Tephrostratigraphy and geological context in paleoanthropology

ARTICLES

Tephrostratigraphy and Geological Contextin PaleoanthropologyCRAIG S. FEIBEL

The products of volcanic eruptionshold a unique place in the establish-ment of a detailed, temporally con-strained record of biotic evolution. Atone level or another, volcanic materi-als contribute much of what we knowconcerning the relative and absoluteordering of biotic events in time. Mostisotopic dates applied to the fossilrecord are measured on crystalserupted from volcanic sources. Thegeomagnetic polarity time scale islargely calibrated with such dates.

Even where materials for direct datingare lacking, relative sequence and tiesto dated localities can be establishedthrough tephrostratigraphy, the geo-chemical correlation of volcanic ejecta(tephra) in stratigraphic sequences.

Tephra have been of considerableimportance in paleoanthropology. Theisotopic dating of sites and long-distance correlation of volcanic asheshave been instrumental in calibratingthe framework for primate and homi-nid evolution.1-6 Even traces of earlyhominids themselves, in the track-ways of Laetoli7,8 and Koobi Fora,9

were formed and preserved in tephra.This paper will explore some of theaspects of tephra records as they relateto paleoanthropological sites in EastAfrica. Both the potential and limita-tions of tephra-based studies will bediscussed as a basis for a better under-standing of this unique component ofmany Plio-Pleistocene localities.

STRATIGRAPHIC PRINCIPLESThe discipline of stratigraphy fo-

cuses on the character and sequenceof rock layers or strata. Character in-cludes the composition and arrange-

ment of the particles that comprise arock. Typically, characterization in-volves the examination of the rock inoutcrop, hand specimen, thin section,or through chemical analysis, but mayalso involve investigation by remotesensing, such as spectral analysis fromsatellite platforms or with various log-ging techniques in boreholes or wells.The sequence of strata can generallybe demonstrated using the Principleof Superposition, which states that inany undisturbed interval of strata, theolder and early-deposited strata under-lie the younger and later-depositedstrata. This can be applied to anyoutcrop or hole in which superposi-tional relationships can be demon-strated.

In many cases, however, throughfaulting or discontinuous exposure,direct superpositional relationships arenot demonstrable, so that we mustrely on correlation to establish tiesbetween rock strata. Correlation in-volves the establishment of equiva-lence of rock units. This equivalenceoften implies contemporeneity, but thisneed not be the case. Lithologic corre-lation relies on the equation of strataor sequences of strata with distinctivelithologic character. The more nearlyunique that character, or the moredistinctive a sequence of lithologies,the more reliable such a correlationbecomes. By the very nature of theproblem, correlations cannot beproven, although different approachesto correlation have varying degrees ofprecision and reliability. Three aspectsof the correlation process are criticalhere: the uniqueness of the propertyused (hence, the likelihood that thecorrelation is correct); the resolutionof the correlative property (the tempo-ral factor, delineating a zone or ahorizon); and the distribution of the

The fossiliferous and artifact-rich sites of East Africa, which are central to ourunderstanding of early hominid evolution, also preserve a detailed record ofexplosive volcanism. The products of these eruptions, ash, lapilli, and pumice, arecollectively known as tephra. They drifted down from the skies or washed downrivers in later rainy seasons, and now provide a key to both dating and correlation,and with them the establishment of a geologic framework for evolution. While sometephra can be directly dated, particularly through mineral phases they contain, theglass component of most ashes has a geochemical fingerprint that is unique to aparticular eruption. That fingerprint defines an isochronous marker, a layer in time.By identifying characteristic geochemical signatures from far-flung localities, geolo-gists can correlate sequences, establish relationships in time, and compile long-term records from local sections. Much of our understanding of the pattern andtiming of Plio-Pleistocene evolution in East Africa is based on this tephrostrati-graphic framework.

Craig Feibel is a geologist in the Depart-ments of Anthropology and Geological Sci-ences at Rutgers University. He has con-ducted research on the geological contextof evolution in the Turkana Basin of Kenya,and other rift valley sites in Tanzania, Ethio-pia, and Israel. His work centers on stratig-raphy, sedimentology, and the reconstruc-tion of paleoenvironments. He received aB.A. in Earth Sciences from DartmouthCollege, an M.S. in Geology from IowaState University, and a Ph.D. in Geologyfrom the University of Utah.

Key words: tephra, stratigraphy; correlation; geo-chemistry; Plio-Pleistocene; dating; paleoanthro-pology

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correlative marker (the spatial factor,defining the geographic range of itsapplicability).

One of the most precise tools forcorrelation is tephrostratigraphy, theuse of deposits of volcanic ejecta asmarkers, and correlation based ontheir distinctive character. The reasonfor the unique utility of tephrostratig-raphy lies in the nature of tephra.10

The complex magmatic history oftephras leads them to be chemicallyunique, and thus to be individuallyrecognizable by geochemical finger-printing. As the product of a singlevolcanic eruption, a tephra providesan isochronous marker, a time plane.The mode of formation of tephras inexplosive volcanic events results inwidespread distribution of the prod-uct.

ORIGIN, TRANSPORT, ANDDISTRIBUTION OF TEPHRA

All tephra begins as a molten mix,a magma, at some depth in the earth.Convection within the magma cham-ber tends to maintain the homo-geneity of this mixture, which can becharacterized as a combination of ele-

ments, many in oxide form, and somecrystals. The elemental componentscan be grouped on the basis of relativeabundance as major elements (present

in abundances typically .5%, includ-ing Si, Al), minor elements (typicallypresent in abundances of 1–5%, includ-ing Fe, Na, K), and trace elements

(comprising 91%, including Ba, Ca,Mn, Nb, Rb, Sr, Ti, Y, Zn, and Zr).Examples of elemental abundances ina range of tephra compositions aregiven in Table 1. While most of thesecomponents exist in a liquid state,volatiles (H2O, CO, SO4, Cl) occur asdissolved gas. In addition, many mag-mas contain large crystals, or pheno-crysts.

