The East Africa Oligocene intertrappean beds: Regional distribution, depositional environments and...

Journal of African Earth Sciences xxx (2013) xxx–xxx

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Journal of African Earth Sciences

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The East Africa Oligocene intertrappean beds: Regional distribution,depositional environments and Afro/Arabian mammal dispersals

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⇑ Corresponding author. Tel.: +39 0552757527, +39 3293811094;fax: +39 055218628.

E-mail address: [email protected] (E. Abbate).

Please cite this article in press as: Abbate, E., et al. The East Africa Oligocene intertrappean beds: Regional distribution, depositional environmenAfro/Arabian mammal dispersals. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.11.001

Ernesto Abbate a,⇑, Piero Bruni a, Marco Peter Ferretti a, Cyrille Delmer b, Marinella Ada Laurenzi c,Miruts Hagos d, Omar Bedri e, Lorenzo Rook a, Mario Sagri a, Yosief Libsekal f

a Dipartimento di Scienze della Terra, Università di Firenze, 50121 Firenze, Italyb Department of Biology and Biochemistry, University of Bath, UKc CNR – Istituto di Geoscienze e Georisorse, Area della Ricerca di Pisa, 56124 Pisa, Italyd Department of Earth Sciences, Mekele University, Mekele, Ethiopiae Department of Geology, Khartoum University, Khartoum, Sudanf National Museum of Eritrea, Asmara, Eritrea

a r t i c l e i n f o a b s t r a c t

Article history:Available online xxxx

Keywords:Intertrappean bedsVolcanic quiescenceOligocene mammal radiationDeinotheriidaeGomphotheriidaeProboscidean datum event

The extensive outpouring of the Oligocene Trap basalts over eastern Africa and western Arabia was inter-rupted by a period of quiescence marked by the deposition of terrestrial sediments. These so-called inter-trappean beds are often lignitiferous and yield recurrent floras and faunas, sometimes represented byendemic mammals. We intended to highlight the peculiar features of these sedimentary intercalationsusing a large-scale approach including eastern Africa and the western Arabian peninsula.

Starting from a new mapping in the Eritrean highland, the intertrappean beds resulted a continuouslevel that was a few tens of meters thick and traceable for some tens of kilometers. They consist of fluvialred, green and gray mudstones and siltstones with subordinate channelized pebbly sandstones, and lig-nite seams. Two new 40Ar–39Ar datings constraint the age of the intertrappean beds between 29.0 Ma and23.6 Ma. The outcrops near Mendefera have yielded the remains of two proboscidean families, the Dei-notheriidae and the Gomphoteriidae. The morphological grade of the two Mendefera proboscideanswould suggest a more derived stage than that of representatives of the same families from other Oligo-cene African sites (e.g., Chilga, Ethiopia). An Oligocene age could be inferred for them. The occurrence ofthe genus Prodeinotherium at Mai Gobro possibly represents the first occurrence of this taxon, while theGomphotheirum sp. might represent the oldest occurrence of this taxon in Africa before its dispersaltowards Asia and Europe. Proboscideans have also been found in the lowland intertrappean beds ofDogali near Massawa. These sediments were contiguous with the Eritrean highland intertrappean bedsduring the Oligocene, but are now tectonically displaced from them by two thousand meters of verticaltopographical distance. Dogali is also known for the occurrence of possible Deinotheriidae remains andthe primitive elephantoid Eritreum. Entering the Ethiopian highland, an inspection of the Agere Selam(Mekele) intertrappean beds revealed the occurrence of lacustrine limestones and diatomites, whichwere contrastingly quite subordinate with respect to the fine clastic sediments found in the nearby AmbaAlaji area. Further south, the intertrappean section in the Jema valley (100 km north of Addis Ababa andclose to the Blue Nile gorge) is 120 m thick with predominant clastic sediments and a few diatomites atthe top. Literature information from 35 additional sites, including northern Kenya, Yemen, Sudan andSaudi Arabia sections, confirms the fluvial and lacustrine depositional environment of the intertrappeanbeds, underlines the interest in their mammal fauna (Chilga, Losodok), and reports exploitable coal seamsfor some of them. As for the vegetal landscape in which the intertrappean beds were deposited, pollenand plant analysis results indicative of a tropical wet forest, similar to that of present-day western Africa.Another common feature of the intertrappean beds is their relatively limited thickness, averaging a fewtens of meters, but reaching a few hundred meters in graben-related basins, such as Delbi Moye in south-ern Ethiopia. In most cases only thin, lens-shaped successions were deposited above the hummockytopography of their volcanic substratum, commonly unaffected by significant faulting. An average dura-tion of the intertrappean beds is from one to three million years. This time interval is commonly matchedby a few tens (or more rarely, hundreds) of meters of sediments left over after erosive episodes or depo-sitional starvation. As to the lateral continuity of the intertrappean beds, the present-day outcrops show

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large differences: from some tens of kilometers in the Mendefera area, to a few tens of kilometres in theJema valley, and to a few hundreds meters in the Agere Selam (Mekele) area. Even if it is difficult to quan-tify the original size of the sedimentation areas, it nevertheless proves that the intertrappean basinsexceed thousands of square kilometers in only a single case (Mendefera), but were quite restricted inmost cases. Their most likely endorheic and local character, together with a regional ill-defined fluvialnetwork, was the effect of a water-course rerouting caused by the progressive rising of the eastern Afri-can and Arabian plateaux. Chronological constraints for the intertrappean beds can be inferred from theage of the hosting Trap succession and by the stratigraphical position that they occupy. Intervolcanic sed-imentary episodes are typically found in the basaltic and subordinately rhyolitic successions that fol-lowed the 31–29 Ma old basaltic widespread paroxysm. With due caveats deriving from thediscontinuous availability of datings specifically dedicated to this issue, we regard the age of the inter-trappean beds as mostly encompassed in the interval from 29 to 27 Ma at the transition between theEarly and Late Oligocene in the Ethiopia/Yemen Trap core. In marginal areas, such as SW Arabia, Eritreaand Kenya, the volcanic activity above the intertrappean beds resumed later, and its quiescence allowed amore prolonged period of sedimentation. The intertrappean beds fall in the second cooling event of theOligocene climatic deterioration. During the contemporaneous apparent drop in the global sea-level andclosure of the Tethyan Ocean between Arabia and southwestern Asia, connections were establishedbetween the African and the Eurasian continents. At that time, southwestern Asia was experiencingsevere aridity with faunal exchanges toward the luxuriously vegetated eastern Africa.

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1. Introduction

A Proterozoic basement overlain by discontinuous Paleozoic toMesozoic sediments, in turn extensively covered by mainly Oligo-cene to Quaternary volcanic rocks, characterizes wide areas of east-ern Africa and western Arabia. During the Oligocene the vertebratefauna in these regions underwent important differentiations, regio-nal radiations and intercontinental exchanges. Yet, the scarcity ofterrestrial Oligocene rock outcrops, particularly in eastern Africa,sets a serious limit to reconstructing the time and the mode of theseevents. To overcome this difficulty, we began with some well-known occurrences of Oligocene sediments and fossils in Eritrea,assessed their possible extension, searched for new fossil faunas,and eventually addressed our field research and literature searchesto nearby regions (Ethiopia, Sudan, Kenya, Yemen and Saudi Arabia).

A common attribute to the Oligocene sediments that we inves-tigated was their position within the Afro-Arabian flood volcanicsuccession. The latter, traditionally described as a Trappean Seriesor Traps (Blanford, 1870), extensively covers the highlands fromEritrea to Kenya and from western Saudi Arabia to Yemen (Figs. 1and 2). Whereas there is a general agreement that this volcanicprovince is because of an exceptional supply of mantle material,the number of involved plumes (whether one or more) and theirlocation (Afar, Kenya, African superswell) are still under debate(e.g. Schilling, 1973; White and McKenzie, 1989; Ebinger andSleep, 1998; George et al., 1998; Ritsema et al., 1999; Mesheshaand Shinjo, 2008). The Traps outpouring has been regarded alsoas a possible cause of climatic deterioration and ensuing massextinctions on a global scale (Courtillot et al., 1988; White andMcKenzie, 1989; Rochette et al., 1998).

In the Eritrean, northern Ethiopian and Yemen highlands, thebulk of the Traps was erupted as flood basalts during the Early Oli-gocene between 31 and 29 Ma (Drury et al., 1994; Hofmann et al.,1997; Ukstins Peate et al., 2005), whereas the overlying volcanites,erupted after a period of quiescence are predominantly Late Oligo-cene to Miocene with rhyolites/trachytes alternating with basaltsof fissural and central activity. This late volcanism has been con-nected to the development of the Afar depression and East Africarift valleys since the Early Miocene.

In southern Ethiopia and northern Kenya, the volcanic activitybegan earlier toward the end of the Eocene (Davidson and Rex,1980; Ebinger et al., 1993; McDougal and Brown, 2008).

The Trap outpouring experienced periods of relative lulls duringwhich terrestrial sediments were deposited. They constitute the‘‘intertrappean’’ beds reviewed in the present paper.

t al. The East Africa Oligoceneci. (2013), http://dx.doi.org/10.

The biological activity was heavily depressed by the harsh eco-logical conditions during the intense and continuous Trap volca-nism. However, faunas and floras prospered and re-colonized theterritories during the volcanic stillness and sediment accumula-tion. The intertrappean beds thus contain good data for recon-structing Oligocene environments, climates, paleogeography, andbiological events.

Intertrappean beds have been reported from many eastern Afri-can regions since the nineteenth century because of their ligniteseams and fossil remains (e.g. Blanford, 1870; Aubry, 1885;Dainelli and Marinelli, 1912; Rodriguez, 1919; Grabham and Black1925; Arambourg, 1933; Stefanini, 1933; Merla and Minucci, 1938;Usoni, 1952; Getaneh and Saxena, 1984). In our review we willexamine the classic Mendefera (formerly called Adi Ugri) fossilifer-ous occurrence in Eritrea (Vialli, 1966), and will describe furtherintertrappean outcrops in the Eritrean highland and lowland. Mov-ing southward, we will take into account similar outcrops in north-ern and southern Ethiopia, adding new data from our fieldobservations (Fig. 2). Interregional correlations and comparisonswill be defined through the results of a field survey in Sudan andsearches of available literature in Yemen, Kenya and Saudi Arabia.

2. Methods

Because the intertrappean beds are commonly thin and poorlyconsolidated, their lithology, aerial continuity, stratigraphical posi-tion, age assignment and fossil contents have been generally disre-garded or poorly studied. For decades, research attention has beengiven mainly to the lignite seams intercalated in these sediments.

In our work, we tried to fill this knowledge gap with new fieldinvestigations and re-assessments of previous studies. We appliedclassical geological field methods, including geological mapping,satellite image analyses with field controls, sedimentary logmeasurements, and dedicated sample collection. In this researcha major issue was the dating of the intertrappean beds. Their fossilcontent is generally poor, and the vertebrate species, althoughremarkably evocative of the Oligocene faunal differentiation andenvironment, are moderately time indicative. A positive conditionis that the intertrappean beds are by definition associated with vol-canic (mostly basaltic) rocks, which can be radiometrically dated,although with some uncertainty. Yet, the relationship betweenthe volcanic rocks and the intertrappean beds covers a wide spec-trum of situations. We list them in descending order of suitability:(1) radiometric dating of basalts immediately over- and/or under-lying the intertrappean sediments (upper and lower basalts); (2)

intertrappean beds: Regional distribution, depositional environments and1016/j.jafrearsci.2013.11.001


20°N

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20°N

10°N 10°N

R E D S E A

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Mekele.

.ASMARA

AdenLakeTana

TurkanaLake

40°EE°63 E°63

.

.

Tekeze R.

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ADEN

Omo R.

Blue Nile

Harar

..

Jima

Massawa. .

.

SANA’A

Jiddah

Awash R.

ADDIS ABABA

DJIBOUTIDese

Paleozoic to Quaternary sedimentary rocks Proterozoic basement

Plio-Quaternary (subord. Late Miocene) volcanic flows and edifices Main Ethiopian Rift volcanic province, Late Miocene to Quaternary

Afar Depression volcanic province, Miocene to Quaternary

Traps: Oligocene to Miocene volcanic flows and edifices (includes Late Eocene volcanites in S.Ethiopia and Kenya)

Fig. 1. The Cenozoic volcanic rocks in NE Africa and SW Arabia (modified after Merla et al., 1979; Davidson and Rex, 1980; Mohr and Zanettin, 1988; Davison et al., 1994;McDougall and Brown, 2008; Stern and Johnson, 2010).

E. Abbate et al. / Journal of African Earth Sciences xxx (2013) xxx–xxx 3

dating of basalts intercalated in the sediments; (3) dating throughcorrelation with dated basalts of nearby areas; (4) age estimationbased on the assumed position of the intertrappean beds withinthe Trap succession.

Please cite this article in press as: Abbate, E., et al. The East Africa OligoceneAfro/Arabian mammal dispersals. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.

