GEOCHRONOLOGY OF THE ADU-ASA FORMATION AT GONA, ETHIOPIA
by
Lynnette Kleinsasser
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2007
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Lynnette Kleinsasser
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Geochronology of the Adu-Asa Formation at Gona, Ethiopia
Lynnette Kleinsasser, 1 Jay Quade, 1 Naomi Levin, 2 Scott W. Simpson,3 William C. McIntosh, 4
and Sileshi Semaw5
1
Department of Geosciences
University of Arizona
Gould-Simpson Bldg. #77
1040 East 4th Street
Tucson, Arizona 85721-0077, USA
2
Department of Geology and Geophysics
University of Utah
135 South 1460 East
Browning Bldg.
Salt Lake City, Utah 84112-0111, USA
3
Department of Anatomy
School of Medicine
Case Western Reserve University
10900 Euclid Ave.
Cleveland, Ohio 44106-4930, USA
4
New Mexico Bureau of Geology and Mineral Resources
New Mexico Institute of Technology
801 Leroy Place
Socorro, New Mexico 87801-4796, USA
5
Stone Age Institute
P.O. Box 5097
Bloomington, Indiana 47407-5097
ABSTRACT
The Gona Paleoanthropological Research Project (GPRP) area includes many rich fossil
localities that are of great consequence to the study of human evolution. The oldest of the
deposits at Gona, the Adu-Asa Formation, crops out across the western half of the GPRP.
The Adu-Asa Formation consists of nearly 200 m of fossil-bearing sedimentary rocks in
thin (≤ 30 m) laterally variable sections interlayered with abundant basaltic lava flows. Across
the project area, these volcanic and sedimentary sequences are tilted gently to the east and are
repeated by north-south trending, mostly west dipping normal faults that accommodated
extension in the Afar rift.
As is typical in continental/transitional extensional settings, the volcanic rocks in the
Adu-Asa Formation are strongly bimodal. Basaltic lavas and tuffs are abundant, but we have
also identified a rhyolite center and seven different silicic, or dominantly silicic, tuffs.
Of these tuff units, we were able to identify and to correlate four major tuffs in the AduAsa Formation at Gona by combining geochemical comparisons with detailed stratigraphic
sections through fossil-bearing deposits. From oldest to youngest, we have named these units the
Sifi, the Kobo’o, the Belewa, and the Ogoti Tuffs. The Sifi and the Kobo’o Tuffs are directly
associated with fossil localities. In addition, there is a distinctive porphyritic basalt that lies
above the Kobo’o Tuff which forms an important stratigraphic marker, and with which many
fossil sites are directly associated.
Anorthoclase from the Kobo’o Tuff is dated by 40Ar/39Ar at 5.45 ± 0.07 Ma (2s), whereas
sanidine from the Belewa Tuff returned a date of 5.52 ± 0.03 Ma (2s). Given the similarity in
dates, we can infer that the Adu-Asa Formation accumulated in a shorter time than can be
resolved using the 40Ar/39Ar method, and that the fossil finds in the Adu-Asa Formation are dated
at 5.5 Ma.
INTRODUCTION
Genetic studies suggest that human and chimpanzee lineages diverged in Africa during
the late Miocene-early Pliocene (Horai et al., 1992; Ruvolo, 1997; Chen and Li, 2001; Patterson
et al., 2006). Patterson et al. (2006) estimates that the human-chimpanzee genome diverged
permanently no earlier than 6.3 Ma, although the authors prefer a younger age of humanchimpanzee speciation, perhaps as young as 5.4 Ma. By contrast, recent hominid finds of
Sahelanthropus tchadensis in Chad, Orrorin tugenensis in Kenya, and Ardipithecus kadabba in
Ethiopia all date to this pre-5.4 Ma time period, suggesting that the hominid-chimpanzee
divergence must have been earlier (Vignaud et al., 2002; Sawada et al., 2002; WoldeGabriel et
al., 2001). Resolving the apparent contradiction between genetic and fossil evidence, as well as
sorting out the phylogenetic relationships between the earliest hominids, rests upon the discovery
and study of new, well dated, fossil material.
The Gona Paleoanthropological Research Project (GPRP), which has previously
produced specimens of Ardipithecus ramidus in the early Pliocene Sagantole Formation (Semaw
et al., 2005), has in recent field seasons made a number of discoveries in the older, and largely
unstudied, deposits of the Adu-Asa Formation. These older hominids are assigned to the species
Ardipithecus kadabba (Simpson et al., 2007). Secure dating of these and similar finds is crucial
to illuminating the earliest chapter of our evolution.
Geographic and geologic setting
The Gona Paleoanthropological Research Project (GPRP), located approximately 300 km
northeast of Addis Ababa, Ethiopia, contains a fossil-rich record of fluvial, lacustrine, and
volcanic deposits spanning much of the last 6 Myr (Fig. 1). The GPRP lies within the Afar
Basin and is 150-200 km west of the current triple junction between the Red Sea rift, the Aden
rift, and the Main Ethiopian rift (Tesfaye et al., 2003). The Afar basin is bounded on the north
by the Danakil horst, on the east by the Somalian escarpment, and on the west by the Western
Ethiopian escarpment. Within this basin, the GPRP area is bounded on the north by the MileBati road, on the east by the Awash river, and on the south by the As Bole drainage. The
western extent of the project area continues into the Ethiopian Escarpment. These westernmost
deposits have previously been referred to as the Dahla series fissural basalts in the volcanological
literature (Barberi et al., 1975; Wolfenden et al., 2005), but here we adopt the term Adu-Asa
Formation. This term was coined by Kalb et al. (1982) and embraced by later workers in the
same area (WoldeGabriel et al., 2001) to encompass interbedded basalts and sedimentary rocks
due south of the western part of Gona. Satellite photos strongly suggest north-south continuity
of this formation along the entire western side of the Awash graben in this region, a correlation
confirmed by radiometric dates that we present in this paper.
The Adu-Asa Formation in the GPRP area is composed of about 185 m of mostly basaltic
lava flows intercalated with thin zones of volcaniclastic, fluvio-lacustrine sedimentary rocks. All
fossil localities are confined to these sedimentary rocks. A rhyolite dome also crops out in the
northern end of the project area and caps the Adu-Asa Formation there. Both rhyolitic and
basaltic tuffs are common throughout the formation. However, the basaltic tuffs are too altered
to use as geochemical markers, so we have used only the silicic tuffs to provide the necessary
chronological control on the fossil localities (Fig. 2). In all but one case, the tuffs are ash fall
units interbedded with sedimentary rocks, and most are reworked to some degree. The exception
is an unwelded ash flow tuff and its related surge deposit associated with the rhyolite dome in the
northern end of the project area, although this unit does have an ash fall component as well.
Structurally, the Adu-Asa Formation at Gona is cut by numerous north-northwest
trending, west-dipping faults that accommodated extension in the Afar rift. Dips on beds are
gentle and generally to the east and virtually never exceed 25º E. Thus, deposits in this
formation tend to decrease in age to the east. Small repetitions of the stratigraphy by normal
faults are common. The abundance of these faults, the similarity in appearance of basalt lava
flows, and locally poor exposure made correlations of outcrops from area to area difficult. Here,
the use of glass compositions from tuffs to correlate between areas—the central focus of this
research—was vital to producing a coherent stratigraphic context for the fossil finds.
The Adu-Asa Formation grades upward into the younger Sagantole Formation to the east.
The contact is conformable and characterized by a shift from mostly volcanic units with a
sedimentary component in the Adu-Asa Formation to dominantly sedimentary units with a
volcanic component in the Sagantole Formation. This lithologic contrast is strongly expressed
topographically over much of the GPRP area. The Adu-Asa Formation is characterized by steep
ridge(=basalt lavas)-and-swale(=intercalated sedimentary rocks) topography, whereas the
sedimentary rocks that dominate the Sagantole Formation weather recessively. The top of the
Adu-Asa Formation is marked by a final topographically high-standing basalt lava flow (Figs. 1,
2). Exceptions to this pattern can be found at the extreme northern and southern ends of the
GPRP, where basalts instead of sedimentary dominate the Sagantole Formation, and the
transition between the Adu-Asa and Sagantole Formations is indistinct.
The Sagantole Formation at Gona is dominantly lacustrine volcaniclastic sedimentary
rocks with intercalated basaltic lavas. While rich in fossils, the deposits of the Sagantole
Formation at Gona are altered and highly faulted, making geochemical correlations of tuff units
difficult. Previous work has constrained specimens of Ardipithecus ramidus from this formation
to between 4.51 and 4.32 Ma, based on 40Ar/39Ar dates from tuffs and basaltic lavas
stratigraphically above and below the fossil localities, as well as paleomagnetic studies of
sedimentary rocks also associated with the fossil sites (Semaw et al., 2005). The base of the
dated portion of the Sagantole Formation at Gona is 4.64 ± 0.06 Ma, based on plagioclase from a
tuff (Quade et al., this volume), providing a minimum age for the Adu-Asa Formation.
The Sagantole Formation is bound on the east by the As Duma fault, a major north-south
trending, east dipping normal fault that has been active post-4 Ma to present. It juxtaposes the
east-dipping Sagantole Formation to the west against largely undeformed and much younger
Hadar and Busidima Formations to the east. The Hadar and Busidima Formations span the
period roughly from >3.5 to ≥0.2 Ma. In the GPRP area, the base of the Hadar Formation is >3.5
Ma, based on (1) an 40Ar/39Ar date of 3.55 ± 0.11 Ma on sanidine from a tuff at As Aela and (2)
on the identification of the Sidi Hakoma Tuff in northeast Gona, which is correlated to the Tulu
Bor Tuff in the Turkana basin (Brown, 1982; Walter and Aronson, 1993; Quade et al., 2004;
Quade etal., this volume). The top of the Busidima Formation is ≥0.2 Ma, based on
tephrostratigraphic and magnetostratigraphic studies (Quade et al., 2004; Quade et al., this
volume). A major disconformity between 2.7 and 2.9 Ma separates the Busidima Formation
from the underlying Hadar Formation (Quade et al., 2004).
METHODS
Field work
The field and lab work for this study were conducted during 2003-2006. Field work
focused primarily on collecting volcanic units suitable for 40Ar/39Ar dating and/or major-element
geochemical characterization using an electron microprobe. At most of the fossil localities, we
also measured stratigraphic sections in order to document the relevant relationships between
fossil-rich beds and the sampled volcanic units.
Most of the geochronological samples taken in the Adu-Asa Formation at Gona are ash
fall tuffs, although several basalt samples were collected along with a few obsidian and ash flow
tuff examples.
For ash fall and ash flow tuff samples, collection focused on obtaining both fresh glass
shards and any juvenile phenocryst populations present in each unit, although almost every tuff
unit encountered in the field was collected.
Virtually all of the ash fall deposits are reworked to some degree, and many form
multiple subunits in outcrop. In these cases, we sampled each subunit in order to be sure that we
had obtained a representative sample. Commonly, one subunit contains a greater density of
phenocrysts, and another contains a greater concentration of fresh glass shards. If any other
populations were present, such as pumice lapilli or obsidian clasts, subunits containing these
populations were also sampled in order to obtain the various components of the tuff. Thus, by
sampling each subunit individually, we were able to account for the sedimentary sorting that may
have separated different portions of a single tuff.
For the basalts, collection efforts focused on obtaining samples that were as fresh and as
little oxidized as possible. In addition, we looked for lava flows with holocrystalline groundmass
and a small percentage of phenocrysts, but we collected hand samples of lavas at many
stratigraphic levels throughout the Adu-Asa Formation at Gona. In practice, only outcrops that
were pervasively argillized and friable were not sampled.
Lab work
Tuffs
Samples were prepared by crushing and sieving each tuff into various size separates,
typically >500mm, 500-250mm, and 250-125mm. If the sample contains unaltered glass shards,
then a portion of the size separate in which the shards were most abundant was used to make a
microprobe mount. Most commonly, this was the 250-125mm size fraction.
Every tuff sample collected in the Adu-Asa Formation at Gona was processed for
analysis on an electron microprobe. All suitable sample separates, whether glass shards,
obsidian, or feldspar, were analyzed on a Cameca SX50 electron microprobe at the University of
Arizona.
For each component studied, we analyzed approximately 20 points, with each point on a
different shard or crystal. In a typical suite of analyses, most shards/crystals proved chemically
homogenous. Often a few grain analyses were rejected prior to statistical analysis as
contaminated, if their compositions were different from each other and from the main
compositional mode.
Many researchers have documented alkali mobility in glass as a result of electron
bombardment during electron microprobe analysis (Hunt and Hill, 1993; Morgan and London,
1996; Nielsen and Sigurdsson, 1981). In determining the best analytical conditions to use, we
followed some of the suggestions of Froggatt (1992) and Hunt and Hill (1993). Froggatt (1992)
suggests that researchers use a beam defocused to at least 10mm across, as well as a lower beam
current when analyzing alkali elements. Hunt and Hill (1993) recommend that researchers
analyze alkalis first, before a sample has time for significant mobilization to occur. After some
experimentation, we settled on the analytical conditions in Table 1. We used these conditions for
all analyses, whether glass or crystal.
At the time, we were unaware of the paper by Morgan and London (1996), which
specifically sought an optimal analytical set-up for dealing with alkali mobility and the
corresponding “grow-in” of Si and Al. As a comparison, we ran newly prepared mounts of
selected samples using the set-up conditions Morgan and London (1996) recommend and
compared those results to the data obtained using the set-up conditions in this study (Tables 1, 2).
Glass. If a size separate contained fresh glass shards, then no further processing was
necessary before creating a microprobe mount. Shards that had partially devitrified or were
otherwise altered were ground away during polishing, as was any rind of clay alteration on
otherwise well preserved glass.
We examined glass shards under a 10-40x binocular scope to establish their general
morphology. We follow the descriptive shard morphology system of Katoh et al. (2000), which
is based on work by Ross (1928), Heiken (1972), and Yoshikawa (1976) (Fig. 3).
The few obsidian samples, either collected as core stones or picked out of a tuff sample
as a clast, were prepared for the electron microprobe by lightly crushing them with a mortar and
pestle and then mounting the pieces using the same process as that of the glass shards.
Phenocrysts. Many of the tuff samples collected also contained phenocrysts, usually
feldspars. After separating a tuff sample into the various size fractions, if it was determined that
feldspars were present, a small number (commonly 50-100) were extracted by hand and were
mounted.
Feldspars were analyzed not only to determine the suitability of the crystals for 40Ar/39Ar
dating, but also as a check on any tuff correlations that were made based on volcanic glass
chemistry. Crystals suitable for 40Ar/39Ar dating should be unaltered and contain ≥ 1% K2O.
