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ABSTRACT

The basal Upper Jurassic unconformity in the Lusitanian Basin of Portugal commonly exhibits an irregular palaeokarstic surface developed in underlying Middle Jurassic carbonates. In the Serra da Arrabida area south of Lisbon, wire cut quarry faces expose a thick Upper Jurassic paleosol complex overlying fissured and brecciated limestone veneered with a calcrete crust and associated with colluvial deposits. The paleosol complex is a red mudstone with calcrete stringers which superficially resembles present-day red Mediterranean Terra Rossa soils. A detailed micromorphological study indicates the absence of any clay illuviation in the paleosol unit, which suggests that it is not comparable to the true Terra Rossa Alfisols, but more closely resembles present-day Aridisols. This difference from true Terra Rossa soils probably reflects formation under a drier climate, which is confirmed by the occurrence elsewhere in Portugal of evaporitic lake deposits of the same age. The paper stresses the role of soil petrography (micromorphology) in interpreting pedogenesis in paleosols.