BP

BP Impact Structure

The BP Impact Structure, also referred to as Gebel Dalma, is a significant geological feature located in southwestern Libya, approximately 165 km northeast of the Kufra Oasis. This deeply eroded impact crater is distinguished by three discontinuous, concentric ridges that rise above the surrounding desert plain. These prominent ridges are composed primarily of deformed sedimentary rocks, predominantly sandstone, siltstone, and minor conglomerate, belonging to the early Cretaceous Nubian Sandstone formation.

Discovery and Confirmation

The structure's potential as an impact site was first recognized in 1967 by Kohman and colleagues, who identified it along with the Oasis Impact Structure through analysis of space and aerial photography. W. M. Johnson also noted both landforms as possible impact structures while engaged in geological mapping of the Gebel Dalma area. A. J. Martin, working for British Petroleum, was the first geologist to visit the site in 1968 and subsequently named it after his company. More detailed field studies conducted in the early 1970s by B. M. French, J. R. Underwood, Jr., and others provided definitive confirmation of its impact origin.

Description and Morphology

The BP Impact Structure exhibits a morphology characterized by highly eroded and discontinuous hills forming three concentric rings. These hills can rise between 20 to 30 meters above the intervening valleys. The innermost ring, approximately 400–600 meters in diameter, is considered the deeply eroded central uplift of a small, complex impact structure. Outcrops of chaotically deformed light-colored sandstones, the oldest rocks exposed in the area, are found in the southern part of this inner ring. The middle ring, with a diameter of about 2 km, displays outward-dipping strata forming a prominent inward-dipping scarp, with dips ranging from 30° to 70°. The outer ring is a semicircular feature with a diameter of 2.8 km, though its southern half is heavily eroded and difficult to identify. Radar imagery has revealed a more complete bulls-eye pattern, suggesting a diameter of 3.2–3.4 km, which is larger than estimates derived from visual imagery.

Shock Metamorphism and Age

Evidence for an extraterrestrial impact is supported by the presence of shock metamorphism in quartz grains found at the site. Initial studies in 1974 by French and colleagues identified multiple sets of planar elements, including shock lamellae, in sandstone samples, indicating high-pressure shock. While some later investigations found only planar fractures, more comprehensive re-sampling and petrographic analysis in 2010 revealed diagnostic planar deformation features, planar fractures, and feather features in quartz grains, unequivocally confirming the impact origin. The absolute age of the BP Impact Structure remains unknown due to the absence of datable rocks and minerals. However, its maximum age is estimated to be between 90 and 120 million years old, based on the presumed age of the Nubian sandstone. It has been speculated that the BP and Oasis impact structures may be of similar age and possibly the result of a double impact event.