Depósitos continentais e marinhos na plataforma litoral da região do Porto. Importância da tectónica na sua organização espacial

Abstract

Keywords: Littoral plalform; marginal relief; alluvial fans; marine deposits; Quaternary; neotectonics. One oflhe most common characteristics of Portuguese littora! is the existence of a planed surface (the so-called "littoral platform", situated at different altitudes and bordered from the inland by a straight relief, strongly contrasting with that planed surface. This one is generally covered wilh several outcrops of the so-called Plio-Pleistocene deposits. Till the eighties this platform has been interpreted as stable staircase of old marine leveis, registering in a passive way the eustatic variations. The rigid step bordering it easterly should be a fossil cliff. However, our study has proved that many of these deposits have a continental origin. These continental deposits have fluvial or alluvial fan facies and they are lying above 40 meters. Marine deposits seem to be quite rare and they only occupy a small westem area, beneath the altirude of 40 meters, and developing into three different marine leveis. There is a rigid step between the two kinds of deposits. We think that the clear geometric separation between these deposits together with this rigid step, indicales a tectonic origin. It seems that the sea must have touched only the westem part of this surface, when neotectonic movements lowered it down. There are more evidences for neotectonic movements: a) there are faults (mainly inverse faults) affecting lhe higher deposits or this litoral platform; b) the sarne marine levei seems to appear at differenl altitudes, developing an irregular pattem wilh a general trend dipping from the North to the South.