Once a magma approaches theearth’s surface, several changes beginto occur. Lithostatic pressure, the bur-den of overlying rock, decreases andgases begin to exsolve. This leads to alocal increase in fluid pressure, whichspeeds the rate of upward movement.Thus, magmas rich in volatiles tend toaccelerate dramatically as they nearthe earth’s surface, then to erupt explo-sively. Low-volatile magmas may alsobehave explosively, but typically withmuch less energy. Expanding exsolvedgases create a magma froth that isanalogous to the foamy head of a beer.The froth is quenched to glass with therapid temperature drop of eruption.(If erupted slowly and some of thegases are allowed to escape, the samematerial could cool more slowly, al-

The complex magmatichistory of tephras leadsthem to be chemicallyunique, and thus to beindividually recognizableby geochemicalfingerprinting. As theproduct of a singlevolcanic eruption, atephra provides anisochronous marker, atime plane.

Glossary

Alkaline—rich in alkaline metals (sodium, potassium).Ash—particle size category for pyroclastic rocks with grains

less than 2 mm in diameter. Most volcanic ash is composed ofglass (vitric ash), but it may include crystals and rock fragments(lithics) as well.

Basalt—a basic igneous rock, usually dark colored. Com-posed chiefly of calcium feldspars and pyroxene in a vitricgroundmass (matrix).

Basic—silica-poor. Said of an igneous rock having roughly 45to 52% SiO2. Basic rocks are generally rich in iron, magnesium,and calcium. Their low silica content gives them a low viscosity, sothey may erupt quietly and produce only small quantities of tephra.

Bentonite—a soft, light-colored rock consisting of clay miner-als and silica formed from the chemical alteration of a vitricvolcanic ash.

Carbonatite—a rare magmatic carbonate and the rock formedwhen it solidifies. Carbonatite lavas and ashes, such as thoseerupted from the volcano Oldoinyo Lengai in Tanzania, aretypically black on eruption but recrystallize and turn whitewhen exposed to water. Carbonatite ashes preserved the homi-nid footprints at Laetoli.

Glass—the amorphous product of rapid cooling of a magma.Known as obsidian when it constitutes a whole rock, it alsooccurs as a component of volcanic ash and as the groundmass(matrix) of a lava. The adjective ‘‘vitric’’ is used to describe glassycomponents.

Lava—a molten extrusive or the rock solidified from it. Lavasare usually composed of a mixture of large and small crystals ina matrix of glass.

Phenocryst—a conspicuously large crystal in a volcanic rock.Phenocrysts typically form early in the crystallization process;hence their large size. Potassium-bearing phenocrysts, such assanidine crystals, are ideally suited for isotopic age determina-tions by K-Ar or 40Ar/39Ar methods.

Pumice—a light-colored, vesicular glassy rock. Usually buoy-ant enough to float on water.

Rhyolite—a silicic igneous rock, typically with phenocrysts ofquartz and alkali feldspar.

Silicic—silica-rich. Usually defined as 65% or more SiO2

content. Silicic magmas tend to be more viscous than basiccompositions, and thus erupt explosively. The major tephra-producing eruptions are of silicic compositions.

Tephra—a general term for pyroclastic material eruptedexplosively from a volcano. As used here, the vitric componentof a tephra has a distinctive geochemical composition, the‘‘fingerprint’’ of that eruption.

Tuff—a sedimentary rock consisting of greater than 50%pyroclastic material such as ash, lapilli, or blocks. The particlescomprising tuff typically give it a characteristic color andtexture and usually make it a bed resistant to erosion, and thus adistinctive stratigraphic marker.

88 Evolutionary Anthropology Articles

lowing crystal growth and the forma-tion of a lava). After eruption, thequenched glass froth still moves withconsiderable force behind it, and mostis immediately shattered to form theglass shards that are a major compo-nent of most volcanic ash (Fig. 1).More substantial portions of the frothmay resist shattering and retain theirintegrity to form pumice clasts. Uponeruption, pumices are highly angularclasts, but the abrasion of subsequenttransport quickly rounds them to theirmore familiar form. In addition to thepredominantly glass or vitric productsof eruption, volcanoes may also ejectlithic and crystal tephra. Lithic tephraare composed of pulverized particlesof preexisting rocks, usually lavas.They can be important in lithologiccorrelation, but generally are not suit-able for geochemical correlation. Crys-tal tephra are composed of mixesusually dominated by feldspar, horn-blende, quartz, and pyroxene, withaccessory zircon, apatite, and otherminerals. These tephra tend to be dis-tributed in the area of the volcanicvent. The feldspars and zircons maybe sources of numerical dates andgeochemical characterization may beused in correlation. By their mineralnature, however, these phases do not

exhibit the chemical variability ofglasses and thus are less commonlydiagnostic.

Tephra comprises the class of volca-nic products explosively ejected fromthe source. The size of such materialranges from extremely coarse blocksand bombs (.64 mm diameter), tolapilli (2–64 mm) and ash (,2 mm).The blocks and bombs generally travel

only a short ballistic route from thesource and thus are deposited on ornear the volcanic edifice, whereas la-pilli and ash can be carried to a consid-erable height by the thermally driventurbulence of an eruptive cloud (Fig.2). As a result, they can drift longdistances before settling out of theatmosphere. Ash and lapilli can alsobe redistributed by fluvial or aeolian

Figure 1. Photomicrograph of glass shards showing characteristic morphology.

