The inner domains of the Iberian Variscides, where both studied regions belong, are composed by thick continuous marine sequences of lower Paleozoic age, that have been settled during the rifting events of the Variscan cycle. As should be expected, in the upper Paleozoic, the situation is completely different. The predominant collision environment didn't favor the sedimentation in the inner domains. Indeed, by that time, the erosion processes are here predominant, due to the major inversion that have occurred in the internal sectors. Sedimentation has been then restricted to some small intramountainous basins, most of the times tectonically controlled (Medeiros et al., 1980; Domingos et al., 1983; Rodrigues, 1997).
In the Marão region the autochthonous meta-sedimentary sequence presents a well exposed succession from Cambrian to Devonian (Fig. 3).
At the bottom, the Desejosa Formation exhibits a turbiditic facies composed by a very monotonous multilayer sequence where millimetric to centimetric phyllite layers are interbedded with siltstones.
Unconformable on the Desejosa Formation rests the volcano-sedimentary Vale de Bojas Formation, of lower Ordovician age. Two predominantly conglomeratic members with an important volcaniclastic contribution can here be individualized: the lower Bojas Conglomerate and the upper Impure Quartzites. Overlaying the previous units is the Quartzite Formation, also of middle Ordovician age, composed by three members. The lower member, Iron-free Quartzites, is composed by decimetric to metric quartzite and conglomerate layers interbedded with pelites. In the middle Iron-Quartzites member the conglomeratic layers are absent and there is abundant magnetite disseminated in the decimetric quartzite horizons. The upper member of this formation is represented by the Upper Psammites, composed by a multilayer of quartzites and pelites of millimetric to centimetric thickness. To the top of the previous formation, the detritic component quickly vanishes; a rather monotonous black pelites sequence is attained giving rise to the middle ordovician Slaty Formation. The presence of centimetric to decimetric lenticular oolitic ironstones interbedded in the previous pelites as well as the occurrence of "pelites with fragments" indicates an upper Ordovician age, more precisely the Kosovian.
Concerning the Silurian, in the northern Portugal autochthon three lithological units could be individualize (Meireles, 2000; Piçarra, 2003). The lower Infraquartzítica Formation is composed by quartzites and carbonaceous black shales with intercalations of lydites. This unit is overlained by the Quartzítica Formation, where the quartzites are dominant. The Silurian ends by the Supraquartzítica Formation, composed by brown shales with intercalations of hematitic siltstones, quartzites, lydites, volcanics and limestones in the upper part. In the Marão sector, only the lower unit is exposed where it is known by the local name of Campanhó Formation (Pereira, 1987).
The Paleozoic metasedimentary succession ends in the Marão region by the Santos Formation of devonian age. While in the lower part of this unit, the gray slates and the siltstones are dominant, in the upper section the phyllites and the metagreywackes are the most common lithologies (Pereira, 1987).
The basal and upper boundaries of the Parautochthonous sequence are two main regional thrust planes, respectively, the already mentioned, Main Trás-os-Montes Thrust and the Lower Allochthonous Basal Thrust (Ribeiro et al., 1990).
The metassedimentary sequence that crops out in the studied area, like the global sequence of the Parautochthon, is composed of siliciclastic low grade rocks, metagreywackes and quartzites. In this sequence, some interbeddings of black cherts and acid metavolcanics occur (Fig. 4).
Although several layer parallel tectonic thrusts along the regional column could be found, one of them, the Palheiros - Vila Flor Thrust, with vast geographic extent, divides the Parautochthon in two main structural domains, being considered a major regional thrust. Thus, it is necessary to consider independently the stratigraphy under and above this thrust plane and, consequently, reformulate the former lithostratigraphy of the Parautochthonous nappe for this sector (table I). The former Pelito-Grauváquica Formation should disappear and two other units are defined separated by the Palheiros-Vila Flor Thrust.
Despite the occurrence of second order thrusts two units are defined in Lower Parautochthonous. The Xistos Inferior Formation (Ribeiro, 1974) rests directly on the Main Trás-os-Montes Thrust. This unit with a strongly deformed quartzitic level in its base, is mainly composed of dark phyllites and siltites with several interbeddings of black cherts and, sometimes, quartzites. The Curros Formation (Ribeiro, M. A., 1998) is a monotonous millimetric layered sequence of greenish phyllites and metapsamites. This unit has a lithogeochemical anomaly in elements from basic source, Sc, V, Cr, Co and Ni (Ribeiro, M. A., 1998). A silurian age for the Xistos Inferior Formation and a Devonian age for the Curros Formation are proposed based only in regional lithological correlations (Pereira, 2000).
In the normal limb of the first order D1 fold preserved in the Serra de Santa Comba, from base to top, four units are defined. The Filitos Cinzentos Formation is a new unit resulting from the regional reinterpretation with the consequent disappearance of Pelito-Grauváquica Formation. It is composed of fine gray phyllites with sparse intercalations of acid metavolcanics with relics of porphyritic textures. Associated with these metavolcanics lenses of feldspatic greywacke occur. The Quartzitos Superior Formation is composed of massive quartzites with cross bedding laminations and schistose quartzites with some intercalations of conglomerate lenses. The rocks reveal a high textural and mineralogical maturity. The Xistos Superiores Formation is very similar to the Xistos Inferiores Formation. It is composed of phyllites derived from very fine pelitic material with large amounts of organic matter. Intercalations of black cherts are abundant. The higher unit in the Upper Parautochthonous sequence is the Xistos e Grauvaques Culminantes Formation, a monotonous succession of phyllites with thin intercalations of siltites and greywackes.
The only chronological constrain in this sequence is the occurrence of poorly preserved graptolites in black cherts from the Xistos Superiores Formation that give a middle Landoverian age (C. Romariz determination). Recent new graptolites discoveries are presently under study.