EAGE Basin Research (2010) 22, 904–917, doi: 10.1111/j.1365-2117.2009.00452.x Closing and continentalization of the South Pyrenean foreland basin (NE Spain): magnetochronological constraints Elisenda Costan, Miguel Garce´ sn, Miguel Lo´ pez-Blancon, Elisabet Beamudw,MiriamGo´ mez- Paccardn and Juan Cruz Larrasoan˜ az1 nGrup de Geodina'mica i Ana'lisis de Conques (GGAC),Universitat de Barcelona. Departament d’Estratigrafia, Paleontologia i Geocie'ncies Marines, Facultat de Geologia, Mart|¤iFranque's s/n, 08028 Barcelona, Spain wLaboratori de Paleomagnetisme (UB-CSIC), Serveis de Suport a la Recerca UB, Istitute of Earth Sciences‘Jaume Almera’,CSIC. Sole¤iSabar|¤s s/n, 08028 Barcelona, Spain zInstitute of Earth Sciences‘Jaume Almera’,CSIC. Sole¤iSabar|¤s s/n, 08028 Barcelona, Spain ABSTRACT This paper presents new magnetostratigraphic results from a1100-m-thick composite section across the marine to continental sediments of the central part of theSE margin of theEbro basin(NE Spain). Integration with existing marine and continental biochronological data allows a robust correlation with the geomagnetic polarity time scale.The resulting absolute chronology ranges from 36.3 to 31.1Ma (Priabonian to Rupelian), and yields an interpolated age of 36.0 Ma (within chron C16n.2n)for theyoungest marine sediments ofthe easternEbro basin.This age is inconcordancewith a reinterpretation of earlier magnetostratigraphic data from the western South Pyrenean foreland basin, and indicates that continentalization of the basin occurred as a rapid and isochronous event. The basin continentalization, determined by the seaway closure that resulted from the uplift of the western Pyrenees, was probably coincident with a mid-amplitude eustatic sea level low with a maximum at 36.2 Ma.The base level drop that followed the basin closure and desiccation does not appear associated to a signi¢cant sedimentary hiatus along the margins, suggesting a late Eocene shallow marine basin that rapidly re¢lled and raised its base level after the seaway closing. Rapid basin ¢lling following continentalization predates the phase of rapid exhumation of the Central Pyrenean AxialZone from35.0to32.0Ma,determined from the thermochronologydata.It is possible then that sediment aggradation at the front of the fold-and-thrust belt could have contributed to a decrease in the taper angle, triggering growth of the inner orogenic wedge through break-backthrusting and underplating. Contrasting sedimentation trends between the western and eastern sectors of the South Pyrenean foreland indicate that basin closing preferentially a¡ected those areas subjected to sediment bypass towards the ocean domain. As a result, sediment ponding after basin closure is responsible for a two-fold increase of sedimentation rates in the western sector, while changes of sedimentation rates are undetected in the more restricted scenario of the eastern Ebro basin. INTRODUCTION namic processes related to the orogenic belt and its asso- ciated subduction systems determine the evolution of the Peripheral foreland basins are wedge-shaped elongated foreland basin (DeCelles & Giles, 1996). This coupling is troughs that form as a £exural response of tectonic plates bi-directional, because sediment load in the in¢lling fore- to continental collision.Bywayof the lithospheric £exure, land can in£uence the deformation style in the orogenic a strong dynamic link exists between the foreland basin belt (Stori & McClay,1995; Mugnieretal.,1997). As a result, and its adjacent orogen (Beaumont,1981),such that geody- a comprehension of the overall collision setting demands an integrated approach, with kinematics of the fold-and- Correspondence: E. Costa, Departament d’Estratigra¢a, Paleon- thrust belt and basin stratigraphy being conveniently in- tologia i Geocie' ncies Marines, Facultat de Geologia, Mart|¤i cluded in a tectonostratigraphic frame. Franque' s s/n, 08028-Barcelona, Spain. E-mail: [email protected] 1 The evolution of peripheral foreland basins often in- Present address: Instituto Geolo¤gico y Minero de Espanìa (IGME). O¢cina de Proyectos de Zaragoza, Manuel Lasala 44, cludes an early phase of rapid deepening of basin £oor as 9B, 50006 Zaragoza, Spain subsidence greatly surpasses sediment yield. Ongoing r 2009 The Authors 904 Basin Research r 2009 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Ebro basin closing and continentalization chronology plate convergence drives widening and thickening of the mentary gaps or sudden changes in sediment accumula- orogenic wedge, with subsequent increase of surface pro- tion occurred during the continentalization process, cesses and of sediment erosion and transport into the fore- magnetostratigraphic analysis maylead