The Permian Moradi Formation of northern Niger: Paleosol morphology, petrography and mineralogy

https://doi.org/10.1016/j.palaeo.2010.11.002Get rights and content

Abstract

Three basic paleosol morphologies, named Type A, Type B and Type C, are described from the middle–upper Permian strata of the Moradi Formation, Tim Mersoi Basin, northern Niger. The Moradi Formation is a typical alluvial redbed succession dominated by red mudrocks with fine to coarse-grained pebbly channel sandstones and matrix-breccias. Type A paleosols are hosted by well-sorted fine to medium grained trough cross bedded and massive sandstones and preserve abundant vertical to horizontal micritic and microspar calcite tubules, interpreted as rhizoliths. Lateral variability of rhizoliths in Type A paleosols, and their close association with fluvial channel-fill sediments suggests they are the roots of grove stands of phreatophytic vegetation that grew within unstable anabranching stream systems. Type B paleosols are hosted by mudrocks and preserve well-developed ped structure, abundant micritic calcite nodules and vertically-stacked micritic calcite nodular bodies, as well as rare calcite with satin-spar texture interpreted as a pseudomorphic replacement of pedogenic gypsum. The morphology of Type B paleosols suggests they were formed in well-drained floodplain deposits on stable landforms. Type C paleosols are similar to Type B but preserve pedogenic structures indicative of soil volume expansion and contraction, as well as more abundant Stage II pedogenic carbonate nodules. The morphology of Type C paleosols suggests that they developed periodically rather than seasonally in poorly-drained deposits that nevertheless occupied a relatively stable part of the landscape such as the plains flanking ephemeral lakes or sabkhas.

X-ray diffraction analysis of the < 2 μm fraction from the Moradi Formation strata indicates that paleosol phyllosilicates are composed of illite, smectite, and occasionally kaolinite and talc. Illite is likely a detrital mineral, whereas smectite and kaolinite are likely pedogenic weathering products. The presence of talc in the Moradi Formation paleosols is unusual. It is limited to paleosol horizons that also preserve evidence for pedogenic gypsum accumulation and is therefore most likely related to a pedogenic weathering process. It is possible that this talc is a relatively low-temperature (~ 50–100 °C) diagenetic alteration product of pedogenic Mg–phyllosilicates such as sepiolite.

The range of morphologies, petrographic textures and mineralogy of the paleosol profiles indicates semi-arid to hyper-arid climatic setting. This paleoclimatic reconstruction is in agreement with Middle and Late Permian conceptual paleoclimate models and quantitative general circulation models. Nevertheless, and in spite of an arid climate, Moradi paleosols and their host strata also indicate a relatively shallow groundwater table. Importantly, this shallow groundwater resource undoubtedly helped to support the moderately diverse fossil vertebrate assemblage and large-stature macrophytes preserved in the Moradi Formation.

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

References (91)

  • WatsonA.

    Desert soils

  • H. Wopfner

    Tectonic and climatic events controlling deposition in Tanzanian Karoo basins

    Journal of African Earth Sciences

    (2002)
  • WrightV.P.

    A micromorphological classification of fossil and recent calcic and petrocalcic microstructures

  • K. Yemane et al.

    Imprints of postglacial climates and paleogeography in the detrital clay mineral assemblages of an Upper Permian fluviolacustrine Gondwana deposit from north-central Malawi

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (1996)
  • R.J. Arkley

    Calculation of carbonate and water movement in soil from climatic data

    Soil Science

    (1963)
  • R.C. Blakey

    Carboniferous–Permian palaeogeography of the Assembly of Pangae

  • R.H. Blodgett

    Calcareous paleosols in the Triassic Dolores Formation, southwestern Colorado

    Geological Society of America Special Paper

    (1988)
  • S. Boggs

    Principles of Sedimentology and Stratigraphy

    (2001)
  • W.H. Bradley

    Lazurite, talc and chlorite in the Green River Formation of Wyoming

    American Mineralogist

    (1964)
  • R. Brewer

    Fabric and Mineral Analysis of Soils

    (1976)
  • BrookfieldM.E.

