Chapter D4 Potential integrated middle miocene stratigraphy in southeastern Spain

Miocene Stratigraphy: An Integrated Approach A. Montanari, G.S. Odin and R. Coccioni, eds. 9 1997. Elsevier Science B.V. All rights reserved.

Chapter D4

POTENTIAL INTEGRATED MIDDLE MIOCENE STRATIGRAPHY IN SOUTHEASTERN SPAIN Ch. Montenat, E Serrano and J.A. Martin-Perez

INTRODUCTION The eastern segment of the Betic Cordilleras includes three major geological zones, from the north to the south: (1) the prebetic autochthonous domain linked to the Iberian Meseta; (2) the subbetic allochthonous units, overthrusting the Prebetic northward; Prebetic and Subbetic are referred to the external sedimentary zones of the Betics; (3) the internal allochthonous zone, composed of a pile of large alpine nappes including sedimentary and various metamorphic rocks. This domain is separated from the External zone by a major ENE-WSW-trending wrench-fault. The Miocene sedimentary sequences discussed below are located within the Internal zone, between Almeria and Alicante (Fig. 1). The Internal nappes recorded a complex polyphase structural evolution, from Mesozoic to Oligocene-Early Miocene times (review in Diaz de Federico et al., 1990). From the Early Miocene (Burdigalian) onwards, the pile of nappes was tom into small blocks by faulting which developed within a large transcurrent fault zone, the Trans-Alboran shear zone, trending NE-SW through the Eastern Betics. Faulting is related to a compressional stress field caused by the Iberia-Africa collision (N-S- to NW-SE-trending direction of compression; Ott d'Estevou and Montenat, 1985). Wrench tectonics, including large-scale lateral movements (Montenat and Ott d'Estevou, 1992), directly controlled the development of Miocene sedimentary basins (Montenat, 1990). A first generation of basin filling, late Burdigalian to Serravallian in age, with an intra-Langhian minor structural phase, was severely deformed and eroded before Tortonian time. The second generation, ranging from Tortonian to Plio-Pleistocene time, is widely distributed in a variety of intra-mountain depressions (Fig. 1). Miocene deposits are predominantly deep marine, planktonic fossils-rich sediments. Near-shore and continental deposits, sometimes abundant with micromammals (de Bruijn et al., 1975; Mein and Agusti, 1990), are present in the well-preserved Late Miocene palaeogeographic sequence. A diversified Neogene magmatic sequence is closely related to the Trans-Alboran shear zone (Hernandez et al., 1987). It includes plutonic, volcanic and metallogenic events which interfered with the tectonic and sedimentary processes.

364

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VOLCANIC SERIES AND RELATED MIOCENE SEDIMENTS The Miocene volcanism is basically restricted to the easternmost part of the Betic Cordilleras and can be split into three major belts. For each of them the stratigraphic correlations, radioisotopic dates, and biostratigraphic data from the associated sediments, make possible a quite confident chronological framework for the magmatic activity (Bellon et al., 1983; Serrano, 1992) (Fig. 2).

