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In November, 1981, NASA's first Shuttle Imaging Radar mission (SIR-A) began producing maplike photographic strips of Earth scenes from orbital altitude. A Saharan radar image acquired over Algeria clearly delineates two sedimentary basins, Erg Occidental and Erg Oriental, separated by an elongated zone of exposed bedrock, the M'Zab Chebka. At the NE margin of the Chebka, rimrocks, slopes, and ejecta deposits of Talemzane meteorite impact crater appear as a distinct two km wide radar-bright ring. This unique circle of strong radar backscatter distinguishes the solitary impact structure from numerous dayas (similarly appearing karstic depressions) which characterize the region. The crater is prominent on radar, but is obscure on optically obtained satellite and aircraft images, as are partly buried fluvial drainage systems and fault-block traces developed in bedrocks of the Chebka. Radar detection of an annular drainage system indicates possible presence of a ring graben at the crater. Brightest radar signals on the image are cultural features at recently developed gas fields near Hassi er R'Mel.

While Part I was devoted to the study of bowl-shaped depressions in Algeria, the present article focuses on multi-ringed structures of possible impact origin on the Algerian Sahara platform. Four structures were selected from orbital, aviation and geologic documents but only three were visited. TIN BIDER (27°36′N; 005°07′E) is a concentric multiple ring structure at least 6 km in diameter. Upper-Cretaceous sedimentary beds outside the structure dip inward a few degrees at the periphery and become extremely folded nearer the center, yet a general circular symmetry is always retained. A clearly exposed contact between upper deformed beds and underlying on-deformed beds is remarkably flat, dipping less than 10° inward. The upper beds display strong centrifugal folding. In the center of the structure Lower-Cretaceous sandstones about 0.5 km above their normal stratigraphic position are exposed. Although no shatter cones, intensive brecciation, or fracturing were observed, there is definite petrographic evidence of shock metamorphism (planar elements) in the quartz grains of the central sandstones. Tin Bider is a probable impact structure. It is the only astrobleme known with such prominent ductile deformations, a characteristic which may be due to the nature of the target materials. Detailed studies are now required to understand the mechanism of deformation of this multi-ringed structure. Its formation may be early Tertiary in age. FOUM TEGUENTOUR (26°14.5′N; 002°25′E) is an 8 km diameter bull's eye ring pattern. Although the high circularity and the morphology are consistent with an impact origin, the prominence of ductile deformation, the nature of the formations (clay-gypsum with sandstone intercalcations), the type of folds, the relationships between the structure and a surrounding plateau, and the lack of any evidence of shock effects better support a diapiric origin. MAZOULA (28°24′N; 007°49′E) is an 800 m diameter multi-ring feature with a 300 m wide anticlinal dome which rises some 30–35 m above the surrounding horizontal strata. The dome is capped by a flat-lying massive carbonate layer dipping on the flanks in accordance to the surface topography. There are no special disturbances, no fractures or breccias. Mazoula is not impact in origin but may be a Cretaceous rudistid reef and/or a salt diapir.