Reply to the comment by Dominique Schwartz on the
paper “Geochronological arguments for a close
relationship between surficial formation profiles and
environmental crisis (c. 3000–2000BP) in Gabon
(Central Africa)”

Denis Thiéblemont, Christine Flehoc, Michel Ebang-Obiang, Christophe Rigollet, Jean-Pierre Prian, François Prognon

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Denis Thiéblemont, Christine Flehoc, Michel Ebang-Obiang, Christophe Rigollet, Jean-Pierre Prian, et al.. Reply to the comment by Dominique Schwartz on the paper “Geochronological arguments for a close relationship between surficial formation profiles and environmental crisis (c. 3000–2000BP) in Gabon (Central Africa)”. Comptes Rendus. Géoscience, 2014, 346 (7-8), pp.209-211. 10.1016/j.crte.2014.05.001 . hal-03670706

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Reply

Reply to the comment by Dominique Schwartz on the paper
“Geochronological arguments for a close relationship
between surficial formation profiles and environmental crisis
(c. 3000–2000 BP) in Gabon (Central Africa)”, by
D. Thiéblemont et al., 2013 [C.R. Geoscience 345, 272–283]

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CrossMark

Denis Thiéblemont<sup>a,*</sup>, Christine Flehoc<sup>a</sup>, Michel Ebang-Obiang<sup>b</sup>, Christophe Rigollet<sup>a</sup>, Jean-Pierre Prian<sup>a</sup>, François Prognon<sup>a</sup>

ARTICLE INFO

Article history:

Received 23 April 2014

Accepted after revision 5 May 2014

Available online 16 June 2014

Our paper and another by **Thiéblemont (2013)** detail the arguments that led us to propose an aeolian origin as being the most probable for the Cover Horizon.

This interpretation is denied by D. Schwartz, but he
and us (as well as most authors) agree on two key-points
of the profiles of surficial formations in western central
Africa:

* the Cover Horizon is an almost continuous unit blanketing the landscape;

* this unit is generally superimposed on a more or less polygenic coarse-grained unit (stone line) carrying a frequent lithic industry.

This organization may be explained in three different
ways (Vogt and Vincent, 1966):

* the stone line has sunk into the Cover Horizon;

* the Cover Horizon came from above and was deposited over the stone line;

* the Cover Horizon came from below and was spread over the stone line.

This latter solution is the one put forward by the many
“authors highlighting the role of bioturbation as a major
process of soil dynamics” cited by D. Schwartz (including
Schwartz, 1996), the second being the one we favor.

As noted by Schwartz (1996), the occurrence of a lithic
industry within the Stone line is a key-point for the origin
of this unit and that of the overlying Cover Horizon.
Schwartz (1996) argued that this industry could be of
Sangoan age, which corresponds to an age of c. 70,000 to
40,000 years. This indicates an age younger than
40,000 years for the Cover Horizon. Thus, 40,000 years is
the larger time-interval that we can assume for the
formation of the Cover Horizon. In this time-interval, the
“biogenic” model (e.g., Schwartz, 1996) postulates that:

* all the matter was transported by termites from the altered rocks-basement to the surface through the Stone line;

* this matter was spread by colluvial processes over the landscape;

* this colluvium was converted into a homogeneous 1- to 2-m-thick layer covering the landscape.

As the Cover Horizon is commonly observed as lying
directly on an unaltered basement (see Fig. 3 in

<footnote><sup>a</sup> Bureau de recherches géologiques et minières (BRGM), B.P. 6009, 45060 Orléans cedex 2, France</footnote>

<footnote><sup>b</sup> DGMG, Libreville, Gabon</footnote>

<footnote>* Corresponding author.
E-mail address: d.thieblemont@brgm.fr (D. Thiéblemont).</footnote>
Thiéblemont, 2013 or p. 11 in Vogt and Vincent, 1966), sometimes at a regional scale, it must also be postulated that the colluvial process would have been so intense that the material accumulated by termites could have been transported far away from its source.

D. Schwartz mentions the work of Vogt and Vincent (1966) (note that these authors provided strong arguments against the above "biogenic" model) who reported the existence of a strong analogy of mineralogical compositions between the Cover Horizon and the underlying altered basement in some (not all) places where the two units were observed on a single outcrop. **Thiéblemont (2013)** and **Thiéblemont et al. (2012)** have demonstrated that the trace element and Nd-isotopic signatures of the fine-grained fraction of the Cover Horizon are almost constant, irrespective of the geological basement on which the Cover Horizon rests.

In our opinion, too many counterexamples exist against the hypothesis of a biological origin of the Cover Horizon. This leads us to envisage and evaluate an alternative model, and as mentioned above, deposition of particles brought by wind appears to us as the most likely.

