Kuiper-binnenwerk 30-09-2003 15:03 Pagina 123 CHAPTER 5 40ar/39ar dating of tephra layers intercalated

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  • 40Ar/39Ar DATING OF TEPHRA LAYERS INTERCALATED IN ASTRONOMICALLY TUNED MARINE, UPPER MIOCENE

    SEDIMENTARY SEQUENCES IN THE WESTERN MEDITERRANEAN.

    CHAPTER 5

    Kuiper-binnenwerk 30-09-2003 15:03 Pagina 123

  • INTRODUCTION

    Steenbrink et al. (1999) confirmed the astronomical theory of climate change for the Pliocene by 40Ar/39Ar dating of volcanic ash layers distributed at several stratigraphic positions in the astronomically tuned sections in Ptolemais, resulting in an average period of 21.7 kyr for a cycle, equivalent to the expected duration of the cycles based on astronomical theory. 40Ar/39Ar data of the volcanic deposits in the Melilla Basin will be used to confirm the astronomical theory of climate change for the late Miocene for the first time with a method (i.e., 40Ar/39Ar) completely independent from astronomical theory.

    Once confirmed, the 40Ar/39Ar ages will be used for comparison with astronomical ages of the ash layers. In chapter 3 a discrepancy between isotopic 40Ar/39Ar and astronomical ages for the eastern Mediterranean was presented, where 40Ar/39Ar ages tended to be systematically younger than the astronomical counterparts. In chapter 4 an astronomical time frame was established for late Miocene sediments in the Melilla Basin, Morocco. This basin is located near the source area of two volcanic complexes active during the late Miocene, resulting in a large number of very coarse grained volcanic deposits intercalated in the astronomically tuned sections. Therefore, the existence (or non-existence) of systematic differences between the 40Ar/39Ar and astronomical tuning methods could be further explored in great detail, while the large crystal size (>1 mm) in some of the Melilla volcanic deposits enabled us to perform single crystal 40Ar/39Ar dating, allowing the detection of contaminating (xenocrystic) grains. To supplement the data for the western Mediterranean a few volcanic ash layers intercalated in the astronomically tuned Sorbas and Nijar Basins (Sierro et al., 2001) have been dated as well, although the crystal sizes were much smaller (125-250 µm) and biotite was the main suitable K-rich mineral.

    GEOLOGICAL BACKGROUND

    The Betic-Rif Cordilleras of southern Spain and northern Morocco constitute the westernmost extension of the Mediterranean Alpine orogenic belt, which formed in response to the convergence between the European and African plates during the Cenozoic. During the early to middle Miocene this convergence caused thrusting and westward migration of the Internal zones over the External zones (Figure 5.1a). Ongoing convergence between Africa and Iberia caused deformation along NE-SW and NW-SE strike slip faults and the formation of intramontane basins in SE Spain (Sanz de Galdeano and Vera, 1992). The Sorbas and Nijar Basins are two of those basins. The oldest Neogene sediments are conglomerates of Serravallian age, which are overlain by turbiditic sandstones. A major unconformity separates the turbiditic sandstones from onlapping shallow marine calcarenites (Azagador member) of latest Tortonian age (~7 Ma). The Azagador member changes upward into marls, clays and diatomites of early Messinian age (the Abad member). The Abad marls are deposited in the relatively deep parts (200-300 m, Troelstra et al., 1980) of the basins and they change laterally and vertically into platform carbonates and reefs. The Abad marls can be divided in two distinct units: the Lower and the Upper Abad marls. The Lower Abad unit consists of an alternation of indurated homogenous whitish marls and soft homogeneous grey marls rich in foraminifera. The Upper Abad is characterized by the intercalation of sapropels and indurated diatom-rich layers (Sierro et al., 2001). The Abad marls in the central part of the basin are overlain by massive gypsum deposits (Yesares member), which is correlated to the Lower Evaporites of the Central Mediterranean (Krijgsman et al., 1999a).

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    CHAPTER 5

    Kuiper-binnenwerk 30-09-2003 15:03 Pagina 124

  • The extension, forming the Melilla-Nador Basin, started in the Serravallian (~14 - 10 Ma) and resulted in the deposition of a folded, molassic clastic wedge (Guillemin and Houzay, 1982), followed by uplift of the Paleozoic metamorphic core due to NS-directed compression. Messinian extension provided accommodation space for marl deposition in the basin and the development of a carbonate platform. The onset of marine sedimentation in the Melilla Basin is coeval to the deepening of the central Betic Corridor basins like Sorbas and Nijar (Van Assen et al., in press, chapter 4). This marine sedimentation phase ended with a lagoonal to lacustrine regressive sequence (Saint Martin et al., 1991). Sedimentary cyclicity in the Melilla Basin starts with bipartite cycles composed of indurated cherty laminites and homogeneous sediments with a change in color from gray to brownish marls coeval to the change in color from the Lower to Upper (more brownish) Abad in the Sorbas and Nijar Basins. Therefore, a change in the bio-lithofacies characteristics in the Melilla Basin is coeval to the division in the Lower and Upper Abad (Van Assen et al., in press, chapter 4). Volcanic horizons suitable for 40Ar/39Ar dating are intercalated in both the cyclic sediments of the Sorbas, Nijar and Melilla Basins.

