Pliocene trace fossils from oyster substrates in the Nijar Basin, Betic Cordillera, southern Spain
Introduction
The Almería-Níjar Basin is an intermontane depression, developed marginal to the Mediterranean (Aguirre, 2017). Its rocks range in age from mid Miocene to Quaternary, and comprise a range of sedimentary and volcanic facies associated with events at the western end of the Alpine belt. The facies, faunas (especially the microfaunas) and the stratigraphy are known in some detail (e.g., Aguirre, 1998), as is the tectonic setting of the region (see, for example, Rutter et al., 2012). Recent palaeontological studies have focussed on the ecology of the early Pliocene transgression through the development of bored surfaces of Mesozoic limestones (Aguirre et al., 2017), and the description and interpretation of hard-substrate communities in the south-west part of the basin (Aguirre and Jiménez, 1997). The lower Pliocene bioclastic sandstones overlie, unconformably, the older Neogene units (normally the Messinian) across the Cenozoic basins in southern Spain (Braga et al., 2006). The sandstones, deposited in an open embayment, are locally fossiliferous and dominated by shelly benthos, commonly characterised by oysters and scallops forming shell beds. The shells are commonly bored and encrusted, each hosting a microecosystem of both epi- and infauna.
The Pliocene crops out on the higher ground around the village of El Argamasón, within the eastern part of the Almería-Níjar Basin. The thick succession of bioclastic sandstones generally contains complete, but disarticulated, shells of the oyster Saccostrea cucullata (Born, 1778) and the scallop Amusium crustatum (Bronn, 1827). The oyster shells are bored and encrusted and these commensal products are the focus of this paper.
Section snippets
Geological setting
The Almería-Nijar Basin is an elongated depression, orientated SW–NE, developed on the Internal Zone of the Alpine Betic Cordillera and filled by Serravallian to Quaternary deposits (Serrano, 1990; Fig. 1 herein). The Neogene, post-Alpine orogenic evolution of the Betic Cordillera is dominated by strike-slip tectonics and is related to the opening of the Alboran Sea (see Lonergan and White, 1997; Vera, 2001). In this context, several rapidly subsiding, narrow and elongated tectonic depressions
Pliocene sedimentary rocks (Cuevas Formation)
Pliocene strata have the widest areal distribution in the Nijar Basin (Fig. 2). The Pliocene sedimentary rocks unconformably overlie Upper Messinian sedimentary (Yesares and Feos formations), and older metamorphic and volcanic basement rocks of the Carboneras Fault Zone (Aguirre, 1998; Braga et al., 2003; Martín et al., 2004; Omodeo Salé et al., 2012; Sendra et al., 2019). The Pliocene Cuevas Formation is the focus of this study (Fig. 2). It is associated with a sharp lithological change from
Fossil and trace fossil assemblages
Burrows in the sediments are rare and where they have been identified are faintly preserved vertical indications of Skolithos isp. reaching a maximum of 100 mm in length and between 5 and 10 mm in diameter. They only occur in the calcarenite beds. Thin-shelled bivalves such as Amusium cristatum and other pectinids are well-preserved in the calcarenites, and abundant broken shelly debris can be found throughout all of the calcarenites (Fig. 4A, B). Oyster shells, Saccostrea cucullata (Born), are
Locality and material
The material was selected from many 10s of shells both in situ and loose at or near the summit of an unnamed hill adjacent to the Río Alías (Fig. 2). Here the Pliocene strata are sub-horizontal and are exposed in two spurs separated by a col. Each shell was examined and 20 of the best-preserved specimens were transported back to Durham, and eight of those chosen for further investigation. The sample of shells was specifically selected for shell borings and encrustations although most shells
Systematic ichnology
In the eight free valves of oysters considered herein six ichnotaxa were identified (Fig. 5, Fig. 6, Fig. 7C–E; Table 1). Additionally to these borings, encrusting invertebrate shells attached to the oyster valves include bryozoans, the acorn barnacle Balanus sp. and cementing bivalves, commonly very incomplete, but most likely juvenile oysters (Fig. 7A, B; Table 1). These body fossils are not discussed further herein.
Ichnogenus Caulostrepsis Clarke, 1908
Type ichnospecies: Caulostrepsis taeniola
Discussion
Many tens of oyster shells occur within this part of the Cueva Formation (see above). The eight selected and well-preserved free valves of the oyster Saccostrea cucullata (Born), specimens RGM.1320202 to 1320209, are considered in the present study. Penetrating these valves were six ichnotaxa (Fig. 5, Fig. 6, Fig. 7C–E; Table 1): Caulostrepsis taeniola Clarke; Entobia isp.; Gastrochaenolites isp. aff. G. lapidicus Kelly and Bromley; Maeandropolydora isp. cf. M. sulcans Voigt; Oichnus
Declaration of competing interest
The authors declare no conflicts of interest.
Acknowledgements
Abigail King thanks Van Mildert College for receipt of a summer research internship. DATH and SJJ thank Ken McCaffrey for stimulating discussions in the field. We thank Alfred Uchman for his comments on a previous draft and making us aware of Łaska et al., while in press. We thank our reviewers, Alfred Uchman and an anonymous referee, for their comments.
References (61)
- et al.
Testing models for the Messinian salinity crisis: the Messinian record in Almería, SE Spain
Sedimentary Geology
(2006) Determination of a Late Miocene rocky palaeoshore by bioerosion trace fossils from the Bozcaada Island, Çanakkale, Turkey
Comptes Rendus Palevol
(2012)- et al.
Neoichnology and implications for stratigraphy of reworked Upper Oligocene oysters, Antigua, West Indies
Proceedings of the Geologists' Association
(2014) - et al.
Late Pliocene benthic foraminifera and mollusks from the Atsipades Section, Central Crete; palaeoecological distribution and use in palaeoenvironmental assessment
Geobios
(2005) - et al.
The Messinian of the Nijar Basin (SE Spain): sedimentation, depositional environments and paleogeographic evolution
Sedimentary Geology
(2003) - et al.
Late Neogene-Recent uplift of the Cabo de Gata volcanic province, Almería, SE Spain
Geomorphology
(2003) - et al.
Structure and geological history of the Carboneras Fault Zone, SE Spain: part of a stretching transform fault system
Journal of Structural Geology
(2012) - et al.
Neogene planktonic foraminiferal biostratigraphy and evolution: equatorial to subantarctic, South Pacific
Marine Micropaleontology
(1981) El Plioceno del SE de la Península Ibérica (Provincia de Almería). Síntesis estratigráfica, sedimentaria, bioestratigráfica y paleogeográfica
Revista de la Sociedad Geológica de España
(1998)Amusium cristatum shell beds in the lower Pliocene deposits of Almería–Níjar Basin (SE Spain)
Spanish Journal of Palaeontology
(2017)