El Kherba is situated in the Ain Hanech area on the edge of the eastern Algerian High Plateaus in the Sétif Province (Figure 1A) 7 km northwest of the city of El Eulma at the southern limit of the Neogene Beni Fouda sub-basin in a sedimentary outcrop cut by the deep ravine of the intermittent Oued Boucherit (895 m a.s.l.). The archeological site is located at 960–970 m a.s.l. on the northeast–southwest escarpment of Guelta Zerga (100 m higher) created by the fluvial networks of Oued Deheb that flows toward the Mediterranean. Morphostratigraphically, the site represents the end of the filling of the tertiary continental Beni Fouda sub-basin by fluvial channels carrying sediments from the margins of the Tellean formations made of gravels, sands, and floodplain muds, forming alluvial plains draining to the southwest toward the El Eulma area.

The site was discovered in 1992 following new investigations launched in the Ain Hanech research area (Sahnouni, 1998; Sahnouni and de Heinzelin, 1998). Large-scale excavations have been carried out at El Kherba yielding a rich and diverse Early Pleistocene fauna associated with Oldowan stone tools. The fauna includes Gasteropoda indet., Mauremys leprosa, Crocodylia indet., Canis primaevus, Crocuta crocuta, Panthera sp., Felis? Lagomorpha, Elephas moghrebiensis, Ceratotherium mauritanicum, Equus tabeti, Equus aff. oldowayensis, Hippopotamus gorgops, Kolpochoerus heseloni, Giraffa pomeli, Sivatherium maurusium, Pelorovis howelli, Gazella pomeli, and Numidocapra crassicornis (Sahnouni and Van der Made, 2009: Sahnouni et al., 2018). Overall, the fauna suggests a more or less open and dry landscape. However, the occurrence of a permanent body of water is indicated by the remains of crocodile, aquatic turtle, and hippopotamus. The faunal paleoecological reconstruction is supported by isotopic evidence of pedogenic carbonates indicative of a savanna ecosystem with expanding grassland vegetation and increasing aridification through time at El Kherba (Sahnouni et al., 2011).

The stone tool assemblages are made primarily of limestone and flint. They comprise cores and core-forms, whole flakes, retouched pieces, and fragments (Figure 2). The cores and core-forms include unifacial and bifacial choppers, polyhedrons, subspheroids, spheroids, and simple cores. The technological and typological characters show that the assemblages of Oldowan are very similar to those from Olduvai Upper Bed I and Lower Bed II in Tanzania, especially in terms of flaking patterns and resultant artifact forms (Sahnouni, 1998; 2006). Evidence, of usewear traces on several artifacts and of cut-marked and hammerstone-percussed bones, indicates that El Kherba was a place for intense subsistence activities by early hominins (Figure 2), including disarticulating and removing meat and breaking bones of large mammals to extract marrow (Sahnouni et al., 2013).

The Sétif region is part of the Maghrebid chain formed during the alpine orogeny. It is composed mainly of marl lands of Middle Cretaceous to Neogene age (Figure 1C) (Djenba, 2015). The El-Eulma area, located 25 km east of Sétif, is a vast depression composed of six sub-basins (Demdoum, 2010), including the Beni Fouda sub-basin that contains the Ain Boucherit–Ain Hanech Plio-Pleistocene deposits (Duval et al., 2021). The deposits of this basin are of variable facies that consist of red clay fluvio-lacustrine sediments overlaid by heterogeneous beds of several tens of meters thick gray–yellow marls containing limestone concretions and interbedded pebbles, gravels, and sands (Demdoum, 2010). This sequence is often covered with a massive calcrete deposit that can reach 3 m of thickness in some places (Djenba, 2015).

