now
this way and now that and changes its name as it
changes direction. - Dante