    NEOGENE VOLCANISM IN THE WESTERN MEDITERRANEAN

    Calc-alkaline, potassic and basaltic volcanism is scattered across the Alboran Sea and Betic-Rif systems (e.g., Hernandez and Bellon, 1985). The Neogene magmatic activity developed in the eastern Betics seems to be closely related to the major strike slip faults. In the Guercif Basin magmatic activity seems also be related to the faults along the border of the basin. The earliest Neogene igneous activity was a basaltic dyke

    125

    CH AP

    TE R

    5

    40Ar/39Ar DATA OF MELILLA, SORBAS AND NIJAR

    ���� ���� ���� ����

    � �

    �� �� ��� �

    ��� RABAT

    TANGER

    SEVILLA

    CADIZ MALAGA

    NIJAR

    GIBRALTAR

    TETOUAN

    MELILLA

    MEKNES M ou

    loy a �LEGEND:

    I B E R I A N M E S E TA

    N

    FORTUNA

    GUERCIF HAUTS

    PLATEAUX MOROCCAN MESETA

    FEZ

    TAZA

    VALENCIA

    Internal Units

    External Units

    Marine Gateways

    Major faults

    Basement

    GRANADA M

    ID DL

    E AT

    LA S

    AT L A N T I C

    O C E A N

    M E D I T E R R A N E A N

    S E A

    B

    C

    A

    � � ��� �� ���

    × Jumilla

    × Cancarix

    × Calasparra

    Mula × ×

    Murcia

    × Fortuna

    Zeneta ×

    Almeria ×

    Cabo de Gata

    × Carboneras

    Vera ×

    × Aguilas

    × Cartagena

    × Barqueros

    × Mazzaron

    0 50 km

    N�calc-alkaline serieslamproitic series alkaline series

    shoshonitic seriesN ija

    r b as

    in Sorbas basin

    SE SPAIN

    7.7 Ma

    2.8 - 2.6 Ma

    7.0 - 5.7 Ma

    8.2 - 6.8 Ma

    7.0 - 5.7 Ma

    15.1 - 7.3 Ma

    Ras Tarf

    Trois Fourches

    × Melilla Gourougou

    Amjar

    Sidi Maatoug

    × Ain Zohra

    Guilliz

    Taourirt ×

    Chaffarines islands

    × Oujda

    ALGERIAMOROCCO

    ~12 Ma

    9.6 - 4.7 Ma

    8.0 - 4.9 Ma

    � � ������

    < 5.9 Ma

    < 5.9 Ma

    < 5.9 Ma

    13 - 7.9 Ma

    5.2 - 1.5 Ma

    ~10 Ma N

    0 50 km

    SORBAS

    FFiigguurree 55..11aa--cc:: GGeeoollooggiiccaall sseettttiinngg aanndd NNeeooggeennee

    vvoollccaanniissmm iinn tthhee BBeettiicc--RRiiff CCoorrddiilllleerraass..

    These figures show the geological setting, the

    distribution of Neogene volcanism and the locations

    of the studied sections in the Western Mediterranean.

    Kuiper-binnenwerk 30-09-2003 15:03 Pagina 125

  • swarm around 22 Ma located in the central and western internal zones of the Betics (Torres-Roldán et al., 1986). Furthermore, the oldest volcanic rocks are mainly calc-alkaline and are restricted to the Cabo de Gata-Carboneras area in Spain (figure 5.1b). In the eastern Rif calc-alkaline series are more widespread (figure 5.1c). Dates obtained for this calc-alkaline volcanic suite range from 15-7 Ma in southern Spain, whereas the Rif-Tell (northern Africa) calc-alkaline mountains range from 13-8 Ma in age (e.g., Bellon et al., 1981; Hernandez and Bellon, 1985). In the Alboran Sea the Alboran Island is a calc-alkaline volcanic edifice with volcanism at 18-7 Ma (Aparico et al., 1991). A second suite of dominantly potassic-ultrapotassic rocks with a wide variety of compositions (shoshonitic to lamproitic) erupted in Spain between 8 and 5 Ma and in North Africa between 9 and 4 Ma. The lamproitic volcanic rocks are widely scattered in southern Spain, but do not occur in northern Africa. The youngest volcanic rocks are alkaline basalts with an age of Pliocene to Quaternary in southern Spain, in North Africa the youngest volcanic rocks are Messinian to Quaternary alkaline basalts.

    Considering the ages of the different Neogene volcanic complexes in the western Mediterranean (figure 5.1b and c), and the relation between tephra thickness and distance from the source (Watkins et al., 1978), the volcanic tephras in the Messadit basin most probably originate from the Gourougou volcano located immediately south of the basin. The Trois Fourches complex, at ~10 Ma immediately north of the basin is too o