Estudio comparativo entre las areniscas paleozoicas y triásicas de la isla de Menorca: evidencias de procesos de reciclado

Abstract

Prehercinian and permotriassic siliciclastic formations (Lower Devonian and Lower Carboniferous) in the Menorca island are bounded by sedimentary unconformities. The aim of this paper is to analyze the recycling processes and the main variations in source area lithologies related with sedimentary unconformities, by the light of petrographic composition of those detrital formations. Three main stages in the accumulation of detrital deposits can be distinguished by analysis of modal composition on a QFR diagram: a) Lower Devonian (Q95, F3, R2) - Carboniferous lower unit (Q52, F13, R35): it represents a significant reactivation of source areas of low-rank metamorphic lithologies. b) Carboniferous lower unit- Lower Buntsandstein (Q97, F1, R2): it represents a period of progressively increasing detrital maturity. In this period important recycling processes are inferred. c) Lower Buntsandstein - Upper Buntsandstein (Q87, F7, R6): it shows a slight but definitive change in source lithology with influence of' granitic-gneissic source rocks. On a QmFLt diagram, sandstones of the second stage defined above display an important in crease in polycrystalline quartz during permian sedimentation. This fact outlines a low-rank metamorphic contribution from the source area. Scarce content of metamorphic fragments in permian sandstones suggests that intense reworking of those sandstones took place during transport. Compositional indices (Qp/Qt, F/Qt and Ch/Qt) permit to corroborate the existence of recycling processes related to unconformities. Finally, a general provenance model of sandstones of Menorca is proposed. In this model the main sedimentary unconformities related with sustantial lithological changes of source areas are the boundaries between Lower Devonian and Carboniferous lower unit, the Carboniferous upper unit and the Permian, and that beated between the Lower and Upper Buntsandstein. In addition, the Carboniferous upper unit and Lower Buntsandstein are the sandstone formations with greatest recycling processes from sandstone formations deposited previously.