D. Schwartz presents his own set of counterexamples to this hypothesis.

## 1. Lack of aeolian dune structures

Away from the recent sandy formations of the eastern (Batéké sands) and western (Cirques series) parts of Gabon, the Cover Horizon is mainly composed of clay and silt. This makes it similar to a loess that is a type of aeolian deposit with no associated dune structures. Possible dune structures are however observed in the southwestern part of Gabon, where the Cover Horizon passes to a 10- to 30-m-thick sandy-clayey material. The radar image (SRTM survey) of these structures is reported in the explanatory note of the third edition of the Geological map of Gabon at a scale of 1:1,000,000 (zone D on Fig. 52, p. 235) (Thiéblemont et al., 2009; **Thiéblemont, 2009**).

## 2. Lack of typical impacts on sand particles

The only work on this topic is from Giresse and Le Ribault (1981), who studied two samples collected in the surface cover close to the Atlantic in SW-Congo. The authors concluded that the material was deposited by aeolian processes during the Holocene, but they noted that because of its fast duration this aeolian event did not produce impact structures on quartz grains.

## 3. Lack of lacustrine silt deposits

As far as Gabon (Giresse et al., 2009) and Congo (Vincens et al., 1998) are concerned, the most recent sections (upper 2 m with ages of less than 2500 BP) of cores collected in lakes sediment are described as organic-rich "muds" or "clays" with a large predominance of fine-grained particles (< 50 µm).

## 4. Poor-sorting of the Cover Horizon

No data or reference is provided by D. Schwartz in support to the assumption that the Cover Horizon would be poorly sorted. From our own data, this is especially untrue for the western parts of Gabon and Congo, where the Cover Horizon overlying the Cirques Series shows the sigmoid-shaped cumulative curve frequently considered as typical of aeolian deposits. It is worth noting that the exoscopic study of Giresse and Le Ribault (1981) mentioned earlier was performed on samples collected in western Congo. Nevertheless, recent studies on aeolian sediment (see Muhs, 2013 for a synthesis) no longer consider sorting as a necessary and sufficient condition to state the aeolian origin of sediment. Most aeolian materials are in fact a combination of different grain-size fractions of more or less distal/proximal origin (McTainsh et al., 1997; Yvard, 1968). Well-documented examples of unsorted aeolian sediments in Africa are provided by Brunotte and Sander (2000) and Brunotte et al. (2009) in the Kaokoland of northern Namibia.

## 5. Archeological constraints

As mentioned above, the stone line displays an abundant lithic industry of old-stone to middle-stone age and different new sites were indeed recovered during our geological mapping in Gabon (Théblemont et al., 2009). As mentioned by Locko (1991), in Gabon the artifacts appear as dispersed elements within the stone line. D. Schwartz mentions sites where "it was sometimes possible to recognize structures such as knapping sites, which demonstrate that the artifacts are strictly in position and that the soil profiles were not disturbed since burial." From the references given by the author, the mentioned site should be the Mokeko site, located near Ouesso in the Sangha region of Congo. In this site, Lanfranchi and Schwartz (1990) first described the artifacts as being "perfectly in place at the top of stone lines" but later, Lanfranchi (1991) considered that the site had been reworked, "as most the sites of old-stone to middle-stone ages (MSA) reported in central Africa". This is a good indication that the emplacement of the Cover Horizon took place after c. 12,000 BP, which is the age postulated for the end of the MSA in Gabon (Locko, 1991). We may thus conclude that the about 2 m of sandy to clayey materials forming the Cover Horizon was accumulated in less than 12,000 years. A detailed discussion of the geochronological constraints related to the timing of the deposit of the Cover Horizon is included in our paper.

## 6. δ¹³C constraints

D. Schwartz argues that the progressive change of the δ¹³C of SOM with depth in savannah soils should be incompatible with the emplacement of this soil by aeolian sedimentation, because such a process would result from distinct episodes. A striking feature of the Cover Horizon is its vertical homogeneity. This characteristic being common to many aeolian deposits and especially loess, thus we
see no contradiction between the continuity of the $\delta^{13}C$ variation (whatever its cause) and the aeolian hypothesis.

The hypothesis of an aeolian origin of the Cover Horizon is not strictly new (see ref. in Vogt and Vincent, 1966), but was apparently considered as “unreasonable” in the second part of the 20th century. This hypothesis is now founded on a large set of new analytical data, as well as on classical field observations. As noted earlier, a large agreement exists amongst scientists on the organization of the profiles of surficial formation in western central Africa. New analyses provide strong arguments for their largely allochthonous nature and abundant geochronological data clearly demonstrate that their formation (and especially that of the Cover Horizon) was a very fast process. These are two key-points that have to be taken into account by any future research.