The Oldowan site of El Kherba forms part of the Ain Boucherit–Ain Hanech Plio-Pleistocene sequence, which consists of a succession of paleontological and archeological deposits estimated to be between 4 and 1.67 Ma in age (Sahnouni et al., 2018; Duval et al., 2021). Other Oldowan localities nearby include Ain Boucherit Lower (AB-Lw), Ain Boucherit Upper (AB-Up), and Ain Hanech (Figure 1B). All the Oldowan deposits are encased in the 29 m thick Ain Hanech formation that comprises six stratigraphic members named, from bottom to top, P, Q, R, S, T, and U (Figure 1D) of fluvial origin made of alternating gravels and sandstone with mudstone (Sahnouni and de Heinzelin 1998; Sahnouni et al., 2018). El Kherba deposits were formed in Member T and slightly in the uppermost part of Member S (Figure 3). Member T is 4 m thick and is mainly a muddy unit, light brown (7.5 YR 6/4) or pink (7.5 YR 7/4) in color with carbonated nodulations in its final 2 m. Based on magnetostratigraphy, ESR dating, and biochronology of large mammals (proboscideans, equids, and suids), Ain Hanech and El Kherba deposits date to 1.78 Ma (Parés et al., 2014). The AB-Lw and AB-Up Oldowan deposits, situated lower in the stratigraphy, date to 2.44 Ma and 1.92 Ma, respectively (Sahnouni et al., 2018; Duval et al., 2021).

El Kherba stratigraphy is studied in three profiles within the excavation set out altimetrically so that the site stratigraphy can be described in its entirety from top to base. These include deposits in quadrant T29 of Block II (1.90 m thick), in quadrant X32 of Block II (0.90 m thick), and in quadrants N32–N33–N34–N35 (2 m thick) of Block I (Figure 3, Supplementary Figure S2). The correlation of these three profiles allowed us to compose a synthetic stratigraphic log of 4.80 m thickness in which 11 layers have been recognized (Figure 3) based on sediment texture, consistency, and color. Additional criteria were also considered in discerning the layers including proportion and granulometry of coarse elements and their nature and degree of wear and alteration, paleontological content, and, if necessary, crusting, bioturbations, and metallic inclusions (iron and manganese). The sedimentary features of the successive layers (TI–TXI), from top to bottom, are as follows.

Layer TI consists of pale brown (10 YR 8/3) homogeneous carbonated clayey silt, which is variably thick (19–27 cm), organized in sheets, friable, and strongly disturbed by rootlets from overlying colluviums; fissures that are horizontally oriented; few small-sized limestone gravels (∼3 cm in diameter); and rare black flint granules. Layer TII is variably thick (15–30 cm) consisting of compact and indurated pale brown clayey silt (10 YR 8/3) and few flint granules. Layer TIII consists of 30 cm thick indurated white gray (7.5 R 7/0) clayey silt and some medium-sized (3–6 cm) limestone pebbles. Horizontal and vertical cracks caused its disintegration in thin plates, and cracked clay clods of various sizes (8–10 cm) appear at the base. Layer TIV is 1.10 m thick pink (7.5 YR 8/4) clayey silt with fine sands in its base. The color of the sediment is heterogeneous due to CaCO₃ spots of variable shapes and sizes (1–5 cm), the consistency is friable, and the texture is polyhedral, flint granules. Layer TV is variably thick (10–40 cm) and bowl shaped, which consists of light reddish brown clayey silt (5 YR 6/3) with some gray clay blots (7.5 R 7/0). Fine sand is more important than in upper layers; the consistency is friable and the structure is in prismatic blocks due to vertical cracks; presence of carbonated nodules (∼1 cm in diameter); this layer is poor in coarse elements. Layer TVI is thicker on the distal ends (40 and 52 cm) than in the middle (23 cm), consists of gray clayey silt (7.5 YR 7/0), has low proportion of sand, is friable and in prismatic blocks, is dotted with CaCO₃ spots and with metal oxyhydroxides in the form of dendrites, and has rare coarse elements. Layer TVII is 40 cm thick. It consists of light reddish brown clayey silt (5 YR 6/3) and gray clay clods (7.5 R 7/0) with fine sand, is friable and in prismatic blocks, and includes metal oxyhydroxides as veneers and carbonate spots. This layer discontinues abruptly toward the west due to a phase of erosion caused probably by a brief intensity storm event. Layer TVIII is 0.45 m thick sandy silt with a large proportion of medium-sized (3–6 cm) and large-sized (6–10 cm) pebbles and granules representing 50% relative to the fine-grained fraction. It consists of small polyhedral blocks due to numerous vertical cracks and is friable and covered with metal oxyhydroxides in the form of spots and 4–5 cm thick layers, giving it variegated color made of gray (7.5 R 7/0) and reddish brown (5 YR 6/3). Layer TIX is 0.25 m thick gray (7.5 R 7/0) and reddish brown (5 YR 6/3) variegated clayey silt with a structure organized in polyhedral aggregates. It is friable and of heterogeneous color due to metal oxyhydroxides in the form of centimetric beds and CaCO₃ spots and contains rare granules. Layer TX is a light gray clayey silt (2.5 Y 7/0) extending over a homogeneous thickness (0.32 m), with the presence of Mn beds (∼10 cm thick) and whitish CaCO₃ spots. The sediment is indurated and fissured, and abundant small (1–3 cm) limestone pebbles (some of which are altered) are primarily concentrated at the bottom of the layer representing more than 70% of the deposit. Layer TXI is 0.90 m thick friable clayey silt breaking down into small polyhedral blocks due to vertical and horizontal cracks. Its color is light reddish brown (5 YR 6/3) and gray (7.5 R 7/0) with some CaCO₃ spots but in a lesser proportion than in the upper layers; few granules are concentrated in the middle of the layer.

