Late Miocene–early Pliocene planktonic foraminifer event-stratigraphy of the Bajo Segura basin: A complete record of the western Mediterranean

Marine and Petroleum Geology 77 (2016) 1010e1027

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Research paper

Late Mioceneeearly Pliocene planktonic foraminifer eventstratigraphy of the Bajo Segura basin: A complete record of the western Mediterranean Hugo Corbí*, Jesús M. Soria Department of Earth Sciences and the Environment, University of Alicante, Apdo. Correos 99, San Vicente del Raspeig, 03080 Alicante, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 March 2016 Received in revised form 2 August 2016 Accepted 3 August 2016 Available online 4 August 2016

The Bajo Segura basin (eastern Betic Cordillera) has one of the most complete late Mioceneeearly Pliocene marine records of the western Mediterranean. An updated planktonic foraminifer zonal scheme based on recent astronomically tuned biozones is presented for this interval, documenting a complete succession of biostratigraphic markers, from biozone MMi9 (earliest Tortonian) to MPl3 (latest early Pliocene), of likely significance for regional-scale correlation throughout the Mediterranean. The findings reveal a series of intrazonal events (some unreported until now in the Mediterranean Neogene basin), including the particularly interesting two influxes of the Globorotalia miotumida group during the Tortonian. These biostratigraphic findings are the basis for a framework of the major allostratigraphic units in the basin based on planktonic foraminifer event-stratigraphy: synthems Tortonian I, Tortonian II, Tortonian-Messinian I, Messinian II, and Pliocene. In addition, the timing of the main tectonosedimentary and palaeogeographic events throughout the basin's evolution has been further constrained. Our results suggest that, at least in the Bajo Segura basin, the late-Messinian barren interval (non-distinctive zone) can be considered an ecobiostratigraphic zone (cenozone) characterized by dwarf fauna of planktonic foraminifera. Consequently, the Bajo Segura composite section can be regarded as a biostratigraphic reference section for Neogene basins in the Betic Cordillera and hence also in the Western Mediterranean. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Biostratigraphy Planktonic foraminifera Tortonian Messinian Salinity Crisis Pliocene Betic Cordillera Bajo Segura basin

1. Introduction For almost two decades, high-resolution integrated stratigraphy research on the Mediterranean margins and Ocean Drilling Program sites (Hilgen et al., 1995, 2000a, 2000b, 2000c, 2005; Krijgsman et al., 1995, 1999a, 1999b, 2001, 2004; Hilgen and Krijgsman, 1999; Iaccarino et al., 1999; Sprovieri et al., 1999; Sierro et al., 2001; Hüsing et al., 2009, among others) has improved the biostratigraphic resolution of the Mediterranean planktonic foraminifer biozonations of Bizon and Bizon (1972), Cita (1973, 1975a, 1975b), Iaccarino and Salvatorini (1982), and Iaccarino (1985). The resulting high-resolution biostratigraphic framework, together with magnetostratigraphic, astro-cyclostratigraphic, and geochronologic data, has allowed researchers to further constrain the chronostratigraphy and geochronology of the late

* Corresponding author. E-mail address: [email protected] (H. Corbí). http://dx.doi.org/10.1016/j.marpetgeo.2016.08.004 0264-8172/© 2016 Elsevier Ltd. All rights reserved.

Mioceneeearly Pliocene Mediterranean (Lourens et al., 2004; Hilgen et al., 2012). This interval spans the entire marine record of the Bajo Segura basin (SE Iberian Peninsula, Betic Cordillera), which has been the site of hydrocarbon exploration in onshore and adjacent offshore areas since the 1960s. In fact, recent reference oilindustry wells drilled within this region have been analysed in the scientific literature in terms of Mediterranean palaeoceanographic evolution (Soria et al., 2008a; Ochoa et al., 2015). Furthermore, this basin is of considerable interest because it represents one of the most complete late-Neogene records of the Mediterranean margins, both as regards the time interval and the variety of depositional environments, from fluvial to deep marine (Soria et al., 2001, 2005, 2008a, 2008b, 2014; Caracuel et al., 2011; García-García et al., 2011). In terms of event stratigraphy, the published planktonic foraminifer zonal schemes for this basin are somewhat outdated as they are based on Bizon et al. (1972) and Montenat (1990) who do not consider the recent astronomically calibrated time-scale. Several planktonic foraminifer studies do consider the recent astronomically tuned time-scale, but are carried out on localized

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stratigraphic sections in the basin (Krijgsman et al., 2006; Soria et al., 2008a), but a biostratigraphic zonal scheme based on foraminifer event-stratigraphy and taking into account modern chronostratigraphic charts has not yet been established for the entire marine record of the Bajo Segura basin. In an aim to cover this gap, we have analysed the complete marine record of this basin in detail, considering the modern zonal schemes reported in Lourens et al. (2004), Gradstein et al. (2012) and synthesized in Iaccarino et al. (2007). The first goal of this paper is to establish the succession of foraminifer events in the marine record of the Bajo Segura basin in order to create an updated zonal scheme (biozonation) for this western Mediterranean reference basin. Several recent studies on the stratigraphy and sedimentary architecture of the Bajo Segura basin record (Soria et al., 2008a, 2008b) serve as a good starting point for this work. The planktonic event-stratigraphy proposed herein serves to advance the paper's second goal d to refine the timeframe of the main tectono-sedimentary and palaeogeographic events in the basin's evolution. Prior unpublished data (Corbí, 2010) and new biostratigraphic evidence indicate that the basin's marine stratigraphic record spans from the earliest Tortonian to the end of the early Pliocene. All the recent, astronomically calibrated, planktonic foraminifer biozones have been documented for this interval. In addition, a series of intrazonal events (some previously unreported) are suitable for regional high-resolution correlation with other upper MioceneePliocene reference sections and ODP sites in the Mediterranean. Regionally significant events serve to correlate the Bajo Segura synthems with their counterpart lithostratigraphic units or formations in several reference sections from the Neogene Betic basins (such as Sorbas, Níjar, and Lorca), as well as with other Mediterranean reference sections (e.g. Gibliscemi and Rosello in Sicily and Monte dei Corvi in northern Italy). Moreover, we discuss the biostratigraphic of our findings in the lateMessinian interval, providing new data for the Messinian Salinity Crisis debate (Hsu et al., 1978; Krijgsman et al., 1999a; Rouchy and Caruso, 2006; Roveri et al., 2014), and exploring the possibility that the late-Messinian barren interval could be considered, at least in the Bajo Segura basin, as the MMi13d subzone. In short, the results show that the Bajo Segura basin composite section, since it includes an almost complete record from the late Miocene (Tortonian) to the early Pliocene, can be regarded as one of the late-Neogene reference sections for the western Mediterranean. 2. Geological setting and stratigraphic architecture of the Bajo Segura basin The Bajo Segura basin is part of the group of Betic Cordillera Neogene basins (Fig. 1A, B). Its basement is composed of metamorphic and sedimentary rocks from the two main domains of this
rride complex) to the south Cordillera: the Internal Zones (Alpuja and the External Zones to the north. Within the basement, lowerand middle-Miocene units (lower Neogene according to Montenat, 1990) discordantly overlie the Internal and External Zones. The genesis of the Bajo Segura basin was at the beginning of the late Miocene, like many other basins in the Betic Cordillera, together called the Post-Orogenic Neogene Basins (Viseras et al., 2004). Its sedimentary infill is Tortonian to Quaternary, with a strong tectonic control due to the activity of the Trans-Alboran shear zone re De et al., 1988), which continues to this day (Alfaro (Larouzie et al., 2012). Tectonically and palaeogeographically, the basin has two clearly defined sectors, north and south (Fig. 1B, C) (Soria et al., 2008b). As can be seen from Fig. 1C, the two sectors are separated by the Río Segura fluvial valley, whose formation is related with the recent activity of the Bajo Segura Fault. The basin's main stratigraphic features were illustrated by