TABLE 1. Chemical Composition of Selected Tephra From East Africaa

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O Ba* Nb* Sr* Y* Zn* Zr*

Carbonatite AshOldoinyo Lengai,

9 Oct. 1960 (z) 0.73 0.03 0.25 1.10 0.55 0.49 25.29 24.68 0.68 8 180 157 12 600 75 354Basaltic Ash

Cindery Tuff, AfarCT-B (y) 52.23 3.45 13.56 16.03 0.21 4.41 8.92 1.81 0.97 298 270 164

unnamed ash,Lenderit K90-4612 (u, v) 45.77 2.55 15.60 12.61 0.21 5.48 10.45 3.49 1.31

Rhyolitic AshTulu Bor Tuff, type

K80-179 (x) 0.16 1.56 0.05 0.32 3.20 135 83 6 63 79 366Tulu Bor Tuff,

K81-529 (w) 76.71 0.17 12.78 1.69 0.03 0.03 0.25 4.63 3.70 143 76 6 60 76 354Sidi Hakoma Tuff,

SHT-41 (w) 77.23 0.18 12.85 1.84 0.03 0.05 0.27 5.41 2.15 151 80 10 59 82 348KBS Tuff, type 77-17

(u, x) 72.98 0.20 10.79 3.16 0.11 0.03 0.18 3.23 3.92 30 194 ,5 134 251 1291KBS Tuff, Lothagam

K91-4720 (u) 73.34 0.17 10.90 3.03 0.11 0.04 0.18 1.25 1.77 17 194 2 140 205 1282Fejej tuff, 89FJ-1 (t) 72.23 0.36 14.24 4.36 0.13 0.09 1.45 3.75 3.28 1074 70 130 89 166 820Cindery Tuff, Afar

CT-R (y) 75.62 0.21 12.32 2.99 1.00 4.09 3.98 833 79 117

aOxide Values are in wt. %, trace elements (*) in ppm.z, from Dawson and coworkers11; y composite of microprobe and INAA data from Walter and coworkers12; x from Cerling andBrown13; w from Hart and coworkers14; v from Haileab15; u from Feibel, unpublished data; t from Asfaw and coworkers.16

Articles Evolutionary Anthropology 89

reworking, in which case they behavelike most small sedimentary particles,and can be carried considerable dis-tances by traction, saltation, and sus-pension. Lapilli to block-sized pumi-ceous tephra can also be distributedover long distances by fluvial activity.Pumice, because of its vesicular (bub-bly) nature, has extremely low densityand high buoyancy, and consequentlyfloats on water. Its behavior is thusoutside the range of normal sedimen-tary particles: Very large clasts can betransported far from their source byfluvial action or may even drift acrossoceans. A consolidated pyroclastic de-posit, whether it be the result of air-fallaccumulation, pyroclastic flow, or flu-vial reworking, is referred to as a tuff.Primary tephra deposits tend to berelatively pure, with contaminationconfined to near-source material en-trained during eruption. Reworkedtuffs, however, often contain a substan-tial component of epiclastic materialthat is unrelated to the eruption andmay be incorporated during transport.

The common chemical origin of di-verse volcanic products, including la-vas, obsidian (massive volcanic glass),

pumice, and ash means that all ofthem preserve a common geochemicalfingerprint inherited from the sourcemagma. This is the basis for the geo-chemical correlation of tephra. It mayalso allow correlation of distal prod-ucts like tephra with more proximalproducts like lavas and obsidians.

CHARACTERIZATION OF TEPHRAThere are many approaches to the

characterization of tephra, some ofwhich are relatively definitive, whileothers are more general in nature.Glass shards, the most commonly in-vestigated component, exhibit charac-teristic shapes, color, and index ofrefraction.17 These properties may beof use, particularly in defining isolatederuptive products, but in a sequencerich in tephra these characteristicstend to be duplicated and lose theirdiagnostic value. The aspect most suc-cessfully employed in tephra character-ization is glass geochemistry.

Geochemical correlation of volcanicglasses depends on the fact that pre-served glass, even when hydrated, re-tains its original chemical composi-

tion for most elements. Because someelements are more mobile than others,certain elements such as sodium andpotassium are usually downplayed forcorrelation purposes.18 The conserva-tive nature of most elements, however,means that the original geochemicalsignature can be determined fromanalysis of a pure glass separate.Therein lies the trick.

Many attempts at geochemical cor-relation have run afoul of the fact thatmost vitric ashes are contaminated.The impurities may include primaryvolcanic crystals or crystallites fromthe original eruption or epiclastic ma-terial, crystals, or lithic fragmentsmixed in during transport and deposi-tion. Because of their limited chemicalmakeup, crystal or lithic contami-nants can overwhelm the chemical

signature of a glass. A single zircon(ZrSiO4) crystal can contain more zir-conium than hundreds of thousandsof glass shards.

There are analytical strategies toavoid contamination problems. Theelectron microprobe is used to analyzeindividual glass shards or grains; con-taminants are easily recognizable bytheir chemistry. In very small samples,however, the microprobe has difficul-ties in detecting trace elements pre-sent in minute quantities. To add thesechemical components to the ‘‘signa-ture’’ of an ash, large numbers ofshards must be combined to produce abigger sample that can be analyzed byX-ray fluorescence spectroscopy,which has detection levels down toparts per million. For this analyticalprocedure, a pure glass separate must be

Figure 2. Origin, transport, and deposition of tephra and related volcanic rocks. The geochem-istry of the magma at depth is preserved in eruptive products, including lavas and obsidian, aswell as tephra. Proximal products of explosive eruptions include ash flows and ballistic fallout.Distal components include aerially dispersed ash and lapilli, as well as water-borne ash, lapilli,and larger pumice clasts.

