Sedimentary and diagenetic markers of the restriction ...

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Sedimentary Geology 121 (1998) 23–55

Sedimentary and diagenetic markers of the restriction in a marine basin: the Lorca Basin (SE Spain) during the Messinian J.M. Rouchy a,Ł , C. Taberner b , M.-M. Blanc-Valleron a , R. Sprovieri c , M. Russell d,g , C. Pierre e , E. Di Stefano c , J.J. Pueyo f , A. Caruso c , J. Dinare`s-Turell b,1 , E. Gomis-Coll b , G.A. Wolff g , G. Cespuglio e , P. Ditchfield e , S. Pestrea h , N. Combourieu-Nebout i , C. Santisteban f , J.O. Grimalt d a

CNRS-ESA 7073, Laboratoire de Geólogie, Museúm National d’Histoire Naturelle, 43, rue Buffon, 75005 Paris, France b Institut de Cie ` ncies de la Terra (I.C.T.-C.S.I.C.), Lluis Sole´ i Sabaris s=n, 08028 Barcelona, Spain c Department of Geology and Geodesy, University of Palermo, Corso Tukory 131, Palermo, Italy d Department of Environmental Chemistry (C.I.D.-C.S.I.C.), Jordi Girona 18–26, 08034 Barcelona, Spain e CNRS-LODYC, Universite ´ P. et M. Curie, 4, place Jussieu, 75252 Paris Cedex 05, France f Departamento de Geoquimica, Petrologıá y Prospeccio ´ n Geolo´gica, Universidad de Barcelona, Z.U. de Pedralbes, 08071 Barcelona, Spain g Department of Earth Sciences, University of Liverpool, Bedford Street North, P.O. Box 147, Liverpool L69 3BX, UK h Geological Institute, Laboratory of Palaeontology, 3, Caransebes, 78344 Bucharest, Romania i CNRS-ESA 7073, Universite ´ P. et M. Curie, 4, place Jussieu, 75252 Paris Cedex 05, France Received 16 September 1997; accepted 8 April 1998

Abstract The Lorca Basin (southeastern Spain) is part of a chain of small marginal Neogene basins located in the structurally active Betic area. The Upper Miocene (Messinian) sequence is composed of a thick diatomite-bearing series (Tripoli Unit) overlain by the Main Evaporites, analogous to the classical succession that records the main events during the Salinity Crisis in the Mediterranean region. The shallow restricted conditions of this region amplified the sedimentary responses to local and global forcings. An integrated approach using sedimentology, micropalaeontology, stable isotope geochemistry and organic geochemistry has been applied to the Tortonian=Messinian succession of the Lorca Basin, in order to obtain a continuous record of the environmental changes. The sediments record two major events which affected the whole Mediterranean: (1) high levels of productivity that led to the formation of the diatomite-bearing deposits in the early Messinian (Tripoli); and (2) the Messinian Salinity Crisis with its two major stages, represented by the Halite and Gypsum Units, both mainly precipitated from marine-derived brines. The rapid reflooding of the Mediterranean by normal marine waters at the base of the Pliocene did not reach the Lorca Basin, nor other basins of this part of the Betic area. Instead, continental sediments were deposited as a consequence of the regional uplift of SE Iberia, which started close to the Messinian=Pliocene boundary. The most prominent feature of this basin concerns the record of its restriction by the time of the deposition of the Tripoli Unit, which led to intercalations of precursor evaporitic layers, consisting of Ca-sulphate deposited in sub-aqueous and sabkha conditions, interbedded with diatomites. This alternation of evaporites and diatomites proves that the Lorca Basin was periodically restricted and reflooded by marine waters, a possible cause Ł Corresponding 1

author. E-mail: [email protected] Present address: Palaeomagnetic Laboratory, Fort Hoofddijk, University of Utrecht, Budapestlaan 17, 3584 CD Utrecht, Netherlands.

0037-0738/98/$ – see front matter  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 7 1 - 2

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J.M. Rouchy et al. / Sedimentary Geology 121 (1998) 23–55

for this being relative sea-level fluctuations in the Mediterranean. This strengthens evidence of diachronism that suggests that the onset of the first Messinian evaporitic deposition was not synchronous, but was dependent on bathymetry and local tectonics. High productivity during the early Messinian in this basin is demonstrated by the thick deposits of diatomites. However, stagnation episodes may have occurred during this interval, as suggested by the preservation of high amounts of organic matter (organic-rich shales) and the extent of bacterial sulphate reduction which apparently occurred during early diagenesis. The formation of organo-sulphur compounds, replacement of sulphates by carbonates and the high levels of elemental sulphur are by-products of diagenetic processes occurring in a restricted hypersaline environment.  1998 Elsevier Science B.V. All rights reserved. Keywords: Messinian; Mediterranean; Betic basins; evaporites; organic matter; sulphur; bacterial sulphate reduction

