Chronostratigraphy and correlation of the Plio-Pleistocene tephra layers of the Konso Formation,...

qTables 1}4 and the Appendix appear on the Quaternary ScienceReviews Website www http://www. elsevier. nl/locate/quascirev.

*Corresponding author. Tel.: #81-795-59-2017; fax: #81-795-59-2007.

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

Quaternary Science Reviews 19 (2000) 1305}1317

Chronostratigraphy and correlation of the Plio-Pleistocenetephra layers of the Konso Formation,

southern Main Ethiopian Rift, Ethiopiaq

Shigehiro Katoh!,*, Shinji Nagaoka", Giday WoldeGabriel#, Paul Renne$,Majorie G. Snow#, Yonas Beyene%, Gen Suwa&

!Division of Earth Sciences, Hyogo Museum of Nature and Human Activities, Yayoigaoka 6, Sanda-shi, Hyogo, 669-1546 Japan"Department of Geography, Nagasaki University, Bunkyo-machi, Nagasaki-shi, Nagasaki, 852-8131 Japan

#EES-1/D462, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545 USA$Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California, 94709 USA

%Department of Archaeology and Anthropology, C.R.C.C.H., Ministry of Information and Culture, P.O. Box 1907, Addis Ababa, Ethiopia&The University Museum, the University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan

Abstract

The Plio-Pleistocene Konso Formation is widely exposed in the Konso area located at the southwestern end of the southern sectorof the Main Ethiopian Rift. It is known for its rich Acheulean archeological occurrences and abundant vertebrate fossils, includingthose of A. boisei and H. erectus. The formation also contains more than 30 tephra layers. The lithologic characterization and majorelement compositions of discrete glass shards of these tephra, combined with single-crystal 40Ar/39Ar dating of seven tu!s, haveestablished the tephrostratigraphy and age range of the 200-m thick formation. The succession at Konso was deposited betweenapproximately 1.9 and 1.4 Ma and is divided into the Sorobo, Turoha, Kayle, and Karat Members in ascending stratigraphic order.Two of the Konso tephra are correlative to the Pliocene KBS and the early Pleistocene Chari Tu!s of the Omo-Turkana Basin alongthe Ethiopian and Kenyan border. This correlation enables precise time-controlled comparisons of faunal and archeological remainsbetween the two regions. ( 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

The Konso (or Konso-Gardula) research area (Fig. 1)replete with abundant mammalian fossil remains andearly Acheulean assemblages was "rst discovered in thefall of 1991 by a paleoanthropological survey project(Asfaw et al., 1992). Preliminary geochronological andbiochronological information suggested a circum 1.4 Maage for most of the paleontological and archeologicalremains. However, follow-up studies indicated the pres-ence of older sediments in the area. The initial investiga-tion was based on limited observations of mammalianfaunal remains and radioisotopic dating results of a fewprominent marker tu!s.

Field work conducted since 1993 has resulted in over7000 identi"able mammalian specimens including a totalof 18 hominid specimens attributed to A. boisei andH. erectus (Asfaw et al., 1992; Suwa et al., 1997). Thetaxonomic and morphological characteristics of thefauna are currently being assessed. Excavations of in situearly Acheulean sites are in progress. Concurrent withthe paleontological and archeological studies, extensivegeologic "eld investigations have been conducted to es-tablish a reliable chronostratigraphic framework for theentire Plio-Pleistocene sequence designated as the KonsoFormation (Nagaoka et al., 2000). Tephra layers haveplayed a key role in this investigation. The results of foursummer seasons of "eld research (1994}1997), each con-ducted after feedback from laboratory analysis of vol-canic glass shards and single-crystal 40Ar/39Ar dating ofmarker tephra layers are reported here.

The purpose of the present study is to (1) outline thelithologic characteristics and stratigraphic relationshipsof the Konso Formation tephra, (2) document the major

0277-3791/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 9 9 - 2

Fig. 1. The Main Ethiopian Rift in the East African Rift System and thelocation of the Konso research area (modi"ed from WoldeGabriel andAronson, 1987): (MER), Main Ethiopian rift; (1) Galana graben; (2)Amaro horst; (3) Ganjuli graben; (4) Ririba rift; (5) Chew Bahir rift; (6)Usno rift; (7) Omo rift; (8) Kibish rift; (9) Gregory rift. Inset map, ERS,Eastern Rift System; WRS, Western Rift System; SRS, Southern RiftSystem.

element compositions of multiple stratigraphic markertephra used to determine and clarify the Konso Forma-tion stratigraphy, and (3) provide a temporal calibrationof the Konso Formation based on single-crystal40Ar/39Ar dates of key tephra layers. Accordingly, a de-tailed tephrostratigraphy and geochronology of theKonso Formation is presented. Moreover, based on twomarker tephra layers, inter-basin correlation between theKonso research area and the Omo-Turkana Basin alongthe Ethiopian and Kenyan border is assessed.