Although plagioclase containing ≤ 1% K2O was dated by the 40Ar/39Ar method, the large
associated errors often compromised the utility of the sample. Extent of alteration was
determined by examining back-scattered electron (BSE) images of feldspars during electron
microprobe work. If significant alteration was detected, it usually appeared as clay growth
within cleavage planes of crystals.
We also analyzed the feldspars as a check on tephra correlations made based on the glass
chemistry. If the glass composition of two tuff samples was identical but the phenocryst
populations proved chemically distinct, then the two samples likely represent different eruptions.
Because the possible range of feldspar compositions is less than in glass, however, comparisons
based solely on the composition of feldspar populations is insufficient for firm correlation.
Tuff samples containing feldspars suitable for 40Ar/39Ar dating were sent to the New
Mexico Geochronology Research Laboratory at the New Mexico Institute of Mining and
Technology. Samples were further processed using magnetic, heavy liquid, and hand-picking
methods in order to obtain a homogeneous population of feldspars. For details on analytical
methods used in obtaining the 40Ar/39Ar dates presented in this study, see Table 3.
Basalts. As with the tuffs, basalt samples were crushed and sieved into various size
fractions. Generally, the 100-120-mm size fraction was further processed by placing the sample
in an ultrasonic cleaner with a dilute HCl solution. Groundmass concentrates from this size
fraction were obtained through further magnetic and hand-picking techniques.
Basalt samples were analyzed with an electron microprobe to determine their suitability
for dating by the 40Ar/39Ar method. BSE images of the basalts are useful in assessing the amount
of clay alteration and glass content, and the compositional data obtained on the electron
microprobe allows characterization of the K content of the basalt. Analytical details on 40Ar/39Ar
dates obtained from basalts are shown in Table 3.
Similarity Coefficients
In order to statistically evaluate our geochemical correlations, we have calculated the
similarity coefficient for all possible pairings of glass analyses as well as feldspar pairs. The
similarity coefficient, or SC, is a statistical measure first developed by Borchardt et al. (1972)
and later refined by Rodbell et al. (2002). Created specifically for comparing the chemical
compositions of glass in tuffs, it is a measure of the similarity of two tuffs based on a suite of
geochemical analyses. If two samples have the same mean and standard deviation for every
oxide included in the analysis, the SC would be equal to 1. In practice, an SC of 0.95 or greater
is generally considered to be a valid correlation (Sarna-Wojcicki et al., 1980; Davis, 1985),
whereas it is common for samples of the same tuff to produce SCs that are slightly lower. An SC
of 0.92 is often taken as the lower limit for an acceptable correlation (Froggatt, 1992).
For these calculations, we used analyses of Na2O, K2O, SiO2, MgO, Al2O3, CaO, MnO,
FeO, and TiO2, where all measured Fe is expressed as FeO. Although we analyzed for additional
elements, we only used the nine oxides listed above in the SC calculations, as the other oxides
were almost always present in amounts at or below the detection limit of the electron
microprobe.
Following the equation as defined in Rodbell et al. (2002), the SC is calculated as:
d(A,B)=[S(Ri *gi )/(Sgi)],
where:
d(A,B)=the similarity coefficient for samples A and B,
Ri= XiA/XiB if XiB > XiA and Ri = XiB/XiA if XiA > XiB ,
XiA = concentration of element i in sample A,
XiB = concentration of element i in sample B,
gi = 1- (((siA/ XiA)2 + (siB/ XiB)2)/E)1/2,
siA = the standard deviation of element i in sample A,
siB = the standard deviation of element i in sample B, and
E = 1 – (detection limit/ (average of XiA, XiB).
Because the detection limit on each oxide varies with every analysis, we calculated the average
detection limit for every oxide in every sample. When determining the SC for samples A and B,
we used whichever detection limit was the larger. Any tuffs that were analyzed but not included
in this statistical analysis were excluded as a result of missing detection limits, which prevented
the SC calculation.
RESULTS
Tuff analyses
Most of the sample analyses (Tables 4, 5, GSA repository item#) are from ash fall units,
but a few were obtained from obsidian that was present either as a clast within an ash fall or ash
flow tuff, or as a core stone from a rhyolite. We calculated the similarity coefficient, or SC, for
each sample pair of glass and phenocryst analyses (Tables 6, 7). In this study, glass correlations
are supported by a SC of 0.92 or higher, except samples for which we did not obtain the
detection limits of the microprobe analyses, which are necessary for the SC calculation.
The felsic glass composition from the Adu-Asa Formation is primarily rhyolitic or dacitic
in character (Figs. 4, 5), although of course this may not be representative of the magma as a
whole, since it does not take into account the contribution of phenocryst chemistry. For bimodal
units, the mafic glass component plots as a basalt or basaltic andesite.
Tuff descriptions
In all, we have identified four major tuffs in the Adu-Asa Formation at Gona, as well as
three minor glassy tuffs and a crystal-rich series of related tuffs. Electron microprobe analyses,
of both glass and feldspar, confirm many of the tentative correlations that were made in the field
based on outcrop appearance and stratigraphic position. From oldest to youngest, the glassy tuffs
are named the Sifi Tuff, the Kobo’o Tuff, the Belewa Tuff, and the Ogoti Tuff. The Hamadi Das
Crystal-Rich Sequence Tuffs includes a number of altered, plagioclase-rich tuffs that lie
stratigraphically below the Sifi Tuff.
Sifi Tuff
The Sifi Tuff is the oldest of the glassy tuffs in the Adu-Asa Formation at Gona and is a
critical marker horizon because it is often associated with fossil localities (Figs. 1, 2). Except for
some minor fault repetitions, the Sifi Tuff can be traced along strike from north to south across
much of the Gona project area (Fig. 2). It appears as lenses in fluvial sedimentary rocks in the
southernmost part of the GPRP area, and crops out at the As Bole Dora (ABD), Bodele (BDL),
and Hamadi Das (HMD) groups of fossil sites, as well as near the Escarpment (ESC) fossil
localities (Figs. 1, 6).
Fossil-rich beds lie both above and below the Sifi Tuff. At the ABD sites, fossil-bearing
beds lie below both the Sifi Tuff and a diatomite, whereas at the BDL 2 site the fossils derive
from gravels above the Sifi Tuff. At the HMD sites, fossil-bearing deposits crop out both above
and below the level of the Sifi Tuff. There, the fossils can be traced to the silstones below the
level of the Sifi Tuff, as well as to a gravel unit above.
In the central portion of the GPRP area, sedimentary rocks containing the Sifi Tuff show
evidence for a shift from a lacustrine to a more fluvial environment. Dark, laminated mudstone
and diatomite beds are common in the lower part of the ABD, BDL and HMD stratigraphic
sections, whereas the BDL and HMD sections contain more sandstones and gravels above the
level of the Sifi Tuff. The Sifi Tuff is heavily reworked into lenses, some of which can be 1-2 m
thick. This suggests that the transition from a lacustrine to a fluvial environment was completed
by the time the Sifi Tuff was deposited.
Glass shards are rhyodacitic and typically ~0.5mm in size with A-type morphology,
although some B-type shards are present (Fig. 3). Glass in the Sifi Tuff is distinguished by a
CaO content of 0.4-0.5%, an MnO content of 0.06-0.08%, and a K20 content of ~2.5% (Fig. 5;
Table 4). We were not able to identify a homogenous population of phenocrysts, so the Sifi Tuff
is not suitable for radiometric dating.
Kobo’o Tuff
The Kobo’o Tuff crops out in fluvial sedimentary rocks in the northern half of the project
area, although it may be present along strike in areas not well surveyed to the south (Fig. 2). The
Kobo’o Tuff, too, is reworked and varies in thickness from 0.5 m to ~5 m in paleochannels. In
common with the Sifi Tuff, the Kobo’o Tuff is repeated by normal faults, with displacements
generally less than 1 km. Whereas the Kobo’o Tuff has only been identified at one fossil site,
ESC 9 (Figure 6), this tuff is repeatedly found near the ESC cluster of sites (Fig. 1, 2). Even at
ESC 9, however, the fossils were not in situ, so it is not possible to determine the exact
relationship between the Kobo’o Tuff and any fossil localities.
The stratigraphic sections containing the Kobo’o Tuff are dominantly fluvial with many
more basalt flows than in the sections containing the Sifi Tuff. In the measured sections
containing the Kobo’o Tuff, sedimentary rocks were typically pale red claystone with
interbedded volcaniclastic sandstone, gravel, and aphanitic basalt lavas.
In outcrop, the Kobo’o Tuff is clearly felsic at the base and strongly bimodal towards the
top. In hand sample, the bimodal portion exhibits a “salt and pepper” appearance, consisting of
approximately 60-75% felsic shards and 25-40% mafic shards. This pattern, as shown in Figure
6, was noted at multiple sample collection sites. In some sample localities, a final felsic layer
caps this felsic to bimodal sequence, but this uppermost felsic layer is not always present. The
striking bimodal nature of the Kobo’o Tuff in outcrop is unique among the tuffs in the Adu-Asa
Formation at Gona. Whereas the Ogoti Tuff also has a mafic component to it, it is not nearly as
obvious in hand sample (see below).
Electron microprobe analysis reveals that the Kobo’o Tuff is actually polymodal, with
two very similar rhyodacite phases in addition to the basaltic/basaltic andesite phase. The major
differences between the two silicic components are the CaO and the FeO contents (Fig. 5; Table
4). In Silicic Mode A, the mean CaO content is 0.65% (n=265), and in Silicic Mode B the mean
is 0.86% (n=93). For the FeO content, Silicic Mode A contains ~2.5% FeO, whereas Silicic
Mode B has ~2.7-2.8% FeO. All other oxides are similar in both modes (Table 4). Silicic Mode
B is concentrated in the same beds as the mafic shards. Silicic Mode A shards are 0.5-1mm in
diameter and are a mix of type A and B morphologies (Fig. 3). Silicic Mode B shards are also
0.5-1mm in diameter but display type D and E morphologies. The mafic shards are up to 1 mm
in size and are dominantly type A morphology.
The Kobo’o Tuff is the only polymodal tephra identified in the Adu-Asa Formation at
Gona (Fig. 5; Table 4). Both Silicic modes contain ~0.22% TiO2 and ~0.11% MnO, which is
unique to the tuffs described here. The mafic component contains ~3% MgO and 7% CaO,
which differs from the mafic component of the Ogoti Tuff.
In contrast to the Sifi Tuff, the Kobo’o Tuff contains a chemically homogenous
population of feldspars with K contents of ~1.4% (Table 5). Sample Gon05 216 (Fig. 7), which
contained anorthoclase, returned an 40Ar/39Ar age of 5.45 ± 0.07 Ma at the 2s level (Table 3; Fig.
8).
Belewa Tuff
The Belewa Tuff is known from two localities only (Fig. 2). The first comprises 10m of
tephra interbedded with pinkish siltstone, sandstone, and gravel (Fig. 9). The tuff at this locality,
from which sample Gon05 262 was collected, contains abundant perlitic obsidian clasts, lapillisize pumice pieces, and ash containing glass shards and sanidine. Individual shards are around 1
mm in diameter, and associated pumice is 1-2 cm. Morphologically, shards were a mix of Btype with some A-type grains (Fig. 3).
The second locality, where sample Gon05 265 was collected, is much finer grained than
its chemical correlate Gon05 262. Sample Gon05 265 is 1-2 m thick and contains coarse ash,
perlitic obsidian fragments, and sanidine phenocrysts. Glass shards in this sample were typically
1 mm in diameter, and perlitic obsidian fragments up to 1mm in diameter were also present.
Shard morphologies in this outcrop are dominantly B-type, with a small amount of A- and F-type
shards (Fig. 3).
In the proximal outcrop of the Belewa Tuff, there is some degree of pedogenic
development between a few of the tuffaceous layers (Fig. 9). The degree of pedogenesis is slight
and is indicated primarily by the angular, blocky jointing with slickensides that are prominent in
the claystone units. Nevertheless, this demonstrates time breaks between eruptions of material
that are chemically identical. These younger layers are thinner and finer grained than those at
the base of the outcrop and likely did not spread material far from the source. It is the thick,
lapilli-sized deposits at the base of the first outcrop that likely correlates to the second, more
distal outcrop where Gon05 265 was sampled.
Chemically, the Belewa is distinguished by a CaO content of ~0.23%, which is the lowest
Ca content of any vitric tuff recovered thus far in the entire formation (Fig. 5; Table 4).
An 40Ar/39Ar date on sanidine (with a K20 content of 6-7%) from sample Gon05 265
returned an age of 5.52 ± 0.03 Ma (2s) for the Belewa Tuff (Table 3; Fig. 8).
Ogoti Tuff
The Ogoti Tuff is the youngest of the four major tuffs in the Adu-Asa Formation and the
only one found to contain more than just an ash fall component. Like the other ash fall samples
collected at Gona, the ash fall part of the Ogoti Tuff is interbedded with sedimentary rocks. The
ash flow overlies the glassy obsidian portion of a rhyolite flow, whereas the surge deposit
directly underlies the micropumicious base of a rhyolite flow. This complex is located in the
northern part of the study area (Fig. 1, 2) and defines the only eruptive center we found in the
Adu-Asa Formation.
The Ogoti Tuff is bimodal, with a minor basaltic component (Table 4). The ash fall tuff
and surge deposits contain glass shards 1-2 mm in diameter as well as lapilli-sized pumice and
cm-sized perlitic obsidian fragments. The ash fall deposit has multiple tuffaceous beds separated
by thin silty interbeds, for a total thickness of ~4 m. The surge deposit is approximately 2 m in
thickness, with cross-bedded layers and a channelized distribution of sublayers. The ash flow
tuff contains glass shards 1-2 mm in diameter and blocks of obsidian and pumice up to 25 cm are
common.
Glass in the Ogoti Tuff is characterized by a CaO content similar to the Sifi Tuff
(~0.45%), coupled with a K2O content of ~6% (Fig. 5; Table 4). The mafic component is
distinguished by a CaO content of 9-10%, an MgO content of ~5%, and a mean MnO content of
0.23% (Table 4).
Rhyolitic glass shards in the Ogoti Tuff display both A and B-type morphologies (Fig. 3).
The basaltic component was only identified as pumice clasts within ash fall tuff sample Escash
13 (Table 4). Additional mafic shards are present in the other samples, but have devitrified and
thus were not suitable for analysis.