Potential integrated Middle Miocene stratigraphy in southeastern Spain

CHRONOLOGY

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366

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Cabo de Gata group The Cabo de Gata group is located in the Cabo de Gata massif (northeast of Almeria), including the off-shore extensions to the east and the southwest. This group represents one of the most important Miocene volcanic complexes of the Mediterranean which includes about 500 km of outcrop, and more than 1000 m of drilled volcanics below sea level. The volcanics were piled up on the eastern side of a large left lateral fault, the Carboneras fault, trending NE-SW (Fig. 1). The Cabo de Gata group includes a series of calc-alkaline sequences referred as sequences A to D (Bellon et al., 1983; see regional mapping of these different units in Bordet, 1985). Sequence A. This sequence is composed of two different petrochemical suites: low-K andesites (Aa suite) overlain by a thick pile of dacites, and rhyolites and ignimbrites enriched in K20 (Ab suite, outcropping in the southem part of the Gata massif). The first manifestations of the Aa member are found in the NW approaches of the Gata massif (north and northwest of the city of Carboneras) where several andesitic flows are interbedded with pelagic marls and turbidites, late Burdigalian-early Langhian in age (see below). About 50 km to the northeast of Carboneras, volcanics referred to as the Aa suite outcrop in the Aguilas area along the beach of Los Terreros. Andesitic agglomerates are interbedded within plankton-rich marls at the Langhian/Serravallian boundary (Montenat et al., 1978; see description below). The Ab suite is not yet dated and is tentatively referred to the Serravallian, but without precise correlation with any sedimentary series. Sequence B. Sequence B is composed of low-K20 and Na20 rocks ranging from frequent andesites to more rare dacites and rhyolites. It is characterized by a thick pile of breccias and domes, widely outcropping in the Gata mountains. Nine K-Ar dates of whole-rocks spread out within the series gave ages ranging from 11.83 -1-0.60 (2or interlaboratory) Ma to 9.34 -1-0.45 Ma (late Serravallian to early Tortonian; Bellon et al., 1983). Rocks presumably correlated to the lower part of the same sequence are dated to between 12.4 Ma and 10.8 Ma (late Serravallian) by Di Battistini et al., (1987). Sediments interbedded within series B are calcarenites abundant with Heterostegina (Los Escullos near San Josr). In some places, like E1 Plomo, the volcanics, dated at 9.624-0.50 Ma (Bellon et al., 1983) are covered by marls with planktonic Foraminiferas, indicative of the base of the upper Tortonian (Serrano, 1992; Montenat and Serrano, Chapter E8). Sequence C. This sequence is an acid suite of ignimbritic rhyolites some of which are locally enriched in K20 and hydrothermalized, that are located in the central part of the massif in what is referred to as the gold ore deposits of Rodalquilar. Whole-rock rhyolite samples taken outside of the altered zone are K-Ar dated at 8.904-0.45 Ma, and 8.75 4- 0.45 Ma, late Tortonian (Bellon et al. 1983). Sequence C is capped by Messinian reefs (Bordet, 1985). Sequence D This sequence corresponds to low-K and Na andesites (similar to sequence B) grouped in several volcanoes located close to the coast (near San Josr, Roldan, and other localities). No stratigraphic relationships between sequences C and D have been observed. Available K-Ar dates from whole-rock samples indicate ages of 8.67 4- 0.45 to 7.90 4- 0.45 Ma (Bellon et al., 1983), and 8.6 Ma to 7.5 Ma (Battistini

Potential integrated Middle Miocene stratigraphy in southeastern Spain

367

et al., 1987). These volcanic rocks are located within the late Tortonian as evidenced by Neogloboquadrina with dextral and sinistral coiling, below and above the volcanics, respectively. The plutonic equivalents of these volcanic granodiorites to gabbros units are evidenced by the way of erupted pebble-dikes (Montenat et al., 1984).

The Mazarrrn group This is another calc-alkaline sequence with high A1 content, derived from shallow crustal anatexis from the Betic metamorphic basement (Zeck, 1970; Venturelli et al., 1986). It includes a variety of dacitic to rhyolitic volcanoes (domes and breccias) located in an outer position (westwards to northeastwards) with regard to the Gata massif (Almeria, Vera, Hinojar, Mazarr6n, Cartagena) (Fig. 2). The volcanics cut through early Tortonian pelagic deposits in the Cartagena area. In the Hinojar and Mazarr6n basins, the breccias and tuffites are interbedded within late Tortonian plankton-rich marls with Neogloboquadrina humerosa. In any case the volcanics were emplaced prior to the Tortonian/Messinian boundary. Only a few reliable radioisotopic K-Ar whole-rock dates are currently available (see Chapter E8): 6.62 4- 0.30 Ma, and 7.20 4- 0.35 Ma (Cartagena); 8.31 4- 0.40 Ma (Vera) (Bellon et al., 1983), 7.6 4- 0.30 Ma (Vera, K-Ar dating of biotites from rhyolite, Nobel et al., 1981).

