Marine bioerosion in rocks of the prehistoric tholos of La Pastora (Valencina de la Concepción, Seville, Spain): archaeological and palaeoenvironmental implications

Abstract

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 (2830–2450 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.