We will see that the most favorable cases are not always recur-rent. When the ages of the embedding basalts were unavailable, weresorted to the nearest dated basalts. Because these basalts areseparated from the base or top of the intertrappean beds by a



Fig. 2. Schematic map of the Oligocene Trap succession (gray) with location of the intertrappean bed sites reviewed in the text (map modified after: Merla et al., 1979including Ashangi and Aiba basalts and Oligocene Alaji rhyolites; Davison et al., 1994 including Late Eocene volcanic rocks in southern Ethiopia and Kenya; and Stern andJohnson, 2010). Triangles: intertrappean beds with fossil proboscideans; circles: intertrappean beds with no reported fossil mammals; underlined numbers indicatesedimentary logs in the text figures. Site names: 1 – Harrat Hadan (Arabia); 2 – Odi basin (Sudan); 3 – Sana’a (Yemen); 4 – West of Sana’a (Yemen); 5 – Lahima region(Yemen); 6 – Dhi Sufal (Yemen); 7 – Osailea (Yemen); 8 – Dogali (Eritrea); 9 – Adi Zerna (Mendefera); 10 – Mai Gobro (Mendefera); 11 – Agere Selam (Mekele); 12 – AmbaAlaji (Mekele); 13 – Wuchale (Dese); 14 – Magdala (Dese); 15 – Atbara valley (Lake Tana); 16 – Chilga (Lake Tana); 17 – Gorgora peninsula (Lake Tana); 18 – Molale (Dese); 19– Debra Sinah (Dese); 20 – Mush Valley (Dese); 21 – Ankober (Addis Ababa); 22 – Cassam Valley (Addis Ababa); 23 – Sululta /Addis Ababa); 24 – Mugher Valley (AddisAbaba); 25 – Jema Valley (Addis Ababa); 26 – Mojo (Addis Ababa); 27 – Nejo-Aleitu Valley (Nekempte); 28 – Didessa Valley (Nekempte); 29 – Dilla valley (Nekempte); 30 –Yayu (Jima); 31 – Lalo Sapo (Jima); 32 – Delbi Moye (Jima); 33 – Gogeb Kida (Jima); 34 – Kella (Guraghe); 35 – Amaro Ridge (Burji); 36 – Agere Salam (Sidamo); 37 – KibreMengist (Sidamo); 38 – Losodock (Lake Turkana); 39 – Nacway (Lake Turkana); 40 – Lokone (Lake Turkana).


certain thickness of volcanites, we had to estimate a time intervalpertaining to the intervening pile of basalts. Considering the pub-lished outpouring rate, it has been possible to obtain approximatedates for the onset or end of the intertrappean bed deposition.

As for radiometric dating, Ar/Ar ages were available for manysites. In a few cases uncertainty arose when only K/Ar ages fromold analyses were accessible. As concerns the error of cited datarefer to the original papers.

In the framework of this contribution, two 40Ar–39Ar dates wereobtained from the Mai Gobro section in Eritrea (Appendix A)


Paleomagnetic studies of some sections offered further supportto the radiometric dating of the intertrappean sediments.

In this paper, we use the numerical time scale from Gradsteinet al. (2012) with the Eocene/Oligocene boundary at 33.9 Ma andthe Oligocene/Miocene boundary at 23.0 Ma.

Large vertebrate fossils were located by surface prospecting,and all bone and dental fragments were collected. The presentdescription of Eritrean proboscideans is based on the originalmaterial kept within the Eritrea National Museum collections.The nomenclature of dental characters follows Tassy (1985,




1996). Comparisons with other African specimens were based onthe published descriptions and figures (Kappelmann et al., 2003;Sanders et al., 2004; Ducrocq et al., 2010).

3. The intertrappean beds in Eritrea: the Mendefera and Dogalistudy areas

3.1. The Mendefera area

The recognition and mapping of a sedimentary level continu-ously outcropping in the Trap succession of the Eritrean highlandadds new data to the history of this volcanic pile, provides novelelements to the geological reconstruction of the Adi Quala-Asmaraarea, and bears significant paleontological and biogeographicalimplications (Abbate et al., 2012).

In the past, there have only been rather patchy reports for thissedimentary intercalation. Clays that are associated with rhyoliteshad first been described and sampled by Dainelli and Marinelli(1912) 15 km west of Mendefera. Merla and Minucci (1938) con-firmed the presence of this sedimentary level in the Traps and itsassociation with liparites. A lignite mine (Adi Zerna mine) discov-ered by Rodriguez (1919) 5 km northeast of Mendefera yielded atooth of a large-bodied mammal determined by Vialli (1966) asDeinotherium cf. hobleyi and assigned to the early Miocene. Thespecimen (a lower m2, kept in the collections of the Museo Geolog-ico G. Capellini of the University of Bologna) is more correctly re-ferred to Prodeinotherium cf. hobley. More recent geological mapsand reports (e.g. Drury et al., 1994; Zanettin et al., 1999) promptedour search for more outcrops of this sedimentary intercalationwithin the Traps between Asmara and Adi Quala (Fig. 3). Thesebasalts are regarded as the northern extension of the volcanic suc-cession widely exposed in northern Ethiopia. In Eritrea they havebeen studied by Zanettin et al. (1999, 2006a,b) who provided anupdated regional stratigraphy summarized as follows. Above theNeoproterozoic basement, discontinuously covered by Paleozoicto Mesozoic sandstones and laterite (see among others, AndrewsDeller, 2006; Ghebreab et al., 2009), there is a widespread Trap car-apace. From north (Asmara) to south (Adi Quala) Zanettin et al.(1999, 2006a) distinguished the Asmara basalt, the coeval Alaji ba-salt and the overlying Adi Ugri basalt. Because of some complexrelations among these units, we chose in this paper to refer to alower basalt succession, including the Asmara and Alaji basalts,and an upper basalt succession corresponding to the Adi Ugribasalt. Each of these new groupings has a thickness of up to400–500 m.

Between the lower and the upper basalts, we traced the inter-trappean beds. This continuous sedimentary level is evident inthe landscape by its different responses to erosion, and can be usedas a regional marker (Fig. 3).

Very good exposures occur from Adi Zerna to the new fossilifer-ous sites of Mai Gobro (10 km east of Mendefera), and along theMendefera-Barentu road. Their thickness averages 20 m andreaches 50 m along the Barentu road.

The predominant lithologies are silt and mudstone, commonlyred (Fig. 4) and subordinately greenish and gray. Basalt flows andsills are also present (Mai Gobro, Haserlibo and along the road toBarentu) as well as rhyolite layers and lenses (Serae rhyolite ofZanettin et al., 2006a). The latter are particularly well exposed20 km NNE of Adi Quala (Fig. 5) where represent a traceable levelin the landscape. A further rhyolite level (up to 20 m thick) occursat the base of the lower basalts in the Adi Quala area.

Coarser clastic lenses are scatterly present and well developedat Adi Zerna (Fig. 3, location 1) and its neighbourhood along thefoot of the eastern escarpment of the upper basalts.


Two relatively favorable exposures in the Mai Gobro area(10 km east of Godefelassi) allowed the detailed measurement ofthe Mai Gobro 1 and Mai Gobro 2 sections (Fig. 6). They entirelycover the approximately 20-m thick intertrappean level.

Mai Gobro 1: This section begins with 3 m of silty mudstonesfollowed by a channelized body (about 10 m thick) with conglom-erates and sandstones overlain by the upper basalt flows through a3 m thick talus. The silty mudstones are massive, reddish to brown,intensely pedogenized with abundant Fe and Mn nodules. Thechannelized body consists of three gravelly to sandy channels witha multi-storey internal architecture. The channels are a few meterthick, about 50 m wide, and show an asymmetric cross section.They are floored by up to one meter of coarse lag deposits, formedby rounded to sub-rounded silty mudstone clasts dispersed in asandy matrix (Fig. 7), or by clast-supported basalt pebbles. In thelower channel the lag deposits are covered by a 40 cm thick setof planar cross-bedded sandstones dipping about 15–20�, trans-verse to the channel axis, and cross laminated sandstones with ahuge clay chip rich in fossil leaves. The lag deposits of the remain-ing two channels are covered by massive or horizontal laminatedconglomerates and sandstones with abundant fossil leaves.

The lower channel yielded the new found mammal fossils (GPS:N14�52.4840; E38�53.0660) (Abbate et al., 2012). These representtwo proboscidean families, Gomphotheriidae and Deinotheriidae.An isolated, complete bilophodont P4 (upper fourth premolar) isreferred to a relatively small-sized deinothere, while a second pro-boscidean, a Gomphotheriidae, is documented by a fragmentaryright maxillary bearing an M2 and M3 and by an isolated molarcone.

Mai Gobro 2: This log (Fig. 6) is located at a distance of about500 m toward north from Mai Gobro 1. A channelized fining-up,7 m thick body of conglomerates and sandstones is interbeddedbetween a few meters thick silty mudstone. The latter is predom-inantly massive, reddish to brown and subordinately gray, mottled,intensely pedogenized with abundant Fe and Mn nodules. Towardthe top a kaolinitic, laminated bed is present. In the lower portionbelow the coarse clastics a thin basalt flow is interbedded. At dif-ference with the Mai Gobro 1 section, only one channel is presenthere. It exhibits a conglomeratic lag followed by conglomerate andsandstone beds generally showing parallel lamination. The upper-most sandy level is intensely bioturbated and pedogenized. Plantremains are abundant in the upper portion of the channel filling.

Two basalt samples collected immediately at the base and andat the top of the intertrappean beds (Fig. 6) provide new precisechronological constraints (see later).

Supplementary sections of the intertrappean beds have beeninvestigated to better define the Mendefera level in the nearbyareas. They include the dismissed mines of Adi Zerna and Mai Car-bon/Enda Manuel, a section close to Haserlibo, and exposuresalong the Barentu road. Although some of the exposures are heav-ily spoiled by the mining operations, the following details werecollected.

At Adi Zerna 10 m of red silty mudstones containing at the basesome thin beds of pinkish and white rhyolites rest below the lig-nite level less than one meter thick according to previous authors.Immediately below the lignite level, the silty mudstones are darkgray with gypsum crystals, siderite and pyrite nodules and abun-dant fossil leaves. In the mudstone layer underlying the ligniteseam Rodriguez (1919) reports the occurrence of sandstones nowpresumably covered by the talus. This section is capped by acolumnar 4.50 m thick basalt flow.

At Mai Carbon half way between Adi Zerna (Fig. 3, location 1)and Mai Gobro (Fig. 3, location 2) a 40 cm thick lignite seam withblack organic shales and well-preserved leaves (Fig. 8) has beenexploited during the last century. The local section starts with5 m of red mudstones, 2 m of yellowish mudstones and 5 m of



Fig. 3. Schematic geological map of the Eritrean highland between Asmara and Adi Quala showing the distribution of the intertrappean beds and the locations of thefossiliferous sites of Adi Zerna (1) and Mai Gobro (2).


basalt flow underlying the lignite seam. The section ends with 6 mof basalt flow covered by 5 m thick reddish silty mudstones. Thelatter include lenticular basalt flows.

Still, NNE of Mendefera, close to Haserlibo village, the inter-trappean beds are well exposed along the local creek (Fig. 9).They consist of a 20-m thick basal portion with alluvial plainmassive pinkish/whitish silts and red pelites, followed by a 3-mthick basalt, most likely a dyke. Overlying sediments are locallenses of conglomerates and sandstones interbedded in 8-m thickbrownish to red pelites. The upper basalts close the localsuccession.

Furthermore, a 50-m thick section 15 km west of Mendeferaalong the Barentu road is particularly interesting due to the pre-dominant massive red silty mudstones stuffed with basalt sillsand flows. The only coarse grained deposits are small channelsfilled by reworked caliche nodules.


Based on the features observed and described above, we canassume that the sedimentary environment during the depositionof the intertrappean beds was represented by flood plains thatwere subjected to intense pedogenesis and crossed by straightand low sinuosity fluvial channels. Swamps, where organic mate-rial was accumulated, and pools with kaolin deposition were inter-spersed in these flood plains. The volcanic activity was drasticallyreduced during the deposition of the intertrappean sediments, onlyinterrupted by rare basalt flows and rhyolite eruptions.

Correlation with the Adi Zerna and the Mai Gobro outcropsshows that the rhyolite within the intertrappean beds rests a fewmeters below the fossiliferous horizon. Thus, the rhyolite datedto 24.6 ± 0.25 Ma (K/Ar) by Zanettin et al. (2006a) provides a max-imum age for the mammal bones.

For the Asmara/Alaji basalt outcrops in the map of Fig. 3 the fol-lowing radiometric datings are available:



S

clgB2

Fig. 4. The intertrappean beds (S) represented by red silt and mudstone with minorsandstone and fine conglomerate (clg) rest below the upper basalts (B2), roadsection between Mendefera and Adi Quala. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

B1

B2S

SR

Fig. 5. The intertrappean beds (S) including a rhyolite body (R) are interposedbetween the lower (B1) and upper basalts (B2), 20 km NNE of Adi Quala.


– Basalt, at Emba Tekera, 26 km southwest of Asmara, base of theTrap succession, ca 32 Ma (whole rock, 40Ar–39Ar) (Teklay et al.,2003 in Teklay et al., 2005).

– Basalt, at Teramni directly above the laterite on the basement,29.6 ± 0.6 Ma (40Ar–39Ar) (Zanettin et al., 2006a).

– Basalt, northeast of Mendefera, 100 m above the laterite,28.5 ± 3.2 Ma (40Ar–39Ar) (Drury et al., 1994).

– Basalt, in the middle-upper portion of the Addi Quala section28.6 ± 0.3 (K/Ar) (Zanettin et al., 2006a).

