- Email: [email protected]
Geochronology and distribution of silicic volcanic rocks of Plio-Pleistocene age from the central sector of the Main Ethiopian Rift
Quaternary International. Vol. 13/14, pp. 6%76, 1992. Printed in Great Britain.
1040-6182/92 $15.00 1992 INQUA/Pergamon Press Ltd
GEOCHRONOLOGY AND DISTRIBUTION O F S I L I C I C V O L C A N I C R O C K S O F P L I O PLEISTOCENE AGE F R O M THE CENTRAL SECTOR OF THE MAIN ETHIOPIAN RIFT
Giday W o l d e G a b r i e l , * R o b e r t C. W a l t e r , t James L. Aronson$ and William K. Hart§ *Earth Environmental Sciences Division, EES-1/D462, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. tGeochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, CA 94709, U.S.A. SDepartment of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, U.S.A. §Department of Geology, Miami University, Oxford, OH 45056, U.S.A.
Plio-Pleistocene silicic volcanoes, calderas, and their eruptive products are common throughout the Main Ethiopian Rift (MER). A compilation of K - A r and ~)Ar/39Ar dating results for the central sector of the M E R demonstrate that the most voluminous and widespread ignimbrites of this region are Pliocene in age, between 4.2 and 3.5 Ma. These units occur in and around: (1) The Awasa and Wagebeta calderas, (2) the Guraghe and Munesa escarpments, where a major buried caldera is believed to occur in the adjacent rift floor, (3) the Addis Ababa-Nazret area, and (4) the foothills and shoulders of the northern sector of the Main Ethiopian Rift. The older units are overlain by less voluminous and, in most cases, rift-bound late Pliocene and Pleistocene pyroclastic rocks. The Pliocene ignimbrites in this area are likely to be the near-source equivalents of widespread ash-fall units associated with fossil hominid deposits of the Turkana and Afar sedimentary basins.
INTRODUCTION The purpose of this report is to document the source, distribution, and age of major Plio-Pleistocene silicic tephras of the central sector of the Main Ethiopian Rift (MER), located between Lake Awasa in the south and Lake Zway in the north (Fig. 1). This study area encompasses the rocks exposed in the rift valley itself, as well as rocks exposed along the escarpments of the rift valley and on the highland regions adjacent to the right margins. The volcanic rocks in the vicinity of the central MER range in age from Miocene to Quaternary. Late Miocene rocks of the Guraghe Mountains consist of basaltic flows (ca. 800 m thick) interbedded with ash-fall deposits, welded ignimbrites, and rhyolitic lavas. This interbedded basalt-rhyolite sequence is overlain by densely welded, crystal-rich Early to Middle Pliocene rhyolitic ignimbrites that may have erupted from calderas within the M E R graben (WoldeGabriel et al., 1990). Proximal silicic deposits throughout the central sector of the MER have been arranged into time-stratigraphic units based on field relationships and K-Ar dating by WoldeGabriel et al., 1990. In this paper, we present new dating results that refine the ages of the silicic pyroclastic rocks from the central sector of the MER. These results can be compared with dates for distal tephra deposits in the Turkana Basin, the Afar Basin, and DSDP cores in the Gulf of Aden and Indian Ocean, for which several chemical correlations have been made (Brown, 1982; Sarna-Wojcicki et al., 1985). For example, the prominent Tulu Bor Tuff from the Koobi Fora Formation of East Turkana, Tuff-B and Tuff U-10 from the Shungura and Usno Formations of Omo Basin, have been chemically correlated with the Sidi Hakoma Tuff 69
(SHT) of the Hadar Formation in the west-central Afar (Brown, 1982; Walter, 1981). Sarna-Wojcicki et al. (1985) recognized a tephra layer in DSDP cores from the Gulf of Aden that matched the SHT-Tulu Bor glass chemistry. The Turkana and Afar Basins are approximately 900 km apart, with the central MER located approximately midway (Fig. 1, inset map).
