THE SITE OF THE BATATA DETECTOR IN MALARGÜE, ARGENTINA: GEOLOGICAL CHARACTERIZATION

CANET MIQUEL, M; MEDINA-TANCO, G.; PI, T.; VILLANUEVA-ESTRADA, R.; MENDIOLA, F.; LOZANO SANTA CRUZ, R.; D'OLIVO, J.C.; SANCHEZ, F.; SUPANITSKY, D.A.; DEL PERAL, L.; RODRIGUEZ FRIAS, D.; REDONDO, A.; PACHECO, N.

Otro; Publicacion Interna Observatorio Pierre Auger (GAP-NOTE); 2008

Observatorio Pierre Auger

he BATATA detector is essentially an hodoscope whose main objective is to characterize the punch-through as a function of depth over the site of the AMIGA extension. Beyond this function, it is also a prototype for several design parameters of the muon counters that will eventually be buried at the side of each surface station of AMIGA. The punch through as a function of depth is strongly correlated with the chemical composition of the soil in which the detector is buried. Ideally, the results of the measurements obtained by BATATA, at a particular geographical position, will be extrapolated to the actual positions of the other AMIGA stations over an area of more than 20 km2. Therefore, it is extremely important to know how representative the soil above the BATATA planes is of the soil above the other muon counters of the AMIGA array that are located at several km of distance from the former. In order to be able to answer this basic question, a minerological, geochemical and grain size characterization of the whole AMIGA area and, particularly, of the BATATA site was performed. The studied samples are gravels (scales > 2 mm) and pebbly sands (scales < 2 mm) deposited in the alluvial plain of Cuenca Amarilla in Malargue. The sands and the sandy matrix of the gravels are coarse, lithic, feldspar-rich and quartz-poor sands. Most pebbles correspond to fragments of granite. The grain size follow, a bimodal asymmetric distribution corresponding to sand with medium to coarse pebbles. The granules are mostly angular. These sands are mostly composed by plagioclase, followed by quartz and, in subordinate amounts, calcite, augite and clay minerals. Chemically, the sand are very homogeneous, showing high contents in silica (58.80 wt. % SiO2), aluminum (15.39 wt. % Al2O3), iron (6.40 wt. % Fe2O3(T)) and calcium (5.82 wt. % CaO). The average composition of sand approaches the composition of granite. The measured density averages 1.7 g/cm3 for sands and 2.1 g/cm3 for gravels. In water saturation conditions, the density rises up to 25 % in sandy sediments. The mineralogical (low quartz/feldspar ratio) and chemical composition, and the grain size distribution and shape indicate that these are immature sediments deposited very close from the source area rocks. Regarding the specific needs and objectives of BATATA, the main results of this study suggest that: (i) the sampled soils are Quaternary, unconsolidated gravel and sandy sediments deposited in an aluvial plain, and are expected to be quite homogeneous in chemical and mineralogical composition up to a few meters depth, although grain size distribution could present considerable changes along the sedimentary deposit; (ii) the chemical composition of the sand matrix closely approaches that of granite, which constitutes most of the coarse fraction (pebble-cobble). The average atomic number and weight of the sediments, stoichiometrically deduced from the average major element composition, is 10.04 and 20.25, respectively.