UNUSUAL BRECCIAS IN THE PROTEROZOIC ATAR GROUP, MAURITANIA-MALI-ALGERIA: POTENTIAL DEPOSITION RELATED TO EXTRATERRESTRIAL IMPACT AND IMPACT-RELATED TSUNAMIS

Proterozoic strata of the Atar Group, Mauritania-Algeria contain the “blue beds,” an unusual breccia horizon that is correlated chemostratigraphically across >1000 km of the West African craton. The Atar Group consists of ~700 meters of intertidal to shallow subtidal, orange-weathering stromatolitic dolostone, which contrasts sharply with the breccia horizon. Along craton margins in western Mauritania and Algeria, the “blue-beds” are represented by a 4-6 meter thick succession of amalgamated blue-grey limestones that include clast-supported breccia, matrix-supported normally graded breccia, matrix-dominated beds with angular to contorted “floating” clasts, and grainstones composed of fine-grained cross-bedded clasts (Stagner et al. 2004). Clasts vary from <1 mm to 30 cm in diameter, and contacts between various facies are typically planar to deeply scoured. Ball-and-pillow structures, microfaulting, and minor sediment fluidization in the 50 cm beneath the “blue beds” led to an interpretation of the “blue beds” as a seismite deposit (Plaziat et al. 1990); although cyclicity among “blue bed” facies better fits an interpretation of cyclic storm-induced loading (Stagner et al. 2004). In cratonal central Mauritania, however, strata laterally equivalent to the “blue beds” are composed of a 2-5 meter thick brown-black polymict breccia. The breccia is clast-supported, with clasts from 2 mm to >1 meter in diameter. Larger clasts are angular, unsorted, and frequently contain internal stress fractures. Smaller clasts are commonly aligned along the margins of larger clasts and in deformation bands indicative of viscous flow. As in craton margin deposits, polymict breccias are based by a single horizon of large (>2 m diameter) ball-and-pillow structures. The lateral extent of this unusual breccia horizon and occurrence in a stable cratonal region makes it unlikely that these beds represent a simple, large magnitude seismic event. Likewise, apparent cyclicity of craton-margin deposits in the absence of more typical storm suggests that these beds do not represent simple storm-induced loading. Rather, we suggest that the Atar "blue beds” bear a striking resemblance to substrate disruption and tsunami deposition generated during a marine impact event and may represent a previously unrecognized impact horizon.