1. Introduction The restriction that affected the Mediterranean during the Messinian (Salinity Crisis) took place through a succession of complex hydrological changes which are documented in the sedimentary record by a sequence of marine marls, diatomitebearing deposits (Tripoli Unit), evaporites and brackish deposits (Lago-Mare) (Cita et al., 1978; Hsu¨ et al., 1978; Rouchy, 1982; Rouchy and Saint-Martin, 1992). Large basin to basin variations resulted from the overprinting of global events (i.e. plate tectonics, glacio-eustatic and climate fluctuations) by regional orogenic activity (Cita and McKenzie, 1986; Rouchy and Saint-Martin, 1992; Butler et al., 1995). The shallow peripheral basins were particularly sensitive to such variations and provide significant information on chronology and pattern of restriction. The Lorca Basin is one of the small marginal basins (200 km2 ) located in the structurally active Betic domain (Fig. 1). The Upper Miocene sedimentary sequence is composed of marine marls, diatomite-bearing deposits (Tripoli Unit) and evaporites (Gypsum and Halite Units) (Geel, 1979; Rouchy, 1982; Ortı´, 1990; Montenat et al., 1990; Benali et al., 1995; Guilleń-Mondejar et al., 1995). The relatively shallow water conditions which prevailed during the Messinian amplified the sedimentary response to local and=or global environmental changes. The study of this basin enables a refinement of: (1) the chronology of the restriction; (2) the sedimentary and diagenetic processes related to organic-rich and hypersaline sedimentation; (3) the evolution of deposition towards continental settings; (4) the role of tectonics on the basin restriction. Firstly, the bio- and magnetostratigraphy, sedi-

mentology, fossil assemblages, organic matter contents and carbonate stable isotopic compositions of a reference section (La Serrata), located in the central part of the Lorca Basin (Fig. 1A), were studied. Secondly, additional sampling was carried out in several parts of the section in order to determine the relationships between organic-rich sediments, diagenetic carbonates replacing sulphates and elemental sulphur. Finally, the information obtained has been contrasted with observations of several sections located in the northeastern sector of the basin (Fig. 1A).

2. Geological background The Lorca Basin is an intermontane depression which belongs to a system of interconnected Neogene basins located in the eastern part of the Betic Mountains. These basins are placed within a complex fault zone which crosses this orogenic system over more than 250 km between Almeria and Alicante (Fig. 1B). Proposed mechanisms of basin formation are: (1) extensional collapse of the orogenic belt (Platt and Vissers, 1989); or (2) strike-slip movement along this faulted system, which in turn is interpreted as a shear zone related to the collision between the African and Iberian plates (Montenat et al., 1987; de Larouzie`re et al., 1988; Sanz de Galdeano, 1990). The Lorca Basin is bounded by two main fault systems oriented NE–SW (North Betic to the north and Alhama de Murcia to the south) and N– S, which remained active during the Neogene and controlled the sedimentation (Montenat et al., 1990; Guilleń-Mondejar et al., 1995). The physiography of the basin, which has existed since the Early Miocene, was modified as a


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Fig. 1. (A) General geologic map with locations of the studied sections (adapted from Geel, 1979). (B) Regional structural framework (from Montenat et al., 1987).

consequence of a tectonic phase during the late Serravallian=early Tortonian (Guilleń-Mondejar et al., 1995). After this period, sedimentation began with early Tortonian shallow water deposits, i.e. conglomerates, siliciclastics and carbonates, including coral reefs. A palaeogeographical differentiation occurred during the Tortonian resulting in two distinct zones: a more stable area to the northwest with continuous carbonate and coarse-grained siliciclastic

sedimentation, and an elongated depocentre parallel to the southeastern margin. In the depocentre, which corresponds to the La Serrata section (Fig. 2), a 1000 m-thick sequence of marls accumulated during the Tortonian and earliest Messinian (Dinare`s-Turell et al., 1998) (Fig. 2). The overlying Messinian series comprises pre-evaporitic laminated deposits (Tripoli Unit), about 130 m thick, and the Main Evaporites (Geel, 1979; Rouchy,

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Fig. 3. (A) Panoramic view of the measured sections of the La Serrata, showing the Tripoli Unit with the three lower layers of sulphur-bearing diagenetic carbonates. The thickness of the interval between layers I and III is 19 m. This view corresponds to the area comprised between the sites labelled 1a and 1c on Fig. 1A. (B) View towards the reef carbonate complex of the Cejo de los Enamorados, on the SW margin of the basin, showing: (1) the interfingering of the carbonate talus with the Tortonian marls; (2) the progressive thinning of the Tripoli Unit over the talus; and (3) the Tripoli Unit and Main Gypsum Unit onlapping the reef talus deposits.