STRATIGRAPHY A regional stratigraphy for the central sector of the MER has been provided by DiPaola (1972), and studies on specific segments of the central sector of the MER have been made by other investigators, who have defined the major stratigraphic units in the region (Laury and Albritton, 1975; UNDP, 1973). Subsequent detailed fieldwork and geochronologic studies further refined the stratigraphy of the central MER and documented the eruptive history of individual volcanic centers within the region (EIGS, 1985; Mohr et al., 1980; WoldeGabriel et al., 1990). Stratigraphic nomenclature and K-Ar ages for the pyroclastic units in the MER are given in Table 1. The oldest rhyolitic units in the region are silicic centers of Mid-Oligocene to Late Miocene age (31 Ma and 10 Ma) and several tuffaceous units ranging from Mid- to Late Miocene age (ca. 17-8 Ma). These silicic tuffs are intercalated with basaltic flows along the rift escarpments and the shoulders of the central sector of the MER (WoldeGabriel et al., 1990). The Miocene silicic rocks are small in volume when compared with the voluminous basaltic flows. The Plio-Pleistocene units of the central MER are attributed to three time-stratigraphic groups: (1) The Butajira Ignimbrites (4.2-3 Ma), (2) the Chilalo
70
G. WoldeGabriel et al.
3oina
otl . \
42°
Ida Ale Gabilima
~yelu
Bora At Dire Dawa "---~Qa"~.
I ~,fdem
_....Harar~
~ ~/llb. ~ f / ~ O" ~ FantaleW'~umbi / Ambo- Lineament __ Yerer Zuqua~lt'"-'~ . o B ° s e t i , ~ °Id°yi " <0 N a z m t /
GURAGHEZB°raO f
8°
/'
"
~'~fL_.
Ziway
~
A/~O_qil~/ QChilalo~&~ ,, /:~:~" MUNES Wagebeta/ #t"~, G~ • ~ C ab~ - ;'~C~ike(~ H~unkuIo /.N L. Awasa.j ~.,/t /~lc" Kaka kfuul°C)<~ \~'~r~ Damot ~,-~ •,, ~-"~s~'Shashemene ,¢.,<%v /
~
/AoDUgeU~lam
.~
~/~W~ ver
abiShebe,
"~ ] ~
6°
BOUNDARYFAULTS •
[D..[
0 I
100 km
RHYOLITECENTERS CaIderasin mostcases
I
I
-
e~
Q I
TRACHYTECENTERS I
FIG. 1. Distribution of Plio-Pieistocene silicic centers in the central to northern Main Ethiopian Rift and the southern Afar Rift. Many of these centers are associated with one or more caldera features. Major rift-bound calderas are shown by dotted circles. Thick stratigraphic sections investigated for the dispersal of Plio-Pleistocene pyroclastic rocks are: (1) Awash River Gorge; (2) Wabi Shebele Gorge; (3) Ambo fault scarp; and (4) Omo River Gorge (inset map). Broken lines encircling rift-bound lakes indicate existing (Awasa) or inferred calderas.
