Journal of Archaeological Science 41 (2014) 435e446
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Marine bioerosion in rocks of the prehistoric tholos of La Pastora (Valencina de la Concepción, Seville, Spain): archaeological and palaeoenvironmental implications L.M. Cáceres a, *, F. Muñiz b, J. Rodríguez-Vidal a, J.M. Vargas c, T. Donaire d a
Departamento de Geodinámica y Paleontología, Universidad de Huelva, Avda. Tres de Marzo, s/n, 21071 Huelva, Spain Facultad de Ingeniería, Geología, Universidad Andrés Bello, sede Concepción, Autopista Talcahuano, 7100 Talcahuano, Concepción, Chile Servicio de Arqueología, Ayuntamiento de Valencina de la Concepción, Sevilla, Spain d Departamento de Geología, Universidad de Huelva, Avda. Tres de Marzo, s/n, 21071 Huelva, Spain b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2013 Received in revised form 26 August 2013 Accepted 2 September 2013
In this study we examined the materials used in the building of the tholos of La Pastora located in the archaeological site of Valencina de la Concepción (Seville, Spain), which represents the largest settlement of the Guadalquivir Valley during the Third millennium BC. Three different types of rock were found: Palaeozoic sandstone-shale, which forms the walls of both the corridor and the chamber; Palaeozoic granite, which makes up part of the roof slabs; and Neogene calcareous sandstone, which forms the remainder of the roof and floor slabs, as well as the jambs, lintels and some blocks of the walls of the chamber. Intense marine bioerosion as a result of the activity of bivalves, annelids and sponges was observed in some elements of the calcareous sandstone. In fact, the remains of some of the tracemakers were unusually well-preserved, as was the case for the bivalve Petricola lithophaga, whose shells remain in many of their burrows. We infer that these bioerosion structures were formed in a shallow coastal environment (intertidal), and that bioerosion was possible until the few moments before its extraction as construction material. Thereby, the environment in which rock extraction took place was likely a coastal wave-cut platform, in which calcareous sandstone levels outcropped and stood out between clay layers. The radiocarbon dates obtained from these bivalve shells provide a time range between 4780 and 4400 cal yr BP (2830e2450 yr BC) as the highest probability for rock extraction and the subsequent building of the tholos. Thus, the date represents a “Terminus Post Quem” for the construction of the monument of La Pastora. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ichnology Bioerosion Third millennium BC Valencina de la Concepción Tholos of La Pastora
1. Introduction The tholos of La Pastora in Valencina de la Concepción (Seville, Spain) is one of the most important megalithic monuments of southern Europe. Valencina de la Concepción represents the largest settlement of the Guadalquivir Valley (Fig. 1) during the Third millennium BC and has been considered as the main political centre of the first hierarchical intersocial structure of Western Europe (Nocete, 2001; Nocete et al., 2005). This megalithic building is located in the southwest area of the settlement (Fig. 2) and it shows evidence of having been a very special sacred place. Such is the case for its unusual astronomical orientation, toward the sunset, while the predominant standard in Iberian megaliths is toward the rising sun (Hoskin, 2002). Or, the * Corresponding author. Tel.: þ34 959 219 850; fax: þ34 959 219 440. E-mail address:
[email protected] (L.M. Cáceres). 0305-4403/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jas.2013.09.001
extraordinary discovery of 27 javelin tips, deposit of which is singular in the prehistory of western Mediterranean (Garcia-Sanjuan, 2010; Hunt et al., 2012). La Pastora was built with three types of lithological materials: blocks of sandstone-shale, calcareous sandstone and granite slabs. Most of the calcareous sandstone slabs show marine bioerosion structures. Bioerosion is the process by which a vertebrate, invertebrate or plant organism carves or penetrates a hard biogenic substrate (shells, bones, amber, coprolites and other mineralized skeletons) or non-biogenic substrate (rocks of several origins) by mechanical destruction and/or chemical dissolution (Bromley, 1970, 1990, 1994; Buatois and Mangano, 2011; Ekdale et al., 1984; Gibert et al., 2004; Neumann, 1966; Santos et al., 2003; Tapanila et al., 2004; Taylor and Wilson, 2003; Wisshak et al., 2005; among others). The resulting structures from the bioerosive activity of these organisms are mainly caused by the combination of several parameters that are closely related: environmental conditions,
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Fig. 1. Geographical and geological framework of the Lower Guadalquivir Basin and closest Iberian Massif, with location of Valencina de la Concepción. Key: 1: Devonian slate, quartzite, conglomerate and limestone; 2: Carboniferous Volcanic Sedimentary Complex; 3: granitoid; 4: basic plutonic rocks; 5: Carboniferous slate, sandstone and conglomerate; 6: Permian volcanic and sedimentary rock; 7: Triassic clay and gypsum; 8: Miocene marl, sandstone and silex stone; 9: Miocene calcarenite and conglomerates; 10: Miocene Blue marl; 11: Transition Facies; 12: Pliocene sand and silt; 13: Quaternary sand (dunes and beaches); 14: Quaternary fluvial deposits (conglomerate, sand and clay); 15: marshland; 16: shoreline of maximum Flandrian flooding.