1982; Ortı´, 1990; Ortı´ and Rosell, 1990; Benali et al., 1995) (Figs. 2 and 3A). The Gypsum Unit (La Serrata Gypsum), up to 50 m-thick, outcrops more or less continuously across the basin. Two boreholes drilled north of the gypsum outcrops crossed a Halite Unit, 200 m-thick, underlying massive gypsum deposits, which could be correlated to the Gypsum Unit (IGME, 1982; Ortı´, 1990). Thus, deposition of the Halite Unit was interpreted as predating that of the Main Gypsum Unit or to be partly equivalent to the lower part of this unit (Figs. 1 and 2). The area with the known maximum thickness of evaporites is considered to represent the more subsident part of the

basin (Ortı´, 1990). The thickness of the outcropping pre-evaporitic formation decreases regularly towards the eastern and western margins of the basin. Onlapping geometries between the Gypsum Unit and the older deposits (Fig. 3B) can be deduced from field observations. In the Lorca depression, the evaporites are covered by continental marls of undifferentiated late Messinian to Pliocene age. In the northwestern part of the basin, in the area crossed by the Guadalentin River (Fig. 1A), the Tripoli and Gypsum Units are not present and instead thick deltaic to fluviatile siliciclastic deposits are found. The Lorca Basin, as it is for the rest of Neogene

Fig. 2. The La Serrata sections in the central part of the basin showing the lithology, biostratigraphy and magnetostratigraphy (magnetostratigraphy is from Dinare`s-Turell et al., 1998). The left-hand section is a simplified stratigraphic log of the whole sedimentary succession and the right-hand section corresponds to a detailed log of the late Tortonian=Messinian pre-evaporitic succession. Numbers I to VIII refer to the sulphur-rich carbonate layers.

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basins in southeastern Spain, records a cortical uplift since the Messinian (Docherty and Banda, 1995). This general uplift might have led to the absence of a record of the Pliocene transgression in this area, after the deposition of the Messinian evaporites. The Sierra de la Tercia range (Fig. 1A) was submerged during most of the Tortonian and early Messinian, forming submarine highs covered by large carbonate platforms, the talus of which locally interfingers with the Tortonian=lower Messinian marls (Fig. 3B). These submarine highs were sub-aerially exposed during the late Messinian and remained as topographically highs, which separated different areas of evaporite deposition, as shown by the presence of evaporites in the Lorca depression and on the opposite side of the Sierra de la Tercia (Fig. 1A) (Montenat, 1973; Rouchy, 1982; Ortı´ et al., 1993).

3. Sampling and methods The evolution of the depositional conditions is constrained chronologically by a biostratigraphic study (planktonic foraminifera and calcareous nannoplankton) together with a palaeomagnetic survey that was carried out along the reference section of La Serrata (Dinare`s-Turell et al., 1998). Biostratigraphic data derive from samples taken ca. every 5 m up to 900 m and approximately every 1 m in the upper part of the section (about 300 m), up to the base of the Main Gypsum Unit. Estimations of water depth and indications of bottom water conditions were obtained from quantitative data on benthic foraminiferal assemblages by analyzing the species variation of 300 specimens in the fraction greater than 125 µm. Quantitative data were assessed by Q-mode factor analysis, using the Cabfac program (Klovan and Imbrie, 1971; Imbrie and Kipp, 1971). Four factors were rotated and the Rotated Factor scores are reported in Table 1. The most significant species indicated by these factors are: Factor 1 (dominated by Heterolepa praecinctata, Spiroplectammina carinata), uppermost slope environment, ca. 150–200 m depth (Parker, 1958; Wright, 1978); Factor 2 (dominated by Bulimina spp. spinose type, Bolivina dilatata), poorly oxygenated bottom conditions and high supply of organic carbon in the bottom waters (Verhallen, 1991; Sen Gupta