In terms of associated archeological remains, fossil bones and stone artifacts are contained throughout the stratigraphy in three distinct levels, namely, from bottom to top, C, B, and A (Figure 3) (Sahnouni et al., 2002). They are encased in layer TX for level C; in layers TVIII and TVII for level B; and in layers TVI, TV, and the lower half of TIV for level A. The thickness of levels C, B, and A is variable including 0.55 m (depth, −483 to −538 cm), 0.62 m (depth, −398 to −460 cm), and 1.27 m (depth, −260 to −387 cm), respectively. Level B is the thinnest, yet it encases the bulk of the archeological material (18.80 finds/m³)) (Figure 3).

Of fifteen samples (Figure 3), thirteen unconsolidated sediments were analyzed. Each raw sample was previously dried at 40°C before quartering. The coarse elements (gravels and pebbles) were separated by water sieving to retain only the particles <2 mm (coarse and fine sands, silts, and clays). The latter were subject to a sedimentological analysis including grain size analysis, carbonate quantification, and pH measurement. However, a brief microscopic observation of the thin sections prior to the particle size analysis revealed the presence of micritic aggregates and localized areas slightly to strongly cemented by micritic calcite. Therefore, it was necessary to use a solution of hydrogen peroxide to destroy the organic matter and 10% diluted hydrochloric acid to remove the carbonates. Then, sodium hexametaphosphate at 5 g/l was added to the sediment that was ultrasonicated for 30 s. The grain size analysis of the decarbonated sediments was carried out wet with the granulometer Malvern Mastersizer 2000 laser diffraction particle size analyzer, using the wet technique because of the silty clayey texture of the samples. In our protocol, we favored the Fraunhofer approximation because it allows for the analysis of heterogeneous sediments with different refractive indices. To optimize the reproducibility of the results, four tests were undertaken on the same sample. The results show no variation in the data and display a perfect superposition of the graphs. The average of the different results was retained. The curve and the grain size parameters were acquired in digital form. The grain size distribution of the samples was processed with the GRADISTAT software (Blott and Pye, 2006).

