Geology Reference
In-Depth Information
The Upper Jurassic is mostly reefs, up to 1,500-m thick. Out of the
Cretaceous, only the upper series is represented. It is over 800-m thick; car-
bonates in the section dominate clastics. Eroded Cretaceous rocks are over-
lain by the Lower-Tortonian marls with tuff, argillite and calcareous clay
interbeds (up to 250 m); by the Lower-Tortonian salt-bearing Tirassa Fm.
(up to 200 m) and sandy-clayey alternation of the Kosov Fm. (up to 1,200
m); and by the Lower-Sarmatian calcareous clays with siltstone and sand-
stone interbeds (the Dashava Fm., up to 3,500-m thick overlying the eroded
Tortonian complex).
The foredeep is highly asymmetric in a cross-section. Its geosyncli-
nal flank is narrow and steep; its platform flank is broad and low-angle.
The axial portion is noticeably offset toward the Carpathian Mountains.
Numerous lengthwise and crosscutting faults were mapped. They cause a
step-block structure of the region.
Two tectonic subzones are identified in the Internal zone of the fore-
deep, the Borislav-Pokut and the Sambor subzones. The former is a com-
plex anticlinorium filled with linear high-amplitude folds extending along
the faults. The folds are often overturned and thrust over one another; they
are cut by overthrusts and crosscutting faults. As a result, the brachian-
ticlinal portions of anticlinal zones may turn out to be in different fault-
blocks offset in the cross direction. The edge southwestern blocks (internal
or Borislav) form ledges separated by large overthrusts (1,500-2,000 m and
greater).
The Sambor subzone forms a synclinorium of several linear rows of folds,
which are thrown one over the other and on the whole over the External
zone of the Foredeep. The Internal zone is an area of strong neotectonic
activity. Local structures in the Internal zone are intensely faulted; nappe
tectonics and significant horizontal offsets are common. It causes drastic
disagreements between the structure of different stratigraphic intervals in
individual step-blocks.
The External zone is simpler: the local structures are not as strongly
faulted, and the structures of different stratigraphic intervals are almost
coincide. The general tectonic plan of the folding is mostly a northwesterly
monoclonal dip.
The commercial oil accumulations in the Internal zone are mostly asso-
ciated with nonuniform discontinuous Oligocene and Eocene lens-shaped
reservoirs. The External zone includes mostly natural gas accumulations
in the Mesozoic interval. The fields in the Internal zone usually comprise
stacked sheet-type, fault-trapped accumulations. The largest fields in the
Internal zone are Borislav, Dolinskoye and Bitkovskoye, and in the External
zone, Rudki, Khodnovichi, Dashava and Ugerskoye.
