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Regional Geology BP and Oasis Impact Structures, Libya: Remote Sensing and Field Studies Christian Koeberl1, Wolf Uwe Reimold2, and Jeff Plescia3 1Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria ([email protected]) 2Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Johannesburg 2050, South Africa ([email protected]) 3MP3-E163, Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA ([email protected]) Abstract. Remote sensing images and recent field studies provide important morphological and structural constraints for the BP and Oasis impact structures in Libya. The BP structure (25°19' N, 24°20' E) is highly eroded and appears on satellite images in the form of a set of concentric rings. The rocks are composed of sandstones of the Jurassic to Cretaceous Nubia Formation. The Landsat image allows division of the structure into inner, middle, and outer rings. The ERS1 and Radarsat radar images do not show much detail, but the rings are more pronounced than in the visible wavelengths, probably because the radar penetrates the sand cover. In the field it became obvious that the cited diameter of 2.8 or 3.2 km for the outer ring does not represent the actual crater diameter, as the “middle ring” represents the actual crater rim with a diameter of about 2 km. The middle ring is characterized by a distinct series of hills of up to about 30 m elevation above the surrounding desert, with rocks dipping at 30 to 50° outwards. Some parts of the rim are characterized by intense folding and faulting. The “outer ring” is a discontinuous ring of low hills with 1-2 m average elevation, dipping at 10° inwards. The “inner ring” is interpreted as a small central uplift structure (ca. 500 m in diameter) with steeply upturned strata that are intensely folded, faulted, and – in places – brecciated. 162 Koeberl et al. For Oasis (24° 35' N, 24° 24' E) diameters of 11.5 km or even 18 km (from radar images) have been suggested in the past. However, the topographically most prominent feature of this structure is a ring of discontinuous (up to 100 m high) hills 5.1 km in diameter. The rocks in this prominent ring dip outward and locally are intensely folded. It can be debated whether or not this feature represents the actual crater rim. 1 Introduction The BP and Oasis impact structures in southeast Libya are highly eroded. They appear as concentric ridges of deformed rocks that rise above the surrounding desert plain by only a few tens of meters. Both structures were first mentioned by Kohman et al. (1967), who described their location based on space and aerial photography. Martin (1969) provided additional detail of the BP structure, which is the smaller of the two. BP and Oasis are centered at 25°19' N and 24°20' E, and 24°35’N and 24°24’E, respectively (Figs. 1, 2); both are located close to the Libyan-Egyptian border. The names resulted from the fact that, in the 1960s, the first research visit to the Oasis structure was made by a field party of the Oasis Oil Company of Libya; BP was visited around that time by a team from the B.P. Exploration Company. The impact origin of the structures was supported in the early 1970s by French et al. (1972, 1974) on the basis of observations of planar deformation features (PDFs) in some of the crater rocks. However, the number of observations of these features was very limited. Underwood and Fisk (1980) suggested that both craters formed simultaneously by a double impact, but apart from the relative proximity (ca. 80 km distance) and a similar degree of erosion, no real evidence to support such a suggestion is available. At this time the absolute ages for these structures are not known due to lack of datable material, although an attempt was made to isolate apatite from sandstone samples for fission track dating (R. Giegengack, pers. comm.); so far not enough material has been available. The ages can only be constrained to post-Nubian; i.e., postdating the Jurassic to Lower Cretaceous target rocks; less than ca. 120 Ma. BP and Oasis Impact Structures, Libya: Remote Sensing and Field Studies 163 1.1 Libyan Desert Glass Based on the proximity of the BP and Oasis structures to the area of the Libyan Desert Glass (LDG), which is found in a limited strewn field about 150 km to the east (Fig. 1), and the lack of disturbed Jurassic to Lower Cretaceous sandstone strata in the area of the occurrence of the glass, several workers (e.g., Martin 1969; Underwood and Fisk 1980; Murali et al. 1988; cf. Koeberl 1997) have considered the possibility that the LDG might be related to one or both of the eastern Libyan impact structures. However, Diemer (1997) speculated that the BP and Oasis structures could Fig. 1. Location map, showing parts of Egypt and Libya, after Abate et al. (1999). The position of the LDG field (in western Egypt) is shown in relation to the BP and Oasis impact structures in neighboring Libya. No LDG has been found in Libya. 