Compositional and textural correlations between Olduvai Gorge Bed I tephra and volcanic sources in...

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Quaternary International 178 (2008) 306–319

Compositional and textural correlations betweenOlduvai Gorge Bed I tephra and volcanic sources in the

Ngorongoro Volcanic Highlands, Tanzania

Lindsay J. McHenrya,�, Godwin F. Mollelb, Carl C. Swisher IIIb

aDepartment of Geosciences, University of Wisconsin-Milwaukee, 3209 N. Maryland Avenue, Milwaukee, WI 53211, USAbDepartment of Geological Sciences, Rutgers University, Wright Labs, 610 Taylor Road, Piscataway, NJ 08854, USA

Available online 30 January 2007

Abstract

Tephra layers from Bed I of Olduvai Gorge, Tanzania, are correlated directly to their source volcanoes in the nearby Ngorongoro

Volcanic Highlands (NVH) based on composition, texture, and age. Tephra in lower Bed I are more silicic than the upper Bed I tephra,

and most contain quartz. The Naabi Ignimbrite from lower Bed I is compositionally similar to an ignimbrite exposed close to

Ngorongoro Crater. None of the sampled tephra from the Ngorongoro Crater wall is an exact match for the Naabi Ignimbrite or other

lower Bed I tephra, although overall composition and mineral assemblages suggest a common source. The uppermost Bed I tephra, Tuff

IF, contains at its base a pyroclastic surge component rich in trachytic lava fragments that match (in texture, mineral assemblage, and

mineral composition) with trachytic lavas exposed in the crater wall of the NVH Olmoti volcano. Tuff IF is compositionally similar to

other upper Bed I tephra (especially Tuffs IB, IC, and IE), so Olmoti is the most likely source for most upper Bed I marker tephra.

Geochemical correlations indicate that widespread ignimbrites and airfall tephra erupted from Ngorongoro during Olduvai lower Bed

I time, reaching at least 40 km from source. Upper Bed I records a switch in NVH volcanic activity from Ngorongoro to Olmoti,

coinciding with the first appearance of stone artifacts in the Olduvai record. Olmoti continued to supply pyroclastic deposits to the

Olduvai Basin until the end of Bed I.

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1. Introduction

Tephra erupted from the Ngorongoro Volcanic High-lands (NVH), Tanzania, are interbedded with fluvial,wetland, and lacustrine hominin-bearing deposits of Old-uvai Gorge. These tephra beds provide a means fordeveloping a stratigraphic and temporal framework forthe Olduvai Gorge deposits. In turn, these deposits allowreconstruction of the development and evolution of land-scapes during the time Homo habilis and Australopithecus

boisei resided in the Olduvai Basin. In addition, thesetephra provide a record of explosive volcanic activity forthe nearby NVH, a record that would be difficult toreconstruct using only the eroded and vegetated volcanoes

remaining today. Further, correlating these tephra directlyto their volcanic sources makes it possible to reconstructthe eruptive history of the NVH, and provides insight intothe magmatic evolution of NVH volcanic centers.Olduvai Gorge and the hominin-bearing Olduvai Beds

are located on the shoulder of the East African Rift System(EARS), only 20 km from the NVH (Fig. 1). Bed I, theoldest and thickest of the Olduvai Beds, contains a welldeveloped volcanic record, including lavas, airfall tephra,ignimbrites, and pyroclastic flow and surge deposits (Hay,1976). Proximal deposits (ignimbrites and surges) withinBed I require a nearby volcanic source, most likely one ofthe three younger NVH volcanoes directly adjacent to themodern Olduvai Basin (Lemagurut, Ngorongoro, orOlmoti, Fig. 1). However, rift-related faulting and sub-sequent deposition of younger material has buried mostproximal deposits, making direct physical mapping be-tween Olduvai and the NVH impossible. In addition, thelower part of Olduvai Bed I is only exposed in the

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1040-6182/$ - see front matter r 2007 Elsevier Ltd and INQUA. All rights reserved.

doi:10.1016/j.quaint.2007.01.004

�Corresponding author. Tel.: +1414 229 3951; fax: +1 414 229 5452.

E-mail addresses: [email protected] (L.J. McHenry),

[email protected] (G.F. Mollel), [email protected]

(C.C. Swisher III).


westernmost part of the gorge, farthest from any of thelikely sources (30–40 km). Compositional differences be-tween the lower Bed I rhyolites and upper Bed I trachytes(Fig. 2) suggest either multiple volcanic sources or a majorshift in magmatic evolution at a single source. All of thesefactors make it difficult to determine the specific volcanicsources for tephra in Olduvai Bed I.

Determining the eruptive history of the NVH is crucialfor reconstructing the landscape and environments inwhich hominins lived during Olduvai Bed I times(2.03–1.79Ma, Walter et al., 1992; Hay and Kyser, 2001).Detailed geochronological and geochemical histories of theNVH volcanoes are in progress by one of us (G.M.) andare beyond the scope of this paper. However, it is clear thatthe elevation of the major volcanic centers would haveaffected temperature and precipitation (as rain shadoweffects), the locations of riparian corridors accessible tohominins, drainage patterns, and available material for themanufacture of stone tools. Hay (1976) suggested that attheir peaks of activity, volcanoes in the NVH (particularlyNgorongoro itself) could have towered over 3 km abovetheir current elevations, providing a landscape drasticallydifferent from that seen today. Identifying which of these

volcanic centers were active during the deposition of theOlduvai sequence will help to constrain past landscapeevolution, and provide information about the likelyvolcanic hazards faced by hominins living nearby.The objectives of this paper are to (1) compare textures

and compositions of Olduvai Bed I tephra to (a)ignimbrites closer to the NVH and (b) ignimbrites andlavas exposed in the crater walls of the most likely sourcevolcanoes, and (2) use new and existing geochronology todetermine which of the NVH volcanoes were active duringthe deposition of Olduvai Bed I. In particular, lavafragments contained within Olduvai Tuff IF will becompared to lavas exposed in the NVH.

2. Regional setting and background

2.1. The Olduvai strata

Olduvai Gorge exposes a 20-km long section of Plioceneto Holocene strata up to 100m thick that contains homininfossils, stone artifacts, and paleoecological indicatorscritical for the study of hominin evolution (Hay, 1976).Tephra layers interspersed in these fossiliferous deposits

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Fig. 1. Map of the Olduvai region, including the Ngorongoro Volcanic Highlands (NVH). Olmoti, Ngorongoro, and Lemagurut are the three younger

volcanoes closest to Olduvai Gorge and the most likely source candidates for the Olduvai Bed I pyroclastic deposits. Sample sites and localities mentioned

in the text at Olduvai, along the First Fault, and at Olmoti and Ngorongoro are marked. Map modified after Ashley and Hay (2002).

