Geology Reference
In-Depth Information
The described patterns suggest considerations regarding the formation
conditions of ground water salinity and component composition, specifics
of their spatial distribution and the processes and mechanisms that caused
them.
The available data reflect the specifics of the region's geologic evolution,
of its tectonic regime, of the paleogeographic environments under which
depositional facies have been formed and distributed. They also suggest
the general direction in the paleo- and modern-day hydrochemical pro-
cesses, which resulted in the generation of basically low-salinity waters of
the sediment cover regardless of their genetic type.
Analysis of the available data suggests that in Pliocene complexes the
source hydrocarbonate-sodiumе type of relatively low salinity syndeposi-
tional formation solutions (11.5
17.5 g/l) was formed as early as in sedi-
mentogenesis and early diagenesis of the deposits. It occurred on contact of
the ooze (pore) solutions with alkali minerals present in elevated amounts
in the enclosing rocks. These alkali minerals formed in the process of gran-
ite weathering in the depositional basin sediment provenance areas.
The formed solutions were subsequently impoverished by the paleo-
infiltration of fresh meteoric waters. The infiltration occurred during the
deposition hiatuses at the times of inversions in the basin's geologic evolu-
tion. These hiatuses happened at the time boundary between Anthropogene
and Late Pliocene and between Late and Early Pliocene (Bakirov, 1959;
Barbat, 1961; Ver Wiebe, 1957).
Further increase in contents of the carbonates and bicarbonates in
ground waters (to 10
−
12 mg.-equiv.) occurred at a later stage of deposit
lithification (during catagenesis). At that time increased release of the
CO
3
−
and
HCO
−
ions from the filtration-leached rocks was implemented within
the elevated temperature zone. That was proved in experimental studies
conducted by Valukonis and Khodkov (1959), Kononov (1965). At the
same stage the increase in water alkalinity and some decline in salinity
were facilitated by hydrochemical effects which accompanied dehydration
of the montmorillonite clay [Kapchenko (1978) model].
Therefore, elevated ground water carbonate and bicarbonate concentra-
tions in the Pliocene series are the integral result of the three processes
stretched in time. At the first stage (diagenesis), at relatively shallow depths,
it was a result of the contact between pore waters from the lithified sedi-
ments and alkali-containing mineral associations in the enclosing rocks.
That determined the initial enrichment in
CO
3
−
and
HCO
−
and, thus, the
hydrocarbonate-sodium type. At the second stage (catagenesis), at much
higher depths (and high temperature) it was a result of the thermochemi-
cal leaching of reservoirs and dehydrating of clay members.
−
