ACTA GEOLOGICAHISPANICA,v. 32 (1997), no 3-4, p. 171-184 (Pub. 1999) Late Miocene alluvial sediments from the Teruel area: Magnetostratigraphy, magnetic susceptibility, and facies organization Sedimentos aluviales del Mioceno Superior del area de Teruel: Magnetoestratigrafía, sus- ceptibilidad magnética y organización de facies M. GARCÉW*, W. KRIJGSMAN(l), J. VAN DA'M(~),J.P. CALVO(^), L. AL CALA(^), and A.M. ALONSO-ZARZA(3) (1)Paleomagnetic laboratovy Fort Hoofddijk, Budapestlaan 17, 3584 CD Utrecht, The Netherlands (2)Department of Geology, Institute ofEarth Sciences, Budapestlaan 4, 3584 CD Utrecht, The Netherlands (3)Departamento de Petrologia y Geoquimica, Universidad Complutense, 28040 Madrid (4)Museo Nacional de Ciencias Naturales, CSIC, José Gutiérrez Abascal2, 28006 Madrid *Now at Institute of Earth Sciences 'Yaume Almera", CSIC, C/Solé Sabaris, S/N, 08028 Barcelona E-mail: [email protected] ABSTRACT We present the paleomagnetic dating of the upper Miocene red alluvial to palustrine sequences of the area north of Teruel. The mag- netostratigraphy of the Masada del Valle and Masada Ruea sections indicates a minimum age of about 11 Ma to the lowermost sedi- ments post-dating the unconformity over the Mesozoic basement. The overall Miocene sequence represents a retrogradation of the al- luvial system, grading upwards into palustrinellacustrine sedimentation. The onset of palustrine deposition is dated in Masada Ruea at about 9.7 Ma. The late Vallesian faunas of Masía del Barbo, of which leve1 2B is the reference locality of biochronozone MNlO, are dated to 9.3 Ma. The anisotropy of magnetic susceptibility (AMS) in the tenigenous sediments suggests that the orientation of the mag- netic ellipsoids represents the paleocurrent direction. Paleocurrent interpretations based on AMS data can be used for a better compre- hension of the facies arrangement in the Neogene sediments of the Teniel basin. Keywords: Teniel Basin. Miocene. Vallesian. Turolian. Vertebrate fossils. Magnetostratigraphy. Magnetic anisotropy. Alluvial sedi- mentation. RESUMEN Se ha realizado un estudio magnetostratigráfico de las sucesiones aluviales que afloran al norte de Teruel (sector de Masada del Va- lle-Masada Ruea) con objeto de precisar la edad de los sedimentos y de los yacimientos de vertebrados fósiles. La correlación de las su- cesiones con la escala de tiempo de polaridad geomagnética indica una edad de 11 Ma para los sedimentos mas antiguos estudiados y que yacen en discordancia sobre el substrato Mesozoico. La iniciación de la expansión de la sedimentación palustre/lacustre ha sido da- tada en Masada Ruea en cerca de 9.7 Ma. La edad de las faunas de Masía del Barbo 2B, localidad de referencia para la biocronozona MN10, es de 9.3 Ma. La anisotropia de la susceptibilidad magnética (AMS) en las facies siliciclásticas indica que la orientación de la fábrica magnética esta controlada por la dirección de paleocorrientes. Los datos de AMS pueden ser útiles para determinar la evolución de las paleocorrientes y contribuir al estudio de la distribución de facies y la evolución del relleno sedimentario de la cuenca. Palabras clave: Cuenca de Teruel. Mioceno. Vallesiense. Turoliense. Vertebrados fósiles. Magnetoestratigrafia. Anisotropia magné- tica. Sedimentación aluvial. INTRODUCTION the westernmost rift basin related to the Neogene exten- sional structure of the eastem Iberia continental margin The Temel basin is a preferred region among verte- (Moissenet, 1984; Anadón et al., 1989a). The geometry brate paleontologists because of the abundance of rich fos- and evolution of the Teruel basin were fimdamentally con- siliferous sites, particularly of late Miocene to Pliocene trolled by tectonic extension and subsidence along its age. The relatively low structural complexity combined basin bounding faults. with extensive and accessible rock outcrops allows accu- rate lithostratigraphic correlations between neighbouring Sedimentation in the Teruel basin took place under en- sections. Paleontologists have succeeded on building a de- dorreic conditions from the early Miocene to late Pliocene tailed biostratigraphic zonation based on unequivocal (early Villafranchian) (Moissenet, 1989). Basin infill was stratigraphic superposition of faunas (van de Weerd 1976; dominated by alluvial red bed sequences altemating with Adrover 1986; Mein et al., 1983; Mein et al., 1989; Daams lacustrine/palustrine limestones and gypsum units. As a et al., 1988; van Dam, 1997; van Dam et al., in prep.). rift basin, lateral and vertical facies distribution was pri- marily controlled by syndepositional tectonic activity Tlie biostratigraphy in the Teruel basin has recently along the basin bounding faults. Terrigenous contributions been coupled with a series of magnetostratigraphic studies were fed into the basin through alluvial fans located on its (Moissenet et al., 1990; Opdyke et al., in press; Krijgsman, active margins, which graded laterally into shallow lacus- 1996; Krijgsman et al., 1996). These studies have provided trine deposits in areas with more