L.J. McHenry et al. / Quaternary International 178 (2008) 306–319 307


were used to establish a local stratigraphic and temporalframework (Hay, 1963, 1967, 1971, 1976, 1990; Walter et al.,1992; McHenry, 2004, 2005). Pyroclastic deposits within thesequence include airfall tephra, ignimbrites, and pyroclasticsurges and flows likely produced by the nearby NVH to theeast and south of the Olduvai Basin (Fig. 1).

The Olduvai stratigraphic record is divided into a seriesof beds, named Beds I–IV, Masek, Ndutu, and Naisiusiufrom oldest to youngest (Fig. 2) (Hay, 1976). The focus ofthis paper is on Olduvai Bed I, which has an age range of2.03–1.79Ma (Walter et al., 1992; Hay and Kyser, 2001).The stratigraphy of Bed I tephra has been used to establishtime horizons between widely separated sites within theOlduvai Basin (Hay, 1976). A revised stratigraphic frame-work based on tephra composition (primarily major andminor elements in phenocrysts) helps link sites in theeastern and western parts of the gorge, between whichphysical correlation is difficult or impossible (Blu-menschine et al., 2003; McHenry, 2004, 2005).

The lower part of Bed I is exposed mainly in the westernpart of the Olduvai Basin and contains predominantlyquartz-bearing rhyolitic/trachytic tephra layers (Fig. 2).The green, rhyolitic Naabi Ignimbrite directly underlieslower Bed I as per Hay (1976) but is here treated as thelowermost tephra layer within Bed I based on its

compositional similarity to the overlying coarse feldsparcrystal tuff (CFCT) and Tuff IA. It is overlain by theCFCT and Tuff IA. The basaltic lavas of Bed I mark thetransition between lower and upper Bed I in the easternpart of the gorge (Hay, 1963, 1976), whereas Tuff IA marksthis transition in the west (Hay, 1976). The terms lower andupper Bed I have different meanings in the archaeologicalliterature (e.g., Leakey, 1971). Archaeologically, theyrepresent the lower and upper parts of the Upper Memberonly (above the basaltic lavas). Here, we use Hay’s (1963,1976) geological nomenclature. Upper Bed I contains sixmajor tephra of trachytic or trachyandesitic composition:Tuffs IB through IE, the Ng’eju Tuff, and Tuff IF(McHenry, 2005). In Olduvai Bed II (directly above TuffIF) tephra composition changes to more silica-under-saturated compositions, such as nephelinite and foidite(Fig. 2), reflecting, most likely, a transition to a differentvolcanic source.The Olduvai strata are offset along northeast–southwest

trending normal faults associated with the EARS (Hay,1976; Dawson, 1992; Foster et al., 1997). The First Fault(Fig. 1) has the greatest offset and surface relief andexposes a series of coarse ignimbrites and pyroclastic flowdeposits at the easternmost margin of the Olduvai deposits.The current depocenter (the Olbalbal Depression) lies

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Fig. 2. Stratigraphic divisions (Beds) within the Olduvai Basin (modified from Hay, 1976) and tephra compositions for Bed I (McHenry, 2004, 2005).

GPTS after Berggren et al. (1995). Specific ages for the Bed I tuffs are based on dates from Hay (1976), Walter et al. (1992), Hay and Kyser (2001), and

Blumenschine et al. (2003). Question marks indicate tephra without well-constrained dates. Olduvai magnetics of Tamrat et al. (1995). Dashed lines

indicate either where insufficient age control is available (for Bed transitions) or important tephra that do not mark Bed transitions (Tuffs IB and IIA). A

transition from rhyolitic and basaltic to trachytic tephra compositions occurs at the boundary between lower and upper Bed I.

L.J. McHenry et al. / Quaternary International 178 (2008) 306–319308


between the First Fault and the NVH, covering anypotential nearer-source deposits to the east (Hay, 1976).

2.2. The Ngorongoro Volcanic Highlands

Seven potential source volcanoes are found within 60 kmof Olduvai in the nearby NVH to the east and south. Thesevolcanoes are located in a bifurcation of the eastern branchof the EARS (Baker et al., 1972). These volcanoes haveerupted materials ranging from silica undersaturatedcarbonatites, nephelinites, phonolites, trachytes, and ba-salts to quartz-bearing rhyolites (Hay, 1976; Mollel, 2002).The volcanoes of the NVH range in age from at least4.5Ma to the present (Gromme et al., 1970; Bagdasaryanet al., 1973; Drake and Curtis, 1987).

The NVH include, from southwest to northeast (and ingeneral descending age), Lemagurut and Satiman, Ngor-ongoro, Olmoti, Embagai, Kerimasi, and Oldoinyo Lengai.Ngorongoro and Olmoti (to the east) are likely candidatesfor Bed I tephra sources not only because of theirproximity to Olduvai (�30 km to the east), but alsobecause of age as measured by the K-Ar and 40Ar/39Arage systems (Hay, 1976; Manega, 1993). Lemagurut (20 kmto the south) is another potential source because of itsproximity, though its age has not been determinedsatisfactorily (Bagdasaryan et al., 1973; Drake and Curtis,1987; Manega, 1993). Embagai and Kerimasi are likelycandidate sources for silica-undersaturated tephra from theyounger Olduvai beds (Hay, 1976), and Oldoinyo Lengai isstill active today (Dawson et al., 1994). All of the volcanoeslie close enough to Olduvai that they could have suppliedthe proximal surge and ignimbrite deposits in addition toairfall tephra.

2.3. Correlating between Olduvai and the NVH

Upper Bed I tephra (Tuffs IB and younger) generallycoarsen toward the east, implying a source at either Olmotior Ngorongoro rather than Lemagurut. Tuff IF is presentas a 10m thick ignimbrite at Locality 200 (Fig. 1). Thickaccumulations of pyroclastic surge deposits of Tuff IF arealso found in a channel deposit adjacent to Lemagurut(Locality 99, Fig. 1), some 32 km southwest of Locality200. This complicates the issue by suggesting the possibilityof a nearby Lemagurut source.

The best hope for correlating the Bed I pyroclasticdeposits to specific sources in the NVH is to compare thecompositions of the Naabi Ignimbrite (lower Bed I) andTuff IF (upper Bed I) to ignimbrites and lava flowsproximal to potential volcanic sources. The Naabi Ignim-brite is welded and may correlate directly with ignimbritespreserved closer to source or at the source volcanoesthemselves. Tuff IF contains abundant lava fragments in itspyroclastic surge component, which might correlatedirectly with lavas exposed within the crater wall of itssource. If these two tephra can be correlated directly totheir sources, then tephra of similar composition and

stratigraphic position can also be attributed to thesesources. Dating lavas exposed on these volcanoes canstrengthen these correlations, confirming the timing of theireruptions in relation to Bed I chronology.Most of the glass shards of Olduvai tephra layers are

devitrified by post-depositional alteration under sali-ne–alkaline conditions. Lavas and ignimbrites closer tosource also lack a fresh glass phase, either because ofalteration or crystallization to a fine-grained (but notglassy) matrix. As a result, the preferred techniques intephra characterization and correlation (involving thecomposition of the glass phase: e.g. Froggatt, 1992; Feibel,1999; Sarna-Wojcicki, 2000) cannot be applied. Instead,the mineral assemblage and major and minor elementcompositions of multiple phenocrysts are used for compar-ison. Mineralogical differences have been used successfullyfor tephra fingerprinting at Olduvai (McHenry, 2004, 2005)and elsewhere (e.g. New Zealand: Bignall et al., 1996;Cronin et al., 1996a, b).

