Paleoalterações e cimentações nos depósitos continentais terciários de Portugal central: importância na interpretação de processos antigos

Abstract

Key words: Tertiary; palaeoweathering; palaeosols; cementation; climate; basin analysis; alluvial deposits; Portugal. This paper describes the palaeoweathering, cementation, clay minerais association and other closely related characteristics of central Portugal allostratigraphic Tertiary units (SLD's), that can be used for palaeoclimatic interpretation and palaeoenvironmental reconstruction. Lateral and vertical changes in palaeosols are of value for improving our understanding of the autocyclic and allocyclic controls on sediment accumulation in an alluvial basin, but they can also have stratigraphic importance. ln some cases it is concluded that the geomorphological setling may have been more decisive than climatic conditions to the production of the palaeowealhering. During late Palaeogene (SLD7-8), surface and near-surface silicification were developed on tectonically stable land surfaces of minimal local relief, under a semi-arid climate; groundwater flow was responsible for some eodiagenesis calcareous accumulations, with the neoformation of palygorskite. Conditions during lhe Miocene (SLD 9-11) were favourable for the smectlzation of the metamorphic basement and arenization of granites. Intense rubefaction associated with basement conversion into clay (illite and Kaolinite); is ascribed to internal drainage during late Messinian-Zanclean (SLDI2). During Piacenzian (SLD I3) intense kaolinization and hydromorphism are typical, reflecting a more humid and hot temperate climate and important Atlantic fluvial drainage. Later on (Gelasian-early Pleistoccne 1; SLD 14), more cold and dry conditions are interpreted, at the beginning of the fluvial incision stage. Silica cementation is identified in the upper Eocene-Oligocene" (SLD 8; the major period of silicification), middle to upper Miocene (SLD 10) and upper Tortonian-Messinian (SLD 11); these occurrences are compatible with either arid or semi-arid conditions and the establishment of a flat landscape upon which a silcrete was developed.