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Ameghiniana

versión impresa ISSN 0002-7014

Ameghiniana vol.46 no.2 Buenos Aires abr./jun. 2009

 

ARTÍCULOS ORIGINALES

Impact of storms on Pliocene benthic foraminiferal assemblages of southwestern Spain

María Luz González-Regalado1, Francisco Ruiz1, Manuel Abad1, Jorge Civis2, José Ángel González Delgado, Juan Manuel Muñoz3, Edith Xio Mara García4, José Gabriel Pendón5 and Antonio Toscano1

1Departamento de Geodinámica y Paleontología, Universidad de Huelva. Avda. de las Fuerzas Armadas, s/n., Campus de El Carmen, 21071-Huelva, Spain.
2Departamento de Geología. Universidad de Salamanca, 37008-Salamanca, Spain.
3Departamento de Estadística e Investigación Operativa, Universidad de Sevilla. Profesor García González, s/n. 41071-Sevilla, Spain.
4Departamento de Botánica y Zoología, Centro Universitario de Ciencias Biológicas y Agropecuarias, Universidad de Guadalajara, 45110-Jalisco, México.
5Departamento de Geología, Universidad de Huelva. Avda. de las Fuerzas Armadas, s/n., Campus de El Carmen, 21071-Huelva, Spain.

Abstract. Lithostratigraphical and faunal analyses of five Pliocene sections located in the southwestern Guadalquivir Basin (S Spain) permit reveal three main facies, which represent shallow marine fairweather conditions (FA-1), storm events (FA-2) and littoral/fluvial environments (FA-3). The vertical disposition of these facies indicates a regressive sequence. The statistical study of foraminiferal populations revealed five infralittoral and circalittoral assemblages. Vertical and horizontal variations among these assemblages, the P/B ratio and four diversity indices, together with the mollusc distributions, suggest the presence of a shallow marine palaeoenvironment in this area in the Lower Pliocene, with an increasing depth towards the southwestern. The main effects from storm events on the foraminiferal faunas are a slightly decrease of individuals and species, and a marked drop in the P/B ratio. The poststorm conditions are characterized by: a) a higher number of individuals; b) a progressive increase in the P/B ratio and c) small increases in the remaining indices.

Resumen. Impacto de las tormentas en las asociaciones de foraminíferos bentónicos del Plioceno del suroeste de España. El análisis litoestratigráfico y paleontológico de cinco secciones pliocenas situadas en el sector suroccidental de la Cuenca del Guadalquivir (Sur de España) permite distinguir tres facies principales, depositadas durante periodos de calma en un medio marino somero (FA-1), en épocas de tor­menta (FA-2) y en medios litorales a fluviales (FA-3). La disposición vertical de estas facies es indicativa de una secuencia regresiva. El análisis estadístico de las poblaciones de foraminíferos diferencia cinco asociaciones, representativas de medios circalitorales e infralitorales. Las variaciones verticales y horizontales de estas asociaciones, el índice P/B y cuatro índices de diversidad, unidas al análisis de la distribución de moluscos, sugieren que este sector estuvo ocupado por un medio marino somero durante el Plioceno Inferior, con un aumento de la profundidad hacia el suroeste. Los principales efectos de las tormentas en las asociaciones de foraminíferos son una pequeña disminución del número de individuos y especies, así como un importante descenso en el índice P/B. Los periodos posteriores a las tormentas se caracterizan por: a) un aumento en el número de individuos; b) un progresivo incremento en el índice P/B; y por c) pequeños aumentos en los índices de diversidad.

Key words. Benthic foraminifera; Storms; Lower Pliocene; SW Spain.

Palabras clave. Foraminíferos bentónicos; Tormentas; Plioceno Inferior; Suroeste España.

