Geologic setting of the Abdur Archaeological Site on the Red Sea coast of Eritrea, Africa

摘要

The Abdur Archaeological Site was defined initially by the discovery and dating (125 +/- ka by Ur/Th) of in situ stone tools within uplifted marine terrace deposits located along the southern Red Sea coast of Eritrea, near the small village of Abdur. These tools represent some of the earliest well-dated evidence for human occupation of coastal marine environments. The site is located on the Buri Peninsula along the eastern shoreline of the Gulf of Zula and covers an area of approximately 7 km by 1 km. Three main stratigraphic units are defined and discussed from oldest to youngest: (1) The Buri Formation is defined herein as a sequence of brackish-freshwater (estuarine) and fluvial-deltaic sediments consisting of mudstones, siltstones, sandstones, conglomerates and limestones with ash and pumice beds. Ar-Ar dating of pumice and tephras puts the time of deposition of this unit from about 0.91 to 0.72 Ma (Early-Middle Pleistocene). Several stone tools were discovered in the Buri Formation, indicating early human occupation of a coastal environment during the Early to Middle Pleistocene. (2) The Abdur Volcanic Complex (AVC) is a small basaltic shield complex that forms the highlands along the eastern part of the Abdur Site and extends to the north and south of the area. Basalts from this center were dated from 2.12 to 0.17 Ma, indicating that the volcanic complex has been tectonically and magmatically active prior to, during and after deposition of the Buri Formation. (3) The Abdur Reef Limestone (ARL) is the remnant of a shallow marine reef system deposited approximately 125 ka (last glacial highstand, isotope stage 5e) along the margins of the Abdur volcanic highlands. The ARL consists of a basal transgressive lag deposit overlain by extensive buildups of mollusks, echinoderms, bioclastic sands and corals up to 11 m thick. Numerous stone tools in the ARL fall into two tool kits, bifacial hand axes of the Acheulian industry and Middle Stone Age-type (MSA) obsidian flakes and blades. Their distribution suggests that foraging activities of early humans varied with environmental setting. The Buri Formation has been folded and faulted prior to deposition of the ARL, with dips as high as 36° and vertical locally along fault zones. The ARL has been uplifted up to 8-15 m and tilted l°-5° in a seaward direction. The area is cut by numerous faults, part of a regional NNW-trending fault system. Occurrences of Buri Formation equivalents to the north along the Buri Peninsula as well as in the area southwest of the Alid volcano suggest the area west of the escarpment was a broad lowland characterized by rivers, coastal lakes, and estuaries along the Red Sea coast during the Early to Middle Pleistocene. Reefs equivalent to the ARL covered large parts of the adjacent Buri Peninsula, the north end of Mt. Ghedem across the Gulf of Zula, and the Dahlak Archipelago to the north.