For estimating the carbonate proportions by infrared spectroscopy, we favored the pellet preparation method because it allows for a quantitative analysis. The employed procedure is that by Fröhlich and Gendron-Badou (2002) that includes the following steps: 1) mechanical grinding of the raw sediment with agate mortar until particles <2 µm are obtained; 2) taking 1 g sample composed of 2.5 mg of ground sediment and 997.5 mg of KBr (potassium bromide), with 10–5 g accuracy; 3) homogenization of the mixture with agate mortar during 5 min; 4) pelleting of 300 mg of the mixture under 10 to 11 tonnes of vacuum pressure during 1:30 min; 5) steaming the pellets for 24 h at 100°C to evacuate atmospheric water; and 6) measuring with an infrared spectrometer.

The pH measurement is realized with a pH meter comprising an electrode and an electronic box. 20 g of <2 mm diameter raw sediment from each sample is diluted in 50 ml of distilled water. The solution is then subjected to magnetic stirring for a few minutes. After calibrating the pH meter with neutral (scale 7), acid (scale 4), and then basic (scale 9) buffer solutions, the electrode is rinsed with distilled water and then introduced into the solution. The pH value is directly displayed on the electronic box screen.

Fifteen micromorphological samples were collected throughout the El Kherba stratigraphic profile (Figure 3). The sampling procedure used is that of plaster blocks described by Courty and Fédoroff (2002), which prevents any disintegration of the sediment during its transport. In the field, the procedure consists of extracting samples in the form of blocks measuring 15 × 10 × 10 cm. The blocks can contain one or more stratigraphic layers depending on the thickness of the latter. All the visible faces of the block were plastered and labeled by site and number, stratigraphic position, and orientation. Once hardened, the blocks were removed, and their internal faces were plastered.

In the laboratory, the process developed by Guilloré (1985) was used to produce thin micromorphological sections. The blocks were opened on only one side, oven-dried at 25°C for 72 h, and cell impregnated with a mixture of polyester resin fluidized with styrene in a proportion of two-thirds/one-third, to which a few milliliters of the Butanox catalyst and 2–3 drops of an accelerator were added. They were indurated in an airy fume cupboard until the resin was completely consolidated. Each block was cut lengthwise into two parts using a diamond saw. While one part is archived as copy, the other part is cut into a 60 mm × 120 mm x 10 mm slab and mounted on a slide glass with polyester resin and thinned to 35–40 µm (for mineral transparency). A total of thirty thin micromorphological sections were made. They were first examined under a petroscope (which offers a wide field vision of the sample constituents) and then under the polarizing microscope at ×1.6, ×5, ×10, ×20, and ×63 magnifications. Plane-polarized light (PPL) and crossed-polarized light (XPL) were used for detailed description of the sediment microstructure and for taking microphotographs at various scales.

The particle size analysis reveals that El Kherba sediments are made up of a high proportion of silt–clay (80%). Sands, averaging 20%, are represented exclusively by fine sands (99%), while clays are significantly less abundant than silts (23 versus 57%) (Figure 4). The latter are relatively stable throughout the stratigraphy, while the sands experience significant variations between the lower part of the stratigraphy (from base up to layer VIII) where they vary between 17 and 43% and the upper part (from layer VIII to the top) where they decrease to 8%. This reduction occurs mainly at the expense of clays, which gradually increase to reach 30% in the upper layers due to a change in the deposition dynamics. Based on Blott and Pye (2006) criteria, the grain size curves are bimodal in eleven samples and trimodal in the samples KHs 2 (layer TII) and KHs 10 (layer TVII). The kurtosis index ranges between 0.65 and 1.09. The highest values are in the lower part (from the base to −370 cm) showing mesokurtic and platykurtic curves. The lowest values are found toward the top part forming mostly platykurtic curves (except sample KHs 2 that is very platykurtic). According to Miskovsky and Debard (2002), all these curves reflect a mixture of one or more sediment populations. Asymmetry (or skewness) shows a tendency toward finest particles in the first half of the infilling and toward fine particles in the second half (Figure 4). Only sample KHs 1 presents a symmetrical curve. The sorting index, whose values are between 0.170 and 0.223, also subdivides the profile into two sets (Figure 4) and indicates a good sediment classification with particles increasingly calibrated toward the top. The ratio of the asymmetry (skewness) and the mean of the grains, on the scale of ɸ (Figure 5A), shows that all the samples, except KHs 1, have positive values. According to Friedman (1961), they correspond to sands of dunes or rivers. Yet, as these values are <1.49 ɸ, a dune origin is ruled out. The ratio of skewness and kurtosis also shows positive values (1.79–3.08 ɸ) and a wider distribution of kurtosis points (Figure 5B). These values also reflect sand from dunes or rivers (Friedman, 1961). Solely KHs 1 has a negative value, which is a sample from a level relatively rich in coarse sands (Figure 4). However, the composition of the sediments, consisting predominantly of fine particles (<40 µm), the notable proportion of fine sands (up to 43%), poor particles’ sorting, and the presence of stratification suggest hydric deposition of the sediments. However, the presence in some samples of more than one mode does not rule out an eolian origin of the sands.