The conservative natureof most elements,however, means that theoriginal geochemicalsignature can bedetermined fromanalysis of a pure glassseparate. Therein lies thetrick.


produced. This can be done by standardseparation techniques, including the useof acid washing, heavy liquids, and amagnetic separator (see Box 1).

The separation and analysis of largepopulations of glass shards brings upanother problem, however. Some vit-ric tephra include shards that repre-sent not a single population, but sev-eral chemical entities. These arebimodal or polymodal compositions.Multiple compositions can be incorpo-rated in a tuff in several ways. Somemay result from mixed-magma erup-tions, in which two chemically distinctmagmas erupt from the same source.Others may result from coincidenteruption from several sources. It islikely, however, that most mixing re-sults from the reworking process, inwhich the products of one or moreearlier eruptions are incorporated intothe products of a later eruption duringtransport and deposition. Reworkingcan mix ash components and may alsoderive pumice clasts from earlier erup-tions.

A standard step in any analyticalprotocol is the initial use of micro-probe analysis to determine the modal-ity of the glass population. X-ray fluo-rescence analysis of polymodal glassreflects its relative mix of constituentmodes. In some cases the mixture ishomogeneous, so that the analysis canbe characteristic. Pumice clasts mustalways be characterized geochemi-cally to prove they correlate with theassociated ash in a tuffaceous deposit.Otherwise an isotopic date obtainedon feldspar phenocrysts from a pum-ice might reflect an earlier eruptiveevent rather than the direct precursorto the depositional event. It is perhapsof greater significance that once adated pumice has been geochemicallycharacterized the associated age canbe exported to sequences where cor-relative tephra have been identified,even in the absence of pumice at thatlocality.

An analytical data set, whether de-rived by electron microprobe, x-rayfluorescence spectroscopy, or othermeans, contains the chemical abun-dances obtained from the glass. Theseabundances are generally reported asweight percent or parts per million,and may be presented as elementalabundances or in terms of oxides. The

differences are simply in reportingstyle, but the numbers will vary accord-ingly. There is no standard or absoluteset of elements to be analyzed. Majorelements are generally measured withthe electron microprobe, as analyticaltotals are used as a measure of thereliability of the results. Major ele-ments, however, are of little diagnosticvalue within a group of similar tephra,and thus x-ray fluorescence spectros-copy analyses often omit major ele-ments. Minor and trace elements aregenerally the most diagnostic, but eachelement will vary in utility dependingon magma type (basaltic versusrhyolitic). Furthermore, each element

has its own analytical difficulties (seeBox 2).

The process of correlation is basedon the recognition of geochemicalsimilarity. Exact equivalence is gener-ally not attainable, even for replicateanalyses of the same material. Thus,similarity and the criteria for correla-tion become a matter of interpreta-tion. There are several problems here.First, although in a geological sense atephra is an isochronous marker repre-senting an eruptive event, that event isnot truly instantaneous; it may, in fact,involve a prolonged eruptive cyclerather than a single eruption. As theduration of an eruptive cycle increases,

so do the odds that the magma chemis-try will evolve or that different parts ofthe magma chamber will be sampled.Many tephra display chemical trends,which mark variation within an erup-tive event. With only a small samplingof such variable tephra, it is some-times difficult to determine whethertwo analyses reflect points on a con-tinuum or distinct entities. This prob-lem can usually be resolved by compar-ing the data from multiple elementsand developing a large comparativedata set. Generally, it is only after alarge data set has been compiled andthe ranges of variation of tephra com-position are understood that the allow-able limits of similarity can be estab-lished.

A less serious problem is the mul-tiple occurrence of characteristicchemical signatures. Just as distantlyrelated organisms may converge mor-phologically to appear quite similar,distinctive chemical compositions maybe repeated over time. And just asbiological analogies can generally berecognized when viewed in a broadercontext, repeat chemistries are typi-cally quite obvious when viewed incontext. Geological context providesthe basis for recognizing miscorrela-tion based on chemical similarity and,therefore, the importance of establish-ing tephrosequences and combiningtephra data with supporting litho-,chrono-, and biostratigraphic data. Atephrosequence is a composite succes-sion of geochemically defined tephra.The sequence is homotaxial, whichmeans that is arranged serially. Oncethis serial relationship is established itbecomes a check on the validity ofcorrelations proposed from individualoccurrences. Thus, once the tephrose-quence A - B - C - D is established, anyproposed correlation placing tephra Dbetween A and B implies either anerror in the documentation of fieldrelationships in the second occur-rence or a problem with the proposedcorrelation. This is the standard trial-and-error method that constantly testsand refines the accepted componentsof a tephrosequence. The most diffi-cult problems are encountered in de-veloping or extending a tephrose-quence when data to test relationshipsare scarce.

Just as distantly relatedorganisms may convergemorphologically toappear quite similar,distinctive chemicalcompositions may berepeated over time. Andjust as biologicalanalogies can generallybe recognized whenviewed in a broadercontext, repeatchemistries are typicallyquite obvious whenviewed in context.


Box 1. Analytical Pathways for Correlation of Vitric Tephra

There are as many different ap-proaches to sampling, preparation, andanalysis as there are researchersworking on tephra. With respect to theultimate correlation of tephra from dif-ferent localities and analyses under-taken by different researchers, twoparts of the process are critical: re-moval of contaminants and accuratedetermination of chemical composi-tion. Two decades ago, both pre-sented significant problems. Today,many laboratories are producing con-sistent results with fairly standard pro-cedures and a diverse array of analyti-cal tools. A few of the typical stagesare summarized here.

