Incised valleys, their filling and mammal fossil record: a case study from Middle-Upper Pleistocene deposits of the Roman Basin (Latium, Italy)

Incised valleys represent important elements of the stratigraphic record. In the last years many studies have been addressed to understand how allogenic and autogenic factors control their filling and geomorphological evolution. Incised valleys are here reported from Middle-Upper Pleistocene deposits of the Roman Basin. Stratigraphic architecture of the incised-valley fills shows a hierarchy of high-frequency depositional sequences formed under the influence of glacio-eustatic sea-level changes, tectonic uplift, and volcanic activity. In the study area, two types of tectonically-controlled valleys, NW-SE and NE-SW oriented, were recognized. The NW-SE oriented valleys represented tributaries of Tiber River, while the other valleys were related to streams flowing directly into the Tyrrhenian Sea. The formation of these valleys was strictly related to local base-level variations in response to cyclic fluctuations of relative sea-level. The filling and the preserved morphology of these valleys indicate that their incision, widening, and filling occurred during the early and late lowstand and transgressive phases. During these phases the shape of the valleys was modified, hosting a sedimentary succession generally characterized by fluvial depositional systems evolving upward to fluvio-palustrine and lacustrine systems. Only in rare cases the top of the valley fills records lacustrine-lagoonal deposits showing an initial marine influence.
Most of the mammal remains in the Rome area are contained in the incised-valley fills. As such, the history of these remains follows the same evolution of the valleys, although the overall composition of the fauna, as well as the structures and the richness in specimens are constrained by the palaeoenvironmental conditions and the taphonomic burial patterns. This to doubt about the definition and the introduction of new biochronological units (e.g. the faunal units, FUs) when sedimentological and physical stratigraphic analyses of the deposits in which faunal remains occur are not available. The integrated sequence-stratigraphic and mammal-biochronology approach to the Middle-Upper Pleistocene deposits of the Roman Basin, proves to be a powerful tool in testing the relationships among physical stratigraphic units, faunal complexes, and faunal units, allowing to establish a local chronostratigraphic framework in which the faunal assemblages can be framed.

Keywords: Incised valleys; Base-level change; Sequence stratigraphy; Mammal faunal units; Taphonomy; Middle-Upper Pleistocene; Roman Basin; Italy.