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«Geological Mapping of Remote Mountainous Regions Using Metric Camera Imagery Initial Experiences with Photogrammetric Space Images *) By Manfred F. ...»

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The depicted area lies within the range of the opening rift valley of the Red Sea.

This is a region of high interest for geologists, providing different types of basement, igneous, volcanic and sedimentary rocks, besides an interesting pattern of faulting tectonics and ring structures.

The Saudi Arabia scene shows the highest quality of the three scenes studied. It exhibits neither cloud cover (save for a 4 X 10 km2 and a 5 X 7 km2 thin cirrus-type patch) nor snow and contains practically no natural vegetation, the artificially irrigated fields representing a negligible percentage of the whole area. Relief and, therefore, shadows are subdued in comparison with the other two regions treated in this paper. Thus, the Saudi Arabia imagery provides ideal conditions for structural and lithological photogeological mapping.

3.2 Geological Image Interpretation Three major surface types dominate the scene (plate 1 and 2): a portion of the Red Sea, the Red Sea coastal plains, and the mountains formed by the western margin of the Arabian Shield (cf. CHAPMAN 1978 a, 1978 b, BAYER 1984), reaching an altitude up to 2404 m in that region. A fourth prominent surface class, extensive lava flows that show in deep black hues, may be separated within the Arabian Shield. The above-mentioned (section 3.1) lineaments and ring structures form other eyecatching elements.

122 Manfred F. Buchroithner Fig. 1: Location map of the STS 9/Spacelab 1 Metrie Camera scene Saudi Arabia. The scene consists of the images 01-0120-03 and 01-0121-03, acquired December 2, 1983. The frame depicts one image or operation strip no. 3.

An overall area of 189 kmX 189 km, i. e. 21 350 km2, has been interpreted as shown in plate 2. This acreage includes the southern 40% of image 01-120-03 that was mapped monoscopicly. A large portion of these 40% are covered by sea, and the rest is almost exclusively flat coastal sand plains. Thus, it seemed justified to add this area to the geological interpretation map as the third dimension would not have yielded so much additional information.

Though the Saudi Arabia images are perfectly suitable for a photogeological analysis, the limitations in identifying the various photogeological rock units without any reference data of the study area showed up drastically. Even with colour as an additional source of information, it was rather hard, and in some places impossible, to make lithological assignments in areas of metamorphic crystalline and various types of igneous rocks. Fortunately, it was possible to get two sheets of the Plate 1: Stereogram of a part of the Saudi Arabia scene. Reproduction of the STS 9/Spacelab 1 Metric Camera photographs 01-0120-03 and 01-0121-03, acquired December 2, 1983, in original scale (approx. 1 : 820 000). Originals are colour-infrared film positives. The city of Medina is situated close to the centre of the right margin of the left stereomate. For further explanation see text section 3.1.

Geological Mapping of Remote Mountainous Regions geological map 1 : 250,000 of the Kingdom of Saudi Arabia, which cover a part of the interpreted area (PELLATON 1979, 1981). H. HÖTZL and H.-J. BAYER (University of Karlsruhe) who provided these maps which can only be obtained in Saudi Arabia. Their courtesy is highly appreciated.

The Red Sea shore over long distances exhibits coral reefs, locally interrupted by major tidal channels. Close to the sea, there are spots with white salt crusts, or sabkha deposits, or mixtures of light coral sands and salt precipitates. In some places the coastal plains seem to carry remnants of elevated older Quaternary surfaces, probably consisting of either gravel or corraline rocks (cf. plate 2).

Dispersely distributed over the coastal plains, one finds fields of prominent eolian sand deposits. These fields become more frequent towards the margin of the Arabian Shield and are quite abundant in front of its foothills (windward side, see below).

Some minor dune fields are also evident in wider wadis inside the mountainous terrain. Most of these dunes - especially those on the coastal plains - belong to the parallel wavy or crescentic, the parabolic, the stringers and, to a lesser extent, to the star or radial type of the five major types of eolian sand deposits established by MCKEEN, BREED & HARRIS 1973 and MCKEEN & BREED 1974 respectively. They are obviously caused by winds predominantly blowing from W and WSW, thus reflecting the conditions in December 1983. Multitemporal studies, perhaps by means of Landsat data, could prove whether these sand fields are stable or shifting.

