BAJADA DEL DIABLO IMPACT CRATER-STREWN FIELD (ARGENTINA): GROUND MAGNETIC AND ELECTROMAGNETIC SURVEYS

PREZZI, C.; ORGEIRA, M. J.; ACEVEDO, R.D.; PONCE, J. F.; MARTINEZ, O.; VASQUEZ, C.; CORBELLA, H.; GONZALES, M.; RABASSA, J. O.

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Año: 2010 vol. 51 p. 121 - 124

0006-6729

Bajada del Diablo impact crater field is located in the Northern Patagonian Massif, Chubut, Argentina (Fig. 1). Impact craters have been identified on two rock types: the Quiñelaf Eruptive Complex and Pampa Sastre Formation (Acevedo et al., 2009). Most of the rocks forming the Quiñelaf Eruptive Complex have been classified as trachytes, but other rocks are present, such as rhyolites, trachyandesites, trachybasalts, and pyroclastic rocks. Pampa Sastre Formation corresponds to conglomerate layers with basalt clasts boulder and blocks in size (up to 50 cm in diameter) in a coarse sandy matrix. The study area (Fig. 1) includes at least 66 impact craters found in Miocene olivine basalts of the Quiñelaf Eruptive Complex and in the Late Pliocene/Early Pleistocene Pampa Sastre conglomerate (Acevedo et al., 2009). It is widely accepted that a key tool in the initial recognition and characterization of terrestrial impact craters is geophysics (e.g. Pilkington and Grieve, 1992; Hawke, 2004). The magnetic signature of craters varies considerably (Pilkington and Grieve, 1992), but an overall circular magnetic low due to demagnetization of the target rocks and reduction in susceptibility is expected (Pilkington and Grieve, 1992; Hawke, 2004). Pilkington and Grieve (1992) established a set of general criteria that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis of impact origin of circular structures. However, such origin can only be confirmed on the basis of geologic evidence. With the aim of further investigate the proposed impact origin of the circular structures identified in Bajada del Diablo (Acevedo et al., 2009), we carried out detailed topographic, magnetic and electromagnetic ground surveys in two craters (8 and A) found in Pampa Sastre conglomerates. Both craters are simple, bowl-shaped structures with rim diameters of 300 m and maximum depths of 10 m (Figs. 2 and 3). They have been partially filled in by debris flows from the rims and wind-blown sands (Acevedo et al., 2009). Total magnetic field was measured at 1563 stations located in and out of craters A and 8, using a Geometrics 856 proton precession magnetometer (Figs. 2 and 3). 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 corresponding magnetic susceptibilities were measured; the intensity of 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 2000 and -1500 nT, are observed (Figs. 4 and 5). Such large amplitude and short wavelength anomalies are not detected out of the craters. Euler’s deconvolution was applied in order to estimate the depth of the sources. The first and the second vertical derivatives, the analytic signal and the curvature attributes of the residual magnetic field, were also calculated with the aim of sharpening and further analyse the detected anomalies. 2.5 and 3D modelling were carried out, considering the existence of induced and remanent magnetizations. The parameters used for each modeled body (i.e. susceptibility and remanent magnetization intensity) were measured in the laboratory and/or estimated taking into account previously published data. For all used frequencies, the electromagnetic profiles show lower apparent electrical conductivities in the crater’s floor, while the rims present notably higher values (Fig. 6). Our results suggest that in the crater’s floors Pampa Sastre conglomerate would be absent or deeply buried. 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 highamplitude anomalies are not present out of the surveyed craters, supports this hypothesis. The morphological, geological and geophysical features of the studied circular structures could only be satisfactorily explained assuming an extra-terrestrial projectile impact.