3. Methods

3.1. Site selection and sampling

Samples were collected from Bed I tephra in the OlduvaiBasin, from ignimbrites exposed along the First Fault, andfrom lavas and ignimbrites exposed in the crater walls ofOlmoti and Ngorongoro volcanoes. Samples of Olduvaitephra were collected as part of a project to construct astratigraphic framework for Bed I (McHenry, 2004).During this process, Tuff IF was sampled across theOlduvai Basin and examined by optical microscopy andelectron microprobe analysis to ensure proper identifica-tion. As Tuff IF is the product of multiple eruptions andconsists of multiple surge deposits (along with one lapilli-rich airfall layer, McHenry, 2005), multiple samples werecollected at some sites (Localities 40 and 99). Lower Bed Itephra layers were collected in the western part of OlduvaiGorge (Naabi Ignimbrite: sample 02-T3, Locality 64;CFCT: sample 61-F, Locality 61; Tuff IA: sample 02-T52, Locality 65).Samples were also collected from two ignimbrites

exposed along the First Fault (between Olduvai and theNVH), south of the mouth of the modern gorge (Locality153). These included a green (lower, sample 153g) and a red(upper, sample 153r) ignimbrite.Lavas and ignimbrites in the NVH (Olmoti, Ngoron-

goro, and Lemagurut) were examined in the field for thepresence of rhyolites and trachytes that might be compar-able to the Olduvai Bed I tephra deposits. Sampling waslimited because of vegetation cover and accessibility. AtNgorongoro Crater most exposures along the ascendingand descending roads showed only basaltic or intermediatecomposition lavas, though one rhyolitic ignimbrite wassampled from a road cut along the descending road(southwest corner). Samples of two trachytic lava flows(Olmoti lavas 1 and 2, samples 03-OLT-1 and 03-OLT-3)

ARTICLE IN PRESSL.J. McHenry et al. / Quaternary International 178 (2008) 306–319 309


were collected from the western crater wall of Olmoti.Extensive sampling at Lemagurut by Mollel (2002)recovered only lavas of intermediate composition, provid-ing no suitable candidates for possible correlation with theOlduvai tephra. The sample sites selected are indicatedon the map in Fig. 1, with GPS coordinates included inTable 1.

3.2. Sample preparation and analysis

Mounts containing glass and phenocrysts from thesamples were prepared for electron microprobe analysisusing the methodology described in McHenry (2005). Thinsections of some Olduvai tephra layers and NVH lavas andignimbrites were also prepared, polished, and carboncoated for electron microprobe analysis. Most sampleswere analyzed by electron microprobe at Rutgers Uni-versity (JEOL JXA-8600 SuperProbe), running at 15 kVand 15 nA with a rastering beam (count times, standards,and data analysis methods reported in McHenry, 2005). Avolatile correction routine was applied to all glass samplesto minimize Na loss during analysis (Donovan, 2000).Additional analyses were conducted by electron microp-robe at the University of Wisconsin-Madison (CamecaXS50) running at 15 kV and 15 nA with a focused electronbeam (for minerals) or a defocused beam (for glasses).Three of the thin sections (both Olmoti lavas and the TuffIF lava fragments) were run on both systems to ensurecomparability of the results. Since no difference wasobserved, results of analyses on both microprobes werecombined. Individual analyses with low totals wereexcluded. For the feldspars, some volatilization wasobserved during analysis, resulting in low Na concentra-tions for some analyses. The feldspar end members (albite,anorthite, orthoclase, and celsian) were calculated from theraw data, and analyses with insufficient albite for stoichio-metric feldspar were excluded. This resulted in theexclusion of several anomalous analyses previously re-ported for Tuff IF anorthoclase at Locality 40 by McHenry(2005). Low totals for the titanomagnetite analyses likelyresult in part from oxidation, but further analyses to

confirm this are beyond the scope of this project. Cr, Zn,and V were not observed in EDS for any sample, Cr wasbelow detection in WDS for the few samples in which itwas determined quantitatively. Their inclusion would notlikely increase the low totals substantially.Compositions were compared to the geochemical ‘‘fin-

gerprints’’ determined for all of the major Olduvai Bed Itephra to ensure proper identification (McHenry, 2005)and to identify potential correlations with near-sourcesamples. Major and minor elemental data for glasses andmineral phases were plotted on bivariate and ternary plotsand compared visually, using the compositional distinc-tions documented in McHenry (2005). Feldspar, augite,amphibole, and titanomagnetite grains were selected fordetailed analysis since these show the greatest between-tephra variability and thus are of greatest use indiscriminating between individual Olduvai tephra units(McHenry, 2005). Other minerals (aenigmatite, sodalite,quartz, and minor accessory phases) were noted but notanalyzed quantitatively since they are less useful forcompositional correlation in the Olduvai region.Backscattered electron images (BSE) were made of lava

fragments from Tuff IF and from the lavas and ignimbritescollected from the NVH to compare volcanic textures. Amagnification of 250� was used for both to comparetextures and grain sizes. All BSE images were acquiredusing a 20 kV, 20 nA electron beam.One lava from the Olmoti Crater wall (Olmoti lava 2,

sample 03-OLT-3) was prepared for dating using the40Ar/39Ar laser incremental heating technique at RutgersUniversity. The lava was crushed, sieved (with the �40 to�60 mesh size fraction retained), and washed in anultrasonic bath of distilled water. The sample wasirradiated at the Oregon State University Triga ResearchReactor (OSTR) facility for 40min. Isotopic interferencecorrections used are 36Ar=37ArCa ¼ 2:72� 0:01ð�10�4Þ,39Ar=37ArCa ¼ 7:11� 0:024ð10�4Þ, and 40Ar=39ArK ¼ 7�3ð10�4Þ (Deino and McBrearty, 2002). Mass discriminationwas monitored during the analyses from replicate aliquotsdelivered from an on-line air pipette, and J was calculatedfrom replicate analyses of co-irradiated Alder Creek