For detailed investigations concerning shape and intra-dune distances, however, the high spatial resolution and stereoscopic coverage of the Metric Camera imagery is far superior to Landsat MSS as well as Thematic Mapper data (cf. section 6.). The pronounced linear transverse dunes in front of the mountains show maximum lengths of some 8 to 10 km. Height measurements of these prominent dunes by means of a Kern DSR-1 analytical plotter yielded maximum values of some 80 m (mean of several measurements). The asymmetric profile is clearly visible, but the cross-sections of small dunes of whatever shape can be defined, as long as their height reaches a certain extent (say some 25 m) when interpreted at approx.

1 : 50 000 magnification.

An intensely dissected and eroded scarp (cf. CHAPMAN 1978 b) forms the southwestern periphery of the Arabian Shield mountains. These mountains mainly consist of Precambrian to Palaeozoic rocks (CHAPMAN 1979 a, BAYER 1984). Upper Proterozoic (PELLATON 1979, 1981) volcanic rock material predominates. These dark series contain zones of prevailing intercalations of light-grey rocks. The latter should be mafic dikes and fine-grained epiclastic sediments. They display clearly visible sedimentary contacts (facial interfingering). They are wedged (± vertical dip) along a NW-SE trending major lineament (graben, cf. below). To the SW of this feature, the above mentioned material shows a minimum thickness of 3-4.5 km (plate 2).

Granitic to granodioritic rocks also occur. They are dealt with in connection with granitic intrusion features further below.

Tertiary volcanism (CHAPMAN 1978 a, PELLATON 1981), due to the plate tectonic activities in the reach of the Red Sea rift system, is represented by an obviously extensive lava field SE of Medina. Locally, flow structures in the range of tens of meters can still be descerned in the Metric Camera stereomodel. This effusive series also occurs in isolated caprocks (and fills of ancient wadis?) farther to the NW of the 124 Manfred F. Buchroithner continuous lava cover. Obviously it represents an equivalent to the Harrah Basalt farther in the SE (cf. NEBERT 1969). The young age of the basaltic effusives can be easily inferred from the "overflow" over the old basement, their "fresh" blackish hue and their still well-preserved flow features on the surface. Volcanic vents/cones as well as small isolated patches of saline playa deposits can be found on and at the margin of the basaltic rock material.

Two major sets of lineaments can be separated. The most eye-catching one of these linear features is a major "fissure" trending NW-SE, ± parallel to the Red Sea axis, which cuts through most of the scene. Locally, this fissure widens to a grabenlike structure up to 3.3 km in width (see Sa'Adin Graben, PELLATON 1981). This feature (and its less prominent perpendicular complementary lineament set) perfectly fits into the supra-regional tectonic pattern of Post-Triassic fractures along the Red Sea (FURON 1968). The prevailing lineaments - which obviously are the surface expression of faults, run ± E-W and ± N-S. A conjugate pair strikes ± NW-SE and ± NE-SW, thus forming faults trending parallel and transverse to the axis of the Red Sea (plate 2). It may be noticeable that it is not the latter pair of faults but the E-W trending one which shows the better expression on the surface. The direction of wadis (and other drainage lines) is largely controlled by these two sets of comparatively very young faults (relative to the age of the rocks, cf. NEBERT 1969).

One of the most eye-catching elements of the whole Saudi Arabia scene is the large ring structure in the N of the stereomodel (plate 1-3, fig. 2). Apart from its asymmetry in morphological expression, it is nearly a textbook example and correlates almost perfectly to the structural elements of the Aorounga ring structure in the Borkou area of the southern Sahara (Tchad). In particular, the "inner and outer sand-filled ring ditches", which have been described from Aorounga by ROLAND 1974 and 1976, are easily identifiable in the eastern portion of the structure.

The whole feature shows an elliptic shape and a diameter of some 35 to 55 km, with its long axis trending ± E-W. For comparison, the Aorounga structure only has a radius of some 8 km, though it displays a rather perfect circular shape. The better discernability in the E might be related to a lower relief and fluviatile erosion in the ring ditches, whereas the western part is formed by the backbone of a mountain chain (plate 1 and 3).

Farther to the W-WSW of the stereomodel, a few more ring structures and circular features can be seen (plate 2). Their diameters reach some 15 km, thus being more or less of the size of the Aorounga structure. These rings, however, are not as clearly expressed as the large one mentioned above.

