Ebro Basin continental sedimentation associated with late compressional Pyrenean tectonics(north-eastern Iberia): controls on basin margin fans and fluvial systems


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    A tectosedimentary unit (TSU) is a type of allostratigraphic unit made up of a succession of strata deposited within an interval of geological time and under a tectonic and sedimentary regime of definite polarity (modified from Garrido-Megías, 1982). Its boundaries (sedimentary breaks) have basinal extent and are generated by inflections or sharp changes in the rate of allocyclic factors that controlled basin fill dynamics (modified from Pardo et al., 1989, and Villena et al., 1996). According to Arenas (1993) and Arenas & Pardo (1994a), in the present study the main allocyclic factor was the uplift rate of the Pyrenean sediment source areas, caused by shortening in the Gavarnie Unit.

C. Arenas, Área de Estratigrafía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, C/Pedro Cerbuna, 12, E-50009 Zaragoza, Spain. Fax: 34 976 76 10 88; e-mail: carenas@posta.unizar.es


ABSTRACT This contribution deals with the External Sierras and a part of the foreland Ebro Basin related to the southern Pyrenean thrust front. The structure of the External Sierras consists of a south-verging thrust system developed from middle Eocene to early Miocene times. Since the end of the early Oligocene, a regional-scale detachment anticline (the Santo Domingo anticline) developed, folding the original thrust system and creating new thrust units.

The molassic fill in this part of the Ebro Basin (Uncastillo Formation) mainly corresponds to an extensive, composite distributary fluvial system, termed the Luna system, which drained the uplifted Gavarnie Unit to the north. Small, marginal alluvial fans originated along the External Sierras and coalesced in the proximal-middle portions of the Luna system.

Three tecto-sedimentary units (TSU), late Oligocene to early Miocene in age, comprise the Uncastillo Formation. Lateral relationships and areal distribution of lithofacies through time have been used to establish sedimentary models for the marginal alluvial fans and the Luna fluvial system. Their sedimentary evolution was controlled by tectonics affecting the drainage basins, and based on mapping and stratigraphic relationships of the TSU, the temporal succession of the marginal alluvial fans and their relationships with each thrust system in the south Pyrenean front can be shown. Alluvial fan formation evolved through time from west to east, in accord with the progressive eastward growth of the Santo Domingo anticline as a conical fold.

The fluvial network of the Luna system appears to have been mainly radial, but near the basin margin its architecture was influenced by the syndepositional Fuencalderas and Uncastillo anticlines developed within the Ebro Basin. These low-amplitude folds originated by layer-parallel shearing caused by rotation of the southern flank of the Santo Domingo anticline. Progressive uplift of these anticlines constrained part of the fluvial discharge to synclinal areas parallel to the basin margin; these areas where characterized by meandering sandy channels. At the peripheral tips of the anticlines the channel system flowed basinward.