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Table 1

GPS coordinates for localities used in this study, West to East

Locality Latitude Longitude Sample

65 S 02157000.20 0 E 35114026.00 0 Tuff IA (sample 02-T52)

64 S 02157007.10 0 E 35114042.50 0 Naabi Ignimbrite (sample 02-T3)

61 S 02156057.50 0 E 35115013.20 0 CFCT (sample 61-F)

99 S 03102041.00 0 E 35116020.30 0 Tuff IF surge deposit near Lemagurut

46 S 02159025.60 0 E 35120056.10 0 Ng’eju Tuff (upper and lower)

40 S 02159032.10 0 E 35121025.00 0 Tuff IF

153 S 03104007.50 0 E 35122041.70 0 1st Fault, south of gorge, 153r and 153g

Ngorongoro S 03109006.50 0 E 35128039.10 0 Ngorongoro caldera wall, descending road

200 S 02152055.60 0 E 35129050.90 0 1st Fault, north of gorge, Tuff IF ignimbrite

Olmoti 2 S 03102008.50 0 E 35139052.40 0 Olmoti crater wall (lava 2, sample 03-OLT-3)

Olmoti 1 S 03101008.30 0 E 35140028.40 0 Olmoti crater wall (lava 1, sample 03-OLT-1)



(AC ¼ 1:194� 0:007Ma) (Renne et al., 1998) at 1.791(� 10�4). The sample was heated in step-wise increments of60 s each using a 50W CO2 laser defocused through a 6mmintegrator lens. The argon isotopes were measured on aMAP215-50 mass spectrometer, and 40Ar/39Ar ages werecalculated using automated software written by Deino.

4. Results

4.1. General results

The mineral content of the Olduvai Bed I tuffs and NVHlavas documented here are reported in Table 2. The NaabiIgnimbrite (sample 02-T3) contains quartz, augite, aenig-matite, and anorthoclase feldspar phenocrysts. Ignimbritesfrom the First Fault exposures south of the mouth ofOlduvai Gorge (near Ngorongoro, samples 153r and 153g)also contain quartz and have similar phenocryst composi-tions (Table 3).

Tuff IF was found to have a consistent compositionacross the Olduvai Basin, in its glass lapilli (wherepreserved), surge-related phenocrysts, and lava fragments(McHenry, 2005). This contradicts Hay’s (1976) descrip-tion of compositional differences in Tuff IF betweeneastern and western Olduvai. This difference between ourand Hay’s results is explained by (a) the presence ofmetamorphic detrital contaminants in Tuff IF at Locality80 (west of the 5th fault) and (b) the previous misidenti-fication of Tuff IB as Tuff IF to the west of the 6th fault(Blumenschine et al., 2003; McHenry, 2005). Both thecontamination and the misidentification would have ledHay (1976) to conclude that Tuff IF had variablecomposition.

The Tuff IF lava fragments have an intergranular textureand trachytic composition with a matrix of anorthoclasefeldspar laths surrounding smaller and less abundant lathsof Na-augite, Na-amphibole, aenigmatite, and titanomag-

netite (Fig. 3). Occasional phenocrysts of sodalite andanalcite are also present. Within the lapilli, the glass isphonolitic (McHenry, 2005) and contains anorthoclasephenocrysts with distinctively high BaO (up to 3.42wt% atthe cores of individual grains) compared to the otherOlduvai tephra (Table 4). All Tuff IF analyses reportedhere are from Hay’s (1976) Locality 40.The two lavas sampled on the western crater wall of

Olmoti have an intergranular texture and trachyticcomposition, with a mineral assemblage similar to that inthe Tuff IF lava fragments (Fig. 3). The ignimbrite sampledon the descending road into Ngorongoro (western craterwall) is rhyolitic.The 40Ar/39Ar plateau and isochron ages of

1.8070.01Ma from Olmoti lava 2 (sample 03-OLT-3) aresimilar and indistinguishable from the integrated age of1.8270.01Ma (Fig. 4). The first five low temperature steps(excluded from the plateau age) are slightly older, likelybecause of low temperature alteration resulting in K loss.

4.2. Comparison of distal deposits to ignimbrites and lavas

at or close to source

4.2.1. The Naabi Ignimbrite, First Fault Ignimbrites, and

Ngorongoro Crater

Feldspar and augite phenocryst compositions for theNaabi Ignimbrite (sample 02-T3) are similar to phenocrystsin the green ignimbrite exposed along the First Fault tothe south of Olduvai (Locality 153, sample 153g) (Table 3,Fig. 5). Petrographically, both are green and containquartz, aenigmatite, augite, and anorthoclase feldspar(Table 2). The anorthoclase feldspar for both the NaabiIgnimbrite and the First Fault green ignimbrite aredistinctively higher in Fe2O3 than the other lower Bed Irhyolitic tephra. Augite from both ignimbrites is distinc-tively higher in FeO, except for a minor secondarypopulation with lower FeO concentrations present in both

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Table 2

Mineral content in Olduvai Bed I tephra

Fayalite Augite Sodic

Augite

Aenigmatite Hornblende Titanomagnetite Ilmenite Plagioclase Anorthoclase Sodalite Quartz Analcite

Naabi – x – xx – to + + x – xxx – x –

CFCT – xx – – Xx x xx – xxx – x –

IA – x – – + x + + xx – x –

Red ignimbrite xx xx – – – x x – xxx – + –

IB – xx – – – to + xx – – xxx – – –

IC – xx – – – to + xx – – xxx – – –

ID – xxx – – – to + xx – to + xxx – – – –

IE – xx – – – to + xx – – xxx – – –

Ng’eju Lower – xx – – xx xx + xxx – – – –

Ng’eju Upper – xx – – x xx + xxx – – – –

Tuff IF surge – xx x – to + xx x – – xxx – – –

Tuff IF lapilli – xx – – + to x xx – – xxx – – –

Tuff IF lava frags. – x xx x xx x – – xxx + – – to +

Ngorongoro Desc Rd xx xx – – – x x – xxx – x –

Olmoti trachytes – to + + to xx xx x x to xx x – – xxx + – – to +

Note: Frequency symbols are: –: absent, +: rare, x: between common and rare, xx: common, xxx: abundant.



the Naabi Ignimbrite and 153g (Fig. 5). Neither augitecomposition is observed in other lower Bed I tephra. Thisphenocryst assemblage has not been observed in otherOlduvai tephra (McHenry, 2005), nor was quartz observedin any NVH eruptive center other than Ngorongoro(Manega, 1993; Mollel, 2002).

