Late Pliocene-Early Pleistocene palaeoenvironmental evolution of the Rieti Basin (Central Apennines)

Abstract

Facies analysis of Late Pliocene-Early Pleistocene deposits in the Rieti Basin is used as the basis for the paleoenvironmental and tectonic reconstruction of the area, in particular, the sedimentary structures, the lithological composition and paleontological content (fresh-water ostracods and continental molluscs) of sediments cropping out in the southern and northern sectors of the basin are discussed. Available data are used to reconstruct the main phases which lead to the origin and evolution of the Rieti Basin. 1) The origin of the intra-apenninic Rieti basin is linked to an extensional tectonic event probably occurring during the Late Pliocene. 2) First infilling phase with semi-arid alluvial fan facies. The sedimentation of this coarse material is due both to tectonic and climatic events (probably during the climatic deterioration near the Plio-Pleistocene boundary). 3) The ancient morphological depression is filled by the beginning of the Pleistocene. 4) Second phase of infilling occurs in a braided plain sedimentary environment. One or more marshes occupied the northern sector, while in the southern sector only an alluvial plain is identified: a) sediments at the base of this sedimentary succession indicate palustrine environments characterised by alternating fresh and oligohaline water-bodies influenced by the nearby sea (lower part of the Early Pleistocene, Santernian substage); b) deposits of the upper part of the sedimentary sequence of the second infilling phase do not show any marine influence. Identified fresh-water palaeoenvironments must still referred to the Early Pleistocene due to the presence of Belgrandia sp., Mammuthus meridionalis (evolute form) and Equus stenonis. 5) Tectonic activity which downthrew the northern sector of the basin, originating the present-day Rieti Basin, is referable to the late Early Pleistocene-early Middle Pleistocene phase.