U- and Th-series disequilibria in coastal infill sediments from Praia da Rocha (Algarve Region, Portugal): a contribution to the study of late Quaternary weathering and erosion

U- and Th-series disequilibria observed in a sequence of infill sediments f rom Praia da Rocha, southern Portugal, were used in combination with geoche mical and particle size data to investigate sediment provenance with a view to resolving the late Quaternary weathering and erosion history of the Alg arve region, and the stratigraphy of coastal karstic exposures of the Faro/ Quarteira (FQ) formation. The red infill units can be distinguished from th e brown and buff units on the basis of their lower residual U- and Th-conce ntrations, their differing post-depositional histories las revealed by U- a nd Th-series disequilibria in sequentially extracted sediment phases), and their greater degree of sediment processing Hence, the buff and brown infil l units appear to be derived locally from weathering of the Miocene limesto ne whilst the red infill may be linked to large-scale mass movement of the FQ formation from further inland, but south of Silves, during the Late Plei stocene. This sequence of events confirms a close association between the f ormation of karat topography and infilling by gravitational slumping, debri s flow and fluvial activity, and, hence, accounts for the complex (three-st age) sediment provenance of the infill material. (C) 1999 Elsevier Science B.V. All rights reserved.