General Geology of BP and Oasis Impact Structures

Both features are located in the Al Kufrah basin of southeastern Libya. At the present erosion level, the rocks that crop out at both structures are sandstones (more or less ferruginous) with minor conglomerates and siltstones. These rocks are strata of probably Early Cretaceous age (Goudarzi 1970; Tawadros 2001; Hallett 2002), which is generally characterized as a ferruginous, fine- to medium-grained sandstone containing abundant cross-beds, mud cracks, ripple marks and silicified wood. There has been some confusion about the name of the rock units. In the literature, the target rocks in the BP-Oasis-Kufra area have been called Nubian Sandstone, or as belonging to the "Nubia Group" or "Nubia Formation" (e.g., Klitzsch 1978). There seems to be quite some discussion regarding the names of these rocks in the general area, as summarized by Tawadros (2001). Regionally, this group/formation is characterized by buttes and mesas of more or less horizontal beds, rising up to some 100 m above the surrounding plains, and some, mostly northwest-trending, ridges. The color of these strata is variably white, yellow, brown, and orange, but in many places it is black, even on a fresh surface. The dark color derives from iron- and manganese-oxide mineralization that is so intense that blocks of Nubian Sandstone can resemble basalt. The thickness of the Nubian Sandstone in the region of these two impact sites is unknown, but it reaches a maximum thickness of 1700 m elsewhere (e.g., Tawadros 2001). The sandstones have experienced low-grade metamorphism and were subject of regional deformation (Underwood 1975, 1976; Underwood and Fisk 1980). Northwest-trending ridges have been suggested to be clastic dikes (Underwood and Fisk 1980), but they probably are differentially weathered zones parallel to northwest-striking fractures. There are regional post-Paleozoic northeast-southwest, as well as a strongly developed post-Lower Cretaceous northwest-southeast, trends. The latter is invariably associated with fault and joint development.

Figures 3a and 3b show aerial photographs of BP and Oasis, respectively. The BP structure was suggested to be of meteorite impact origin by Kohman et al. (1967) and Martin (1969). Earlier, it had occasionally been referred to as the "Jebel Dalma" structure. The only geological description available, until now, is that by Underwood and Fisk (1980), who noted that BP is a relatively small structure defined by two discontinuous rings of hills and a central peak (Fig. 3a). According to these authors, the inner ring has a diameter of 2 km with an average height of 30 m, and the outer ring, which is characterized by strata that dip inwards at fairly low angles, has a diameter of 2.8 km and a maximum relief of only ca. 15 m. The rocks at

Fig. 3. Aerial photographs of the BP (a left) and Oasis (b right) impact structures. The image of the BP impact structure shows the two rings of hills and the central peak, whereas for Oasis only the inner ring of hills (diameter: 5.1 km) is clearly visible. The outer set of disturbed strata (11.5 km diameter) is partly visible on the bottom part of the image. North is up in both images.

Fig. 3. Aerial photographs of the BP (a left) and Oasis (b right) impact structures. The image of the BP impact structure shows the two rings of hills and the central peak, whereas for Oasis only the inner ring of hills (diameter: 5.1 km) is clearly visible. The outer set of disturbed strata (11.5 km diameter) is partly visible on the bottom part of the image. North is up in both images.

the center of the structure show intense jointing, and bedding is difficult to discern. Although the outer ring has a diameter of 2.8 km, McHone et al. (1995a,b, 2002) speculated, based on interpretation of space shuttle radar studies, that disturbed beds, which are covered by a thin veneer of sand, extended to a diameter of 3.2 km.

According to Underwood and Fisk (1980), the Oasis structure is a ring-shaped feature with a prominent circular range of hills approximately 100 m high, which forms an inner ring with a diameter of 5.1 km (Fig. 3b). In contrast to BP, the Oasis structure lacks a well defined central peak. Most of the rocks within the central ring of hills are intensely folded, and beds may be vertical or overturned. Previously, a diameter of 11.5 km was assigned to the Oasis structure, based on an outer zone of disturbed strata that is visible in the field (Underwood and Fisk 1980) and from the air (Fig. 3b). However, disturbed strata, covered by a thin veneer of sand, are suggested to extend to a diameter of 18 km, based on Space Shuttle radar imagery (McHone et al. 1995a,b, 2002).

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