Tertiary deposits of the north-western Iberian Peninsula are heterogeneous because they occur in several morpho-structural positions as isolated and dispersed basins and outcrops. The quality of the palaeontological record is usually very poor and there are scarce data. Correlation depends on a wide range of criteria which are not always equivalent: palaeontology, mineralogy and petrology, geomorphology, tectonics and comparison with better-known and better-dated facies in regions nearby. The results lack homogeneity and there are notable discrepancies.
Tertiary sediments of Galicia, Bierzo and the Cantabrian Range (north-western Spain) occur in small basins and isolated outcrops.
Many geographical features and the Tertiary basins of Galicia are related to fault-systems trending N–S, NNE–SSW, NE–SW, E–W and WNW NW–ESE SE (Fig. 1).
The most important outcrops of Tertiary sediments in Galicia are located in Lugo, between the central peneplain and the eastern mountains. The main basins (Terra Cha, Sarria and Monforte) are asymmetrical: Tertiary sediments rest unconformably upon Palaeozoic rocks in the west and are separated from crystalline basement (igneous rocks) by N20–30 and N50–60 faults (Fig. 2).
The sedimentary fills of the Sarria and Monforte basins, up to 200 m thick, have been divided in various stratigraphical schemes (Birot & Sole, 1954; Brell & Doval, 1974, 1979; Virgili & Brell, 1975; Martin-Serrano, 1979, 1980, 1982; del Olmo, 1986; Vergnolle, 1984, 1985, 1987, 1990) with several lithostratigraphic units which were grouped into the Monteforte Formation (Vergnolle, 1988).