de STEFAN AIRINEI, RADU BOTEZATU
Universitatea Bucuresti
GEOPHYSICAL CONTRIBUTIONS TO THE GEOLOGICAL KNOWLEDGE OF DOBROGEA
(Abstract)
After a short review concerning historical and general considerations on
the geological and geophysical knowledge of Dobrogea, there are examined the
main contributions of the applied geophysics to the delineation and expression
of new geological problems in the following five directions: the "horst" character
of Central Dobrogea; the conceptual framework of the evolution and the relations
between the sedimentary cover and its basement; the hidden magnetic bodies;
the structure of the Danube Delta; the mineral resources of Dobrogea.
The "horst type" of Central Dobrogea – initially conceived
as a geological model – was fixed and enriched in the last two decades
by the results of the gravity, magnetic and seismic researches. The crystalline
basement was divided in the first row into three chief compartments of the
first order, heterogeneous as concerns their petrographical composition as
well as their geological age; the first ones are also separated into compartments
of the second and third order appearing this time as horsts, grabens and steps.
The horst of Central Dobrogea is comprised between the crustal dislocation
Pecineaga – Camena to the north and Capidava – Ovidiu to the south,
with regard to the two other chief compartments (Northern Dobrogea and Southern
Dobrogea) that are in the lower position. This structural situation is maintained
the same both in the continental platform of the Black Sea and in the terrestrial
territory situated to the west and north-west with respect to the Danube. Moreover,
the controlled source seismology attests that these relations are essentially
preserved as far as the Moho discontinuity.
The chief concept concerning the evolution and the relations between the basement
and the sedimentary cover of Dobrogea subsequently extended to the geological
interpretation of the whole territory of Romania, consists in the fact that
the structure of the sedimentary cover is dependent on the metamorphic basement
and, more generally expressed, that the structural layers are preponderantly
dependent in a successive order from top to bottom. There are outlined four
distinct basements, differentiated from the point of view of the geological
age and of the petrographical constitution, separated by crustal dislocations
that seem to have the role of a probable prejurassic system of transform faults,
as well as important differences regarding the thickness and the structure
of the sedimentary cover.
The hidden magnetic bodies, both intrusive and effusive, piercing either partially
or entirely the crystalline basement of Dobrogea, were revealed by the local
magnetic anomalies lines: four lines associated to the hercynian structure
of Northern Dobrogea and one along the Baikalian basement of the northern part
of Southern Dobrogea.
The Danube Delta and the Sinoe – Razelm lagoonary zone show, as concerns
their crystalline basement, step structures disposed as horsts and grabens
sustaining a sedimentary cover including beds Paleaozoic to Quaternary in age,
with discontinuous successions and variable thickness even for the low distances
as a result of the frequent vertical movements. The deltaic deposits rest on
a predeltaic relief that has represented, during the gulf and prelagoonary
phases of the delta, a system of isthms and islands emerged as far back as
the Antiquity. The deltaic deposits have successively settled on the marine
paleobeach of the delta that permanently goes forward from west to east.
Regarding the discovery of new ores or the extension of the known ones, the
geophysical methods have also brought important contributions to: iron (Iulia
and Palazu Mare), base metals (Altân-Tepe and Somova), barite (Somova),
sands with Ti and Zr (the marine beach from Sulina to Gura Portita), the knowledge
of the geothermalism and the distribution of the thermal waters (Mangalia).
The geophysical information, analysed and interpreted in correlation with all
the geological results, have created a new conceptual framework showing that
Dobrogea represents nowadays one of the better known territories of Romania
as concerns the geological structure and, on the other hand, the specific kind
in which the structural characteristics manifest themselves in the geophysical
anomalies.