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Montenat (1990) and subsequently modified and completed by Corbí et al. (2016), Lancis et al. (2015), Soria et al. (2014, 2008a, 2008b, 2005), Corbí (2010), and Marínez del Olmo and Serrano~ ate (2000). Combining the information of these authors, the On marine record of the Bajo Segura basin is divided into five allostratigraphic units (synthems) bounded by unconformities represented by erosional surfaces that correspond to palaeogeographic and tectonosedimentary changes. Note that this stratigraphic scheme does not consider the upper Pliocene and Quaternary units (mainly continental or coastal lagoonal), which are not suitable for biostratigraphic analysis with planktonic foraminifera. Fig. 2, based on Soria et al. (2014), illustrates the marine stratigraphic architecture of the Bajo Segura basin, as well as the correlation of the synthems and their unconformities with the eustatic curve of Haq et al. (1987). The main stratigraphic and sedimentological features of these marine allostratigraphic units (extensively reported in Corbí, 2010) are given below.
n, and Atalaya formations) Synthem Tortonian I (Garres, Paredo marks the onset of marine sedimentation in the basin. On the whole, this synthem defines a transgressive-regressive cycle, which consists of a lower deepening-upward sequence (from alluvialdeltaic deposits to pelagic basin marls), and an upper, shallowing-upward sequence (from pelagic basin marls to shallow marine sandstones). Synthem Tortonian II (separated from Synthem Tortonian I by the intra-Tortonian discontinuity, expressed by an angular unconformity) is formed by two shallowing-upward superimposed sequences (Columbares and Puj
alvarez formations), both dominated by basin marls topped by shallow marine sandstones. Synthems Tortonian-Messinian I and Tortonian II are separated by the end-Tortonian unconformity, which is a truncation surface. Synthem Tortonian-Messinian I (Torremendo and La Virgen formations) comprises a single shallowing-upward sequence, with similar features to the underlying unit. Its upper boundary corresponds to the intra-Messinian unconformity and was generated by the first sea-level fall related with the Messinian Salinity Crisis. Synthem Messinian II (Garruchal and San Miguel formations) includes both Messinian evaporites and Lago Mare-like sedimentation. The limit with the next synthem (Pliocene) is represented by the end-Messinian unconformity, expressed by an erosional surface with palaeovalleys. This unconformity was generated by the second sea-level fall related to the salinity crisis and followed by earlyPliocene reflooding. According to Lancis et al. (2015), the Pliocene synthem (Hurchillo and Rojales formations) is represented by two transgressive-regressive marine sequences (the lower and upper Hurchillo sedimentary cycles), which led to the basin's definitive continentalization. 3. Materials and methods This study focuses on the foraminifer biostratigraphic analysis of the marine record from the Bajo Segura basin, for which the stratigraphic architecture was recently established by Soria et al. (2008a, 2008b). In our analysis, only the most complete and representative sections were examined. As shown in Fig. 1C, the study sections are, from west to east: the Albatera/Les Moreres composite and Santa Pola sections on the basin's northern margin,
lvarez, La Pedrera, and San Miguel and Los Garres-Garruchal, Puja de Salinas composite in the southern sector. Note that, for the biostratigraphic analysis, we integrate the data provided by Lancis et al. (2010) from the Albatera/Les Moreres composite section, for which the planktonic foraminifer biostratigraphic analysis was performed by the authors of the present work. A complete stratigraphic and facies description of these sections is extensively reported in Corbí (2010). A total of 340 samples were collected with a

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Fig. 1. Geological context of the Bajo Segura basin, with the location of the study sections. A) Location of the Bajo Segura basin in the western Mediterranean. B) Geological synthetic map of the Eastern Betic Cordillera Neogene basins showing the location of the Bajo Segura basin. C) Geological map of the Bajo Segura basin (based on Montenat, 1990), indicating the position of the analysed sections.

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Fig. 2. Marine stratigraphic architecture of the Bajo Segura basin (modified from Soria et al., 2014). Note that synthems and their bounding unconformities have been correlated with the eustatic curve of Haq et al. (1987).

sample spacing averaging 5 m, with higher-resolution sampling (less than 1 m) performed where necessary. All samples were washed to collect the >125 mm fractions, following the standard sieve techniques. Planktonic foraminifera from the >125 mm sieved fraction were analysed to determine the presence and relative abundance of upper Miocene and Pliocene marker species. Abundance and dominant coiling direction ratios (dextral and sinistral) of Neogloboquadrina acostaensis were also determined by counting all the individuals in one representative gram of the 125 mm fraction (n/g, see Figs. 3e5). Scanning electron microscope photos of the representative foraminifer markers (Figs. 3e9) were taken in the technical support laboratories of the Faculty of Science of the University of Alicante. The proposed planktonic foraminifer zonal scheme for the late Mioceneeearly Pliocene in the Bajo Segura basin is mainly based on the recent astronomically calibrated charts compiled by Lourens et al. (2004), Anthonissen and Ogg (2012), and Hilgen et al. (2012), and synthesized by Iaccarino et al. (2007). In each zone description, the following items have been documented: (1) formal definition specifying the reference section and levels in the Bajo Segura basin marine record, and (2) remarks on the specific features of each biozone with special emphasis on other intrazonal events detected, typical benthic foraminifer assemblage, and magnetostratigraphic calibration (where applicable). The biohorizons noted in this work are: first occurrence datum, first common occurrence, first regular occurrence, last occurrence, and last common occurrence, as well as biohorizons distinguished by the abundance fluctuations of certain index species, influx of key species, and the peak abundance of significant taxa (acmes).