– A new 40Ar–39Ar dating at the top of the Asmara/Alaji basalts inthe Mai Gobro 2 section gave an age of 29.04 ± 0.57 Ma (2r)(Fig. 6, Appendix A. Fig. A.1).

From the screening of published and new data, we assume thatthe Asmara/Alaji basalts below the intertrappean beds startedsince 30 Ma and ended before 29 Ma.

Three published radiometric ages can be used to date the AdiUgri Basalt above the intertrappean beds.

– Basalt, at Emba Tekera, 26 km southwest of Asmara, from thetop of a 500 m Trap succession, ca. 20 Ma (whole rock,40Ar–39Ar) (Teklay et al., 2003 in Teklay et al., 2005).

– Basalt (two samples), Adi Zerna mine, 22.72 ± 0.33 Ma (K/Ar)and 22.15 ± 0.18 Ma (40Ar–39Ar) (Zanettin et al., 2006a).


– A new 40Ar–39Ar dating at the base of the Adi Ugri Basalt in theMai Gobro 2 section gave an age of 23.55 ± 0.45 Ma (2r) (Fig. 6,Appendix A Fig. A.2).

In conclusion, the intertrappean beds in the Mendefera area canbe chronologically placed between 29.0 Ma and 23.6 Ma across thelongest time span of all reviewed sedimentary intercalations.

3.2. The Dogali area

Moving to the Eritrean lowland, sedimentary intercalations inOligocene basalts yielding proboscidean remains are also reportedat Dogali a few tens of kilometers west of Massawa (Sagri et al.,1998; Shoshani et al., 2006) (Fig. 2, site 8). There, a ca. 500 m thicksuccession of basalts and associated alternation of sandstones, con-glomerates, lacustrine silts, volcanoclastic sediments and coarseclastics has been described by Kazmin (1973) and Drury et al.(1994) and called the Dogali Formation. The same authors recog-nized that this unit rests through a laterite above the Neoprotero-zoic basement and that the basalts could be correlated with theEritrean plateau basalts. Before the development of the southernRed Sea marginal escarpment these basalts were contiguous tothose of the Eritrean highland.

The Dogali basalts have the lowermost flows dated to28.0 ± 2.5 Ma (40Ar–39Ar) (Drury et al., 1994).

In the Dogali-Desset area Sagri et al. (1998) distinguished abovethe basal basalts (‘‘lower basalts’’) 150 m of predominant siliceoussediments (intertrappean beds) conformably capped by about150–200 m of basalts (‘‘upper basalts’’). The intertrappean sedi-ments cover a 20 by 10 km area and a log has been measured threekilometres northwest of Dogali (Figs. 10 and 11).

In the lower 40 m of the section the prevailing lithofacies is rep-resented by thinly laminated siliceous deposits from yellowish toblackish, alternating with basalt flows and tuffs in beds from fewcm to 10 m thick. The overlying 40 m consist of dark gray lami-nated siliceous mudstones with subordinate graded siltstones inbeds up to 10 cm thick. The section continues with a channelizedbody, up to 10 m thick, including well-rounded basalt pebbles, fol-lowed by pebbly sandstones, horizontally laminated sandstonesand siliceous mudstones. The channel fill contains silicified treetrunks and is overlain by 5 m of graded sandstones and siltstonesin beds up to 30 cm thick with abundant plant remains. Upward,a partially covered interval consists of thick basalts and tuffs with90 cm of thinly laminated siliceous mudstones and a 150 cm thickacidic ash layer. 12 m of laminated silty mudstones alternatingwith graded fine sandstones and siltstones follow. The section con-tinues with 8 m of basalt flow and 5 m of siltstones alternatingwith graded fine sandstones with convolute lamination. After7 m of talus the section shows about 150–200 m of basalt flowsand tuffs (upper basalts).

The basal siliceous deposits, referable to diatomites, indicate alacustrine environment occasionally interested by pyroclasticmaterial and basalt flows. A gradual supply of terrigenous materialpreceded an episode of lake desiccation marked by a fluvial chan-nel. The lacustrine environment resumed with the deposition ofsiliceous mudstones and graded sandstones interbedded with vol-canic materials. If compared with the once contiguous Mendeferaintertrappean beds, the lacustrine deposits are here significantlymore abundant.

A partial Deinotheriidae tooth was found by Sagri et al. (1998)along the road Asmara-Massawa in the coarse grained channelizedbody. From the same layer, but 1500 m southeast, comes the typematerial of the primitive elephantiform Eritreum melakeghebrek-ristosi (a left and right mandibular rami with m2 and m3, and por-tions of mandibular symphysis with remnants of tusk alveoli)described by Shoshani et al. (2006).



Fig. 6. Stratigraphic log of the intertrappean beds in the Mai Gobro area (site 2 in Fig. 3) with new geochronological data.


A basalt flow intercalated in the sediments overlying this fossil-iferous layer has an 40Ar–39Ar age of 26.8 Ma ± 1.5 (Shoshani et al.,2006) and is followed by more than 50 m of sediments.

Taking into account the age reported by Drury et al. (1994) forthe base of the lower basalts and the 26.8 Ma old intercalated ba-salt, we can assign a tentative age of 27.5 Ma to the base of theintertrappean sediments. The age of the basalts immediately abovethe sediments is uncertain, but can be estimated to be approxi-mately 24 Ma assuming a correlation with the Mendefera upperbasalts outcropping in the nearby Eritrean highland.


3.3. The new fossil findings

We offer here the fossil remains recently found in the area ofMendefera, together with remarks on their significance in theframe of the Afro/Arabian proboscidean evolution.

– Deinotheriidae. An upper right bilophodont cheek tooth fromthe Mai Gobro 1 section is attributable to a deinothere(Fig. 12B and C). With a quadrangular, almost square outlinein occlusal view, it is relatively small (Table A.1; Fig. 13A), with



Fig. 7. Lower portion of the Mai Gobro 1 section: fluvial channel carved in thepedogenized red mudstone and filled by coarse sandstone and pebbles. (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

B

ß

ßS

S

Fig. 8. The Adi Ugri basalts (upper basalts, B) above the siltstones and black shalesof the intertrappean beds (S) intruded by a sill (b). Near Mai Carbon 8 km NE ofMendefera.

B1

B2

S

S

Fig. 9. The intertrappean beds (S) between the lower Asmara/Alaji basalts (B1) andthe upper Adi Ugri basalt (B2) 3 km north of Adi Zerna (site 1 in Fig. 3). Prevailingreddish mudstones and siltstones and rare fine sandstones make up the intertrap-pean beds. A basalt intercalation, probably dike, is interposed in the succession (b).Two encircled persons give the scale.


its ectoflexus and entoflexus reduced to absent. The lingual wallof the tooth bears no sign of a cingulum, whilst a crenulatedlabial cingulum can be observed between the protoloph andmetaloph. The protoloph is slightly higher than the metaloph;in both lophs, the lingual cones (protocone and hypocone) aremore worn than the labial ones, showing a distinct triangulardentin area. A strong post-paracrista extends posteriorly fromthe paracone and joins with a distinct pre-metacrista in theinterloph, forming a distinct centrocrista.

The thickness of the enamel observed on the worn lophs, to-gether with the presence of a worn surface on its anterior wall(demonstrating the presence of another tooth anteriorly) lead usto conclude that this tooth is not a dP3. Its size, square outlineand the absence of an observable trace of a convolute (sensuGräf, 1957) on the metaloph suggest that this tooth is a right P4.The development of the ectoloph regularly observed on P4 of dei-notheres is known to be very variable (Delmer, pers. obs.) and thestrong centrocrista present on this tooth is compatible with thisidentification. However, since the metaloph is much worn, it isimpossible to completely rule out the presence of a convolute on


this tooth before its wear. This tooth could therefore alternativelybe identified as a M2 of a very small-sized deinothere (Fig. 13B).

Due to the preservation of the fossil, to its specificities describedabove and to the lack of other data available on this taxon in thislocality, we refer to this specimen here as ‘‘cf Prodeinotherium’’.Its size and primitive characteristics (absence of a true ectoloph)do suggest its referral to the genus Prodeinotherium. However,there is too few data available to date to suggest that it belongsto any species described for this genus, As this tooth is non typicalfor a P4 of a deinothere, and as we cannot completely rule out itsidentification as a M2 that would be even less typical, we favorhere a putative referral to the genus Prodeinotherium. We expectthat additional material referable to this taxon to be discoveredin this locality or in similar horizons will contribute to the identi-fication of this specimen at a species level in the future.

– Gomphotheriidae. A fragmentary left maxillary with a completeM2 and a partial M3 represents a trilophodont gomphothere(Table A.1). The M2 (Fig. 12A) possesses three lophs and a welldeveloped posterior cingulum. Each halfloph is composed by alarger, stocky abaxial cone (main cone) and a smaller mesocon-elet. The first and second pretrite halflophs possesses a largeand low anterior central conule (acc). The pretrite acc1 formsa labially directed crest that joins the anterior cingulum. Pre-trite acc2 extends labially to the anterior side of the posttritemesoconelet. A bulging on the anterior side of the third pretrite



Fig. 10. Geological sketch map of the Dogali area (site 8 in Fig. 2) with the locationswhere proboscideans were collected (1: Sagri et al., 1998; 2: Shoshani et al., 2006)in the intertrappean beds.


halfloph might be interpreted as an incipient acc3. No posteriorpretritre central conules are present. There are no central con-ules on the posttrite halflophs. The anterior cingulum continuesboth labially and lingually, forming robust labial and lingualcingula. There is no trace of cement. Of the M3, only the firsttwo lophs are preserved, with the second one (metaloph) miss-ing the posttrite half. The lophs structure is similar to that of theM2.

The size of the M2 falls at the lower end of the size range ofEarly-Middle Miocene Gomphotherium from Eurasia. The specimenis primitive with respect to typical Middle Miocene G. angustidensin being relatively small, with robust, low cones, absence of ce-ment, absence of pretrite posterior central conules and the occur-rence of a strong labial and lingual cingulum rimming the base ofthe tooth crown. Some of these traits are shared with species ofthe so-called G. annectens group (Tassy, 1994), the earliest andmost primitive representative of the genus Gomphotherium in Eur-asia. No direct comparison with the cf. Gomphotherium from Chilgais possible, as only lower molars are known from that site (Sanderset al., 2004). The two forms differ in size, with the Chilga gompho-there being smaller, a possible primitive trait.

4. The intertrappean beds in northern Ethiopia: the Agere Selam(Mekele), Amba Alaji and Jema valley study areas

Among the numerous intertrappean sites known in Ethiopia(Fig. 2) we carried out stratigraphic sections and maps in three ofthem for which only some generic information was available. Fromnorth to south, they are: Agere Selam, Amba Alaji and the JemaValley.


4.1. Agere Selam (Mekele)

Limited outcrops of a few hundred meters of Trap basalt uncon-formably overlie laterites and Cretaceous sandstones of the MekeleMesozoic outlier (Merla and Minucci, 1938) (Fig. 2, site 11). Withinthese basalts near Agere Selam, a village 40 km west of Mekele,Merla and Minucci (1938) and Arkin et al. (1971) reported lacus-trine deposits with silicified limestones containing diatomitesand gastropods (Melanopsis). On the northern slope of a hill NE ofAgere Selam the first one hundred meters of the Trap basalt succes-sion are exposed. Above these basalts we recognized a ca. 100 mthick, partially covered, sedimentary level (Fig. 14) of which wemeasured two sections totaling 60 m.

The lower section (Fig. 15) directly above the basalt (lower bas-alts) starts with a few meters of gray shales with fish scales, fol-lowed by massive diatomites (4 m) (Fig. 16) with gastropods andgreen massive to laminated shales with ostracods and fish scales.After 8 m of talus, the section includes massive to laminated diat-omites with cherty nodules and Mn coatings, and 4 m of greenmassive to laminated shales with fish scales. A 60 cm thick fine tuffwith vitreous shards and small crystals and a 120 cm thick thinlystratified calcarenite close the section. These calcareous beds, lo-cally with wavy lamination and cherty nodules, are oolitic withgastropods and ostracods.

The upper section, 30 m thick, is just below the resumption ofthe basalt (upper basalts) outpourings (Fig. 15). It consists ofapproximately 30 m of laminated diatomite beds with mm to cmthick greenish shaly interbeds and rare intercalations of oolitic cal-carenites and slumped diatomite with gastropods, pelecypods andoogons.

The abundance of diatomites, generally laminated, alternatingwith thin greenish shales indicates a lacustrine environment farfrom terrigenous inputs for these intertrappean sections. Lake mar-gin instabilities could have caused occasional slumps and rework-ing of the oolites from marginal areas.

Because no dates are available for the Agere Selam lower andupper basalts we must rely on the closest dated sections of Adigrat(80 km NNE of Agere Selam) showing similar reduced Trap thick-ness. The age of the base of the Adigrat Trap is 30.4 ± 0.4 Ma(40Ar–39Ar age, Hofmann et al., 1997) and 29.2 ± 0.7 Ma(K/Ar age, Jones and Rex, 1974), and the base of the upper basaltsshould not be younger than the top of the Adigrat section that isdated to 22.8 ± 0.7 Ma (K/Ar age, Jones and Rex, 1974). A tentativetime span between 29 Ma and 24 Ma, particularly uncertainbecause of the distance from the dated sections, could be assumed.

