engleski

The Agoudal (High Atlas Mountains, Morocco) shattered limestone: petrographical and geochemical studies and additional evidence of impact

Introduction Agoudal is the first and the only confirmed impact structure in Morocco so far, by the existence of well-preserved outcrops of shatter cones in a Jurassic marly limestone formations. Its size and its age are still debated, although estimates based on the denudation rate of the High Atlas Mountains and distribution of shatter cones has led to constrain the minimum size between 1 and 3 km and a minimum age of 1 My, which is quite older than the Agoudal meteorite. Additionally to the Agoudal shatter cones, another macroscopical structural feature related to the impact was observed in the field: vertical to overturned strata trending N150-N160 that are not related to the High Atlas tectonics. Methods and analysis Samples of shatter cones and breccia were collected during the field missions to the Agoudal impact site. Thin sections of Agoudal SCs were prepared and studied under the optical microscope (plane polarized light, PPL). Acetate peels of Agoudal breccia were made. XRD analyses were conducted to compare the Agoudal shattered limestone to a fragment of a un-shattered limestone included in a sample of breccia. Additional analysis such as calcimetry, were performed on the Agoudal shattered limestone and breccia. Results The shattered limestone consists of 85% of micrite, cemented by calcite, with a relatively low but somewhat variable amount of quartz grains with no traces of microscopical mineral deformation such as PDFs. The breccia is poorly sorted and rarely subrounded fragments with clastic matrix, containing lithic and mineral clasts that locally carry very thin calcitic veins. The finest-grained material appears, as a mixture of phyllosilicate alteration and felsic mineral particles. These gray-brownish colored, very finegrained, matrix bearing vein samples comprises very angular clasts of lithic fragments (1 mm to 1 cm in size). No evidence of possible melt components has been detected in this breccia. No breccia in breccia neither meteorite in breccia were observed. A new high temperature mineral phase magnesiowüestite [(Mg, Fe) O] was discovered in the shattered limestone. This mineral is not common in terrestrial sediments ; it is formed almost exclusively by high-temperature metamorphism of dolomites and magnesian limestones. It was suggested that the only plausible mechanism of its formation is by fractionation processes related to vaporization and/or condensation of ejecta during a major impact event (eg. Cretaceous/Tertiary boundary). Calcimetry analysis has provided indication of the total carbonate content of 80% in the Agoudal limestone and of 90.7% in the Agoudal breccia, which was compared to the content of calcite in Steinheim shattered limestone which is of 75%. Further comparison with other shattered limestones from other impact craters would give an idea about the amount of calcite that should be contained in a limestone material to form shatter cones, and if this amount plays a role in their formation. Conclusions The absence of shock deformation features in the studied samples of Agoudal shattered and unshattered limestone requires further petrographical and geochemical analysis. In this purpose, 65 samples from the Agoudal impact site were collected and are going to be studied using the optical microscope and geochemical analysis.

impact, shattered limestone, XRD, Agoudal breccia

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