4. Results: planktonic foraminifer zonal scheme of the Bajo Segura basin In this work (with minor modifications discussed in the text), we adopted a combination of the recent astronomically calibrated biozonation schemes summarized in Lourens et al. (2004), Iaccarino et al. (2007), and Gradstein et al. (2012). Figs. 3e8 illustrate the biozones, bioevents, and allostratigraphic units for each study section. The Bajo Segura composite section, with the position of the most relevant bioevents, is given in Fig. 9.

4.1. Tortonian 4.1.1. MMi9 Paragloborotalia siakensis zone The MMi9 Paragloborotalia siakensis interval zone occurs between the last common occurrence of Globigerinoides subquadratus and the last occurrence datum of Paragloborotalia siakensis (from 11.54 to 11.19 Ma; Hilgen et al., 2000c, 2005; Lourens et al., 2004). This biozone has been determined exclusively in the northern sector of the Bajo Segura basin, with the Albatera/Les Moreres being the reference composite section (samples 1 to 12; Fig. 3). It is characterized by the absence of Globigerinoides subquadratus, whose last occurrence marks the beginning of the biozone, and the scarce, scattered presence of Neogloboquadrina acostaensis with variable coiling direction from the base of the biozone. The most relevant intrazonal event is the first (brief) influx of Globorotalia mediterranea at the base of the biozone (see Fig. 3).

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Fig. 3. Biostratigraphy of the Albatera/Les Moreres composite section, with the position of the studied samples, bioevents, biozonation of planktonic foraminifera, and allostratigraphic units or synthems represented in the section.

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Fig. 4. Biostratigraphy of the Los Garres/Garruchal section, showing location of the studied samples, bioevents, biozonation of planktonic foraminifera, and allostratigraphic units or synthems in the section.

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lvarez section, with position of studied samples, bioevents, biozonation of planktonic foraminifera, and allostratigraphic units or synthems in the Fig. 5. Biostratigraphy of the Puja section.

4.1.2. MMi10 Globigerinoides obliquus zone The MMi10 Globigerinoides obliquus interval zone ranges from the last occurrence datum of Paragloborotalia siakensis to the first regular occurrence of Neogloboquadrina acostaensis s.s. (from 11.19 to 10.57 Ma; Hilgen et al., 2000c, 2005; Lourens et al., 2004). As with the previous biozone, the Albatera/Les Moreres composite is the representative section for this zone (Fig. 3, samples 13 to 16). It is marked by the scarce, irregular record of Neogloboquadrina acostaensis until sample 16, where this species occurs regularly (top of the biozone). 4.1.3. MMi11 Neogloboquadrina acostaensis zone The MMi11 Neogloboquadrina acostaensis interval zone occurs between the first regular occurrence of Neogloboquadrina acostaensis and the first occurrence of Globigerinoides extremus (from

10.57 to 8.27 Ma; Sprovieri et al., 1999; Hilgen et al., 2000c). The Albatera/Les Moreres composite continues to be the reference section for this zone (Fig. 3, samples 16 to 33), although it is also identified at the Los Garres/Garruchal section (samples 1 to 33; Fig. 4), where there is no evidence of a lower limit record for the zone, and there is no limit also of the last occurrence of Globigerinoides extremus. The most significant intra-zonal event is the notable coiling direction change of Neogloboquadrina acostaensis, shifting from dextral in the lower part to sinistral in the upper part of the biozone (samples 30 and 31 at the Albatera/Les Moreres composite section and samples 29 to 30 at the Los Garres/Garruchal section; see Figs. 3 and 4). This significant event, dated by Hilgen et al. (2000c) at 9.54 Ma in the Monte Gibliscemi section (Sicily), allows us to separate subzones MMi11a and MMi11b. This event was also detected by

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Fig. 6. Biostratigraphy of the La Pedrera section, with location of studied samples, bioevents, biozonation of planktonic foraminifera, and allostratigraphic units or synthems in the section.

Serrano et al. (1995) in the Atalaya section, which is equivalent to the lower part of the Los Garres/Garruchal section. Within this zone, several noticeable bioevents have been identified, some unambiguously correlated with other Mediterranean reference sections (e.g. the Gibliscemi composite section in Sicily, Hilgen et al., 2000c). From base to top, the most significant are: (a) an interval at the base of the biozone with predominantly sinistral coiling in Neogloboquadrina acostaensis (only detected in samples 17 to 20 at the Albatera/Les Moreres composite section; Fig. 3) and calibrated in the Mediterranean at 10.05e9.90 Ma (Hilgen et al., 2000c); (b) The last occurrence datum of Globorotalia partimlabiata (sample 17 at the Albatera/Les Moreres section), calibrated in the Mediterranean by Hilgen et al. (2000c) at 9.94 Ma; (c) the last common occurrence of Neogloboquadrina atlantica (large and small forms) (sample 19 at the Albatera/Les Moreres composite section; sample 21 at the Los Garres/Garruchal section); and (d) the first influx of Globigerinoides extremus and Globorotalia suterae/exserta, both recorded at the Los Garres/Garruchal section (samples 27 and

28, respectively). Note that in this section (Fig. 4), we place these latter bioevents (first influx of G. extremus and G. suterae/exserta) in MMi11a. Consequently, these bioevents predate the first occurrence of these species, which characterize the MMi12 zone. 4.1.4. MMi12 Globigerinoides extremus zone The last Tortonian zone falls within the first occurrence of Globigerinoides extremus and the first common occurrence of Globorotalia miotumida group (from 8.27 to 7.24 Ma; Sprovieri et al., 1999; Lourens et al., 2004). Within this interval zone, whose reference
lvarez (samples 1 to 52; Fig. 5), the first occurrence of section is Puja Globorotalia suterae points to the existence of two subzones (MMi12a and b), with a Mediterranean boundary calibrated at 7.84 Ma by Sprovieri et al. (1999). The most significant event of this zone is two influxes of Globorotalia mediterranea, preceded by two continuous record intervals of small-sized Globorotalia miotumida (Fig. 5). Note that both influxes occur in a Globorotalia menardiidominated interval, the distinctive keeled globorotalid of the

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Fig. 7. Biostratigraphy of the San Miguel de Salinas composite section, with position of studied samples, bioevents, biozonation of planktonic foraminifera, and allostratigraphic units or synthems represented in the section.

