Peñarroya, a strike-slip controlled basin of early Westphalian age in Southwest Spain


Authors: Wagner RH

Published in: Bulletin of Geosciences, volume 74, issue 2; pages: 87 - 108;

Keywords: Tectonics and sedimentation, strike-slip faults, Langsettian, Duckmantian, Peñarroya, SW Spain,

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The sedimentary and tectonic history is discussed of the Peñarroya-Belmez-Espiel Coalfield in the province of Córdoba, Sierra Morena, SW Spain. This refers to an elongate, narrow intramontane basin, the Peñarroya Basin, at least 50 km long and perhaps 2 to 3 km wide originally, which formed as a result of strike-slip movements on a possible transform fault immediately NE of the Badajoz-Córdoba shear zone. Two successive depocentres existed in late Langsettian/earliest Duckmantian and late Duckmantian times, respectively. Depocentre I, in the western part of the coalfield, suffered tectonic deformation before Depocentre II came into existence. The latter was emplaced slightly to the SE of the earlier depocentre, on which it impinged marginally. A comparison with the Tertiary strike-slip controlled basins in California allows the conclusion that continual movements on the probable transform fault produced releasing bend conditions leading to fracturing and subsidence of a more passively reacting plate segment on the NE side of the major strike-slip fault, followed by restraining bend conditions provoking compressional tectonic deformation of the depocentre engendered previously. This sequence of events is repeated for the second depocentre. Both depocentres expanded and shifted SE-wards.
Tectonic deformation of the lower Westphalian sediments (of both depocentres) is most intensive parallel and close to the possible transform fault, and shows the effects of thrusting and shearing. NE-directed thrusting also affects synthetic shear planes within the basin. The role of underlying and adjacent Precambrian and Cambrian (?) basement with the overlying Mississippian strata of quite a different, marine basin, is discussed in some detail in the context of the regional geology. The SW basin margin is hidden from view by post-Duckmantian thrusting, but reasons are given for assuming that this basin margin coincides more or less with a steeply angled thrust fault bringing Precambrian schists and gneisses to the surface not far to the SW of the Peñarroya-Belmez-Espiel Coalfield. Although Depocentre I shows a larger proportion of lacustrine strata, the general distribution of facies is similar for both depocentres. Facies belts occur parallel to the strike of the basin, with alluvial fan deposits on the NE basin margin passing laterally into fluviatile, braided river sediments which grade into alluvial plain deposits and lacustrine sediments to the SW. A more or less permanent lake seems to have existed in this direction, and it is assumed that loss of cohesion at the strike-slip fault line may have produced tilting of the basin floor SW-wards. Coal petrographic data are briefly alluded to in so far as these are relevant to the history and special conditions of this strike-slip controlled basin. The significance of sinistral strike-slip faults in the Hesperian Massif of the Iberian Peninsula is briefly discussed in a plate tectonic context.