Publication Date:
2021-03-29
Description:
In the extensive sandstone complex between Bardai and Aozou (Tibesti Mts.‚ Central Sahara) two separate areas have been plotted on a ZEISS-Stereotop alter establishing a net of control points by radial triangulation (slotted templet method). The aerial photographs used were contact prints at a scale of 1 :20 000 and 1 : 50 000.
The purpose of this study was to demonstrate that stratigraphic and tectonic problems of unknown regions may be solved by means of aerial photographic interpretation if it is possible t0 get ground truth from a small test site. In the investigated area the conformity between test site and unknown region was so good, that we can speak of an „Analogon Aozou“ to the „Model Bardai“; this means that the same photo interpretation key may be applied t0 both areas.
The photographic tone is of special importance among the various parameters of interpretation keys as in arid climates it is mainly an expression of desert varnish intensity. This varnish normally masks the original rock colour but it enables the interpreter to detect different resistance against erosion in rocks. Therefore he may deduce different grain size in bedrocks as well as the decrease of consolidation in ignimbrites depending on the distance from the eruption—point or silified zones of jointing, fault breccias, mylonites etc. Furthermore it accentuates the contact metamorphism at volcanic vents, dikes and sills. The latter could be identified only in that way.
Since the desert varnish is also a function of time, age relations (e. g. between depositional terraces or alluvial fans) can be recognized.
In addition t0 the photographic tone homogeneity, surface texture, morphology, topographic position, fracture density and drainage density have been used for the differentiation of rock units. The last two parameters are discussed in regard t0 the comparability of data collected by different authors.
The geologic aerial photographic interpretation was carried out in the following steps: identification — comparison — deduction, and in mapping large areas it proved an ideal method compared with the traditional way of field work.
In the isolated sandstone complex, surrounded by volcanic rocks and metamorphic schists, four different sandstone units could be discerned and mapped for the first time.
The igneous rocks were distinguished by their chemical composition (acid, intermediate, basic), by their form (Stock, volcanic vent, dike, plateau), or by their mode of emplacement (intrusion, extrusion, effusion). In addition to the tertiary—quaternary phase one can deduce a formerly unknown precretaceous magmatic phase. Finally, the great tectonic structures were mapped and the directions of movement and the azimuths of fractures were determined.
The results of the aerial photographic interpretation from an area of more than 1000 km2 are presented in two separate photogeological maps (scale 1 : 50 000).
Description:
research
Description:
DFG, SUB Göttingen, FU Berlin
Keywords:
556
;
910.02
;
Zentralsahara {Geologie}
;
Sahara
;
Tschad
;
Tibesti
;
Tektonik
;
Geomorphologie
;
FID-GEO-DE-7
Language:
German
Type:
monograph_digi
Format:
73
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