Trachytes (3-1.6 Ma), and (3) the Wonji Group (< 1.6 Ma), which crop out along the rift escarpments, fault scarps within the rift, and caldera walls (WoldeGabriel et al., 1990). Several prominent sections exposing these units are briefly summarized below. The voluminous Butajira Ignimbrites (4.2-3 Ma), the focus of this study, erupted from the Wagebeta caldera complex of the western rift shoulder just along the eastern slopes of the Omo River, the Awasa Caldera on the rift floor and from inferred calderas buried beneath the ShallaLangano-Abiata Lakes of the rift floor between Chabi and Aluto (Fig. 1, and inset map). Analytical Methods Analytical methods for the K-Ar dating study discussed here are summarized by WoldeGabriel et al. (1990). New laser-fusion 4°Ar/39Ar results were obtained from samples irradiated for 0.5 hr at 8 MW at
the Omega West research reactor of the Los Alamos National Laboratory, which has a fast neutron fluence of 5.7 × 10]3 n cm -1 sec -1. Cadmium shielding was used to reduce the thermal neutron production of 4°Ar. After irradiation, the samples were transferred to a copper holder and loaded onto the extraction line for overnight bakeout at ca. 200°C. Single grains of glass (Munesa Vitrophyre) and single alkali feldspar grains (Butajira Ignimbrites) were fused with an 8 W argonion laser. The volume of the feldspar grains were < 1 x 10-3 cm3, corresponding to a sample mass of < 2.5 mg per grain. Abundances of the Ar isotopes were measured on a MAP-215 noble gas mass spectrometer fitted with a Johnston electron multiplier operating at a gain of about 30,000. The analyses and data collection are fully automated, with one analysis lasting about 20-30 min. Typical system blank volumes of 4°Ar, 39Ar, 3BAr, 37Ar and 36Ar, which were automatically measured
Plio-Pleistocene Silicic Rocks
71
TABLE 1. Stratigraphy of the Central Sector of the Main Ethiopian Rift Aluto Area (Rift floor) Geothermal Well LA-3 (EIGS, 1985) (WoldeGabriel et al., 1990)
Ethiopian Rift Valley 7°-8°40 lat. north and adjacent plateau (Di Paola, 1972)
Central sector of main Ethiopian Rift (WoldeGabrielet al., 1990)
Awasa Caldera: Eastern wall (WoldeGabrielet al., 1990)
Alluvium and lacustrine sediments (recent)
Silicic and mafic rocks of the Wonji Group (~< 1.6 Ma)
Silicicand mafic rocks of the Wonji Group (<~ 1.6 Ma)
Silicic, trachytic and basaltic units of the Wonji Group (~< 1.6 Ma)
Chilalo trachytes and silicic rocks (Late Pliocene, 3-1.6 Ma)
Chilalo trachytes (Trachytictuff) (1.85 Ma)
Chilalo trachyte (Late Pliocene)
Butajira ignimbrites (Pliocene, 4.2-3 Ma) Guraghe basalts plus silicics (Miocene, 11-5 Ma) Shebele trachytes (MidMiocene, 17-12Ma) KeUabasalts (Oligocene, 31-29 Ma) Pre-Tertiary units (Mesozoic and Precambrian)
Butajira ignimbrite (3.55-3.69 Ma) Guraghe basalt and silicicflows (8.7-8.8) (Late Miocene)
Butajira ignimbrite (Early to Mid-Pliocene)
Alkaline and peralkaline silicic flows (Holocene) Trachyte flows and basaltic tuff (Recent Pleistocene) Alkaline and peralkaline silicic rocks (Early Pleistocene-- Late Pliocene) Alkaline and peralkaline silicic rocks (Pliocene) Basalts and ignimbritesplateau trap series (Pliocene-Early Eocene)
every 3 samples, were 4, 2, 0.08, 0.3 and 0.3 x 10 -iv mol, respectively. Sanidine from the Fish Canyon Tuff was used as the neutron fluence monitor, which has a reference age of 27.84 Ma (Deino et al., 1990). Errors for individual analyses (lo) reflect errors in J as well as in the determination of Ar isotopic ratios, which in turn propagate errors in discrimination (D) and errors in Ar beam intensities from sample and blank (Deino et al., 1990). The weighted mean age and uhcertainty, which are the preferred age and error estimates (Table 2), are computed using an inverse variance weighting factor, which uses deviations about a weighted mean to determine the weighted uncertainty (Deino et al., 1990; Samson and Alexander, 1987). The Ca/K ratio of the sample, and an additional tool for compositional correlation, is derived by dividing the measured 37Arca/ 39ARK ratio by 0.51 (Table 2).
Results Samples selected for this study are discussed with respect to their geographic location and geologic context. For example, most of the Pliocene ignimbrites are well exposed on the faulted, uplifted escarpment walls that form the western and eastern margins of the MER. The Pleistocene ignimbrites, on the other hand, are predominantly exposed within the rift valley itself. The composition of the Pliocene ignimbrites on the margins is substantially and characteristically different from the Pleistocene rhyolites in the rift valley (Walter et al., 1985), suggesting that changing tectonic conditions may have influenced the composition of silicic magma generation during the evolution of the MER.