2. Geologic and geographic setting

The Konso Formation is exposed north of Konsotown at the southern end of the Ganjuli graben of thesouthern sector of the Main Ethiopian Rift (Fig. 1).Exposures range in altitude from 1100 to 1500 m alongthe drainage system of the Iyanda and Gato Rivers, themain tributary system of the Segen River (Fig. 2). Theformation is unconformably underlain by Tertiary ma"clava and Precambrian crystalline basement rocks, and isunconformably overlain by #uvial deposits of middle

Pleistocene to Holocene age. A series of rift-oriented andtransverse faults cut and displace the formation.

The Konso Formation has a total thickness of morethan 200 m, and is divided into four members: the Sor-obo, Turoha, Kayle, and Karat Members, in ascendingstratigraphic order (Nagaoka et al., 2000). The bound-aries of these members were set at the base of distinct andwidespread marker tu!s. The formation is dominated bysedimentary rocks mainly composed of alternating bedsof dark-gray to brown clay (lacustrine or marshy deposi-ts), whitish-gray to red-brown silt and sand (#oodplaindeposits), and limited and/or localized brown sand andgravel (channel beds or slope deposits). Fossils and stonetools are mostly derived from the whitish-gray silt andsand beds, but also occur in the other sediments.

The fossiliferous volcaniclastic deposits, "rst namedthe Konso-Gardula research area (KGA) by Asfaw et al.(1992), currently include 21 fossil-collecting localities des-ignated as KGA1 to 21 (Fig. 2). The entire research areais divided geographically into the eastern, central, andwestern subareas, which are separated from each otherby normal faults running parallel to the rift margins(Fig. 2). The Iyanda and Kayle Rivers further divide theeastern subarea into the northern and southern parts(ESNP and ESSP), and the central subarea into thenorthern and southern parts (CSNP and CSSP), respec-tively. The western subarea consists of the northern(WSNP), central (WSCP), and southern (WSSP) parts,each separated by erosional and terrace surfaces exten-sively developed along the Turoha and Kayle Rivers.

This study focuses on the CSSP, CSNP, and WSCPareas because the most proli"c paleontological and ar-cheological sites are located there, thus necessitating themost detailed geologic investigation. Moreover, theKonso Formation is generally thicker and better exposedat these areas, providing important type sections forestablishing the stratigraphic sequence of the deposits.

3. Analytical methods

3.1. Lithologic characterization

The general features of the tephra layers and sampleswere described with respect to distribution, thickness,color, grain size, structure, morphology of volcanic glassshards, mineral phases, and stratigraphic relationships.Morphological features of glass shards were used forlocal correlation among the various tephra units. Follow-ing Ross (1928), Heiken (1972), and Yoshikawa (1976),glass shards were classi"ed into six morphological types:frothy, "brous, bubble-wall stretched, plate, bubble-junc-tion, and others (types A}F, respectively, in Fig. 3). Foreach tephra sample, 200 randomly selected glass shardsprepared from 125 to 250 mesh sizes were examinedusing a polarizing microscope, and the frequency of each

1306 S. Katoh et al. / Quaternary Science Reviews 19 (2000) 1305}1317

Fig. 2. Geology and geographic subdivision of the Konso research area (modi"ed from Davidson, 1983): (1) Middle Pleistocene to Holocene #uvialdeposits; (2) Early to Middle Pleistocene erosional surface deposits; (3) Plio-Pleistocene Konso Formation; (4) Tertiary ma"c lava; (5) Precambriancrystalline basement rocks; (6) Faults (downthrown side shown by ticks); (7) Numbers of the 21 fossil-collecting localities of KGA1 to 21, numbered byorder of discovery. ESNP, Northern part of eastern subarea; ESSP, Southern part of eastern subarea; CSNP, Northern part of central subarea; CSSP,Southern part of central subarea; WSNP, Northern part of western subarea; WSCP, Central part of western subarea; WSSP, Southern part of westernsubarea.

of the six morphological types were estimated. Mineralphases are generally sparse in the tephra samples andwere not quanti"ed.

3.2. Major element compositions of discrete volcanic glassshards

In the "eld, comparatively well-preserved portions ofeach tephra were identi"ed and sampled. These sampleswere carefully disaggregated and washed in ultrasonicbath to remove altered fractions, and dried. The cleanedfractions were sieved to obtain coarser glass shards(63}250 mesh sizes). For weathered tu!s containing fewerfresh volcanic glasses, shards were handpicked undera binocular microscope.

The major element compositions of discrete glassshards were obtained using an X-ray electron micro-

probe analyzer (EPMA). The total number of tephrasamples currently analyzed is more than 50. EPMA ana-lytical methods are given in WoldeGabriel et al. (1994).For each tu! sample, glass shards with analytical totalsgreater than 90 wt% were selected. The mean and stan-dard deviation of the discrete glass shard results of eachsample were calculated for each element oxide and ananalytical total. Glass shards with one or more elementoxide falling outside three standard deviations of themean were excluded, and the means and standard devi-ations recalculated.

