Planation Surfaces in Northern Ethiopia

Geomorphology 89 (2007) 287–296www.elsevier.com/locate/geomorph

Planation surfaces in Northern Ethiopia

M. Coltorti a, F. Dramis b,⁎, C.D. Ollier c

a Department of Earth Sciences, University of Siena, Via di Laterina 8, 53100 Siena, Italyb Department of Geological Sciences, “Roma Tre” University, Largo S. Leonardo Murialdo 1, 00146 Rome, Italy

c School of Earth and Geographical Sciences, University of Western Australia, Nedlands WA 6009, Australia

Received 7 November 2006; received in revised form 19 November 2006; accepted 19 December 2006Available online 22 December 2006

Abstract

Planation surfaces are an old-fashioned topic in geomorphology, but they are nevertheless important where they make upmuch of the landscape. Northern Ethiopia is largely a stepped topography, caused by differential erosion. Exhumation of oldplanation surfaces that were preserved under sedimentary or volcanic cover is an important process in landscape evolution. Theoldest planation surface is of early Palaeozoic age (PS1); the second is Late Triassic (PS2); and the third is of Early Cretaceousage (PS3). The Oligocene Trap Volcanics buried a surface (PS4) of early Tertiary age, which is now widely exposed by erosionas a surface that, where flat enough, is an exhumed planation surface. The surfaces do not relate to the supposed Africa-widepediplain sequence of King [King, L.C., 1975. Planation surfaces upon highlands. Z. Geomorph. NF 20 (2), 133–148.], eitherin mode of formation and age. Although the region is tropical, there is scarce evidence of deep weathering and few indicationsthat the surfaces could be regarded as etchplains. These surfaces indicate that eastern Africa underwent long episodes oftectonic quiescence during which erosion processes were able to planate the surface at altitudes not too far from sea level. Onlyafter the onset of rifting processes, uplift became active and transformed a vast lowland plain into the present Ethiopianhighlands, largely exceeding 2500 m a.s.l. Some hypotheses and speculations on the genesis of these surfaces are consideredhere.© 2007 Elsevier B.V. All rights reserved.

Keywords: Planation surface; Exhumation; Long-term geomorphic evolution; Ethiopia

1. Introduction

Planation surfaces, once a basic concern of geomor-phologists, have been largely neglected in recent years,and some even deny their existence (Smailes, 1960;Hack, 1973). But they are real features and it isimpossible to make sense of planated landscapeswithout recognising and trying to understand them.

⁎ Corresponding author.E-mail addresses: [email protected] (M. Coltorti),

[email protected] (F. Dramis), [email protected](C.D. Ollier).

0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2006.12.007

There is increasing debate on the mechanism offormation of planation surfaces, and they play animportant role in deciphering the origin of mountains(Ollier and Pain, 2000) and landscape evolution onpassive continental margins (e.g. Peulvast and Sales,2005).

Here we briefly describe the landscape of northernEthiopia, which is dominated by planation surfaces.They do not fit into the schemes of pediplain oretchplain, and seem to follow, at least in part, theclassical peneplanation model of Davis (1899). In anycase, they reveal significant geomorphological detailsand provide data for an up-to-date discussion and

mailto:[email protected]



http://dx.doi.org/10.1016/j.geomorph.2006.12.007

Fig. 1. Map of northern Ethiopia with the localities mentioned in the text.

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interpretation of long-term landscape evolution of thispart of East Africa.

1.1. The study area

The northern Ethiopian Highland (Fig. 1) is a regionof high plateaus, located on the western side of thetriple junction between the Nubian (African), Somalianand Arabian Plates (Mohr, 1967; Ebinger, 1989). It isgenerally over 2000 m a.s.l. with isolated hills andvolcanic relief up to 4620 m at Mt. Ras Dashan, thehighest peak in Ethiopia. The region is bordered to theeast by the stepped fault/fault-line escarpments (Figs. 2and 3), mostly trending N–S, that lead down to the Afarlowlands making a link between the Red Sea faultsystem and the Main Ethiopian Rift (Ebinger, 1989;Abbate et al., 2002). To the west, there is a progressivedecrease in mean elevation to the Sudan and the Nile.The main rivers valleys (Takeze River, Gash River,Angerb River, Abbay River) which deeply incise theplateau are part of the Blue Nile drainage basin. Theirchannels have a stepped long profile, alternating gentlysloping segments and deep gorges with waterfalls attheir head. The valley slopes usually show a stepped

morphology due to selective erosion. This steppedmorphology, known locally as “amba” landscape, isrelated to strong contrasts between hard and soft rocks(Fig. 4). Some contrasting types of rock are conform-able, but some are related to major unconformities andconstitute exhumed planation surfaces.