The lamproites group Lamproites are uncommon ultra-K and magnesium-rich volcanic rocks of deep mantle origin. They are generally found as small pipes, dikes, sills, cones and flows, scattered over a large area to the west and northwest of the preceding groups. Lamproitic intrusions are found in marine Tortonian or Messinian basin deposits at Fortuna, Jumilla, and Murcia, and other locations. In some places like Barqueros and Vera, the volcanic flows are interbedded within marine or continental Messinian deposits (see Chapter E8). Available data indicate K-Ar isotope ages on whole-rock ranging from 7.0 4- 0.3 Ma to 6.2 4- 0.3 Ma at Barqueros (Montenat et al., 1975,) and at Fortuna from 6.16 4- 0.30 Ma (Bellon et al., 1983) to 5.67 4- 0.30 Ma (Bellon et al., 1981). POTENTIALLY USEFUL MIDDLE MIOCENE SECTIONS Two sections belonging to the Gata group suitable for integrated stratigraphy, are described below (Fig. 3).

North of Carboneras (Rambla Granadilla and Torre del Pe~on) Various outcrops of a late Burdigalian-early Langhian succession are preserved within the Carboneras fault zone. They suffered significant deformations due to wrench tectonics prior to the Late Miocene (Tortonian). This sequence consists of pelagic planktonic microfossil-rich deposits, often associated with volcanic flows (especially to the north of Carboneras).

Ch. Montenat, E Serrano and J.A. Martin-Perez

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Rambla Granadilla (Cerro del Marques). A large strip of Miocene rocks, trending NE-SW, appears faulted within dark crystalline schists of the Betic basement. It is a succession of light-grey to green silty marls and brownish turbidite sandstones alternating with andesitic flows some of which are 5 to 20 m in thickness. The total thickness of this sequence is about 200 m. Most of the marls yield a late Burdigalian microfauna including Globigerinoides bisphaericus, G. altiaperturus, G. subquadratus, G. gr. trilobus, Globoquadrina dehiscens, G. baroemoenesis, G. langhiana, Globorotalia (Fohsella) peripheroronda, Praeorbulina glomerosa sicana, etc., which probably indicate the uppermost part of the Burdigalian. Nannoplankton assemblages contain Coccolithus miopelagicus, C. pelagicus, Cyclicargolithus abisectus, C. floridanus, Dictyococcites productus, D. scrippsae, Discoaster deflandrei, D. druggi, D. variabilis, Helicosphaera ampliaperta, H. carteri, H. euphratis, Pontosphaera multipora, Reticulofenestra daviesii, R. gartneri, Sphenolithus heteromorphus. The presence of Sphenolithus heteromorphus and Helicosphaera ampliaperta, and the absence of Reticulofenestra pseudoumbilica, further indicate that these levels belong to the latest Burdigalian. The uppermost part of the series contains an early Langhian microfauna, including

Potential integrated Middle Miocene stratigraphy in southeastern Spain

369

the preceding species (with the exception of G. altiaperturus), and P. glomerosa. The primitive form of P. glomerosa (P. glomerosa curva type), seems to indicate an earliest Langhian age. In the calcareous nannoplankton assemblages Reticulofenestra pseudoumbilica is present, indicating a Langhian age. Isotopic dating of the volcanic rocks (andesite and amphiboles) has not been carried out. It is important to note that a southwestern part of the same faulted zone in La Serrata, at the Archidona quarry, basaltic andesite related to the same sedimentary sequence yielded a Langhian age (15.15 4-0.75 Ma, K-Ar on whole-rock; Bellon et al., 1983). Torre del Pe~on. The Miocene grey marls exposed near the coastline include the previously quoted calcareous microfauna (late Burdigalian and earliest Langhian). They are capped by massive andesitic volcanic rocks (suite Aa). Volcanic debris and tuffites with amphiboles, interbedded within the uppermost part of the marls, make it possible to radioisotope-date the lower Langhian near its basal boundary.