It is evident that the ring structures and the lava flows in the Saudi Arabia scene have a genetic relation to the rifting mecanism of the Red Sea. ROLAND 1976 mentions exactly 20 different geological phenomena which might cause circular features. Ring structures of the Aorounga type and that one described in the present paper, however, can be best explained by granite diapirism (cf. SORGENFREI 1971).

Alkalic granite intrusion plugs of circular or subcircular shape are known from the environment of the whole East African-Arabian rift system. PELLATON 1981, based

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^ Fig. 2: Structural photogeological map of the large ring structure depicted in plate 3, some 55 km NW of Medina. Letters A, B and C indicate ends and corner point respectively of fig. 3 top. The dip signatures indicate vertical to very steep (±70°) dips.

–  –  –

Fig. 3: E—W/S—N cross section through the ring structure of plate 3 and fig. 2 respectively. Location of section parts AB and BC is shown in fig. 2. Below a profile of the Aorounga ring structure (Tchad), adapted to about the same size, is given for comparison (after ROLAND 1976).

126 Manfred F. Buchroithner on field mapping, named the feature W of Medina "Jabal 'Adamar Batholith", formed by "alkalic to calc-alkalic granitoid". This large ring structure may count as one more classical example for granite diapirism.

Several major and minor lineaments cut the ring structures (plate 1-3; fig. 2).

These non-radial linear faults probably represent rejuvenated old planes of inhomogeneity. Based only on space image analysis, it is not so easy to make genetic statements in this respect, though slight vertical as well as horizontal movements along the lineaments before and after the intrusion seem to be rather likely.

As far as major lineaments are concerned, evidence for vertical movements along parallel trending faults is given in the central and north-western portion of the scene. This may lead to the concept of a horst-graben structure similar to that one described by NEBERT 1969 in a region some 100 km to the SE (cf. plate 2).

Presently, the U.S. Geological Survey and the Deputy Ministry for Mineral Resources in Saudi Arabia are carrying out a cooperative program that should lead to a geological mapping at 1 : 250 000 scale of the whole Saudi Arabian territory by means of Landsat MSS data. Ground control for the project was established by the USGS personnel using a helicopter and a Geoceiver (DOYLE 1984). None of the first results achieved within this project were available to the author. Although no resources were available to support field checking, from the geometrical point of view the present photogeological mapping (plate 2) should be more accurate than all previous geological reconnaissance mappings (including the one just mentioned) of that region carried out using either Landsat data or any other space imagery. The study was conducted very efficently demonstrating methodology that has considerable potential for a sound geological mapping when completed with ground reference data from integrated field work.

In 1969 H. E. C. van der MEER MOHR made a geological interpretation of a nearly vertical Gemini IV photograph acquired in 1965 of a part of the Hejaz area centered around the city of Medina with a scale of 1 : 500 000 (cf. also van der MEER MOHR in MEKEL 1978). He points out that, for this Gemini image, the amount of visible detail can only be appreciated when the photograph is compared with the geological map (U. S. Geological Survey 1963). Such a comparison, according to van der MEER MOHR, shows that most of the geological details of the map can be identified on the image, and that in some places photo evidence strongly suggests that the existing map could be revised. As to the topographical detail, the photograph definitely compares favourably with the map and the advantages of synoptic terrain photography are demonstrated dramatically. Naturally, a comparison of the Metric Camera space imagery and the Gemini photograph was of high interest; not surprisingly the new photogrammetric space imagery was far superior to these hand-held photographs in all respects.

Plate 4: Stereogram of the city of Medina (Al Madinah) and surroundings. Enlargement of a part of plate 1 to 1 : 100 000. Location see fig. 1 and plate 1 and 2. Strong photographic magnification, printing and the loss of colour result in a considerable degradation of this stereopair.

Moreover, the left stereomate is situated at the very right margin of the original Metric Camera photograph. Still, stereoscopic view yields an obvious surplus of information. Various details mentioned in the text (section 3.2) can be identified.

Geological Mapping of Remote Mountainous Regions The geographic information provided by the Metric Camera stereopair of images is substantial. In basaltic terrain, highways, roads and tracks show as clear, bright lines, probably covered with wind-blown sand. In the sand plains and saline playa deposits, too, traffic routes and also the famous Hejaz railway come out as comparatively lighter lines, locally also as bundles of whitish beaten tracks (plate 1).

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