The eastern side of the northern Ethiopian Highlandsincludes the Mekelle Plateau, a gently rolling region ofabout 8000 km2 with a mean elevation of 2000–2200 ma.s.l. rising southwards, out of the study area, to over3000 m a.s.l. at the Amba Alagi volcanic range. TheTigre Plateau to the north is separated from the Mekelleoutlier by a set of fault scarps some hundred of metreshigh. The Mekelle outlier (Fig. 2) is an almost circulartectonic sedimentary basin where a thick sedimentarysequence spanning from the Palaeozoic to the Tertiaryis preserved (Fig. 3). This allows us to better definethe relationships between large scale tectonics, sedi-mentation and unconformities, which in the past wereplanation surfaces. The 1:250,000 geological maps ofMekelle (Arkin et al., 1971) and Adigrat (Aklilu et al.,1978) provide a unique opportunity to observe therelationship between bedrock formations and unconfor-mities over a very wide area.

Fig. 2. Geomorphological sketch map of the study area with traces of the cross sections of Fig. 3. 1. Precambrian; 2. Palaeozoic; 3. Late Triassic–Jurassic; 4. Cretaceous; 5. Oligocene Trap Volcanics; 6. Rift escarpment; 7. Afar Neogene Deposits; 8. PS1; 9. PS2; 10. PS3; 11. PS4; 12. Edge of themain Rift escarpment; 13. Major faults; MK. Mekele; AD. Adigrat; EN. Enticho; EA. Edaga Arbi. In the Mekele outlier, where the Mesozoicsedimentary rocks are preserved, the four planation surfaces mentioned in the text are easily recognisable.

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Fig. 3. Simplified geological cross sections of the study area. 1. Precambrian; 2. Palaeozoic; 3. Late Triassic–Jurassic; 4. Cretaceous; 5. Oligocene Trap Volcanics; 6. Afar Neogene Deposits; 7. PS1; 8.PS2; 9. PS3; 10. PS4. In the northern sector both the PS1 and PS are visible. To the east the PS1 is displaced by the Rift fault system. In section 3, cutting across the northern part of the Mekele outlier,all the planation surfaces are present and clearly recognisable.

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2. Geological evolution, bedrock stratigraphy andlarge scale geomorphology

The stratigraphic sequence of the northern EthiopianHighlands ranges from the Pre-Cambrian to theHolocene (Merla and Minucci, 1938; Garland, 1980;Bosellini et al., 1997; Fig. 2). Widespread unconfor-mities related to phases of prevailing erosion interruptedthe major depositional events allowing us to recognisethe occurrence of long periods of limited tectonicactivity.

2.1. Precambrian

Strongly deformed low grade metavolcanics andmetasediments of Precambrian age (Mock et al., 1999)make up the basement in northern Ethiopia (Figs. 2and 3). North of Mekelle (e.g. between Wukro andHauzien) the metamorphic basement is intruded bybatholiths and stocks of Late Precambrian–EarlyPaleozoic age, dated to 600 Ma by Garland (1980).

2.2. The pre-Ordovician planation surface (PS1)

The planation surfaces are very obvious in the field,but in addition to morphology we examined relatedstratigraphy, sedimentology and structures to help de-termine their mode of formation, their relative age, andwhere possible the stratigraphic age. This depends tosome extent on the occurrence of suitable exposures,

Fig. 4. “Amba” landscape

and involved our own field work as well as that recordedin the literature.

The oldest extensive planation surface (PS1) trun-cates the pre-Paleozoic basement and cuts clearly acrossthe Precambrian granite batholiths (Fig. 2). This surfaceis very flat (Figs. 2, 3 and 5) and crops out due toexhumation north of Mekelle, where it reaches eleva-tions of about 3000 m a.s.l., and in western Tigray(Tembien Plateau), where it declines to ca. 2000 m dueto normal faults and a slight westward tectonic tilting. Inthe Mekelle outlier, this surface is deeply buried under athick sedimentary sequence and it is probably locatedalmost at present sea level (Fig. 3).

