PLIOCENE-QUATERNARY SEDIMENTATION IN THE NORTHERN APENNINE FOREDEEP AND RELATED DENUDATION

The deposits of the Pliocene-Quaternary foredeep of the Northern Apenn ine cover at present an area of 103 000 km(2). The original boundaries of the basin are not known, since marginal deposits have been eroded, in particular those of the inner, southwestern border. During Pliocen e times the basin area was reduced by thrust tectonics and the amount of shortening may be tentatively estimated. The present volume of Plio cene and Quaternary sediments may be inferred with good approximation from the maps of the base of the Pliocene and of the Quaternary (base of the Hyalinea balthica Zone) successions. The Pliocene volume has be en corrected adding the estimate of the underthrust sediments, while n o correction has been attempted for the eroded marginal deposits. The estimates of 97 000 and 95 000 km(3) reflecting the present volume of the Pliocene and Quaternary deposits, are therefore significantly less than the volumes originally deposited. Present volumes have been tran sformed in 'net' (0% porosity) volumes, in order to obtain the relativ e net supply rates: 0.021 (Pliocene) and 0.047 (Quaternary)km(3)/a. Ot her unmeasurable factors (volume variations due to the weathering of s ilicates, loss of leached carbonates) may induce a probably unimportan t underestimate of the supply rates. Available data allow an approxima te estimate of the range of the net volume of the Holocene sediments d eposited during the last 6 000 a BP (221-276 km(3)) and of the relativ e net supply rate (0.037-0.046 km(3)/a), that is not significantly dif ferent from the Quaternary one. Applying a porosity correction, these supply rates may be related to the Quaternary source area (128 000 km( 2)) to obtain the relative denudation rates: 0.41-0.46 mm/a (Quaternar y) and 0.36-0.51 mm/a (Holocene). Present supply and denudation rates, deduced from the direct measurements of the bed load and suspended lo ad of the apenninic and alpine rivers, do not differ significantly fro m the Quaternary and Holocene ones. Available data do not allow a reli able estimate of the Pliocene source area, and consequently of the Pli ocene denudation rate. However, a minimum of 160 000-177 000 km(3) has been eroded during Pliocene and Quaternary times. Assuming, as a work ing hypothesis, that the Pliocene source area did not significantly di ffer from the present one, an average thickness of 1240-1390 m could h ave been eroded since the beginning of Pliocene. This estimate is in a greement with the values obtained from the measurements of coalificati on of vegetal organic matter in the outcrops, and suggests that post-o rogenic successions and 'higher' thrust sheets may have been completel y removed in vast areas.