Sampling is perhaps the most con-sistent step. Tephra workers generallyprefer coarser material, such as vitricash of fine to medium sand grade, butoften they must work with whatever anoutcrop has to offer. Once in the labo-ratory, initial disaggregation of the sam-ple may require crushing it with a

mortar and pestle or, to begin with, thesample may be uncemented. Washingin water allows part of the clay contami-nation to be decanted off prior tofurther treatment. A second washing inacid (HNO3) removes carbonate, whichoften occurs as a cement or over-growth. Washing with a stronger acid(HF), usually in an ultrasonic bath,helps to dislodge additional clays. Thisstep may need to be repeated severaltimes to clean the shards of all adher-ing clay.

The flow chart (see Figure) showstwo typical paths of separation andanalysis. Microscopic examination (up-per left) of the dried separate revealsthe proportions of glass shards andcontaminating mineral grains. As longas enough glass is present, thiscleaned separate can be mounted andexamined with the electron micro-probe to determine the chemistry ofindividual glass shards and mineralgrains. Further washing, coupled with

magnetic and/or heavy-liquid tech-niques, are required to obtain a pureglass separate. Magnetic separation,usually performed with the Frantzy

Isodynamic Separator, takes advan-tage of slight differences in the mag-netic properties of glass shards andmineral grains to split them apart.Heavy-liquid separations use densitydifferences between particles to sinksome and float others. This was longconsidered hazardous duty, as manyof the ideal liquids, such as tetrabro-methane, are potent carcinogens.However, recent additions to the mar-ket, such as sodium polytungstate,now offer safer alternatives. The result-ing pure glass separate can bepressed into a pellet for x-ray fluores-cence analysis of the glass shardpopulation. Ultimately, all data endsup in a computer for the final identifica-tion of characteristic glass composi-tions and, it is hoped, for correlation.


THE TURKANA BASIN: ANABUNDANCE OF VITRIC TEPHRA

By far the best example of the use oftephrostratigraphy in paleoanthropol-ogy involves the Plio-Pleistocene de-posits of the Turkana Basin in Kenyaand Ethiopia. These strata preserve atephrosequence of more than 135 indi-vidual tephra. Primarily through the

prodigious efforts of F. H. Brown ofthe University of Utah, some 120 ofthese have been geochemically charac-terized. The stratigraphic studies un-dertaken by geologists throughout thebasin over the past three decades haveestablished the placement of most ofthese geochemically defined tephrawithin local sections. The compilation

of serial relationships established inthese local sections has allowed theconstruction of a master tephrose-quence for the basin. The existence ofnumerous widespread tephra has al-lowed good control on the relation-ships of intervening tephra with lim-ited areal exposure, but the exact serialrelationships of all these local tephra

Box 2. Geochemical Similarity and Correlation

The millions of glass shards thatmake up a tuff may reflect a variety ofpopulation structures in their geochem-istry (see Figure). Some display littlevariability for most elements, and clus-ter tightly on a bivariate plot, as in theTulu Bor alpha shown here. Thus thereis a small range of variation in eachelement (or at least most elements),and a small amount of associatedanalytical error in the determination,and the tephra can be defined asshowing a range of values from x to yfor any element. These tephra arereferred to as unimodal compositions.In other tephra, two or more relativelyinvariant populations can be recog-nized, as in the Lokochot Tuff. Eachdisplays the generally tight clusteringseen in a unimodal population. Whenthey are found together in the sametuff, they define a bimodal or polymo-dal composition. In some cases, thesedifferent modes characteristically oc-cur in association with one another.The Lokochot Tuff always has thesame modes, and they recur in consis-tent proportions so that the overallcomposition is characteristic of thetuff. More commonly, though, severalmodes may be mixed in varying pro-portions, so that an analysis of theentire population is a function of therelative mix, and is not characteristic.This most commonly results from mix-ing of tephras during transportationand deposition, and is one of the mainreasons that both x-ray fluorescenceand electron-microprobe analyses areundertaken.

Deciding when two similar chemis-tries actually represent the sametephra is a matter of experience, but alarge comparative data set helps tre-mendously. Tephra correlation is aclassic exercise in the scientific

method. Once a worker decides thattwo samples represent the same en-tity, a correlation is proposed, and itcan be tested. The simplest test is oneof stratigraphic consistency. Do thetwo tuffs share stratigraphic relation-ships with adjacent tephra? Chemicalsimilarity can be tested both within thepool of prospective correlates andagainst other, noncorrelative tephrathat share some chemical characteris-tics. How tight is the chemical similar-ity? As stated earlier, no correlationcan be proven. But when adequatetesting has not refuted the correlation,it can be accepted as a solid workinghypothesis. Particularly in the case ofcontentious correlations that may af-fect the age of hominid specimens,considerable ‘‘proof’’ may be de-manded. In the case of the Tulu Bor-

Sidi Hakoma correlation, for example,the geochemical similarities that wereobvious to geologists were not nearlyas apparent to paleoanthropologists,and many trees were killed in theensuing struggle. In the end, addi-tional analyses by different methodsonly strengthened the original casethat these are indeed products of thesame eruption. Good statistical testsare available to support correlationbased on large numerical databases.72

The most important support for anycorrelation, however, comes from alarge enough collection of data charac-terizing the eruption, so that the ac-ceptable range of variation is wellunderstood. This, combined with con-sistency of stratigraphic relationships,generally determines the validity of acorrelation.


are not always demonstrable (see Box3).