Euhedral anorthoclase crystals 1-2 mm in diameter are abundant in all phases of the
Ogoti Tuff. An 40Ar/39Ar date on these phenocrysts indicates that the Ogoti Tuff is 5.90 ± 0.11
Ma (2s). However, the age-probability plot is very broad and multi-peaked (Fig. 8), as reflected
in the high MWSD of 15.70 (Table 3). We take this to indicate contamination by older feldspar
populations. The youngest feldspars are around 5.65 Ma, which we view as the maximum age of
the tuff (Fig. 8).
It is important to note that we use the term Ogoti Tuff to include any tuff unit that
displays the characteristic chemical composition, regardless of whether the units are directly
equivalent temporally. This clarification is necessary, as the ash fall, ash flow, and surge
deposits sampled may not have all been deposited at precisely the same time. The obsidian
clasts in the ash flow were chemically identical to the pumice and glass shards in the ash flow,
yet the obsidian clasts had to have been deposited as part of a rhyolite unit prior to being
included in the ash flow. Indeed, obsidian corestones collected from below the ash flow have a
chemical composition indistinguishable from the glass within the pyroclastic units (Table 4).
While this indicates that the various components of the Ogoti Tuff were not deposited at exactly
the same time, the amount of time lapsed was certainly smaller than the error range associated
with even the most precise 40Ar/39Ar date.
Other glassy tuffs
Besides the four major tuffs in the Adu-Asa Formation, we have analyzed three other
geochemically distinct ash fall deposits (Table 4), each of which is known from only a single
outcrop. The paucity of other samples from these tuffs may be due to a lack of survey in the area
and is not necessarily a reflection of the extent of deposits. All three units crop out near the top
of the Adu-Asa Formation, just west of the boundary between the Adu-Asa Formation and the
early Pliocene Sagantole Formation.
Glass composition of sample Gon05 300 is dacitic, while samples Gon05 301 and Gon05
302 are rhyolitic (Fig. 4). All three have an average shard size of approximately 0.5 mm. Gon05
300 and Gon05 301 have dominantly C-type morphology, while Gon05 302 contains A-type
shards with some B-type shards as well (Fig. 3). Sample Gon05 300 is distinguished by a lower
silica content (~68% SiO2) than the other silicic tuffs in the Adu-Asa Formation at Gona, as well
as a higher FeO, CaO, MgO, and MnO contents (Fig. 5; Table 4). Sample Gon05 301, with the
exception of Gon05 300, has the highest CaO content of any of the silicic tuffs in the formation
at 1% CaO (Table 4). Finally, Gon05 301 has a CaO content similar to that of the Sifi and Ogoti
Tuffs (~0.4% CaO); however, the FeO and Al2O3 contents of sample Gon05 301 are higher than
that of either the Sifi or Ogoti Tuffs (Table 4).
Only Gon05 302 contains an obvious population of phenocrysts (Table 5), but the sample
is as of yet undated.
Hamadi Das Crystal-Rich Tuff Sequence
A sequence of plagioclase-rich tuffs, which we refer to as the Hamadi Das Crystal-Rich
Tuff Sequence (HMDS), is found at several locations below the Sifi Tuff. In most cases, the Sifi
Tuff lies 10-30 m above this tuff sequence, such as at the HMD sites (Fig. 6). Many of the tuffs
in this sequence are heavily reworked and may contain non-juvenile populations of plagioclase.
Glass in all HMDS tuffs are too altered for analysis, probably because most of the tuffs in this
part of the Adu-Asa Formation fell into a lake. Without glass analyses, it is impossible to sort
out the exact stratigraphic relationships and correlations for each of these units.
For example, Figure 10 shows a slump deposit disrupting older beds at the BDL 2 fossil
site. The slump deposit, from which sample Gon05 230 was taken, disrupted a thin double layer
from which sample Gon05 229 was collected. Based on the outcrop relationships, the slump
must be younger than the doublet. However, it is possible that the slump contained older
tuffaceous material that was later re-deposited in the slump.
Although these tuffs contain abundant phenocrysts, the low K in the crystals, apparent
recycling of phenocrysts, and/or loss of radiometric Ar hinders precise dating using the 40Ar/39Ar
method. For example, plagioclase from sample Gon05 246, collected from the South Gona
section, returned an 40Ar/39Ar date of 5.73 ± 0.64 Ma (2s) (Table 3; Fig. 6). This determination,
however, was made using only 2 of 15 analyses (Table 3).
Basalts
The basalt flows in the Adu-Asa Formation at Gona are typically blue-gray in color,
holocrystalline, and range in thickness from 1 to 10 m. In general, both the number and
thickness of basalt lava flows increases between the level of the Sifi Tuff and the Kobo’o Tuff
(Fig. 6; see also Quade et al., Fig. 3b, this volume). This trend of increased volcanism and/or
decreased sedimentation continues through to the top of the Adu-Asa Formation at Gona,
although there is a shift to silicic volcanism as represented by the rhyolite dome in the northern
end of the project area (Fig. 2). Although not well surveyed, it appears that below the level of
the HMDS tuffs is another large section of basalt lavas, which appears as an area of high
topographic relief west of the Kasa Gita-Chifra Road and the HMD fossil sites in Figure 1.
Alteration of basalt units can be substantial, especially near faults, with argillization of the matrix
resulting in a friable, sand-like texture. In many cases, relatively unweathered “corestones” can
be found within an otherwise pervasively altered unit.
As many different basaltic lava flows look similar in the field, we have focused on the
tuffs as stratigraphic markers. We nonetheless still sampled many of the basaltic lavas as a
supplement to the geochronological information obtained from the tuffs. Samples Gon05 226
and 227 are from a blue-gray basalt with a fine-grained groundmass and occasional dispersed
plagioclase phenocrysts up to 0.5 cm in diameter (Fig. 11a, b). This unit caps the sedimentary
rocks at the BDL fossil localities and is similar both in description and stratigraphic placement to
basalt sample Gon05 235, which caps the ABD fossil localities (Figs. 1, 2, 6, 11c). Similar bluegray aphanitic units, with no visible phenocrysts reported, are found capping the HMD fossil
sites (Escash 19), underlying the ESC 3 site (Escash 17), and at the base of stratigraphic section
Gon05 219 (Gon05 218 and Gon05 220) (Figs. 1, 2, 6, 11d, e). The presence of a clinker bed
between samples Gon05 218 and Gon05 220 indicates the existence of at least two different
blue-gray aphanitic units (Fig. 6). Although the exact number of basalt units in this part of the
Adu-Asa Formation is not entirely clear, the relationship of these units to the fossil localities is
unambiguous.
We were able to consistently identify one basalt lava flow in the field. This unit is
porphyritic, with numerous plagioclase crystals over 2 cm in length and a black holocrystalline
groundmass. It lies 25-50m above the Kobo’o Tuff in our measured stratigraphic sections (Fig.
6). Consistent identification of this unit is important as it crops out prominently in section above
many of the ESC fossil sites, including ESC 8 (Gon05 213) and the ESC 1, 2, and 3 fossil
localities (Escash 18) (Figs. 1, 2, 11 f, g). Except for ESC 9, the ESC sites are not in section
with any unaltered felsic tuffs, so this porphyritic basalt is the only available stratigraphic marker
that can constrain the age of these sites. In all cases, the ESC sites are stratigraphically below the
porphyritic basalt.
Comparison of electron microprobe analytical conditions
We reanalyzed selected samples of the Kobo’o Tuff using the setup conditions
recommended by Morgan and London (1996) and compared those analyses with the results
obtained using the setup conditions in this study (Tables 1, 2). Although the number of shards
used in this comparison is small, the results highlight important differences in electron
microprobe analytical conditions. For the mafic mode, there is no significant difference between
the two sets of analytical conditions. For the silicic modes, however, the measured amounts of
Na2O and K2O are lower and the measured amounts of SiO2 and Al2O3 are higher for glass shards
analyzed using the setup conditions in this study as compared to the analytical conditions
suggested by Morgan and London (1996). This is a typical pattern during alkali mobilization, as
electron bombardment causes alkalis to shift away from the electron beam and Si and Al
preferentially shift towards the ensuing open space (Hunt and Hill, 1993; Morgan and London,
1996; Nielsen and Sigurdsson, 1981).
While the compositions reported in this study do reflect some alkali mobilization, the
differences are largest in analyses of the Na2O and Al2O3 contents of silicic analyses, and are
only significant at the 2s level for Na2O. This should be taken into account when considering
tephrostratigraphic correlations between these tuffs and those elsewhere in the region.
DISCUSSION
Source of tuffs
In the GPRP area, the only volcanic source thus far identified in the Adu-Asa Formation
was the silicic center in the northernmost part of the project area (Figs. 1, 2). As the Ogoti Tuff
includes an ash flow tuff and surge component, which are primary deposits, the source of this
tuff must be this silicic center.
We can also attribute the Belewa Tuff to this silicic center. Chemically, the Ogoti Tuff is
very similar to the Belewa Tuff. When comparing a sample of the Ogoti Tuff to a sample of the
Belewa Tuff, the average SC=0.84, and one of the sample pairs gives an SC as high as 0.90
(Table 6). Moreover, grain size contrasts between the two outcrops of the Belewa Tuff also
point to a nearby silicic center as the source. The first outcrop sample (Gon05 262) locality
(Figs. 2, 9) of the Belewa Tuff, is much coarser-grained than the outcrop where the second
sample was collected (Gon05 265, Fig. 2), and thus more proximal to the source. Because the
first outcrop is located ~12 km north of the second, more distal outcrop, the source of the Belewa
Tuff must be the rhyolite flow-dome identified in the northern end of the GPRP area (Figs. 1, 2).
Geologic Synthesis
By combining the measured stratigraphic sections in the Adu-Asa Formation with
outcrop patterns of the various volcanic units, we were able to construct a composite
stratigraphic section for the upper part of the Adu-Asa Formation at Gona (Fig. 12). Our
estimate of the composite thickness of the Adu-Asa Formation east of the Kasa Gita-Chifra Road
(Figs. 1, 2) is about 185 m.
Below the level of the Sifi Tuff, the sequence is dominantly lacustrine, as indicated by
the presence of diatomite beds and laminated mudstone. Fluvial deposition had taken over by
the time the Sifi Tuff was erupted, based on the obvious reworking and channelized variations in
thickness of the Sifi Tuff. Dominantly fluvial sedimentation continues through the rest of the
section, as the sedimentary units above the level of the Sifi Tuff are typically red or pinkish
mudstone, interbedded with cross-bedded sandstone and gravel.
As for the volcanic units, the base of the section contains either basalt lava flows or
altered basaltic tuffs. Above the level of the Sifi Tuff, the basalt flows become more voluminous
and numerous, and the tephras shift to a silicic or bimodal rhyolitic-basaltic composition. Near
the top of the Adu-Asa Formation is the rhyolite flow-dome and associated pyroclastic rocks.
We developed a geologic cross section in Figure 13 that reflects all of the
tephrostratigraphic and structural constraints available. This section represents only the top
portion of the Adu-Asa Formation at Gona, as older deposits to the west are mostly unstudied.
However, we observed rhyolite in the north end of the project area directly underlying sediments
of the Sagantole Formation, so we can be sure that this composite section does contain the top of
the formation, at least for the northern end of the project area.
As samples Gon05 300, Gon05 301, and Gon05 302 are tuffs that were encountered once
each, we have not included these units in either the composite stratigraphic section or crosssection. It is likely that these units are younger than the Belewa Tuff, however, because they
crop out to the east of Belewa sample Gon05 265 and 2-3 km west of sedimentary rocks of the
Sagantole Formation. Thus, these units are near the top of the Adu-Asa Formation.
Age of Fossil Localities
The fossil localities in the Adu-Asa Formation at Gona fall into three temporal clusters.
The oldest grouping (Sites ABD 1, 2, HMD 1, 2; Figs. 1, 2, 6, 12) is stratigraphically below the
Sifi Tuff, the middle cluster is above the Sifi Tuff (Sites HMD 2, BDL 1, 2; Figs. 1, 2, 6, 12),
and the third and youngest cluster (Sites ESC 1, 2, 3, 8, and 9; Figs. 1, 2, 6, 12) is around the
level of the Kobo’o Tuff and the porphyritic basalt.
Sites included in the first and second temporal clusters crop out directly in section with
the Sifi Tuff. The ABD 1, 2, HMD 1, and HMD 2 sites contain fossil-bearing deposits that are
below the level of the Sifi Tuff. At the ABD sites, the fossils are confined to the sedimentary
rocks below the diatomite layer (Fig. 6), although given the low topographic relief at these sites
it is has not been possible to determine exactly which stratigraphic layer contains the fossils. At
the HMD sites, fossils have been traced to a siltstone ~7 m below the level of the Sifi Tuff (Fig.
6). However, at HMD 2, fossils also crop out in the gravel unit ~14 m above the level of the Sifi
Tuff (Fig. 6). At the BDL sites, the gravels and sands ~ 6 m above the level of the Sifi Tuff
contain the fossils (Fig. 6). It is these fossil-bearing gravel and sandstone units above the Sifi
Tuff that comprise the second temporal cluster.
The HEN 1 site was not observed directly in section with any of the major stratigraphic
markers discussed here. However, an altered, plagioclase-rich tuff (Gon05 261) was collected
near HEN 1 and the composition of this tuff is similar to the HMDS tuffs (Table 5; Figs. 1, 2).
Thus, the fossils from HEN 1 may be similar in age to the oldest temporal cluster of sites.
The third and youngest temporal cluster of sites includes the ESC 1, 2, 3, 8, and 9 fossil
localities. These sites are associated with the porphyritic basalt and/or the Kobo’o Tuff and are
younger than the sites associated with the Sifi Tuff (Figs. 1, 2, 6, 12). In all cases, sites included
in this third cluster are below the level of the porphyritic basalt. We observed this relationship
directly for sites ESC 1, 2, 3, and 8. ESC 9 is associated with the Kobo’o Tuff, which we have
observed in two measured sections to be below the level of the porphyritic basalt (Fig. 6), so
ESC 9 must also predate the porphyritic basalt. However, the exact relationship between the
fossils at ESC 9 and the Kobo’o Tuff is not clear, as none of the fossils was found in situ and
may derive from either the siltstone below the Kobo’o Tuff or a sandstone unit above (Fig. 6).
The fossils at ESC 1, 2, and 3 are from a gravel layer below the porphyritic basalt, and at
ESC 3 there is an altered tuff at the base of the stratigraphic section. While we cannot
demonstrate this for sure, it is likely that this altered tuff is the Kobo’o Tuff, which would place
the ESC 1, 2, and 3 sites between the level of the Kobo’o Tuff and the porphyritic basalt.
