Geophysical Signatures of two craters at Bajada del Diablo Crater Strewn Field, Argentine.

PREZZI, CLAUDIA BEATRIZ; MARÍA JULIA ORGEIRA; ACEVEDO, ROGELIO; PONCE, FEDERICO; MARTÍNEZ, OSCAR; CARLOS VÁSQUEZ; CORBELLA, HUGO; GONZÁLEZ, MAURICIO; RABASSA, JORGE

Foz do Iguassu, Brazil

Congreso; 2010 The Meeting of the Americas (2010 American Geophysical Union Joint Assembly Meeting); 2010

  Bajada del Diablo impact crater field is located in the Northern Patagonian Massif, Chubut, Argentine. Impact craters have been identified on two rock types: the Quiñelaf eruptive complex and Pampa Sastre Formation. In the first case, craters are placed in olivine basalts. On the other hand, Pampa Sastre Formation corresponds to conglomerate layers with basalt clasts boulder and block in size (up to 50 cm in diameter) in a coarse sandy matrix. The Filu-Có area includes at least 66 impact craters found in the Miocene basalts and the Late Pliocene/Early Pleistocene Pampa Sastre conglomerate. We carried out detailed topographic, magnetic and electromagnetic ground surveys in two craters (8 and A) found in Pampa Sastre conglomerates in Filu-Có area. Both craters are simple, bowl shaped structures with rim diameters of 300 m and maximum depths of 10 m. They have been partially filled in by debris flows from the rims and windblown sands. Total magnetic field was measured at 1563 stations located in and out of craters A and 8, using a Geometrics 856 proton precession magnetometer. The obtained data were corrected for the diurnal variations in the Earth’s magnetic field and the IGRF value was subtracted. Basalts boulders, sandy matrix and infilling sediments were collected and the correspondent magnetic susceptibilities were measured; the remanent magnetization of basalt boulders was also measured. 20 profiles were surveyed at crater 8 with a GEM-2 small broadband electromagnetic sensor using 5 different frequencies. Detailed crater topography was determined using a total station. 726 topographic points were surveyed in craters A and 8. The magnetic anomalies show a circular pattern with magnetic lows (-100 to -200 nT) in the crater’s floors, characteristic of impact structures. Furthermore in the crater’s rims, high-amplitude, conspicuous and localized (short wavelength) anomalies, ranging between 3000 and -1500 nT, are observed. Such large amplitude and short wavelength anomalies are not detected out of the craters. For all used frequencies, the electromagnetic profiles show lower apparent electrical conductivities in the crater’s floor, while the rims present notably higher values. From the detailed topography, morphometric parameters were calculated, suggesting an apparent depth of » 45 m and a true depth of » 110 m for both craters. Our results suggest that in the crater’s floors Pampa Sastre conglomerate would be absent. On the contrary, the crater’s rims exhibit high-amplitude, localized magnetic anomalies and higher apparent electrical conductivities, which would be related to the anomalous accumulation of basalt boulders and blocks remanently magnetized (probably due to shock and heat effects). The fact that such high-amplitude anomalies are not present out of the surveyed craters, supports this hypothesis. The morphological, geological and geophysical features of the studied circular structures can only be explained by means of an extra-terrestial projectile impact.