Element oxides with relatively large variance amongsamples were considered to be useful for tephra correla-tion in various sections of the Konso Formation. Thiswas numerically assessed in relation to within-samplevariance by the simpli"ed method outlined in Table 1 (seehttp://www.elsevier.nl/locate/quascirev).

S. Katoh et al. / Quaternary Science Reviews 19 (2000) 1305}1317 1307

Fig. 3. Morphological classi"cation of volcanic glass shards of theKonso Formation tephra: (A-type) frothy glass shard that consists ofmany bubbles much smaller than the shard size; (B-type) "brous glassshard that is a mass of many slender glass threads; (C-type) bubble-wallstretched glass shard that has 3 to 7 elongate vesicles forming side wallsof cylindrical bubbles; (D-type) plate glass shard that is a part of thebubble-wall with much larger curvature than the shard size and hasoften one or two ridges on its surface; (E-type) bubble-junction glassshard that is a plate shard with an asterisk- or H-shaped ridge; (F-type)other type glass shards including small spherical bubbles and relativelythick blocky shards.

3.3. Single-crystal 40Ar/39Ar dating

The tu! samples for single-crystal 40Ar/39Ar datingwere collected from sections that appeared to yield rela-tively coarse-grained fractions. Grains greater than 47 or63 mesh sizes were obtained using wet sieving. Plagioc-lase or sanidine crystals for dating were handpicked fromthe sieved fractions.

The ages were calculated relative to the Fish Canyonsanidine (FCs) standard (27.84 Ma), and the decay con-stants and 40K isotopic abundance of Steiger and JaK ger(1977). Renne et al. (1998a) has shown that the age of theFCs is slightly older (28.02 Ma), but the slightly youngerage (27.84 Ma) value is used herein to facilitate compari-son with published 40Ar/39Ar data. Correcting the ageswith the revised standard age would add 0.009 to0.012 Ma to samples with measured ages of 1.4 to 1.9 Ma.

Neutron #uence monitoring used for the analyses wasoutlined in Renne et al. (1998a), and all samples werebracketed by four distinct positions occupied by the FCs.The arithmetic mean of the neutron #uence parameter`Ja calculated from these four positions and used tocalculate ages and their errors is 2.627]10~4$

4.1]10~6.Dates for each tu! were calculated as the inverse vari-

ance weighted mean of individual grain analyses, usingthe hierarchical approach outlined by Renne et al. (1996),which accounts for the systematic error in `Ja. In thisprocedure, we "rst calculate the weighted mean40ArH/39Ar and its standard error, then propagate this

and the error in the neutron #uence parameter `Ja intothe age equation. Age uncertainties do not include sys-tematic errors in the age of the standard or 40K decayconstants (Renne et al., 1998a, b).

Some of the tu!s contain obvious xenocrysts or detri-tal contaminants, mostly of Precambrian age, derivedfrom the underlying crystalline basement rocks. To ex-clude such data and other outliers from the age calcu-lations, we included all analyses yielding 40ArH/39Arvalues within 2 standard errors of the weighted mean.Also excluded were analyses with less than 5]10~16

mole of 40Ar.

4. Results

The Konso Formation contains more than 30 tephralayers. Table 2 (see http://www.elsevier.nl/locate/quas-cirev) outlines the lithologic characteristics of the Konsotephra, including color and morphology of glass shardsand associated mineral phases. The tephra layers are allsilt to sand-sized vitric ashes with distinct glass shardmorphologies. Table 3 (see http://www.elsevier.nl/lo-cate/quascirev) summarizes the major element composi-tions of discrete glass shards of the Konso Formationtephra, including those of the dated tephra samples. De-tailed analytical results of these and other analyzed tu!samples are presented in Appendix 1 (see http://www.elsevier.nl/locate/quascirev). Table 3 also gives theweighted-mean single-crystal 40Ar/39Ar ages of seventephra layers (TRT, KYT-2, HAT, TBT, LHT, KRT, andBWT). Individual grain analyses are presented in Appen-dix 2 (see http://www.elsevier.nl/locate/quascirev)

For local correlation among the Konso Formationtephra, four element oxides consisting of Al

2O

3, FeO,

MgO, and CaO were most useful (Table 1). In particular,the less mobile Al

2O

3and FeO were considered the most

reliable. Na2O and K

2O were least useful, not only

because of their relatively large variance among shards ofthe same sample, but also because of the distinctly highervariation noted among multiple samples of the same tu!(Table 3 and Appendix 1). This is most likely related tothe higher mobility of these oxides. The variation dia-gram depicting the mean contents of Al

2O

3and FeO

(Fig. 4) outlines the most conspicuous aspects of thechemical characteristics of the Konso Formation tephra.

