Stratigraphy, palaeoenvironments and model for the deposition of the Abdur Reef Limestone: context for an important archaeological site from the last interglacial on the Red Sea coast of Eritrea

Abstract

Stone tools discovered within uplifted marine terraces along the Red Sea coast of Eritrea at the Abdur Archaeological Site, dated to 125±7 ka (the last interglacial, marine isotope stage 5e), show that early humans occupied coastal areas by this time [Walter et al. (2000) Nature 405, 65–69]. In the present paper the stratigraphy, facies types and faunal composition from 25 measured sections of the tool-bearing Abdur Reef Limestone (ARL) are documented in detail and interpreted to provide a palaeoenvironmental context for the stone artefacts and a model for the deposition of the ARL. The ARL represents a complex marine terrace sequence. Erosional surfaces indicative of interrupted sedimentation are locally observed at two levels within the ARL. They subdivide the complex into three subunits, named 5e₁, 5e₂, and 5e₃, representing different stages of the marine isotope stage 5e sea level highstand, comprising six depositional phases (I–VI) of the ARL. Subunit 5e₁ begins with the initial transgression of the 5e sea level highstand leading to the deposition of widespread lag gravels on which rich oyster beds developed in shallow water (phase I). It further records rapid deepening accompanied by the deposition of low-energy carbonates with scarce corals (phase II), and later shoaling characterised by local development of a fringing reef tract in a sedimented environment (phase III). Subunit 5e₁ is capped locally by a burrowed hardground that is laterally equivalent to depositional discontinuities, interpreted as caused by a globally recognised mid-5e sea level low stand (phase IV). Extensive reef build-up in response to sea level rise and improved conditions for coral growth characterises subunit 5e₂ (phase V). A possible second sea level drop during the 5e highstand is inferred from the oyster-encrusted upper surface of subunit 5e₂. Subunit 5e₃ encompasses restricted coral patches that developed on the upper surface of the underlying subunit during the last stage of the 5e marine high stand (phase VI). Two different toolkits are found in the ARL. One consists of bifacial hand axes and cores of the Acheulian industry, typically associated with the oyster beds encrusted on the transgressive lag deposits. The other consists of Middle Stone Age (MSA)-type obsidian flakes and blades, mainly found in the nearshore and beach environments alongside debris from marine invertebrates and large land mammals. The distribution of these tools suggests that foraging activities of early humans varied with environmental setting. The Abdur Archaeological Site represents a late example of the Acheulian/MSA transition, seen as a benchmark for early modern human behaviour, and is, to date, the earliest well-dated example of early human adaptation to marine food resources.

Introduction

Stone tools within emerged reef terraces on the western shoreline of the Buri Peninsula, Red Sea coast of Eritrea, dated to the last interglacial, 125±7 ka, are the earliest well-dated evidence for human occupation of coastal marine environments (Walter et al., 2000). This archaeological site, termed the Abdur Archaeological Site, is located ca. 60 km southeast of the port town of Massawa near the village of Abdur on the eastern edge of the Gulf of Zula (Fig. 1).

Numerous bifacial hand axes of the Acheulian tradition were found in situ along with obsidian flakes and blades of the early Middle Stone Age (MSA) tradition in various facies of a complex of Pleistocene marine terraces, which we previously named the Abdur Reef Limestone (ARL) (Walter et al., 2000). The discovery of MSA artefacts within last interglacial reef terraces is unique and sheds important new light on human adaptive strategies and migration paths out of Africa (Stringer, 2000, Walter et al., 2000). From the abundance of edible resources found in proximity to these tools, the implication is that early humans were using them to harvest shallow marine food resources and possibly to butcher large land mammals on the ancient shoreline. Occurrences of Acheulian-type bifaces near raised beaches in the central Danakil Depression (Faure and Roubet, 1968) and along the coasts of the northern Red Sea (Montenat, 1986) and eastern Mediterranean (Fleisch and Sanlaville, 1974), suggest that this strategy may have evolved before the last interglacial, but evidence is still insufficient to confirm this hypothesis.

Together with the proof of coastal occupations at Klasies River Mouth in South Africa, dated to at least 116–90 ka (Grün and Stringer, 1991), our discoveries suggest that early humans were well adapted to exploiting coastal marine resources by the last interglacial period. This behaviour implies a significant change from traditional terrestrial feeding habits (e.g., around East African rivers and lakes), possibly in response to hyper-arid conditions and dwindling water resources inland caused by glacio-climatic stress (Walter et al., 2000, Weyhenmeyer, 2000, Faure et al., 2002). Climatically driven human adaptations to coastal marine environments may have led to continental shores becoming routes for dispersal out of Africa and migration along the shorelines of Arabia into southeast Asia during sea level low stands (Stringer, 2000, Walter et al., 2000).

Available fossil hominid evidence from East Africa (Clark, 1988) and recent mitochondrial DNA studies (Ingman et al., 2000) predict an origin for anatomically modern humans in sub-Saharan Africa at 170±50 ka ago. Hence, the Middle to Late Pleistocene is a crucial period for ultimately resolving debates about the origin and evolution of our species. This period is not well represented in the terrestrial sedimentary record of East Africa (Clark, 1988). Uplifted Pleistocene marine terraces, like those around Abdur, are a novel and untapped geological resource for the study of human evolution and behaviour. Last interglacial (marine isotope stage, MIS, 5e) reef terraces are among the commonest landforms of the Red Sea coastline (Taviani, 1998, and references therein), while older interglacial terraces (MIS 7, 9, and 11) are exposed less frequently.

The purpose of this paper is to provide a palaeoenvironmental context for the artefacts that were discovered in the last interglacial reef terraces at Abdur. Detailed information about the stratigraphy, facies types, and faunal composition of the ARL is presented and interpreted to provide a model for the deposition of the reef terraces near Abdur during the MIS 5e sea level highstand. Furthermore, the geologic setting and distribution of different types of stone tools discovered in these terraces are discussed in the context of landscape evolution along this portion of the Red Sea coast. Such details are critical for evaluating the palaeoecological history of this important archaeological site, as well as for guiding future explorations – at Abdur and elsewhere along the coast of the Red Sea – in search of artefacts and fossils relating to the origin, evolution, and migration paths of modern humans. The geologic and structural setting of the ARL and the background of this research project are discussed in more detail by Walter et al. (2000) and Ghebretensae (2002).