Elsevier

Geomorphology

Volume 89, Issues 3–4, 15 September 2007, Pages 287-296
Geomorphology

Research paper
Planation surfaces in Northern Ethiopia

https://doi.org/10.1016/j.geomorph.2006.12.007Get rights and content

Abstract

Planation surfaces are an old-fashioned topic in geomorphology, but they are nevertheless important where they make up much of the landscape. Northern Ethiopia is largely a stepped topography, caused by differential erosion. Exhumation of old planation surfaces that were preserved under sedimentary or volcanic cover is an important process in landscape evolution. The oldest planation surface is of early Palaeozoic age (PS1); the second is Late Triassic (PS2); and the third is of Early Cretaceous age (PS3). The Oligocene Trap Volcanics buried a surface (PS4) of early Tertiary age, which is now widely exposed by erosion as a surface that, where flat enough, is an exhumed planation surface. The surfaces do not relate to the supposed Africa-wide pediplain sequence of King [King, L.C., 1975. Planation surfaces upon highlands. Z. Geomorph. NF 20 (2), 133–148.], either in mode of formation and age. Although the region is tropical, there is scarce evidence of deep weathering and few indications that the surfaces could be regarded as etchplains. These surfaces indicate that eastern Africa underwent long episodes of tectonic quiescence during which erosion processes were able to planate the surface at altitudes not too far from sea level. Only after the onset of rifting processes, uplift became active and transformed a vast lowland plain into the present Ethiopian highlands, largely exceeding 2500 m a.s.l. Some hypotheses and speculations on the genesis of these surfaces are considered here.

Introduction

Planation surfaces, once a basic concern of geomorphologists, have been largely neglected in recent years, and some even deny their existence (Smailes, 1960, Hack, 1973). But they are real features and it is impossible to make sense of planated landscapes without recognising and trying to understand them. There is increasing debate on the mechanism of formation of planation surfaces, and they play an important role in deciphering the origin of mountains (Ollier and Pain, 2000) and landscape evolution on passive continental margins (e.g. Peulvast and Sales, 2005).

Here we briefly describe the landscape of northern Ethiopia, which is dominated by planation surfaces. They do not fit into the schemes of pediplain or etchplain, and seem to follow, at least in part, the classical peneplanation model of Davis (1899). In any case, they reveal significant geomorphological details and provide data for an up-to-date discussion and interpretation of long-term landscape evolution of this part of East Africa.

The northern Ethiopian Highland (Fig. 1) is a region of high plateaus, located on the western side of the triple junction between the Nubian (African), Somalian and Arabian Plates (Mohr, 1967, Ebinger, 1989). It is generally over 2000 m a.s.l. with isolated hills and volcanic relief up to 4620 m at Mt. Ras Dashan, the highest peak in Ethiopia. The region is bordered to the east by the stepped fault/fault-line escarpments (Fig. 2, Fig. 3), mostly trending N–S, that lead down to the Afar lowlands making a link between the Red Sea fault system and the Main Ethiopian Rift (Ebinger, 1989, Abbate et al., 2002). To the west, there is a progressive decrease in mean elevation to the Sudan and the Nile. The main rivers valleys (Takeze River, Gash River, Angerb River, Abbay River) which deeply incise the plateau are part of the Blue Nile drainage basin. Their channels have a stepped long profile, alternating gently sloping segments and deep gorges with waterfalls at their head. The valley slopes usually show a stepped morphology due to selective erosion. This stepped morphology, known locally as “amba” landscape, is related to strong contrasts between hard and soft rocks (Fig. 4). Some contrasting types of rock are conformable, but some are related to major unconformities and constitute exhumed planation surfaces.

The eastern side of the northern Ethiopian Highlands includes the Mekelle Plateau, a gently rolling region of about 8000 km2 with a mean elevation of 2000–2200 m a.s.l. rising southwards, out of the study area, to over 3000 m a.s.l. at the Amba Alagi volcanic range. The Tigre Plateau to the north is separated from the Mekelle outlier by a set of fault scarps some hundred of metres high. The Mekelle outlier (Fig. 2) is an almost circular tectonic sedimentary basin where a thick sedimentary sequence spanning from the Palaeozoic to the Tertiary is preserved (Fig. 3). This allows us to better define the relationships between large scale tectonics, sedimentation and unconformities, which in the past were planation surfaces. The 1:250,000 geological maps of Mekelle (Arkin et al., 1971) and Adigrat (Aklilu et al., 1978) provide a unique opportunity to observe the relationship between bedrock formations and unconformities over a very wide area.