4.2. Amba Alaji

Merla and Minucci (1938) in their ‘‘epoch-making work’’ (Mohr,2009) on the geology of Tigrai described at the Amba Alaji (Fig. 2,site 12), a prominent pyramidal peak 60 km south of Mekele(Fig. 17), ca. 60 m of marlstones, thinly laminated whitish mud-stones and brownish sandstones with abundant gastropods (Mel-anopsis) interbedded in the basalt flows a few meters below thefirst acidic volcanic rocks. The latter gave a K/Ar age of27.8 ± 0.3 Ma (Merla et al., 1979). Merla and Minucci (1938)mapped this sedimentary level for approximately 10 km and re-ported four fossil sites. Ten km southwest of Amba Alaji at the Boravillage, a sample collected near the base of the lower basalt sectiongave an 40Ar–39Ar age of 30.86 ± 0.12 Ma (Kieffer et al., 2004).

We revisited the Amba Alaji site and found that the extensionand continuity of the intertrappean deposits, even if covered by awidespread talus, resulted in a pronounced morphological evi-dence above more than one thousand meters of basalts.

Because of widespread talus, we could measure only the upper25 m of the sedimentary sequence below the overlying volcanic



Fig. 11. Measured stratigraphical section of the intertrappean beds between lower and upper basalts three kilometers NW of Dogali (site 8 in Fig. 2).


rocks (Fig. 18). This section begins with 3 m of an alternation ofpredominant greenish to brown, organic-rich shales and rare len-ticular sandstones and marlstones. Following a ca. 2.5 m thick talusare 4 m of alternating brown and green shales and siltstones. Theshales are still organic-rich and contain abundant gastropods, fishscales and, less frequently, pelecypods (Fig. 19). The section endswith ca. 4 m of laminated diatomites crossed and deformed by abasalt dyke. The abundance of terrigenous and organic materialstogether with the fossiliferous contents point to a marginal lakeenvironment.


The Amba Alaji intertrappean beds can be reasonably placed be-tween 29 Ma (taking into account the thick basalt pile interposedbetween the dated base at Bora and the Amba Alaji sediments)and 27.8 Ma.

4.3. Jema Valley

A ca. 500 m thick carapace of Trap basalts dominates the Paleo-zoic to Mesozoic 1200 m thick section beautifully exposed alongthe Blue Nile gorge and described in great detail since 1885 by



Fig. 12. Proboscidean teeth from the Oligocene intertrappean beds of Eritrea. (A)Gomphotherium sp. from Mai Gobro, left M2, occlusal view. (B and C) cf.Prodeinotherium from Mai Gobro, right P4 in B, occlusal and C, labial views. (D)Prodeinotherium cf. hobleyi from Adi Ugri, right m2, occlusal view. (E) Eritreummelakeghebrekristosi from Dogali, left m2, occlusal view. For all teeth, anterior is tothe left. Scale bar = 4 cm.

B2

S

B1

Fig. 14. The intertrappean beds (S) between the lower (B1) and upper (B2) Trapbasalts, NE of Agere Selam (Mekele). The log of fig. 15 has been measured in thisoutcrop.


Aubry. Within the abrupt Trap escarpment satellite image analysesand field surveys revealed a morphological break traceable forsome tens of km along the Jema River, a left tributary of the BlueNile, close to Debra Libanos (Fig. 2, site 25; 100 km north of AddisAbaba) (Figs. 20 and 21).

Our field control showed that the slope rupture is because of acontinuous level of sedimentary rocks referable to intertrappeandeposits resting above the first two hundred meters of the Trapsuccession. In 1994, the late Getaneh Assefa, former chairman ofthe Department of Geology, Addis Ababa University, showed usan outcrop of this level containing undetermined mammalianfossils. During our 2011 survey, we visited again this sedimentarylevel which is particularly well exposed close to the village of Cabialong the road Lemi-Jema River. Our two sections (Fig. 22) covermost of the approximate 120 m thickness of the sedimentary

Fig. 13. Scatter plots of tooth total length versus greatest width for P4 (A) and M2 (B) o(closed diamond) is compared to both the P4 and M2 samples. Original measurements arbavaricum; open triangle, D. giganteum.


interval, characterized by a fining upward trend with prevalentcoarse-grained deposits passing upward to fine-grained terrige-nous sediments and diatomite beds.

Above the basalts, the section begins with 24 m of amalgamatedconglomerate beds up to 6-m thick with subangular to subroundedpebbles of a maximum size of 30 cm, dispersed in a coarse-grainedsandy matrix. The conglomerates are exclusively composed of ba-salt clasts, and those of greater size are concentrated in the upperportion of the beds (Fig. 23) indicating a non-cohesive debris-flow(sensu Shanmugan, 1996).

The conglomerate body is covered by fining upward alternatingbeds of sandstones, siltstones and shales. The sandstones are mas-sive, plane or cross laminated in beds up to 2 m thick. Fine sand-stones and siltstones show ripple lamination with recurrentorganic debris. Upward, the section includes 5 m of massive ortrough-laminated pebbly sandstones interbedded with fine sand-stones and shales containing coquina beds and Equisetum. A4.5 m thick conglomerate still with clasts of greater size atthe top passes abruptly into massive reddish coarse siltstones,

f African and European deinothere species. The Mai Gobro deinothere upper toothe from Delmer (unpublished data). Symbols: closed circle, P. hobleyi; open square, P.



Fig. 15. Measured stratigraphical section of the intertrappean beds between lowerand upper basalts NE of Agere Selam (Mekele) (site 11 in Fig. 2).

Fig. 16. Intertrappean massive diatomite beds east of Agere Selam.


followed by 8 m of massive to crude horizontally laminated pebblysandstone in beds up to 3 m thick, and fine sandstones and silt-stones. Vegetal debris including Equisetum are common. The suc-cessive 16 m are characterized by erosive-based, massive orhorizontally laminated pebbly sandstones in beds up to 3 m thick.Thin beds of siltstones and shales with vegetal remains separatethe arenaceous layers. A 40 cm thick tephra layer rests below theuppermost arenaceous bed that closes the lower Lemi section.

The upper section begins after ca. 30 m of talus. It consists ofreddish and gray siltstones cut by an asymmetric channelized are-naceous body, up to 150 cm thick and ca. 30 m wide (Fig. 24). Thedeepest portion of the channel is floored by a lag of coarse-grainedmassive sands overlain by epsilon cross-bedded medium to finesands dipping up to 10� toward the deeper part of the channel.


The latter is plugged by 40 cm of gray siltstones with abundantleaves. Then, the section records the appearance of laminatedand massive diatomites, 85 cm thick, followed by 80 cm of massivered siltstones with thin beds of fine sandstones. Ten meters cov-ered by basalt debris separate this upper section from the in situupper basalts.

As for the environmental interpretation of the two sections, thelower very coarse-grained sediments of the first section record cat-astrophic mass-flow triggered by heavy rain episodes that wereprobably connected to rapid climate changes. Upward, the envi-ronment is characterized by braided to low-sinuosity watercourses intersecting vegetated flood plains with rare mass-flowepisodes.

The section in the upper portion of the sedimentary level showsa meandering channel cut in overbank deposits followed by waterpools or shallow-water lakes with a diatom bloom, and muddyflood plains.

As for the lateral extension of the intertrappean beds, we foundthat they are absent in the section along the road across the BlueNile gorge, that is westward from the Jema valley. Their pinchingout toward the Nile took place through thin lenticular outcropsof clay with silicified wood reported by Gani et al. (2009). Towardthe east and north, the intertrappean beds can be traced for at least25 km from Lemi before continuing within the basalt pile of thenorthern Ethiopian plateau.

It is worth noting that the huge basalt clast component in thelower Jema section suggests significant erosional episodes at theexpense of nearby elevated areas (Blue Nile section?).

For the radiometric datings of the basalts above and beneaththe Lemi intertrappean beds, no local ages are available. Yet, wecan correlate the Lemi basalts with those of the Blue Nile succes-sion (40 km to the west) with the base dated to 29.4 ± 0.3 Ma



Fig. 17. The basalts (upper basalts) of the Amba Alaji pyramidal peak rest above the soft intertrappean beds.

v

vv

v

uupperbasalts

lowerbasalts

vv

silt sand pebbly sand

10

20

30

40

0 m

V

VV

< < < basalt dike

Trap BasaltVVV

laminated diatomite

fish

gastropod

leaves

Fig. 18. Measured stratigraphical section of the intertrappean beds between lowerand upper basalts one kilometer north of the Amba Alaji (site 12 in Fig. 2).

Fig. 19. Silicified coquina with mollusc shells of the Amba Alaji intertrappean beds.

JL

KSB1

SB2

Fig. 20. Sightseeing of the southwestern slope of the Jema valley exposing thesection from the Jurassic Limestones (JL) and Cretaceous Sandstones (KS) to theCenozoic basalts. The flat morphology between the lower (B1) and upper (B2)


(40Ar–39Ar, Hofmann, 1997 in Rochette et al., 1998) and the topdated to 26.7 ± 1 Ma (K/Ar, Merla et al., 1979). Assuming this cor-relation and taking into account the stratigraphic/chronologicaldistance from the dated basalts and the intertrappean beds, we in-fer an age between 29 Ma and 27 Ma.

basalts corresponds to the more easily erodible intertrappean sediments (S).

5. Mammal-bearing intertrappean beds in Ethiopia and Kenya

In addition to the studied Eritrean sites with mammal fossils(Mendefera and Dogali), the literature reports two renowned sitesin western Ethiopia (Chilga) and northern Kenya (Losedok) (Fig. 2).In addition we will provide some information for a contentiousoutcrop of sedimentary intercalation in the basalt pile of the Amar-o ridge (Southern Ethiopia).


5.1. Chilga, Ethiopia

This intertrappean sequence outcropping for many tens ofsquare kilometers near Chilga (35 km north of Lake Tana) (Fig. 2,site 16) and deposited in a small NS trending graben, has receivedlong lasting attention for its lignite seams since the first reports by



b2

itit

b1

b1

Mes

2600m

Mes

Mes

2200m

1800

m

1600m

1600

m

LEMI

CABI

2 km

N9°48'

E38°54'

to Jema river

to Fiche

Fig. 21. Geological sketch map showing the intertrappean beds of the Lemi-Cabiarea near Fiche in the Jema Valley. Mes: Mesozoic sandstones, limestones andshales; b1: lower basalts; it: intertrappean beds; b2: upper basalts. Dotted line:edge of the Lemi plateau; star: log location.


Steudner (1864) and Heuglin (1868). After the investigations ofnumerous later prospectors, the first detailed study was performedby Yemane et al. (1987). They recognized in the 34 m thick Haugasection a basal interval (2 m) with gravel and sand followed by10 m of silt and clay capped in turn by 13 m of lignite levels withclay and silt interbeds. The section ends with 3 m of volcanic ash.A shallow silica-saturated freshwater lake environment, precededby fluvial episodes and followed by volcanoclastic events, has beensuggested by the authors. Ferns were the first colonizers after vol-canic episodes destroyed the forest vegetation. The absence ofconifers (Podocarpus and Juniperus) indicates a paleoaltitude of900–1000 m lower than at present (Yemane et al., 1987). In addi-tion to mollusc shells and insect fragments and a few diatoms,Bonnefille (2010) reports trees, presently found silicified, ofapproximately 20–35 m in height, typical of a densely forestedhabitat.

Some short sections studied with great detail by Garcia Massiniet al. (2010) confirmed the fluviatile characters of the sediments inthe lower part of the succession. A fern-dominated flora, indicativeof a disturbed environment under the likely influence of volcanism,has been found in the middle and upper portion. Currano et al.(2011) reported numerous paleosols entering a swampy/fluviatiledeposit characterized by strong floral variability, even in the samelayer.

A Late Oligocene mammal assemblage of paenungulate herbi-vores was collected and studied by Kappelmann et al. (2003). Theystressed that the Chilga fauna documented the oldest occurrence ofdeinotheres and that was important to understand the faunal ex-change between Afro-Arabia and Eurasia (see also Sanders et al.,2004).

Pollen and plant analyses performed by Yemane et al. (1987)and Bonnefille (2010) are indicative of a tropical wet forest similarto that of the present-day Western Africa Guineo-Congolean areawith a well-defined short dry season. Geochemical techniquesthrough carbon, oxygen and hydrogen stable isotopes providedwarmer soil paleotemperatures (29 ± 3 �C), whereas paleobothani-cal proxies gave a mean annual temperature of 24 ± 3 �C, muchhigher than today (Currano et al., 2011).

The Chilga sediments are intercalated in the upper portion ofthe local ca. 600-m thick Trap pile. A basalt flow 80 m below thebase of the sediments has been dated to 32.4 ± 1.6 Ma (K/Ar age),whereas an intercalated ash layer is 27.36 ± 0.11 Ma old (40Ar–39Arage) (Kappelmann et al., 2003). The fossil mammals were collecteda few meters above and below this ash layer. Paleomagnetism inthe sedimentary section constrains the age of the intertrappean


beds between 27 and 28 Ma, i.e., in the magnetic chron C9n(Kappelmann et al., 2003; Currano et al., 2011).

Moving 800 km away from Chilga toward the south, mammal-bearing reddish fluvio-lacustrine deposits are reported in the ba-salt pile of the Amaro ridge in southern Ethiopia (Woldegabrielet al., 1991; Ebinger et al., 1993) (Fig. 2, site 35). They are 10 to20 m thick and lie above 42–35 Ma old basalts and beneath 18 to11 Ma old basalts (Ebinger et al. 1993). Woldegabriel et al.(1991) assigned an age of 15–17 Ma to the primitive Choeroloph-odon elephantoid found in the intervolcanic sediments.

