AURICA DAMIAN
(ABSTRACT)
Each structural whole bears the mark of the major geological events it has
gone through.
Data concerning longitudinal seismic waves velocities in the units of the Eastern
Carpathians bend and in front of it may help to the better understanding of
their dynamics. On the basis of interval velocity data “Vint” of
seismic and sonic loggings, in this paper problems concerning the dynamism
of the External Dacides, the Moldavides, the Foredeep and the Moesian platform
were approached.
In the overlapping area of the Ceahlau nappe on the convolute flysch unit,
it was noted that Cretaceous formations, with different lithology, burial depth
and structural belonging (from the two units) were brought, under the action
of the strong tectonic stress of the Mesocretaceous, Laramic and newer phases,
to a high degree of compaction and diagenesis. This implies high seismic velocities,
of close values, of the order of 4400-4800 m/s.
In the overthrusting area of the Tarcau unit over the marginal folds unit,
velocities were determined in formations of rather different ages, from the
Cretaceous (100-66 My) to the Miocene (23-15 My). Here the major influence
in the formations compaction and diagenesis was that of the tangent tectonic
stress in the old Styrian, new Styrian and newer phases. The values of the
seismic velocities are between 3600-4800m/s.
In the Sub-Carpathian nappe, velocity data were monitored for the Burdigalian-Helvetic
formations at the bore-hole 1 Jitia. Even if these formations are young enough
(17-12 My), here also it was assessed a high degree of compaction-diagenesis
under the action of the tangent stress, of the Old Styrian, New Styrian and
subsequent phases, with a tendency of homogenization of the formations velocities,
in the whole column, towards the value = 4000 m/s.
In the internal foredeep (the situation in the 55 Barbuncesti borehole), the
compaction and the seismic velocities of the formations situated on a mobile
(Orogenic) basement were influenced also by the tangent tectonic compression,
the velocity values in the Upper Miocene-Pleistocen formations being situated
between 2500 and 3900 m/s.
In the medium part of the foredeep (the situation in the 6001 Mizil borehole)
in formations of the same age, under the regime of very quick subsidence, sedimentation
and burial, the compaction of the formations was delayed. Some Pelitic bodies
were buried and were preserved in an under-compacted state. There are no indications
of any contribution of the tectonic stress to the formations’ compaction.
The dynamics of the area consisted of a strong sinking tendency who explains
the presence in these formations of much lower velocities (2100-3000 m/s),
occasionally abnormally low.
On the external side of the foredeep (the situation in the 11 Baraitaru borehole)
in equivalent formations, velocities (2200-3100 m/s) are low. The compaction
was made without the influence of the tangent tectonic compression.
In the foreland of the Carpathians the preservation of some clayish formations
with low compaction (relatively low seimic velocity) in the Calarasi 2881 borehole
even if they suffered only the influences of the lithostatic pressure at 4250-4500
m depths and of the Palaeozoic (Devonian) age, show the absence of the tangent
stress contribution to compaction. Generally, in the Foreland compartments,
the indications of influences of the lithological structure, the lithostatic
pressure and age and the lack of indications of any influence of tangent tectonic
compression on compaction and seismic velocities, show that cover sedimentary
formations were insensitive to tangent tectonic stress. This was due to the
lack of freedom of direct action of the tangent stress on cover formations
supported on and solidary with the platform rigid basement. Without contributing
to the sedimentary formations compaction, the tangent compression was supported
by the tectonic compartment as a whole.
The compartments’ response to tangent compression has been different
depending on the stage, the sense and the amplitude of the relative displacements
they have undergone.
All seismic velocity data prove that the Eastern Carpathians bending area and
its foreland form a whole whose numerous units and structural compartments
are characterized by an accentuated diversity of the amplitude of the tendencies
and of the dynamics stages.
Key words: seismic, sonic logging, velocity data, orogen, foredeep, platform,
different dynamics.