None of the lower Bed I tephra or First Fault ignimbritesis entirely mineralogically consistent with the NgorongoroCrater wall ignimbrite sampled. The closest is sample 153r,the red ignimbrite from the First Fault. It contains asimilar mineral assemblage: similar anorthoclase andaugite compositions (Fig. 5), quartz, titanomagnetite,

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Table 3

Lower Bed I and Ngorongoro mineral compositions (electron microprobe analyses)

Sample Pop n SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O BaO Sum

Augite

Tuff IA 1 3 51.46 0.93 2.49 7.31 0.16 15.85 21.43 0.42 n.d. n.d. 100.05

St. dev. 0.63 0.29 0.24 1.92 0.06 1.02 0.88 0.13

Tuff IA 2 3 54.21 0.33 1.34 3.79 0.10 18.22 21.88 0.39 n.d. n.d. 100.26

CFCT 18 49.61 0.33 0.30 26.55 1.28 2.83 18.40 0.67 n.d. n.d. 99.96

St. dev. 0.46 0.06 0.09 1.35 0.06 0.90 0.23 0.11

Naabi 1 4 47.95 0.34 0.13 29.61 1.27 0.65 16.75 1.57 n.d. n.d. 98.26

St. dev. 0.82 0.05 0.00 0.23 0.05 0.17 1.02 0.50

Naabi 2 2 51.49 0.38 0.24 23.19 1.54 5.14 17.42 0.89 n.d. n.d. 100.30

153g 1 10 47.53 0.41 0.13 30.75 1.25 0.59 17.36 1.48 n.d. n.d. 99.50

St. dev. 0.98 0.13 0.01 0.32 0.11 0.07 0.50 0.27

153g 2 1 46.54 0.53 0.31 22.27 1.36 5.40 19.05 0.71 n.d. n.d. 96.16

153r 23 47.58 0.39 0.43 25.11 1.27 4.35 18.60 0.60 n.d. n.d. 98.34

St. dev. 1.03 0.06 0.11 1.81 0.07 1.31 0.18 0.15

Ngorongoro 17 49.43 0.43 0.52 23.46 1.27 4.95 18.89 0.52 n.d. n.d. 99.48

St. dev. 0.59 0.05 0.15 1.94 0.10 1.51 0.39 0.09

Sample Pop n SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O BaO Sum

Feldspar

Tuff IA 1 5 67.02 n.d. 19.48 0.21 n.d. n.d. 0.40 7.66 4.95 0.19 99.98

St. dev. 1.10 0.47 0.18 0.39 0.42 0.66 0.16

Tuff IA 2 2 65.66 n.d. 21.47 0.31 n.d. n.d. 2.49 8.14 2.30 0.41 100.82

CFCT 19 66.73 n.d. 19.61 0.36 n.d. n.d. 0.11 7.52 6.09 0.08 100.56

St. dev. 0.35 0.25 0.08 0.07 0.29 0.26 0.05

Naabi 12 67.55 n.d. 18.22 0.74 n.d. n.d. 0.04 7.25 6.16 0.07 100.07

St. dev. 0.66 0.29 0.20 0.05 0.20 0.35 0.06

153g 18 67.59 n.d. 18.83 0.75 n.d. n.d. 0.01 7.03 6.21 0.05 100.50

St. dev. 0.31 0.26 0.02 0.20 0.23 0.04

153r 31 67.26 n.d. 19.05 0.34 n.d. n.d. 0.16 7.62 5.92 0.10 100.47

St. dev. 0.46 0.35 0.06 0.13 0.28 0.46 0.07

Ngorongoro 12 65.46 n.d. 18.77 0.34 n.d. n.d. 0.29 7.49 5.65 0.14 98.15

St. dev. 1.22 0.49 0.10 0.26 0.35 0.73 0.12


Titanomagnetite

CFCT 4 0.17 23.53 1.51 68.11 1.19 1.48 0.09 n.d. n.d. n.d. 96.06

St. dev. 0.13 0.55 0.07 1.84 0.16 0.41 0.18

153g 2 0.47 24.64 0.53 68.51 1.67 0.01 0.18 n.d. n.d. n.d. 95.99

153r 9 0.05 20.16 0.28 73.97 0.91 0.05 0.15 n.d. n.d. n.d. 95.55

St. dev. 0.06 1.66 0.11 1.46 0.27 0.06 0.15

Ngorongoro 5 0.38 24.85 0.56 67.07 1.28 0.26 0.04 n.d. n.d. n.d. 94.43

St. dev. 0.20 1.25 0.20 2.72 0.10 0.12 0.05

Pop: population, n: number of grains analyzed.



fayalite, and ilmenite. Ultimately a direct match betweenthe two ignimbrites is unlikely because of a significantdifference in titanomagnetite composition in FeO (74% in153r vs. 67% in the Ngorongoro sample) and TiO2 (20%vs. 25%). This does not imply that the First Faultignimbrites and lower Bed I tephra were not erupted fromNgorongoro, rather that this particular ignimbrite from thecrater wall was not preserved in the Olduvai section.Ngorongoro remains the only probable source because theFirst Fault ignimbrites are proximal to Ngorongoro, andquartz has not yet been found at other eruptive centers inthe NVH.

4.2.2. Tuff IF and Olmoti Crater

The intergranular textures observed in the Olmoti Craterwall lavas are similar to those observed in the lava

fragments of Tuff IF at Olduvai (Fig. 3). Both consistpredominantly of an anorthoclase lath matrix with smallerNa-rich mafic minerals filling in the spaces between thefeldspar laths. The size (typically 40–100 mm) and shape(thin and elongate) of the laths are also consistent. Somesamples from other NVH volcanoes (including Ngoron-goro) also demonstrate intergranular textures, but containabundant plagioclase and generally lack the abundant Na-rich mafic phenocrysts and sodalite present in Olmoti lavasand Tuff IF lava fragments (Mollel, 2002).In addition to textural similarities, Olmoti trachytic lavas

and lava fragments within Tuff IF also have similarmineral compositions (Table 4). Both contain the sameassemblage of Na-rich minerals (anorthoclase, Na-richaugite, Na-rich amphibole, aenigmatite, with trace soda-lite). Large phenocrysts of anorthoclase have Ba-rich

ARTICLE IN PRESS

Fig. 3. Backscattered electron images (BSE) of Tuff IF lava fragments from the lower surge at Locality 40 (A and B) and a trachytic lava flow from the

wall of Olmoti Crater (Olmoti lava 1, sample 03-OLT-1) (C and D). The darkest minerals in these images include anorthoclase (dominant) and sodalite

(rare), the brightest are titanomagnetites, and the intermediate shades represent Na-augite, Na-amphibole, and aenigmatite. All images are at the same

scale. Textural and mineral assemblage similarities between Tuff IF lava fragments and Olmoti lavas make Olmoti a likely source for the upper Bed I

Olduvai tephra.



compositions, a feature so far only observed in Tuff IFlapilli in the Olduvai Basin tephra (Fig. 6) and NVHvolcanic sources. In addition, the Tuff IF lava fragmentscontain Na-rich augite, titanomagnetite, and Na-richamphibole compositions consistent with the Olmoti tra-chytic lavas and distinct from other Bed I tephra (Fig. 6).The Tuff IF surge (lava fragments and phenocrysts) andOlmoti lava 1 (sample 03-OLT-1) have similar low-Ti, low-Al titanomagnetite compositions compared to other Bed Itephra layers. The Tuff IF lava fragments and the Olmotilavas also contain high-Na, low-Al ferrorichterite amphi-boles unlike any in the Olduvai Bed I tephra (including theTuff IF lapilli pargasites and ferroedenites) (Fig. 6). Theamphiboles and augites from the Tuff IF lava fragments

and the Olmoti lavas are compositionally variable, withsignificant grain-to-grain variability in Na2O, TiO2, andAl2O3 contents (Fig. 6). However, their compositionalranges are consistent between samples. The presence ofsodalite and analcite and the absence of quartz in theOlmoti trachytes (McHenry, 2005) distinguish them fromthe more felsic quartz-bearing rhyolites of Ngorongoro.The only inconsistency between the Tuff IF lava fragmentsand the Olmoti trachytic lavas is the presence of fayalite inone of the lava samples (Olmoti lava 1, sample 03-OLT-1).However, this fayalite is a minor component found only inone part of one thin section analyzed and does not appearto be a common constituent. Fayalite is not observed in theother lava (03-OLT-3).