This time assignment pushes the age of these sediments withinthe early Miocene and suggests that they cannot be encompassedin the stratigraphic framework of the Oligocene intertrappean se-quences described in this paper. However, if we take into accountthe age of the lower and upper basalts, the Amaro ridge intertrap-pean beds should cover a surprisingly long time interval, at leastfrom 35 to 18 Ma. It is possible that a portion of this extended per-iod of sedimentation should be assigned to unfossiliferous (untilnow) Oligocene deposits.

5.2. Western Turkana, Kenya

The late Eocene to Miocene Turkana-Tertale fissural basaltsextensively cover northern Kenya, and are regarded as a prolonga-tion of the southern Ethiopia Traps. This region has yielded well-known fossil mammals since the investigations of Arambourg(1933, 1943) and is presently explored for oil (e.g., Morley et al.,1999).

At Losodok (10 km west of Lake Turkana in the Lothidok range)(Fig. 2, site 38) the Kalakol Basalts (785 m thick) expose in their ba-sal portion a 35–50 m thick intervolcanic sedimentary unit (Eraga-leit Beds) comprising terrestrial reddish conglomerates,sandstones and subordinate claystones, cyclically arranged in fin-ing-upward sequences up to 10 m thick (Boschetto et al., 1992).Near the top, the Eragaleit beds yielded primates and proboscideawith other abundant mammals (Leakey et al., 2011). The basaltsbelow this sedimentary unit gave K/Ar ages of 27.9 ± 0.3 Ma and26.9 ± 0.2 Ma. The overlying basalts are 24.2 ± 0.3 (K/Ar) Ma old(Boschetto et al., 1992; Brown and McDougall, 2011). Boschettoet al. (1992) inferred an age for the Eragaleit Beds spanning from27.5 Ma to 24.2 Ma.

Still, in northern Kenya, two sites located 50 km southwest ofLake Turkana both in the Lokichar Basin (Nakwai and Lokone,Fig. 2, sites 39, 40) exhibit coarse and fine terrestrial sandstonesrich in Late Oligocene mammals, among which there are Deinothe-riidae, and molluscs (Ducrocq et al., 2010; Leakey et al., 2011). Dif-ferent from the Losidok intertrappean beds, the Lokichar sequencesare covered by massive flood basalts, but rest immediately abovethe Proterozoic basement without intervening basalts. Borings inthe Lokichar basin met Oligocene lacustrine shales (partly oilshales) with pollens indicating paleoclimatic and paleogeographi-cal conditions similar to those found in the coeval sediments ofthe Chilga basin (Talbot et al., 2004; Vincens et al., 2006).

6. Further intertrappean deposits in East Africa andsurrounding regions

In Fig. 2 we list the intertrappean beds known to us from the lit-erature or from first-hand field surveys and we show their locationin the map. Their wide distribution in East Africa and surroundingregions (Yemen and Sudan) testifies to the large-scale expansion ofsimilar paleogeographical and paleoecological conditions duringthe stagnation of volcanic activity. In this paragraph we reviewthose sites without reported evidence of Tertiary mammals. Formost of them only scanty data are available as for paleontological




0 m

10

20

30

40

50 100

90

80

70

60

100

120

130

undeterminedbone

llowerbasalts

coarse sand with sparse granules tuff


Trap Basaltbasalt conglomerate and pebbly sand

leaves

50

upperbasalts

Equisetum


diatomite ripples gastropod

LEMI

Fig. 22. Stratigraphical section of the intertrappean beds between lower and upper basalts measured along the Lemi-Cabi road (site 25 in Fig. 2).


and radiometrical datings and sedimentological interpretation. Anexception is the Magdala site, where an Oligocene age is welldocumented (see below). Some other cases are challenging. For in-stance, the paleofloras of the Mush, Molale and Debra Sina sites(Fig. 2, sites 18, 19, 20) in northern Ethiopia have been consideredto be of Miocene affinity (Lemoigne et al., 1974), but Bonnefille(2010) underlines the need of further studies. Moreover, the scar-city of local radiometric dates for the embedding volcanites ismanaged with correlatable dated samples that were collected evenat distances of tens of km.

6.1. Northern Ethiopian sites

In the historical locality of Magdala (60 km northwest of Dese,Fig. 2, site 14) Blanford (1870) described approximately 30 m ofbrown coarse sandstones, whitish and black shales with silicifiedtrunks, alternating with volcanic ashes and basalts along the Jittacreek and within the upper unit of the Trappean Series (Magdala


Group). From this site Unger (1866) reported the remains of fossilwood from Nicolia aegyptiaca.

Nearby datings in the Wegel Tena area provide 30.2 ± 0.1 Ma(40Ar–39Ar) for the lower basalts, and 28.2 ± 0.1 Ma (40Ar–39Ar)for the upper basalts (Hofmann et al., 1997; Rochette et al.,1998). A rhyolite above the intertrappean sediments has an ageof 27.7 ± 1.25 Ma (K/Ar) (Nicoletti and Petrucciani, 1973). Availabledata and stratigraphic positions of the intertrappean beds in re-spect to the dated basalts suggest an approximate age of 30 Maand 28.5 Ma.

Further south, in the Mush Valley (150 km northeast of AddisAbaba) the intertrappean section reaches approximately 20 mand comprises a basal sandstone followed by an alternation oflacustrine mudstones and siltstones with tuff interbeds and twoseams of lignite. The lower seam is a few cm thick, and the upperreaches 2 m. The coal-bearing lacustrine sediments are predomi-nantly composed of sandstones, siltstones, shales rich in plant re-mains, and coal seams (Usoni, 1952; Lemoigne et al., 1974;



Fig. 23. Amalgamated conglomerate beds with basalt clasts of larger size concen-trated in the upper portion, covered by fine sands and silts. Lower portion of theJema intertrappean beds (Fig. 22).

Equisetum

ss

Fig. 24. Lemi upper section (see log Fig. 22): diatomite and siltstone with anasymmetric channelized arenaceous body (ss) with epsilon cross-bedding. In theinset Equisetum remains.


Wolela, 2007). The floral assemblage mainly consists of angiosper-mes with some elements indicating a lacustrine-palustrine envi-ronment and a Miocene affinity (Lemoigne et al., 1974).

At Wuchale (50 km north of Dese) in the 25 m thick intertrap-pean sediments two lignite seams are intercalated within coalbearing sediments (Getaneh and Saxena, 1984). The latter consistof sand, carbonate shales and oil shales (Wolela, 2007).

Intertrappean sedimentary successions occur at Molale andDebra Sinah, two small towns 100 km and 130 km south of Dese,respectively (Gros, 1992; Bonnefille, 2010); the sediments are rep-resented by kaolinite beds and lignite seams with fossil woods ofprobable Miocene affinity (Lemoigne et al., 1974).

Francaviglia (1940) described numerous sedimentary intercala-tions with diatomites in the upper rhyolitic portion of the volcanicsuccession in the Cassam valley, 75 km east of Addis Ababa, (Fig. 2,site 22).


In the northwestern Ethiopia plateau, close to Chilga, Minucci(1938a,b) reported two intertrappean sedimentary occurrences,the upper reach of the Atbara river with gastropods, and the LakeTana Gorgora peninsula with plant remains (Fig. 2, sites 15, 16).

Other intertrappean successions with sand, mud, marl and lig-nite seams, are reported by Usoni (1952) and Getaneh and Saxena(1984) at Ankober (100 km northeast of Addis Ababa), Mugher val-ley near Mulo Farm (100 km west of Addis Ababa), Sululta (nearAddis Ababa) and Mojo (50 km east of Addis Ababa).

6.2. Southern Ethiopian sites

The Trap succession in southern Ethiopia starts approximatelyten million years before that in the northern Ethiopia/Yemen prov-ince, that is during the late Eocene (Dainelli, 1943; Mohr, 1962;and, among others, in more recent time Ebinger et al., 1993). Rela-tively few localities with intertrappean beds are reported in the lit-erature for this area (Fig. 2), and updated information is availableonly for two sites (Yayu and Delbi-Moye Basin, Fig. 2, sites 30,32), mainly concerning stratigraphy and sedimentology.

In the Yayu area (100 km northwest of Jima) a 165 m thick se-quence of intertrappean sediments (Fig. 25) has been investigated(Tadesse et al., 2003; Wolela, 2007, 2010). They rest on approxi-mately 100 m of basalt flows floored by basement rocks and canbe traced for ca. 35 km. They terminate against a recent fault(Fig. 8, in Wolela, 2007). In the lower part of the sedimentary suc-cession sandstones from a meandering fluviatile environment passupward into lacustrine oil shales. The middle portion comprisescoal-bearing sediments laid down in a lacustrine and swampyenvironment. Upward, the lacustrine oil-shales with rich spore,pollen and algae assemblages are capped by fluvial sandstones.

The age of the base of the volcanic succession can be extrapo-lated from the dated basalts outcropping approximately 60 kmsouthwest of Yayu (base of Mekkonen basalts of Davidson andRex, 1980). They cluster approximately 31.1 Ma. For the Yayuupper basalts, a K/Ar age of the Nekempte plateau basalts(100 km NE of Yayu) of 27.5 ± 0.8 Ma (Abebe et al., 1998) can be as-sumed. Because of the stratigraphical/chronological distance fromthe dated basalts, we can assume a 30 Ma to 28 Ma age for theintertrappean beds.

In the Gogeb valley (20 km south of Jima) the Delbi-Moyesedimentary succession outcrops for a few square km, reaches amaximum thickness of 378 m, and rests on Trap basalts. Its lowerportion consists of 278 m of predominantly muddy lacustrinedeposits with oil shales and coal, and fluvial episodes. They arecapped by 80 m of basalt followed by 20 m of clay with abundantplant remains of a fluvial and swampy environment, shales, andpyroclastic sediments. The section ends with a hundred metersof basalts and overlying rhyolites (Wolela, 2007).

The K/Ar age of the basalts underlying the sediments can be as-sumed to be 31.6 ± 1.0 Ma from a sample collected along the Omoriver 80 km to the northeast of Delbi (Merla et al., 1979). As for theage of the base of the overlying volcanites, a rhyolite from the Omovillage, 40 km east of Delbi, gave a K/Ar age of 26.6 ± 0.8 Ma (Merlaet al., 1979). Because of the stratigraphical/chronological distancefrom the dated basalts we can assume an age of 30 Ma to 27 Mafor the intertrappean beds.

Wolela (2007) reported similar fluvio-lacustrine successions inthe surroundings of Delbi at Lalo-Sapo and Gogeb-Chida.

Other sites of intertrappean sediments with lignite seams arereported by Getaneh and Saxena (1984) at Nejo Aleitu (120 kmnorthwest of Nekempte, Fig. 2, site 27), with lignite interbeddedin 10 to 20 m thick sands and conglomerates; in the Dilla valley,near Nejo (120 km west of Nekempte), with lignite interbeddedin green–blue shales; in the Didessa valley (110 km southwest of




Nekempte), such as with 50 cm of lignite associated withsands, shales and marls; at Kebre Menghist (210 km southeast ofJima).

Further intertrappean beds were reported by Woldegabrielet al. (1990) at the foot of the Guraghe escarpment along the Kellasection and near Agere Selam (Sidamo) (Fig. 2, sites 34, 36). TheKella section exposes crystalline basement and Mesozoic sedi-ments capped by 32 Ma old basalts overlain by 2 to 10 m thick flu-vial deposits unconformably truncated by 4.2 Ma old basalts. In theAgere Selam section, lacustrine strata are intercalated in a ca.30 Ma old volcanic succession consisting of basalts, trachybasaltsand rhyolites.

6.3. Eastern Sudan, Yemen and Western Arabia

The eastern Sudan, Yemen and Western Arabia Trap basaltsconstitute the outer margins of the Ethiopian flood basalt provincetoward the north and northeast (e.g., Kenea et al., 2001; UkstinsPeate et al., 2005) and include sedimentary intercalations (Figs. 1and 2).

In the Sudanese Odi Basin (Langheb-Hamshkoreib district,southern Red Sea Hills; Fig 2, site 2) Whiteman (1971) reportedtwo hundred meters of Trap basalts above a ca. 15 m thick fluvialto lacustrine ‘‘infrabasalt’’ succession of Paleogene age. It includesmarl, sandstone, oolitic limestones with ostracods and stromato-lites (Sengör, 2001 and new data of present authors). The overlyingTrap basalts are dated to 30.8 ± 1.0 Ma (K/Ar) at their base and29.9 ± 0.3 Ma (40Ar–39Ar) at their top (Kenea et al., 2001). Theycontain a few meters of intertrappean sandstones followed bymudstones, gastropod-bearing limestones and dolostones. Theage of these sediments is tentatively placed between the two dat-ings of the host basalts.

In Yemen, the Trap succession south of Sana’a comprises onelacustrine lignitiferous intercalation in its middle portion at Osai-lea and Dhi Sufal (Lipparini, 1954) (Fig. 2, sites 6, 7) with freshwa-ter gastropods, leaves, silicified trunks, and diatomites. 40Ar–39Ardatings bracket the age of the Osailea basalts between30.71 ± 0.26 Ma and 26.51 ± 0.12 Ma (Baker et al., 1996). Becauseof the uncertain position of the intertrappean beds with respectto the dated basalts, we can only infer an age of 30 to 27 Ma.