restricted clastic supply: the continental biostratigraphy with a high-resolution ab- either central areas far beyond the terrigenous influence or solute chronology.This is the basis for further regional and protected areas behind paleogeographic elevations, acting global correlations, allowing a better understanding of the as efficient sediment traps or inducing particular direc- significance of the events recorded in the basin. tions of detrital transport (Broekman, 1983). Despite the resolution already achieved in the se- The Teruel area shows a complicated structure due to quences of the Temel basin, some aspects of the chronol- the concurrente of: 1) the Neogene NNE-SSW fault lin- ogy of late Miocene faunas are still to be refined. In this eation; 2) the reactivating of the pre-Neogene NW-SE study we present the magnetostratigraphic dating of some structural trend of the Iberian chain (Concud fault); and 3) alluvial sequences in the Temel area. We provide age con- the diapiric manifestations of Triassic evaporites (El Salo- straints for: 1) the beginning of the subsidence and clastic bral, NE Teruel). The initiation of the sedimentation in the sedimentation in this area, and 2) the onset of the palus- Temel area is represented by the upper Miocene Tejares trine and lacustrine conditions after a period of wide- Formation, informal denomination for the red alluvial se- spread alluvial sedimentation. This study also yields an quences near Temel (Anadón et al., 1990; Alcalá et al., age for the late Vallesian locality Masía del Barbo 2B, the 1994), also referred to as part of the Peral Formation (van reference site of the MN 10 zone (de Bmijn et al., 1992). de Weerd, 1976). The Tejares Fm. lies in unconformity over the diapiric Triassic evaporites and the Jurassic lime- stones (Adrover et al., 1986). Biostratigraphic information THE TERUEL AREA: GEOLOGICAL SETTING for the Tejares Fm. is limited to its upper levels (upper Vallesian of la Salle, Adrover et al., 1982; Alcalá et al., The Teruel basin is a narrow NNE-SSW striking half- 1994) and laterally equivalent palustrine/lacustrine units graben that cuts the megastructures of the Iberian chain (e.g.: upper Vallesian-lower Turolian in Los Aguanaces, La along more than 100 km (Fig. 1). It can be considered as Gloria section, Krijgsman et al., 1996; van Dam, 1997; Jurassic limestones Figure 1. Geological sketch of the Teruel area (adapted from Anadón et al., 1990) and location of the sites mentioned in the text: MDV: Masada del Valle; MRU: Masada Ruea; MB: Masía del Barbo; GL: La Gloria; BK: El Bunker de Valdecebro. Figura 1. Esquema geológico de los alrededores de Teruel (adaptado de Anadón et al., 1990) y situación de las localidades nombradas en el texto: MDV: Masada del Valle; MRU: Masada Ruea; MB: Masía del Barbo; GL': La Gloria; BK: El Bunker de Valdecebro. van Darn et al., in prep.). The Tejares Fm. is lithostrati- Anadón et al., 1989b). North and East of Teruel, the Te- graphically equivalent to the lower terrigenous member of jares Formation is overlain by the white limestones of the the Rio Turia Fm. (Broekman et al., 1983), that extends Alfambra Forrnation (van de Weerd, 1976). The distinct south of Teruel along the western side of the Teruel-Ade- time-transgressive character of the transition from red muz basin. The base of Rio Turia Fm. is dated as Aragon- beds to carbonates is demonstrated by direct field obser- ian-early Vallesian at Casas Bajas (Broekman et al., 1983), vations of lateral lithofacies transitions and by biostrati- and lies unconformably over the alluvial sediments of the graphic information of different sites at the base of the Al- Umbria de la Muela and Val de la Sabina formations, fambra Fm. (van de Weerd, 1976; van Dam, 1997). which are dated as late Aragonian-early Vallesian at Casas Altas as well (Broekman et al., 1983). The terrigenous se- quences represented by the lower Rio Turia and Tejares MAGNETIC STRATIGRAPHY Formations have the greatest surface-extension of al1 units distinguished in the Teruel basin and represent a major re- In this shidy, we present the magnetostratigraphic re- treat of pre-existing lacustrine systems (Libros sequence, sults from the Tejares Fm. and its transition to the over- Masada Ruea * ,-----A e-----4 0 i el 1, -,*.*/ : 1 ** / 180 306 -60 O 60 -$o O I 90~ decl ination inclination VGP latitude Figure 2. Magnetostratigraphy of the Masada Ruea section (see fig. 6 for legend). Figura 2. Magnetoestratigrafía de la sucesión de Masada Ruea (ver leyenda en fig. 6). lying Alfambra Fm. in the Masada Ruea (Fig. 2) and the the absence of suitable lithologies and the chances for oc- Masada del Valle (Fig. 3) sections. The studied succes- currence of hiatuses in the palustrine facies of the Alfarn- sion outcrops along the eastern slopes of the Alfambra bra Fm. (as indicated in the Bunker de Valdecebro section, valley, 2-3 km. north of Teruel. The classic late Vallesian Krijgsman et al., 1996). The sampling was concentrated faunas of Masía del Barbo are found at the transition in the terrigenous Tejares Fm.