substrate type, producer type and behaviour. Therefore, from their analysis and observation, we can draw conclusions related to ancient environmental conditions, sea-level changes and the reconstruction of ancient shorelines (Bromley, 1992; Bromley and Asgaard, 1993a; Cachão et al., 2009; Domènech et al., 2001; Martinell and Domènech, 1995; Santos et al., 2010; Warme, 1975; among others). The study of bioerosion structures provides very valuable information to other research fields such as ecology, ethology, zoology, geochemistry, sedimentology, biostratigraphy, palaeobiology and evolutionary palaeoecology (Rodríguez-Tovar et al., 2010). However, it has not been widely used in archaeology (Gautier, 1993; Mikulás, 1999; Rodríguez-Tovar et al., 2010), except for the study of traces of animals on bones, which allow the palaeoecologic reconstruction of behaviour (Gautier, 1993; Haynes, 2002; Noe-Nygaard, 1989), or the study of microbial bioerosion as an agent of bone deterioration (Jans et al., 2004). In this paper, we analyse and interpret the set of bioerosion structures found in different architectural elements of the tholos of La Pastora. This analysis provides data of great interest from an archaeological, palaeoenvironmental and geomorphological perspective. The results of bioerosion in archaeological remains are detailed, mainly related to the lithological nature of the materials, their origin, their exploitation and the relationship between man and the environment. An attempt to date the bioerosive activity has been also carried out, taking into account the probable limitations of the method and, therefore, with all possible caution.
Fig. 2. Extension of the archaeological site of Valencina de la Concepción (after Vargas, 2004). Red: Area of housing occupancy and working; Yellow: Necropolis area. Numbers and black dots are the location of main megalitic monuments: 1: La Pastora; 2: Matarrubilla; 3: Montelirio; 4: Ontiveros. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. The Arqueological zone of Valencina and the architecture of the “tholos de La Pastora” The settlement of Valencina de La Concepción is located about 8 km west of the city of Seville (Figs. 1 and 2), SW Spain, at the
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plateau of Aljarafe, which rises about 150 m above sea level (m a.s.l.) and about 145 m from the current floodplain of the Guadalquivir river. With over 4,600,000 m2 of archaeological surface (Vargas, 2003), the settlement represents the most extensive example of the recent prehistory of the southern Iberian Peninsula (Nocete, 2001). Its surface is divided into an area of housing occupancy and working, with 2,356,000 m2, and an exclusive area for the necropolis, with 2,332,000 m2 (Vargas, 2003, 2004), both on the same plateau about 150 m a.s.l. An occupation of such dimensions contrasts with other settlements of this region of the same age, where the usual extensions were in the range of 10,000e50,000 m2, with scarce examples of greater extension, which do not exceed 1,000,000 m2 in any case (García-Sanjuan and Hurtado, 1997). The necropolis has several main megalithic buildings (La Pastora, Matarrubilla, Ontiveros and Montelirio, Fig. 2) and smaller buildings distributed among them, such as the Roquetito (Murillo et al., 1990) or the more recently excavated tombs in the northern part of Montelirio (Mora Molina et al., 2012). The excavated buildings are mostly of tholoi type, with a circular chamber of 1e 4.4 m in diameter and corridors of several tens of metres in the monumental ones and of a few metres in smaller tombs. The tholos of La Pastora consists of a long corridor and a circular chamber, which together reach 45.65 m in length (Figs. 3 and 4). The circular chamber shows a diameter of 2.60 m with walls built of little slabs of sandstone and shale. On the upper section of this wall and by layer approximation (false dome), there are several slabs laid, on which a bigger slab of granite is laid, as a closure. The corridor has a trapezoidal section (Fig. 3) and is divided into three sections separated from each other with protruding slabs by way of doors, with jamb and lintel preserved in the second and third section, and also with a threshold in the section with access to the chamber. The walls are masonry of sandstone and shale and both the floor and the roof are big slabs which, from the petrographic point of view, correspond to carbonated sandstone where the studied bioerosion structures of this work were identified, and granite (Figs. 3 and 4). The covering tumulus was originally more enhanced, thereby, at the moment of its discovery the accumulation of earth above the chamber was about 2 m. Regarding its total area, this was calculated to be about 2300 m2 (Vargas, 2004).