Table 1 Varimax factor score matrix Var. Allomorphina trigona Ammonia beccarii Bolivina alata Bolivina dilatata Bolivina punctata Bulimina spp. (costata type) Bulimina spp. (spinose type) Cancris oblongus Cassidulina carinata Cassidulina crassa Cassidulina oblonga Chilostomella spp. Cibicidoides pachyderma Cibicidoides ungerianus Florilus boueanus Fursenkoina schreibersiana Gavelinopsis praegeri Globobulimina spp. Gyroidinoides altiformis Gyroidinoides neosoldanii Gyroidinoides umbonatus Hanzawaia rodhiensis Heterolepa praecinctata Hopkinsina bononiensis Lenticulina spp. Marginulina spp. Martinottiella spp. Melonis spp. Nonionella turgida Oridorsalis umbonatus Orthomorphina spp. Pullenia spp. Quadrimorphina spp. Rectuvigerina spp. Sigmoilopsis schlumbergeri Siphonina planoconvexa Sphaeroidina bulloides Spiroplectammina carinata Stainfothia complanata Textularia mexicana Textularia ponderosa Uvigerina spp. (costata type) Valvulineria spp.

1

2 0.057 0.020 0.016 0.011 0.053 0.039 0.007 0.016 0.041 0.006 0.008 0.012 0.225 0.034 0.109 0.000 0.010 0.082 0.020 0.112 0.085 0.012 0.737 0.015 0.241 0.020 0.005 0.024 0.020 0.032 0.005 0.018 0.004 0.012 0.016 0.002 0.019 0.498 0.000 0.127 0.015 0.159 0.058

3 0.025 0.113 0.026 0.325 0.018 0.064 0.898 0.006 0.003 0.029 0.024 0.001 0.025 0.042 0.100 0.016 0.068 0.020 0.002 0.012 0.045 0.022 0.018 0.059 0.032 0.001 0.004 0.045 0.003 0.042 0.005 0.040 0.005 0.025 0.005 0.005 0.003 0.066 0.009 0.000 0.000 0.174 0.042

4 0.005 0.166 0.008 0.105 0.838 0.064 0.032 0.002 0.004 0.012 0.017 0.002 0.139 0.051 0.041 0.014 0.310 0.018 0.018 0.026 0.013 0.011 0.198 0.020 0.002 0.009 0.012 0.024 0.009 0.028 0.003 0.032 0.001 0.003 0.006 0.004 0.000 0.285 0.008 0.049 0.005 0.076 0.024

0.016 0.362 0.001 0.143 0.048 0.047 0.000 0.018 0.007 0.006 0.004 0.004 0.129 0.026 0.817 0.014 0.040 0.008 0.012 0.004 0.011 0.011 0.011 0.019 0.053 0.001 0.002 0.003 0.003 0.280 0.026 0.208 0.001 0.024 0.007 0.006 0.003 0.102 0.038 0.031 0.001 0.120 0.061

and Machain-Castillo, 1993); Factor 3 (dominated by Bolivina punctata and Gavelinopsis praegeri), the latter species is indicative of an outer shelf environment, ca. 100 m depth (Parker, 1958; Wright, 1978); Factor 4 (dominated by Florilus boueanus, Ammonia beccarii), inner shelf environment, ca. 50–60 m


depth (Parker, 1958; Wright, 1978). Diatom assemblages were studied in 25 samples characteristic of the different intervals, with 300 to 500 specimens counted for each sample according to the method used by Schrader and Gersonde (1978a). The palaeomagnetic results are based on a study including 45 sampling sites distributed through the Tortonian=lower Messinian marls and the lower third of the Tripoli Unit (Dinare`s-Turell et al., 1998). The data have been correlated with the Geomagnetic Polarity Time Scale of Cande and Kent (1995), using the astronomical time scale of Krijgsman et al. (1995) and Hilgen et al. (1995) as age constraints for the reversal boundaries and biostratigraphic events. The bulk mineralogy and carbonate contents of 25 samples from the homogeneous Tortonian marls, 330 samples from the pre-evaporitic late Tortonian=Messinian deposits, and 30 samples from the intra- and post-gypsum marls of the La Serrata section have been analyzed. The carbonate content of each sample was measured on 100 mg of powdered sediment using a manocalcimeter (MCM). The bulk mineralogy was determined by X-ray diffraction using a Siemens D-500 instrument (Ni filtered Cu Kα radiation). Additional samples were obtained from sections from the northeastern margin (Carretera de Caravaca, Casas del Mellado, Cortijo de las Colegialas, Cortijada del Pozuelo) and from a sequence of organic-rich shales (labelled ORS in Fig. 2) associated with the sulphur-bearing limestone bed III. Clay minerals from some selected samples have also been studied. These are representative of the main intervals i.e. twelve samples from the homogeneous Tortonian=lower Messinian marls, fourteen from the Tripoli Unit and eight from the post-evaporitic marls. The clay fraction was analyzed by the method of free-carbonate oriented pastes (fraction less than 2 µm in size) which were glycolated and heat-treated at 570ºC for 90 min (Brindley and Brown, 1980). Fine-grained carbonates, biosiliceous deposits and diagenetic carbonates were characterized by normal light microscopy and scanning electron microscopy. Total organic carbon (TOC) and total sulphur content were measured on 48 representative samples using the analytical procedure described in Russell et al. (1997). Analysis of organic geochemical molecular markers was performed on selected samples, utilizing gas chromatography (GC) and