164 Koeberl et al. Fig. 2. Regional Landsat image, showing the BP structure at the upper edge of the image, the Oasis structure at the center. Inset shows the structures at slightly higher resolution. North is at the top for all satellite images. be older than the LDG. Giegengack and Underwood (1997) also discussed that the target rocks currently exposed at the BP and Oasis structures could have been covered at the time of crater formation under about 400 meters of younger sediment. Libyan Desert Glass is an enigmatic natural glass that is found in an area of about 3000 to 6000 km2 (the exact numbers vary between different BP and Oasis Impact Structures, Libya: Remote Sensing and Field Studies 165 publications), similar to lag deposits, on the floor of corridors between sand dunes of the southwestern corner of the Great Sand Sea in western Egypt, near the Libyan border (Fig. 1). The glass occurs as centimetre- to decimeter-sized, irregularly shaped, and strongly wind-eroded pieces. The total preserved quantity of the glass has been estimated at 1.4·109 grams, but it is quite likely that the original mass was much larger (Barnes and Underwood 1976; Diemer 1997). Attempts to determine the age of the LDG were made using the K-Ar and fission-track methods. Due to the low K content of the glass, the errors on these K-Ar ages are too high to be meaningful (Matsubara et al. 1991; Horn et al. 1997). The only precise ages of the LDG come from fission track age determinations, which gave 28.5 ± 2.3 Ma (Storzer and Wagner 1971) to 29.4 ± 0.5 Ma (plateau age; Storzer and Wagner 1977), and 28.5 ± 0.8 Ma (Bigazzi and de Michele 1996; see also Bigazzi and de Michele 1997, and Horn et al. 1997) ages. The origin of the LDG has been the subject of much debate since its discovery (cf. Clayton and Spencer 1934; Weeks et al. 1984; Diemer 1997), and a variety of exotic processes were suggested, e.g., a hydrothermal sol-gel process (Jux 1983 and Feller 1997); or a lunar volcanic source (Futrell and O'Keefe 1997). However, there is abundant evidence of an impact origin of these glasses, including the presence of schlieren (texture with discontinuous almost parallel bands) and partly or completely digested minerals, such as lechatelierite (a high temperature SiO2 melt phase); baddeleyite (a high temperature break-down product of zircon; Kleinmann 1969; Storzer and Koeberl 1991; Horn et al. 1997); and the likely existence of a meteoritic component (Murali et al. 1987, 1988, 1989, 1997; Rocchia et al. 1996, 1997; Barrat et al. 1997). This is also in agreement with Os isotopic data of dark bands in the LDG (Koeberl 2000). The source material of the glass remains a mystery. Storzer and Koeberl (1991) suggested from Zr/U and REE data that none of the sands or sandstones from various sources in the region are good candidates to be the sole precursors of the LDG. Compositional data for surface sands (Koeberl 1997) show significant differences from the average LDG composition. There is some chemical and isotopic similarity to rocks from the BP and Oasis impact structures in Libya (Abate et al. 1999). However, a recent Rb-Sr and Sm-Nd isotopic study of the LDG (Schaaf and Müller- Sohnius 2002) suggests that “Nubian” rocks are not likely precursors of the LDG. To complicate things even further, Barakat (2001) and Kleinmann et al. (2001) found some shocked quartz-bearing breccias in the LDG strewn field, but – so far – no evidence for an actual crater has been found in this area. The possible connection to the origin of the Libyan Desert Glass led us to start an investigation of BP and Oasis. We quickly realized the lack of 166 Koeberl et al. detailed geological and structural information, and of representative samples from these structures. The only geological studies were those of some oil exploration geologists and of Underwood and co-workers in the 1960s and 1970s, and all previous workers spent only up to a few hours at each site. Thus, in the late 1990s plans for an expedition were made by two of us (CK, WUR) to study both structures and obtain fresh samples. Unfortunately a number of logistical and bureaucratic problems delayed the actual fieldwork until January 2002.
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