Introduction

In the last few decades, several investigations have been focused on the geological record of high-energy events, with distinctive deposits recognized in the Palaeozoic (Hurst, 1979; Gastaldo et al., 1990; Emig and Gutiérrez Marco, 1997), Mesozoic (Calvet and Tucker, 1988; Fürsich and Pandey, 1999; Schulte et al., 2006) and Caenozoic (Júarez-Arriaga et al., 2005) eras. In Holocene sequences, different interdisciplinary analyses can distinguish among sedimentary facies derived from the action of storms (Tsutsui et al., 1987), hurricanes (Halfar et al., 2004), cyclones (Woodroffe and Grime, 1999) and tsunamis (Clague and Bobrowsky, 1994).
In numerous cases, there is remarkable sedimentological and/or macrofaunal evidence of these events, with the presence of shell-bearing, coarse-grained beds rich in large foraminifera, bivalves, brachiopods and gastropods (Randazzo et al., 1990; Reynaud et al., 1999), which can even change the
shell-banding patterns after these events (Creese and Underwood 1976). In addition, these events affect significantly the distribution of different microorganisms, with measurable effects on the distribution of diatom and calcareous nannoplankton assemblages (Troelstra, 1987; Hemphill-Haley, 1996), changes in the agerelated population structure of ostracods (Ruiz et al., 2003; Ruiz et al., 2005) or increased planktic foraminiferal growth rates (Schiebel et al., 1995). Moreover, differences in the ichnological assemblages (Eyles et al., 1992), as well as floral changes in pollen or vascular plants (Hughes et al., 2002) aid in determining the impact of storms or tsunamis.
Benthic foraminifers are generally among the most prominent tracers of high-energy events and indicate (palaeo-)environments such as coastal ponds (Dix et al., 1999), barrier islands (Vance et al., 2002; Luque et al., 2002), gulfs (Cundy et al., 2000) or inner shelf settings (Tapiero et al., 2003). Faunal studies include analyses of the taxonomical composition of foraminiferal assemblages and the calculation of various diversity indices (e.g., Drinia et al., 2005). However, most of such studies only differentiate the physical features of the storm-induced deposits and few distinguish the main faunal characteristics produced during storm events from typical post-storm assemblage distributions.
In this paper, we analyse the impact of storms on Pliocene benthic foraminiferal assemblages of southwestern Spain. For this purpose, we statistically define the main assemblages, infer their palaeoecological implications and track the vertical variations of several diversity indices within Huelva province.

Pliocene palaeogeography of southwestern Spain: the Neogene of Huelva

In southwestern Spain and northern Morocco, the two main seaways (North Betic and South Rifian corridors) connecting the Mediterranean Sea and the Atlantic Ocean during the Tortonian-Messinian were progressively closed, culminating in the temporal isolation of the Mediterranean Sea (Hsü et al., 1973; Krijgsman et al., 1999). During the Pliocene, two large gulfs of Atlantic influence (the Guadalquivir basin of Spain -figure 1.1.- and the Gharb of Morocco) occupied the western part of these corridors.


Figure 1. Geographical and geological settings of Southwestern Spain, with description of the regional lithostratigraphical column and location of the five Pliocene sections studied in this paper / localización geográfica y geología del suroeste de España, con descripción de la columna litoestratigráfica regional y situación de las cinco columnas pliocénicas estudiadas en este trabajo.

In the western sector of the Guadalquivir basin four Neogene formations lying unconformably on a Palaeozoic-Mesozoic substrate are defined (figure 1.2):

a) Niebla Formation (Baceta and Pendón, 1999). This Tortonian unit has a variable thickness (0-25 m) and consists of fluvial conglomerates, littoral sands and shallow marine calcarenites. These calcarenites contain common bivalves (Gigantopecten, Ostrea, Crassostrea, Spondylus), echinoderms (Clypeaster, Echinolampas), red algae (Melobesidae) and nummulitids (Heterostegina).
b) Gibraleón Clay Formation (Civis et al., 1987). This is a monotonous lithofacies consisting of gray-blue marls and clays, except for the presence of a condensed, silty glauconitic layer near the base. The planktonic foraminiferal fauna indicates an upper Tortonian to Zanclean age for these materials (Sierro, 1985). These fine sediments were deposited in upper bathyal to outercircalittoral palaeoenvironments, with maximum water depth (>300 m) near the base and progressive shallowing toward the top (Sierro and Civis 1987; González-Regalado and Ruiz, 1990).
c) Huelva Sand Formation (Civis et al., 1987). The overlying deposits are formed of silty sands with a basal glauconitic layer that contains a rich fauna of selachians (Ruiz et al., 1998). In its upper part, this formation consists of several lenticular, sometimes lumachellic layers of mollusc shells interbedded with massive, bioturbated beds. This paper is focussed on the foraminiferal record of this formation.
d) Bonares Sand Formation (Mayoral and Pendón, 1986). The lowermost part is represented by coarse-grained sands with some molluscrich beds and abundant bioturbation. Near the top, this facies is replaced by conglomerates.