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gas chromatography–mass spectrometry (GC–MS) (Russell et al., 1997). The isotopic composition (18 O, 13 C) of calcite was determined on 180 samples from the Tripoli Unit and eight samples from marls immediately underlying this unit. Sampling resolution ranged from 15 to 150 cm, with an average of 50 cm. The isotopic composition of dolomite was determined on 51 samples coming from the opal-CT-rich dolomitic layers, from the Tripoli Unit and from the uppermost dolomitic interval underlying the Main Gypsum. Five additional samples from the dolomite associated with the precursor gypsum interbeds present in the marginal sections were analyzed. Stable isotope analyses were performed on both calcite and dolomite. The ‘calcite’ value is taken as corresponding to the fraction of CO 2 produced after 20 min reaction at 25ºC with phosphoric acid. ‘Dolomite’ from carbonate mixtures was isolated by selective attack with acetic acid 1 N for 20 min and later reacted with phosphoric acid at 25ºC for 4 days. The CO2 gas was analyzed on a triple collector mass spectrometer (VG-SIRA 9). The Ž values are expressed in ‰ relative to the PDB reference and the Ž18 O values of dolomites are corrected by 0.8‰ for the fractionation effect during the phosphoric acid reaction (Sharma and Clayton, 1965).

4. Stratigraphy The onset of deposition of the marl series underlying the Tripoli Unit is not accurately dated, as the first datum (Globigerinoides obliquus extremus First Occurrence, 8.26 Ma) (Sprovieri et al., 1997) is identified only 420 m above the base of the section (Fig. 2). The next biostratigraphical event identified in the sequence is the First Occurrence (FO) of Globoratalia praehumerosa at 597 m. Between 640 and 775 m, Gt. praehumerosa was not found, but Gld. obliquus extremus and Gt. continuosa are present, indicating that this segment represents a repetition of that in the 420 and 590 m interval, probably as the result of submarine mass redeposition on the platform slope. The presence of this repeated interval and the frequent inclusion, in the marls, of large carbonate blocks reworked from the shelf areas, indicate that these processes of resedimentation were common during the Tortonian (Dinare`s-Turell et al., 1998) (Fig. 2). Al-

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though the FO of Gt. praehumerosa is known to occur within the chron C4n.2n at 7.69 Ma (Sprovieri et al., 1997), its presence here in a reversed palaeomagnetic interval (Fig. 2) is interpreted as being due to the local environmental conditions, which make its appearance here slightly later than in deeper Mediterranean basins. The Tortonian=Messinian boundary (Globorotalia conomiozea and Reticulofenestra rotaria FO, 7.15 Ma) occurs 25 m below the first typical diatomitic level considered as the base of the Tripoli Unit and correlates with chron C3Bn (Fig. 2). The increase of dextral Neogloboquadrina acostaensis is recognized 11 m above the base of the Tripoli Unit (Fig. 2). In the upper part of the section, about 20 m below the Main Gypsum Unit, the foraminiferal assemblage is composed only of species reworked from the Cretaceous and Eocene. From these age constraints a first approximation on the variation of sedimentation rates through the studied series can be proposed. In the interval between the FO of Gl. obliquus extremus and the FO of Gt. praehumerosa, the sedimentation rate can be estimated at ca. 34 cm=ka. In the lower part of the Tripoli Unit, the reduced thickness of the chron C3Ar., which is ca. 5 m for an interval of 400 ky (Hilgen et al., 1995), implies a much lower sedimentation rate (1.2 cm=ka). A gap in sedimentation occurring near the base of the Tripoli Unit could explain this very low rate although no evident erosional surface has been observed in the field. The age of the base of the Tripoli Unit cannot therefore be accurately and unambiguously defined. The absence of palaeomagnetic results (interval not sampled) and the lack of biostratigraphical markers in the upper two thirds of the Tripoli Unit do not allow the accurate definition of the age of the base of the Gypsum Unit. The lack of indigenous foraminifera and nannofossils in the continental post-evaporitic deposits also precludes the recognition of the Mio–Pliocene boundary.