In the following sections, for each of the three focalareas described here (WSCP, CSNP, and CSSP), wepresent the stratigraphic relationships of the tephralayers together with brief accounts of "eld setting andlithologic characteristics. The stratigraphy was estab-lished from a combination of observed "eld relationships,lithologic characteristics, and major element composi-tions of discrete glass shards. Within the limits of analyti-cal error, the 40Ar/39Ar dating results were consistentwith the stratigraphic constraints.


Fig. 4. Variation diagram showing mean alumina and iron contents ofindividual tephra layers in the Konso Formation. Each point representsthe mean composition of an individual sample listed in Appendix 1.

4.1. Central part of the western subarea (WSCP)

Several northeast-striking normal faults run throughthe WSCP, dividing it into three smaller blocks (Fig. 5).Displacements were noted along other cross cutting nor-mal faults. The Kayle and Karat Members crop out onTertiary ma"c lava and Precambrian crystalline base-ment rocks that are cut by rhyolite and basalt dykes. Therich fossil-collecting sites of KGA7 to KGA14 are locatedwithin the WSCP (Fig. 5).

The Kayle Member in the WSCP contains more than8 tephra layers, including the three major tu!s represent-ed by the Trail Bottom Tu! (TBT), Ivory Tu! (IVT), andLehayte Tu! (LHT) in ascending stratigraphic order(Fig. 6). The TBT continuously crops out along the banksof the tributaries of the Kayle and Turoha Rivers atKGA 11 and 13 (Sections 29}33 and 38 in Fig. 6). Atthese localities, the TBT is a well-laminated gray tobrownish-gray volcanic ash layer that contains ac-cretionary lapilli less than 0.5 cm in diameter. This tephrais discontinuously exposed at KGA 8, 9, and 14 (Sections11, 14, 15, and 28 in Fig. 6), and is more stronglyweathered there than at KGA11 and 13. The TBT isthicker (1.55 m at maximum) compared with the rest ofthe Konso tephra and is characterized by gray glassshards of bubble-wall stretched and plate types (Table 2).The IVT occurs about 5 m above the TBT at KGA11 andmore than 15 m below the Karat Tu! (KRT) at KGA8(Sections 30 and 11 in Fig. 6). This tu! is whitish and welllaminated at KGA11. Unlike the TBT, the tu! is domin-

ated by whitish to clear glass shards of frothy and "broustypes (Table 2). The pinkish gray LHT lies 0.5 to 5 mabove the TBT at KGA9 and 11 (Sections 15 and 29 inFig. 6). It discontinuously crops out below the sand andgravel beds at KGA10 that lie about 2 m above the IVT(Sections 18}20 in Fig. 6). These beds have yielded manyvertebrate fossils including those of A. boisei and H.erectus (Asfaw et al., 1992; Suwa et al., 1997). The LHTis composed of dominant plate type and sparse "brousand bubble-wall stretched types of clear glass shards(Table 2).

Three local tu!s occur below the TBT at KGA13 and14 (Sections 28, 32, 33, and 38 in Fig. 6). These aredesignated as the Thick Pink Tu! (TPT), Hope Tu!(HPT), and Droplet Tu! (DRT), in ascending strati-graphic order. The sediments below the DRT are strong-ly weathered to reddish-brown, suggestive of a relativelylong time interval between the DRT and HPT. The TPTis only observed at the KGA13 section (Section 32 inFig. 6). It shows clear lamination and is more stronglyweathered than the HPT. In the Kayle Member, anunnamed tu! 9711-66 lies between the TBT and IVT atKGA11 (Section 30 in Fig. 6). Another local tu!, the PinkTu! (PNT), occurs from 8 to 10 m below the KRT atKGA12 and 14 (Sections 24 and 27 in Fig. 6). Thestratigraphic relationships of the PNT to the other tu!sbelow the KRT is not presently clear.

The Karat Member in the WSCP contains more than10 tephra layers, including three major tu!s representedby the Karat Tu! (KRT), Bench Tu! (BNT), and BrightWhite Tu! (BWT) in ascending stratigraphic order(Fig. 6). The KRT, a whitish-gray or light-brown volcanicash layer, is important because it is datable and wide-spread in the WSCP (Fig. 6). The boundary between theKayle and Karat Members is set at the base of this tu!(Nagaoka et al., 2000). The BNT is a pinkish-gray vol-canic ash layer that occurs 1}5 m above the KRT. It iswidely traceable like the KRT (Fig. 6). These two tu!s arecharacterized by light-gray glass shards of dominantplate and bubble-wall stretched types, with less abundantbubble-junction glass shards (Table 2). The KRT andBNT often occur in the same section, and are very dis-tinct marker tephra layers for local correlation.

The BWT is also extensively exposed in the WSCPexcept at KGA7, 10, and 11 (Fig. 6). It occurs in themiddle level of the Karat Member, about 2}5 m abovethe BNT or KRT. The BWT is a whitish-gray volcanicash, characterized by abundant light-gray glass shards ofplate and bubble-wall stretched types with less abundantfrothy, "brous, and bubble-junction shards (Table 2). Atsome sections of KGA8, 12, and 14 (Sections 9 and 10, 24,and 26 in Fig. 6), there are many thinner and "ner whiteto whitish-gray tu!s between the BWT and BNT. Theyare designated as the White Tu! Series (W-S), and aresimilar to the BWT in color and morphology of glassshards (Table 2).


