First magnetic stratigraphy results of some continental formations of the Upper Aterno Valley (Central Italy)

Abstract

The Upper Aterno Valley is a wide tectonic depression, NW-SE oriented, consisting of a large plain valley floor and a system of deeply dissected carbonatic slopes. Within the valley floor, alluvial sediments have been deposited, locally terraced, whereas on the slopes breccias and debris deposits crop out No chronological data to constrain the age of the studied units are available, because the sediments do not contain materials suitable for biochronological valuation, as also occurs for most of the Central Apennine Pleistocene continental deposits. To determine the chronology of the upper Aterno valley deposits, we paleomagnetically sampled four continental stratigraphic units (G, H, I and L) at five different sites. In each site we collected 10-15 samples, pushing plastic boxes into the clayey horizons or drilling cores with a portable drill in the calcareous cemented beds. The two sites from I and H units show a reverse magnetic polarity whereas the other two (from G and L units) show a normal polarity. Breccias (unit I) sampled at sites AA04 and AA05 show a reverse polarity and are attributed to the early Pleistocene (reverse Chrons C1r and C1r.1r, age between 780 and 1770 kyrs according to Cande and Kent, 1995). These breccias are correlated to the formation named "brecce di Bisegna" by Bosi and Messina (1990), that represents a reference element in the stratigraphic and evolutive setting of the whole Central and Southern Apennines. The fluvial deposits sampled at site AA03 from H unit belong to the same time interval and assume the same stratigraphic position compared to other relevant sedimentary episodes recognized in the Abruzzi intermountain basins. As the stratigraphic unit L (site AA02) is more ancient than unit H and is characterized by normal polarity, we assume that unit L is older than 1770 kyrs and refer it to a generic Pliocene. Regarding unit G (site AA01), more recent than unit H, and characterized by normal polarity, we suppose that it has been deposited during the Brunhes normal Chron, and therefore that it is younger than 780 kyrs. Information, coming from paleomagnetic data integrated with stratigraphic, morphological and tectonic features, could be used to define a more complete geological and tectonic Quaternary evolution of the upper Aterno valley.