INVESTIGADORES
GONZALEZ GUILLOT Mauricio Alberto
congresos y reuniones científicas
Título:
Bajada del Diablo impact crater-strewn field (Argentina): ground magnetic and electromagnetic surveys
Autor/es:
PREZZI, C.; ORGEIRA, M.J.; ACEVEDO, R.D.; PONCE, J.F.; MARTÍNEZ, O.; VÁZQUEZ, C.; CORBELLA, H.; GONZÁLEZ GUILLOT, M.; RABASSA, J.
Lugar:
Mar del Plata
Reunión:
Congreso; GeoSur - International Geological Congress on the Southern Hemisphere; 2010
Resumen:
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 boulders and block 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 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 (A and 8) 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 Earths magnetic field and the IGRF value was
subtracted. Basalt 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 craters floors, characteristic of
impact structures. Furthermore, in the craters 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. Eulers deconvolution was
applied in order to estimate the depth of the sources. The first and second vertical derivates, 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 craters floor,
while the rims present notably higher values (Fig.6). our results suggest that
in the craters floor the Pampa Sastre conglomerate would be absent or deeply
buried. On the contrary, the craters 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 could only be satisfactory
explained assuming an extra-terrestrial projectile impact.