Stratigraphic studies in the 1960sand 1970s employed the predomi-nantly fluvially reworked tuffs of thebasin as lithostratigraphic markers.19-22

Correlations were based on outcroptracing, lithologic features, and associ-ated stratigraphic sequence. This ap-proach yielded mixed results. In therelatively continuous and laterally ho-mogeneous strata of the lower OmoValley, the lithologic approach wasgenerally successful, but in the KoobiFora region it proved disastrous. Thestratigraphic complexity and discon-tinuous exposures in this region led tocritical miscorrelations, with resultantmismatches in compiling the regionalstratigraphic sequence. Amid the fall-out of this confusion was the wellknown ‘‘KBS controversy.’’23

In 1979, geochemical correlation ofthree tephra between the Koobi Foraand Shungura Formations establishedthe possibility of a tephrostratigraphicframework on a basinal scale.24 Inten-sive study of the Koobi Fora tephraproduced a major revision in correla-tions for that region25 and a broad-based characterization of the majortephra there.13 The tephra sequence ofthe lower Omo Valley had been usedas a basis for lithostratigraphic subdi-vision in the 1960s and early 1970s,but research there was halted longbefore the establishment of geochemi-cal correlation as a tool in East Africa.Samples in the collections of F. H.Brown became the basis for early cor-relations,24 which were followed bymore thorough characterization.26,27

The ramifications of this early workquickly spread to a much larger scale.Brown28 recognized the fingerprint ofthe Tulu Bor Tuff in Hadar’s SidiHakoma Tuff (as characterized by R.C. Walter29). Despite protracted argu-ment,30–35 this correlation withstoodscrutiny and established a basis forlong-distance correlation using geo-chemical signatures. Quick on theheels of this discovery, Sarna-Wojcickiand coworkers36 broadened the scopefurther with correlations to tephra indeep-sea cores from the Gulf of Aden.This advance not only provided a strati-graphic link between the terrestrialand marine records, but allowed theimportation of isotopic and biostrati-

graphic records from the oceans intothe context of terrestrial evolution.

Field work in 1985–1986 by Brownand Haileab amassed a substantialcollection of the Omo Valley tephrasdocumented stratigraphically by deHeinzelin.37 Analysis of this materialproduced three significant contribu-tions.15 Although the relatively undis-turbed, well-exposed and homoge-neous exposures of strata in the lowerOmo Valley provided an ideal settingfor lithostratigraphic correlation, mi-nor adjustments were necessary fol-lowing geochemical investigation ofthe tephra. This underlines the prob-lems inherent in lithostratigraphic cor-relation when the field character of atuff reflects depositional setting moreit does than any other factor. With thecharacterization of numerous addi-tional tephra from the Omo sequence,

further correlations to other TurkanaBasin localities were also established.And with a large data base for compari-son, Haileab15 was able to documentmajor tephra groupings, reflecting theevolutionary history of the volcanicsource terrane.

The scope of long-distance correla-tions continued to broaden, reachingthe Western Rift,38 the Baringo Ba-sin,39,40 additional sites in the MainEthiopian Rift,41–43 and deep-sea coresfrom the western Indian Ocean.44 Atthe same time, work within the Tur-kana Basin focused on the applicationof tephra to problems of chronologyand fine-scale stratigraphic resolu-tion. McDougall3 established a robustand detailed geochronology for theKoobi Fora region, relying primarilyon feldspar crystals extracted from

pumice clasts. This chronologic frame-work could be applied to newly re-corded localities such as West Tur-kana,45,46 along with well-known sitessuch as Lothagam.47

The Nariokotome hominid wasdated by a combination of isotopicdating and geochemical correlation.48

McDougall and coworkers49 datedpumices collected from Koobi Forathat could be geochemically related totephra at the hominid site,50 where nopumices existed. An important resultof this study was the clear demonstra-tion that datable pumice clasts can beincorporated in tephra layers havingglass chemistry, based on the ash com-ponent, that differs from that of thepumice composition. In this situation,the chemical fingerprint indicates thatash and pumices did not originate inthe same eruption. The simplest expla-nation is that the numerically domi-nant component of the tuff (the mil-lions of ash shards) represents theeruptive event, whereas the minoritycomponent (dozens of pumice clasts)resulted from an earlier eruption. Inthis case, the pumices represented anearlier eruption that is recorded by anash stratigraphically lower in the sec-tion with which the pumices could bechemically matched.51,52 Although thetemporal difference between the twocomponents was insignificant in thiscase, their demonstration serves as acaution with respect to the applicationof isotopic dates on feldspars frompumice clasts. The glass geochemistryof pumice clasts from the TurkanaBasin is routinely investigated beforeage determinations are undertaken inorder to establish the relationship ofthe clasts to the encasing tephra.

By the late 1980s, advances in thegeochronology and tephrostrati-graphic framework of the Turkana Ba-sin had resulted in a wholesale over-haul of the geologic framework for theregion. Integration of tephrostrati-graphic, isotopic, and paleomagneticdata allowed a reevaluation of thetemporal control on the hominid re-cord.5 The implications of tephra cor-relation for the paleogeographic evolu-tion of the basin led to a revised modelfor basin evolution.53 Even features ofsedimentation and habitat charactercould be tied to patterns of tephraeruption and distribution.54 A tabula-tion of eruptive events recorded in the

In the relativelycontinuous and laterallyhomogeneous strata ofthe lower Omo Valley,the lithologic approachwas generallysuccessful, but in theKoobi Fora region itproved disastrous.


Box 3. Tephrosequences Large and Small

Using the information of sequenceestablished from outcrop studies andthe characterization of tephra basedon geochemistry, it is possible to estab-lish the relationships among tephraover long temporal spans and tremen-dous geographic ranges. This is ex-actly analogous to the development ofthe geological time scale based onbiotic evolution. Within a small area,the details of sequence and characterallow the establishment of a tephro-stratigraphic framework in which a fewmajor widespread tephra control therelationships of intervening minortephra, although the exact relation-ships among these minor tephra arenot always demonstrable. The sameprinciple holds for large-scale applica-tion and regional correlations, where afew major tephra, usually representingvoluminous explosive eruptions, estab-lish a framework, but minor or local

tephra between them cannot alwaysbe precisely related.