South of Aguilas (Los Terreros beach) The northernmost extension of the Gata group volcanics may be found in some islets located near the shoreline, to the south of Aguilas along the beach at Los Terreros. In the Los Terreros area, outcrops of a thick Miocene sequence, including pelagic marls, turbidite sandstones, and debris-flows, contain planktonic microfaunas indicative of early and late Langhian ages (Praeorbulina and Orbulina zones; Montenat et al., 1978). The top of the sequence is made of grey marls and includes volcanic debris of andesite, tuffites with amphiboles, and conglomerates. In these levels, the Langhian/ Serravallian boundary is located at the first appearance of Globorotalia praemenardii (Serrano, 1992). The nannoplankton assemblages contain Sphenolithus heteromorphus, Reticulofenestra pseudoumbilicus and Calcidiscus macintyrei, which characterize zone NN5 of S. heteromorphus (Martini, 1971). The disappearance of Discoaster kugleri occurs in the same level as the appearance of G. praemenardi. These volcaniclastics mark the beginning eruptions of massive andesitic volcanics (breccias and domes) which crop out in the islets and on the shoreline at Cuatro Calas north of Los Terreros. Radioisotope dating of the andesite of Cuatro Calas (K-Ar whole-rock) give an age of 14.1 4- 0.7 Ma for the lowermost Serravallian. CONCLUSIONS This short review on the chronology of Betic Miocene volcanic rocks, and related sediments, points out the good correlation of available radioisotope datings in a robust chronological framework. Nevertheless, it is important to note that most of these dates were produced ten or fifteen years ago using the K-Ar isotope dilution technique applied to whole-rock samples. Additional interdisciplinary studies taking into consideration the recent progress in radioisotopic analysis, regional geology, and biostratigraphy, will certainly lead to a more accurate and precise determination of the integrated stratigraphy of this important volcano-sedimentary sequence.

370

Ch. Montenat, E Serrano and J.A. Martin-Perez

As for the Middle Miocene interval, several different sections in the Gata-Aguilas area contain suitable sections (volcaniclastic layers and volcanic flows with amphiboles, associated with plankton-rich sediments) to bracket the Burdigalian/Langhian and Langhian/Serravallian boundaries (Fig. 3). SOMMAIRE - - POTENTIEL POUR LA STRATIGRAPHIE INTEGRt~E DU MIOCI~NE MOYEN EN ESPAGNE DU SE (Manuscrit soumis: Fgvrier 1994; rgvisg: Juin 1994; rgdacteur responsable: GSO) Le segment oriental des Cordill~res bftiques se compose de trois unitfs gfologiques du nord au sud: (1) le domaine prfbftique autochtone; (2) les unitfs subbftiques allochtones charrifes vers la nord; (3) la zone interne allochtone composfe de nappes empilfes comprenant des roches sfdimentaires et diverses roches mftamorphiques. Les sfquences sfdimentaires mioc~nes situfes dans cette zone interne remplissent des bassins en deux temps: (1) du Burdigalien supfrieur au Serravallien; les sfdiments ont ensuite 6t6 dfformfs et en partie 6rodfs avant le Tortonien; (2) du Tortonien au PlioP16istoc~ne; elles sont alors largement rfparties dans des dfpressions intramontagneuses. Les facies reprfsentfs sont marins, profonds et fiches en fossiles planctoniques mais on rencontre aussi d'intfressants bassins continentaux parfois fiches en micromammiferes. La prfsence commune de roches volcaniques interstratififes depuis le Burdigalien supfrieur jusqu' au Messinien constitue le caract~re le plus remarquable pour une 6tude de stratigraphie int6grfe. Au Miocene moyen, le 'Groupe' volcanique de Gata subdivis6 en 'Sfries' para~t &re un bon objectif de recherche. Les profils les plus prometteurs concernent la partie inffrieure du Miocene moyen: les deux limites du Langhien pourraient ainsi ~tre caractfrisfes ~ la base dans les profils de Rambla Granadilla et Torre del Pefion, au sommet dans celui de la plage de Los Terreros. Ces 6tudes modernes permettant de prfciser les connaissances dfj?a acquises pour localiser dans le temps des 6vfnements biostratigraphiques marins et continentaux. (Sommaire propos~ par les r~dacteurs, GSO)