    Temporary desert lake deposits, Lower Permian (Rotliegendes) Southern Scotland, U.K.

  • S.W. Buol et al.

    Soil Genesis and Classification

    (1997)
  • J.P. Calvo et al.

    Authigenic clay minerals in continental evaporitic environments

    Special Publications of the International Association of Sedimentologists

    (1999)
  • CamporealeC. et al.

    Hydrological and geomorphological significance of riparian vegetation in arid lands

  • J.S. Choudhari

    Relationship between pedogenetic manifestations in some arid soil and age of the landforms

    Journal of Earth System Science

    (2008)
  • CollinsE.W.

    Styles of Deformation in Permian strata, Texas Panhandle

    Bureau of Economic Geology Geological Circular 84-4, Austin, U.S.A

    (1984)
  • E.A. Davis

    Metamorphosis in the culture market of Niger

    American Anthropologist

    (1999)
  • P. Deutz et al.

    Morphology and stable and radiogenic isotope composition of pedogenic carbonate in late Quaternary relict soils, New Mexico: an integrated record of pedogenic overprinting

    Journal of Sedimentary Research

    (2002)
  • W.A. DiMichele et al.

    Climate and vegetational regime shifts in the late Paleozoic ice age earth

    Geobiology

    (2009)
  • C.R. Fielding et al.

    Stratigraphic imprint of the Late Palaeozoic ice age in eastern Australia: a record of alternating glacial and nonglacial climate regime

    Journal of the Geological Society of London

    (2008)
  • K.S. Frederiksen et al.

    Sequential architecture and cyclicity in Permian desert deposits, Brodick Beds, Arran, Scotland

    Journal of the Geological Society

    (1998)
  • G.M. Friedman

    Occurrence of talc as a clay mineral in sedimentary rocks

    Nature

    (1965)
  • M.T. Gibbs et al.

    Simulations of Permian climate and comparisons with climate-sensitive sediments

    Journal of Geology

    (2002)
  • L.H. Gile

    Morphological and genetic sequences of carbonate accumulation in desert soils

    Soil Science

    (1966)
  • GileL.H. et al.

    Soils and geomorphology in the Basin and Range area of southern New Mexico

    Guidebook to the Desert Project: New Mexico Bureau of Mines and Mineral Resources Memoir 39

    (1981)
  • GreigertJ. et al.

    Essai de description des formations géologiques de la République du Niger

    Editions du Bureau de Recherches Géologiques et Minières, Paris

    (1967)
  • R.J. Hugget

    Soil chronosequences, soil development, and soil evolution: a critical review

    Catena

    (1998)
  • IsbellJ.L. et al.

    Timing of the late Palaeozoic glaciation in Gondwana: was glaciation responsible for the development of northern hemisphere cyclothems?

  • JCPDS International Centre for Diffraction Data

    Mineral Powder Diffraction File Data Book

    (1980)
  • H. Jenny

    Factors of Soil Formation

    (1941)
  • F. Joulia

    Les Séries primaires au N et au NW de l'Aïr (Sahara central). Discordances observées

    Bulletin de la Societe Geologique de France

    (1959)
  • C.F. Klappa

    Calcified filaments in Quaternary calcretes; organo-mineral interactions in the subaerial vadose environment

    Journal of Sedimentary Research

    (1979)
  • C.F. Klappa

    Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance

    Sedimentology

    (1980)
  • J.E. Kutzbach et al.

    Pangean climates: megamonsoons of the megacontinent

    Journal of Geophysical Research

    (1989)
  • J. Laity

    Aeolian destabilization along the Mojave River, Mojave desert, California: linkages among fluvial, groundwater, and aeolian systems

    Physical Geography

    (2007)
  • Cited by (0)

    View full text