PhD thesis presented at University of Minho by Diamantino I. Pereira, 1998

Abstract
The objective of this thesis is the analysis of the Cenozoic sedimentary cover of eastern Trás-os-Montes in north-east Portugal. This has involved sedimentological characterization and the study of the geomorphological positioning of the deposits as well as their correlation with known units in basins or depressions closest to the area of study. The sedimentological characterization was based both on the description of lithofacies and other surface data and also on laboratory data such as granulometric analysis, quartz sand surfaces textures (SEM), heavy minerals and < 2µm mineral associations. Objectives, geological and geomorphological setting, methodologies and previous studies are described in Ch. 1.

The sediments occur in small outcrops; mostly in depressions resulting from tectonic accidents in Mirandela and Bragança-Vilariça-Manteigas, depressions currently geomorphologically evident, or in depressions filled and levelled by the surface of the Miranda do Douro Upland. The description and characterization of the sediments (Ch. 2), the more detailed study of certain sedimentological features (Ch. 3) and the knowledge of recorded units in surrounding regions (Ch. 4), has enabled the definition of a range of lithostratigraphic units as well as the morfo-sedimentogenic conditions and stratigraphic correlations outlined in Ch. 5 with the following conclusions:

• The Vale Álvaro Formation, with a maximum recorded thickness of 23 meters, is formed of coarse conglomerates, with barely altered and rounded clasts of mafic and ultramafic rocks from the Massifs of Bragança and Morais over which they crop out. The conglomerates are intercalated with red sand layers and immature marl. The <2µm mineral association is composed of smectite and smectite+palygorskite. This formation is originated in alluvial fans confined to narrow depressions. In Bragança, its deposition is evident in a subsident block associated with the leftward NNE-SSW strike fault caused by the Bragança-Vilariça-Manteigas accident in conjunction with tectonic movements which could be related to the Pyrenean phase. These formations occurred under semi-arid climatic conditions, possibly during the Oligocene.
• The Bragança Formation, with a maximum thickness of 80 meters, fills incised-valleys and developed as an erosive response to tectonic impulses and the subsequent rise relative to the mountainous areas dating from the middle Tortonian. A series of intramountainous depressions were formed as a result of these movements, with drainage occurring in the direction of these depressions which were filled with immature sediments. Within the Bragança Formation, two members can be distinguished. The lower Castro Member, fining-upward, is formed by conglomerates at the base which correspond to the channel lags, below gravel-sandy sediments which suggest a fluvial braided model with low sinuosity and incised-valleys. Towards the top, fine sediment predominates, with dominant smectites. The upper Atalaia Member has a gravel-sand composition and corresponds to a succession of episodes in a fluvial braided model with low sinuosity and with momentary transitions to a more sinuous style. The Bragança Formation suggests temperate to warm conditions with one particularly rainy season. Characterization and correlation with other units support the hypothesis of an upper Tortonian to lower Messinian age for the Castro Member and upper Messinian to Zanclean for the Atalaia Member.
• The Mirandela Formation is formed of a succession of conglomerates with a sandy matrix, intercalated with sand layers and rare lutites, indicators of a high energy regime. It has a predominantly quartzose and kaolinitic character. With a maximum recorded thickness of 30 meters, it is limited to the Mirandela depression, fills narrow, deep paleovalleys and should be related to a tectonic impulse which opened up that depression to an exorheic regime, precursor of the current Atlantic drainage pattern. It could be argued that the Mirandela Formation is related to the Ibero-Manchega I phase (3.5 Ma) and the relatively hot and humid conditions of the Late Pliocene.
• The Aveleda Formation is formed of superficial reddish deposits. They occur essentially over a level surface which marks a discontinuity with the older formations. They develop to a greater extent at the base of relieves. Conglomeratic lithofacies predominate, with sub-angular quartzose clasts, supported by a predominant mud, kaolinitic-illitic matrix. They originate in debris flow, intercalated with occasional mud flow. Characteristics and correlation with neighbouring units suggest that the sediments were deposited in response to the Ibero-Manchega II (2.6 Ma) tectonic phase, in an alluvial fan environment, in hot arid or semi-arid conditions.
• The Sampaio Formation, limited to the Vilariça depression, is formed of predominantly reddish or brown conglomeratic sediments. These deposits, which are of varied composition throughout the valley, reveal a model of alluvial fans perpendicular to the depression. These are connected to the main incisions which can currently be observed and are set out over the surface that supports the plistocenic Douro river terraces. The terraces lie between 50 and 30 meters above the thalweg. The characterization of the < 2µm mineral association also shows evidence of similarities between this unit and the Douro terraces. The data suggests the Pleistocene period for the Sampaio Formation.