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Precambrian and Palaeozoic materials of the Iberian Massif (Fig. 1). The Guadalquivir Basin was filled of sediments from a great wedgeshaped, elongated depression in a WSW-ENE direction, during the Neogene and the Quaternary. The oldest materials are located north of the Valencina site and outcrop in a band composed of conglomerate and sandstone of carbonated cement. Over them and further south, but still north of Valencina, the Blue Marl Unit outcrops. It is a thick sequence of marine, clayey, greyebluish, more or less carbonated materials with levels of gypsum and iron oxides (Civis et al., 1987). The consistency of these materials is generally soft. The unit known as Transition Facies outcrops closer to the site (Mayoral and González, 1987), which constitutes a gradual shift from the lower unit of blue marl and the overlying, sandy unit. It consists of a rhythmic alternation of clayeyesilty and sandye sandstony terms, in the upper sections of which there are 10e 70 cm thick strata of carbonated sand and sandstone alternate with another 10e30 cm thick strata of clay (Fig. 5G). The age of this sandstone corresponds to the Upper Miocene (Mayoral and González, 1987). Above the previous formation, the Yellowish Sand and Silt Unit is located, which originated in the early Pliocene. It is a sequence of subtidal coastal siltyesandy sediments (Civis et al., 1987) with some levels of sandstones intercalated in the upper sections. With a weaker nature regarding its cementation, this unit constitutes the substrate on which most of the prehistoric site of Valencina de la Concepción is arranged. All of these geological units are slightly inclined southward by less than 2%. This geometrical arrangement causes all these materials to be topographically lower in that direction. Located to the north, the precambrian and palaeozoic terrains of the Iberian Massif have important associated mineral reservoirs. It must be considered that one of the most important mining districts on earth, the Iberian Pyrite Belt (Saez et al., 1996), spans to the NW of the Guadalquivir BasineIberian Massif boundary, in the vicinity of Valencina. So, in the metallurgic smelting quarter of Valencina, Nocete et al. (2008) identified ores from different sources areas: massive sulphides from the Pyrite Belt, at a distance of between 20 and 30 km (Marcoux, 1998; Sáez et al., 1989) and vein mineralizations from the Iberian Massif, north of Valencina at a distance of 30 km (Marcoux and Sáez, 1994). 4. Materials and methods
3. Geological setting of the “tholos de La Pastora” The prehistoric site of Valencina is located in the Guadalquivir Basin, near its northern boundary. Northward, there are outcrops of
From the architectural elements (slabs [floor and roof], lintels, jambs and walls) preserved in the corridor and chamber of the tholos of La Pastora (Fig. 4), bioerosion structures have been
Fig. 3. Views from inside the tholos of La Pastora. A. View towards the inside with the light of the setting sun of summer solstice illuminating the interior of the monument. Note the lithological difference between the walls (blocks of sandstone shale), roof (calcareous sandstone and granite slabs) and floor (calcareous sandstone slabs). B. View outward and upward, note the prominent calcareous sandstone slabs of the roof.
