Figure 3: Quaternary formations of the lower Isser valley (Chemlal 1983)
Although there are several clues to the possible origin of knapped rocks in the scientific literature devoted to Algerian prehistory, systematic research into the precise origin of lithic raw material outcrops has not yet been carried out. For the study area, it was therefore necessary to start a field survey to verify and complete the information found in the geological literature at both local and regional levels.
The inventory of petrographic sources of the study area contains evidence based mainly on macroscopic and microscopic descriptions of lithic material. The purpose of this inventory is to help to locate raw material outcrops. The descriptions listed in a data sheet will be used to develop a reference 'lithothèque'.
The late Quaternary deposits of northern Algeria have been the subject of numerous studies and many attempts have been made to establish terrace chronologies for parts of the country, including the Lower Isser valley (de Lamothe 1899; Vita Finzi 1967). According to the analysis of the various Wadi Isser alluvial deposits in terms of fluvial hydraulics, as well as the tectonic examination, it is well known that the recurrent tectonic activity of the Thénia fault, occurring ever since the Villafranchian and continuing until the present day, is responsible for the evolving local palaeogeography (Boudiaf 1996; Chemlal 1983; Déverchère et al. 2005). This recurring tectonic activity has been associated with the accumulation of coarse gravels, which are constituents of seven alluvial terraces with altitude decreasing from west to east (Fig. 3).
These terraces are evidence for the successive displacement of Wadi Isser stream beds occurring during the Quaternary. The initial palaeoflow of Wadi Isser was not far from Courbet-Marine, but has since moved eastwards. The first terrace, at 200m elevation, forming the Ouled El Bor plateau is, then, a local source of various lithic raw materials and highlights one of the factors influencing the choice of this site.
Figure 3: Quaternary formations of the lower Isser valley (Chemlal 1983)
In this article, the term flint is used to describe varieties of chert containing both microcrystalline and microfibrous quartz with conchoidal fracture, occurring in chalk and marly limestone formations (Cailleux 1929). The local geological structure is marked by sedimentary formations and Precambrian metamorphic rocks, crossed locally by acid and basic intrusions. This mineralogical diversity was exploited by the Iberomaurusians, who had knapped several types of rocks available both locally and regionally (Fig. 4).
The field survey into this region revealed several siliceous rock outcrops of different ages. Flint pebbles are available in alluvial terraces, such as Ouled El Bor, situated about 3.5km to the south of Courbet-Marine. Concerning the primary outcrops, Jurassic flint appears in the north flank of Djebel Bou Zegza as discontinuous beds within folded chalk formation of 10m thickness belonging to Middle and Upper Lias. Moreover, lower Cretaceous flint occurs as benches 200mm thick within the bedded chalk formation located south of Djebel Bou Zegza.
Eocene flint primary outcrops are localised at about 30km south of Courbet-Marine. This flint occurs as large nodules into Eocene marl formations, found to a depth of ten metres, along the road from Keddara to Palestro. Nevertheless, the nearest outcrops are those of olistrostrome conglomerates, termed 'Oligo-Miocene Kabyle' (Tefiani 1967), which were formed during lower Burdigalian (Middle Miocene) and from which material came from the underlying tellian and flysch formations. The coating of these conglomerates, including brown flint, is constituted of coarse greso-ferruginous cement. Another variety of Eocene flint occurs as black nodules in the marl and chalk formations encountered at Dellys locality, about 15km to the east of the Courbet-Marine site. Nevertheless, this black flint was not among the lithic pieces found at this site.
Figure 4: Map of local and regional geological formations
Macroscopic characterisation of studied flint was done using a binocular microscope (GX 16) on both archaeological and geological material. This examination considered the association of different criteria such as colour, zonation, texture, opacity, grain and inclusions. The description of textural characters refers to the carbonate rocks classification established by Dunham (1962), which was adapted and used by many archaeologists (Demars 1982; Féblot-Augustins 1997; Bressy 2003). Flint varieties were grouped into five categories based on their textural and structural characteristics. Furthermore, this classification was supported by other methods of characterisation, such as petrographic and X-ray diffraction analyses. The flint macroscopic features are mainly as follows:
Apart from the studied flint morphological features, optical microscope techniques provide further information from which we can deduce sedimentary environments and the mechanism and rate of silicification. The analysis of thin-sections taken from 15 samples selected from different flint types of the study area was carried out in the laboratory of stratigraphy at the Centre de Recherches et de Développement at Boumerdès (Algiers). The results obtained confirm the existence of a wide range of microfacies (Fig. 5). This evidence suggests that the silica precipitation conditions were neither uniform nor homogeneous, and occurred on various original matrices.
The samples are represented by veined and whitish flint types. Observations taken from veined flint thin-sections indicate a silicified micritic-bioclastic matrix with ferruginous-microgranular quartz. Small-sized benthic foraminifera, of which Involutina liasica is well preserved (Fig. 5A, see frame), are numerous. This micro-texture indicates a deep marine environment belonging to the Middle Lias.
Thin-section from whitish flint shows patterns of calcitic initial matrix with microgranular quartz appearing locally. Apart from rare bivalves and relics of lamellibranches, shellfish coprolites, in which only holes are silicified, are well preserved. It is likely that this sample came from what was deposited in an aquatic setting such as a beach environment of the Upper Lias (Fig. 5B).
Banded flint micro-texture is characterised by an oxidised silicified-micritic matrix rich in radiolarians filled with chalcedony, sponge spicules and ostracod valves. However, recrystallised foraminifera are not common (Fig. 5C). This silicification may have occurred in a deeper marine environment belonging to Valanginian facies.
Brown flint initial matrix indicates a patchy silicification of bedded ferruginous clay (Fig. 5D-F). This flint is an oxidised facies characterised by the occurrence of hematite and ankerite crystals as well as dolomite rhombs and carbonate. Moreover, the presence of planktonic foraminifera relics with ostracod valve fragments entirely silicified is well attested. This flint texture is characteristic of Eocene facies linked to old deposits of deep marine environments.
Figure 5: Thin-sections of flint samples
The diffraction measurements were performed on a diffractometer X'PERT Pro MPD at room temperature using a monochromatic beam of copper (wavelength 0.15405 nm). All data points were recorded between 10 and 80°2θ, with 0.02° of step size (scanning rate 0.04°/s). Figure 6 illustrates X-ray diagrams of flint mineral powder. The analysis using a database PDF-2 [International Center of Diffraction Data (ICDD) PDF-2 release 2004, licence du Centre de recherche Nucléaire de Birine (C.R.N.B)], shows that the brown flint has a higher ratio of dolomite with very small amount of calcite then the other samples. Further investigation by ICP/MS (Inductively Coupled Plasma Mass Spectrometry) should be undertaken to determine trace elements.
Figure 6: X-ray diffraction analysis of Lias and Eocene flint samples
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Last updated: Wed Jul 29 2009