Geology Reference
In-Depth Information
The role of undercompacted clay rocks in the emergence and formation
of structures was reviewed by many scientists. As early as 1935, Mirchink
identified in the Meso-Cenozoic complex over the southeastern plunge
of the Caucasus Major competent (Mesozoic, Pliocene) and incompetent
(Paleogene-Miocene) intervals. The former, in his view, develop relatively
quiescent large folds. The latter are the expressions of bed plasticity and
crushing, fine and complex folding. The role of plastic clays in the structure
formation was illustrated by Mirchink with the most typical example of
Kobystan. Developed there are numerous local highs of a complex struc-
ture with clearly-expressed squeezing-out of some formations, their push-
ing up through the fragmented crests of the folds, with frequent disruption
of the beds to form overthrust scales, with clear manifestations of twisting
forces, and other distinctive features pertaining to the disharmonious fold-
ing of plastic incompetent masses (Mirchink, 1935).
Similar cases resulting, in particular, in the “twisted” lamination of
(“crushed”) clays due to temporary increase in the pore pressure were
also described by Dott, Rich, Bentor and Froman. The latter two authors
showed the causative connection between the “twisted” lamination in the
Ordovician clay within the Licein marls in the Canadian Northwestern ter-
ritories (buried underneath the younger pebble-stone) with their defor-
mation from short-term load of the upper sequence (Gretener, 1969).
Discussing the same mechanism Agabekov identifies intraformational
or inter-bed plastic clay mass, which creates a disharmonious folding
by injecting it from the deepest part of a depression to its periphery. He
believed that the major factors were exogenous processes and the force of
gravity. The same circumstances, in his view, explain the fault direction
toward the plunge (Agabekov, 1963).
The study of the displacement mechanisms in plastic clays from the
deposit-overloaded synclinal zones into the underloaded zones of a weak
pressure (into the structural crests) indicates that such displacements
(including the vertical mass movement) are possible mostly under condi-
tions of maximum abnormally high pore pressure and its gradients arising
as a result of various geologic processes.
Substantial role of high pore pressure and load alleviation of uncom-
pacted elastic clay rocks in the folds' initiation and formation may be illus-
trated by real-life models. One such model may be clay behavior within a
restricted volume, and another one, the heaving of a construction pit bot-
tom while building large hydrotechnical facilities and tall buildings.
Gretener described the former case. When bentonite clay was placed
in salt water and then gradually loaded with sand, a semblance of the dia-
pir structure immediately emerged and the sand layer was pierced. When