Methodology

In the western sector of the Guadalquivir Basin, five sections of the Huelva Formation (figure 1.1: El Rompido, Moguer, Lucena, Bonares, and Casa del Pino) were selected for this study, all are Pliocene age (Globorotalia margaritae-Globorotalia puncticulata Biozone; Sierro, 1985). Descriptions of the lithostratigraphy and facies associations, and analyses of major macrofaunal groups were accomplished in the field.
A systematic micropaleontological sampling (table 1: 56 samples) was undertaken at intervals of 0.5 m. From each sample (250 g dry weigth), the wash residue > 100 µm was examined and up to 300 individuals were picked (Buzas, 1979), with a later extrapolation to the whole sample to estimate both the total number of individuals and the percentages of the constituent species. In addition, the P/B ratio (planktic foraminifera/total foraminifera) was calculated and the following diversity indices were determined: Margalef (1982), Shannon-Weaver (1963), Equitability (Buzas, 1979) and α (Fisher et al., 1943), determined by the graphic method of Murray (1973).

Table 1. Quantitative foraminiferal distribution (in %) in the sections / distribución cuantitativa de los foraminíferos (en %) en las secciones.

In a second step, the Pearson correlation matrix was obtained using SYSTAT 11.0 TM (p > 0.01: white numbers in table 2; p > 0.05: underlined numbers in table 2). Finally, a cluster analysis (R-mode; Euclidean distance) was performed on the thirty most abundant species in order to delineate the main assemblages. Each assemblage was characterized as follows: a) a main species, well represented in several sections; b) some additional species, highly correlated with the previous one and among themselves (p > 0.01 in most cases); and c) some secondary species, well correlated with either the main species or some additional species (p > 0.05 in most cases). Finally, the percentages of each assemblage in each sample were calculated.

Table 2. Pearson´s correlation matrix, with inclusion of the thirty main species. White numbers: p > 0.01; underlined numbers: p > 0.05) / matriz de correlación de Pearson, con inclusion de las 30 especies principales.

Results and discussion

Facies: Description, foraminiferal distribution and interpretation

Three main facies may be distinguished (figure 2): Silty-clayey sands (FA-1). This facies consists of yellow to brown well-sorted, very fine sands (75-120 µm in most cases). Percentages of silts and clays are variable (6-47 %), with smectites the main clay mineral (Castaño et al., 1988). The calcium carbonate contents vary between 6% and 25%, owing to the presence of local accumulations of molluscs. These sands often have decreasing glauconitic contents toward the top of the local series, where they are interbedded between the bioclastic layers of FA-2. Bioturbation is variable, with locally high abundances of Thalassinoides and Ophiomorpha burrows. According to Droser and Bottjer (1986), the ichnofabric index oscillates between 2 and 4.


Figure 2. Lithostratigraphy, faunal contents and facies associations of the selected sections / litoestratigrafía, contenido paleontológico y asociaciones de facies de las secciones seleccionadas. C-1, sample / C-1, muestra.

