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

CARLES CANET; GUSTAVO MEDINA-TANCO; TERESA PI; RUTH E. VILLANUEVA-ESTRADA; FABIOLA MENDIOLA; RUFINO LOZANO SANTA CRUZ; JUAN CARLOS DOLIVO; FEDRICO A. SANCHEZ; A. D. SUPANITSKY; LUIS DEL PERAL; DOLORES RODRIGUEZ-FRÍAS; ALMUDENA REDONDO; NOELIA PACHECO

Malargüe

Otro; Reunion de la Colaboración Internacional Pierre Auger; 2008

Colaboración Internacional Pierre Auger

The BATATA detector is essentially an hodoscope whose main objective isto characterize the punch-through as a function of depth over the site ofthe AMIGA extension. Beyond this function, it is also a prototype forseveral design parameters of the muon counters that will eventually beburied at the side of each surface station of AMIGA. The punch through asa function of depth is strongly correlated with the chemical composition of the soil in which the detector is buried. Ideally, the results of themeasurements obtained by BATATA, at a particular geografical position,will be extrapolated to the actual positions of the other AMIGA stations over an area of more than 20 km^2 . 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 characterizaterization of the whole AMIGA areaand, particularly, of the BATATA site was performed. The studied samplesare gravels (scales > 2 mm) and pebbly sands (scales < 2 mm) depositedin the alluvial plain of Cuenca Amarilla in Malargüe. The sands and thesandy matrix of the gravels are coarse, lithic, feldspar-rich and quartz-poor sands. Most pebbles correspond to fragments of granite. The grain sizefollow, a bimodal asymmetric distribution corresponding to sand withmedium to coarse pebbles. The granules are mostly angular. These sandsare mostly composed by plagioclase, followed by quartz and, in subordinate amounts, calcite, augite and clay minerals. Chemically, thesand are very homogeneous, showing high contents in silica (58.80 wt. %SiO_2 ), aluminum (15.39 wt. % Al_2 O_3 ), iron (6.40 wt. % Fe_2 O_{3(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/cm 3 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 objectves of BATATA, the main results ofthis study suggest that: (i) the sampled soils are Quaternary, uncosolidated gravel and sandy sediments deposited in an aluvial plain, and are expected to be quite homogeneous in chemical and mineralogicalcomposition up to a few meters depth, although grain size distributioncould present considerable changes along the sedimentary deposit; (ii) the chemical composition of the sand matrix closely approaches that ofgranite, which constitutes most of the coarse fraction (pebble-cobble). The average atomic number and weight of the sediments, stoichiometricallydeduced from the average major element composition, is 10.04 and 20.25,respectively.