Geology of the Upper Ribagorzana and Tor valleys, Central Pyrenees, Spain Sheet 8, 1 : 50,000

The geology of the region of map sheet 8, Ribagorzana-Tor, of the 1 : 50,000 geological maps of the Central Pyrenees, is described. The map covers part of the southern axial zone, the Nogueras zone, and the northern part of the Southern Cretaceous zone. The stratigraphie sequence consists of marine Palaeozoic rocks from the Cambro-Ordovician to the Carboniferous, unconformably overlain by mainly fluvial deposits ranging in age from the Upper Westphalian to the Lower Triassic. The Middle and Upper Triassic is represented by a lagunal evaporite sequence. The fully marine Mesozoic rocks, mainly limestones and marls, range in age from Liassic to Upper Cretaceous. The entire sequence is again unconformably overlain by fluvial piedmont deposits of the Upper Eocene-Oligocene. Hercynian and Alpine orogenies acted on this intercontinental mountain chain. In the axial zone at least five individual deformation phases with different trending axes can be attributed to the Hercynian stress field. The second, or major, folding phase produced a generally northward-dipping axial-plane slaty cleavage. Granodiorite batholiths and numerous dykes intruded almost at the end of this compressional phase. The major Alpine deformation shortened the axial zone by means of north to south up- and overthrusts along the Hercynian cleavage or fault planes, which at the same time caused asymmetric folding of the post-Hercynian strata above the unconformity. The Nogueras zone is interpreted as a steep flexure zone that collapsed due to the vulnerability of the easily deformed Keuper series filling the space between this flexure and the rigid mass of the overlying Mesozoic calcareous rocks; the latter is thought to have moved towards the south mainly under the influence of the gravitational pull. The post-Miocene erosion following the Alpine uplift led to various aplanation levels, some remnants of which are preserved in our area. Glacial forms developed during the Pleistocene, and subsequent river erosion modified the glacial morphology

The geology of the Upper Ribagorzana and Baliera valleys, Central Pyrenees, Spain

In the mapped area there is a well-exposed low-grade metamorphic marine sequence from Ordovician to Lower Carboniferous, unconformably overlain by Permo-Triassic continental deposits. Determinable fossils are rare. The Ordovician consists of a quartzite/shale sequence with one marly limestone intercalation in the upper part. The Silurian is developed as a classical graptolite-bearing black shale facies with an Orthoceras limestone near the top. The Devonian rock-sequence in the north differs from that in the south. The northern or Sierra Negra facies area consists of a thin (120—250 m) alternation of mainly limestone and slate; the southern, Baliera facies area, is thicker (340—780 m), shows more individual limestone-slate units, and is moreover characterized by a conspicuous quartzite member (0—50 m) in the middle part of the Devonian. The Devonian sequence in both areas is subdivided into four or five separate formations which have been mapped individually. The Carboniferous consists in both areas of micaceous slates with a low sand content. The continental Permo-Triassic is developed in the Germanic facies of red mud- and silstones, sandstones, and conglomerates at the base, followed by a non-fossiliferous limestone/dolomite (Muschelkalk) and gypsum-bearing, vividly coloured marls of the Keuper. The major structural movements, which probably began already in early Carboniferous times, increased in strength towards the Westphalian B. Several phases of deformation have been recognized. The first deformation produced concentric, open to tight asymmetric folds without cleavage development. Their axial planes have a general E-W to ESE-WNW trend in the north (Sierra Negra Unit), a constant NE trend in the centre (Baliera Unit), and an E-W and NW-SE trend in the south (Ribagorzana Unit). The second deformation, representing the main phase, was caused by a N-S compression and is characterized by tight to isoclinal folds with a steep northward-dipping axial plane cleavage in the north, the dip becoming more moderate in the centre and south. Fold axes and \u03b4-lineations show a girdle distribution. A third deformational phase bent the entire structure around a NNE-trending axis coinciding with the bed of the Ribagorzana River. The Maladeta granodiorite and accompanying dykes intruded parallel to the general cleavage trend and caused a metamorphic aureole of moderate width. Near its southern border, gravity folds were formed locally. The economic occurrence of galena, exploited near the village of Bono, is related to this igneous activity. A fourth deformational phase was produced by a renewed N-S compression, causing local folding of the first or main-phase cleavage, also showing a weak secondary axial plane cleavage (fracture or crenulation cleavage), and local thrust movements along the earlier cleavage plane. This deformation might be a late Hercynian or an Alpine phase. The Alpine orogeny initiated or rejuvenated important northward-dipping overthrusts and minor thrust movements along the main phase cleavage in the Palaeozoic, which at the same time caused asymmetric folding of the Permo-Triassic strata above the unconformity. The post-Miocene erosion following the Alpine uplift led to various aplanation levels, some dubious remnants of which are preserved in our area. Glacial forms developed during the Pleistocene, and subsequent river erosion modified the glacial morphology

Lithostratigraphic subdivision of Post-Hercynian deposits in the South-Central Pyrenees, Spain

On the existing geological maps of the South-Central Pyrenees such as those of Dalloni (1910, 1930), Misch (1934), Almela & Rios (1947) and Alastrue, Almela & Rios (1957) the units distinguished and the colours used represent stratigraphie time intervals. Many stage boundaries, however, fall at levels which may lie anywhere in a homogeneous rock sequence. To overcome this difficulty the practice up to now has been to substitute the nearest marked lithological change for the unmappable time-stratigraphic limit. This simplification hardly matters in small-scale geological maps (scales of 1 : 175,000 and 1 : 200,000). The errors introduced become unjustifiable, however, when mapping on scales of 1 : 10,000 to 1 : 50,000. Pronounced facies changes and diachronism as have been shown to apply to many of the deposits under consideration (Souquet, 1967), add a further complication. Mapping of the Central Pyrenees has been undertaken by the Geological Institute of the University of Leiden and to date the first six, 1 : 50,000, map sheets of the projected series of ten maps have been published. In the course of field studies carried out by the present authors in the South-Central Pyrenees, a lithostratigraphic subdivision was developed which is here published in a condensed form. The subdivision is largely based on physical criteria, recognizable in the field. The units distinguished are formations in the sense of the Code of Stratigraphical Nomenclature (1961)