Disposition of macrofauna is irregular, with several lenticular patches of variable thickness (5-40 cm) and limited lateral continuity. These patches consist mainly of bivalves, with well preserved specimens of Glycymeris insubricus, Acanthocardia paucicostata, Spisula subtruncata, Corbula gibba and ostreids. Taphonomic analysis indicates the presence of several species in life position (Pelecyora brocchii, Panopea glycymerys, Atrina pectinata), the observation of all growth stages, good preservation of delicate structures (spines, fine ribs), and the absence of preferential orientation in both scaphopods and gastropods. This mollusc assemblage is very similar to the biocoenosis of well sorted, fine sands that occur in the inner shelf areas of the Mediterranean Sea and the adjacent Atlantic coasts (Peres and Picard, 1964; Andrés, 1982; González Delgado 1983; Castaño et al., 1988).
Seventy-one foraminiferal species were found in this facies. However, samples with more than 20 species are concentrated mainly in the Moguer and El Rompido sections (table 1). In the Moguer section, the basal and uppermost samples are clearly dominated by Florilus boueanum (33.6-71.6%) and, to a lesser extent, Ammonia tepida (3.5-22.7%), whereas the remaining samples exhibit a high diversity (17-28 species) and important percentages of Fursenkoina schreibersiana (3-7.9%), Globobulimina auriculata (3.9-9.2%), Orthomorphina tenuicostata (1.6-10.8%) or Textularia articulata (5.5-28.1%). These more diverse samples have the highest P/B ratio of all samples studied (16.2-31.1%), moderate to high values of the Fisher (2.5-5.5) and equitability (0.5-1) indices, and low to moderate values of the remaining indices (Margalef: 1.7-2.2; Shannon-Weaver: 1.6-2.8). These four last species mentioned and these index values are characteristic of recent Iberian circalittoral to bathyal environments with a depth range between 50 and 2500 m, though they are more frequent in outer neritic zones. In these modern Iberian areas, Ammonia tepida is well represented in very shallow (0-30 m) environments, whereas Florilus boueanum is more common in circalittoral areas (Sánchez-Ariza, 1979; Villanueva, 1994; González-Regalado et al., 2000). These data indicate a depth increase in the intermediate part of this section.
In the El Rompido section foraminifera are abundant in the basal and uppermost samples (45,000-65,000 individuals/sample), whereas their numbers diminish considerably in the intermediate part (Fig. 3: <8,000 individuals/sample). Nevertheless, the number of species is relatively constant throughout this section (20-29 species). Florilus boueanum is the dominant species (20-54.6%) in this facies, together with moderate percentages of Ammonia tepida (3.55-18.2%), Fursenkoina schreibersiana (3.36-9.34%), Bulimina elongata (2.88-10.2%), and Heterolepa bellincionii (2.66-17.3%). P/B ratio is very low to low (figure 3, 4.5-12%), whereas the diversity indices show moderate to high values in relation to the rest of the sections (Margalef: 2.5-4; Shannon-Weaver: 2.5-5.5; Equitability: 0.55-0.7; Fisher: 2.5-4). The presence and abundance of infralittoral (Ammonia tepida), ubiquitous (Florilus boueanum) and inner circalittoral (Fursenkoina schreibersiana, Bulimina elongata) species, and the index values suggest an outer infralittoral environment (20-40 m depth).



Figure 3. Vertical changes of faunal parameters (species richness , P/B ratio and diversity indices) in the Pliocene foraminiferal populations of the five sections. Dark grey: storm beds (Facies FA-2) / cambios verticales de los parámetros faunísticos (número de especies, índice P/B e índices de diversidad) en las poblaciones de foraminíferos pliocénicos de las cinco secciones. En gris: depósitos de tormentas (Facies FA-2).