5. The Tortonian and lower Messinian marls 5.1. Bulk mineralogy These marls display an homogeneous composition with an average amount of 43% carbonates

(calcite and dolomite), 20% clays, 8% quartz, 1% feldspars and a significant proportion of amorphous material (organic matter, amorphous silica). Dolomite constitutes about 10% of the bulk carbonate. Illite represents around 70% of the clay fraction, the rest being chlorite and smectite in equal amounts. In the uppermost 50 m, several sandstone beds and dolostone=opal-CT-rich dolomitic layers, up to 50 cm thick, are found intercalated within the homogeneous marls (Fig. 4). The nodular and irregular bedding suggests a diagenetic origin for the opal-CTrich dolomitic layers. Locally (Carretera de Caravaca section), this interval includes some bioturbated carbonate mud mounds, up to 1 m in thickness, which contain serpulid and bivalve accumulations. On SEM examination, the opal-CT-rich dolomitic layers appear mainly without structure, with abundant voids due to dissolution of diatom frustules (Fig. 5A); however, scarce well-preserved diatom frustules are present. Lepispheres of opal-CT are common (Fig. 5B). The presence of diatoms and of cherts that formed from diatom silica remobilization, suggests that several episodes of high biosiliceous productivity occurred before the deposition of the Tripoli Unit. With respect to the underlying marls, the carbonate fraction shows a significant increase in the percentage of dolomite, which is usually the only carbonate present in the opal-CT-rich layers (Fig. 4). 5.2. Organic matter The TOC values range between 0.06 and 0.6% in the Tortonian marls, but increase to 1–2% above the Tortonian=Messinian boundary (Fig. 6). Total sulphur contents are also low in the Tortonian marls, and again increase above the Tortonian=Messinian boundary. This increase in sulphur is paralleled by an increase in the abundance of organo-sulphur compounds (OSC), though compared to the more organic-rich samples of the overlying Tripoli Unit, these are still of minor importance. There are no molecular indicators of hypersalinity in this part of the section, but almost all the samples contain minor amounts of C25 highly branched isoprenoid (HBI) thiophenes, which indicate that there was an input from certain species of diatoms to these sediments (Volkman et al., 1994).

J.M. Rouchy et al. / Sedimentary Geology 121 (1998) 23–55 31

Fig. 4. Mineralogical composition of the Messinian deposits of the La Serrata section. Note that by comparison with the underlying deposits the mineralogy changes rapidly in the Tripoli Unit. The percentage of gypsum includes both primary and secondary gypsum resulting from sulphur oxidation. Halite and thenardite are related to surficial precipitation as efflorescences.

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Fig. 5. Scanning electron microscope (SEM) images of the opal-CT-rich dolomitic layers intercalated within the Tortonian=lower Messinian deposits. (A) Structureless fabric with elongated ghosts resulting from the dissolution of diatom frustules. (B) Dolomite crystals cemented by aggregates of opal-CT.

5.3. Micropalaeontological markers The planktonic foraminiferal assemblage is generally poor throughout the sequence and is essentially represented by Globigerina bulloides, Neogloboquadrina acostaensis (left coiling) and Globigerinoides spp. Globorotalia menardii (left coiling) is sporadically present up to about 900 m. Calcareous nannofossil assemblages are poor and affected by moderate to strong reworking of taxa from Cretaceous–Eocene to Middle Miocene deposits. Reticulofenestrids, Coccolithus pelagicus, Calcidiscus spp. and Helicosphaera stalis are the most frequent autochthonous taxa. Discoaster variabilis, D. surculus and D. brouweri are also present in the best diversified assemblages. Ceratholiths and other Late Miocene stratigraphic markers are absent in this interval. In the rich and well diversified benthonic assemblages, only Factor 1 (see definition of factors used for environmental interpretation above in Section 3) is dominant in the segment below the repeated interval, with small fluctuations. Consequently, the depth of deposition is estimated to be about 150–200 m. The fairly constant sedimentation depth implies that the accumulation rate equalled the subsidence rate during the deposition of sediments in this interval. A significant change occurs above the repeated interval, where species characteristic of Factor 3 dominate the assemblages, being indicative of a water depth of about 100 m, up to just above the FO of Gt. conomiozea. In the upper part of this segment, episodic intercalations of assemblages with high val-

ues of Factor 2 are indicative of periods of poorly oxygenated bottom water conditions, which appear simultaneously with the precursor episodes of high biosiliceous productivity recorded by the opal-CTrich dolomitic layers.

