Influence of inherited topography on the Holocene sedimentary evolution of coastal systems: An example from Arno coastal plain (Tuscany, Italy)

摘要

Understanding the relative control exerted by autogenic factors, such as changes in sediment supply, local subsidence and inherited topography, is of crucial interest for a thorough comprehension of the sedimentary evolution of late Quaternary coastal systems. Through an example from the Arno coastal plain, in Tuscany, we show to what extent sedimentation of the Holocene succession, even after the time of maximum marine ingression, was influenced by the presence of a buried 5-7 km-wide incised-valley system, generated by the Arno River during the late Pleistocene in response to the last glacial sea-level fall. A prominent stratigraphic marker (locally known as "pancone"), ca. 5-15 m thick, made up of extremely soft and homogeneous organic-rich clays, is reported from the upper 20-30 m of the mid-Holocene succession buried beneath the Arno coastal plain. High-resolution stratigraphy, integrated with microfossil investigations, enables detailed fades characterization and mapping of this marker horizon. At proximal-most locations, "pancone" consists of thin lagoonal deposits sandwiched between backswamp clays. Around the city of Pisa and at more distal locations, this stratigraphic marker is made up almost entirely of lagoonal clays with lateral transition to backswamp deposits. The development of "pancone" was triggered by an abrupt transgressive pulse that led to the ultimate flooding of the Arno paleovalley. This transgressive event occurred shortly after a widespread episode of sub-aerial exposure and climate deterioration related to the 8200 yr BP cold event. Confinement of the mid-Holocene ("pancone") lagoon into an area formerly occupied by the post-glacial Arno incised-valley system, along with transition to backswamp facies in coincidence of the former interfluves, is interpreted to reflect the higher compaction of the less indurated valley-fill relative to the adjacent, stiff Pleistocene substratum. The local increase in land subsidence contributed to the prolonged persistence of a 6 km-wide lagoonal system, encompassing the peak of Holocene transgression and the early phases of decelerated sea-level rise (up to ca. 6000 cal yr BP). The Arno coastal plain thus offers a good illustration of how inherited topography, due to the presence of formerly active incised valleys, may influence the Holocene evolution of coastal plains, favoring local development and preservation of topographic lows.