The carbonates are nodules of a few centimeters in diameter, or in the form of cement. Infrared spectroscopy analysis of the raw sediments indicates that their proportion varies between 11 and 30% with the lower part of the deposits being less supplied (Figure 4). Their evolution, which is close to that of fine sands in the lower half of the infilling, becomes, thereafter, parallel to that of the clays in the upper half. This concomitant evolution of carbonates and sands in the lower layers shows a common detritic origin of the two fractions, while in the upper layers, they are in a secondary position following the recrystallization of the calcite within the mass (see Micromorphological Analysis).

The sediments have an alkaline pH with minor values (7.66–7.94) in the lower levels of the infilling than in the upper levels (8.09–8.59) (Figure 4). Their evolution is related to carbonates, which allows carbonate stability (Karkanas and Goldberg, 2019) and creates favorable conditions for a good preservation of faunal remains (Stephan, 2000; Berna et al., 2004; Karkanas, 2010). Despite the alkalinity of the sediments, we note the presence of some organic residues, which require a humid environment and an acidic pH ~ 3 (Fraysse et al., 2004; Shahack-Gross et al., 2004). This antinomy is explained by the contribution of water rich in calcium carbonate transforming the acidic medium into a basic medium (Shahack-Gross et al., 2004) and would have contributed significantly to the decomposition of organic matter (Karkanas, et al., 2000; Fraysse et al., 2009).

To sum up, El Kherba grain size data reveal a stability in the evolution of the proportion of silts and variations in those of sands and clays depending on the depth of the stratigraphic profile. These differences allow us to subdivide the profile into two sedimentation phases. The first phase, from the base to −370 cm, is somewhat high-energy deposit as it is formed of a coarse fraction of pebbles and granules along with a fine fraction comprising 56% silt and 24% sand. The second phase, which extends to the top of the profile, is relatively stable and characterized by more homogeneous sediments and a silty clayey texture (61% silt and 30% clay). The different parameters (kurtosis, skewness, sorting) confirm this subdivision with distinct values in the two parts of El Kherba stratigraphy suggesting two modes of deposition. The first mode characterizes its lower part that is silty–sandy with a high concentration of gravels and cobbles. These deposits correspond to alluvium of bottom load generated by a high-energy regime. The second mode concerns its upper part, which consists of an exclusively silty clayey texture suggesting an attenuation of the intensity of the water regime with the deposition of fine particles. Fine-grained particles and the alternation of lenses of fine sands and silts (see Micromorphological Analysis) rather suggest their transport by low energy currents and their settlement by suspension during periodic flooding characteristic of floodplain setting, which is known to be deposited at lower flow speeds.

The micromorphological features of the El Kherba sediments are summarized and comprehensively described with their paleoenvironmental implications, respectively, in Supplementary Table S1 and Supplementary Text (Supplementary Material). Here, we highlight the results of the micromorphological study concerning the lithological facies and post-depositional development of the sediments.