Tortonian. Also noticeable is the influx of Neogloboquadrina acostaensis with dextral coiling (sample 44 at the Albatera/Les Moreres
lvarez section), in a composite section and sample 30 at the Puja sinistral-dominated coiling interval, the characteristic feature of the zone. 4.2. Messinian 4.2.1. MMi13 Globorotalia miotumida group zone The sole Messinian zone, the MMi13 Globorotalia miotumida group, develops between the replacement of the Globorotalia

menardii group (last common occurrence) by the Globorotalia miotumida group (first common occurrence) and the reoccurrence of open marine and normal salinity conditions in the Bajo Segura basin after the Messinian Salinity Crisis (biozone calibrated in the Mediterranean between 7.24 and 5.33 Ma; Hilgen et al., 2000b; Lourens et al., 2004). Our biostratigraphic results reveal four subzones (MMi13aed), which are best represented at the Los Garres/ Garruchal, La Pedrera, and San Miguel de Salinas composite sections (Figs. 4, 6 and 7, respectively). It is interesting to note that the Globorotalia miotumida group, the marker species of this zone, is recorded differently in the two basin sectors (northern and

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Fig. 8. Biostratigraphy of the Santa Pola section, indicating position of studied samples, bioevents, biozonation of planktonic foraminifera, and allostratigraphic units or synthems in the section.

southern). In the northern sector, this globorotalid group, typical of deep water settings, is vary scarce (Colmenar and Torrellano sections; Corbí, 2010), undoubtedly because the northern margin reflects generally shallow marine depositional systems (Soria et al., 2008b). In contrast, in the southern margin of the basin, where deeper marine depositional systems dominate, the Globorotalia miotumida group is more common and is represented by Globorotalia conomiozea and Globorotalia mediterranea, the deep forms of this group. The MMi13a Globorotalia miotumida group-Globorotalia nicolae subzone develops until the first occurrence of Globorotalia nicolae, an event calibrated at 6.83 Ma in the Mediterranean (Lourens et al., 2004). This biostratigraphic marker is extremely scarce in the Bajo Segura basin, having been identified solely in samples 50 to 52 at the Los Garres/Garruchal section (Fig. 4). This is probably due to ecological factors, in addition to its short appearance interval (6.83e6.72 Ma). Considering the magnetostratigraphic data of Soria et al. (2008a) for the Garruchal section, the interval appearance of Globorotalia nicolae is chrons C3Ar and C3An2r. The MMi13b Globorotalia nicolae interval subzone spans between the first occurrence datum of Globorotalia nicolae and the coiling change of Neogloboquadrina acostaensis (from sinistral to dextral), an event calibrated at 6.35 Ma (Lourens et al., 2004). This subzone, recorded at the Los Garres/Garruchal and the La Pedrera sections (samples 53e59 and 1e10, respectively; Figs. 4 and 6) is characterized by the scattered and always scarce presence of the Globorotalia miotumida group. The abundance of Bolivina (mainly Bolivina spathulata) and Uvigerina (dominated by Uvigerina cylindrica) benthic foraminifera is also a significant feature with ecobiostratigraphic value of this subzone. Based on our findings and considering the magnetostratigraphic data of Soria et al. (2008a),

the subzone falls between the upper part of Chron C3Ar and the lower part of Chron C3An1r. The MMi13c Turborotalita multiloba interval subzone spans between the coiling change of Neogloboquadrina acostaensis and the predominance of dwarf fauna of planktonic foraminifera. Note that the Mediterranean planktonic foraminifer zonal scheme (Iaccarino et al., 2007) establishes the upper limit of this zone at the beginning of the barren interval, considering that the MMi13c spans between 6.35 and 5.96 Ma (Krijgsman et al., 1999a; Lourens et al., 2004). This subzone is represented at the Los Garres/Garruchal and La Pedrera sections (samples 60e68 and 10e21, respectively; Figs. 4 and 6). It is characterized by the common (but intermittent) presence of Turborotalita multiloba, with an abundant influx (more than 50% of the total foraminifer assemblage) in the lower part of the biozone (La Pedrera section, Fig. 6). The magnetostratigraphic data of Soria et al. (2008a) classifies this subzone in the C3An1r-C3r chrons interval. A notable bioevent is the first occurrence datum of Bulimina echinata at the San Miguel de Salinas composite section (sample 1; Fig. 7), dated at 6.29 Ma in the Mediterranean (Kouwenhoven et al., 2006). The last subzone of the Messinian (MMi13d) is an assemblage zone (cenozone) spanning between the predominance of dwarf fauna of planktonic foraminifera and the reoccurrence of open, normal salinity conditions after the Messinian Salinity Crisis in the Bajo Segura basin. This subzone (for which the representative section is the San Miguel de Salinas composite; samples 11e40; Fig. 7) is equivalent to the Mediterranean barren interval (nondistinctive zone) (Iaccarino et al., 2007), thus giving an age of 5.96e5.33 Ma (Lourens et al., 2004), falling into chron C3r in the Bajo Segura basin (Corbí et al., 2016). During this interval, the evaporites related with the Messinian Salinity Crisis are deposited

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(Corbí et al., 2016). Our findings indicate this late-Messinian interval is an ecozone or assemblage zone characterized by dwarf fauna of planktonic foraminifera, mainly represented by Globoturborotalita decoraperta, Globigerina bulloides, and Neogloboquadrina spp. Interestingly, this dwarf fauna record reveals an absence of notable reworking, as shown by the good preservation of the tests and absence of abrasion, dissolution, or fragmentation in almost all the specimens. 4.3. Pliocene 4.3.1. MPl1 Sphaeroidinellopsis s.l. zone The MPl1 Sphaeroidinellopsis s.l. zone, best recorded at the La Pedrera section (Fig. 6), ranges between the re-occurrence of open, normal marine waters in the Bajo Segura basin after the Messinian Salinity Crisis and the first common occurrence of Globorotalia margaritae (5.33e5.08 Ma; Lourens et al., 2004). Within the zone, the following events (most ecobiostratigraphic) typical of the Mediterranean MPl1 biozone (Iaccarino et al., 1999, 2007) occur: (a) an abundance of Globigerinoides spp. (b) The acme of Sphaeroidinellopsis spp. (dominated by S. seminulina and S. paenedehiscens), an event calibrated in the Mediterranean at 5.31e5.30 Ma (Lourens et al., 2004). This event was only determined at the La Pedrera section, as this species is typical of pelagic environments underrepresented in the Pliocene marine record of the Bajo Segura basin. (c) an abundance of Globoturborotalita nepenthes, an event identified in Italy (Zachariasse and Spaak, 1983; Trenkwalder et al, 2008) and in the western Mediterranean (Iaccarino et al., 1999). And, finally, (d) an abundance of Globoturborotalita apertura/decoraperta. 4.3.2. MPl2 Globorotalia margaritae zone The MPl2 Globorotalia margaritae zone is an interval between the first common occurrence of Globorotalia margaritae and the first occurrence datum of Globorotalia puncticulata (5.08e4.52 Ma; Lourens et al., 2004). In the Bajo Segura basin, this zone is characterized by the rare and scattered presence of Globorotalia margaritae. The biozone may be represented at the La Pedrera section, where some individuals of G. margaritae have been identified after the Sphaeroidinellopsis acme (Fig. 6), and should probably be considered as the common occurrence since the range of this species is continuous only in pelagic successions (Iaccarino et al., 2007). This biostratigraphic interpretation is consistent with the fact that the Pliocene marine record of the Bajo Segura basin is mainly dominated by shelf depositional systems (e.g. the La Pedrera and San Miguel de Salinas composite section; Figs. 6 and 7). In the San Miguel de Salinas composite section, some individuals of G. margaritae have been identified (Fig. 7), but the Sphaeroidinellopsis acme has not been found, probably due to ecological factors. Therefore, our planktonic foraminifer data cannot establish the exact distribution of the biozones represented in this part of the section. However, recently Corbí et al. (2016) reported the occurrence of Reticulofenestra cisnerosii at the Pliocene base of this section, proposed as a complementary event to define the base of the Pliocene in the Mediterranean (Chron C3r, Lancis and Flores, 2006). Therefore, we regard the lower part of the Pliocene in the San Miguel de Salinas section as the MPl1 biozone (Fig. 7). As can be seen in this figure, it is important to note that, close to the G. margaritae occurrence, we have identified a few individuals of Globorotalia mediterranea, with no signs of reworking such as abrasion, dissolution, or fragmentation. 4.3.3. MPl3 Globorotalia margaritae-Globorotalia puncticulata zone This is a concurrent range zone between the first occurrence

