Can mima-like mounds be Vertisol relics (Far North Region of Cameroon, Chad Basin)?
Introduction
Non-anthropogenic earth mounds occur in many parts of the world and the questions related to their origin make them exciting features (Cramer et al., 2012). The debate is generally centred upon the discussion of their abiotic or biotic origin. For example, earth mounds observed in South Africa (locally termed heuweltjies; Moore and Picker, 1991, Francis et al., 2012) and Brazil (Funch, 2015) are attributed to termite activity, but others provide some evidence they can result from differential erosion (Cramer et al., 2012). In North America, the geomyid pocket gopher may be involved in the mound (or mima mound) formation (e.g. Cox, 1984), but the debate on their real capability to produce them is still open (e.g. Butler, 1995). Silva et al. (2010) proposed that erosive processes combined with vegetation patterns are the cause of earth mounds (campos de murundus) observed in Brazil instead of resulting from termite activity. The origin of earth mounds changes from one region to another, but they have some common features, such as their regular spatial distribution or their size (Cramer et al., 2012). It was recently proposed that many of these different earth mounds, termed mima-like mounds, can be explained by the combination of erosion and/or aeolian processes with vegetation patterns (Cramer and Barger, 2014, Cramer and Midgley, 2015).
All along the Sudano-Sahelian belt, carbonate nodules have been observed in non-carbonate watersheds (Fig. 1A). In northern Cameroon (Fig. 1B), these nodules are associated with non-anthropogenic earth mounds (Fig. 2). These new features have not been studied up until now. In the Far North region of Cameroon (Chad Basin) they are frequently observed in the Diamare piedmont (Fig. 1B), and are particularly well developed within stream networks (10 m < diameter < 30 m and height up to 1 m). On Google Earth images (Fig. 1C), they are distributed in eroded areas as regularly spaced (i.e. over-dispersed) round features. They can be defined as mima-like mounds due to their size and their spatial distribution. However, vegetation patterns do not play a main role in their formation, whereas erosion processes do. Generally, mima-like mounds are associated with sparse vegetation or none at all (Figs. 2A, 3). Consequently, the conditions under which they form still remain unclear.
The clay- and carbonate-rich association composing the mima-like mound sediments was already described in the Diamare piedmont (Martin, 1961, Sieffermann, 1963, Brabant and Gavaud, 1985, Morin, 2000) and also farther away in the Chad Basin (Erhart, 1954, Pias, 1962, Bocquier, 1973). Compilation of former and new field observations raises questions on the potential links that could exist between these multiple locations. The clay-rich sediment, enriched in carbonate nodules, can be interpreted as a layer (or multi-layer?) forming residual mima-like mounds in highly eroded areas. Regarding the soils developing from this clay-rich parent material, there are mainly Planosol, Solontez and Vertisol (Brabant and Gavaud, 1985). However, these soils are often described and mapped as degraded soils (Brabant and Gavaud, 1985). Interestingly, mima-like mounds are typically observed in these degraded zones. This is consistent with their presence in eroded areas. However, could pedogenesis have a specific role in their genesis?
The aim of this study is to characterize sedimentological and geomorphological settings of mima-like mounds from the Far North region of Cameroon and to propose a possible origin of their formation. To achieve this goal, a multiple scale approach is used compiling (1) field observations, (2) grain-size distribution and clay mineralogy data at the mound scale, and (3) thin-section observations. Roles of pedogenesis and later erosion in the formation of mima-like mounds are investigated. Two main hypotheses are proposed: (i) mima-like mounds observed in northern Cameroon are relics of Vertisols associated with a gilgai micro-relief; and (ii) they result from abiotic processes related to the succession of pedogenesis and erosion-deposition phases associated to Late Quaternary regional climatic changes. They may also be useful as paleo-environmental records for the Late Pleistocene-Holocene period in the Chad Basin.
Section snippets
General settings
Mima-like mounds are observed in the Diamare piedmont, spreading between the Mandara Mountains and the Yayres floodplain (Fig. 1B). The Mandara Mountains are mainly composed of granite and gneiss. Metamorphic rocks (greenschist facies), known as the green rocks of Maroua, constitute only a minor part of the massif around Maroua (Brabant and Gavaud, 1985). The transition from the Mandara Mountains to the Diamare piedmont is abrupt, with a decreasing slope from about 10% to 2%, which is a typical
Sampling
Soil profiles are described according to the guidelines provided by the Food and Agricultural Organization of the United Nations (FAO, 2006). Depth, percentage and components of the skeleton (> 2 mm), structure, relative abundance of carbonate (tested with 10% hydrochloric acid) and colour (determined in the field with a Munsell colour chart) are given in Table 1. Samples were collected at two main sites (Fig. 1B, stars). Trenches were opened through selected mounds. Soil profiles were described
Field and soil descriptions
The studied mima-like mounds are located in the Mayo Tsanaga granitic watershed (Fig. 1B). They are distributed within stream networks and are regularly spaced (Fig. 1C) with R = 1.72 (see Appendix, Fig. A.1) meaning over-dispersion (Clark and Evans, 1954). They are easily identified because of their sparse vegetation cover (Fig. 2A) and their darker colour (Fig. 3). The size of the mounds is up to 10 m in diameter and up to 1 m in height (Fig. 2B). Their nature can be referred to as the clay-rich
Discussion
The presence of mima-like mounds in eroded areas from the landscape supports that erosion processes are involved in their morphology. Field observations show that vegetation patterns do not have a role in mound formation (Figs. 2A, 3). The question of another biotic factor (termite activity?) may be involved is not totally ruled out, especially as the over-dispersion spacing of the mounds may support a biotic origin (Cramer and Midgley, 2015). Nevertheless, up until now, observations pointed to
Conclusions
Investigations on newly observed earth mounds in the Chad Basin led to an original interpretation of their formation. According to their size and spatial distribution, they can be defined as mima-like mounds (Cramer and Barger, 2014). Recently it was postulated that mima-like mound shapes result from the combination of erosion and/or aeolian and vegetation patterns. Mima-like mounds observed in northern Cameroon are strongly linked to erosion, and there is no evidence that vegetation patterns
Acknowledgements
The authors want to thank Dr. Jean-Louis Rajot and the other members of the Hydraride program (2011), who drew our attention to the mounds of the Sudano-Sahelian belt and collected the first samples from Niger and Burkina Faso. They also made the first observations in Northern Cameroon. The PEERS MORDRED insured the logistical aspects during the initial fieldwork in 2012. The LMI PICASS'EAU from the IRD and the members of Water and Environmental Sciences lab from the University of Ngaoundéré
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