In the remaining three sections (Lucena, Bonares, and Casa del Pino), the foraminiferal abundance is very variable (2,000-65,000 individuals/sample), and two infralittoral species (Ammonia tepida, Florilus boueanum) constitute 64-96% of the total fauna. P/B ratio (<4 %) and diversity indices (Margalef: 1.2-2.8; Shannon-Weaver: 0.8-2.6; Equitability: 0.5-0.8; Fisher: 0.6-3.4) are very low to moderate in comparison with the two previous sections. These data and the mollusc assemblages suggest a Pliocene infralittoral palaeoenvironment in the easternmost part of the Huelva province.
The general palaeonvironmental interpretation of the Lucena, Bonares and Casa del Pino sections is also supported by the ostracode faunas, which in-clude numerous infralittoral species (Paracytheridea depressa, Carinocythereis whitei, Hiltermannicythere spp., Basslerites berchoni, Neocytherideis fasciata, Callistocythere rastrifera, Costa punctatissima, Pseudocytherura calcarata, Loxoconcha tumida) that live in shallow, sandy modern environments (0-50 m depth) of the adjacent Cádiz Gulf (Ruiz et al. 1997; 2000). In addition, several circalittoral taxa (Ruggieria tetraptera, Pterigocythereis jonesii, Cythe-rella spp.) are abundant in both the Moguer and, to a lesser extent, El Rompido sections, whereas the Bonares section contains abundant individuals of Cyprideis and Myocyprideis, genera usually associated with brackish and freshwater conditions (Nascimento, 1983).
Bioclastic sands (FA-2). This facies association is characterized by the presence of fossilbearing, shell-supported sandy-pelitic sediments arranged in sub-horizontal layers (10-90 cm thickness) over erosional base. The matrix is composed of yellow, well sorted fine to very fine sands (90-160 µm) with significant percentages of silts and smectitic clays (15-50%) and moderate calcium carbonate contents (18-28%). Some horizontal laminations have been observed near the base of these layers. Bioturbation is very scarce to virtually absent.
This facies occurs in all sections and is represented by two to five bioclastic layers (figure 2), with up to 150 species of bivalves and gastropods and more than 80 individuals/kg of each group in some of these layers. Major species of bivalves are Corbula gibba, Nuculana pella, Nuculana fragilis, Myrtea spirifera, Spisula subtruncata and Glycymeris insubricus, where-as the most abundant gastropods are Ringiculina buccinea, Neverita josephinia, Lunatia macilenta, Naticarius tigrinus and Hinia planicostata. These molluscs are generally well preserved, as indicated by the presence of spines, ribs or remains of the original colors. Percentages of articulated bivalves are variable (5-40%), whereas fragmented valves are common and eroded carapaces are rare. In addition, numerous individuals of some large species (Pelecyora brocchi, Panopaea glycymeris, Ostrea lamellosa) were found in life position.
The foraminiferal record is similar to Facies FA-1, though some small differences may be observed. Only 5 to 15 foraminiferal species were found in these layers, as some species (Dorothia gibbosa, Bolivina arta) show higher percentages and others (Cancris auriculatus, Lenticulina cultrata, Ortomorphina tenuicostata) are absent or poorly representated in relation to Facies FA-1. In the shallower zone (Lucena, Bonares and Casa del Pino sections), the P/B ratio is very low (< 1% in most samples), whereas the diversity indices show similar values or are only slightly lower than Facies FA-1.
The ostracod assemblage is similar to Facies FA-1, although the population age structure shows a remarkable change. Adults to A-6 moults are well represented in Facies FA-1, reflecting a low-energy (paleo-)biocoenosis structure (Whatley, 1988), whereas only adults and the last juvenile moults are observed in Facies FA-2, suggesting a high-energy thanatocoenosis structure (Ruiz et al., 2003).

Both microfaunal data and the mollusc distribution suggest these deposits were formed by to the sporadic action of storms which led to remobilization of the bottom and a redeposition of molluscs near their initial locations, whereas Facies FA-1 represents the normal palaeobiocoenosis deposited during fair-weather times and eroded periodically. Similar interpretations have been indicated for shelly beds observed in Tertiary sections of Greece or Austria (Drinia et al., 2005; Zuschin et al., 2005).
Medium sands and microconglomerates (FA-3).
Sediments overlying the Huelva Sand Formation consist of either clayey, medium sands or sandy microconglomerates (~ 20 m thick) with orange shades. They have an erosive base developed over Facies FA-1 or FA-2 overlain by sediments that contain parallel laminations in the lower few meters. In the upper part, they include planar and/or tabular crossbedding, low-angle crossbedding and megaripples. The macrofauna is limited to the lower sandy beds, with frequent specimens of bivalves (Cardium, Tellina, Solen, Paphia) and an abundant ichnofauna (Ophiomorpha, Gyrolithes).
Preliminary analyses of some basal deposits showed a few foraminiferal molds belonging probably to Florilus boeuanum and Ammonia tepida and no ostracode valves or carapaces, whereas no traces of either group were found in the upper, coarse-grained sediments. The lower materials were deposited in very shallow littoral, even intermareal environments, whereas the upper coarse-grained beds represent a transition toward fluvial conditions (Rodríguez Vidal et al., 1985; Mayoral and Pendón 1986).

