Características diferenciales y génesis de los niveles silíceos explotados en el yacimiento arqueológico de Casa Montero (Vicálvaro, Madrid)

Abstract

The research during the construction of the M-50 highway has allowed the study of the silica levels that constitute the most important Neolithic flint mine in the Iberian Peninsula. A group of more than 3.800 extraction pits were realized to obtain cherts and opals that appear among Mg smectites beds (Aragonian deposits). The description of the nodular and stratiform silica levels and the study of their types and distribution have been done at geological sections obtained during the archaeological excavations. Three petrological types of siliceous rocks (chert, opal and opaline chert) are established and defined according to their mineralogy and microfacies. Cherts are constituted only by quartz and locally minor amounts of moganite. Opals contain opal CT, Mg smectites and minor quartz. Opaline chert is defined when the proportion of quartz is higher than 50%. All the types of siliceous rocks reproduce bioturbation structures as well as other structures found in the host rocks (grains, intraclasts, brecciation). The silica levels at the base of the columns are constituted only by chert and were formed by the silicification of nodular and lenticular dolomite levels included in the Mg-smectites beds. The rest of the siliceous levels are composed of opals and opaline cherts and were formed by silicification of Mg-smectites. The outer part of the opaline chert has a higher content in opal and clay relicts whereas the inner part is mainly microcrystalline quartz. This change is consequence of the recrystallization of the opal (aging) producing a more compact silica rock. The main silicification occurred under the groundwater table because the structure of the host rocks is conserved in the silica rocks. A general question is whether the silica that precipites is introduced by groundwater or it is released from the dissolution of clays during their replacement