Western Rift Margin at Guraghe The Guraghe Mountains are exposed on the highlands west of the M E R escarpment (Fig. 1). Riftward, step-faulted blocks are composed of Pan-African crystalline basement rocks, Mesozoic sedimentary rocks, Oligocene basalts and voluminous Miocene basalts and interbedded silicic units that are unconformably overlain by the widespread Butajira Ignimbrites (WoldeGabriel et al., 1990). At the Guraghe Escarpment, the Butajira Ignimbrites are dominantly represented by a single 200-250 m thick unit composed of a welded, crystal-rich, ash-flow tuff. This unit, termed the Crystal Tuff, overlies several thin (ca. 5 m), crystal-poor, moderately welded, lithic-rich and localized ignimbrite sheets. The Crystal Tuff is overlain by moderately welded pumiceous and crystal-rich flows. Conventional K - A r ages on several samples from the western escarpment at Guraghe are between 4.2 and 3.5 Ma (WoldeGabriel et al., 1990). New single-crystal laserfusion (SCLF) 4°Ar/39Ar data on sanidine from the dominant Crystal Tuff collected from the Guraghe escarpment are consistent with the K - A r data, and show that the Crystal Tuff erupted about 3.5 Ma (Table 2). Three sanidine separates from the topmost units collected along the Guraghe escarpment yielded K - A r ages of 2.6 and 2.72 Ma and belong to the late Pliocene Chilalo Trachyte units (WoldeGabriel et al., 1990). Eastern E s c a r p m e n t at Munesa The Munesa escarpment is located 75 km across the rift valley, opposite the Guraghe escarpment (Fig. 1). The Munesa escarpment is composed of a moderately welded crystal-rich tuff, petrographically and chemi-
G. WoldeGabriel et al.
72
TABLE 2. Radiometric Ages on Whole Rocks and Feldspars from the Central Sector of the Main Ethiopian Rift I. K/Ar data K:O (wt%) 4°Ar* (10-~1 tool/g)
Sample
Location
Rock Type
ET-1 ET23~ ET23:~ ET-21:~ ET-9:~ BT-755 W83-1 W83-43B
Addis Ababa Munesa Munesa Munesa Guraghe Guraghe Bora ~it Asebot
Tuff Tuff K-feldspar K-feldspar K-feldspar K-feldspar Obsidian Obsidian
Sample
L o c a t i o n Material
89MK-1B BT-87B BT-87B ET-9 BT-75 BT-87A
Guraghe Sanidine M u n e s a Anorthoclase
0.0006 0.0439
9.6111 10.3865
83 97
Guraghe Sanidine G u r a g h e Sanidine M u n e s a Sanidine
0.0049 0.0053 0.0006
1.3436 10.5536 1.3656
97 95 95
ET-21 89W6-1B ET-25 84N-1
Munesa Awasa Guraghe Nazret
0.0028 0.0020 0.0036 0.026
1.3768 10.4513 1.4033 1.4255
95 96 92 90
Sanidine Sanidine Sanidine Anorthoclase
4.55 2.0275 4.60 2.0639 6.68 3.1276 6.72 3.2413 5.95 3.0126 6.45 3.2708 5.67 3.7104 4.57 3.4464 1I. 4°Ar/39Ar single crystal laser-fusion age data Ca/K 4°Ar*/39Ar 4°Ar* (%) 1
4°Ar* (%) 87 78 60 86 79 92 51 84 N
Age (Ma)
1.885 x 10-4 1.83 x 10 4
5 4
3.27 + 0.03 3.40 + 0.07
1.42 X 10 -3 1.83 x 10 4 1.42 X 10 -3
6 5 6
3.44 + 0.02 3.49 + 0.03 3.51 + 0.03
6 1 6 4
3.51 + 0.02 3.55 + 0.08 3.58 + 0.01 3.63 _+ 0.02
1.42 1.89 1.415 1.415
x x × x
10 3 10-4 10 3 10-3
Age (Ma)f 3.10 + 3.1 + 3.25 + 3.35 + 3.5 + 3.50 + 4.53 + 5.23 +
0.14 0.15 0.15 0.15 0.16 0.15 0.23 0.24
K/Ar§ (Ma)
2.83 + 0.17 2.95 + 0.2 WR 3.5 + 0.16 3.5 + 0.15 3.51 + 0.17 3.53 -+ O.2 WR 3.35 + 0.15 3.6 + 0.2 3.32 _+0.0611 3.11 + 0.06¶
*Radiogenic argon. fDetermined from decay constants and isotopic abundance of 4°K according to Steiger and J/iger (1977). ~:Crystal tuff. §K/Ar data (WoldeGabriel et al., 1990). IIFeldspar separate (Morton et al., 1979). ¶Whole rock without feldspars (Morton et al., 1979). N Number of feldspars analyzed.