ARTICLE IN PRESS

Table 4

Olduvai Tuff IF and Olmoti lava mineral compositions (electron microprobe analysis)


Augite

Tuff IF lava fragments 1 5 50.22 0.97 0.93 25.12 0.78 1.05 9.36 7.40 n.d. n.d. 95.82

St. dev. 1.01 1.03 0.26 1.14 0.25 0.41 2.93 1.91

Tuff IF lava fragments 2 3 50.76 1.10 2.26 17.63 1.02 5.45 16.44 3.11 n.d. n.d. 97.77

St. dev. 0.96 0.16 0.52 0.36 0.06 1.06 1.52 0.35

Tuff IF lapilli 21 50.93 0.93 1.45 13.48 0.80 11.20 20.27 0.82 n.d. n.d. 99.87

St. dev. 0.58 0.26 0.39 0.45 0.07 0.39 0.31 0.10

Tuff IF surge 1 8 50.93 0.50 1.23 14.96 0.91 9.22 20.78 1.15 n.d. n.d. 99.68

St. dev. 0.98 0.05 0.25 1.37 0.10 1.21 0.67 0.48

Tuff IF surge 2 2 50.94 0.32 0.69 21.52 1.18 4.55 17.95 2.51 n.d. n.d. 99.65

Olmoti lava 1 (03-OLT-1) 1 9 50.07 1.83 1.78 20.93 0.93 3.58 14.49 3.91 n.d. n.d. 97.52

St. dev. 1.83 0.45 0.94 1.26 0.14 0.90 1.79 0.76


St. dev. 0.68 1.08 0.31 0.65 0.14 0.24 2.00 1.11


St. dev. 1.84 0.27 0.17 1.19 0.11 0.52 1.98 1.34


St. dev. 2.08 0.56 0.59 0.81 0.09 0.89 0.72 0.12


Amphibole

Tuff IF lava fragments 8 48.42 0.97 3.17 18.54 0.90 10.08 7.22 5.06 1.50 n.d. 95.86

St. dev. 1.31 0.24 1.18 2.77 0.10 1.68 0.82 0.40 0.11

Tuff IF surge 1 12 42.66 3.01 8.36 20.44 0.75 8.76 10.36 3.11 1.02 n.d. 98.47

St. dev. 1.44 0.24 0.52 1.60 0.12 0.80 0.37 0.45 0.11

Tuff IF surge 2 2 50.16 1.13 2.72 20.86 0.96 9.30 6.60 5.12 1.28 n.d. 98.12

Ng’eju Upper 4 42.73 3.83 9.33 19.81 0.55 8.53 10.42 2.55 1.00 n.d. 98.75

St. dev. 0.55 0.30 0.28 3.59 0.13 2.16 0.10 0.04 0.13

Ng’eju Lower 6 42.00 4.04 9.60 16.95 0.49 9.92 10.55 2.57 0.91 n.d. 97.03

St. dev. 0.60 0.13 0.33 0.53 0.05 0.20 0.04 0.07 0.03

Olmoti lava 1 (03-OLT-1) 1 2 50.10 1.02 1.91 23.22 0.95 6.55 3.28 7.33 1.56 n.d. 95.91



St. dev. 0.35 0.59 0.34 1.40 0.05 0.72 0.62 0.49 0.04




The correlation between Tuff IF and Olmoti volcano issupported by the new 1.8070.01Ma 40Ar/39Ar age forOlmoti lava 2. This age is comparable to the ages reportedfor upper Bed I tephra (Tuff IB: 1.84570.002Ma,Blumenschine et al., 2003; Tuff IF: 1.79Ma, Hay andKyser, 2001). The upper Bed I age also agrees with theoverlying normal to reverse paleomagnetic transitioncorrelated with the top of the Olduvai subchron(1.785Ma: Berggren et al., 1995; Olduvai magnetics ofTamrat et al., 1995).

5. Discussion

5.1. Consistency with previous age determinations

Published dates on lava and feldspar from NgorongoroCrater include 2.4570.15 to 2.15Ma (no error reported)(K-Ar: Gromme et al., 1970; Curtis and Hay, 1972),2.0270.02Ma (40Ar/39Ar single crystal laser fusion,Manega, 1993), and 1.83 to 1.9870.03 Ma (whole rockK-Ar of slightly altered ignimbrites, Manega, 1993). A

reversal in the magnetostratigraphy in the lower part of theNgorongoro Crater wall was originally correlated to theGauss/Matuyama polarity reversal (Gromme et al., 1970)now dated at 2.47Ma (Berggren et al., 1995), 2.581Ma(Cande and Kent, 1992), or 2.59Ma (Gradstein et al.,2004). The more recently derived younger dates forNgorongoro suggest that this reversal may be the end ofthe �2.1Ma Reunion event instead (Mollel and McHenry,2004). The younger dates bring Ngorongoro more in linewith the ages for lower Bed I (2.03Ma at the base, Walteret al., 1992).Ages derived from both K-Ar and single-crystal laser

fusion (SCLF) 40Ar/39Ar on lavas and ignimbrites pre-sumably from Olmoti yielded ages of 1.10–1.85Ma,consistent with upper Bed I and younger Olduvai Beds(Manega, 1993). The single date for the Olmoti Crater walllava in this study is consistent with previous studies andcomparable to ages for upper Bed I tephra. Furthergeochronological refinements of the NVH may help tonarrow these time intervals and provide an additional testfor source correlations.

