Late Miocene satrochronology and basin evolution of the Atlantic side of the Betic Corridor linked to the Messinian Salinity Crisis

  1. Bastiaan van Den Berg
Supervised by:
  1. Francisco Javier Sierro Sánchez Director
  2. José Abel Flores Villarejo Director

Defence university: Universidad de Salamanca

Year of defence: 2016

Committee:
  1. Francisco Javier Hernández Molina Chair
  2. Juan C. Larrasoaña Secretary
  3. Frederik J. Hilgen Committee member
Department:
  1. GEOLOGÍA

Type: Thesis

Abstract

This thesis gives an overview in understanding the Late Miocene evolution of Mediterranean-Atlantic gateways in general and new results and insights for the Betic Corridor in particular. Betic Corridor research was done in two ways: • By constructing a high-resolution age model for the Atlantic side of the corridor and linking changes in depositional environment to basin or corridor evolution and events in the Late Miocene Atlantic or Mediterranean region. • By improving the chronostratigraphic framework of Upper Miocene sediments in the western Betics and relating changes in depositional environment to corridor evolution. During the Late Miocene, when the strait of Gibraltar probably did not exist yet, the Betic Corridor through southern Spain formed, together with the Rifian corridor through northern Morocco, the marine connection between the Atlantic Ocean and the Mediterranean Sea. Progressive restriction and finally closure of these marine gateways led to catastrophic changes in both Mediterranean sea-level and salinity. This event is known as the Messinian Salinity Crisis (MSC). Despite decades of research, the exact timing of closure, the geometry of the corridors, and the patterns of exchange through them still remain uncertain. The first chapter of this thesis gives a comprehensive overview of the evolution of these Late Miocene gateways and the nature of Mediterranean-Atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins. It also considers the possible impact of evolving exchange on both the Mediterranean and global climate and highlights the main enduring challenges for reconstructing past Mediterranean-Atlantic exchange. The rest of this thesis focusses on the Atlantic side of the Betic Corridor. The eastern part of the Betic Corridor was divided in several sea branches, of which each closed separately in the late Tortonian due to sedimentary infill and tectonic uplift. However the timing of closure of the most western of these branches and probably the last existing, the Guadalhorce corridor, is still disputed. The western part of the Betic Corridor was formed by the Guadalquivir Basin. The entire water mass that passed through the branches mentioned above also passed through this basin, which opened to the Atlantic, so any change in the corridor geometry must have (indirectly) also influenced the water mass in this basin and hence the sedimentation. This basin is the missing link in Late Miocene Mediterranean-Atlantic gateway evolution research. In Chapter 2 and 3 a high-resolution age model is constructed for the entire Messinian of the Guadalquivir Basin by tuning cyclical changes in elemental composition of the sediments, visualized through XRF, to the astronomical target curves using magnetobiostratigraphic tie-points. Based on the age model the following observations were done and their regional or global relevance through possible linkage with Atlantic or Mediterranean events were considered. • Two significant changes in sedimentation rate were found synchronously in both cores. One at 5.55 Ma coinciding with the onset of the final stage of MSC and a major deglaciation, and one at 5.33 Ma coinciding with the Miocene-Pliocene boundary and the reestablishment of fully marine conditions in the Mediterranean, terminating the MSC. This suggests that MSC events not only affected the Mediterranean Basin but probably also the Spanish Atlantic margin. The mechanism explaining these interbasinal correlations is unclear but some potential scenarios based on tectonics or changes in ocean currents are suggested. • The benthic oxygen isotope record correlates well with the previously published Atlantic records of Ain el Beida, Loulja and ODP Site 982, suggesting the presence of North Atlantic Central Water and no direct influence of Mediterranean Outflow Water (MOW) on these sediments. However, an offset is recognized between the benthic oxygen isotope records of this margin and the Moroccan Atlantic margin for the late Messinian glacial period (5.9-5.5 Ma), concurrent with the first stages of the MSC. This might be the result of the decrease/cessation of the MOW in the Gibraltar Strait region. • A gradual change in the nature of the typical fluctuations in geochemical composition of the sediments was recognized, which is associated with a gradual change in depositional environment from distal to proximal as the Guadalquivir Basin infilled. In the distal setting biogenic carbonate was diluted by cyclic influxes of detrital material; while in the proximal setting sedimentation is dominated by cyclic influxes of coarser grained terrestrial input. In Chapter 4 the chronostratigraphic framework of the Ronda Basin and Antequera region, which are located adjacent to the Guadalhorce corridor, is improved. These key areas probably formed the northern part of that gateway and both indicate a shallowing upward sedimentary succession from marls to calcarenites. Biostratigraphic dating of the marls resulted in all cases in a late Tortonian age, older than 7.58 Ma. These observations suggest that the Guadalhorce corridor probably closed during the latest Tortonian-earliest Messinian. This is in line with results from the Arcos Basin and the late Tortonian closure of the Granada Basin, suggesting that the late Tortonian period of tectonic uplift in the eastern Betics, closing the eastern branches of the Betic Corridor, continued westward and also uplifted the western Betics. This resulted in a latest Tortonian-earliest Messinian closure of the Betic Corridor, well before the onset of the Messinian Salinity Crisis.