BP Impact Structure

General

The BP structure is located in the Gebel Dalma area of Southern Cyrenaica, 165 km northeast of Kufra Oasis, and is centered at 25°19' N and 24° 20' E. It occurs in a broad northeast-trending syncline of Nubian Sandstone. Flanking the syncline are hills composed of Upper Paleozoic shelf clastics, in which neither volcanics nor evaporites have been observed. French et al. (1974) and Underwood and Fisk (1980) described BP as consisting of two discontinuous rings that surrounded a central bloc; the outer ring of hills is 2.8 km in diameter with about 20 m of maximum relief (our observations - see below - indicate only about 10 m maximum relief and, on average, not more than 2 m relief), with sandstone beds dipping inward at 3°-15°. The inner ring of hills is more deformed, has a diameter of about 2 km, and an average relief of 30 m. Most of these beds dip outward at 20°-40°. These two rings form an asymmetric (wider on the eastern side) ring syncline. According to Underwood and Fisk (1980), the inner ring also shows numerous gently plunging folds with axes tangent to the structure and dipping outwards by as much as 70°. The central block is 0.6 km in diameter and has about 38 m of relief. Beds are intensely jointed, and the eroded southern half of the block exposes the oldest rock in the area, a light-colored (purplish to whitish) sandstone that has been complexly folded.

McHone et al. (1995a,b, 2002) noted that much of the surface at BP is sand-covered. As Shuttle radar penetrates the sand, it reveals the underlying bedrock; the data show a nearly complete radar-bright pattern that, according to these authors, was typical of central uplifts. McHone et al. suggested that the radar data indicated a crater diameter of up to 3.2 km. Three radar-bright rings were observed and suggested to be composed of eroded blocks of Nubian sandstone. The structure reportedly is surrounded by a subtle fluvial drainage, not seen in visible imaging.

Within the structure itself, Underwood and Fisk (1980) reported only Nubian Sandstone. These authors noted that the structure consisted of three near-circular concentric rock outcrops. The innermost ring had high-angle and chaotic dips forming a mass of craggy outcrop. The middle ring consisted of uniformly outward-dipping (30°-50°) strata. The third ring dipped inward at 5°-15° and formed a low scarp, for most of its extent barely protruding above the surface. Underwood and Fisk (1980) interpreted these outer two rings to define a ring syncline.

Landsat Image

On Landsat imagery, the BP structure exhibits three concentric rings with diameters of about 425, 2075, and 3135 meters. Figure 4 illustrates the structure in Landsat images (see also color plate 1). The radar and Landsat images provide various levels of detail. In the visible wavelength images (e.g., Fig. 4 and color plate1), the rings appear to be separated; however, in the radar image (Fig. 5) they appear to be more continuous.

Fig. 5. Radarsat image of BP impact structure. The more continuous nature of the circumferential "rings" when compared with the visible image (Fig. 4) is apparent. Note also the interruption of the northwest-trending structure. North is at the top.

Inner Ring: The inner ring (the "central block" of Underwood and Fisk [1980]) is continuous around ~270° of arc, but the southeastern sector is missing. The ring is about 425 m in diameter. In infrared (IR) wavelengths, the ring appears more continuous. The ring is formed by a massif, which is about 150-160 m wide and surrounded by a 145 m wide comparatively flatter region with a few isolated, largely sand-covered hills - especially on the southern side. It is possible that bright sand on the southern side of the ring that obscures the southern half of the ring at visible wavelengths.

Middle Ring: The middle ring (the "inner ring" of Underwood and Fish [1980]) is composed of discontinuous hills of variable width and morphology. It is about 2075 m in diameter. In the visible wavelengths much of the middle ring appears to be composed of two concentric ridges separated by 150-200 m. In the south-southeast and north-northwest quadrants, the ring becomes wider with apparently more complicated structure. The south side has a large (409 x 495 m) massif, and in the northwest a massif is about 500 m across. The bedrock ridges have alluvium extending away for variable distances. In most places the bedrock ridges are less than 100 m across.

Fig. 6. Landsat images of the Oasis structure: CU indicates the Central Uplift / Inner Ring; MR indicates Middle Ring; OR indicates the Outer Ring. CP are the clastic plugs mentioned by Underwood and Fisk (1980) and discussed in the text.
Fig. 7. Radarsat image of Oasis impact structure, projected so that north is up. Compare to the visible image (Fig. 6).

On the south-southeast side (at azimuth 155°) there is a major drainage that crosses the middle ring from the interior. At 211°, there is a minor drainage, which seemingly also crosses the middle ring. It certainly drains the hills associated with the middle ring. At azimuth 322°, there is a northwest (N54°W) trending structure that cuts across the middle ring for about 450 m. Farther out are two more linear features that are offset from this feature and trend in the same northwest direction.

BP Structure

Fig. 8. Approaching the BP structure in the field from the west.

Outer Ring: There is an outer low-relief ring with a diameter of about 3135 m. This outer ring is defined only by low arcuate hills, although in the southeast and northwest there are larger hills. This outer ring is also of variable distance (1350 to 1900 m) from the center. At azimuth 285° there is a large hill that has a northerly trend, which would be on strike with the hills farther north and outside of the structure. However, in the radar scene (Fig. 5) it has a curved appearance as if it were part of the outer ring. In the field (see below) it is clear that this ring has a topography of only a few meters, with shallowly inward dipping strata, i.e., this cannot be the crater rim.

To the northwest of the crater, at a distance of about 2200 m from the center, there are a set of north-northeast (N20°E) trending ridges that do not appear to be affected by the deformation related to the formation of the structure. These features provide a limit on the extent of deformation. The hill to the northwest (N54°W), at 1742 m from the center of BP, does appear to be affected though; it has a slightly more easterly orientation than the other parts of the unit by about 10-20°, particularly at its southern end.

Throughout the rings, the bedrock exposed has very similar spectral character. The largest differences are observed for various surficial units, such as alluvium or sand cover of variable thickness. The material exposed in the middle and inner rings appears similar, and there is some variation in the outer ring, which may be due to aeolian materials.

Was this article helpful?

0 0

Post a comment