to uncertain corre- land. In consequence, a late evolution of peripheral lations with the time scale. foreland basins often culminates with a transition from The South Pyrenean foreland basin in NE Spain is a an under¢lled to a ¢lled or an over¢lled depositional state particular case among the alpine foreland basins because as sediment supply surpasses accommodation in the fore- it is limited by active margins that underwent su⁄cient deep zone. In ancient settings, the reference frame for the uplift to cause isolation of the basin from the open ocean. ¢lling state is the sea level, so that transition from marine The continentalization of the basin was preceded by a to continental sedimentation normally equates with the phase of progressive restriction of its marine connections, over¢lled stage (Sinclair,1997). leading to precipitation of relatively thick salt deposits in The timing and rates at which continentalization oc- two distinct depocentres. In this paper, we provide con- curs in a foreland basin are dependent on the interplay of straints on the evolution of the South Pyrenean foreland a number of factors such as the rate of thrust wedge propa- basin by establishing a precise chronology for its complete gation, the £exural rigidity of the underlying plate and the continentalization.We contribute with new magnetostra- rate of sediment supply from the mountain range (Sinclair, tigraphic data from the eastern Ebro basin, and integrate 1997). Marine^continental transition is often thought to our results with a reinterpretation of existing magnetos- be diachronous, achieved by ¢lling of the basin along its tratigraphic records along the South Pyrenean foreland axis and following the regional depositional gradient. basin.We analyse the timing and the sedimentation trends However, if continental collision ultimately leads to a across the marine^continental transition and discuss the land-locked palaeogeographic con¢guration, the transi- overall scenario of basin continentalizationwithin the tec- tion from open to closed drainage may result into a ba- tonostratigraphic framework of the South Pyrenean fold- sin-wide isochronous continentalization. and-thrust belt. It reveals that unravelling the palaeogeographic scenar- io and driving mechanism of basin continentalization is of relevance for constraining geodynamic models of the GEOLOGICAL SETTING thrust wedge-foreland basin system. Achieving this goal requires a basin-wide correlation of the stratigraphic units The Ebro basin is a triangular-shaped basin surrounded associated to the marine^continental transition, which by three alpine ranges: the Pyrenees to the N, the Iberian can be accomplished through accurate dating. Di⁄culties Range to the SWand the Catalan Coastal Ranges to the SE often arise, however, when searching for age constraints in (Fig. 1). This basin represents the latest evolutionary stage shallow marine to continental sedimentary environments of the South Pyrenean foreland, whereas earlier stages of (Bera et al., 2008). Restricted marine environments pre- foreland basin evolution are now incorporated as piggy- ceding continentalization often fail to provide reliable back basins on top of allochtonous thrust nappes (Ori & biostratigraphic markers, while vertebrate fossil ¢ndings Friend, 1984; Puigdefa' bregas et al., 1992). In this sense, the in continental sediments are typicallyscarce.Also, if sedi- Ebro basin is considered the autochthonous part of the Fig. 1. Geological map of the South 2°W 0° 2°E Pyrenean foreland basin.Distribution of the Neogene Massif Central Atlantic Upper Eocene evaporites based on outcrop, Tertiary in foreland (Ebro basin infill) mine and borehole data (simpli¢ed from Ocean Eocene piggyback basins Rosell & Pueyo,1997). Locations of sites: (1) Aquitanian Mesozoic and Lower Tertiary allochthonous units Basin Paleozoic basement Maians^Rubio¤composite location of sites magnetostratigraphic section; (2) 43°N limits of the Eocene marine facies Castellfollit del Boix hydrocarbon borehole Jaca-Pamplona Basin known limits of potash evaporite formations (IGME,1987); (3) Vic magnetostratigraphic normal fault thrust blind thrust section (Burbank et al., 1992; Taberner et al., 1999; Cascella & Dinare' s-Turell, 2009); (4) Pyrenees Santpedor fossil locality (Sa¤ez,1987; Arbiol &Sa¤ez,1988; Anado¤n et al.,1992); (5) Jorba^ La Panadella section (Feist et al., 1994); (6) Iberian Range Rocafort^Vinaixa composite magnetostratigraphic section of Barbera' et Duero Ebro Basin al. (2001); (7) Oliana magnetostratigraphic Basin 6 5 section (Verge¤s & Burbank,1996); (8) Arguis magnetostratigraphic section (Hogan & Catalan Coastal RangesMediterranean Burbank,1996); and (9) Salinas 41°N Sea magnetostratigraphic