Cluster analysis: assemblages and autoecological parameters

Analysis of a first dendrogram (figure 4.1) delimits two first monospecific assemblages, which constitute up to 80% of the total foraminiferal fauna. An additional dendrogram (figure 4.2) was made using only the remaining species, in order to determine new assemblages that previously may have been masked by the strong statistical weight of Ammonia tepida and Florilus boueanum. These two dendrograms and the Pearson´s correlation matrix (table 2) permit to define five main assemblages: Ammonia tepida assemblage (Ass. 1). No additional or secondary species. This species is very abundant in the three westernmost sections (Lucena, Bonares and Casa del Pino), constituting 18-50% of the total foraminiferal faunas in most samples. In these sections, the highest percentages (up to 50%) were found in Facies FA-1, with an increase after storm periods (especially in Casa del Pino). In both the El Rompido and Moguer sections, its percentages are clearly lower (3.5-22.7%). This species is abundant in recent infralittoral environments of the Cádiz Gulf (Villanueva, 1994).


Figure 4. Cluster analysis, with inclusion of the thirty most abundant species (A) and exclusion of Ammonia tepida and Florilus boueanum (B). For abbreviations, see table 2 / análisis de agrupamiento, con inclusión de las 30 especies más abundantes (A) y con la exclusión de Ammonia tepida y Florilus boueanum (B).

Florilus boueanum assemblage (Ass. 2). No additional or secondary species. Florilus boueanum is the most representative species of the area studied, with high to very high percentages (20-69%) except in four intermediate samples of Moguer (M-3 to M-6) and some isolated samples of El Rompido and Casa del Pino. It is generally dominant in Facies FA-2 and in the basal samples of all sections, being partially replaced later by other assemblages in the deeper water sections (El Rompido: assemblage 3; Moguer: assemblage 6) and sharing predominance with Ammonia tepida in the rest. These data indicate a broader depth distribution of Florilus boueanum compared to Ammonia tepida, which is more restricted to coastal areas, and a slightly preferential concentration in storm-induced deposits in comparison with A. tepida. In other Lower Pliocene sections of south-eastern Spain, Florilus boueanum has been collected even in middle shelf facies (Pérez-Muñoz et al., 2000), whereas this species is a constituent of tsunami-induced deposits along the Indian coasts (Nagendra et al., 2005).
Globobulimina auriculata assemblage (Ass. 3). Additional species: Cassidulina laevigata, Dorothia gibosa, Lenticulina calcar, Melonis padanum, Orthomorphina tenuicostata and Textularia articulata. Secondary species: Marginulina costata and Heterolepa praecincta. This assemblage is dominant (26.4-38.7%) in the intermediate samples of Moguer, coinciding with the highest P/B values of all sections (16-32%). These species coincide with circalittoral environments, though some (e.g., Cassidulina laevigata) live even in bathyal areas (Sgarrella et al., 1983; Bizon and Bizon 1984; Debenay and Basov, 1993; Villanueva, 1994). Fursenkoina shreibersiana assemblage (Ass. 4). Additional species: Bulimina aculeata, Bulimina elongata, Cancris auricula, Heterolepa bellincionii, Hopkinsina bononiensis. Secondary species: Hanzawaia boueana, Lobatula lobatula, Planorbulina mediterranensis, Protel-phidium granosum. This assemblage is well represented in the El Rompido section (17.2-40.1%), being dominant in some samples belonging to Facies FA-1 (figure 5). It is of lesser importance in the remaining sections, with percentages that do not surpass 12.8% in Moguer and are low to very low (< 5.1%) in both the Lucena and Casa del Pino sections. These species coexist generally in the inner circalittoral zone, at depths between 20 and 100 m on modern southwestern European marine shelves (Mateu, 1970; Pujos, 1976; Bizon and Bizon 1984; Martins, 1997). Reusella spinulosa assemblage (Ass. 5). Additional species: Elphidium advenum, Elphidium crispum and Vaginulina striatissima. Secondary species: Ammonia beccarii, Elphidium excavatum and Eponides antillarum. This assemblage constitutes <4% of the fauna in almost all samples of Moguer and El Rompido, but has variable percentages in the remaining sections (Lucena: 1.8-25.4%; Bonares: 2.6-14.8%; Casa del Pino: 0.1-10.6%). In modern Iberian shelves, these species are found generally at depths down to 70 m (Colom 1952; Galhano 1963; Levi et al. 1993; Villanueva 1994; Levi et al. 1995), although they do occur to 200 m depth in the Mediterranean Sea (Mateu, 1970; Sánchez-Ariza, 1979; Ribes and Gracia, 1991). Some of them (Ammonia beccarii, Elphidiun advenum, Elphidium excavatum) may inhabit in low salinity areas of the inner shelves, tidal in-lets or marshes (Donnici and Barbero, 2002; Nagendra et al., 2005; Hippensteel et al., 2005; Abbene et al., 2006).