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Fig. 4. The tholos of La Pastora, view in longitudinal and plan section. The different lithologies that constitute the monument can be distinguished. The slabs of the roof are represented and ordered with numbers (see text) from the entrance in the plan view.
identified in 11 slabs of the roof (numbers 6, 9, 10, 12, 13, 15, 16, 17, 19, 23 and 25; Fig. 4), as well as in the lintel (number 14) and in the jambs of the chamber entrance, all of them of calcareous sandstone. The analysis and systematic identification of these structures were carried out according to direct observations, and their relative concentration on the visible faces from the chamber and the corridor. Specimens of the bivalves preserved within the perforations were collected for systematic identification, which monospecifically corresponded to Petricola lithophaga. Likewise, three samples of these bivalve shells were taken from two of the most affected slabs for radiocarbon dating (Fig. 4). Two samples were collected from one of these over two different faces of the rock. The radiocarbon analysis by AMS was performed in the “Centro Nacional de Aceleradores” (CNA) in Seville (Spain). Conventional radiocarbon ages have been converted to calibrated ages (cal yr BP) with the CALIB 5.0.1 program (Stuiver and Reimer, 1993), using the calibration dataset Marine04 (Hughen et al., 2004) for marine samples (Table 1). An additional regional marine reservoir correction (DR) of 135 20 yr was also applied for marine samples with ages 4900 BP, of 100 100 yr for marine samples with ages between 2500 and 4600 BP, and for the interval 4600e 4900 BP a DR value taken from the graphic corresponding to the conventional marine radiocarbon date closer to the one to be calibrated (Soares and Dias, 2006; Soares, 2008; Soares and Martins, 2010). Ages discussed below are expressed as the highest probable age of the 2s calibrated range (Table 1). Separately, samples were taken for petrographic study. A Nikon Eclipse LV-100 POL petrographic microscope connected to a Nikon DS-Fi1 camera, with a 5.24 megapixel 2/3 inch sensor, was used to obtain the images of thin layers. The camera is connected to an Intel Pentium 4 2.66 GHZ with the NIS-Elements image capture software. 5. Results 5.1. Petrography of the construction materials of the tholos In the tholos of La Pastora there are three different types of lithological materials: blocks of sandstone-shale, calcareous sandstone and granite slabs. There is also a block of diabase in the roof (roof slab number 5 in Fig. 4); however, regarding its exclusiveness
and the fact that it was placed above all the other slabs, its origin is doubtful and, therefore, it could most probably be the recent seal of an opening made at the time of its discovery. The walls of the corridor and part of the chamber consist of blocks or slabs of a rock formed by alternating bands of finegrained sandstone and shale (Fig. 5A). These slabs are mainly rhombohedric and decimetric in size. Petrographically, this sandstone is a well-sorted quartzarenite, essentially consisting of quartz grains and, in lower proportion, plagioclase, chlorite, white mica and opaque minerals, included in a matrix that is mostly sericitic (Fig. 5B). The shale shows similar mineralogical composition, with greater abundance of sericite. They come mainly from the materials of the Iberian Massif located further north of the tholos. Granite, which constitutes 22% of the roof slabs of the tholos of La Pastora (Fig. 5C), has equigranular, hypidiomorphic texture, with average grain size between 1 and 2 mm, and is composed essentially of quartz, plagioclase, alkali feldspar and minor biotite. The quartz and feldspar crystals show a characteristic micrographic texture (Fig. 5D). They include biotite, apatite, opaque minerals and zircon as accessory minerals, and as secondary minerals there are chlorite, which appears altering biotite crystals, and sericite, as a result of feldspar alteration. Petrographically, this rock is similar to the carboniferous granite described in the Northern Seville Mountains (De la Rosa, 1992), which also belong to the Iberian Massif. Finally, of the 27 roof slabs preserved, 74% of them and practically all the slabs of the floor, correspond to calcareous sandstone (Fig. 5E). They are classified as carbonated fossiliferous quartzarenite (Fig. 5F), with very fine grain size (around 0.1 mm), a good to very good degree of selection and essentially consist of quartz grains (from angular to subangular, around 60% of the total), carbonated fossil (foraminifera, gastropods and bivalves mostly, around 20%) and minor grains of plagioclase, white mica, tourmaline and opaque minerals. All these grains are included in a carbonated cement (20%). This sandstone comes from the described strata with the same composition located in the Transition Facies of the Guadalquivir Basin. The fundamental difference between this rock and the sandstone that makes up the walls of La Pastora is their mineralogical composition, the presence of Neogene fossils (Fig. 5F) and ichnofossils, and the carbonated cement.