Multiple outcrops of the Kobo’o Tuff occur near many of the ESC sites (Figs. 1, 2), so it is
plausible that this altered tuff is the Kobo’o Tuff. At ESC 8, the fossil-bearing units are derived
from gravels and are below the porphyritic basalt and above an aphanitic basalt flow. For these
reasons, we have placed the third temporal cluster of sites (specifically, sites ESC 1, 2, 3, and 8)
in a gravel unit at ~83 m on the composite stratigraphic section (Fig. 12). We have included
ESC 9 in the third temporal cluster of sites due to its association with the Kobo’o Tuff, but this
site most likely predates the ESC 1, 2, 3, and 8 sites since the fossils from ESC 9 are within a
few meters of the Kobo’o Tuff, while the rest of these sites are above the level of the Kobo’o
Tuff.
Potential for correlations with other paleoanthropological projects
To date, late Miocene and early Pliocene deposits in this region have only been studied in
the Middle Awash project area (Renne et al., 1999; WoldeGabriel et al., 2001; Kalb et al., 1982).
40
Ar/39Ar dates from the Adu-Asa Formation there are late Miocene in age and thus close to the
age of the Adu-Asa Formation at Gona (WoldeGabriel et al., 2001). In the Middle Awash area, a
tuff near the base of the Sagantole Formation returned an 40Ar/39Ar date of 5.55 ±0.9 Ma (Renne
et al., 1999). Later work in the Middle Awash area constrains the Adu-Asa Formation in that
area to between 5.54 ± 0.17 Ma and 5.77 ± 0.08 Ma, based on 40Ar/39Ar dates on groundmass
from a basaltic lava flow and a basaltic tuff, respectively (WoldeGabriel et al., 2001). However,
tuffs with published descriptions from the Middle Awash area are largely basaltic and thus are
chemically dissimilar to the tuffs characterized here, with one exception. The Witti Tuff is a
bimodal tuff from the Middle Awash that is chemically similar to the Kobo’o Tuff (Table 8).
However, low-K plagioclase from the Witti tuff yielded an average age of 5.6 Ma, whereas the
Kobo’o Tuff contains higher-K anorthoclase and returned a slightly younger date of 5.45 ± 0.07
Ma (2s). In addition, the mafic component of the Witti Tuff is significantly higher in CaO,
MgO, FeO, and TiO2 (Table 8). Although these may not be the same tuffs, they may have come
from the same source. If this is the case, then it is likely that the Adu-Asa Formation at Gona
slightly predates the published portions of the Adu-Asa Formation as described in the Middle
Awash project (WoldeGabriel et al., 2001).
CONCLUSIONS
The Adu-Asa Formation at Gona is ~185 m thick and is composed largely of stacked
basalt flows interbedded with fluviolacustrine sediments and numerous ash fall tuffs. Within the
main sedimentary interval, environments shifted from lacustrine at the base to fluvial above. At
the same time, the composition of the volcanic units also shifted, from basaltic lava flows and
tuffs to a greater component of silicic material.
We have identified seven different silicic, or dominantly silicic, tuffs in the Adu-Asa
Formation at Gona, as well as a series of altered, crystal-rich basaltic tuffs and a distinctive
porphyritic basalt unit. Of the silicic tuffs, four form major stratigraphic markers, which in
conjunction with the crystal-rich sequence of tuffs and the porphyritic basalt, have allowed us to
correlate fossil-bearing deposits and clarify the overall stratigraphy of the deposits in the AduAsa Formation.
As a result of this work, we have determined that the fossil localities in the Adu-Asa
Formation at Gona are grouped into three major clusters, and all date to ~5.5 Ma. The oldest
and middle cluster of sites is associated with the HMDS Tuffs and the Sifi Tuff. The oldest
cluster lies between the HMDS Tuffs and the Sifi Tuff, while the middle cluster is above the
level of the Sifi Tuff. Localities included in these clusters are the ABD, BDL, and HMD groups
of sites. The youngest cluster of fossil localities is associated with the porphyritic basalt unit that
is stratigraphically above the Kobo’o Tuff. This group of sites includes ESC 1, ESC 2, ESC 3,
and ESC 8. Based on the rapid apparent deposition rates of the Adu-Asa Formation, we estimate
the ages of the fossils to be between 5.5-5.6 Ma.
It is likely that the portion of the Adu-Asa Formation described here is younger than the
deposits of the Adu-Asa Formation as described in the Middle Awash study area south of Gona.
A test of this proposal awaits publication of the entire sections of the Sagantole and Adu-Asa
Formation at the Middle Awash and characterization of the tuffs they contain.
ACKNOWLEDGMENTS
We thank K. Schick and N. Toth at the Center for Research into the Archeological
Foundations of Technology (CRAFT) for their support of this project, and Ambacho Kebeda,
Soloman Kebede, Haptewold Habtemichael, and Yonas Beyene for help with permits. We also
thank ARCCH of the Ministry of Youth, Culture, and Sports Affairs of Ethiopia for the field
permit. Financial support was provided by the LSB Leakey Foundation, National Geographic,
Wenner-Gren Foundation, and the National Science Foundation (SBR-9910974 and RHOI). Melanie Everett, Steve Frost, Bill Hart, Lisa Peters, Mike Rogers, and Dietrich Stout are all
warmly acknowledged for all their help and interesting scientific exchanges. Our special thanks
to Asahmed Humet and many other Afars who in various ways facilitated this research. Kleinsasser also thanks the Department of Geosciences at the University of Arizona and the Bert
Butler Foundation for funding, as well as Ken Domanik, Nelia Dunbar, Eric Seedorff, Joaquin
Ruiz, and Christa Placzek.
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TABLE 1. ELECTRON MICROPROBE ANALYTICAL CONDITIONS
Beam size
Accelerating voltage
Current
Time
(m)
(kv)
(nA)
(s)
Na,K
10
15
8
10
Si, Mg,Al,Ca,Mn,Fe,Ti
1-3 (spot)
15
20
20
Na,Si,Al,K
20
15
2
20
Mg,Ca,Mn,Fe,Ti
20
15
20
20
Elements
Condition A
Condition B*
Notes: Unless otherwise noted, all electron microprobe data collected in this study was analyzed using
Condition A.
*Condition B from Morgan and London (1996).
TABLE 2. COMPARISON OF ELECTRON MICROPROBE ANALYTICAL CONDITIONS
Kobo'o Tuff Condition* n Na2O K2O SiO2 MgO Al2O3 CaO MnO FeO TiO2 Total†
Silicic A
AVERAGE
A
263 2.04 1.91 71.54 0.01 12.06 0.65 0.11 2.49 0.22 91.27
σ
0.65 0.22 1.33 0.01 0.32 0.04 0.02 0.11 0.04 1.60
AVERAGE
B
21 3.44 2.10 71.44 0.01 11.56 0.68 0.12 2.51 0.23 92.28
σ
0.19 0.12 1.90 0.01 0.21 0.04 0.02 0.10 0.03 1.96
Silicic B
AVERAGE
A
95 1.88 1.95 70.26 0.03 12.37 0.85 0.12 2.76 0.23 90.70
σ
0.52 0.32 1.32 0.01 0.34 0.04 0.02 0.10 0.05 1.46
AVERAGE
B
3 3.26 2.16 68.64 0.03 11.76 0.87 0.12 2.66 0.25 90.01
σ
0.02 0.12 2.08 0.01 0.13 0.01 0.02 0.10 0.09 2.19
Mafic
AVERAGE
A
36 2.42 1.30 51.77 3.16 12.93 6.98 0.39 13.25 2.85 95.33
σ
0.61 0.08 1.57 0.25 0.29 0.25 0.04 0.76 0.15 1.79
AVERAGE
B
7 2.39 1.39 53.11 2.75 12.34 6.63 0.40 13.24 2.63 95.09
σ
0.63 0.05 2.08 0.34 0.27 0.41 0.05 0.63 0.18 1.79
Notes: Comparison of electron microprobe analytical conditions and their effect on the apparent
composition of samples.
*Conditions A and B given in Table 1
†probe measured total
§summed total of oxides shown
Total§
91.03
1.60
92.08
1.94
90.46
1.45
90.01
2.19
95.06
1.76
94.88
1.79
TABLE 3. 40Ar/39Ar ANALYTICAL DATA
ID
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(x 10 )
39
ArK
-15
(x 10
K/Ca
mol)
40
Age
±1σ
(%)
(Ma)
(Ma)
Ar*
GONO5 216B, Sanidine, J=0.0007451±0.05%, D=1.003±0.001, NM-192K, Lab#=55993
*01
11
15
08
12
13
14
10
16
06
03
07
05
02
04
09
11.39
4.485
4.250
4.303
6.538
5.398
17.58
4.690
4.435
4.309
4.370
5.227
5.500
5.540
7.766
48.17
2.86
5.36
5.38
5.39
5.43
5.43
5.49
5.49
5.54
5.57
5.62
5.84
6.01
6.16
6.50
7.81
1.169939
0.09
0.05
0.09
0.62
0.11
0.19
0.06
0.28
0.07
0.14
0.45
0.48
0.34
0.36
4.15
5.45
0.07
-0.59
0.72
1.51
2.64
3.04
3.75
3.90
4.18
4.78
5.34
5.72
7.91
19.27
20.70
21.91
5.73
2.049845
3.13
2.32
0.67
3.11
4.94
0.94
0.72
1.83
0.74
0.32
8.07
2.63
1.23
9.18
.64
GON05 265C, Sanidine, J=0.0007444±0.06%, D=1.003±0.001, NM-192K, Lab#=55994
05
4.674
0.1847
2.530
1.326
2.8
84.3
5.29
01
4.264
0.0356
0.7659
2.683
14.3
94.8
5.42
07
4.242
0.0313
0.5709
1.917
16.3
96.1
5.47
06
4.386
0.0306
1.052
2.712
16.7
93.0
5.47
04
4.273
0.0365
0.6429
1.891
14.0
95.6
5.48
15
4.366
0.0392
0.9537
2.601
13.0
93.6
5.48
02
4.320
0.0375
0.7934
2.867
13.6
94.6
5.48
11
4.284
0.0333
0.6529
1.170
15.3
95.6
5.49
14
4.269
0.0275
0.5639
1.108
18.5
96.2
5.50
09
4.274
0.0317
0.5435
3.524
16.1
96.3
5.52
03
4.312
0.0266
0.6403
7.500
19.2
95.7
5.53
12
4.335
0.0355
0.6779
2.849
14.4
95.4
5.55
08
4.241
0.0235
0.3244
2.212
21.7
97.8
5.56
10
4.271
0.0410
0.3911
1.734
12.4
97.4
5.58
13
5.374
0.0489
4.103
5.029
10.4
77.5
5.59
Mean age ± 2σ
n=14
MSWD=0.57
15.4 ±5.9
5.52
0.069898
0.09
0.05
0.09
0.05
0.09
0.08
0.07
0.08
0.07
0.03
0.08
0.10
0.05
0.05
0.03
Mean age ± 2σ
6.702
0.0505
0.0345
0.0075
0.8835
0.7071
0.1386
0.0345
0.5532
0.0248
0.0087
0.5366
0.4911
0.8506
0.6869
18.04
33.25
1.665
0.8164
0.9858
8.684
4.774
45.68
2.033
1.201
0.5534
0.6106
3.118
3.610
3.452
10.09
148.6
n=7
MSWD=1.32
0.074
1.002
1.855
0.965
0.133
0.752
1.459
1.733
0.306
1.451
0.576
0.181
0.169
0.242
0.239
0.020
0.076
10.1
14.8
67.8
0.58
0.72
3.7
14.8
0.92
20.6
58.5
0.95
1.0
0.60
0.74
0.028
18.6
89.1
94.4
93.2
61.9
74.9
23.3
87.3
93.0
96.3
95.9
83.2
81.3
82.9
62.3
11.9
27.2 ±50.5
GONO5 246, Sanidine, J=0.0007479±0.05%, D=1.003±0.001, NM-192K, Lab#=55991
15
9.681
5.575
35.80
0.042
0.092
-4.5
02
10.41
5.539
34.97
0.032
0.092
5.1
14
38.05
11.22
128.1
0.041
0.045
2.9
12
5.955
1.174
13.87
0.137
0.43
32.8
01
10.53
25.21
35.18
0.032
0.020
21.1
06
4.267
8.794
7.555
0.022
0.058
64.7
13
13.68
7.077
38.54
0.097
0.072
21.0
11
47.59
2.257
151.2
0.373
0.23
6.5
07
7.658
1.347
14.30
0.057
0.38
46.3
10
10.08
3.289
21.63
0.135
0.16
39.3
09
5.868
0.4316
5.602
0.312
1.2
72.4
03
117.3
21.41
383.5
0.013
0.024
4.9
05
13.99
6.844
0.9114
0.043
0.075
102.1
04
18.82
2.344
12.20
0.104
0.22
81.9
08
14.63
14.29
-1.2241
0.011
0.036
110.6
Mean age ± 2σ
n=2
MSWD=0.07
0.60 ±1.64
TABLE 3. 40Ar/39Ar ANALYTICAL DATA
Gon05 271, Sanidine, J=0.00089994±0.05%, D=1.002±0.001, NM-196H, Lab#56258
03
5.389
0.2152
6.493
7.390
2.4
64.7
08
4.893
0.2058
4.736
3.387
2.5
71.7
06
4.713
0.2400
4.017
4.817
2.1
75.2
05
4.059
0.2933
1.705
7.835
1.7
88.2
04
4.546
0.2488
3.205
3.055
2.1
79.6
02
4.138
0.2673
1.787
1.683
1.9
87.8
12
4.281
0.2577
2.199
4.910
2.0
85.3
01
3.680
0.3411
0.1190
1.578
1.5
99.8
09
5.567
0.2299
6.471
3.409
2.2
66.0
07
5.377
0.2534
5.635
4.785
2.0
69.4
13
4.535
0.2175
2.542
4.612
2.3
83.8
14
5.975
0.2776
7.367
3.667
1.8
63.9
15
5.550
0.1207
5.832
4.518
4.2
69.1
11
4.946
0.2900
3.400
2.769
1.8
80.2
10
4.731
0.2390
2.236
1.954
2.1
86.4
Mean age ± 2σ
n=15
MSWD=15.70
2.2 ±1.2
5.65
5.69
5.75
5.80
5.86
5.88
5.92
5.95
5.95
6.05
6.16
6.19
6.22
6.42
6.62
5.90
0.04
0.07
0.05
0.03
0.07
0.11
0.05
0.10
0.07
0.05
0.05
0.08
0.07
0.08
0.11
0.11
Notes: Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for
interfering reactions.
Errors quoted for individual analyses include analytical error only, without interfering reaction or J
uncertainties.