Also the huge Trap pile west and north of Sana’a (Fig. 2, sites 3,4), with an age spanning from 30.9 Ma to 26.5 Ma (Baker et al.,1996), exhibits lenticular sedimentary intercalations. They reacha maximum thickness of 20 m and consist of fluvial-lacustrinemudstones, claystones and calcareous sandstones with abundantfreshwater gastropods and silicified plant remains.

In the Lahima region between Sana’a and Hudaydah (Fig. 2, site5), a few meters of sediments are intercalated in the lowermostportion of the basalt succession (Davison et al., 1994). Moreover,in the slopes surrounding the west of the Sana’a plain, repeatedsedimentary intercalations and acidic volcanics are found above450 m thick lower basalts (Ukstins Peate et al., 2005). The sedi-ments/acidic volcanics alternations that underlie the upper portionof the Trap pile are ca. 280 m thick and range in age from29.9 ± 0.2 Ma to 28.0 ± 0.1 Ma (Baker et al., 1996; Ukstins Peateet al., 2005).

Among the Harrats of Western Arabia, the Hadan volcanic prov-ince (Fig. 2, site 1) is characterized by 150 m of basalt flows corre-latable to the Trap basalt succession. They include three lacustrineintercalations resting above a basalt dated to 27. 8 ± 1.4 Ma (K/Ar)(Arno et al., 1980). This age can be taken for the base of the localintertrappean beds, whereas for the upper basalts a flow in thenearby Harrat Al Bukum-Nawasif has been dated to 22.8 ± 1.1 Ma(K/Ar) (Arno et al., 1980). The age of the intertrappean beds canbe assumed between 27.5 Ma and, less confidently, 24 Ma.


7. Chronology and paleobiogeography of Oligo-Mioceneproboscideans from East Africa

The intertrappean Oligo-Miocene sediments from Eritrea, Ethi-opia and Kenya that are discussed in this paper, contain a diverseproboscidean fauna. Although fragmentary, the available materialclearly indicates that this time interval was crucial for the originof Neogene proboscideans and helps fill the gap between the olderPaleogene record of the order in Africa and the Neogene record.

Three main fossil localities, representing time successive mam-malian fossil assemblages, are of particular interest as far as pro-boscidean evolution is concerned.

The oldest one is Chilga, Ethiopia, dated to 28–27 Ma (Sanderset al., 2004). The Chilga fauna does not include Eurasian immi-grants, thus predating the Oligo-Miocene faunal interchange. Inaddition to the typical Paleogene proboscideans, such as Phiomiaand Palaeomastodon (known from the classical sites of the Fayum),Chilga records the first occurrence of the families Deinotheriidaeand Gomphotheriidae, previously known only from the Neogene.At the broadly coeval site of Dogali in Eritrea (27 Ma), the basal ele-phantimorph Eritreum (Fig. 12E) was collected (Shoshani et al.,2006). This taxon is at present only known from this site. It isimportant from an evolutionary point of view in that it testifiesto the origin of a typical elephantoid trait: the horizontal toothreplacement. Losodok and Nakwai are two younger sites fromWest Turkana, Kenya dated to ca. 25–24 Ma, that contain the firstoccurrence of a northern immigrant in Africa, a carnivore (Leakeyet al., 2011). Roughly coeval is a fragmentary elephantimorph tuskfrom Pakistan (Antoine et al., 2003) indicating that early, intermit-tent, faunal exchanges between Africa and Eurasia previouslybegan in the Late Oligocene. The Losodok/Nakwai proboscideanfauna still contains primitive taxa as Chilgatherium, althougharchaic forms, such as Palaeomastodon and Phioma no longer occurr(Leakey et al., 2011). Significantly, these sites record the first occur-rence of the family Mammutidae, with the new genus Losodokodonand a more derived deinothere, an intermediate betweenChilgatherium and the Miocene Prodeinotherium (Rasmussen andGutiérrez, 2009; Gutierrez, 2011). Fragmentary remains of a gom-phothere are also present.

The most recent localities with proboscidean fossils from theintertrappean beds of East Africa are those from the Mendeferaarea in Eritrea: Adi Ugri (Adi Zerna mine) and Mai Gobro. Asdiscussed above, the proboscideans at Mai Gobro are tentativelyassigned to Gomphotherium sp. and Prodeinotherium sp. The small,very lophodont deinothere lower m2 (Fig. 12D) from Adi Ugri(Adi Zerna mine) described by Vialli (1966) compares very wellwith those of P. hobleyi, and might suggest that also the MaiGobro deinothere should be referred to that species. However,at present it cannot be excluded that both specimens actuallyrepresents a new species, similar but more primitive than P. hob-leyi. Gomphotherium is known from several Early Miocene sites inEast Africa approximately 20 Ma and makes its first occurrence inEurasia at 18 Ma with primitive species of the annectens group(Tassy, 1990). The Mai Gobro Gomphotherium sp. shows affinitywith this last group of forms and might represent the oldestoccurrence of this taxon in Africa before its dispersal toward Eur-ope and Asia as part of the so-called Proboscidean Datum Event(Madden and Van Couvering, 1976). Prodeinotherium apparentlyalso originated in East Africa (Sanders et al., 2010a,b), with MaiGobro possibly representing the first record of this taxon. Prodei-notherium then became widespread in Africa in the Early Miocenewith the species P. hobleyi, and is known from North Africa toUganda and Namibia in the south and west. Prodeinotherium alsotook part in the Proboscidean dispersal out of Africa in the earlyMiocene, spreading to Europe and southern Asia approximately




17–16 Ma and producing the vicariant species P. bavaricum and P.pentapotamie, respectively.

8. Discussion

The study of the intertrappean beds provides key elements forthe reconstruction of the paleogeographical, paleoecological andbiogeographical conditions of a wide area from western Arabia toKenya, in which the Oligocene sediments are poorly represented.This is demonstrated by the paleogeographical maps which showwide exposed lands during the Oligocene (Fig. 27). They were theconsequence of the uplift of the Afro-Arabian dome (e.g., Burkeand Gunnell, 2008; Avni et al., 2012), the geodynamic reassessmentof the Afro-Arabian and Eurasian plates and coeval sea-levellowering.

In addition to the intertrappean beds, only a few Oligocenedeposits are preserved on land besides those met by borings in riftbasins. For instance, Oligocene continental sediments outcrop inSomalia only as small patches along the Indian Ocean (Mudugh:Pozzi et al., 1985; Abbate et al., 1994) and the Gulf of Aden (Daban:Abbate et al., 1993).

8.1. Features of the intertrappean beds

In a large-scale overview, the examined intertrappean sites aredistributed in an elongated NS ellipse with a 2000 km major axisand a 1000-km minor axis (Fig. 27). This wide distribution matchesthat of the Traps’ outcrop area (Davison et al., 1994; Kenea et al.,2001; Ukstins Peate et al., 2005) and is clear evidence of the sur-prisingly broad occurrence of the intertrappean beds.

Although very distant, the examined sections exhibit some com-mon features. One is related to their stratigraphic position withinthe Oligocene Trap pile since they are placed between two differentvolcanic stages. The first stage is commonly marked by some hun-dred meters thick basalt flows with a bulk accumulation betweenca. 31 and 29 Ma, and corresponds to the lower basalts of this paper.They reach the outstanding thickness of more than a thousandmeters in the Lima-Limo section (Hofmann et al., 1997) and theBora/Alaji area (Rochette et al., 1998) because they profusely fedby dikes through a fault system in their substratum. In the casesof reduced thicknesses, this is most likely because of erosion beforeintertrappean beds deposition (e.g., Yayu, Jima, Fig. 2) or due to astarving volcanic supply in marginal areas (Harrat Hadan in SaudiArabia, Odi basin in Sudan, Losidok in Kenya) peripheral to the mainTrap outpouring bulge of Ethiopia and Yemen.

The second stage overlies the intertrappean beds and comprisesa typical alternation of basalts with recurrent rhyolites and

Fig. 25. Lacustrine fine sandstones and black shales in the intertrappean beds of theYayu area (site 30 in Fig. 2).


trachytes. They were extruded during a time span starting from27 and 23 Ma. In a regional frame, the second stage was followedby the Neogene shield volcano activity.

Further common characteristics of the intertrappean beds con-cern their lithology and sedimentary environment. The sedimentsare continental typically consisting of reddish, dark gray, black,beige to brown and greenish siltstones and mudstones with occa-sional lignite seams. These fine-grained deposits are attributable toalluvial plain and palustrine environments. Additionally, a lacus-trine deposition is indicated by diatomites, greenish shales richin ostracods, freshwater molluscs and abundant plant remains.Layers or lenses of sandstones and conglomerates are frequentlypresent and interpreted as fluvial channels and overbank flooddeposits.

Significant amounts of coal seams (mainly lignite varieties) arefound in Eritrea (Mendefera) and in Ethiopia (Chilga, Mush Valley,Yayu and Delbi Moye, e.g., Usoni, 1952). Oil shales are reported inthe Yayu, Delbi Moye, Lalo-Sapo, and Wuchale basins (Wolela,2007).

The thickness of the intertrappean beds is at maximum a fewhundred meters (ca. 380 m at Delbi Moye, in southern Ethiopia),but, on average, less than 50 m. Although the thickness is limited,the sedimentary sequences have been preserved because they aresandwiched within volcanic flows.

Thicker accumulations are generally related to subsiding gra-bens, which is well documented in some sections (e.g. Chilga, Yayu,Delbi Moye, Losodok). In other cases, the unevenness of the volca-nic floor has generated shallow sags filled by sediments. This oc-curs for the intertrappean beds with variable thickness from 20to 50 m at a short distance, traceable at the base of the 20 by40 km upper basalts tabular carapace in the Mendefera area(Fig. 3). This mesa-like structure is a wide erosional remnant notbounded by normal faults.

Basalt flows, less frequently rhyolites and ash layers, are a com-mon occurrence in the intertrappean beds. In some cases, the vol-canic component prevails on the sediments (Harrat Hadan in SaudiArabia, and west of Sana’a, Yemen).

8.2. Paleolandscape reconstruction

The lithological and sedimentological evidence provided by thesections studied in more detail suggests a landscape with smoothreliefs crossed by a poorly defined drainage. The rivers were small,with low-sinuosity to meandering and frequently endorheic intoswamps and lakes. In these water-rich environments, vegetal accu-mulations produced organic-rich sediments and often coal seams.Additionally, the silica availability, because of the sporadic volcanicactivity, favored frequent diatom blooms in the lakes.

The development of this poorly defined drainage can be relatedto the reorganization of the African river network during the Oligo-cene (Goudie, 2005; Stankiewicz and de Wit, 2006). In particular,for eastern Africa, the impingement of the Afar plume, successiveregional basement uplift, and associated continental basalt outp-ourings produced the NS-elongated elliptical bulge of the Eritreanand Ethiopian highlands. This upheaval caused an eastward shift ofthe eastern Africa continental divide. Consequently, rivers origi-nally flowing toward the Indian Ocean were gradually capturedby the Mediterranean-directed paleoNile system (Said, 1993;Abdelkareem et al., 2012). Because of this stream-rerouting, themorphology of the Eritrean and Ethiopian highlands varied fromtime to time, and local restricted basins developed during volcanicquiescence.

Environmental and ecological information is derived also frompaleobothanical studies performed in the Chilga, southern Ethiopiaand Lokichar basins (Yemane et al., 1987; Vincens et al., 2006;Wolela, 2007; Bonnefille, 2010). Pollen and plant analyses are




indicative of a tropical wet forest similar to that of the present-dayWestern Africa Guineo-Congolean area, with a well-defined shortdry season, and rainfall greater than 1000 mm/y. For the Chilgasections, geochemical techniques indicate warmer soil paleotem-peratures and higher mean annual temperature than today(Currano et al. (2011)).

The Chilga area was characterized by 20- to 35-m high trees,and both in Chilga and in the Turkana basins, ferns were the firstcolonizers after volcanic episodes had destroyed the forest vegeta-tion. Additionally, the absence of conifers (Podocarpus and Junipe-rus) indicates a paleoaltitude lower than at present (Yemaneet al., 1987; Vincens et al., 2006). Generally, the intense rainfalland the wet forests were linked to the equator position at the cur-rent-day latitude of Addis Ababa during the Oligocene (Smith et al.,1994; Swezey, 2008; Abdelkareem et al., 2012).

Concerning fossil mammalian assemblages at Chilga, the overalloccurrence of arsinoitheres, hyracoids, and paleomastodontsmostly corresponds to that of the faunas from older Paleogene sitesknown within the African continent (Kappelmann et al., 2003;Sanders et al., 2004). The occurrence of this fauna in the highlandsof Ethiopia suggests that these herbivores had a widespread distri-butional range across Afro-Arabia, possibly occupying generalistniches with broad ecological tolerances (Jacobs et al., 2005).

8.3. Geometry of the intertrappean sedimentary level

The assessment of the geometry of the intertrappean beds iscrucial to determine whether we are dealing with a single, moreor less coeval and widely extended interval or with lens-shapedintercalations of various ages and stratigraphic positions. The an-swer bears paleogeographic and paleontological implications.