cally equivalent to the Crystal Tuff exposed at Guraghe (WoldeGabriel, 1987). At Munesa, the Crystal Tuff is more than 300 m thick and forms a single flow unit capped by a thin layer (ca. l m) of densely welded and partially weathered vitric tuff and a basalt flow. Northward the Crystal Tuff is overlain by Pleistocene silicic tuffs of the Wonji G r o u p ( < 1.6 Ma). Several rift-oriented horsts west of Munesa are entirely composed of the Crystal Tuff. New SCLF 4°Ar/39Ar analyses on sanidines from two samples of the Crystal Tuff at Munesa, BT-87A and ET-21, yield ages of 3.51 + 0.03 and 3.51 + 0.02 Ma (Table 2). These samples yielded K - A r dates of 3.53 and 3.35 Ma on whole-rock and alkali feldspar, respectively (Table 2) (WoldeGabriel et al., 1990). The spuriously young K - A r date measured on ET-21 is attributed to incomplete extraction of radiogenic 4°Ar from the sanidine, a problem that can occur in high-K volcanic feldspars (McDowell, 1983). Samples from the graben floor adjacent to the faulted horsts of Crystal Tuff are slightly younger, with apparent ages ranging from 3.48 to 3.19 Ma (WoldeGabriel et al., 1990). Alkali Feldspar and whole-rock K - A r ages of 2.83 Ma and 2.95 Ma were obtained on a glassy ignimbrite, termed the Munesa Vitrophyre, stratigraphically above the Crystal Tuff at Munesa (WoldeGabriel et al., 1990). Laser microprobe 4°Ar/39Ar dating of individual handpicked glass fragments from this unit yield a date of 2.88 + 0.3 Ma, in close agreement with K - A r results
(Table 2). Hand-picked anorthoclase grains from the same sample, however, yield a significantly older SCLF 4°mr/39Ar date of 3.406 + 0.007 Ma (Table 2). The younger anorthoclase K - A r age is likely due to incomplete extraction of 4°Ar, as has been noted for sample ET-21 (Table 2). The discordance between glass dates and SCLF anorthoclase dates may be caused by open-system chemical exchange in the glass, such that K enrichment and/or Ar depletion during diagenesis would clearly lower the measured K - A r age. Therefore, the SCLF anorthoclase age of 3.406 + 0.007, is considered a more reliable estimate for the age of the vitrophyre due to the consistency and quality of the measurements, and the known dependability of alkali feldspar compared to glass as a 4°Ar/39Ar chronomoter (Walter et al., 1990). Eastern R i f t E s c a r p m e n t at A wasa The number and magnitude of fault displacements along the eastern escarpment decreases southward, such that the faults are barely visible in the Shashemene area (Fig. 1). Here, the eastern wall of the Awasa caldera merges with the eastern rift margin, where more than 500 m of Late Miocene (9.7 Ma), Pliocene (3.7-1.9 Ma), and Pleistocene (1.6-0.5 Ma) volcanic rocks are exposed (WoldeGabriel et al., 1990). The section is dominantly composed of Plio-Pleistocene pyroclastic rocks. The Pliocene units are composed of crystal-rich, densely welded tuffs, which are exposed in