6. The Tripoli Unit 6.1. Distribution of the deposits The Tripoli Unit, the lower boundary of which is conventionally placed at the base of the first prominent diatomite bed, comprises two main members (Fig. 2). (1) The Lower Member is composed dominantly of diatomite beds, several metres thick, interbedded with silty marls. The thickness of the unit decreases from 90 m in the basin centre to less than 30 m at the margins, due to the thinning of the marl intervals, whereas the total thickness of the massive diatomites stays constant between 18 and 22 m, except in the most marginal areas to the southwest and northeast where only few diatomite layers, only a few centimetres thick, occur. These diatomites and marls are also interbedded with different sediments, e.g. fine- to coarse-grained sandstones, commonly containing clasts of basement rocks, fine-grained limestones or dolostones, laminated and crystalline gypsum, gypsarenites with fibrous gypsum veins. The most prominent feature of the Lower Member of the Tripoli Unit is the presence of intercalated lay-


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Fig. 6. Total organic carbon (TOC) and sulphur content of the marls and diatomites in the late Tortonian=Messinian pre-evaporitic interval. H D levels where biomarker assemblages indicate hypersalinity; D D presence of diatoms indicated by biomarkers (DC in significant amounts).

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ers of sulphur-bearing deposits, mostly carbonates, and gypsum. The gypsum layers are developed along the margins of the basins, especially in the NE area between Casas del Mellado and up to about 1 km to the northeast of Cortijada del Pozuelo, but are not apparent in the La Serrata area, where the gypsum has usually been replaced by carbonates (Fig. 1). At Cortijada del Pozuelo seven layers of gypsum, few tens of centimetres to 2 m in thickness, and one of sulphur-rich carbonate with gypsum relics (fourth layer from the base) are cyclically intercalated within the marls and diatomites (Figs. 7 and 8). The sections studied in this marginal area, especially at Cortijada del Pozuelo (Fig. 8), record episodic gypsum deposition intercalated within the marine Tripoli Unit. This provides evidence that high salinity deposition occurred well before the precipitation of the main Messinian evaporitic unit (Main Gypsum and Halite Units). This, and the adjacent Fortuna Basin, are the only places in the Mediterranean area where such interbedding of marine diatomites and gypsum is clearly exposed. In the central part of the study area, between Casas del Mellado to the northeast and the Guadalentin River to the southwest, the majority of these gypsum beds grade laterally into sulphurbearing deposits, mainly composed of carbonates (Figs. 8 and 9). At the La Serrata section, they are located below the base of the Gypsum Unit, at 108 m (layer I), 100 m (layer II), 88 m (layer III), 87.7 m (layer IV), 84 m (layer V1 ), 56 m (layer VI), 48 m (layer VII) and 36 m (layer VIII). These layers can be correlated between La Serrata and Cortijada del Pozuelo (Fig. 1), except for the stromatolite-like peloidal limestone labelled V1 which was not clearly identified in the marginal sections. In contrast, layer V2 which is well represented at the margins could be represented in the La Serrata section by a thin layer of powdery carbonate associated with secondary gypsum, located 59 m below the Main Gypsum. In the La Serrata area, layer VII corresponds to a fine-grained limestone that overlies a breccia composed of sandstone boulders and fragments

of Tortonian marls. Most of these carbonates were mined for sulphur exploitation in the first half of this century, the most intensely worked being layers III and IV. The three uppermost layers are not present in the central part of the studied outcrops (Carretera de Caravaca) which corresponds to an area of greater siliciclastic input, represented by thick beds of reddish coloured sandstones. These sandstones sometimes display basal erosional contacts and reduced lateral continuity, and are interpreted as channel-fill deposits intercalated within red marls and clays. Generally, in the area between Casas del Mellado and Cortijada del Pozuelo, the replacement of gypsum by carbonate occurs in different levels. The carbonate layers are more widespread in the lower part of the section (layers I to IV) than in the upper part (VI to VIII). Layers IV and VI, as well as the stromatolite-like layer V, are usually found intercalated within slumped diatomite and marl sequences throughout their areal extension, with layer V itself also being slumped in some areas. (2) The Upper Member has a maximum thickness of 33 m. It is composed mainly of silty marlstones showing an upward increase in thin layers, several centimetres to tens of centimetres in thickness, of reddish sandstones and containing dispersed thin diatomitic layers in the lower half of the member. The topmost part contains sandstone beds that are thicker than those of the Lower Member. Below the Gypsum Unit, the Upper Member ends with a 2 m-thick interval, composed of fine-grained carbonate (mostly dolomite) beds (Fig. 3A). This uppermost carbonate interval is not found in the SW and NE marginal areas where a breccia composed of gypsum, carbonate and sand fragments is locally observed just below the Main Gypsum (Cejo de los Enamorados, Cortijo de las Colegialas). 6.2. Structure and composition of the sediments Compared to the underlying marls, the Tripoli Unit is characterized by sharp variations in lithology and mineralogy (Fig. 4). The diatomitic deposits

Fig. 7. Lithological correlations between the central area (La Serrata section) and sections of the northeastern margin (Casas del Mellado, Cortijada del Pozuelo), illustrating the lateral transition of the main sulphur-bearing diagenetic carbonates to massive gypsum along the basin margin. The letter g or c after each layer number indicates gypsum or carbonate.