Fig. 5. Geology of the central part of the western subarea (WSCP): (1) Latest Pleistocene to Holocene #uvial deposits; (2) Middle to Late Pleistoceneterrace deposits; (3) Early to Middle Pleistocene erosional surface deposits; (4}8) Plio-Pleistocene Konso Formation (4, Karat Member; 5, KayleMember; 6, Turoha Member; 7, Sorobo Member; 8, undivided Konso Formation); (9) Tertiary ma"c lava; (10) Precambrian crystalline basementrocks; (11) Faults (downthrown side shown by ticks); (12) Locations of stratigraphic sections.

Three more localized tu!s named the Piso Tu! (PST),Hard Gray Tu! (HGT), and Boleshe Tu! (BOT) occur inthe upper part of the Karat Member. The PST liesbetween the BWT and HGT at KGA12, and also occursat KGA13 (Sections 23 and 34 in Fig. 6). It containsabundant accretionary lapilli less than 1 cm in diameter.The HGT crops out 5}10 m above the BWT or its equiv-alent stratigraphic level at KGA9, 12, and 14 (Sections 13and 22, 23, and 26 in Fig. 6). The BOT lies more than 5 mabove the stratigraphic horizon equivalent to the BWTat KGA7 (Sections 1, 2, 4, and 5 in Fig. 6). The tu! isstrongly weathered and lacks glass shards in many sec-tions. In the study area, it is not currently known if theBOT, PST, and HGT layers crop out in the same section.Two localized altered tu!s occur at KGA7 and 8 abovethe BOT (Sections 2 and 4, 5, and 7 in Fig. 6).

4.2. Northern part of the central subarea (CSNP)

The CSNP includes a hilly terrain, representing thefossil-collecting locality of KGA4. The CSNP is sub-divided into three small fault-bounded blocks bynorth}south and northwest}southeast striking normalfaults (Fig. 7).

The western block occupies the footwall and consists ofthe Sorobo and Turoha Members (Fig. 7). Two majortephra layers consisting of the Blue Tu! (BLT) andTuroha Tu! (TRT) occur in this block (Fig. 8). The BLT isa well-laminated bluish-gray volcanic ash layer that lo-cally occurs in the Sorobo Member. It crops out directlyabove Tertiary ma"c lava at the westernmost part of theblock (Section 53 in Fig. 8). This tu! is dominated by clearto light-gray glass shards of frothy type (Table 2).


Fig. 6. Stratigraphic sections in the central part of the western subarea (WSCP): Locations of stratigraphic sections are shown in Fig. 5. Tu! sampleslisted in Tables 2 and 3, and in Appendices 1 and 2, are indicated by sample number and arrows.

The TRT is a widely traceable, white to whitish-grayvolcanic ash layer, which is exposed mainly towardthe top of the KGA4 hill, occurring 5}10 m above theBLT (Sections 46}52 in Fig. 8). The boundary betweenthe Sorobo and Kayle Members is set at the base ofthe TRT (Nagaoka et al., 2000). Upward "ning andlamination at many sections characterize the TRT.Pumice grains less than 1 cm in diameter and minerals,including feldspar, quartz, and magnetite are concen-trated in the lowest part, whereas the upper and middleparts are vitric and strongly altered to pinkish white. Thistu! is characterized by clear to light-gray plate glassshards (Table 2).

The triangular southern block exposes a portion of theKayle Member (Fig. 7). Two localized tephra layers arepresent in this fault block. One of them, the Hand AxeTu! (HAT) is an important stratigraphic marker becauseit occurs directly below a sand bed that contains rich

Acheulean artifacts including abundant handaxes andpicks (Sections 43}45 in Fig. 8). This whitish-gray to graytu! is characterized by a subequal mixture of frothy,"brous, and bubble-wall stretched types of light grayglass shards (Table 2).

Another tephra layer (954-4 tu!) was identi"ed on topof a ridge near the northeastern margin of the fault block(Section 42 in Fig. 8). Lithologic and chemical character-istics of this tu! are similar to those of the TBT mappedin the WSCP and CSNP (Tables 2, 3, and Appendix 1).The 954-4 tu! occurs only at this single section andits dip and strike markedly di!er from those of otherstrata in the member. Furthermore, it is downwarpedand fractured. These observations suggest that the tu! isreworked and unconformably overlies the HAT andsuperjacent sediments.

