Palaeogeography, Palaeoclimatology, Palaeoecology
The Permian Moradi Formation of northern Niger: Paleosol morphology, petrography and mineralogy
Introduction
The past few decades have seen significant advances in understanding of Permian terrestrial climate evolution across northern mid-latitude (Newell et al., 1999, Rees et al., 2002, Yakimenko et al., 2004), equatorial (e.g. DiMichele et al., 2009, Nairn and Smithwick, 1976, Parrish, 1982, Parrish, 1993, Schneider et al., 2006, Steel, 1974, Tabor et al., 2007, Tramp et al., 2004, Ziegler et al., 1997, Ziegler et al., 2003) and extreme southern mid-latitude and polar landscapes (Fielding et al., 2008, Isbell et al., 2003, Retallack et al., 2003, Smith, 1990, Wopfner, 2002, Wopfner and Kreuser, 1986). Permian climate history from south-central Pangean regions, including sub-Saharan Africa and northern South America (Fig. 1A), remains especially poorly known (e.g., Rees et al., 2002, Ricardi-Branco, 2008) in large part because of a paucity of Permian-age sedimentary rocks in the region. Therefore, the few basins that are known to have occupied south-central Pangea with outcrops that contain sedimentary and biological records of Permian time are critically important for paleoclimate studies. They provide the only geologically-based proxy data for paleoclimate reconstructions, they help to fill the spatial and temporal knowledge gap that exists between the Permian tropics and southern high-latitudes, and they provide the “ground-truth” needed to test, modify and refine conceptual and quantitative paleoclimate models.
The results from study of alluvial paleosol profiles preserved within the Permian Moradi Formation of the Tim Mersoi Basin, Niger are presented herein. Field-scale observations of paleosol morphology, in conjunction with petrographic and mineralogical analyses provide, for the first time, geologically-based proxy data for paleoclimate from south-central Pangea.
Section snippets
Geological background
Middle and Upper Permian rocks of the Tim Mersoi sub-basin of the Iullemeden basin are exposed in a southwest-northeast outcrop belt in north-central Niger (Greigert and Pougnet, 1967, Kogbe, 1981; Fig. 1). The tectonic setting of this basin is not well understood, although it has been proposed that compressional forces resulting from late Paleozoic collision of Gondwana and Laurasia caused reactivation of strike slip faults in the Pre-Cambrian basement, uplift of the Aïr Massif, and down
Field
Although there is very little vegetative cover in the study area, a combination of low-lying topography, low regional dip (< 2°) and a modern regolith mantle often exceeding 400 mm thick contributes to a poor quality of surface exposures of Moradi rocks. As a result, the measured stratigraphic sections of the Moradi Formation include numerous covered intervals (Fig. 3), and many parts of the stratigraphy were described from excavation pits up to 1.5 m deep. Moreover, accurate stratigraphic
Results
Paleosol profiles were subdivided into horizons on the basis of down-profile changes in macro- and micromorphological features (Fig. 4, Fig. 5). Three major types of paleosol morphologies (Types A, B and C) exist in the Moradi strata based on the presence of distinctive characteristics that include soil structure, accumulation of carbonate and gypsum, slickensides, fossil root traces and color (Fig. 4, Fig. 5). Later, we first describe the morphology, mineralogy (Fig. 6) and petrography (Fig. 7
Paleosol interpretation
Paleosol morphologies, mineralogy, petrographic textures and fabrics among the three different types of paleosols are used to classify each type of paleosol according to the paleosol taxonomy of Mack et al. (1993). For each paleosol type we also consider the closest estimated soil taxon within the context of the USDA Soil Classification System (Soil Survey Staff, 1975, Soil Survey Staff, 1996). Paleosols assigned to each Order are interpreted as being the product of similar soil-forming
Summary and conclusions
Paleosol profiles in the Middle–Upper Permian Moradi Formation of the Tim Mersoi Basin, Niger include (1) calcic Protosols, (2) Calcisols and gypsic Calcisols, and (3) calcic Vertisols and gypsic calcic Vertisols. As mentioned earlier, each of these paleosol morphologies, and their mineralogy, provide persuasive evidence for pedogenesis under dry, semi-arid to hyper-arid climate. Comparison of the pedogenic gypsum and mg-phyllosilcates within the Moradi paleosol profiles with similar modern
Acknowledgements
We are indebted to the Niger Ministry of Mines and Energy (Niamey, Niger) for access to the field sites and permitting the collection of samples for laboratory analysis. We thank Stephanie Thomas (SMU) for assistance in the field. Mary Milleson (SMU) helped with X-ray diffraction analyses of clay samples. Professor Finn Surlyk served as editor for this manuscript, and Professors Steven Driese (Baylor University) and Paul McCarthy (University of Alaska Fairbanks) reviewed an earlier version of
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