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Fig. 8. Precursor gypsum layers (arrows) intercalated within the Tripoli Unit (Cortijada del Pozuelo section). The thickness of the interval between the lowermost and uppermost gypsum layers is 65 m.

vary from pure diatomite to diatomitic marlstones or siltstones (Fig. 10A,C) and are usually laminated due to variations in the amount of diatoms, siliciclastics and carbonates that commonly correspond to calcareous nannofossils. Some of the laminae are composed of marly diatomites containing angular flakes of diatomitic laminae, indicating episodes of reworking (Fig. 10B). Layers of dark laminated chert (composed of massive opal-CT), up to 10 cm thick, are commonly intercalated within the diatomites (Fig. 10A,D). The composition of the carbonate fraction of the Tripoli Unit is variable, dolomite being common and even the exclusive carbonate component in some samples. A layer, up to 10 cm in thickness, composed of aragonite is present just below carbonate layer I, whereas thin layers of laminated limestones composed either of aragonite or calcareous nannoplankton are intercalated in the marls and diatomites below carbonate layers II, III, IV and V. In the topmost carbonate beds, the carbonate fraction is composed of dolomite, except for the thin and intensely deformed layers underlying the Gypsum Unit, which also contain calcite. Dolomite commonly appears as a cement or as aggregates of small crystals up to 20 µm (Fig. 11A,B). The terrigenous fraction in these sediments is mainly composed of quartz, clays and minor amounts of feldspars. The abundance of smectite is significantly higher than in the Tortonian marls, commonly reaching 30 to 60% of the clay fraction, which also contains illite and minor amounts of Fe-chlorites, kaolinite, mixed layers and palygorskite. Small

amounts of pyrite are common in the samples, although they are below XRD detection limits. 6.3. Sulphur-bearing carbonates and associated sediments These deposits display common sedimentary features, e.g. high porosity, in-situ brecciation and collapse structures which are responsible for marked variations in thickness, from a few centimetres to about 1 m. The carbonate layers are composed of variable amounts of calcite and dolomite, locally associated with aragonite. Gypsum was a major depositional component as indicated by abundant pseudomorphs of gypsum crystals that have been replaced by sparry calcite and dolomite. Gypsum pseudomorphs are surrounded by a dolomicritic matrix and consist of isolated or rosette aggregates of lenticular crystals up to a few millimetres in size (Fig. 12) and random or vertically oriented aggregates of prismatic or lenticular crystals up to 4 cm in length. In particular, layer III shows large undulations at its bottom and top. These might represent the external shapes of domal aggregates of selenite with their basal nucleation cones. Celestite locally constitutes aggregates of centimetre-sized crystals, whilst barite and chert are present in layer III. Elemental sulphur appears as disseminated granules in the carbonate matrix, millimetre to centimetre nodules or aggregates of large crystals infilling cavities after gypsum dissolution and veins bounding the breccia clasts. Locally, centimetre-sized nodules of alunite are found on top of the marls which


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Fig. 9. Sketch summarizing the correlations between the sulphur-bearing deposits and the gypsum throughout the basin.

are in contact with the base of layer III. This mineral is reported in similar settings in the Middle Miocene of the Gulf of Suez (Rouchy and Pierre, 1987), where it is thought to have been produced by the reaction of clay minerals with sulphuric acid derived from the oxidation of sulphides at a redox boundary. Secondary gypsum may be formed recently by oxidation of elemental sulphur; the former locally impregnates the surface of the carbonates at outcrop, or sporadically forms discontinuous diagenetic layers. The carbonate layers are interbedded with centimetre- to decimetre-thick diatomite and diatomitic marl beds, containing thin layers of organic-rich shales, in particular below layer III. Millimetreto centimetre-thick layers of laminated carbonates, pure white in outcrop, are also occasionally inter-

calated within the diatomites associated with the sulphur-bearing carbonates. These laminated carbonates are composed either of aragonite (Fig. 13A) or, most commonly, of monospecific and dwarf nannofossils (