The eastern block contains a portion of the KayleMember. It is in normal fault contact with the western


Fig. 6. (continued )

and southern blocks that occupy the footwalls (Fig. 7).The TBT and another localized tu! (964-26) are presentin this fault block. The TBT occurs topographically to-ward the base of the KGA4 hill and stratigraphicallyoccupies the uppermost part of the member (Sections39}41 in Fig. 8). The 964-26 tu!, which occurs 3}4 mbelow the TBT, is currently recognized at only two sec-tions (for example, Section 41 in Fig. 8). It is whitish andweakly cemented by calcareous precipitation. The up-per-half of the tu! is "ner and more strongly weatheredthan its lower-half, which contains accretionary lapilliless than 0.5 cm in diameter.

4.3. Southern part of the central subarea (CSSP)

The CSSP (KGA6 and adjacent areas) is subdividedinto two normal fault-bounded blocks, with the smaller

western block forming the footwall (Fig. 7). The Turohaand Kayle Members overlie Tertiary ma"c lava andPrecambrian crystalline basement rocks in the westernblock, while the Kayle and Karat Members occur in theeastern block. Middle to late Pleistocene terrace depositscover a substantial portion of the CSSP (Fig. 7).

In the western block, three major tephra layers lie atthe lower part of the Kayle Member (Fig. 9). These arethe Kayle Tu!-1 (KYT-1), the Kayle Tu!-2 (KYT-2), andthe Kayle Tu!-3 (KYT-3), in ascending stratigraphicorder. The KYT-1 is a white to whitish-gray volcanic ashlayer, which consists of a subequal mixture of frothy,bubble-wall stretched, and plate types of clear to pale-brown glass shards (Table 2). It crops out continuouslywithin the western block, and thus the boundary betweenthe Kayle and Turoha Members is set at the base of thistu! (Nagaoka et al., 2000). The KYT-2 is a gray to


Fig. 7. Geology of the southern and northern parts of the centralsubarea (CSSP and CSNP). For legends, see Fig. 5.

Fig. 8. Stratigraphic sections in the northern part of the central subarea(CSNP). Locations of stratigraphic sections are shown in Fig. 7. SeeFig. 6 for details of legends.

brownish-gray volcanic ash layer that lies 2 to 3 m abovethe KYT-1. It is thicker and well laminated at its typelocality (Section 60 in Fig. 9). The tu! is characterized byabundant gray to light-gray plate shards (Table 2). Thebrownish-gray KYT-3 is dominated by bubble-wallstretched and plate types of pale brown glass shards(Table 2). The KYT-1 and KYT-2 have contrasting colorand glass shards and occur together at many sections ofthis fault block (Sections 56}60 in Fig. 9), providingdistinct stratigraphic markers for local correlation.

In the eastern block, the Kayle Member is dominatedby weathered clay and silt beds, and contains a localizedtu! (956-10). The 956-10 tu! is discontinuously exposedalong the right bank of the Kayle River (Section 61 inFig. 9). It occurs at the lowest level of the local sectiondominated by weathered brown to red-brown sandy clayand silt beds. The tu! appears vitric but the glass shardsare almost totally altered.

The Karat Member of the eastern block consists ofalternating beds of laminated clay and thin diatomitebeds with a widely traceable dark-gray lignite bed(Sections 61}65 in Fig. 9). The member contains alocal tu! (956-13) and the BWT in ascending strati-graphic order. The 956-13 tu! is an altered volcanic

ash layer interbedded within the clay and diatomitelayers, at least 20 m above the 956-10 tu! (Sections 62,64, and 65 in Fig. 9). The BWT, which is lithologicallyand chemically equivalent with the BWT of the WSCP,lies 5 to 7 m above the 956-13 tu! (Sections 63}65 inFig. 9).

Finally, an unnamed tu! (956-11) occurs in a silty claylayer above 3-m thick sand and gravel beds, about 11 mabove the BWT. These beds are discontinuously exposedon top of the highest terrace of the eastern block. Tu!956-11, which is part of the terrace deposits, is recognizedonly at Section 65 (Figs. 7 and 9). It is characterized bya coarser unit that contains pumice grains less than 1 cmin diameter at its base.

5. Discussion

In the WSCP, the relative stratigraphic positions of sixmajor tu!s (TBT, IVT, LHT, KRT, BNT, and BWT)were unambiguously established using "eld evidence


Fig. 9. Stratigraphic sections in the southern part of the central subarea (CSSP). Locations of stratigraphic sections are shown in Fig. 7. See Fig. 6 fordetails of legends.

(Fig. 10). These tu!s are well-correlated within the WSCPon the basis of color, morphology, and major elementcompositions of discrete glass shards (Tables 2, 3, andFig. 4).

The relative stratigraphy of the more localized tu!s,TPT, HPT, DRT, tu! 9711-66, W-S, PST, and HGT arealso clearly established from "eld evidence (Fig. 10).However, based on lithologic and chemical information,no tephra unit correlative to the PNT was found in theWSCP (Tables 2, 3, and Fig. 4). Instead, the tentativestratigraphic placement of the PNT directly above theLHT was determined by comparing the thickness of the

sediments between the KRT and PNT layers. Moreover,the stratigraphic relationships between the BOT and thePST/HGT is currently unresolved because of variablesediment thickness between the BWT (and equivalentstratigraphic level) and these tu!s (Fig. 6).