2.3. The post-PS1 Ordovician depositional cycle

The oldest rocks that cover the PS1 planation surfacebelong to the Early Paleozoic. They are represented by awide variety of continental sandstones of alluvial andaeolian facies (Enticho Sandstone Formation; Arkinet al., 1971) that in their upper part interfinger withtillites and coastal sediments (Edaga Arbi Glacials;Beyth, 1971; Garland, 1980). These deposits, whosetotal thickness is few hundred metres, are thought to beof Middle-Upper Ordovician age. In fact, paleomagneticdata confirm that during the Ordovician–Silurian theSouth Pole was located in North Africa so that theEthiopian glacial cover could have reached the dimen-sions of a continental ice-sheet (Saxena and Assefa,1983).

in northern Tigray.

Fig. 5. The pre-Ordovician planation surface (PS1) between Wukro and Adigrat.


These rocks crop out at the northern margin of theMekelle outlier and, as small remnants, in different partsof the northern Highlands where they make up minorridges and hills rising slightly above the PS1.

2.4. The Late Triassic planation surface (PS2)

This unconformity, that covers at least the wholePermian, corresponds to a planation episode called the“First Erosional (Peneplain) Cycle” by Merla andMinucci (1938). The amount of vertical erosion, whichfollows a previous uplift phase (Guiraud and Bosworth,1999), roughly ranges between some hundreds to nomore than one thousand metres. Although of greatstratigraphic significance, this unconformity has littlegeomorphological expression because in many places itmerges with the previous PS1 surface. Moreover, theplanation surface is rarely exhumed due to weakness ofthe underlying rocks. The best places where it isrecognisable are the Mekelle outlier and in the Adigratarea (Figs. 2 and 3) where it is preserved at around2300–2500 m a.s.l.

2.5. The Mesozoic depositional cycle

The first Mesozoic sediments in the area are theAdigrat Sandstone (also known as “Lower Sandstone”)of Late Triassic–Early Jurassic age (Merla and Minucci,1938; Blanford, 1869). The sandstones crop outextensively all around the Mekelle outlier and as limited

remnants in different parts of the Tigre Plateau andalong the Eastern Escarpment, lying unconformablyover the older Palaeozoic deposits or, further to thenorth, directly on planated Precambrian basement (Merlaand Minucci, 1938). The sandstones are fluvial, withcross-bedding and fossil wood, indicating a wide alluvialplain crossed by meandering channels (Bosellini et al.,1997). A gradual transition to coastal marine depositswas observed by Garland (1980). The thickness of theAdigrat Sandstone is variable, possibly as a result ofsyn-sedimentary faulting. In the northern EthiopianHighlands it reaches about 700 m at Abi Adi, south–west of Mekelle, and it thins out to the east (Beyth,1972). Around Adigrat and in Eritrea the top of theAdigrat Sandstone is partially covered by a thick lateriticcrust (Merla and Minucci, 1938; Abul-Haggag, 1961).

The boundary between the Adigrat Sandstones andthe overlying marine Antalo Supersequence is transi-tional through 20–30 m thick shales with some cal-carenite and sandstone intercalations (Bosellini et al.,1997; Sagri et al., 1998). The occurrence of laterite soilsand ferruginous hard ground layers testifies the repeatedfluctuations of sea level during the early stages ofthe transgression (Bosellini et al., 1997). The age ofthe transitional beds is Late Callovian (?) — EarlyOxfordian.

The Antalo Supersequence (Late Jurassic) is ashallow water transgressive succession made up ofalternating limestone and shale (Bosellini et al., 1997). Itlargely outcrops in the Mekelle outlier. The lower part

Fig. 6. The Cretaceous planation surface (PS3) north of Mekele. Mt. Amba Aradam is visible in the background.


of the supersequence (Antalo Limestone; Merla andMinucci, 1938) includes massive limestone layersalternating with shales; the upper part besides shaleincludes marl, coquina limestone, sandstone andgypsum (Agula Shales; Merla and Minucci, 1938).

The transgression came from the south–east (Soma-lia), where the deposits are thicker and the facies re-veals a deeper sedimentary environment (Merla andMinucci, 1938; Bosellini et al., 1995, 1997). It likelyextended over a much larger area than that of theoutcropping sediments whose present limits are due tosubsequent erosion. The dissection of this sedimentarysequence produced the best conditions for selectiveerosion, creating stepped slopes and the typical mesa/amba morphology.