Mean age is weighted mean age of Taylor (1982). Mean age error is weighted error of the mean (Taylor,
1982), multiplied by the root of the MSWD where MSWD>1, and also incorporates uncertainty in J factors and
irradiation correction uncertainties.
Decay constants and isotopic abundances after Steiger and Jäger (1977).
*Analyses in italics are excluded from calculations.
Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma.
Decay Constant (LambdaK (total)) = 5.543e-10/a
Correction factors for Gon05 216b, Gon05 Gon05 246, and Gon05 265c:
(39Ar/37Ar)Ca = 0.0007 ± 5e-05
(36Ar/37Ar)Ca = 0.00028 ± 1e-05
(38Ar/39Ar)K = 0.0125
(40Ar/39Ar)K = 0 ± 0.0004
Correction factors for Gon05 271:
(39Ar/37Ar)Ca = 0.000676 ± 4e-06
(36Ar/37Ar)Ca = 0.000277 ± 2e-06
(38Ar/39Ar)K = 0.0126
(40Ar/39Ar)K = 0 ± 0.0004
TABLE 4. SUMMARY OF ADU-ASA FORMATION GLASS ANALYSES
Number
Marker tuff
Major element composition (weight percent)
Sample number
of
shards
Na2O
SiO2
ZrO2
23
16
20
17
19
21
16
19
15
8
20
19
20
19
2.61
2.59
2.48
2.37
2.34
2.41
2.29
2.28
2.69
2.61
1.96
2.68
1.23
1.32
2.99
2.99
2.44
2.38
2.57
2.94
2.98
2.91
2.98
4.03
2.82
1.82
2.72
2.26
73.18
72.99
73.64
72.98
73.41
73.25
72.82
73.39
72.58
75.88
72.75
72.23
73.42
73.74
MgO
0.01
0.01
0.03
0.02
0.01
0.01
0.01
0.01
0.02
0.09
0.01
0.01
0.01
0.01
Al2O3
Gonnl 59
Gonnl 61
Gon05 215a
Gon05 215b
Gon05 228
Gon05 231b
Gon05 231c
Gon05 231d
Gon05 234b
Gon05 234c
Gon05 238
Gon05 243
Gon05 251a
Gon05 251b
F
0.12
0.07
0.04
0.04
0.06
0.08
0.06
0.06
0.05
0.06
0.03
0.05
0.02
0.03
K2O
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
Sifi
11.87
11.88
12.05
12.31
12.01
11.67
11.63
11.61
11.50
12.14
11.88
11.91
11.94
12.01
0.04
0.05
0.05
0.05
0.04
0.04
0.04
0.03
0.06
0.06
0.05
0.05
0.05
0.04
CaO
0.47
0.46
0.41
0.36
0.45
0.47
0.46
0.47
0.45
0.42
0.49
0.48
0.49
0.49
Cl MnO
0.03 0.07
0.04 0.06
0.04 0.06
0.04 0.06
0.04 0.07
0.04 0.08
0.04 0.06
0.03 0.07
0.03 0.06
0.02 0.08
0.03 0.08
0.03 0.07
0.03 0.09
0.03 0.08
FeO
1.84
1.85
1.74
1.73
1.84
1.85
1.85
1.86
1.88
1.57
1.94
1.93
1.95
1.98
0.16
0.17
0.14
0.14
0.17
0.18
0.17
0.17
0.19
0.28
0.19
0.19
0.19
0.18
TiO2 BaO
0.04
0.04
0.06
0.02
0.05
0.05
0.05
0.05
0.04
0.07
0.05
0.05
0.06
0.06
Total
93.44
93.21
93.18
92.50
93.05
93.05
92.46
92.96
92.52
97.30
92.30
91.50
92.19
92.21
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Kobo'o (Silicic A)
Escash 11a
Gonnl 52
Gonnl 53
Gon05 216a
Gon05 216b
Gon05 216d
Gon05 219a
Gon05 219b
Gon05 224a
Gon05 224b
Gon05 225
Gon05 257a
Gon05 257b
Gon05 258
21
19
17
9
10
16
17
20
18
17
18
40
20
17
2.24
2.05
2.54
2.07
2.08
2.31
0.82
0.86
1.57
1.72
1.76
2.86
2.62
2.22
0.05
0
0
0.07
0.01
0.04
0.04
0
0
0.07
0.02
0.04
0.03
0.05
2.26
2.05
1.77
1.94
1.86
2.26
1.90
1.93
1.91
2.04
1.56
1.75
1.71
1.99
70.32
70.26
70.12
72.36
71.98
73.29
71.77
71.62
71.19
71.29
71.27
72.25
72.82
71.10
0.01
0.01
0.00
0.02
0.02
0.01
0.02
0.02
0.02
0.02
0.02
0.01
0.01
0.01
11.48
12.02
12.23
12.10
11.95
12.18
12.13
12.15
12.12
12.11
12.06
12.01
12.14
12.21
0.09
0.06
0.07
0.08
0.08
0.07
0.08
0.09
0.07
0.07
0.08
0.07
0.07
0.07
0.69
0.66
0.61
0.70
0.69
0.62
0.68
0.71
0.67
0.65
0.69
0.61
0.63
0.62
0.04
0.04
0.05
0.04
0.05
0.06
0.04
0.05
0.04
0.04
0.05
0.06
0.07
0.06
0.13
0.11
0.10
0.12
0.11
0.09
0.11
0.12
0.11
0.11
0.12
0.10
0.10
0.10
2.51
2.50
2.42
2.60
2.66
2.45
2.52
2.65
2.49
2.49
2.53
2.43
2.45
2.40
0.22
0.23
0.21
0.25
0.22
0.22
0.21
0.23
0.23
0.23
0.23
0.20
0.21
0.23
0.08
0.04
0.06
0.04
0.05
0.07
0.08
0.06
0.09
0.06
0.07
0.08
0.08
0.06
90.11
90.08
90.23
92.39
91.76
93.68
90.41
90.54
90.53
90.91
90.46
92.46
92.94
91.11
Kobo'o (Silicic B)
Kobo'o (Silicic B)
Kobo'o (Silicic B)
Kobo'o (Silicic B)
Escash 10
Escash 11b
Gonnl 50
Gon05 216c
13
29
29
17
1.99
1.90
1.68
2.11
0.03
0.05
0.07
0.04
1.70
2.19
1.71
2.23
69.27
70.12
70.40
70.99
0.03
0.03
0.03
0.03
11.99
12.52
12.49
12.33
0.07
0.08
0.08
0.07
0.85
0.86
0.86
0.86
0.04
0.05
0.05
0.05
0.11
0.12
0.12
0.11
2.81
2.72
2.77
2.84
0.23
0.23
0.23
0.24
0.07
0.05
0.07
0.07
89.17
90.94
90.55
91.98
Kobo'o (Mafic)
Kobo'o (Mafic)
Escash 11b
Gon05 219c
12
16
2.11
2.52
0.13 1.26
0.05 1.33
52.70
50.95
3.34
2.94
12.98
12.79
0.15
0.16
6.99 0.02
6.92 0.02
0.36
0.42
12.58 2.94 0.00 95.55
13.69 2.78 0.00 94.58
Gon05 262a1
Gon05 262a2
Gon05 262b
Gon05 262b obs
Gon05 262c
Gon05 262d
Gon05 262d obs
Gon05 262e
Gon05 262f
Gon05 262f obs
Gon05 262h
Gon05 262h obs
Gon05 265a
Gon05 265b
Gon05 265c obs
15
13
13
15
16
32
16
15
15
12
14
17
20
20
21
1.69
1.52
2.06
2.48
1.76
1.82
2.20
1.70
1.66
2.20
1.84
2.45
1.94
2.01
1.40
0.05
0.08
0.07
0.09
0.07
0.10
0.04
0.07
0.08
0.08
0.03
0.08
0.04
0.06
0.07
5.43
5.10
5.65
5.58
5.53
5.67
5.77
5.38
5.41
5.68
5.64
5.82
4.91
5.17
5.24
74.85
75.31
74.83
75.53
74.93
74.30
75.10
75.74
75.58
75.34
75.06
74.93
72.20
72.37
73.66
0.01
0.01
0.01
0.01
0.01
0.01
0.00
0.01
0.01
0.01
0.02
0.01
0.00
0.00
0.00
11.33
11.38
11.44
11.63
11.40
11.42
11.46
11.48
11.46
11.46
11.74
11.71
10.92
10.87
11.29
0.10
0.09
0.08
0.05
0.06
0.08
0.07
0.07
0.07
0.07
0.06
0.05
0.08
0.07
0.07
0.24
0.23
0.21
0.23
0.22
0.23
0.22
0.24
0.23
0.23
0.28
0.26
0.21
0.21
0.21
0.06
0.05
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.05
0.06
0.06
0.06
0.06
0.03
0.03
0.03
0.03
0.02
0.03
0.02
0.03
0.03
0.02
0.03
0.04
0.04
0.03
0.02
2.05
2.02
1.81
1.77
1.75
1.89
1.81
2.01
1.82
1.91
1.94
1.90
1.78
1.74
1.76
0.18
0.17
0.15
0.15
0.15
0.16
0.15
0.18
0.15
0.16
0.18
0.17
0.14
0.13
0.15
0.03
0.03
0.02
0.02
0.01
0.02
0.02
0.02
0.00
0.03
0.03
0.01
0.03
0.01
0.02
96.06
96.02
96.43
97.64
95.97
95.78
96.94
96.99
96.55
97.26
96.91
97.48
92.36
92.74
93.98
Escash 13 F1
Escash 13 M2
Escash 13 G1
Escash 13 F3
Gon05 273b
Gon05 273c
21
8
18
15
20
17
1.56
2.05
2.07
1.64
1.87
1.92
0.08
0.07
0.13
0.06
0.07
0.09
5.37
6.36
6.19
5.57
5.83
6.12
73.51
73.11
73.34
73.71
73.00
73.71
0.02
0.02
0.02
0.02
0.01
0.01
11.43
11.30
12.04
11.99
12.08
12.44
0.02
0.03
0.04
0.03
0.03
0.04
0.44
0.44
0.45
0.45
0.41
0.47
0.04
0.05
0.05
0.04
0.07
0.06
0.04
0.05
0.04
0.04
0.04
0.05
1.57
1.65
1.65
1.63
1.78
1.95
0.14
0.16
0.15
0.15
0.15
0.17
0.05
0.05
0.03
0.06
0.05
0.06
94.29
95.34
96.20
95.39
95.38
97.10
Belewa
Belewa
Belewa
Belewa*
Belewa
Belewa
Belewa*
Belewa
Belewa
Belewa*
Belewa
Belewa*
Belewa
Belewa
Belewa*
Ogoti†
Ogoti†
Ogoti†
Ogoti†
Ogoti
Ogoti
TABLE 4. SUMMARY OF ADU-ASA FORMATION GLASS ANALYSES
Number
Marker tuff
Major element composition (weight percent)
Sample number
of
shards
Ogoti#
Ogoti#
Ogoti†#
Ogoti#
Ogoti#
Ogoti*#
Gon05 283
Gon05 284b
Gon05 284c
Gon05 284d
Gon05 286a
Gon05 286b
15
19
19
20
20
16
1.98
2.05
2.06
1.84
2.26
2.08
0.05
0.09
0.09
0.07
0.05
0.06
6.27
5.90
5.99
5.97
6.07
5.52
74.55
74.35
73.45
72.98
74.91
72.09
0.02
0.02
0.02
0.02
0.03
0.04
11.84
12.12
12.03
11.98
12.13
11.83
0.03
0.03
0.02
0.03
0.03
0.04
0.40
0.48
0.44
0.43
0.47
0.48
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.05
0.04
1.56
1.76
1.66
1.71
1.73
1.74
Ogoti†
Escash 13 M2
13
2.28
0.07 0.82
50.45
5.05
12.94
0.02
9.77 0.02
0.23
12.49 2.52 0.01 96.71
Obsidian §
Obsidian §
Obsidian §
Obsidian §
Obsidian §
Gon05 270
Gon05 272
Gon05 281
Gon05 285
Gon05 287
13
18
20
13
20
2.08
2.90
2.49
2.43
2.35
0.12
0.10
0.08
0.07
0.08
6.48
5.39
5.70
5.99
5.90
74.26
72.61
70.90
71.93
72.13
0.02
0.02
0.03
0.15
0.03
12.66
11.39
11.94
11.57
11.32
0.04
0.04
0.04
0.04
0.03
0.66
0.40
0.56
0.57
0.42
0.03
0.04
0.04
0.04
0.04
0.03
0.04
0.04
0.04
0.04
0.79
1.67
1.88
1.85
1.50
0.17
0.12
0.17
0.25
0.13
Unnamed 1
Unnamed 2
Unnamed 3
Gon05 300
Gon05 301
Gon05 302
11
18
12
1.69
3.20
2.74
0.05 1.92
0.08 3.88
0.09 3.64
67.57
71.90
72.79
0.31
0.02
0.02
13.02
13.09
12.22
0.09
0.03
0.04
1.29 0.05
1.02 0.04
0.41 0.12
0.13
0.06
0.06
3.34
2.00
2.10
0.39 0.04 89.88
0.13 0.06 95.51
0.12 0.04 94.40
Na2O F
K2O SiO2
MgO
Al2O3
ZrO2
CaO Cl
MnO
FeO
TiO2
0.13
0.15
0.13
0.13
0.13
0.16
BaO Total
0.02
0.03
0.06
0.07
0.04
0.05
0.06
0.07
0.07
0.03
0.04
96.94
97.06
96.04
95.32
97.93
94.17
97.39
94.79
93.95
94.96
94.02
Notes: Summary of glass analyses from the Adu-Asa Formation at Gona, with total Fe expressed as FeO. Unless otherwise noted, all samples are
of glass shards in ash fall tuffs. Samples were analyzed on a Cameca SX50 electron microprobe at the Lunar and Planetary Laboratory, University of
Arizona using the set-up conditions listed in Table 1. Sample locations are shown in Figure 2.