datum of Globorotalia puncticulata and the last occurrence datum of Globorotalia margaritae (4.52e3.81 Ma; Lourens et al., 2004) that is under-represented at the Bajo Segura basin. However, the Santa Pola section contains continuous and abundant Globorotalia puncticulata and typical specimens of Globorotalia margaritae and Globigerinoides elongatus (Fig. 8). Note that our biostratigraphic observations in other representative sections (e.g. Guardamar; Corbí, 2010) suggest that the Pliocene in Santa Pola falls into the MPl3 zone, which is congruent with the nannoplankton biostratigraphy data of Lancis et al. (2015). As shown in Fig. 8, the first specimens of Globigerinoides ruber occur in the upper part of the Santa Pola section, which could be interpreted as the beginning of the Mediterranean MPl4 zone (Iaccarino et al., 2007). 5. Discussion The stratigraphic architecture framework and sedimentary facies analysis provided by Soria et al. (2008a, 2008b) in combination with the proposed planktonic foraminifer zonal scheme presented in this work reveal a complete biozone record from the earliest Tortonian to the end of the early Pliocene. Comparing this temporal interval with other planktonic foraminifer biostratigraphic records of the Betic Cordillera Neogene basins (e.g. Granada, Sorbas, Níjar, Lorca, Fortuna, and Vera) allows us to propose the “Bajo Segura basin composite section” as a biostratigraphic reference section for the western Mediterranean (see Fig. 9). This composite section comprises eight biozones (from MMi9 to MPl3) with their distinctive bioevents, together with other significant intrazonal events that can be useful for regional-scale correlation with other Neogene basins of the Betic Cordillera, as well as with other Mediterranean Neogene reference sections. In addition, the proposed planktonic foraminifer event-stratigraphy scheme provides the first detailed biostratigraphic approach (based on the astronomically calibrated geological time-scale) of the chronostratigraphic evolution of the entire marine record of this reference basin. Due to its regional significance, particular attention is paid to the late-Messinian interval, exceptionally recorded in the basin. 5.1. Succession of bio-events and regional correlation in the Mediterranean All the planktonic foraminifer biozones for the Tortonian period (from MMi9 to MMi12) have been recognized. Within this period, 14 succession events have been identified (see Fig. 10). This figure summarizes the bioevent correlation with the most reference Tortonian sections from the Mediterranean in Spain (Lorca and Granada basins; Krijgsman et al., 2000; Corbí et al., 2012), Sicily (Falcolara and Gibliscemi; Krijgsman et al., 1995; Sprovieri et al., 1999; Hilgen et al., 2000c), northern Italy (Monte dei Corvi; Hilgen et al., 2003), Greece (Metochia, Gavdos; Krijgsman et al., 1995), and Cyprus (Pissouri basin; Kouwenhoven et al., 2006; Morigi et al., 2007). Especially bioevents are the first and second Tortonian influxes of the Globorotalia miotumida group, which precede the first common occurrence of this group at the beginning of the Messinian. Unlike other Neogene basins of the Betic Cordillera, the Bajo Segura basin is subject to marine conditions throughout practically the entire Tortonian period. In the Betic Cordillera domain, the Granada, Lorca, and Fortuna basins record the late-Tortonian restriction event, which is characterized by evaporitic precipitation (Krijgsman et al., 2000; Corbí et al., 2012). As pointed out by Corbí et al. (2012), the late-Tortonian Bajo Segura marine record (Tortonian II synthem) is probably correlated with the Hondo and Serrata formations of the Lorca basin, as well as with the transition between Units I and II of the Granada basin. In this

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Fig. 9. Planktonic foraminifer biostratigraphy of the Bajo Segura composite section, with the locations of the main bioevents identified.

1022 H. Corbí, J.M. Soria / Marine and Petroleum Geology 77 (2016) 1010e1027 Fig. 10. Biostratigraphy-based correlation between the Bajo Segura basin bioevents and their equivalents in the Betic Cordillera Neogene basins (Sorbas, Níjar, Lorca, and Granada), and in representative sections from Morocco, Sicily, Northern Italy, the eastern Mediterranean (Cyprus and Greece), and ODP sites from the centre of the Mediterranean.