ARTICLE IN PRESS

Sample Pop n SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O BaO Sum

Feldspar

Tuff IF lava fragments St. de. 10 65.12 n.d. 18.52 0.67 n.d. n.d. 0.34 7.28 6.23 0.07 98.24

St. dev. 0.83 0.34 0.12 0.30 1.07 1.60 0.09

Tuff IF lapilli 1 16 66.32 n.d. 19.15 0.46 n.d. n.d. 0.80 8.23 3.51 0.70 99.32

St. dev. 0.56 0.43 0.10 0.17 0.26 0.35 0.37

Tuff IF lapilli 2 3 64.65 n.d. 20.10 0.45 n.d. n.d. 1.16 7.46 3.26 2.67 100.15

St. dev. 0.28 0.58 0.04 0.51 0.36 0.52 0.71

Tuff IF surge 10 63.63 n.d. 20.74 0.30 n.d. n.d. 1.12 8.41 3.70 0.42 98.43

St. dev. 1.83 0.85 0.11 0.49 0.46 0.94 0.24

Olmoti lava 1 (03-OLT-1) 15 65.69 n.d. 18.87 0.94 n.d. n.d. 0.55 7.55 5.36 0.13 99.28

St. dev. 1.20 0.77 0.32 0.31 0.66 1.04 0.14

Olmoti lava 2 (03-OLT-3) 1 9 64.62 n.d. 19.47 0.65 n.d. n.d. 0.57 7.83 4.54 0.63 98.54

St. dev. 1.36 0.71 0.22 0.16 0.67 0.63 0.55

Olmoti lava 2 (03-OLT-3) 2 5 65.57 n.d. 21.08 0.25 n.d. n.d. 1.12 8.47 2.85 1.45 100.98

St. dev. 0.37 0.91 0.07 0.22 0.50 0.35 0.38

Titanomagnetite

Tuff IF lava fragments 11 0.33 15.10 0.35 75.85 1.67 0.05 0.06 n.d. n.d. n.d. 93.42

St. dev. 0.35 1.37 0.20 2.01 0.12 0.03 0.11

Tuff IF lapilli 14 0.24 21.27 1.02 65.78 1.61 1.32 0.07 n.d. n.d. n.d. 91.30

St. dev. 0.36 0.31 0.11 1.50 0.06 0.14 0.04

Tuff IF surge 1 9 0.24 14.26 0.57 73.86 1.59 0.04 0.11 n.d. n.d. n.d. 90.66

St. dev. 0.16 0.99 0.36 1.58 0.20 0.08 0.11

Tuff IF surge 2 5 0.05 21.63 1.02 69.22 1.59 1.18 0.01 n.d. n.d. n.d. 94.71

St. dev. 0.02 0.69 0.21 1.14 0.20 0.31 0.01

Olmoti lava 1 (03-OLT-1) 23 0.24 15.03 0.63 76.23 1.54 0.22 0.01 n.d. n.d. n.d. 93.89

St. dev. 0.15 1.18 0.11 1.21 0.12 0.09 0.02

Olmoti lava 2 (03-OLT-3) 30 0.18 19.34 0.37 72.34 1.94 0.14 0.02 n.d. n.d. n.d. 94.32

St. dev. 0.24 2.08 0.26 2.01 0.19 0.14 0.03

Pop: population, n: number of grains analyzed. All Tuff IF samples from Locality 40, Olduvai Gorge.

Table 4 (continued)



5.2. Implications for Olduvai Bed I tephra sources

Tuff IF is similar in composition (in glass and mineralassemblage) to other Bed I tephra, especially Tuffs IB, IC,and IE (Fig. 6). An analysis of similarity coefficients (SC,after Borchardt et al., 1972) calculated using the upper BedI tephra glass compositions (using data from McHenry,2005) reveals the compositional similarities between thesetephra. TiO2, Al2O3, FeO, MnO, MgO, and CaOconcentrations as analyzed by electron microprobe of glass

shards were used to create the matrix (Table 5). A moreaccurate SC determination would be possible using traceelements, but the frothy and microlite-rich nature of theOlduvai Bed I glass shards makes it difficult to conducttrace element analysis. Using the major and minor elementdata available, Tuffs IB, IC, ID, IE, and the Lower Ng’ejuTuff all have SC values between 0.82 and 0.89 compared toTuff IF glass. This level of correspondence is below the0.92 generally required for equivalence, but is within therange expected for tephra derived from a common eruptive

ARTICLE IN PRESS

Lower Bed I and Ngorongoro feldspar: Fe2O3 vs. K2O

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

2

K2O (wt. % oxide)

Lower Bed I and Ngorongoro augite: FeO vs. Al2O3

Fe

O (

wt.

% o

xid

e)

Al2O3 (wt. % oxide)

18

20

22

24

26

28

30

32

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fe

2O

3 (

wt.

% o

xid

e)

3 4 5 6 7

a b

Fig. 5. Comparison of Olduvai lower Bed I rhyolitic tephra (including the Naabi Ignimbrite) phenocryst compositions to the First Fault ignimbrites (153r:

red ignimbrite, 153g: green ignimbrite) and a sample from an Ngorongoro Crater wall ignimbrite. (A) Feldspar compositions, Fe2O3 vs. K2O. (B) Augite

compositions: FeO vs. Al2O3. Tuff IA is omitted because its augite composition lies outside the range of the other tuffs.

0

40

80

-10

0

10

0 10 20 30 40 50 60 70 80 90 1000

0.5

1.0

App

aren

t Age

(M

a)

1.5

2.0

2.5

3.0

B

C DE F G I J K

Plateau Age = 1.80 ± 0.01 Ma

Integrated Age = 1.82 ± 0.01 Ma

40Ar/39Ar Laser Incremental Heating Spectrum for Olmoti Lava 03-OLT-3

%40Ar*

Ca/K

Cumulative %39Ar Released

Isochron Age = 1.80 ± 0.01 Ma40Ar/36Ar Intercept = 296 ± 3MSWD = 1.7

H L

Fig. 4. Age spectrum of incrementally heated whole rock (lava) from lava 2 (sample 03-OLT-3) from the Olmoti Crater wall. The plateau age is

indistinguishable from that of the isochron. The slightly older integrated age is likely because of low temperature alteration of the lava.



source (Sarna-Wojcicki et al., 1984). It is therefore likelythat these tephra were also erupted from Olmoti, but asairfall tuffs, surges, and flows without many lava frag-ments. Thus, Olmoti was active from at least 1.86 until1.79Ma (dates for Tuffs IB and IF, Walter et al., 1992; Hayand Kyser, 2001). In lower Bed I, both the CFCT (2.01Ma:Walter et al., 1992) and Tuff IA (1.98Ma: Walter et al.,1992) contain rhyolitic components similar to the NaabiIgnimbrite (especially glass, feldspar, and quartz; Fig. 5),making it likely that these tephra have the same source:Ngorongoro Crater.Mollel (2002) indicated that Lemagurut lavas are inter-

mediate in composition and are therefore unsuitable forcorrelation with the trachytes and rhyolites of Olduvai BedI, which is consistent with our field observations. While wecannot rule out a potential role for Lemagurut as a source

ARTICLE IN PRESS

Table 5

Similarity coefficients for Bed I tephra glasses

Tuff IF Lapilli Tuff IF surge

Loc 40 Loc 40

Tuff IB Loc 6 0.89 0.89

Tuff IC Loc 64 0.84 0.82

Tuff ID Loc 40 0.84 0.82

Tuff IE Loc 6 0.85 0.84

Ng’eju L Loc 46 0.86 0.86

Ng’eju U Loc 46 0.65 0.64

Tuff IF lapilli Loc 40 1 0.97

Tuff IF surge Loc 40 0.97 1

Similarity coefficients using electron microprobe data from glasses

reported in McHenry (2005).