*Obsidian clast
†Pumice clast
§Glassy rhyolite
#Ash flow tuff or surge deposit
TABLE 5. SUMMARY OF ADU-ASA FORMATION FELDSPAR ANALYSES
Marker tuff/
Locality
Stratigraphic
position
Sample
number
Number
Major element composition (weight percent)
of
MnO FeO TiO2 BaO
Total
Hamadi Das
below Sifi Tuff
Gon05 236a
13
2.05 0.04 0.04 48.06 0.15 32.40 0.00 16.02 0.01 0.01 0.65 0.05 0.02
99.50
Hamadi Das
below Sifi Tuff
Gon05 236b
18
2.57 0.04 0.08 49.21 0.15 30.21 0.00 14.63 0.02 0.01 0.67 0.05 0.03
97.66
Hamadi Das
below Sifi Tuff
Gon05 239
17
5.99 0.02 0.28 56.89 0.04 26.96 0.00
0.01 0.01 0.32 0.05 0.05
99.51
South Gona
above Sifi Tuff
Gon05 244
10
3.84 0.02 0.14 52.81 0.15 29.22 0.00 12.65 0.01 0.01 0.67 0.09 0.02
99.63
South Gona
below Sifi Tuff
Gon05 246
15
5.98 0.05 0.28 57.51 0.03 27.49 0.00
8.90
0.01 0.01 0.35 0.04 0.03 100.70
South Gona
below Sifi Tuff
Gon05 248
15
5.94 0.02 0.28 57.70 0.02 26.65 0.01
8.72
0.01 0.01 0.32 0.04 0.02
99.75
Bodele
below Sifi Tuff
Gon05 229
13
3.76 0.03 0.13 51.70 0.12 29.46 0.00 12.90 0.02 0.01 0.79 0.06 0.02
99.01
Bodele
below Sifi Tuff
Gon05 230
13
6.32 0.03 0.49 57.93 0.02 26.59 0.01
7.99
0.01 0.01 0.39 0.04 0.05
99.90
As Bole Dora
below Sifi Tuff
Gonnl 62
20
7.67 0.03 1.80 63.02 0.02 22.95 0.00
4.07
0.01 0.01 0.55 0.05 0.19 100.38
As Bole Dora
below Sifi Tuff
Gon05 233
18
8.22 0.03 0.58 62.79 0.01 23.66 0.00
4.70
0.01 0.01 0.19 0.02 0.12 100.37
ESC 9
below Kobo'o Tuff
Gon05 217
17
3.02 0.04 0.10 49.91 0.14 30.71 0.01 14.21 0.02 0.01 0.65 0.07 0.04
Kobo'o
Kobo'o Tuff
Gon05 216b
18
8.72 0.03 1.39 65.55 0.00 22.28 0.00
2.69
0.01 0.01 0.27 0.02 0.22 101.20
Belewa
Belewa Tuff
Gon05 262a1
8
6.60 0.08 7.05 66.92 0.00 19.61 0.00
0.10
0.01 0.02 0.22 0.01 0.08 100.69
Belewa
Belewa Tuff
Gon05 262a2
12
6.33 0.03 7.03 66.74 0.00 19.26 0.00
0.11
0.01 0.01 0.22 0.03 0.07
99.84
Belewa
Belewa Tuff
Gon05 262i
6
6.31 0.03 6.62 65.04 0.00 19.53 0.01
0.43
0.01 0.01 0.19 0.01 0.81
99.00
Belewa
Belewa Tuff
Gon05 265c
18
6.60 0.04 6.31 65.26 0.01 19.94 0.01
0.37
0.00 0.01 0.20 0.02 0.40
99.17
Ogoti
Ogoti Tuff
Gon05 271*
17
8.01 0.04 3.43 65.22 0.01 21.18 0.01
1.90
0.00 0.01 0.23 0.01 0.50 100.56
Ogoti
Ogoti Tuff
Gon05 273b
17
7.18 0.06 5.29 65.36 0.01 20.06 0.01
0.76
0.00 0.01 0.20 0.02 0.64
Ogoti
Ogoti Tuff
Gon05 283*
22
7.54 0.04 4.04 65.37 0.00 20.95 0.01
1.44
0.01 0.01 0.23 0.02 0.51 100.17
Ogoti
Ogoti Tuff
Gon05 284a*
17
7.90 0.05 3.84 65.71 0.01 21.69 0.00
1.71
0.01 0.01 0.23 0.01 0.47 101.64
Ogoti
Ogoti Tuff
Gon05 286a*
18
7.96 0.03 3.15 65.73 0.00 21.56 0.00
2.06
0.01 0.00 0.23 0.02 0.46 101.23
Ogoti
Ogoti Tuff
Gon05 286c*
19
8.23 0.06 3.05 63.47 0.01 21.04 0.01
2.18
0.01 0.01 0.25 0.02 0.39
Gon05 302
4
8.46 0.00 3.17 66.99 0.01 20.27 0.01
1.06
0.02 0.01 0.32 0.00 0.41 100.71
Gonnl 64
17
7.84 0.04 1.78 63.42 0.02 22.96 0.01
4.08
0.00 0.01 0.58 0.05 0.20 101.00
Gon05 241
13
4.26 0.02 0.20 51.96 0.12 29.56 0.00 12.45 0.02 0.01 0.82 0.09 0.00
99.53
Gon05 253
13
2.20 0.06 0.05 46.03 0.16 30.69 0.00 16.28 0.00 0.01 0.61 0.05 0.02
96.17
Gon05 259a
4
6.71 0.04 0.38 57.23 0.03 24.78 0.02
7.75
0.00 0.01 0.42 0.02 0.04
97.43
Gon05 259b
16
7.02 0.05 0.45 57.55 0.01 24.49 0.00
7.37
0.00 0.01 0.36 0.02 0.04
97.39
Gon05 261
16
6.76 0.05 0.66 57.11 0.02 24.87 0.00
7.65
0.01 0.01 0.34 0.03 0.07
97.59
Dekora Kante
below Sifi Tuff
grains
Na2O
F
K2O
SiO2 MgO Al2O3 ZrO2
CaO
8.88
Cl
98.90
99.58
98.71
Notes: Summary of feldspar analyses from the Adu-Asa Formation at Gona, with total Fe expressed as FeO. Unless otherwise noted, all samples are of
feldspar crystals in ash fall tuffs. Samples were analysed on a Cameca SX50 electron microprobe at the Lunar and Planetary Laboratory, University of
Arizona using the set-up conditions listed in Table 1. Sample locations are shown in Figure 2.
*Ash flow tuff or surge deposit
Gonnl 61
215a
215b
234b
238
243
251a
251b
Esc. 11a
Gonnl 52
Gonnl 53
216a
216b
216d
219a
219b
224a
224b
257a
Gonnl 59
Gonnl 61
215a
215b
234b
238
243
251a
251b
Esc. 11a
Gonnl 52
Gonnl 53
216a
216b
216d
219a
219b
224a
224b
257a
Esc. 10
Esc. 11b*
Gonnl 50
216c
Esc. 11b*
219c
262a1
262a2
262b
262c
262d
262e
262f
262h
262b obs†
262d obs†
262f obs†
262h obs†
265a
265b
265c obs†
Esc. 13 F1§
Esc. 13 M2*§
Esc. 13 G1§
Esc. 13 F3§
283**
286a**
286b†**
Esc. 13 M2*§
270#
272#
281#
285#
287#
300
301
302
Gonnl 59
TABLE 6. ADU-ASA FORMATION SIMILARITY COEFFICIENTS (GLASS ANALYSES)
1.00
0.99
0.93
0.90
0.97
0.92
0.90
0.87
0.86
0.82
0.79
0.82
0.77
0.78
0.84
0.72
0.70
0.76
0.77
0.82
0.74
0.75
0.71
0.76
0.41
0.43
0.78
0.78
0.78
0.78
0.79
0.79
0.77
0.82
0.81
0.79
0.80
0.83
0.80
0.79
0.77
0.82
0.84
0.84
0.83
0.80
0.86
0.88
0.42
0.75
0.85
0.89
0.85
0.84
0.62
0.82
0.91
1.00
0.92
0.90
0.98
0.92
0.90
0.87
0.86
0.82
0.79
0.82
0.77
0.78
0.84
0.72
0.71
0.76
0.77
0.82
0.74
0.75
0.72
0.77
0.40
0.42
0.78
0.77
0.78
0.77
0.79
0.80
0.77
0.82
0.80
0.79
0.80
0.83
0.80
0.79
0.76
0.82
0.84
0.84
0.83
0.80
0.86
0.88
0.41
0.75
0.85
0.88
0.82
0.83
0.61
0.82
0.91
1.00
0.97
0.92
0.88
0.89
0.84
0.86
0.84
0.80
0.81
0.78
0.78
0.84
0.72
0.71
0.76
0.78
0.81
0.74
0.76
0.72
0.78
0.42
0.43
0.76
0.75
0.79
0.78
0.78
0.77
0.78
0.79
0.81
0.80
0.79
0.82
0.80
0.80
0.77
0.82
0.83
0.84
0.93
0.81
0.85
0.85
0.42
0.72
0.86
0.84
0.79
0.85
0.62
0.79
0.90
1.00
0.89
0.86
0.87
0.82
0.85
0.82
0.79
0.80
0.77
0.78
0.83
0.72
0.71
0.76
0.77
0.79
0.74
0.76
0.72
0.78
0.43
0.44
0.77
0.77
0.80
0.80
0.80
0.78
0.79
0.80
0.83
0.82
0.80
0.83
0.82
0.81
0.78
0.80
0.82
0.83
0.82
0.82
0.83
0.84
0.43
0.72
0.84
0.83
0.79
0.83
0.63
0.78
0.87
1.00
0.91
0.91
0.87
0.85
0.81
0.79
0.82
0.77
0.78
0.84
0.71
0.71
0.76
0.77
0.82
0.85
0.75
0.71
0.76
0.43
0.45
0.79
0.78
0.78
0.77
0.79
0.80
0.77
0.82
0.80
0.79
0.80
0.82
0.80
0.79
0.76
0.82
0.83
0.84
0.83
0.80
0.86
0.86
0.44
0.73
0.87
0.87
0.86
0.84
0.64
0.83
0.92
1.00
0.90
0.94
0.91
0.84
0.84
0.83
0.83
0.83
0.88
0.77
0.76
0.82
0.84
0.83
0.79
0.81
0.78
0.81
0.44
0.42
0.80
0.79
0.79
0.77
0.80
0.81
0.77
0.82
0.77
0.78
0.79
0.79
0.81
0.79
0.76
0.82
0.84
0.84
0.83
0.81
0.83
0.84
0.43
0.78
0.79
0.85
0.83
0.80
0.67
0.79
0.86
1.00
0.86
0.89
0.83
0.83
0.90
0.83
0.85
0.87
0.79
0.77
0.82
0.82
0.90
0.81
0.78
0.78
0.79
0.46
0.47
0.74
0.73
0.72
0.71
0.73
0.75
0.71
0.76
0.74
0.73
0.74
0.77
0.74
0.72
0.70
0.76
0.77
0.78
0.77
0.74
0.79
0.81
0.45
0.71
0.80
0.83
0.82
0.77
0.69
0.79
0.85
1.00
0.95
0.80
0.80
0.80
0.79
0.79
0.85
0.81
0.80
0.82
0.82
0.80
0.75
0.78
0.76
0.78
0.40
0.39
0.79
0.80
0.73
0.75
0.75
0.80
0.76
0.79
0.73
0.73
0.74
0.74
0.75
0.73
0.78
0.81
0.78
0.78
0.82
0.75
0.77
0.82
0.40
0.72
0.75
0.80
0.78
0.75
0.66
0.75
0.81
1.00
0.82
0.82
0.81
0.81
0.81
0.86
0.82
0.81
0.84
0.84
0.82
0.77
0.80
0.78
0.80
0.41
0.40
0.78
0.79
0.72
0.74
0.74
0.79
0.76
0.78
0.72
0.72
0.73
0.74
0.74
0.73
0.77
0.81
0.78
0.78
0.81
0.75
0.78
0.81
0.40
0.72
0.74
0.80
0.75
0.74
0.66
0.74
0.81
1.00
0.95
0.90
0.94
0.94
0.94
0.87
0.87
0.91
0.92
0.87
0.89
0.91
0.87
0.92
0.45
0.45
0.71
0.69
0.69
0.68
0.70
0.71
0.68
0.73
0.71
0.70
0.71
0.73
0.71
0.70
0.67
0.71
0.73
0.73
0.72
0.71
0.73
0.77
0.46
0.73
0.72
0.80
0.77
0.73
0.74
0.76
0.78
1.00
0.92
0.97
0.96
0.93
0.90
0.89
0.95
0.97
0.88
0.93
0.93
0.90
0.95
0.47
0.46
0.70
0.67
0.69
0.68
0.70
0.70
0.67
0.71
0.67
0.70
0.71
0.69
0.69
0.70
0.66
0.71
0.73
0.72
0.70
0.70
0.72
0.75
0.47
0.74
0.70
0.76
0.76
0.70
0.77
0.74
0.75
1.00
0.91
0.93
0.93
0.87
0.85
0.91
0.91
0.95
0.89
0.86
0.87
0.87
0.49
0.50
0.69
0.67
0.67
0.66
0.68
0.69
0.66
0.70
0.69
0.68
0.69
0.70
0.68
0.67
0.65
0.69
0.71
0.71
0.70
0.68
0.71
0.74
0.48
0.72
0.73
0.79
0.79
0.71
0.75
0.76
0.79
1.00
0.98
0.91
0.91
0.90
0.95
0.95
0.88
0.93
0.92
0.91
0.96
0.47
0.45
0.69
0.59
0.68
0.67
0.69
0.69
0.67
0.70
0.66
0.69
0.70
0.68
0.68
0.69
0.66
0.70
0.72
0.71
0.69
0.69
0.71
0.74
0.47
0.73
0.69
0.74
0.73
0.69
0.77
0.73
0.86
1.00
0.92
0.91
0.90
0.95
0.94
0.89
0.94
0.91
0.91
0.93
0.46
0.45
0.69
0.67
0.68
0.67
0.69
0.70
0.67
0.70
0.66
0.69
0.70
0.68
0.68
0.69
0.66
0.70
0.72
0.71
0.69
0.70
0.71
0.75
0.46
0.74
0.69
0.75
0.72
0.69
0.74
0.73
0.74
1.00
0.85
0.84
0.90
0.91
0.92
0.87
0.88
0.84
0.90
0.46
0.46
0.72
0.70
0.70
0.69
0.71
0.72
0.69
0.74
0.71
0.71
0.72
0.73
0.71
0.71
0.68
0.73
0.74
0.74
0.73
0.72
0.75
0.78
0.47
0.75
0.73
0.80
0.82
0.74
0.74
0.76
0.80
1.00
0.97
0.93
0.91
0.84
0.86
0.86
0.87
0.86
0.41
0.40
0.67
0.65
0.62
0.63
0.63
0.67
0.64
0.66
0.61
0.62
0.63
0.62
0.63
0.62
0.65
0.68
0.65
0.65
0.67
0.63
0.64
0.67
0.40
0.66
0.63
0.69
0.66
0.62
0.72
0.69
0.68
1.00
0.92
0.90
0.82
0.88
0.88
0.89
0.87
0.41
0.40
0.66
0.65
0.61