Aportion of the large-scale tephrose-quence linking the Turkana Basin,Awash Basin, and the Gulf of Aden(Fig. A) (after Brown70). Note that evensome major tephra are not present (orare not yet recognized) in all threeareas.

A generalized tephrosequence fromthe Turkana Basin (Fig. B).5 This corre-lation diagram shows only a few of themore than 135 tephra documentedfrom the basin.

A local tephrosequence from theexposures around Nariokotome (Fig.C).71 All of the 20 or so tephra relatedhere fall between the Morutot (5J-4)and Koobi Fora Tuff (5K-1).

Tephra Names

For the most part, tephra samplescome back from the field with unas-

suming sample numbers, such as K91-4720 or 89FJ-1. Although field work-ers often have a good guess as to theidentity of a tuff when it is sampled, it isonly after laboratory analysis that anidentity is demonstrated. Once atephra has been isotopically dated, isinvolved in a significant correlation, orbecomes an important local marker, itis common practice to apply a name tothe unit. In order to facilitate communi-cation and understanding, names aregenerally chosen from local geo-graphic features or from a local lan-guage, but many different schemeshave been employed. When twonamed tephra are correlated, it is com-mon to indicate the equivalence, as insome of the examples from the Tur-kana Basin. For example, Black Pum-ice Tuff (5J-7). This makes clear thestratigraphic relationships in the termi-nology of related local sequences.


Fig. A Fig. B

Fig. C

strata of the basin reveals a significantcyclical evolution of the volcanic prov-ince in the Ethiopian Highlands (Fig3). Peaks of eruptive activity burieddepositional systems and caused radi-cal reorganization of rivers and associ-ated habitats.55

While the bulk of the vitric tephrawithin the Turkana Basin Plio-Pleis-tocene sequence are derived from theEthiopian Highlands, there are othersources. The Kanapoi Tuff, a vitrictephra in the lower Kerio Valley se-quence, is most likely derived from asource near the Baringo Basin.56 Nam-wamba39 has suggested that the Ka-napoi Tuff may correlate with an ashfrom North Kipcherere in the BaringoBasin exposures. The extensive occur-rence of this ash at Kanapoi and no-where else within the Turkana Basinsequence strongly suggests that this isa tephra derived from the headwaterregion of the Kerio River. Local basal-tic volcanism in the Turkana Basin

produced ashes that typically are acrystal-lithic-vitric mixture. Theseashes are associated with the evolu-tion of the Mt. Kulal basaltic shieldvolcano in the southeast corner of thebasin, as well as with the later develop-ment of the volcanic centers of theBarrier along with South, Central, andNorth Islands. Other crystal-lithicashes do occur in the Turkana Basinsequence, notably in deposits of theLonyumun Lake (ca. 4.1 Ma), but bet-ter examples of these tephra are re-ported from the Hadar region of theMiddle Awash Valley in Ethiopia.

THE MIDDLE AWASH: PROXIMALAND DISTAL TEPHRA

In contrast to the rich and well-controlled tephrosequence of the Tur-kana Basin, the tephra story of theMiddle Awash Valley has been slow tounfold. This reflects, in part, the geo-graphic focus of investigations, with

relatively small targets like the Hadarregion, as well as the more local char-acter of many of the tephra layers.Although vitric tephra do occurthroughout the region,57–59 they havenot been used extensively in correla-tion. Most interest in the tephras ofthis region has focused on their utilityfor isotopic dating.60,61 Many of thevitric tephra of the Middle Awash dis-play complex polymodal composi-tions, which, in part, reflect magmamixing.12 Even localities with goodlocal tephrosequences, like the Arti-fact Site Tuffs of the Gona29,61–63 haveproven difficult to correlate on morethan a local scale. The crystal tephra

of the Middle Awash Valley haveproven an important source of rawmaterial for isotopic dating, but thepotential for a detailed tephrostrati-graphic framework has yet to be tested.

BEYOND CORRELATION ANDDATING: TEPHRA ARCHIVES

No discussion of tephra would becomplete without mention of the spe-cial significance of volcanic ash inrecording short-term events in the past.Geologists refer to tephra as isochro-nous markers because, from a geologi-cal perspective, the eruption, trans-

Figure 3. History of silicic volcanism from the Ethiopian Highlands based on tephra deposits ofthe Turkana Basin. The number of individual tephra is plotted in 250-Ka increments. Three cyclesof increasing eruptive activity, each culminating in a major blow-out and a drop in activity, arerecorded. The greatest frequency of explosive eruptions occurs around 1.6 Ma, and isrecorded in the Okote Tuff Complex at Koobi Fora.

By the late 1980s,advances in thegeochronology andtephrostratigraphicframework of theTurkana Basin hadresulted in a wholesaleoverhaul of the geologicframework for the region.Integration oftephrostratigraphic,isotopic, andpaleomagnetic dataallowed a reevaluationof the temporal controlon the hominid record.


port, and deposition of a tephra isinstantaneous. In real time, however,the geologist’s concept of ‘‘instanta-neous’’ may range from a few minutesto a matter of years. But, by anymeasure, the short-term nature ofthese events, with a rapid accumula-tion of often voluminous tephra depos-its, produces some unique records.