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interval, the late-Tortonian influx of Globorotalia mediterranea calibrated by Krijgsman et al. (1995) at 7.89 Ma, the first occurrence datum of Globorotalia suterae dated in the Mediterranean at 7.84 by Sprovieri et al. (1999), and an influx of dextral-coiled Neogloboquadrina acostaensis have all been recognized in the Bajo Segura basin. This late-Tortonian N. acostaensis influx has also been reported in the Lorca basin by Rouchy et al. (1998), Krijgsman et al. (2000), and Corbí et al. (2012), as well as in the Granada basin (Corbí et al., 2012). We propose that this influx (also recognized in the Bajo Segura basin) is a recognizable regional-scale bioevent useful for regional correlation, at least in the Neogene basins of the Betic Cordillera. Some of the most representative events that can be correlated throughout the Mediterranean have been reported in the Messinian record. Fig. 10 shows the correlation of the Messinian Bajo Segura basin events with the astronomically calibrated sections from the Sorbas and Níjar basins (Sierro et al., 2001), the Rif and Atlantic Morocco (Melilla basin in Van Assen et al., 2006; TazaGuerzif basin in Krijgsman et al., 1999b; Ain el Beida section in Krijgsman et al., 2004; Oued Akrech in Hilgen et al., 2000a), Sicily (Capodarso, Gibliscemi, and Falconara Sicilian sections in Hilgen and Krijgsman, 1999), northern Italy (Monte dei Corvi section in Hilgen et al., 2003; Hüsing et al., 2009), Greece (Metochia section in Gavdos in Krijgsman et al., 1995), and Cyprus (Pissouri basin in Kouwenhoven et al., 2006; Morigi et al., 2007). The most significant Messinian events recorded in the Bajo Segura basin, as can be seen in Figs. 9 and 10, are: (1) the replacement of the Globorotalia menardii group by the Globorotalia miotumida group (7.24 Ma; Hilgen et al., 2000c); (2) the first and last occurrence data of Globorotalia nicolae (6.83e6.72 Ma; Lourens et al., 2004); (3) the sinistral-to-dextral coiling change of Neogloboquadrina acostaensis (6.35 Ma; Lourens et al., 2004); (4) the first occurrence datum of Bulimina echinata dated by Kouwenhoven et al. (2006) at 6.24 Ma; (5) the first common occurrence of Turborotalita multiloba, preceding a significant influx of this species; and (6) the dominance of dwarf fauna of planktonic foraminifera. It is important to note that we have considered in our planktonic foraminifer zonal scheme that the late-Messinian barren interval (non-distinctive zone) is characterized in the Bajo Segura basin by dwarf fauna of planktonic foraminifera. We suggest that these dwarf fauna are a possible distinctive ecostratigraphic tool for regional correlations throughout the Mediterranean (see last section of the discussion for details). Finally, our biostratigraphic results indicate that the Messinian interval in the Bajo Segura basin, which is better expressed at the Los Garres/Garruchal section, can be correlated with the Abad composite section of the Sorbas and Níjar basins (Sierro et al., 2001, Fig. 10). Of special importance is the lack of the first common occurrence of Globorotalia menardii 5 (dextral coiling), a bioevent calibrated by Hilgen et al. (2000a) at 7.36 Ma. In contrast, this bioevent, which precedes the TortonianeMessinian boundary, has been identified by Krijgsman et al. (2006) at the Venta la Virgen section in the southern margin of the Bajo Segura basin. The absence of this marker species in Los Garres-Garruchal section (Fig. 4) could be related to a possible hiatus/erosion originated by the emplacement of the turbidites, which can also cause taphonomic distortion. For the Pliocene, all the Zanclean biozones have been recognized (Fig. 9). Our biostratigraphic data show that the La Pedrera and San Miguel de Salinas composite sections, together with the Cuevas de Almanzora reference section at Vera basin (Benson and Bied, 1991; Van de Poel, 1992; Fortuin et al., 1995), present the most complete marine records of the early Pliocene in southeastern Spain. The first biozone, represented by the acme of Sphaeroidinellopsis spp., marks the restoration of normal marine conditions after the Messinian salinity crisis, corresponding with the Pliocene reflooding of the

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Mediterranean. Fig. 10 presents the bioevent correlation with the Rossello composite section in Sicily (Van Couvering et al., 2000; Lourens et al., 2004) and with the ODP sites (Iaccarino et al., 1999). The representative biostratigraphic features are (from base to top): (1) the acme of Sphaerodinellopsis, calibrated at 5.30e5.21 Ma (Lourens et al., 2004), (2) an abundance of Globoturborotalita nepenthes, (3) an abundance of Globoturborotalita apertura/decoraperta, (4) scarce presence of Globorotalia mediterranea, (5) the first occurrence of Globorotalia margaritae, (6) the first occurrence datum of Globorotalia puncticulata, (7) the first occurrence datum of Globigerinoides elongatus, and (8) the first occurrence datum of Globigerinoides ruber. 5.2. Chronology of the marine record of the Bajo Segura basin The planktonic foraminifer biostratigraphic analysis provides the basis for the chronostratigraphic framework of the marine allostratigraphic units represented in the basin (Fig. 9). Synthem Tortonian I, marking the onset of marine sedimentation, starts in the earliest Tortonian. The finding of Paragloborotalia siakensis, with a last Mediterranean occurrence dated at 11.19 Ma (Hilgen et al., 2000c), and the scattered presence of Neogloboquadrina acostaensis prior to its regular occurrence (dated at 10.57 Ma, Hilgen et al., 2000c), confirm that the first entry of the sea into the basin took place during the earliest Tortonian (biozone MMi9, from 11.54 to 11.19 Ma; Fig. 9). As pointed out by Lancis et al. (2010), the absence of Globigerinoides subquadratus, whose last occurrence marks the beginning of the Tortonian in the Mediterranean (11.54 Ma; Lourens et al., 2004), is consistent with this interpretation. Synthem Tortonian I culminates with the intra-Tortonian discontinuity, expressed by an angular unconformity within the MMi11 subzone (Los Garres-Garruchal section; Fig. 4). Our findings indicate that the intra-Tortonian discontinuity in the Los Garres/Garruchal section is slightly older than the bioevent of the change in coiling direction of N. acostaensis (from dextral to sinistral), which separates the MMi11 subzones (Figs. 4 and 9). Consequently, this discontinuity must be slightly older than the 9.52 Ma at which that bioevent has been calibrated in the Mediterranean. Above the intraTortonian discontinuity is the Tortonian II synthem, composed of two shallowing-upward superimposed sequences. This synthem (Tortonian II) mainly occurs during the late Tortonian (from MMi11a to MMi12b zones; Figures from 3 to 5, synthesized in Fig. 9), thus marking the end-Tortonian discontinuity, just slightly below the top of the Tortonian, where the replacement of Globorotalia menardii by the to Globorotalia miotumida group occurs. As synthesized in Fig. 9, synthem TortonianeMessinian I, composed of a single shallowing-upward sequence, covers from the end of the Tortonian (zone MMi12b) to almost the entire Messinian (zone MMi13). The magnetostratigraphic data of Soria et al. (2008a) for the Los Garres/Garruchal section assigns this synthem to Chron 3Br2n (late Tortonian) to Chron C3r (late Messinian). Based on the San Miguel de Salinas composite section, Corbí et al. (2016) established that the upper boundary of the synthem coincides with the base of the barren interval (Iaccarino et al., 2007) (or the Dwarf Fauna of Planktonic Foraminifer subzone proposed in this work, MMi13d), leading to an end-Messinian discontinuity date of 5.96 Ma. Above this discontinuity lies synthem Messinian II, recording the Messinian evaporites and Lago-Mare facies and Terminal Carbonate Complex (Esteban, 1979) of the upper Messinian. This synthem has recently been extensively documented palaeoenvironmentally and sedimentologically (Corbí et al., 2016). According to these authors, it develops completely during Chron C3r, which is contained within the Mediterranean barren interval or MMi13d subzone of this work (Fig. 9). The upper boundary of the Messinian II synthem is marked by