2.6. The Cretaceous planation surface (PS3)

At the end of the Jurassic, an episode of intensetectonic deformation and uplift, possibly associated withthe onset of a proto-Rift in the Aden Gulf (Bosellini,1992), occurred over a large area and generated some ofthe largest basins in northern Ethiopia as well as in otherparts of east Africa (Bosellini, 1992; Fantozzi, 1998;Fantozzi and Ali Kassim, 2002). In this context, WNW–ESE faults displaced the Mesozoic sequence (Arkinet al., 1971; Aklilu et al., 1978; Russo et al., 1996).

Subsequently, a new planation episode (PS3) cut acrossthe deformed rocks and the WNW–ESE faults (Figs. 2,3 and 6). The amount of denudation is of the order of1–2 km, that is the thickness of the Triassic–Jurassicsequence. West and south of Mekelle (Hegere Selam,Mt. Amba Aradam), at about 2500–2700 m a.s.l., thisplanation surface is a key marker. Elsewhere it isrecognisable as an unconformity within the sedimen-tary sequence.

2.7. Cretaceous — the Amba Aradam Formation

The Amba Aradam Formation, also known as“Upper Sandstone”, consists of a 100–200 m thickquartz sandstone, gravels and silts with occasional thinlaterite levels. It crops out around Mekele, unconform-ably overlying the previous units. Due to the absence offossils, the formation has been referred to the Aptian–Albian by correlation with similar deposits outcroppingin south-eastern Ethiopia (Gortani, 1973; Beauchamp,1977). The sedimentary environment was recentlyrelated to an alluvial plain (Bosellini et al., 1997).However, the presence of layers of laminated well-rounded quarz pebbles and sands at the base of theformation seems to indicate the occurrence of beachdeposits intercalated with coastal marsh and alluvialplain sediments. The upper part of the sedimentary


sequence is often deeply weathered into a reddish violetiron-rich horizon, indicating prolonged subaerial weath-ering in tropical conditions.

2.8. The Trap Volcanics and the sub-Trap planationsurface (PS4)

In the Mekele region the Amba Aradam Formation isoverlain by the Trap Volcanics, a pile of basalt lavaflows more than 2000 m thick, overlying the Mesozoic,Paleozoic and pre-Paleozoic formations (Merla andMinucci, 1938; Blanford, 1869; Zanettin and Justin-Visentin, 1973; Chernet et al., 1998; Coulié et al., 2003).The lower group (Ashangi Group) is composed almostentirely of basalts, and the upper group (MagdalaGroup) includes some rhyolites. Around Mekele, theTrap Volcanics crop out only locally. Ar/Ar dating andmagnetostratigraphy indicate that they were emplaced innorthern Ethiopia between 30.5 and 26 Ma (Chernetet al., 1998; Coulié et al., 2003). In large parts of thehighlands (i.e. in the whole Mekelle outlier) this phaseof eruption was associated with the emplacement a largenumber of doleritic dykes, sills and laccolites within theAntalo Supersequence. The sills are often perfectlyconformable with the strata they intrude, especially inthe Agula Shales, and are up to 50–60 m thick. Theygive rise to prominent steps in the landscape when theoverlying soft rocks are eroded away, and locally are animportant component of the stepped “amba” landscape.

A regional unconformity was firstly recognisedby Blanford (1869) at the base of the basalts. It cor-responds to an extensive erosion surface (Figs. 2 and 3),called by Mohr (1962) “Pre-trappean peneplanation”,which followed a period of tectonic deformation andregional uplift (Abul-Haggag, 1961; Omar and Steckler,1995; Ghebreab and Talbot, 2000). However, the dif-ferences in thickness within the overlying volcanicsseem to indicate an underlying gently rolling landscapewith wide flat sectors rather than a true planation sur-face. Erosion of the basalt cover could lead to exhu-mation of the underlying surface, giving rise to fairlyflat elements in the landscape such as at Mt. AmbaAradam, where a flat erosional surface is preserved ataround 2500 m a.s.l.

2.9. The post-Trap widespread erosion and tectonics

The late Tertiary–Quaternary uplift which followedthe onset of rifting processes (Faure, 1975; Almond,1986; Mohr, 1986; Balestrieri et al., 2005) transformed avast lowland plain into the present Ethiopian highlands,whose altitude largely exceeds 2500 m a.s.l. In this

context, a further important erosional episode extensivelyremoved the Trap Volcanics as well as the underlyingsedimentary rocks, causing the widespread exhumationand the incision of the previous planation surfaces.