Figure 5. Vertical variations (in %) of the five foraminiferal assemblages. Dark grey: storm beds (Facies FA-2). Highlighted numbers represent the most abundant assemblage in each sample / variaciones verticales (en %) de las cinco asociaciones de foraminíferos. En gris: depósitos de tormentas (Facies FA-2). Los números reseñados representan la asociación más abundante de cada muestra.

Paleoenvironmental reconstruction of southwestern Spain during the Pliocene

Combining the facies and cluster analysis data a palaeoenvironmental reconstruction of the investigated area is suggested (figure 6), with a inner circalittoral (Moguer sectior) to outer infralittoral environment for the southwestern area and an inner infralittoral environment for the northeast area (Lucena, Bonares, Casa del Pino sections).


Figure 6. An approximate palaeoenvironmental reconstruction of southwestern Spain during the Pliocene / reconstrucción paleoambiental aproximada del suroeste de España durante el Plioceno.

The deeper areas to the southwest are characterized by Florilus boueanum and a coincidence of assemblages 3 and 4, which include several species (Buliminidae, Cassidulina laevigata, Hopkinsina bononiensis) associated with low-oxygen environments (Pérez Muñoz et al., 2000; Levi et al., 1995; Debenay and Redois, 1997). Faunas include the highest P/B ratio values (up to 5% in most cases) and diversity indices are similar to those observed in the inner shelf (20-50 m depth) of the adjacent Cádiz Gulf (Villanueva, 1994).
In the shallowest areas, two species (Florilus boueanum, Ammonia tepida) represent up to 80% of the total foraminiferal faunas. Planktonic foraminifera are very scarce and the diversity indices show lower values in relation to the deeper zones. This positive correlation between both the P/B ratio or the α-index and depth has been already found in numerous marine shelves (e.g., Hayward, 1982; Murray, 1991). Dissolved oxygen content was probably normal, with scarce representation (<4%) or absence of Cassidulina, Bolivinidae or Buliminidae.
This inner infralittoral sector was under the influence of freshwater inputs, with the presence of abundant populations of brackish-water ostracods and scarce, reworked individuals of freshwater species (Ruiz and González-Regalado, 1996). Some of the most frequent foraminiferal species (Ammonia beccarii, Ammonia tepida, Elphidium crispum, Florilus boueanum, Protelphidium) also may inhabit in low-salinity environments (Murray, 1973, 1991; González-Regalado et al., 2001).

Foraminifera and storms

Effects of storms

In these littoral paleoenvironments, periodical storms eroded the bottom, producing the accumulation of shelly beds. The basal part of these shelly beds shows the following differences in relation to the underlying fairweather conditions:

a) A slightly lower number of individuals and similar or lower number of species. In the North Pacific region, foraminifera are significantly less diverse in some energetic areas subjected to frequent storms than in relatively quiet shallow sites (Kaminsky, 1985);
b) Higher percentages of some species (Bolivina arta, Dorothia gibbosa), accompanied by lower abundances of others (Cancris auriculatus, Ortomorphina tenuicostata) and the absence of some (Lenticulina cultrata);
c) A decrease in the P/B ratio, probably due to the destruction of globigerinids, the most important planktonic foraminiferal group (Sierro, 1985). The fragile tests of this group and some benthic species (e.g., Martin and Wright, 1988) are most susceptible to destruction. Moreover, rounded and thin tests have low traction velocities, lower or similar to benthic species with roughly equidimensional tests (Snyder et al., 1990), which permit easy transport along the bottom.
d) No significant differences or slightly lower values in the diversity indices. As pointed out by Levin et al., (2001), strong erosive currents do not necessarily depress diversity and may even increase it in harpacticoid copepods (Thistle 1998). Nevertheless, numerous investigations link severe storms to a drop in species richness among macrobenthos (Boesch et al., 1976), bivalves (Allen and Sanders, 1996), polychaetes (Gage, 1997) and nematodes (Lambshead et al., 2001);
e) Percentages of circalittoral assemblages (Ass. 3 and 4) are similar to or lower than those observed in fairweather conditions, indicating limited sedimentary transport from the outer shelf to the shallower areas during these high-energy events.

All these data suggest a limited impact of storms on the foraminiferal populations, which is a remarkable datum because examples of the reverse are clearly more abundant. Storms or hurricanes are capable of eroding sediments rich in foraminifera in relatively deep waters, and redepositing them in shallower marine areas and marshes (Collins et al., 1999; Hippensteel et al., 1999), resulting even in inverted sequence in some coastal areas (Nigam and Chaturvedi, 2006).

The post-storm conditions

With few exceptions, the transition toward quieter conditions is marked by:

a) An increasing number of individuals and similar or a slightly higher number of species;
b) A progressive increase in the P/B ratio. This increase has been also found in planktonic diatoms, which settle out of the water column during fair-weather conditions and are deposited in sandy bottoms of lower shoreface environments (Campeau et al., 1999);
c) A small increase in the diversity indices;
d) A predominance of Florilus boueanum in the post-storm samples, being progressively replaced by Ammonia tepida during subsequent fairweather conditions in the shallowest areas or near the transition to Facies FA-3. This change may be due to environmental adaptability of the first species and progressive depth decrease, respectively.

In addition, bioturbation increases progressively in Facies FA-1 under fairweather conditions, where-as Facies FA-2 shows scarce evidence of bioturbation. This transition has been also observed in Cretaceous deltaic deposits of Wyoming and Utah (Gani et al., in press).

Conclusions

An integrated study of the southwestern Spanish Pliocene record revealed three main facies. Facies FA-1 (silty-clayey sands) was deposited in infralittoral to circalittoral palaeoenvironments during fair-weather conditions, whereas Facies FA-2 (bioclastic sands) resulted from storm activity and Facies FA-3 (medium sands and microconglomerates) represented the transition toward intertidal/fluvial conditions.
Five foraminiferal assemblages were recognized by cluster analysis and the Pearson's correlation matrix, all of them well represented in recent continental shelves of both Europe and Africa. Both the Ammonia tepida and Reusella spinulosa assemblages occur generally in inner infralittoral areas (< 40 m depth), where-as the Fursenkoina schreibersiana and Globobulimina auriculata assemblages are found generally in middle and even outer shelves. The remaining assemblage (Florilus boueanum) corresponds to inner and middle shelf environments. The mollusc record and foraminiferal distributions permit palaeoenvironmental reconstruction of the Pliocene of southwestern Spain, with the presence of circalittoral (Moguer), outer infralittoral (El Rompido) and inner infralittoral (Lucena, Bonares, Casa del Pino) areas, these latter subjected to periodic freshwater inputs.

Effects of storms on the foraminiferal populations are limited in relation to those observed on populations of other micro- and macrofaunal groups subjected to periodical high-energy events. Storms caused a slight decrease in the number of individuals, number of species, P/B ratio, diversity indices and bioturbation, with a subsequent recovery during the later fairweather conditions. These data and the taphonomic analyses of Facies FA-2 indicate a small-scale, localized transport of these storm-induced shelly beds from their original pale-obiocoenosis.

Acknowledgments

This work was supported by two Spanish DGYCIT Projects (CTM2006-06722 and CGL2006-01412) and the RNM-238 group of the Andalusia Board.

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Recibido: 4 de diciembre de 2008.
Aceptado: 7 de marzo de 2009.