Section snippets

Geological evolution, bedrock stratigraphy and large scale geomorphology

The stratigraphic sequence of the northern Ethiopian Highlands ranges from the Pre-Cambrian to the Holocene (Merla and Minucci, 1938, Garland, 1980, Bosellini et al., 1997; Fig. 2). Widespread unconformities related to phases of prevailing erosion interrupted the major depositional events allowing us to recognise the occurrence of long periods of limited tectonic activity.

Discussion

Exhumed surfaces are well-known in different parts of the world and, as in northern Ethiopia, they originate well back in geological time, though they are now part of the present scenery (Ollier et al., 1988, Twidale, 1994, Lidmar-Bergstrom, 1995). Elsewhere in Africa exhumed planation surfaces have been recognised where overlying volcanics have been stripped away. In Uganda partial stripping of the Miocene Mount Elgon has exposed a very flat and deeply weathered sub-volcanic surface (Ollier

Conclusion

Apart from the uncertainties on genetic mechanisms and models, the planation surfaces of northern Ethiopia provide a fine example of long-term geomorphological evolution in a region affected by recurrent phases of intense tectonic activity, responsible for the emplacement of vast sedimentary basins and significant relief, alternating with prolonged periods of tectonic quiescence, during which planation surfaces were modelled at relatively low altitude. They provide enough information to show

References (50)

  • Y. Abul-Haggag

    A Contribution to the Physiography of Northern Ethiopia

    (1961)
  • Aklilu, A., Amenti, A., Beyth, M., Dow, D.B., Garland, C.R., Temesgen, H., Hailu, T., 1978. Geological map of Adigrat...
  • Arkin, Y, Beyth, M., Dow, D.B., Levitte, M., Temesgen, H., Hailu, T., 1971. Geological map of Mekele, Sheet ND 37–11...
  • Beauchamp, J., 1977. La serie sedimentaire en Ethiopie centrale et orientale. Ph. D. Thesis, Universiy Claude Bernard,...
  • Beyth, M., 1971. The geology of central and western Tigre. Unpublished report, Ministry of Mines, Addis...
  • M. Beyth

    Paleozoic–Mesozoic sedimentary basin of Mekele outlier, Northern Ethiopia

    Am. Assoc. Pet. Geol. Bull.

    (1972)
  • W.T. Blanford

    On the geology of a portion of Abyssinia

    Q. J. Geol. Soc. Lond.

    (1869)
  • A. Bosellini

    The Continental Margins of Somalia

    Am. Assoc. Pet. Geol. Mem.

    (1992)
  • A. Bosellini et al.

    Il Calcare di Antalo nella regione di Macallè (Tigray, Etiopia settentrionale)

    Rend. Fis. Accad. Naz. Lincei, Ser. 9,

    (1995)
  • A. Bosellini et al.

    The Mesozoic succession of the Mekele outlier (Tigre province, Ethiopia)

    Mem. Soc. Geol.

    (1997)
  • W.M. Davis

    The geographical cycle

    Geogr. J.

    (1899)
  • C.J. Ebinger

    Tectonic development of the western branch of the east African rift system

    Geol. Soc. Amer. Bull.

    (1989)
  • P.L. Fantozzi

    Transition from continental to oceanic rifting in the gulf of Aden: structural evidence from field mapping in Somalia and Yemen

    Tectonophysics

    (1998)
  • P.L. Fantozzi et al.

    Geological mapping in northeastern Somalia (Midjiurtinia region): field evidence of the structural and paleogeographic evolution of the northern margin of the Somalia plateau

    J. Afr. Earth Sci.

    (2002)
  • D.A. Foster et al.

    Structural framework and denudation history of the flank of the Kenya and Anza Rift, East Africa

    Tectonics

    (1996)
  • Cited by (56)

    • Tropical Mountain Rivers

      2022, Treatise on Geomorphology
    • Characteristics and susceptibility zonation of landslides in Wabe Shebelle Gorge, south eastern Ethiopia

      2021, Journal of African Earth Sciences
      Citation Excerpt :

      Generally, based on the field observation carried out, the primary causes of rockfall in the study area are uncontrolled blasting, presence of weathered geological materials, presence of jointed and fractured rocks, seepage of water through weathered and fractured rocks, a high slope gradient in natural slopes, and a steep or vertical road slope cuts (on average 70–85 degrees). The geology of Ethiopia has been studied by different authors (Gibson, 1969; Coltorti et al., 2007; Kazmin, 1972; Kazmin and Warden, 1975; Mohr and Zanettin, 1988) and major rock types (from the oldest to the youngest) include basement rocks, sedimentary and volcanic. The Wabe Shebelle gorge is dominated by sedimentary and volcanic rocks of various types.

    View all citing articles on Scopus
    View full text