The assumption of a single interval can be assumed for theMendefera area where a detailed mapping has allowed the tracingof the intertrappean beds for more than 40 km (Fig. 3).

When outcrops of the intertrappean beds are discontinuous, itis critical to determine whether the exposures terminate by origi-nal pinching out, faulting, recent erosion or talus cover. Thus, var-ious conditions can be met. For instance, in the Dogali area,eastward of Asmara, the sedimentary interval outcrops continu-ously for 20 km. Similar to what happens in the Mendefera region,there is no field evidence of depositional pinching out, and thepresent-day outcrops are erosional remnants of a more extendedhorizon.

Amba Alaji is different because in Merla and Minucci’s (1938)map the intertrappean sediments are shown as a 10-km elongatedbody sandwiched within the volcanic rocks with an original lentic-ular geometry.

In the Jema area, the intertrappean beds have been traced forsome tens of kilometers and pinch out toward the Blue Nile, but,as previously mentioned, they are continuous toward the eastand north for at least 25 km from Lemi before plunging withinthe basalt pile of the northern Ethiopian plateau.

As for the outcrops of the Chilga, Yayu, Delbi Moye and Losod-ok/Lokichar section in western Ethiopia and northern Kenya, avail-able maps show that their NS-elongated structure is connectedwith graben development controlled by transversal and/or longitu-dinal fracture systems. Their present-day extension, exceeding 40by 60 square kilometers suggests that the original basins were ofa relevant size.

In addition to the aforementioned occurrences, the map in Fig. 2shows many other outcrops of the intertrappean beds. The major-ity of the sections is not accompanied by detailed maps or reportsand cannot provide information on the extension of the sedimen-tary level.

In conclusion, it is likely that the intertrappean beds cannot bereferred to an uninterrupted body that is representative of a single


wide basin. Our field observations and available literature, rather,indicate discrete basins exceeding thousands of square km. Theirhummocky volcanic substratum and relative tectonic quiescencewere responsible for a poor accommodation space in many areas.The result was relatively thin successions that are subject to lateralpinch outs. Larger thicknesses can be found limited to some rift ba-sins, such as at Delbi Moye and western Turkana.

In some Trap successions the absence of intertrappean sedi-ments could be related to morphological highs of the volcanic sub-stratum characterized by non-deposition and/or erosion. In thiscase, the Trap pile would record a gap (lacuna) in the volcanic accu-mulation (e.g., Blue Nile and Lima Limo sections). Moreover, themissed report of intertrappean beds in some published descrip-tions of the Trap succession could also be because of unfavorableexposures, particularly in highly forested areas, such as southernEthiopia.

8.4. Age of the intertrappean beds

We are aware that any effort to produce a firm chronologicalframe for the intertrappean beds suffers from the scarcity of spe-cific radiometric datings. Additionally, some of the dating was per-formed decades ago with the K/Ar method. However, we areimpressed by some broad time correspondences among sectionsthat are separated by wide areas and located in various geologicalcontexts. Following this approach and taking into account the con-siderations discussed in the Methods section, we chose thirteen ofthe forty sites that are included in the map of Fig. 2 which moreconfidently allowed the compilation of the diagram of Fig. 26.

The selected sites are arranged from north (left) to south (right)according to an African/Arabian plate pre-drift reconstruction. Foreach site, we portray the estimated time covered by the intertrap-pean beds based on the age of the lower and upper basalts and, in afew cases, on the age of the volcanic bodies intercalated in theintertrappean sediments. Stratigraphical supplementary consider-ations (see ’’Methods’’) were used when the dated basalts werenot immediately below or above the sediments. Notably, the argu-ments selected for the age estimation, their reliability and signifi-cance have been previously illustrated for each site description.

The diagram shows that the age of the base of the intertrappeanbeds is between 30 and 27.5 Ma, clustering approximately 30/29 Ma for the sites in Ethiopia and Yemen. Additionally, theresumption of the volcanic activity above the intertrappean bedswas at 27/28 Ma contemporaneous for most of the Ethiopian andYemeni sites. In these areas, the duration of the intertrappean bedsis 1 to 3 Ma.

Somewhat younger) ages, from 23.6 to 24.2 Ma, are met for thebase of the upper basalts in the Hadan Harrats, Dogali, Mendefera,Agere Selam (Mekele) and Losodok sites. It is relevant that they oc-cupy a marginal position with respect to the bulk of the Traps thaterupted in the Ethiopian and Yemen highlands. In these marginalsites, the resumption of the magmatic activity after the sedimentdeposition has been delayed for some million years and resultedin an extended time between the lower and upper basalts filledby only a few tens of meters of sediments. This reduced sedimentthickness can be referred to a very low accumulation rate with pro-fusely pedogenized red siltstones and mudstones, often undergo-ing erosive episodes recurrent in continental environment.

It should also be noted that the age attributions derived fromthe thirteen selected sites find possible support from, or at leastare compatible with, many of the remaining 27 sites included inthe maps of Fig. 2. For most of them the information from the lit-erature provides a broad stratigraphic placement which is consis-tent with the age that we assumed from the selected sites.

Compared with the global climate-change chronology based ondeep-sea oxygen isotope records, the time span covered by the



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29.0

24 24 24

Fig. 26. The age of selected intertrappean beds from SW Arabia to northern Kenya and the deep-sea oxygen isotope variation curve. Circled numbers above columns arelocation site numbers of Fig. 2. Each bar displays in orange the age span of the intertrappean beds, and in green the age span of the lower and upper basalts. Poorly constrainedtime- extent in basalt effusion or sediment deposition is shown by white intervals in the columns. Available radiometric datings on the left side of the bars with new ages inrectangles (Mai Gobro section). Estimated age of the intertrappean beds on the right side of the bars in bold. The hatchured rectangles at the base of the bars refer to sectionswhere the onset of the lower basalt above the Mesozoic or Proterozoic substratum is relatively well dated. Oxygen isotope curve after Zachos et al. (2001).

Fig. 27. Late Oligocene paleogeographic map showing possible Eurasia/Africa-Arabia connections and African and Eurasian mammals geographic extent through Africa andArabia (modified after Dercourt et al., 1993, Schandelmeyer and Reynolds, 1997 and Avni et al., 2012).


Please cite this article in press as: Abbate, E., et al. The East Africa Oligocene intertrappean beds: Regional distribution, depositional environments andAfro/Arabian mammal dispersals. J. Afr. Earth Sci. (2013), http://dx.doi.org/10.1016/j.jafrearsci.2013.11.001



intertrappean beds falls during the Oligocene deterioration (Milleret al., 1991; Zachos et al., 2001). This cooling is regarded by someauthors to be connected with the coeval huge Trap emission, whichalso produced widespread effects on the biosphere (Courtillotet al., 1988; Haggerty 1996). In particular, the intertrappean bedswere deposited during the second cooling event (Oi2; Pekaret al., 2002) of the Oligocene deterioration. Nevertheless, the influ-ence of this cooling was mild and had no apparent with no sensibleeffect on the environment of Ethiopia, Yemen and Kenya becauseof the low-latitude position of these areas during the Oligocene.

8.5. Is there any similarity with the intertrappean beds of the DeccanTraps?

The eastern Africa/Arabia intertrappean beds find a possiblecounterpart in the sedimentary levels intercalated within the Dec-can Traps of India from which they derive their name.

The Maestrichtian to Danian Deccan Traps constitute a volcanicprovince of comparable size with that of the Africa/Arabia region(e.g. Jerram and Widdowson, 2005). The largest outcrops are foundin the northwestern portion of peninsular India, and a few smalloutcrops occur on the eastern side close to Rajahmundry in AndhraPradesh. For this area, a more extensive subsurface occurrence hasbeen shown by numerous borings that were made for gas and oilexploration (Keller et al., 2011).

In the up to 3500-m thick basalt pile three phases (‘‘megapuls-es’’) from approximately 67.5 Ma to 64.5 Ma, have been recognized(Chenet et al., 2009). The second phase is the thickest (80% of thetotal Traps) and erupted in a very short time (Courtillot et al.,1988). As noted in the previous paragraphs, similar huge and rapideruptions have been reported for the Ethiopian flood basalts byHofmann et al. (1997).

The Cretaceous-Tertiary (K-T) boundary recognized at the top ofthe second phase of the Deccan Traps establishes a close link be-tween voluminous volcanism and mass extinctions at this bound-ary (Duncan and Pyle, 1988; Keller et al., 2008; Chenet et al., 2009).

The Deccan intertrappean beds (also called the Intertrappeansin the Indian geological literature) have been famous since the19th century (Coulthard, 1829; Blanford, 1867) for their paleonto-logical remains, including dinosaur eggshells, teeth and bones, aswell as associated abundant fossil floras and faunas of continentaland, more rarely, marine environments (see review in Rana, 1990).

The Intertrappeans mainly consist of terrigenous fluvio-lacus-trine sequences with marl, limestone and chert, rich in gastropodsand ostracods. Their thickness is rather limited ranging from a fewmeters to ten meters (Rana, 1990) with a lateral extent not exceed-ing more than a few kilometers (e.g. Choubey, 1973). In the Jhilmiliand Rajahmundry regions (central and eastern peninsular India,respectively), the intertrappean beds exhibit marine sedimentsreferable to shallow- to open-sea environments together with ter-restrial deposits (Keller et al., 2008). The so-produced continental-marine-continental cycle reaches an outstanding thickness of 60 mand a lateral extent, documented by correlatable sections, ofapproximately 600 km from Rajahmundry to Jhilmili. Plankticforaminifera of early Danian age are found in the marine depositsabove the K-T boundary. The exceptional lateral extent and rela-tively large thickness of these intertrappean sections are linkedto their position within a transcontinental graben structure (theNarmada rift and Krishna-Godavari graben of the Trans-DeccanStraits, Keller et al., 2011).

In addition to the sedimentary intertrappean beds, reddenedprominent horizons, called ‘‘bole beds’’ or ‘‘red boles’’, are dissem-inated between the basalt flows. The bole beds consist of red (ormore rarely green) clays hosting spheroidally weathered piecesof basalts. They are commonly few centimeters thick, of limitedlateral extent (up to 2 km) and recurrent in the entire Trap pile.


The bole horizons developed during short quiescence betweentwo eruptions, and their duration has been estimated to be lessthan 1000 years by Gérard et al. (2006). They have been variouslyinterpreted as weathered basalts, weathered pyroclastics, soils,metasomatized baked horizons and sediments (Chenet et al.,2009). Similar horizons, although only cursorily mentioned, occurrepeatedly at various levels also in the Afro/Arabia Traps.

As for the stratigraphic distribution within the three volcanicphases composing the Trap pile, the red bole horizons are diffuselyinterlayered in the entire pile, while the much more rare Intertrap-peans characterizing the first phase, are not reported in the inter-mediate thicker phase (e.g. Rama Rao, 1936), but are again presentat the transition between the second and the third phase. Further-more, the intertrappean beds of the first phase are apparently lessthick and less continuous than those of the second/third phasetransition. Although the number of the sedimentary episodes inthe first phase is uncertain, it is likely that they were short becauseof the short duration of the first phase.

When comparing the Indian Intertrappeans with their Afro/Ara-bia analogs, it seems more significant to take into account the se-quences at the second/ third volcanic phase transition in theDeccan Traps and the intervolcanic beds between 27 and 29 Maof the Afro/Arabian province.

The time covered by the Intertrappeans in the Rajahmundry andJhilmili sections is approximately 0.25 Ma (Keller et al., 2008; 2009)shorter than that surmised for the Afro/Arabia intertrappean beds(ca. 1 to 3 Ma). However, they were deposited with a similar aver-age accumulation rate (approximately 10 cm/1000a) much lowerthan that of the stacking of the underlying basalts. Despite their rel-atively limited thickness, they both have regional lateral continuity,and become thicker in connection with graben structures where,limited to the Deccan Intertrappeans, marine episodes are reported.

Both the upper Deccan Intertrappeans and the Afro/Arabianintertrappean sediments testify an environmental deterioration:the first at the K-T boundary and the second at the Early-Late Oli-gocene transition. In both areas, harsh ecological conditions werecaused by the huge rapid eruptions before the deposition of theintertrappean levels.

The consequences at the K–T boundary were more dramatic be-cause of the joint effect of the Deccan Trap activity with an extra-terrestrial body impact causing a global biota mass extinction(Courtillot et al., 1988).

8.6. Africa/Eurasia land-connections and mammal exchanges

Africa was probably the first land mass that became an isolatedcontinent as Gondwana broke up. After the last connection withSouth America (Mid-Cretaceous), Africa joined to Eurasia continentalthough terrestrial contact was not established until the Late Oli-gocene / Miocene. In the long time span between these two events,the Afro-Arabian continent remained biogeographically isolated bythe intervening Tethys. This oceanic domain has been dramaticallyreduced since the Late Eocene Eurasia-Arabia collision (e.g. Allenand Armstrong, 2008). In this context, shallow marine environ-ments, together with local emergent areas, progressively replacedthe Tethyan deep-water realms. Oligocene paleogeographic mapsof the Eurasia/Arabian areas (Dercourt et al., 1993; Schandelmeyerand Reynolds, 1997) depict this new scenario with shallow marineenvironments interrupted by spatiotemporally discontinuous landbridges (Fahrs areas: Braud 1989 in Dercourt et al., 1993) (Fig. 27).The latter were aerially more extended during global sea-level low-erings when southwestern Asia was experiencing a harsh coolingclimate with increasing aridity (Dupont-Nivet et al., 2007). By con-trast, moist forests and woodlands were predominant over aridgrasslands in eastern Africa (Bobe, 2006; Bonnefille, 2010) andcould attract faunas from Eurasia. Connected with these paleogeo-




graphic conditions is the major unresolved question of the origin ofthe African placental colonization (phylogeny and paleobiogeogra-phy, geodynamic context).The timing of the initial radiation of en-demic African placentals (including tubulidentates, hyracoids,embrithopods, proboscideans, sirenians, hyaenodontid ‘‘creo-donts’’, primates, macroscelideans and tenrecoidea), is also one ofthe most important questions in contemporaneous mammalianpaleontology (Asher and Seiffert, 2010). During the Late Creta-ceous-Paleogene time span, Africa was prominently geographicallyisolated (resulting in strong endemic mammal faunas) and hosteda noticeably reduced faunal diversity. In this scenario, contrary toother continental provinces, the ecological niches were relativelyfree and offered evolutionary opportunities for new eventualimmigrants.