Plio-Pleistocene Silicic Rocks an isolated fault block that may be part of the original caldera wall. The basal Pliocene unit in this section, which occurs unconformably above a Late Miocene (9.7 Ma) welded tuff, contains abundant fiamme and minor amounts of basaltic lithic clasts. This basal unit is overlain by a series of cliff-forming, intensely welded turfs (> 75 m thick). A sample from the top of this fault block (89W6-1B) is strongly welded and lithic-free. This isolated fault block does not contain the late Pliocene trachytic and Pleistocene silicic tephras that crop out along the main rift wall just a few kilometers (> 5 km) to the north and east of the block. The Pleistocene rocks of the Awasa-Corbetti calderas belong to the Wonji Group (< 1.6 Ma). The southern wall of the Awasa caldera is less prominent and consists of basalts (1.6 Ma) capped by a crystal-rich welded tuff (1.1 Ma). Along the western wall, a major fault exposes more than 250 m of welded and nonwelded tephra. A welded tuff from the middle of the western section yielded a K-Ar date of 1.27 Ma (WoldeGabriel et al., 1990). The Corbetti caldera walls and the two resurgent domes within the caldera consist of several layers of pyroclastic flows and aphyric and crystal-rich obsidian units. Several samples from sections within the nested Awasa caldera yield K-Ar results ranging from 3.69 to 0.021 Ma (WoldeGabriel et al., 1990). A new SCLF 4°Ar/39Ar date on one of the welded turfs above the 3.69 Ma fiamme-rich welded tuff yielded an age of 3.55 Ma which is within the age range of the Crystal Tuff of the Guraghe and the Munesa sections of the northern part of the central sector. Apparent chemical differences between the 3.55 Ma tuff at Awasa and the Crystal Tuff (WoldeGabriel, 1987) suggest that the two units either erupted contemporaneously from two separate centers or that the chemical variations are due to eruption from a chemically zoned magma system. Western Rift Escarpment at the Wagebeta Calderas A major east-west lineament projects through the southern part of the central sector of the rift, incorporating the Wagebeta calderas of the western rift shoulder north of Sodo and the nested Awasa-Corbetti calderas (Fig. 1) (Merla et al., 1979). Three or more calderas of the Wagebeta caldera complex and associated silicic domes are aligned along this feature, which dissects a narrow volcanic plateau between the rift floor to the east and the deep canyon of the Omo River about 15 km to the west (Fig. 1). The Wagebeta calderas are smaller and older than the Awasa and Corbetti calderas. The main Wagebeta caldera, situated between two smaller east-west aligned calderas, is a well-defined, steep-walled, circular depression at least 5 km in diameter. Feldspar separates from the welded tuff from the eastern caldera wall yielded a K-Ar age of 4.2 Ma, and a rhyolite dome ca. 2 km south of Wagebeta is dated to 3.6 Ma (WoldeGabriel et al., 1990). The southern part of the caldera complex is characterized by rhyolite domes, trachytic lavas, and silicic tephras
73
