QUATERNARY FLUVIAL GEOMORPHOLOGY AND HISTORY OF THE LOWER MONDEGO RIVER VALLEY, 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 river. A set of slope deposits and two sets of alluvial terraces present on both sides of the lower Mondego valley have been identified, mapped and sampled. Analysis of sedimentary units present in gravel pit exposures situated in these landforms reveals an interesting fluvial history in this valley.

The slope deposits are the oldest known Quaternary deposits and are situated high above the oldest terraces. These deposits are mainly composed of massive poorly sorted gravel units without bedding structures with bi-modal grain size distributions. The highest and oldest alluvial terraces named the Ameal-Santo Varão terraces are mainly composed of massive poorly sorted gravel units with minor bedding structures with bi-modal grain size distributions. The lowest and youngest alluvial terraces named the Tentugal-Gabrielos terraces are by far the most interesting alluvial bodies. They are composed of massive poorly sorted gravel units revealing minor bedding structures with bi-modal grain size distributions. These are overlaid by interbedded sand and gravel units overlaid by well sorted and bedded sand units with ripple lamination and cross bedding with unimodal grain size distributions. Brownish-gray clay units overlay the sand units.

The present day alluvial stratigraphy of the lower Mondego valley indicates three cycles of aggradation and downcutting caused mainly by eustatic sea-level and climate changes which occurred during Pleistocene times. The first cycle is pre-Illinoian in age and was responsible for the deposition and downcutting of the slope deposits. It seems to reveal tectonic uplift and deposition in a poorly drained environment with the absence of a main river channel followed by downcutting to form the main Mondego river channel. The second cycle occurred most likely during Yarmouth and Illinoian times and was responsible for the genesis of the oldest terraces. It indicates the presence of a braided stream environment with a strong torrential regime. The third cycle occurred during Sangamon and Wisconsin times and was responsible for the genesis of the youngest terraces and indicates the presence of a meandering river environment very close to the present day Mondego.