Elements used: Al2O3, TiO2, FeO, MgO, MnO, and CaO.

Calculation using similarity coefficient formula of Borchardt et al. (1972).

BaO

(w

t. %

oxid

e)

Upper Bed I and Olmoti alkali feldspar: BaO vs. CaO

CaO (wt. % oxide)

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2 2.50

2

4

6

8

10

0 10 15 20 25 30

Upper Bed I and Olmoti augite: Na2O vs. FeO

FeO (wt. % oxide)

Na

2O

(w

t. %

oxid

e)

Upper Bed I and Olmoti titanomagnetites: Al2O3 vs TiO2

TiO2 (wt. % oxide)

Al 2

O3

(w

t. %

oxid

e)

0

0.5

1

1.5

2

2.5

12 14 16 18 20 22 24 26 28

2

3

4

5

6

7

8

0 4 10 12

Upper Bed I and Olmoti amphibole: Na2O vs. Al2O3

Al2O3 (wt. % oxide)

Na

2O

(w

t. %

oxid

e)

5

2 6 8

a b

c d

Fig. 6. Comparison of Olduvai upper Bed I tephra (including Tuff IF) phenocryst compositions to the Olmoti lavas (lava 1: sample 03-OLT-1 and lava 2:

sample 03-OLT-3). Data for the other Olduvai Bed I tephra is fromMcHenry (2005). Each data point represents the composition of a single mineral grain.

(A) Alkali feldspar compositions, BaO vs. CaO. Tuff ID and the Ng’eju Tuff are excluded from this diagram because they contain only plagioclase

feldspar. (B) Augite compositions, Na2O vs. FeO. (C) Titanomagnetite compositions, Al2O3 vs. TiO2. The Upper Ng’eju Tuff is omitted because its

titanomagnetite Al2O3 concentrations are substantially higher than the other Bed I tuffs (Table 4, McHenry, 2005, Fig. 7C). (D) Amphibole compositions,

Na2O vs. Al2O3.



for some Bed I tephra, such a possibility is unlikely given theabsence of petrologically similar volcanic rocks on Lema-gurut in addition to our direct correlation of Tuff IF lavafragments to lavas found in Olmoti Crater. This means thatthe thickness (more than 3m) and coarseness of the Tuff IFsurge deposits at Hay’s Locality 99 (proximal to Lemagurut,distal to Olmoti) are not due to proximity to source. Asthese deposits are concentrated within channels, it is likelythat the paleotopography at the time of eruption includeddrainages that channeled pyroclastic surges originating fromOlmoti in a more southerly direction toward Lemagurut.Animal footprints found on the top surfaces of individualsurge packages at this site indicate that the landscape wasnot completely devastated at this distance (�30km) fromsource and that there were local ‘‘survivors’’ of thisotherwise devastating volcanic eruption.

5.3. Using rock fragment composition for tephra correlation

Using the texture and composition of rock fragments intephra that were entrained during eruption is a promisingmethod for tying tephra to their specific volcanic sources.Explosive volcanic eruptions typically entrain fragments ofpre-existing rocks along with the primary volcanic materi-al. These rocks may be products of previous eruptions ormaterial that in-filled the vent between eruptions. Theserocks may be easier to identify at the volcanic source thanthe tephra fallout, because lavas are more coherent andmore difficult to erode. In the case of Tuff IF, thepyroclastic surges contained mostly fragments of pre-existing lavas. In the case of airfall tephra, lava fragmentsare less common but are nonetheless present, and could beused for correlation. Lava fragments containing multiplemineral grains are unlikely to be transported far because oftheir size and density. Thus, this technique is most usefulonly with proximal tephra layers and flows and would notbe as successful for more distal tephra layers.

The lava fragments in Tuff IF do not imply that Tuff IFhas the exact same age as the lavas themselves. The lavascould have been deposited at any time previous to theeruption of Tuff IF, as they needed to be in place to befragmented and entrained in the Tuff IF pyroclastic surges.The difference in composition between the lava fragments(with Na-rich augite, aenigmatite, and rare sodalite) andthe primary glass and phenocryst assemblage (with augiteand titanomagnetite) makes it unlikely that the lavafragments represent primary material from the sameeruption. Thus, the slightly older age (1.8070.01Ma) forOlmoti lava 2 (03-OLT-3) compared to Tuff IF (1.79Ma,Hay and Kyser, 2001) is consistent with expectations,although the ages are not significantly different if probableerrors are considered.

6. Conclusions

Using the textures and mineral compositions of lavafragments preserved in Tuff IF in Bed I Olduvai we infer

that Tuff IF’s source is the Olmoti Crater �30 km away.The Naabi Ignimbrite in lower Bed I is compositionallysimilar to an ignimbrite exposed along the First Fault closeto Ngorongoro Crater. None of the other lower Bed Itephra layers is correlative with the Ngorongoro Craterignimbrite. Mineralogical similarities (particularly thepresence of quartz) suggest that Ngorongoro is the volcanicsource for the lower Bed I tephra, as other potential centersare silica-undersaturated or entirely intermediate or basal-tic (Manega, 1993; Mollel, 2002). The transition from thequartz-bearing lower Bed I rhyolites to the upper Bed Itrachytes most likely represented a change in volcanicsource, especially since Tuff IF (upper Bed I) is under-saturated in silica. This compositional change most likelyrepresents a switch in volcanic source from Ngorongoro toOlmoti. The earliest signs of hominin occupation atOlduvai are found near the transition between lower andupper Bed I, and Olmoti was the dominant active volcanothroughout upper Bed I.

Acknowledgments

We thank the Tanzania Commission for Science andTechnology and the Tanzania Antiquities Department forgranting us permission to conduct research at OlduvaiGorge. Richard Hay, Gail Ashley, Jeremy Delaney, JohnFournelle, Harald Stollhofen, Ian Stanistreet, RobertBlumenschine, and other members of the Olduvai Land-scape Archaeology and Paleoanthropology Project (direc-ted by R. Blumenschine and F. Masao) providedinvaluable support in both the field and analytical portionsof this project. Frank Brown and Jeffrey Knott providedinsightful and helpful reviews of the initial manuscript.Funding was provided in part by the L.S.B. LeakeyFoundation (grants to McHenry and Mollel), the Geolo-gical Society of America, Sigma Xi, NSF (Ashley: EAR9903258; Swisher: BCS-0109027), and the University ofWisconsin- Milwaukee.

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