Plio-Quaternary geological evolution of the high Salto River Valley (Central Italy): the Marano De’ Marsi Unit

This paper summarizes the results of multidisciplinary researches carried out in a wide span of time on the earliest continental deposits of the high Salto River valley. The study led us to characterize sedimentary environments, to analyze their relationships and to propose a chronological attribution. Facies are referable to a complex depositional system consisting of a Gilbert-type lacustrine delta and of slope-type partly interfingered fan deltas coming from the eastern border of the basin. The geological data collected allowed us to refer the succession to a single sedimentary cycle; in Marano de’ Marsi area the paleodrainage and progradation directions were also recognized. To better constrain the chronological, paleoclimatic and paleoenvironmental context, detailed investigations have been conducted on pelitic facies of the deltaic-lacustrine system in two sampling sites (an artificial outcrop at Marano de’ Marsi village and a borehole near Borgorose village). Paleomagnetic analyses show that Marano de’ Marsi and Borgorose successions have a normal polarity and generally low magnetic susceptibility values. Pollen diagrams from both sites mainly record a mid to high elevation forest pollen rain. The existence of trees from different vegetation belts suggests the presence of a well developed mountain system in the surroundings. In the case of Marano de’ Marsi section, these vegetation phases alternate with sudden, strong and short spreads of temperate and subtropical taxa. These alternations suggest that important climate changes occurred, with cyclical forest variations typical of glacial / interglacial periods of Pliocene and Early Pleistocene. Pollen and paleomagnetic data led us to refer both records to the Olduvai subchron, at the end of the Pliocene; nevertheless, the possibility that the records deposited during the Gauss chron cannot be, definitely, excluded. Field geological data and magnetic fabric results suggest that the Marano de’ Marsi unit sedimentation took place in a basin developed under extensional tectonic regime.