ALLUVIAL TERRACES OF THE LOWER MONDEGO RIVER VALLEY: PRODUCT OF LATE QUATERNARY EUSTATIC AND CLIMATIC FLUCTUATIONS IN CENTRAL PORTUGAL

With a drainage area of 6,772 km² and a length of 220 km the Mondego river situated in west-central Portugal is the largest Portuguese originating river. A set of slope deposits and two sets of alluvial terrace deposits are present on both margins of the lower Mondego river valley and have been identified, mapped, and sampled. Analysis of sedimentary units in gravel pits and road cut exposures present in these deposits revealed an interesting and complex late Quaternary alluvial history in this valley.

The slope deposits are the oldest known Quaternary deposits and are located high above the oldest alluvial terraces. These deposits are composed mainly of massive sub-mature poorly sorted gravel units with Gmg and Gp lithofacies and with bi-modal grain size distributions. The highest and oldest alluvial terrace deposits present in the Mondego valley are composed of massive sub-mature poorly sorted gravel units with Gp and Gt lithofacies and with bi-modal grain size distributions. The lowest and youngest alluvial terrace deposits present in this valley are by far the most interesting sedimentary bodies. These are composed of massive sub-mature poorly sorted gravel units with a Gp lithofacie and with bi-modal grain size distributions overlaid by sub-mature moderate sorted and bedded sand units with Sp and Sr lithofacies and with unimodal grain size distributions. In some locations, an immature poorly sorted silt unit with an Fm lithofacie and a bi-modal grain size distribution overlies the sand units.

Two sets of marine terrace deposits located on the northern and southern margins of the Mondego estuary are believed to be correlative in age with the two sets of alluvial terrace deposits. The present day alluvial sedimentology and stratigraphy of the lower Mondego river valley indicates that at least two cycles of aggradation and downcutting caused mainly by eustatic sea level and climatic fluctuations occurred during late Quaternary times. The alluvial lithofacies also show the presence of both braided and meandering channel environments during these two late Quaternary constructional cycles.