0.62
0.62
0.66
0.63
0.65
0.60
0.61
0.62
0.61
0.62
0.61
0.64
0.67
0.64
0.64
0.66
0.62
0.63
0.66
0.40
0.65
0.62
0.67
0.65
0.61
0.73
0.68
0.67
1.00
0.97
0.87
0.90
0.91
0.92
0.90
0.44
0.43
0.72
0.70
0.67
0.69
0.68
0.73
0.70
0.72
0.64
0.66
0.67
0.66
0.68
0.67
0.68
0.73
0.70
0.70
0.73
0.69
0.69
0.72
0.43
0.72
0.67
0.73
0.71
0.67
0.76
0.72
0.72
1.00
0.87
0.90
0.92
0.92
0.92
0.46
0.44
0.72
0.69
0.68
0.70
0.70
0.72
0.70
0.73
0.65
0.68
0.69
0.67
0.69
0.69
0.68
0.73
0.72
0.71
0.72
0.71
0.70
0.74
0.45
0.74
0.68
0.74
0.73
0.68
0.78
0.72
0.74
1.00
0.86
0.82
0.84
0.84
0.47
0.48
0.69
0.67
0.67
0.66
0.67
0.69
0.65
0.70
0.68
0.67
0.68
0.70
0.68
0.67
0.65
0.70
0.71
0.71
0.70
0.68
0.71
0.74
0.46
0.71
0.75
0.79
0.78
0.71
0.72
0.78
0.79
262h obs†
265a
265b
1.00
0.96
0.95
0.91
0.91
0.93
0.92
0.94
0.92
0.94
0.87
0.85
0.84
0.87
0.85
0.83
0.86
0.39
0.75
0.83
0.83
0.82
0.82
0.61
0.74
0.79
285#
262f obs†
262e
1.00
0.94
0.96
0.95
0.95
0.97
0.98
0.95
0.97
0.96
0.94
0.86
0.88
0.87
0.87
0.88
0.86
0.88
0.41
0.76
0.85
0.85
0.85
0.85
0.61
0.73
0.79
281#
302
262d
1.00
0.96
0.94
0.98
0.94
0.96
0.96
0.96
0.92
0.96
0.96
0.96
0.89
0.87
0.87
0.89
0.88
0.86
0.87
0.38
0.75
0.86
0.82
0.82
0.85
0.59
0.72
0.77
272#
262d obs†
262c
1.00
0.97
0.97
0.92
0.96
0.93
0.96
0.99
0.98
0.94
0.97
0.98
0.95
0.85
0.88
0.87
0.86
0.88
0.87
0.89
0.41
0.76
0.86
0.84
0.84
0.86
0.59
0.72
0.78
270#
301
262b
1.00
0.91
0.93
0.93
0.97
0.95
0.93
0.91
0.90
0.92
0.90
0.94
0.91
0.95
0.86
0.82
0.81
0.85
0.82
0.81
0.83
0.37
0.73
0.81
0.81
0.80
0.80
0.58
0.72
0.78
Esc. 13 M2*
262b obs†
262a2
1.00
0.97
0.93
0.95
0.95
0.99
0.96
0.95
0.92
0.92
0.94
0.92
0.94
0.93
0.94
0.86
0.84
0.83
0.86
0.84
0.83
0.85
0.39
0.73
0.84
0.83
0.82
0.82
0.61
0.73
0.80
286b†**
300
262a1
1.00
0.37
0.36
0.39
0.37
0.39
0.38
0.38
0.36
0.39
0.39
0.39
0.38
0.39
0.39
0.38
0.35
0.36
0.36
0.36
0.38
0.36
0.40
0.79
0.37
0.38
0.38
0.40
0.38
0.46
0.40
0.39
286a**
262h
219c
1.00
0.93
0.39
0.38
0.41
0.38
0.41
0.39
0.40
0.38
0.38
0.40
0.41
0.38
0.40
0.42
0.39
0.36
0.38
0.38
0.38
0.40
0.37
0.42
0.82
0.39
0.37
0.37
0.39
0.37
0.47
0.39
0.38
283**
287#
Esc. 11b*
1.00
0.45
0.44
0.68
0.65
0.67
0.66
0.68
0.68
0.66
0.69
0.65
0.68
0.69
0.67
0.67
0.68
0.64
0.68
0.70
0.69
0.68
0.68
0.70
0.72
0.46
0.71
0.68
0.74
0.70
0.68
0.76
0.76
0.73
Esc. 13 F3§
1.00
0.95
0.86
0.87
0.87
0.85
0.87
0.86
0.88
0.42
0.75
0.86
0.83
0.84
0.85
0.59
0.73
0.79
216c
1.00
0.97
0.95
0.85
0.87
0.86
0.85
0.87
0.85
0.87
0.40
0.75
0.85
0.83
0.84
0.84
0.59
0.74
0.80
1.00
0.92
0.46
0.44
0.69
0.66
0.65
0.67
0.67
0.69
0.67
0.70
0.62
0.65
0.65
0.64
0.65
0.65
0.65
0.69
0.68
0.67
0.68
0.67
0.66
0.69
0.44
0.69
0.64
0.71
0.66
0.65
0.78
0.73
0.69
Esc. 13 G1§
1.00
0.93
0.93
0.91
0.85
0.88
0.87
0.86
0.87
0.88
0.90
0.39
0.78
0.88
0.89
0.88
0.89
0.56
0.76
0.82
Gonnl 50
1.00
0.96
0.97
0.97
0.94
0.85
0.88
0.86
0.86
0.87
0.87
0.89
0.42
0.76
0.86
0.86
0.86
0.86
0.59
0.74
0.80
1.00
0.95
0.96
0.45
0.43
0.69
0.66
0.67
0.67
0.69
0.69
0.66
0.70
0.64
0.67
0.68
0.66
0.68
0.68
0.65
0.69
0.70
0.69
0.68
0.69
0.68
0.72
0.44
0.71
0.66
0.73
0.69
0.67
0.77
0.76
0.72
Esc. 13 M2*§
1.00
0.98
0.95
0.96
0.98
0.94
0.85
0.88
0.88
0.86
0.88
0.88
0.89
0.41
0.76
0.86
0.85
0.85
0.87
0.58
0.73
0.79
Esc. 11b*
1.00
0.97
0.97
0.95
0.95
0.96
0.94
0.86
0.87
0.87
0.86
0.87
0.88
0.88
0.39
0.75
0.89
0.86
0.86
0.88
0.56
0.75
0.81
1.00
0.94
0.95
0.96
0.47
0.45
0.67
0.64
0.66
0.65
0.67
0.67
0.64
0.68
0.64
0.67
0.67
0.66
0.66
0.67
0.63
0.67
0.70
0.69
0.67
0.68
0.68
0.72
0.46
0.70
0.66
0.72
0.68
0.66
0.76
0.75
0.70
Esc. 13 F1§
1.00
0.92
0.92
0.94
0.94
0.93
0.92
0.92
0.89
0.88
0.88
0.90
0.89
0.88
0.90
0.38
0.78
0.87
0.88
0.83
0.86
0.58
0.75
0.81
Esc. 10
1.00
0.93
0.95
0.95
0.95
0.91
0.96
0.96
0.98
0.89
0.85
0.85
0.89
0.86
0.84
0.86
0.40
0.74
0.84
0.81
0.82
0.83
0.60
0.72
0.77
Esc. 10
Esc. 11b*
Gonnl 50
216c
Esc. 11b*
219c
262a1
262a2
262b
262c
262d
262e
262f
262h
262b obs†
262d obs†
262f obs†
262h obs†
265a
265b
265c obs†
Esc. 13 F1§
Esc. 13 M2*§
Esc. 13 G1§
Esc. 13 F3§
283**
286a**
286b†**
Esc. 13 M2*§
270#
272#
281#
285#
287#
300
301
302
265c obs†
262f
TABLE 6. ADU-ASA FORMATION SIMILARITY COEFFICIENTS (GLASS ANALYSES), CONTINUED
265c obs†
Esc. 13 F1§
Esc. 13 M2*§
Esc. 13 G1§
Esc. 13 F3§
283**
286a**
286b†**
Esc. 13 M2*§
270#
272#
281#
285#
287#
300
301
302
1.00
0.88
0.84
0.83
0.87
0.84
0.83
0.85
0.38
0.72
0.84
0.80
0.81
0.82
0.58
0.71
0.76
1.00
0.93
0.93
0.98
0.92
0.92
0.92
0.36
0.80
0.90
0.85
0.81
0.92
0.57
0.73
0.82
1.00
0.98
0.93
0.94
0.95
0.94
0.38
0.86
0.90
0.89
0.85
0.94
0.57
0.74
0.82
1.00
0.95
0.95
0.97
0.95
0.38
0.86
0.91
0.89
0.85
0.95
0.56
0.75
0.84
1.00
0.92
0.94
0.94
0.38
0.81
0.90
0.88
0.86
0.91
0.59
0.74
0.83
1.00
0.93
0.91
0.40
0.83
0.90
0.86
0.87
0.95
0.58
0.73
0.82
1.00
0.96
0.38
0.83
0.93
0.90
0.87
0.95
0.54
0.76
0.85
1.00
0.42
0.85
0.91
0.93
0.92
0.92
0.61
0.78
0.84
1.00
0.39
0.38
0.38
0.41
0.39
0.46
0.39
0.89
1.00
0.77
0.85
0.79
0.82
0.57
0.71
0.72
1.00
0.88
0.85
0.93
0.54
0.79
0.89
1.00
0.90
0.89
0.56
0.82
0.85
1.00
0.88
0.62
0.75
0.79
1.00
0.54 1.00
0.75 0.62 1.00
0.85 0.57 0.85 1.00
TABLE 6. ADU-ASA FORMATION SIMILARITY COEFFICIENTS (GLASS ANALYSES), CONTINUED
Notes: Similarity coefficients (SCs) for glass analyses from the Adu-Asa Formation at Gona. SCs of .95 or greater are outlined and SCs between
.92 and .94 are shown in bold. Samples labeled with only a number (e.g. 215a) have had the sample prefix "Gon05" omitted. The prefix "Esc." is
short for Escash. Unless otherwise noted, electron microprobe analyses are of glass shards in ash fall tuffs.
*Sample contains both a felsic and a mafic population. The higher SCs denote the felsic split.
†Obsidian clast
§Pumice lapilli
#Glassy rhyolite
**Ash flow tuff or surge deposit
Gonnl 62 Gonnl 64
Gonnl 62
1.00
Gonnl 64
0.99
1.00
216b
0.84
0.84
217
0.52
0.53
229
0.52
0.53
230
0.71
0.70
233
0.77
0.77
236a
0.51
0.51
236b
0.54
0.55
239
0.65
0.64
244
0.55
0.56
246
0.66
0.65
248
0.66
0.65
253
0.50
0.50
259a
0.79
0.77
259b
0.74
0.73
261
0.74
0.73
262a1
0.60
0.59
262a2
0.59
0.58
262i
0.64
0.63
265c
0.65
0.64
271*
0.74
0.74
273b
0.65
0.64
283*
0.72
0.71
284a*
0.74
0.73
286a*
0.75
0.75
286c*
0.76
0.76
302
0.73
0.72
216b 217
229
TABLE 7. ADU-ASA FORMATION SIMILARITY COEFFICIENTS (FELDSPAR ANALYSES)
230 233 236a 236b 239 244 246 248 253 259a 259b 261 262a1 262a2 262i 265c 271* 273b 283* 284a* 286a* 286c* 302
1.00
0.47
0.48
0.68
0.80
0.45
0.50
0.67
0.50
0.66
0.67
0.44
0.73
0.72
0.74
0.63
0.63
0.67
0.68
0.80
0.69
0.76
0.79
0.81
0.85
0.80
1.00
0.63
0.51
0.80
0.86
0.65
0.93
0.65
0.65
0.79
0.65
0.61
0.57
0.40
0.40
0.44
0.43
0.43
0.41
0.43
0.44
0.43
0.44
0.40
1.00
0.76
0.59
0.63
0.88
0.66
0.89
0.89
0.56
0.95
0.94
0.92
0.55
0.56
0.60
0.60
0.60
0.56
0.60
0.61
0.60
0.61
0.61
1.00
0.89
0.61
0.49
0.90
0.96
0.61
0.92
0.62
0.61
0.89
0.63
0.59
0.55
0.39
0.39
0.43
0.42
0.42
0.39
0.42
0.43
0.42
0.43
0.39
1.00
0.47
0.51
0.71
0.53
0.70
0.71
0.45
0.77
0.80
0.80
0.61
0.61
0.68
0.69
0.73
0.68
0.71
0.73
0.74
0.75
0.68
1.00
0.93
0.58
0.84
0.58
0.58
0.95
0.57
0.57
0.53
0.38
0.38
0.42
0.41
0.41
0.38
0.41
0.41
0.41
0.42
0.38
1.00
0.62
0.90
0.62
0.62
0.92
0.61
0.62
0.57
0.41
0.41
0.46
0.45
0.44
0.42
0.44
0.45
0.45
0.45
0.41
1.00
0.67
0.97
0.99
0.57
0.89
0.85
0.85
0.55
0.56
0.60
0.60
0.59
0.56
0.60
0.61
0.60
0.61
0.62
1.00
0.67
0.63
0.83
0.67
0.64
0.60
0.42
0.43
0.47
0.48
0.45
0.43
0.45
0.46
0.46
0.46
0.42
1.00
0.97
0.58
0.90
0.86
0.86
0.54
0.55
0.59
0.59
0.58
0.58
0.58
0.59
0.59
0.60
0.60
1.00
0.58
0.89
0.86
0.86
0.55
0.56
0.60
0.60
0.59
0.56
0.60
0.61
0.60
0.61
0.62
1.00
0.58
0.55
0.51
0.37
0.37
0.41
0.40
0.40
0.37
0.40
0.41
0.40
0.41
0.37
1.00
0.95
0.95
0.61
0.60
0.66
0.67
0.66
0.63
0.66
0.67
0.67
0.67
0.67
1.00
0.93
0.56
0.55
0.60
0.61
0.64
0.60
0.64
0.65
0.64
0.65
0.64
1.00
0.58
0.57
0.61
0.63
0.64
0.60
0.64
0.65
0.65
0.66
0.66
1.00
0.97
0.90
0.90
0.72
0.79
0.75
0.74
0.71
0.69
0.70
1.00
0.91
0.89
0.71
0.79
0.75
0.74
0.71
0.69
0.69
1.00
0.97
0.77
0.90
0.81
0.79
0.77
0.74
0.75
1.00
0.79
0.91
0.83
0.81
0.78
0.76
0.76
1.00
0.81
0.93
0.96
0.97
0.94
0.87
1.00
0.86
0.84
0.80
0.77
0.81
1.00
0.96
0.91
0.88
0.86
1.00
0.95
0.92
0.86
1.00
0.96
0.87
1.00
0.88 1.00
Notes: Similarity coefficients (SCs) for feldspar analyzes from the Adu-Asa Formation at Gona. SCs of .95 or greater are outlined and SCs of .92 to .94 are shown in bold. Samples labeled with only a
number (e.g. 216b) have had the sample prefix "Gon05" omitted. Unless otherwise noted, electron microprobe analyses are of feldspars in ash fall tuffs.