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from the ichnotaxonomic perspective, they are related to ichnogenus: Gastrochaenolites, Caulustrepsis, Maeandropolydora and Entobia.
Fig. 5. Building rocks of the tholos of La Pastora. (A) Palaeozoic sandstone-shale, rocks of the corridor walls and the chamber. (B) Thin layer of the rock above, textural characteristics. (C) Palaeozoic granite, roof slab number 24. (D) Granite thin layer view showing its characteristic micrographic texture. (E) Neogene calcareous sandstone, roof slab number 13. (F) Thin layer of the rock above, texture and neogene marine microfossil (centre). (G) Field outcrop of Transition Facies: Sa: calcareous sandstone; Cl: clay.
5.2. Bioerosion traces in the calcareous sandstone The bioerosion traces are geologically recent and affect exclusively the calcareous sandstone of the late Miocene, used in the tholos as slabs, lintels and jambs (Fig. 6). They have been observed both in the faces (surfaces), seen from inside the tholos, and in some of the lateral faces of the rocky blocks (Fig. 6B). In general, the distribution of the bioerosion structures is irregular and they appear isolated, juxtaposed and/or cut by each other. The borings recognized show four morphological types. They correspond to the perforating activity of marine, lithophage, and endobenthic organisms (bivalves, polychaete annelids and clionid sponges), and
5.2.1. Bivalvia borings: Gastrochaenolites The most abundant bioerosion structures are those related to the activity of lithophage bivalves. In longitudinal section, they present clavate, drop or tear shape morphology (Fig. 7A), with a simpler opening, smaller than the main chamber. Their morphology in cross section varies between heart-shaped (Fig. 7B) and circular, and they are oriented perpendicularly to slightly inclined to the plane of colonization. The complete specimens usually preserve clavate shape morphology; other specimens are truncated near the base, like shallow, circular (Fig. 7E), cross sections (Santos and Mayoral, 2009). The maximum values obtained for diameter and depth are 1.5 and 8 cm, respectively. The concentration of bores observed in the rocks varies among slabs, the maximum concentration present in slab number 6 with 285 bores/m2 (Fig. 6A). The described structure is equivalent to the ichnogenus Gastrochaenolites Leymerie, 1842 (amended diagnosis in Donovan and Hensley, 2006) and it is interpreted as an inhabitation structure performed by suspension feeder, endolithic bivalves. Mytilidae, Gastrochaenidae, Pholadidae, Teredinidae, Myidae, Petricolidae, Tridacnidae, Arcidae and Clavagellidae are current families of boring bivalves that live mostly in rocky substrates (loose rocks or relatively wide surfaces) of shallow marine environments (Kelly and Bromley, 1984). Particularly, members of the family Gastrochaenidae, the genus Lithophaga (Mytilidae) and several subgenera of Clavagella (Clavagellidae), are boring bivalves restricted to calcareous substrates (Belaústegui et al., 2012). Other organisms could produce structures similar to those of Gastrochaenolites, such as gastropods, annelids or sipunculids (Bromley, 2004). In many of the clavate-shaped Gastrochaenolites, the producer bivalve was determined by the exceptional preservation of its valves. This bivalve belongs to the Petricola lithophaga Retzius species (Fig. 7C and D) (O. Veneroida, Fam. Petricolidae). On the other hand, the remains of the producer are never preserved in truncated Gastrochaenolites. In limestones of South Italy from the Pleistocene, Bromley and D’Alessandro (1987) define the ichnospecies Gastrochaenolites cor, which they directly associate with the activity of P. lithophaga; therefore, we can establish that clavateshaped Gastrochaenolites with or without preserved remains of P. lithophaga are classified at ichnospecific level as G. cor (Fig. 7A, B and C), while those Gastrochaenolites that are truncated as shallow circular sections (Fig. 7E) are established with an uncertain nomenclature: Gastrochaenolites isp. 5.2.2. Polychaete borings: Caulostrepsis and Maeandropolydora Two structure types were identified: i) Galleries as short and sinuous grooves (Fig. 7F), occasionally branched, 60 mm in maximum observed length and