By far the most spectacular of theserecords for paleoanthropology is thehominid trackway at Laetoli (Fig. 4).The tracks of three hominids form twotrails across a paleolandscape associ-ated with a diverse record of theirPliocene community.7,8 For nearly twodecades, these prints stood as the old-est evidence of upright bipedalism inhominids. The individual ashfall sur-faces of the Footprint Tuff preserve atrace fossil sample of a communitycollected over a matter of days, devoidof the effects of time-averaging socommon in the fossil record. In thecase of the Laetoli footprints, thechemistry of the volcanic ash is againof special significance. The source vol-cano, Sadiman, erupted carbonatiteash that partially dissolved and recrys-tallized on wetting, then set as a natu-ral variant of concrete. The Laetolifootprints were thus preserved in ahighly durable form. This contrastssharply with similar footprints discov-ered at Koobi Fora,9 where a hominid

trackway was preserved in a rhyoliticash because the ash was wet at thetime of formation. This tuff, however, isvery poorly lithified to this day, as therhyolitic glass has remained unaltered.

The use of tephra as markers acrosslandscapes can provide a wealth ofdetail about ancient geographies. Thelarge-scale patterns of landscape evolu-tion in the Turkana Basin were primar-ily reconstructed using tephra mark-ers to map out ancient channels andfloodplains.53,64 The characteristics ofindividual outcrops can sometimesprovide further details such as thewidth and depth of paleochannels; theyalso can document the magnitude ofshort-term accumulation rates (Fig.5). Although East Africa has yet toproduce clear evidence of catastrophicdeaths related to tephra accumula-tion, the blanketing of ancient land-scapes with tuffs several meters inthickness would surely have had eco-logical impacts. The duration and mag-nitude of these ecological effects iscurrently under investigation.

PROBLEMS, PITFALLS,AND PROMISES

To appreciate fully the utility oftephra, it must be understood thatthey provide two fundamentally differ-ent types of data. One is numerical age

control; the other is correlation. Rela-tively few tephra provide materialsthat are adequate for isotopic ageanalysis, while most are amenable togeochemical characterization and po-tential use in correlation. But evenwithout a measured numerical age,each identifiable tephra provides anisochron. And these isochrons, whenviewed in sequence and in associationwith isotopic ages, magnetic polaritytransitions, and the fossil record, estab-lish a framework in which to viewevolutionary and cultural records in atightly controlled context.

Although the contributions of teph-rostratigraphy to paleoanthropologi-cal investigations are significant, thereare problems in both method and ap-plication. One limitation on the corre-

lation of tephras based on glass geo-chemistry is the alteration of that glass.Volcanic glass is highly reactive.65–67

Moreover, particularly in the alkalinediagenetic environments of many EastAfrican locales, primary glass is rap-idly reduced to secondary clay miner-als. By this process, a tuff is trans-formed into a rock called bentonite.Sites such as Olduvai Gorge, with itsrich tephra record, have been beyondthe capabilities of geochemical charac-terization for this reason. There aresome techniques that may bring eventhese localities within the range oftephra correlation. Primary mineralphases such as feldspars may engulftiny droplets of magma during crystalgrowth. These melt inclusions pre-serve the same geochemical signature

Figure 4. Hominid tracks at Laetoli (1995). These prints of three individuals were formed in acarbonatite ash from the volcano Sadiman. There is no evidence that the ash was wet at thetime the hominids passed, but a gentle rainfall shortly afterward caused the recrystallization ofthe ash and preservation of the prints.

Although East Africa hasyet to produce clearevidence of catastrophicdeaths related to tephraaccumulation, theblanketing of ancientlandscapes with tuffsseveral meters inthickness would surelyhave had ecologicalimpacts.


as a glass phase, and are currently thefocus of interest in altered tephrasfrom East Africa,15 as well as somebentonites of great antiquity else-where.68 This technique holds greatpromise for expanding the applicationof geochemical characterization of al-tered tephra.

Miscorrelation is perhaps the mostserious pitfall of tephra studies. Geo-chemical signatures can be mislead-ing, and apparent similarities do notalways reflect valid correlations. Thisis particularly the case when data ob-tained by different analytical tech-niques are compared or when workersrely on a small comparative data base.Because correlations may carry withthem associated isotopic age control,the incentive to accept a correlationon weak evidence may be strong. Typi-cally, however, such miscorrelationsare readily apparent on close inspec-tion of the data.

In spite of the inherent difficulties ofanalysis and interpretation, tephro-stratigraphy remains a powerful toolto control and expand the geologiccontext of paleoanthropological stud-ies. Besides their day-to-day utility inlocal and regional work, tephra stud-ies permit unequaled precision in long-distance correlation. The continuedaccumulation of geochemical data on

tephra from paleoanthropological sitescan only draw tighter the skein ofreliable correlations and precise tem-poral control.

ACKNOWLEDGMENTS

This paper grew over years of discus-sions with geologists and paleoanthro-

pologists interested in problems re-lated to tephra. Most of the practicaldetails of tephra work accumulatedover a long and productive apprentice-ship with Frank Brown, who has myheartfelt thanks. Harry Merrick pro-vided boundless support and encour-agement, and engendered the initialformulation of this paper in lecturesfor his Koobi Fora Field School. Thecuriosity and questions of my studentshave kept me on my toes and helped tofocus this presentation. Much of thebackground to the paper developed inthe course of research sponsored bythe National Science Foundation andthe Leakey Foundation. I thank themfor their generosity.

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● Paleoanthropology of South AsiaKenneth A. R. Kennedy

● The Changing Face of the Genus HomoBernard Wood and Mark Collard

● Pleistocene Human Colonization of Siberia and Peopling of the Americas: An Ecological ApproachTed Goebel

● Recent Work on the Paleolithic of Central AsiaRichard S. Davis and Vadim A. Ranov

● Hunting Down the Hunter-GatherersRobert Foley

● The Thoughtful Ape: Politics, Language, Intelligence and CognitionMichele L. Goldsmith

● A New Look at Old Teeth: Dental variation in the earliest AmericansJoseph F. Powell

● Postcranial Remains and the Origin of Modern HumansOsbjorn M. Pearson


Documents

Tephrostratigraphy and geological context in paleoanthropology