*Ash flow tuff or surge deposit
TABLE 8. KOBO'O AND WITTI TUFF COMPARISON
Na2O K2O SiO2 MgO
Al2O3
CaO MnO FeO
TiO2
P2O5
Total†
263
95
36
2.04
1.88
2.42
1.91
1.95
1.30
71.54
70.26
51.77
0.01
0.03
3.16
12.06
12.37
12.93
0.65
0.85
6.98
0.11
0.12
0.39
2.49
2.76
13.25
0.22
0.23
2.85
NA*
NA
NA
91.03
90.46
95.06
38
39
2.59
2.30
4.79
1.19
70.00
50.93
0.00
4.36
12.00
12.66
0.90
8.04
0.10
0.24
2.38 0.20
14.11 3.65
n
Kobo'o Tuff
Silicic A
Silicic B
Mafic
Witti Tuff
Silicic
Mafic
0.02 92.97
0.65 98.11
Notes: Comparison of glass analyses on the Kobo'o Tuff at Gona and the Witti Mixed Magmatic Tuff from the
Middle Awash. Analytical conditions for the Kobo'o Tuff are given in Table 1 (Condition A). The Witti Tuff was
analyzed at Los Alamos National Laboratory using a Cameca SX50 electron microprobe with a 15 nA current,
accelerating potential of 15 kV, and a 10 µ beam size. All Fe is expressed as FeO. Data on the Witti Tuff is
from WoldeGabriel et al. (2001).
*NA is not analyzed
†summed total of oxides shown
FIGURE CAPTIONS
Figure 1. Locations of paleontological sites within the Adu-Asa Formation at Gona. Note the
rhyolite dome in the northern end of the project area. Inset shows the location of the GPRP area
within Ethiopia. Paleontological sites in the neighboring Sagantole, Hadar, and Busidima
Formations not shown.
Figure 2. Locations of samples from volcanic rocks (tuffs, flows, pumice blocks, etc) from the
Adu-Asa Formation at Gona. For samples containing only a number, the prefix “Gon05” has
been omitted. In cases where multiple samples were collected from the same locality, we have
omitted markers for altered tuffs, obsidian samples, or basalts to aid clarity. Geochronological
samples from the neighboring Sagantole, Hadar, and Busidima Formations not shown.
Figure 3. Morphological classification of glass shards used in this study: (A-type) frothy glass
shard with intra-shard bubbles; (B-type) glass shard composed of slender, fibrous threads; (Ctype) bubble-wall with stretched glass texture containing side-walls of cylindrical vesicles; (Dtype) platy glass shard as part of a bubble wall much larger than the shard, may contain 1-2
ridges; (E-type) platy glass shard with bubble-wall junctions; (F-type) miscellaneous glass shards
including blocky shards and whole bubbles. In this study, shards identified as F-type are blocky
and thick. Figure modified from Katoh et al. (2000), based on previous studies by Ross (1928),
Heiken (1972), and Yoshikawa (1976).
Figure 4. Total alkali-silica diagram of tephra analyses from the Adu-Asa Formation. Note that
these analyses represent only the vitric ash component of the tuffs. Nomenclature after Best
(2003).
Figure 5 a,b,c,d. Compositional biplots of tuff analyses from the Adu-Asa Formation.
Figure 6. Measured stratigraphic sections containing the Sifi Tuff and the Kobo’o Tuff, with
fossil localities indicated. Scale in meters.
Figure 7. Photograph of the Kobo’o Tuff type locality. Sample Gon05 216b (Fig. 2) from this
outcrop returned an 40Ar/39Ar date of 5.45 ± 0.07 Ma (2σ) (Fig. 8; Table 3). The Kobo’o Tuff
here and elsewhere is bimodal and consists of mainly silicic ash layers at the base and is more
mafic at the top. Person for scale.
Figure 8. Age-probability plot, K/Ca ratio, percent radiogenic Ar, and moles of 39Ar for each
sample dated using the 40Ar/39Ar method (excluding Gon05 246 as this analysis is not reliable).
Points in gray were excluded from the age calculation as they are outliers. See Table 3 for
40
Ar/39Ar analytical data on each crystal.
Figure 9. Photographs (a,b; pencil in c 15 cm) and measured stratigraphic section of the type
locality for the Belewa Tuff and where sample Gon05 262 was collected. Figure 9c is the more
distal correlate where Gon05 265 (Fig. 2), c, was sampled and returned an 40Ar/39Ar date of 5.52
± 0.03 Ma (2σ) (Fig. 8; Table 3).
Figure 10. Photographs of the HMDS Tuffs at the BDL fossil localities (Fig. 1) in lacustrine
mudstone. These phenocryst-rich, altered tuff units occur below both the Sifi Tuff and the level
of the fossils at the BDL sites. Sample Gon05 230 (shown in detail in Figure 10b) is a slump
deposit containing 0.5 cm-scale plagioclase crystals and lapilli-sized pumice pieces. Sample
Gon05 229 comprises a double layer of mm-scale plagioclase that is disrupted by the slump
deposit from which sample Gon05 230 was taken. Hammer for scale in Figure 10a is
approximately 40 cm. End of pencil in Figure 10b is ~1 cm.
Figure 11a-g. Back-scattered electron (BSE) images of basalt samples dated using the 40Ar/39Ar
method. Note the areas of clay alteration in some samples (for example, a. Gon05 226) and the
prominent porphyritic texture in f. Gon05 213.
Figure 12. Composite stratigraphic section of the Adu-Asa Formation at Gona.
Figure 13. East-west composite cross section through the Adu-Asa Formation at Gona. See
Figure 2 for location.
40 20’E
40 15’E
oad
Rhyolite
Dome
Fa
u
ESC-2
lt
ESC-3
ESC-1
ESC-10
Du
m
a
11 15’N
Fo Ha
rm da
at r
io
n
ti R
-Ba
e
l
i
M
ESC-9
As
ESC-8
Kasa Gita-Chifra Road
Bu
sid
ima
HEN-1
Adu-Asa Formation
HMD-1
HMD-2
Busidima
Formation
ABD-1
ABD-2
Sifi
BDL-2
BDL-1
11 05’N
Sagantole
Formation
Kleinsasser et al., Fig. 1
11 10’N
is
Gaw
11 00’N
0
km
Red Sea
300
ve
Ri
h
as
Aw
Addis
Ababa
r
Gona
Ethiopia
10 55’ N
Paleontological Locality
Normal Fault
Formation Boundary
4 km N
40 20’E
40 15’E
11 10’N
258
B’
B
259
Gonnl 53,
257
262
Gonnl 30
261
283
281
Sagantole
Formation
Adu-Asa Formation
Escash 19,
236-241
Gonnl 60
Busidima
Formation
Gonnl 61-62,
233-235
A’
A
11 05’N
286, 287
284, 285
Sifi
Kleinsasser et al., Fig. 2
Kasa Gita-Chifra Road
sid
ima
Fa
ult
Escash 18 Escash 17 273, Escash 13
219
Gonnl 50
C’
271, 272
Escash 10
216, 217
270
Escash 11
C
Gonnl 52, 224
213
225
215 Bu
As
Du
m
a
11 15’N
ad
Fo Ha
rm da
at r
io
n
M
o
ati R
B
e
il
226-231,
Gonnl 59
255
254
251-253
is
Gaw
265
250
301
302
300
11 00’N
Ogoti Tuff
Belewa Tuff
Kobo’o Tuff
Sifi Tuff
Miscellaneous Tuff
Basalt sample
Normal Fault
A
10 55’ N
243-248
Formation Boundary
A’ Line of cross section
4 km N
A-type
B-type
A
A
B
C-type
B
C
C
C
D
D-type
E
E
D
E-type
Kleinsasser et al., Fig. 3
F
F
F
F-type
16
Ogoti Tuff, Silicic
Ogoti Tuff, Mafic
14
Belewa Tuff
Kobo'o Tuff, Silicic A
12
Kobo'o Tuff, Silicic B
Na2O+K2O (wt%)
Kobo'o Tuff, Mafic
Rhyolite
Trachyte
Sifi Tuff
10
Gon05 300
Trachyandesite
Gon05 301
8
Gon05 302
6
Basaltic
Trachyandesite
Trachybasalt
4
2
Basalt
0
37
41
45
49
Basaltic
Andesite
53
57
61
SiO2 (wt%)
Kleinsasser et al., fig. 4
Dacite
Andesite
65
69
73
77
4.0
3.5
FeO (wt%)
3.0
2.5
Ogoti Tuff, Silicic
Belewa Tuff
2.0
Kobo'o Tuff, Silicic A
Kobo'o Tuff, Silicic B
Sifi Tuff
1.5
Gon05 300
Gon05 301
Gon05 302
1.0
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.00
1.20
1.40
1.60
CaO (wt%)
81.0
79.0
77.0
SiO2 (wt%)
75.0
73.0
71.0
69.0
67.0
65.0
0.00
0.20
0.40
0.60
0.80
CaO (wt%)
Kleinsasser et al., Fig. 5a,b
14.0
13.5
13.0
Al2O3 (wt%)
12.5
12.0
11.5
11.0
10.5
10.0
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
CaO (wt%)
0.22
Al2O3 /SiO2
0.20
0.18
0.16
0.14
0.12
0.00
0.20
0.40
0.60
0.80
CaO (wt%)
Kleinsasser et al., fig. 5c,d
1.00
1.20
1.40
1.60
Gon05 219
N
S
75
Kleinsasser et al., Fig. 6
70
*******
*** *
*******
GON05-222
ESC-3
20
65
10
15
GON05-221
ESC 2
GON05-400
10
ESC 1
60
5
10
55
GON05-402
GON05-401 5
ESC 8
5
40
50
0
0
Gon05 224
20
0
15
Escash 17
* **
** *
GON05-213
35
45
**********
*** *
** * *
GON05-403
10
30
40
5
25
35
0
20
30
15
25
Escarpment 9
Kobo’o Tuff
20
GON05-219c
GON05-219b
GON05-219a
10
Kobo’o tuff
10
GON05-224a
5
15
5
10
0
GON05-218
GON05-217
0
N
S
South Gona
5
25
GON05-247
HMD, continued
55
0
Escash 19
GON05-220
20
BDL 2
20
50
15
15
GON05-409
45
ABD 1,2
meters
5
10
10
Sifi Tuff
GON05-234a-d,
412
Sifi Tuff
40
0
35
HMD
GON05-246
GON05-248
GON05-231a-d
5
35
GON05-239
30
30
25
25
20
20
GON05-417
GON05-245
Bodele Tuff
GON05-229,230
green marker bed
GON05-244
GON05-232
0
0
GON05-415
GON05-414
Gravel
Sand
Silt
Paleosol
Diatomite
Tuff
15
GON05-237
10
GON05-236b
GON05-236a
5
**
Basalt
Porphyritic basalt
Carbonate
Sifi Tuff
occurs laterally
Fossil site
Correlation
0
Possible
correlation
GON05-224b
Kobo’o tuff
GON05-216a-d
Bimodal layer
Silicic layer
Kleinsasser et al., Fig. 7
GONO5 216B
Moles 39Ar
(x10-14)
10
% Radiogenic
0.1
0.001
100
50
0
80
K/Ca
40
0
Weighted Mean Age = 5.42±0.07 Ma
Relative Probability
MSWD = 1.41
2
3
4
5
6
7
8
9
Age (Ma)
GON05 265C
Moles 39Ar
(x10-14)
10
% Radiogenic
0.1
0.001
60
K/Ca
20
0
Weighted Mean Age = 5.51±0.03 Ma
Relative Probability
MSWD = 0.73
4.0
4.4
4.8
5.2
5.6
Age (Ma)
6.0
6.4
6.8
GONO5 271
50
0
6
2
K/Ca
% Radiogenic
0.1
100
Moles 39Ar
(x10-14)
1
-2
Weighted Mean Age=5.90±0.11 Ma
Relative Probability
MSWD = 15.7
4.0
4.5
5.0
5.5
6.0
Age (Ma)
6.5
7.0
7.5
8.0
Kleinsasser et al., Fig. 8
100
Belewa Tuff
15
GON05-263
GON05-262i
10
GON05-262h
GON05-262f
GON05-262e
GON05-262d
GON05-262c
a
GON05-262b
GON05-262a1, 262a2
meters
5
0
b
c
Kleinsasser et al., Fig. 9
a
Gon05 230
Gon05 229
Gon05 229
b
Kleinsasser et al., Fig. 10
a. Gon05 226
b. Gon05 227
c. Gon05 235
clay
alteration
100 µm
100 µm
d. Escash 19
f. Gon05 213
e. Escash 17
100 µm
200 µm
200 µm
100 µm
c. Escash 18
Kleinsasser et al., Fig. 11
200 µm
Composite Stratigraphic Section,
Adu-Asa Formation, Gona, Ethiopia
100
95
185
* * * * * ***
* * * * * ** *
* * ** * * * *
* * * ********
180
Porphyritic basalt
90
175
85
Ogoti Ash flow Tuff
170
80
165
Ogoti Ash fall Tuff
75
160
70
65
155
Kobo’o Tuff
5.45 ± 0.07 Ma
150
60
145
55
meters
meters
5.52 ± 0.03 Ma
140
50
45
40
35
Belewa Tuff
135
130
125
Sifi Tuff
120
30
115
HMDS Tuffs
25
110
20
105
15
100
10
5
0
Kleinsasser et al., Fig. 12
Gravel
Silt
Diatomite
Tuff
Basalt
Rhyolite
**** Porphyritic basalt
Fossil site
1 km
SW
1250
1000
750
A
Kleinsasser et al., Fig. 13
meters
* * * * * * * * *
Rhyolite
Ogoti Ash fall Tuff
Belewa Tuff
Porphyritic basalt
Kobo’o Tuff
Sifi Tuff
HMDS Tuffs
Normal Fault
A’
B
Composite Cross Section
Adu-Asa Formation, Gona, Ethiopia
C
B’
*
*
***
***
* **
* * **
**
*
NE
1250
1000
750
C’