I depositi del Pleistocene inferiore della bassa Val di Cecina (Toscana, Italia): ricostruzione stratigrafico–deposizionale e proposta di suddivisione in Unita’ Allostratigrafiche

Abstract

G. Sarti et.al., Stratigraphic and depositional reconstruction of the Lower Pleistocene deposits from lower Val di Cecina
(Tuscany, Italy): a proposal of subdivision in Allostratigraphic Units. (IT ISSN 0394-3356, 2007).
In the area comprised between the villages of Riparbella and Casale Marittimo (lower Cecina Valley, Tuscany, Italy) a large amount of
Pleistocene deposits outcrops. Through a detailed geological survey integrated with facies analysis five litostratigraphic units, grouped
in three Allostratigraphic Units (UA), have been detected. The oldest (UA7), lower pleistocenic in age, is formed by two litostratigraphic
units (“Conglomerati delle Ginepraie” and “Sabbie ed argille ad Arctica islandica”) showing latero-vertical relationships and are organized in a transgressive-regressive cycle. The “Conglomerati delle Ginepraie” are arranged in a fining upwards trend and were deposited in small coalescent deltaic environments. Three depositional facies are recognized within the “Sabbie ed argille ad Arctica islandica” recording a fining-coarsening and deepening shallowing upward trend. The UA7 lies in unconformity above pre-quaternary deposits.
The second (UA6), lower pleistocenic in age, is also constituted by two heteropic and transgressive litostratigraphic units
(“Conglomerati delle Lame” and “Arenarie e Sabbie di Montescudaio”) lying in angular unconformity on the UA7. The “Conglomerati
delle Lame”, showing a well developed imbrication, were deposited in a foreshore to upper shoreface depositional environment. The
“Arenarie e Sabbie di Montescudaio” are characterized by a high to low planar-cross stratification and sometimes by overturned stratification and were deposited in a shallow marine environment. The regressive phase of this UA is not preserved due to erosive processes. The youngest unit (UA4, “Conglomerati di Serra all’Olio”), was instead deposited in fluvial-braided environment and lies in angular unconformity on the others UA. The middle pleistocenic age of this unit has been inferred on the basis of its stratigraphic position. This stratigraphic reconstruction strongly conflict from previous interpretations. Important climatic and geological remarks derive from the finding of two taxa (Arctica islandica and the coral Cladocora caespitosa) at different stratigraphic positions within the oldest unit (“Sabbie ed argille ad Arctica islandica”). The Arctica islandica testifying cold temperatures (4°C to 13°C) characterizes the first 5 meters of the unit, while beds of Cladocora caespitosa (15°C to 22°C the temperature range of this coral) only occur upwards. We retain that the deposition of the “Sabbie ed argille ad Arctica islandica” takes place during a warming phase and that the development of Cladocora caespitosa beds could be related to the maximum flooding conditions at the end of the transgressive phase. This interpretation leads to some notable chronostratigraphic implications.