exposed by faulting and streams that flow into the spectacular and deep (ca. 1.5 km) Omo River Canyon. Discussion The Crystal Tuff of the Butajira Ignimbrite sequence is well exposed at the Guraghe (200--250 m thick) and Munesa (250-300 m thick) escarpments. At Guraghe and Munesa, the Crystal Tuff is correlated on the basis of age, chemistry, and petrography. We suspect that the rift floor adjacent to Guraghe and Munesa, roughly 75 km apart, is part of a major rift-bound caldera now mostly buried beneath younger volcanic and volcaniclastic deposits of the central MER. A conservative estimate of the volume of Crystal Tuff erupted is more than 1100 km 3 (WoldeGabriel et al., 1990), suggesting that the evacuation of such a large volume of magma from beneath the M E R may have induced a phase of rifting 3.5 Ma ago, subsequently destroying the caldera itself. Four sections outside the central sector of the MER have been thoroughly investigated to document the areal distribution of the Crystal Tuff: (1) The Wabi Shebele River Gorge (Fig. 1 and inset map) 50 to 60 km southeast of Munesa contains neither the Crystal Tuff nor any of the other Butajira Ignim-. brites. Instead, the time period of the Butajira Ignimbrites (4.2-3.1 Ma) is represented by an unconformity, where Mid-Miocene (17 to 16 Ma) trachyte and silicic lavas and tephras are overlain by Late Pliocene basalts (2.86 to 2.82) (WoldeGabriel et al., 1990). (2) The Ambo fault escarpment (Fig. 1) about 150 km northwest of the Guraghe section consists of Oligocene basalts capped by lateritized soils. (3) The Omo River Gorge in the southern part of the central sector west of Sodo (Fig. 1, inset map) is more than 1.2 km deep and exposes volcanic rocks ranging in age from 17 Ma to 2 Ma (WoldeGabriel and Aronson, 1987). There, a 30 m thick vitric ash occurs below a 3.97 Ma basalt. The vitric tuff section is 50-60 km south of the Wagebeta caldera complex (Fig. 1), but none of the rocks so far examined from Wagebeta is chemically similar to the > 3.97 Ma vitric tuff in the Omo Gorge. (4) In the northern sector of the Main Ethiopian Rift, the Awash River Gorge (Fig. 1, inset map), cuts through Pliocene basalts (5.6-2.0 Ma) with no tuffaceous units in the time range of the Butajira Ignimbrites (WoldeGabriel, 1987). The absence of the Crystal Tuff in the deep Omo, Wabi Shebeli and Awash gorges suggests that the Butajira Ignimbrites were confined to a pre-existing structural depression, perhaps a precursor to the present-day rift graben (WoldeGabriel et al., 1990). However, a 30-50 m thick crystal-rich tuff occurs on top of the Guraghe Mountains at an elevation of greater than 3400 m a.s.l. (BT-119; Table 3) (WoldeGabriel, 1987), which is identical in age and petrography to the Crystal Tuff on the Munesa and Guraghe escarpments. The absence of the Crystal Tuff in adjacent areas may be due to erosion. Although it is somewhat surprising that the Crystal
74
G. WoldeGabriel et aL
o
=~
:
<~.-
&
,-H m ,-H ,-,.-; 4
.E
~.N ¢.,,1 c,,l ~
o
m
(D t--- ~o
+1 +1 ~
,.~< < ?
~'-~£ .~,~
k~
@
E
+1 +1 +1 +1 +1 o +1 +1 +1 +1 +1 ~ ~ + l + l ~ m ~ ~
~ m
.~
. ~
e-, e.,
o ca
~
a4
+t
+~ +~
o
+1 +1
eg~
e~
o
@
.o
0 e~
e"-
~g
=.= ,
O.ID
~'~<~
+1
+1 +1
ee~
+1
o~o.
g.
