Adobes from the Aguada Culture (c. 300-1100 bc) and their possible clay sources at Ambato Valley (Catamarca, Argentina)

ZIMMERMANN, UDO; MARCOS R. GASTALDI; BERTOLINO, SILVANA

Granada

Congreso; XVI International Clay Conference - ICC 2017; 2017

Sociedad Española de Arcillas - SEA

Although the studies of pre-hispanic architecture in northern Argentina are widespread, very little is known of the adobes used in architectural elements (walls, seats, etc.). During the first millenium of the Cristian era at the Ambato Valley (Catamarca, Argentina), the residential buildings were made combining adobe walls and stone colums. The walls were covered with either rock, clay plaster or red paint. Adobes from Piedras Blancas (PB) and Cerco de Palo 065 (CP65) residential sites of the Aguada Culture (4th to 12th c. AC) have been examined as well as local possible raw materials within the Ambato valley. Based on the size, building complexity and other features that reflect the social status of the inhabitants, PB is the site with higher hierarchy and complexity. CP65 on the other hand, locates 800 m north of PB and represents one of low hierarchy. This investigation will help to characterize these materials, to identify the clay source and to evaluate the availability, consumption and procurement of raw materials in the ordinary life of the Aguada people. Mineralogical and geochemical studies were conducted by XRD, FE-SEM-EDS, XRF and lithium metaborate/tetraborate fusion ICP combined with ICP/MS method on both the adobes and the possible source materials. The latter were also preliminary evaluated on basic physical and technological properties; their mineralogy, geochemistry and properties have been previously reported. There are no commercial clay deposits in the area. Local clay sources were selected as possible candidates from: clays of fault gouges derived from crystalline Precambrian to Low Paleozoic basement rocks (Humaya y Los Escobales), altered epiclastic rocks (Los Escobales) and Quaternary loessic sediments (Cerco de Palos 65a-b, El Rodeo, Los Varela, La Aguada, Los Escobales) that filled the valley and crop out close to the archaeological sites. At PB the adobes are fairly homogeneous in mineralogy and composed of quartz, micas (phlogopite, muscovite and biotite) and plagioclase along with hornblende, hypersthene, scarce chlorite and a clay suite of dominant illite, irregular I/S, and traces of kaolinite associated with minor calcite. Only the adobe from the perimeter wall (PB802) do not contain amphyboles and hypersthene but higher proportions of expandable clays (I/S). The adobe from CP65 is similar but with lower proportions of hornblende. This mineralogy matches that of the Quaternary loessic sediments one of which is nowadays used to make adobe. Particularly minerals like hornblende, hyperthene, phlogopite and kaolinite are present in both the adobes and the sediments. Comparing the chemical composition of the adobe material with the sampled clays then all but the samples from the fault gouge are possible candidates for the origin of the adobe. These clay samples are strongly enriched in Bi, Sb, Pb and Cs. The adobe samples do match nearly perfectly the Quaternary sediments and source material samples, besides one fractionated sample (a gravel) from El Rodeo. Slight differences can be observed only in a minute depletion in the adobe samples for Fe2O3 and TiO2 and enrichment in K2O (which is variable in the sources), Th and Cs - besides the significant enrichment in Nb and Ta. Hence, these source samples have been most probably used for the manufacturing of the adobe material. However, the mentioned source samples are not covering the entire composition of the adobe material as those are enriched in Cs, Nb and Ta; a certain additional input of Cs, Nb and Ta enriched material is necessary as all indicated source samples here show lower concentration of those three elements than the adobe, yet a mixing with the highly enriched samples from the fault gouge is not possible as then other trace elements should be enriched in the adobe, like Sb, Bi, Pb and Zr. The trend of using local Quaternary sediments for the adobe is strongly suggested by the mineralogy and the geochemistry. There are no significant differences in the use and supply of raw materials between sites of different hierarchy.