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an erosional surface with palaeovalleys in different parts of the basin (e.g. the Crevillente-Elche sector, Soria et al., 2005, and the La Pedrera-San Miguel sector, Corbí et al., 2016), related with the Messinian Salinity Crisis erosion. Above this erosional surface is the Pliocene synthem, which marks the restoration of open marine and normal salinity conditions in the basin. The Sphaeroidinellopsis acme in the lower part of the synthem (La Pedrera section; Figs. 6 and 9), an event calibrated at 5.30 to 5.31 in the Mediterranean (Lourens et al., 2004), is the most representative event of the MPl1 zone (base of the Pliocene). This data suggests that the Pliocene marine reflooding of the basin occurs simultaneously (or in close timing) with the Mediterranean Messinian/Pliocene boundary (5.33 Ma). It is worth mentioning that, although all the early Pliocene zones (from MPl1 to MPl3) have been determined (Fig. 9), a slight temporal discontinuity inside this synthem cannot be ruled out since a continuous section with all the early Pliocene biozones has not been found. Indeed, the nannoplankton biostratigraphy data of Lancis et al. (2015) suggest a discontinuity between the two transgressive sequences in the synthem (both considered as highstand systems tracts). These authors correlate this discontinuity (dated at 4.1e4 Ma) with the two lower Pliocene seismic sequences of the Gulf of Lion in the western Mediterranean (Lofi et al., 2003; Leroux, 2013). Integrating these results in our chronostratigraphic scheme, we propose that the lower Hurchillo (first transgressiveregressive sequence; Lancis et al., 2015) is located within the Pliocene MPl1 and MPl2 zones of this work. On the other hand, the upper Hurchillo (the second transgressive-regressive sequence), represented in this work in the Santa Pola section (Fig. 8), falls into the MPl3 zone and probably also into the MPl4 zone. It should be noted that the upper Hurchillo sequence, represented at the La Pedrera and San Miguel composite sections (upper part of the sections; Figs. 6 and 7), although very poor in planktonic foraminifer biostratigraphic markers, could be assigned (based on the nannoplankton data of Lancis et al., 2015) to the MPl3 and MPl4 zones. Finally, our biostratigraphic data indicate that the end of marine sedimentation in the basin occurred in the upper part of the MPl3 zone or close to the beginning of the MPl4a zone (3.81 Ma). 5.3. Stratigraphic distribution of Globorotalia miotumida group: palaeoenvironmental discussion The Globorotalia miotumida group (Sierro et al., 1993) or Globorotalia conomiozea group (Zachariasse, 1979; Krijgsman et al., 2006) is composed mainly of Globorotalia miotumida s.s., Globorotalia mediterranea, and Globorotalia conomiozea. This globorotalid group is one of the most significant biostratigraphic markers of the late Miocene from the Mediterranean since their first common occurrence correlates with the Tortonian/Messinian boundary at 7.24 Ma (Lourens et al., 2004). The biostratigraphic data of this work (summarized in Figs. 9 and 10) reveals four notable events for this group, some correlated throughout the Neogene Mediterranean records. The first influx of the Globorotalia miotumida group (GM1) occurs in the earliest Tortonian (Figs. 9 and 10) and is mainly represented by Globorotalia mediterranea, the most convex form (conical) of the group. This event, exclusively recorded at the Albatera/Les Moreres composite section (Fig. 3), is located in the Paragloborotalia siakensis Interval Zone (MMi9), indicating an age of 11.54e11.19 Ma. This event represents the oldest influx of this group recorded in the Bajo Segura basin and, in fact, in the entire Mediterranean. The second influx of the Globorotalia miotumida group (GM2) occurs in the late Tortonian (Figs. 9 and 10). As shown in Fig. 5, it is
lvarez section and placed in the Globigerinoides recorded at the Puja extremus-Globorotalia suterae Interval Subzone (MMi12a). Within

this event, we have included two clear influxes or levels of Globorotalia mediterranea, preceded by an interval with continuous small Globorotalia miotumida s.s. (a relatively flat form). Note that a significant influx of Neogloboquadrina acostaensis dextral is recorded between these two intervals. Either of these influxes (or even both) may be correlated with the late-Tortonian Globorotalia miotumida influx, which precedes the Tortonian/Messinian boundary in the Mediterranean. This influx is calibrated by Krijgsman et al. (1995) at 7.892 Ma in the sections of Metochia (Gavdos Island, Greece), Kastelli (Crete), and Gibliscemi (Sicily). Indeed, these authors documented an influx of conical representatives of the Globorotalia miotumida group (Globorotalia conomiozea) at two levels very close in time, which may be correlated with the two recorded levels in the Bajo Segura basin. Recently, Hüsing et al. (2009) proposed a recalibration of the event at 7.861 Ma based on a biostratigraphic analysis of the Monte dei Corvi section. Moreover, this late-Tortonian influx of the Globorotalia miotumida group was also identified in the Lorca (Krijgsman et al., 2000; Corbí et al., 2012) and Granada basins (Corbí et al., 2012). It is worth noting that, in the Bajo Segura basin, this event precedes the first occurrence datum of Globorotalia suterae, a bioevent astronomically dated at 7.84 Ma in the Gibliscemi and Falconara sections (Sicily) by Sprovieri et al. (1999). The following event is the first common occurrence of the Globorotalia miotumida group, which replaces the Globorotalia menardii group (GM3; Figs. 9 and 10). This event, documented in
lvarez sections (Figs. 4 and 5), the Los Garres-Garruchal and Puja marks the beginning of the Messinian stage in the Bajo Segura Basin and in the Mediterranean (Sierro, 1985; Sierro et al., 1993). According to Sierro et al. (1993) and Krijgsman et al. (1995), the replacement of the Globorotalia menardii group (a typical species of tropical and subtropical regions) by the Globorotalia miotumida group (representative of temperate areas) is the result of the migration of the North Atlantic bioprovinces to the Mediterranean in response to increasing climate cooling. Considering our data, we suggest that the first influx of the Globorotalia miotumida group (GM1) (at 11.54e11.19 Ma in the Bajo Segura basin in the MMi9 zone) may reflect significant faunal changes in the Mediterranean as a result of early-Tortonian climating cooling (Miocene isotopic event MI-5; Miller et al., 1991). This climate event has been calibrated by Turco et al. (2001) at 11.8e11.3 Ma. Moreover, this interval witnessed the arrival of neogloboquadrinids and the real differentiation between Neogloboquadrina atlantica and Neogloboquadrina acostaensis (Turco et al., 2001), events also observed in the marine record of the Bajo Segura basin (Fig. 3). On the other hand, and following this line of argument, the late-Tortonian Globorotalia miotumida group influx (GM2) may reflect the restriction phase related to the Tortonian salinity crisis in the eastern Betics, in which evaporitic deposition is dated at 7.8 Ma, with a duration of 200 kyr (Krijgsman et al., 2000). Indeed, Kouwenhoven et al. (2003) suggest that the Tortonian salinity crisis may also be recorded in the benthic assemblages of the deeper Mediterranean sections (Monte Gibliscemi, Metochia, and Monte dei Casino). Furthermore, the finding in the Bajo Segura basin of an interval with Neogloboquadrina acostaensis (dextral coiling) close to the GM2 influx could be correlated with the late-Tortonian influx of this species also recorded in the Lorca basin (Krijgsman et al., 2000; Corbí et al., 2012). The last Globorotalia miotumida event (GM4) is the slight influx of this group at the beginning of the Pliocene, in particular at the San Miguel de Salinas composite section (Fig. 7), where a few individuals of this group have been detected. These specimens show no obvious signs of reworking (good preservation of the tests and an absence of abrasion, dissolution, and fragmentation). Moreover, we have taxonomically clearly identified these specimens