Although the TBT is recognized in both the CSNP andthe WSCP (Tables 2, 3, and Fig. 4), the stratigraphicrelationships between the underlying 964-26 tu! in theCSNP and the DRT, HPT, and TPT in the WSCP is notcurrently known. The thickness of the deposits betweenthe TBT and each of these four tu!s (Figs. 6 and 8)together with the relatively long time gap inferred


Fig. 10. Local and composite tephrostratigraphy and chronostratigraphy of the Konso Formation, with member division and weighted-meansingle-crystal 40Ar/39Ar ages of seven tu!s with 1-sigma errors. Sample numbers represent unnamed tu!s. The stratigraphic levels of the BOT and TPTare tentative and those of the PNT and tu! 946-26 are not conclusive. Correlation of tu! 956-10 with the PNT will be checked with trace element data.

between the DRT and HPT, suggest that the 964-26 tu!lies between the DRT and HPT (Fig. 10).

The HAT stratigraphically occurs below the TBTbecause the unconformably overlying tu! 954-4 islithologically and chemically correlated with the TBT(Tables 2, 3, and Fig. 4). Although the 40Ar/39Ar result ofthe HAT (ca. 1.5 Ma) is not conclusive, it is consistent

with this stratigraphic interpretation. Furthermore, notephra layers correlative to the HAT are currently recog-nized in the WSCP between the TBT and TPT. There-fore, the HAT probably lies stratigraphically below theTPT (Fig. 10).

In the CSNP, the TRT and BLT occur in the Turohaand Sorobo Members, respectively. Both occur in the


footwall in normal fault contact with the block thatcontains the HAT of the Kayle Member. Based on the40Ar/39Ar ages of the TRT (1.91$0.03 Ma) and theHAT (ca. 1.5 Ma), it appears that the TRT and BLTstratigraphically occur below the HAT (Fig. 10).

The stratigraphic relationships of the tephra layers ofthe Kayle Member exposed in the CSSP to tu!s in otherparts of Konso is not clearly delineated at present. How-ever, the 40Ar/39Ar age of the KYT-2 (1.72$0.03 Ma)indicates that the KYT-1, -2, and -3 are chronostratig-raphically located between the TRT and HAT (Fig. 10).

According to lithologic and chemical similarities(Tables 2, 3, and Fig. 4), the BWT in the eastern block ofthe CSSP was correlated to tu! units in the WSCP. Theglass shard chemistry of the underlying tu! 956-10 issimilar to that of the PNT, IVT, tu! 964-26, and KYT-1(Table 3). However, the lithologic characteristics of the956-10 tu! are similar to those of the PNT comparedwith those of the other three tu!s (Table 2). Despite theabsence of the KRT and BNT in the CSSP sections, the956-10 tu! is currently considered to be correlative tothe PNT (Fig. 10). However, without trace element dataon the two samples, the suggested correlation cannotbe con"rmed.

Based on the lithologic and chemical correlation andthe 40Ar/39Ar results of selected tephra layers in theWSCP, CSNP, and CSSP sections, a composite chrono-stratigraphic column is established for the KonsoFormation (Fig. 10). This stratigraphic information sug-gests that most of the Konso Formation was deposited inthe focal areas of the present study between approxim-ately 1.9 and 1.4 Ma.

The weighted-mean single-crystal 40Ar/39Ar apparentages of some of the tephra layers from the Konso Forma-tion tu!s (corresponding to the TRT, TBT, KRT andBWT of the present study) were reported by Asfaw et al.(1992). The neutron #uence monitoring scheme used inthe published 40Ar/39Ar data is unclear, hence the hier-archical error propagation employed herein cannot beapplied to the previous data. Because one of the objec-tives of this study is to establish regional correlationbetween the Konso tephra and those from the Omo-Turkana Basin along the Ethiopian and Kenyan border,it is critical to account for possible neutron #uence vari-ations. Thus, the 40Ar/39Ar data reported here is usedinstead of the published ages for the temporal regionalcorrelation of the various distal tephra units.

The results obtained here on the TBT (1.43$0.02 Ma)are consistent with previous dates of 1.44$0.01 Ma ona sample from KGA13 (T-1) and of 1.44$0.05 Ma ona sample from KGA4 (19-91). The new 40Ar/39Ar ages ofthe KRT (1.41$0.02 Ma) and BWT (1.40$0.02 Ma)are, however, slightly older than those previouslyreported by Asfaw et al. (1992), although all are withintwo standard error (KRT: 1.34$0.04 Ma, and BWT:1.35$0.02 Ma, 1.37$0.03 Ma, and 1.38$0.07 Ma).

The newly measured ages are based on carefully chosensamples with a larger number of analyzed grains. Forthese reasons and those previously cited, we consider ournew age determination of the KRT and BWT to be morereliable.