3. Discussion

Exhumed surfaces are well-known in different partsof the world and, as in northern Ethiopia, they originatewell back in geological time, though they are now partof the present scenery (Ollier et al., 1988; Twidale,1994; Lidmar-Bergstrom, 1995). Elsewhere in Africaexhumed planation surfaces have been recognisedwhere overlying volcanics have been stripped away. InUganda partial stripping of the Miocene Mount Elgonhas exposed a very flat and deeply weathered sub-volcanic surface (Ollier and Harrop, 1960). A generalscheme of Africa-wide pediplain surfaces was providedby Lester King (1962, 1975), who also includedEthiopia in his scheme but never visited the area. Inreality, the exhumed surfaces of northern Ethiopia donot relate to the King (1975) model, either in mode offormation or age. King believed in a series of pediplainsof world-wide extent, each induced by global epeiro-genic uplift. This is quite different from surfaces relatedto local tectonics, as in Ethiopia, and any correspon-dence with King's ages would be purely coincidental.

More recently, thermo-chronological investigationhave recognised in Kenya episodes of denudation at 55–60 Ma, 100–110 Ma and 180–200 Ma (Foster andGleadow, 1996). These three episodes could fit wellwith the modelling of the PS4, PS3 and PS2 surfaces,respectively.

The planation surfaces of northern Ethiopia wereoriginally unconformities between major rock units withdifferent resistance to the erosional processes. Throughtheir investigation, we can state that the study areaunderwent a series of cyclic events characterised byphases of deposition, where the accommodation spacewas provided by large scale tectonic deformation,alternating with periods during which the previoussediments were firstly uplifted and then planated.

A major phase of deformation occurred before theOrdovician, when the PS1 was moulded across thestrongly deformed basement. Tectonic deformation anduplift occurred before the Late Triassic, when themodelling of the PS2 took place and, to a greater extent,before the Late Cretaceous, when the PS3 was cut acrossthe faulted sedimentary sequences of the Mekelle outlierand the Nile gorge. Uplift also occurred in the EarlyTertiary before the PS4 and the deposition of the TrapVolcanics.


The genetic models and mechanisms of planationare difficult to ascertain. Etchplanation (Wayland, 1933)is not a significant feature in northern Ethiopia, andalthough there are places with fairly deep weathering andduricrusts, none of the investigated surfaces could bedescribed as etchplains. In fact, they are very flat and donot develop in correspondence to weathering horizonseven if older deeply weathered horizons are interlayeredin the sequences and are also intercepted in places. Nordoes the evidence from northern Ethiopia support thesuggestion, made by King (1962) and supported morerecently by Partridge (1998), that planation surfaces canbe created simultaneously at high levels in the landscapeand at the coast because all Ethiopian surfaces have asingle distinct age. Of course, at the present day, currenterosion processes are extending planation surfaces atdifferent levels simultaneously, but when we talk of theage of an exhumed planation surface we refer to the timewhen it originated, not to the time of its exposure. Usingthe normal laws of stratigraphy, the planation surface ageis like an unconformity surface, younger than the rocks itcuts across and older than the overlying sediments.

A prominent genetic role of fluvial processes for PS1,PS2 and PS3 is suggested by the sedimentary facies ofthe overlying deposits (Bosellini et al., 1997) even ifglacial erosion could have contributed to the modellingof PS1.

Marine erosion could also have been responsible forplanation, as suggested by the possible occurrence ofmarine sediments over the PS3, at the base of the AmbaAradam sedimentary sequence. It is worth recalling that inthe nineteenth century many planation surfaces wereinterpreted as surfaces of marine planation, as by Ramsay(1846, 1872), but with the advent of Davisian geomor-phology many were re-interpreted as peneplains. For aninteresting discussion of these ideas see Johnson (1985).

In any case, the Ethiopian planation surfaces seem toindicate that erosional processes repeatedly flatteneddown to the base sea-level a previously deformed anduplifted landscape thus following, at least in part, theclassical cycle of Davis (1899).

4. Conclusion

Apart from the uncertainties on genetic mechanismsand models, the planation surfaces of northern Ethiopiaprovide a fine example of long-term geomorphologicalevolution in a region affected by recurrent phases ofintense tectonic activity, responsible for the emplace-ment of vast sedimentary basins and significant relief,alternating with prolonged periods of tectonic quies-cence, during which planation surfaces were modelled at

relatively low altitude. They provide enough informa-tion to show that conventional models such as pene-planation and pediplanation are inadequate, on theirown, for describing landscape evolution. Here, thesurfaces do not fit into global models such as that ofKing (1975), but relate to local or regional tectonics.

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Planation Surfaces in Northern Ethiopia