This may account for the success of various mammal groups thatimmigrated from Eurasia and became established in Africa, withsuccessful African radiations (Gheerbrant and Rage, 2006). The his-tory of trans-Tethyan Paleogene mammal dispersals records four tosix Dispersal Phases of placentals and marsupials between Africaand Laurasia during the Paleogene (Gheerbrant, 1990, 2001; Gheer-brant and Rage, 2006). These interchanges between Africa and Eur-asia were, however, primarily limited and filtered dispersals.

The Paleogene Afro-Arabian mammalian evolution has a scantyfossil record when compared with those from northern continents(Seiffert et al., 2010). Until the end of the Oligocene (23 Ma), Africawas geologically isolated from Eurasia, so that the Late Oligoceneof the African continent is characterized by the occurrence of ende-mic lineages among Afrotheres. The isolation of Africa was how-ever broken intermittently by discontinuous filter routes thatlinked it to some other Gondwanan continents (Madagascar, SouthAmerica, and perhaps India), but mainly to Laurasia. However, thefinding of scanty remains referable to primitive elephantoids in theLate Oligocene of Pakistan (Antoine et al., 2003) appears to demon-strate that early, possibly intermittent, dispersals between Africaand Southern Asia occurred before the Early Miocene.

Oligocene African faunas were dominated by poor diversity andendemic Afrotheres (including proboscideans, embrithopods andhyraxes) as well as a few non-Afrotherian groups, such as anthra-cothere artiodactyls, primates, and rodents (see, among others,Asher and Seiffert, 2010; Gheerbrant and Rage, 2006; Holroydet al., 2010; Sanders et al., 2010a,b; Rasmussen and Gutiérrez,2009, 2010; Seiffert et al., 2010; Winkler et al., 2010). During Africaapproach to Eurasia, many of the taxa common in the Late Oligo-cene disappeared, and the early Miocene faunas mark a dramaticturnover, being dominated by northern Eurasian immigrants.

Our knowledge of the Oligocene vertebrate fossil record of Africahas increased dramatically because of many recent discoveries inseveral parts of Africa (Egypt: Simons et al., 2008; Eritrea: Shoshaniet al., 2006; Abbate et al., 2012; Ethiopia: Kappelmann et al., 2003;Tanzania: Stevens et al., 2008; Kenya: Leakey et al., 2011).

At the time of faunal exchange between Eurasia and Afro-Arabiaat the end of the Oligocene, the migration from Africa to Arabiawas intermittently viable because of the terrestrial conditions ofthe presently intervening Red Sea. From Afro-Arabia to Eurasia,the terrestrial connections were impeded by the MediterraneanTethys-Indian Ocean seaway, but this marine belt became locallyand temporarily emergent (Fahrs) with land bridges because ofthe thrust of the Iranian margin on the Arabian plate (Dercourtet al., 1993). Land bridges were lengthier and remained aroundlonger during the Chattian sea-level low-stand.

9. Conclusions

Our field and literature survey of the sedimentary occurrenceswithin the Oligocene Trap succession in East Africa and western Ara-


bia has resulted in the following conclusions: (1) the intertrappeanbed occurrences that we reviewed can be consistently framed in asingle sedimentary event widespread from east Africa to westernArabia; (2) the intertrappean beds, more frequent than expected,were deposited in fluvial to lacustrine environments; (3) they are lat-erally continuous from a few tenths of meters to some (unpredicted)hundreds of kilometers, and are a few meters to a few hundreds ofmeters thick; (4) their basins were bounded by numerous interfluvi-al highs determined by uneven volcanic substratum or tectonicstructures, and they were often endorheic as an effect of an ongoingfluvial network rerouting because of the progressive raising of theAfrica/Arabia plateau; (5) as shown by the radiometric datings ofthe encasing basalts, they are concentrated at the transition of theEarly/Late Oligocene in a stratigraphic layer ranging from 29 to27 Ma; (6) the intertrappean units, each one lasting an average 1 to3 Ma, are positioned in the middle to upper portion of the Trap pileafter intense and rapid basalt flood activity, and are covered by fur-ther, sometimes acidic, floods, and/or by volcanic edifices; (7) thevery low rate of deposition of the intertrappean beds contrasts withrapid and voluminous Traps emission; (8) the intertrappean Oligo-cene sediments from Eritrea, Ethiopia and Kenya yielded a diverseproboscidean fauna. Although fragmentary, the material at handclearly indicates that this time interval was crucial for the origin ofNeogene proboscideans and helps fill the gap between the olderPaleogene record of the order in Africa and the Neogene record.The most recent localities with proboscidean fossils from the inter-trappean beds of East Africa are those from the Mendefera area in Eri-trea: Adi Ugri (Adi Zewrna mine) and Mai Gobro. As discussed above,the proboscideans at Mai Gobro are tentatively assigned to Gompho-therium sp. and cf. Prodeinotherium. The Mai Gobro Gomphotheriumsp. shows an affinity with this last group of forms and might repre-sent the oldest occurrence of this taxon in Africa before its dispersaltoward Europe and Asia in the so-called Proboscidean Datum Event;(9) in the best studied sections in central Ethiopia and northern Ken-ya. pollen and plants indicate a tropical wet forest similar to the pres-ent-day Guineo-Congolean areas; (10) the time interval covered bythe intertrappean beds falls within the Oligocene second coolingevent of the global climate scale; (11) during this time, southwesternAsia was experiencing severe climatic conditions because of increas-ing aridity, while moist forest and woodlands were prevailing inAfrica; (12) these contrasting ecological situations, together withthe exposure of discontinuous land bridges between southwesternAsia and Afro/Arabia, triggered important intercontinental mammalexchanges; (13) significant analogies between the OligoceneAfro/Arabia intertrappean beds and those of the Cretaceous/TertiaryDeccan Traps of peninsular India show common stratigraphical andsedimentological features and record environmental deterioration,ecological variations and biological modifications set offby huge and rapid volcanic eruptions preceding sedimentdeposition.

Acknowledgments

The authors would like to thank the Italian Ministry of ForeignAffairs, Archeological Mission Project (Responsible P. Bruni, Uni-versity of Florence) for financial support; the technical staff ofthe Central Petroleum Laboratory, Khartoum and of the NationalMuseum of Eritrea, Asmara for scientific assistance. LR support isgranted by PRIN national Program 2010X3PP8J_003 (ResponsibleD. Rio, University of Padova). The authors gratefully acknowledgecomments from Mike Rampino, New York University, This papergreatly benefited from helpful suggestions of two anonymousreviewers.



Table A.1Age data of analysed samples. Steps used to calculate plateau and isochron ages are inbold. LP: Laser power; 40Ar(r) (%): percentage of radiogenic 40Ar over total 40Ar;39Ar(K) (%): percentage of K-derived 39Ar released in each step.

LP (W) Age ± 2r (Ma) 40Ar(r) (%) 39Ar(K) (%) K/Ca±2r

MGB1 J = 0.00573720 ± 0.0000219 (±1r)0.15 30.43 14.99 26.6 0.2 0.18 0.020.30 28.01 11.59 23.1 0.9 0.22 0.040.40 33.28 3.40 49.4 3.0 0.27 0.030.50 31.24 2.30 59.5 3.6 0.27 0.030.60 30.53 1.40 68.9 5.9 0.27 0.030.70 28.51 1.17 76.3 7.3 0.24 0.030.80 27.98 0.73 86.6 9.1 0.19 0.020.95 27.67 0.66 90.3 7.5 0.18 0.021.10 28.50 0.73 91.5 5.5 0.18 0.021.40 29.42 0.77 89.5 5.4 0.15 0.021.70 30.09 1.66 93.2 3.2 0.07 0.012.00 28.10 1.63 88.6 6.1 0.06 0.013.00 30.55 1.85 95.9 7.8 0.05 0.014.00 28.19 1.70 97.6 7.5 0.05 0.016.00 29.10 1.58 96.6 7.5 0.06 0.01

14.00 29.43 1.51 96.8 19.4 0.06 0.01

MGB5 J = 0.00576681 ± 0.0000223 (±1r)0.20 17.27 46.02 3.8 0.3 0.07 0.030.40 30.10 6.32 19.4 2.6 0.09 0.010.50 29.65 1.96 47.4 5.3 0.11 0.010.65 26.68 1.30 59.7 11.0 0.11 0.010.75 23.40 0.85 82.1 15.3 0.12 0.010.85 23.37 0.85 93.8 13.6 0.12 0.010.95 23.69 0.85 93.3 10.1 0.12 0.011.10 23.56 1.02 86.7 8.4 0.10 0.011.30 23.56 1.07 83.4 7.3 0.10 0.011.70 23.54 1.51 83.6 6.1 0.07 0.012.10 24.69 3.28 86.3 4.0 0.03 0.004.00 25.36 4.67 87.1 4.8 0.02 0.00

14.00 25.50 4.31 89.4 11.2 0.02 0.00

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50

Cumulative 39Ar R

Age

[ M

a ]

Fig. A.1. Sample MGB1. Age (left axis and continuous line) and K/Ca (right axis and dasheused to calculate the plateau age. See text for the age error.

23.

15

20

25

30

35

40

0 10 20 30 40 50

Cumulative 39Ar

Age

[ M

a ]

Fig. A.2. Sample MGB5. L



Appendix A. New 40Ar–39Ar dating

40Ar–39Ar dating was performed on two lava flows sampled inthe Mai Gobro area. Both samples were broken in small chips,and the freshest ones were further grinded and sieved. Grains wereselected at the microscope from the fraction >250 lm. Sampleswere leached with CH3COOH 1 M (60 min) and HNO3 1 N(60 min) and then thoroughly washed with deionised water. Sam-ples were irradiated for 60 h in the core of the 250 kW TRIGA reac-tor of Pavia University, along with fluence monitor FCT sanidine(28.03 Ma, Jourdan and Renne, 2007).

Samples were step-heated using the defocused beam of a diode-pumped Nd-YAG infra-red continuous wave laser. Argon was mea-sured with a MAP215-50 mass spectrometer, operated in electronmultiplier mode (see Laurenzi et al., 2007, for further analytical de-tails). Ages were calculated using the decay constants of Steigerand Jäger (1977) (Table A.1). All displayed errors are at the ±2r lev-els, unless otherwise stated. The errors mentioned in the discus-sion on each sample data are analytical, while the error assignedto the sample proposed age comprises the uncertainty on neutronflux variation and on the age of the fluence monitor, without theuncertainties on 40K decay constants.

Both samples display slightly disturbed age spectra at low tem-perature, but identify plateau ages in the intermediate and hightemperature steps. The reason for the discrepant steps might ten-tatively be due both to internal 39Ar recoil (from K-rich fine grainedgroundmass to more retentive mineral phases) and to alteration orto the combination of both causes. A tiny amount of excess 40Armight affect MGB5. All significative steps of both samples havehigh radiogenic 40Ar yields.

29.04 ± 0.57 Ma

60 70 80 90 100

eleased [ % ]

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

K/C

a

d line) spectra. Error boxes are ±2r analytical. The horizontal bar indicates the steps

55 ± 0.45 Ma

60 70 80 90 100

Released [ % ]

0.00

0.05

0.10

0.15

0.20

K/C

a

egend as in Fig. A.1.




MGB1 has a plateau age of 29.04 ± 0.50 Ma, MSWD = 1.44, com-prising 8 steps with 62.6% of 39Ar release, an integrated total age(equivalent to a K/Ar age) of 29.19 ± 0.45 Ma, equal within errorto the plateau age. The isochron age calculated on the plateau stepsis slightly younger but barely overlapping within error to the pla-teau age, 28.04 ± 0.83 Ma, MSWD = 1.28, with a poorly defined ini-tial 40Ar/36Ar ratio of 418 ± 121. The age proposed for this sample is29.04 ± 0.57 Ma (Fig. A.1).

MGB5 has a plateau age of 23.55 ± 0.39 Ma, MSWD = 0.29, com-prising 9 steps with 80.9% of 39Ar release, an integrated total age of24.69 ± 0.66 Ma, slightly older than the plateau age. The isochronage calculated on the plateau steps is equal to the plateau age,23.46 ± 0.47 Ma, MSWD = 0.32, with an initial 40Ar/36Ar ratio of304 ± 40, atmospheric within error. The age proposed for this sam-ple is 23.55 ± 0.45 Ma (Fig. A.2).

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