## References

* Brunotte, E., Sander, H., 2000. Loess accumulation and soil formation in Kaokoland (Northern Namibia) as indicators of Quaternary climatic changes. Global Planet. Change 26, 67–75.

* Brunotte, E., Maurer, B., Fischer, P., Lomax, J., Sander, H., 2009. A sequence of fluvial and aeolian deposits (desert loess) and palaeosoils covering the last 60 ka in the Opuwu basin (Kaokoland/Kunene Region, Namibia) based on luminescence dating. Quat. Int. 196, 71–85.

* Giresse, P., Le Ribault, L., 1981. Contribution de l'étude exoscopique des quartz à la reconstitution paléogéographique des derniers épisodes du Quaternaire littoral du Congo. Quat. Res. 15, 86–100.

* Giresse, P., Makaya Mvoubou, Maley, J., Ngomanda, A., 2009. Late-Holocene equatorial environments inferred from deposition processes, carbon isotopes of organic matter, and pollen in three shallow lakes of Gabon, west-central Africa. J. Paleolimnol. 41, 369–392.

* Lanfranchi, R., 1991. Synthèse régionale des âges de la pierre ancien et moyen. In: Lanfranchi, R., Clist, B. (Eds.), Aux origines de l'Afrique centrale, âges de la pierre ancien et moyen - Old-Stone Age (O.S.A.)

et Middle-Stone Age (M.S.A.). Centres culturels français d'Afrique centrale, Centre international de civilisations Bantu, Libreville, Gabon. Sepia 89–95.

* Lanfranchi, R., Schwartz, D., 1990. Évolution des paysages de la Sangha (R.P. du Congo) au Pléistocène supérieur. Bilan des observations archéologiques, géomorphologiques, pédologiques et paléobiologiques. In: Lanfranchi, R., Schwartz, D. (Eds.), Paysages quaternaires de l'Afrique centrale atlantique, Quatrième partie - Faunes, flores, paléoenvironnements continentaux, pp. 248–259.

* Locko, M., 1991. Gabon. In: Lanfranchi, R., Clist, B. (Eds.), Aux origines de l'Afrique centrale, âges de la pierre ancien et moyen - Old-Stone Age (O.S.A.) et Middle-Stone Age (M.S.A.). Centres culturels français d'Afrique centrale, centre international de civilisations Bantu, Libreville, Gabon. Sepia 67–70.

* McMains, G., Nickling, W.G., Lynch, A.W., 1997. Dust deposition and particle size in Mali, West Africa. Catena 29, 307–322.

* Muhs, D., 2013. The geologic record of dust. Aeolian Res. 9, 3–48.

* Schwartz, D., 1996. Archéologie préhistorique et processus de formation des stone lines en Afrique centrale (Congo-Brazzaville et zones péripériques). Seminar on Geo-archaeology in Tropical and Mediterranean Regions, Brussels, April 1996. Royal Academy of Overseas Sciences. Geo-Eco-Trop 20 (1-4), 15–38.

* Thiéblemont, D., 2009. Carte géologique et des ressources minérales de la République gabonaise à 1/1 000 000, troisième éd. DGMG Éditions, ministère des Mines, du Pétrole, des Hydrocarbures, Libreville, Gabon.

* Thiéblemont, D., 2013. Evidence for an aeolian origin of the Holocene lateritic surface cover of Gabon. Quat. Int. 296, 176–197.

* Thiéblemont, D., Castaing, C., Billa, M., Bouton, P., Prétat, A., 2009. Notice explicative de la carte géologique et des ressources minérales de la République gabonaise à 1/1 000 000. DGMG Éditions, ministère des Mines, du Pétrole, des Hydrocarbures, Libreville, Gabon (384 p.).

* Thiéblemont, D., Ebang-Obiang, M., Flehoc, C., Guerrot, C., Negrel, P., Rigollet, C., Prognon, F., 2012. Sédimentation éolienne et crise environnementale Holocène supérieur dans l'Ouest de l'Afrique centrale. In: Atelier Climat et Impacts, Université d'Orsay, 19-20 novembre 2012.

* Vincens, A., Schwartz, D., Bertaux, J., Elenga, H., de Namur, C., 1998. Late-Holocene climatic changes in western Equatorial Africa inferred from pollen from Lake Sinnda, southern Congo. Quat. Res. 50, 34–45.

* Vogt, J., Vincent, P.L., 1966. Terrains d'altération et de recouvrement en zone intertropicale. Bull. BRGM 4, 1–111.

* Yvard, J.C., 1968. Formations éoliennes des plateaux de la région de Tours. Bull. Assoc. Fr. Et. Quat. 5 (2), 101–110.