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(scanning electron microscopy images in Fig. 9), together with an open marine assemblage with planktonic (Globigerinoides spp., Globigerina spp., Globoturborotalita spp.) and benthonic foraminifera (Bolivina, Cibicides dutemplei, Elphidium, Lenticulina, Nonion commune, Pullenia bulloides, Textularia gramen, Uvigerina bononiensis, and Valvulineria bradyiana). In the Bajo Segura basin, this scattered presence of Globorotalia mediterranea at the beginning of the Pliocene may represent the last specimens to survive to that period (although a reworked origin cannot be completely ruled out). 5.4. Implications for the late-Messinian biostratigraphy of the Mediterranean This study also provides insights into the late-Messinian interval, which has been thoroughly researched in the Bajo Segura basin. Our results show a complete succession of Messinian bioevents, revealing that the Bajo Segura basin has one of the most complete marine records of the pre-, syn-, and post-evaporitic phases of the salinity crisis. During the late-Messinian interval (Chron C3r), the Evaporitic Unit, the Lago-Mare facies, and the Terminal Carbonate Complex (Esteban, 1979) were all deposited in this basin (synthem MII; Soria et al., 2008a). The biostratigraphic results of this study, as pointed out by Corbí et al. (2016), indicate that the laminated marls interbedded with the Messinian evaporites (San Miguel de Salinas section) are characterized by a typical assemblage dominated by dwarf fauna of planktonic foraminifera, mainly represented by Globoturborotalita decoraperta, Globigerina spp. (mainly G. bulloides), and Neogloboquadrina spp. (Neogloboquadrina sp. and Neogloboquadrina acostaensis mainly with dextral coiling). Taphonomically, individuals in this particular assemblage are generally well preserved, with no abrasion, dissolution, or fragmentation. These taphonomic observations support the notion that this characteristic assemblage is autochthonous, excluding a reworked origin for the dwarf planktonic foraminifer fauna. Consequently, dwarf fauna can be the record of anomalous marine water conditions in the water column regarded to the Messinian Salinity Crisis (Corbí et al., 2016). Regarding the biostratigraphic zonal scheme, we propose that, at least in the Bajo Segura basin, the Mediterranean barren interval (Non-distinctive Zone) defined by Iaccarino et al. (2007) may be considered as the MMi13d subzone represented by an assemblage zone or eco-zone (cenozone). This new biostratigraphic subzone would be characterized by dwarf fauna of planktonic foraminifera, mainly represented by Globoturborotalita decoraperta, Globigerina spp., and Neogloboquadrina spp. (Figs. 9 and 10). Assuming this interpretation for the entire record of dwarf fauna throughout the Mediterranean late-Messinian interval (note that this peculiar fauna has been detected in several onshore sections and ODP sites; e.g. Hsu et al., 1978; Cita et al., 1978; Van de Poel, 1992; Comas et al., 1996; Iaccarino and Bossio, 1999; Iaccarino et al., 1999; Aguirre and
nchez-Almazo, 2004; Braga et al., 2006), we address the possiSa bility of a reassessment of the upper Messinian Mediterranean biostratigraphy, in particular the “barren interval”, which could be renamed the MMi13d subzone. 6. Conclusions This paper has established an updated planktonic foraminifer zonal scheme for the entire marine record of the Bajo Segura basin (a relevant hydrocarbon exploration region of the western Mediterranean) based on the recent astronomically calibrated charts of the Mediterranean. The study has shown how the Bajo Segura composite section could be considered a late Mioceneeearly Pliocene reference section of the western Mediterranean since it

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contains a complete record of the most relevant bioevents of this interval. Based on this planktonic foraminifer event-stratigraphy framework, the following conclusions can be drawn: - The chronostratigraphic framework of the marine allostratigraphic units in the Bajo Segura Basin (synthems Tortonian I, Tortonian II, Tortonian-Messinian I, Messinian II, and Pliocene) has been established. This has allowed us to constrain the age of the beginning and ending of these marine stages, as well as to determine the timeframe of the main discontinuities in the basin. The results indicate that the basin's marine sedimentary record spans from the earliest Tortonian (MMi9 biozone) to the end of the Pliocene (MPl3 biozone). - A succession of bioevents (planktonic foraminifer eventstratigraphy) from the earliest Tortonian to the end of the early Pliocene has been recognized. This biostratigraphic framework has permitted correlation of the Bajo Segura basin synthems with the astronomically tuned sections of the Mediterranean. Some of the intrazonal events detected have not been documented until now in the Mediterranean, including the particularly noteworthy Tortonian Globorotalia miotumida influxes. The first influx occurs in the MMi9 zone (earliest Tortonian), representing the oldest record of this species in the Bajo Segura basin and, thus, the first record in the Mediterranean. The second influx, in the MMi12a zone, can be correlated with the late-Tortonian influx of this species detected in the Lorca and Granada basins, which is also calibrated at 7.892 Ma in several other Mediterranean reference sections. - A complete planktonic biostratigraphic framework has been established for the upper-Messinian interval. The findings of this study suggest that the barren interval of the Bajo Segura basin record, or even throughout the Mediterranean, may be considered as a subzone, specifically an ecobiostratigraphic zone characterized by dwarf fauna of planktonic foraminifera, since no taphonomic evidence of reworking has been found. Further work is needed to establish the detailed, precise palaeoenvironmental, taphonomic, and biostratigraphic significance of these dwarf fauna in the Bajo Segura basin and in other upper-Messinian Mediterranean sections. This research will serve as a basis for future high-resolution integrated stratigraphy studies in the Bajo Segura basin. A follow-up of the present study could refine the chronostratigraphic framework of this representative western Mediterranean basin by combining planktonic foraminifera and nannoplankton biostratigraphy together with magnetostratigraphic, astro-cyclostratigraphic, and geochronological data. Acknowledgements This paper was written in the context of the Paleoenvironmental Changes research group (University of Alicante, VIGROB-167) and supported by project GRE14-05 (University of Alicante). The authors wish to thank the two anonymous reviewers for their valuable suggestions and critical comments, which have improved the quality of this paper. We appreciate the scientific inputs given by
benes during the different phases of this work. Thanks Alfonso Ye also to Christine Laurin for revising the English. References Aguirre, J., S
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