RAPPORTI TRA MORFOGENESI CARSICA ED EVOLUZIONE PALEOGEOGRAFICA NELLE AREE COSTIERE DELLA TOSCANA MERIDIONALE

Abstract

: N. Iandelli & L. Piccini, Relationships between karst morphogenesis and palaeogeographic evolution in coastal areas of
Southern Tuscany. (IT ISSN 0394-3356, 2007).
In Southern Tuscany, close to the border with Lazio Region, wide outcrops of carbonate breccias (Calcare Cavernoso) display e relief
characterised by top horizontal surfaces and slope terraces with karst landforms. Morphometric analysis, based on a DTM with a resolution of 10 m, has allowed us to identify some low-gradient surfaces displaced from 0 to 300 m a.s.l.. Erosional surfaces are of two
different kinds: old summit surfaces and marine terraces. The formers occur mainly on Poggio del Leccio area, as smooth-relief areas
not controlled by lythologic surfaces. Presently, these surfaces are located around 250 m a.s.l., up to 300 m a.s.l., whereas marine terraces are located between 0 and 70 m a.s.l..
The most peculiar feature of this area is the occurrence of surface karst and caves that can be related to palaeo-coastal environments.
Caves show two different patterns: chamber-shaped relict caves, occurring mainly on low gradient surfaces, and horizontal maze
caves. Those of first kind can be the result of the collapse of caves formed at the water table, in a coastal platform anciently located
just a few tens of meters above the sea level, similar to those of many present coastal tropical karsts. Horizontal maze caves can be
grouped into two generations. The first one regards only few caves located around 250 m a.s.l.. The second generation is located near
the present sea level and is probably due to mixing solution in epiphreatic condition. Submerged caves, occurring up to 50 m below
the present s.l., must be related to sea low-stands, because vadose speleothems occur inside them.
The morphologic features of the area, together with the geomorphic features of caves, indicate some of the evolutionary steps that fit
well with the palaeo-geographic reconstructions proposed by Authors for Pliocene and Pleistocene. During the Pliocene transgression
a wide marine platform develops on carbonate rocks. The platform is fragmented and differentially raised during Late Pliocene and
Quaternary. During the emersion of this platform, caves form at the water table just near the topographic surface, whereas flank margin caves are developed along the coast. The uplift moves some portion of this old marine platform up to 250-300 m a.s.l.. During MiddleLate Pleistocene, relict epiphreatic caves are exhumed and destroyed by surface denudation, whereas a new generation of flank margin caves is forming in the zone of eustatic variation of sea level.