The 40Ar/39Ar ages of the TBT, LHT, KRT, and BWTindicate that all tu!s from the TBT to BWT are approx-imate time-equivalents of the Chari Tu! at Koobi Fora,northern Kenya, dated to 1.39$0.01 Ma (McDougall,1985). Among these tu!s, the BWT and W-S are com-positionally correlative to the Chari Tu! and its strati-graphic equivalents in the Omo-Turkana Basin (Table 4)(see http://www.elsevier.nl/locate/quascirev). The BWTand W-S are also similar in color and glass shard type(Table 2) to the Chari Tu! which consists of dominantclear bubble-junction and plate glass shards (e.g., Brownand Feibel, 1986). Since the W-S is more localized inoccurrence and thinner than the BWT, it is concludedthat the BWT is the likely equivalent of the Chari Tu!.

Samples 4A-91 and 4B-91 from tu!s correlative to theTRT yielded ages of 1.66$0.03 Ma and 1.87$0.04 Ma,respectively (Asfaw et al., 1992). We consider the newlymeasured age of the TRT (1.91$0.03 Ma) to be morereliable for the reasons given above. This new age esti-mate indicates that the TRT is temporally correlative tothe KBS Tu! at Koobi Fora, which yielded an age of1.88$0.02 Ma (McDougall, 1985). The major elementcomposition of discrete glass shards from the two sam-ples is also identical (Table 4).

According to Cerling and Brown (1982), the KBS Tu!consists of a subequal mixture of bubble-wall stretched,plate, and frothy types of glass shards. It is reported thatmost of the KBS shards are generally clear and some ofthem grade from light-gray to dark-brown. The basalportion is described to occasionally lack the dark shards,similar to Tu! H2 of the Shungura Formation in theOmo Basin, a correlative of the KBS Tu!.

The glass shard color of the TRT (Table 2) coincideswith that of the KBS Tu! and its correlatives in theOmo-Turkana Basin. However, there are some di!er-ences in shard morphology. The TRT contains moreplate type shards. This di!erence in glass type composi-tion is likely to be a function of transport processesduring the deposition of the tu!s. The relatively poorlysorted KBS Tu! may be the result of transport anddeposition in a #uvial environment. The TRT is probablyfallout initially transported and sorted by wind e!ects,resulting in dominant light plate shards. Thus, on thebasis of shard morphology, age, and chemistry, the TRTis correlative to the KBS Tu!.

6. Conclusions

The Konso Formation at the southern end of theMain Ethiopian Rift, Ethiopia has yielded rich


Acheulean archeological resources and abundant verte-brate fossils, including those of A. boisei and H. erectus.Moreover, the formation contains more than 30 tephralayers. Extensive "eld investigations, mapping, chemicalcorrelation, and single-crystal 40Ar/39Ar dating of se-lected tephra have led to the establishment of a com-posite tephrostratigraphy and chronostratigraphiccolumn for the Konso Formation. According to thisstudy, the depositional ages of the various members ofthe formation range from approximately 1.9}1.4 Ma.This temporal constraint provides detailed chronologicalframework for the paleoanthropological remains thatoccur intermittently throughout the sedimentary se-quence.

Among the dated tu!s, the Turoha Tu! (TRT)and Bright White Tu! (BWT) are correlative to theKBS and Chari Tu!s, respectively, of the Omo-TurkanaBasin in northern Kenya and southwestern Ethiopia.Inter-basin correlation of other Konso Formation tephrais currently in progress, and would greatly contribute tothe spatial and temporal comparisons of the paleon-tological and archaeological remains in the Konso andother adjacent rift-bound fossiliferous sedimentarydeposits.

Acknowledgements

Fieldwork for the Konso PaleoanthropologicalResearch Project has been accomplished underpermission from the Center of Research and Conserva-tion of Cultural Heritage (C.R.C.C.H.), Ministry ofInformation and Culture, Ethiopia. We thank theC.R.C.C.H. and the National Museum of Ethiopiafor allowing us to undertake the necessary researchactivities, and the Bureau of Culture and Information ofthe Southern Nations, Nationalities, and the People'sFederal Regional Government of Ethiopia for support-ing the project. We are grateful to the sta! of the Bureauof Culture and Information and of the C.R.C.C.H. forparticipation in and contribution to the "eldwork. Wealso thank the administration of the Konso Special Ad-ministrative District and the Konso people for support-ing our project. Special acknowledgements are due toAnita Grunder and Robert Duncan for their constructivereview. Support and access to the microprobe facility ofthe Geology/Geochemistry Group of the Earth and En-vironment Sciences Division, Los Alamos National La-boratory is greatly appreciated. The MitsubishiFoundation and the Japan Ministry of Education,Science, Sports and Culture provided the "nancial sup-port for this research.

The following references appear in the Tables andAppendices online, See http://www.elsevier.nl/locate/quascirev. Brown et al. (1992); Cerling et al. (1979); Renne(1998); Renne et al. (1999).

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