7
0 II
7
,..D
+1
0 •
e~
=-6
~
II
.~
+1 +1 + 1 ~ +1 '~
+1~+1
0
+1
Ca E e~ o
o
~
~=~
~
~
.1
e~ <
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
]
I
I
I
I
Plio-Pleistocene Silicic Rocks Tuff is not found in the adjacent plateaus, a geothermal well drilled into the Quaternary caldera of Aluto within the central M E R axis (Fig. 1) bottomed at a depth of 2,143 m (ca. - 2 5 0 m b.s.1.) into a > 500 m thick crystalrich tuff that is petrographically similar to the Crystal Tuff on the adjacent rift escarpments (WoldeGabriel et al., 1990). If this correlation is correct, the difference in elevation between the Crystal Tuff on the escarpments and in the graben suggest a vertical displacement along the rift margins of over 3,000 m. Furthermore, the great thickness of the Crystal Tuff at > 2,000 m depth in the rift floor may represent the confined 'fallback' accumulation of the pyroclastic eruption within the caldera itself. Sedimentary sequences in the west-central Afar Basin of northeastern Ethiopia and the Turkana Basin of southwestern Ethiopia and northern Kenya contain numerous tuffaceous units in the temporal range of the Butajira Ignimbrites and other Pliocene tephras in the vicinity of the M E R (Bro~vn, 1982; Haileab, 1988; Hart et al., in press; McDougall, 1985; Walter and Hart, 1988). The sedimentary basins are fed by the Awash and the O m o Rivers, respectively, and share a narrow drainage divide in the Guraghe Mountains along the western escarpment of the Main Ethiopian Rift (Fig. 1, inset map) (Brown, 1982). The glass chemistry of the 30 m thick vitric tuff in the Omo Gorge, for example, is chemically similar to the Moiti Tuff of the Koobi Fora Formation (Turkana Basin) and to a vitric tuff, VT-1, at the base of the Pliocene section in the Middle Awash deposits (Afar Basin) (Hart et al. in press). The Moiti Tuff is dated to < 4.1 Ma (McDougall, 1985), which is consistent with the estimated age of the vitric tuff in the Omo Gorge. In addition, the Munesa vitrophyre is temporally and chemically equivalent to the SHT-Tulu Bor Tufts in the Hadar Formation and Koobi Fora Formations, respectively (Brown, 1982; Walter and Aronson, submitted; Walter et al., 1990). These examples indicate that the M E R region has played a significant role as the source of tephra deposits in the Awash and Turkana Basins. Several Miocene to Pliocene volcanoes are aligned along the southern margin of the Afar depression. According to Kazmin and Behre (1978), these centers, such as Bora'at and the Gara Adi, Afdem, Asebot, Gumbi and Woldoyi (Fig. 1), formed along the western border fault of an ancestral rift graben that runs parallel and adjacent to the present-day eastern escarpment. Two obsidian samples from Bora'at and Asebot yield K - A r dates of 4.53 and 5.23 Ma, respectively (Table 2), and an age of 6.1 Ma has been reported from the Gara Gumbi silicic center (Kunz et al., 1975). The Nazret Group, mostly composed of pyroclastic rocks, is confined to the eastern part of the northern sector of the Main Ethiopian Rift, and is dated to between 9 and 3 Ma (Kazmin and Behre, 1978; Kazmin et al., 1980). In the vicinity of Addis Ababa, along the western edge of the northern M E R system, several tephra layers occur near trachytic and silicic centers that range from 5.3 to 3.1 Ma (Morton et al., 1979). K - A r dates from the
75
silicic flows of Mount Yerer, about 25 km southeast of Addis Ababa, are between 3.6 and 3.3 Ma (Morton et al., 1979; Morton and Rex, 1975). Morton et al. (1979), described porphyritic obsidians flows collected along the Addis A b a b a - N a z r e t road, southwest of Mount Yerer yielding K - A r dates of 3.11 and 3.32 Ma on a feldspar-free matrix and feldspar separates, respectively. SCLF 4°Ar/39Ar analyses on anorthoclase grains from a porphyritic obsidian recently collected from this locality yields a mean date of 3.63 Ma (Table 2). Silicic rocks near Mount Yerer, south of Addis Ababa, are similar in age to the Crystal Tuff of the Butajira Ignimbrite and the densely welded vitrophyre that occurs in the Munesa section above the Crystal Tuff. CONCLUSION Plio-Pleistocene silicic centers and their eruptive products are widespread throughout the rift systems of Ethiopia, but the central sector of the Main Ethiopian Rift is still regarded as a main source area for many widespread silicic pyroclastic units found in the fossiliferous deposits of the Turkana and Afar ~asins. As shown in this paper and elsewhere (Hart et al. in press; Walter et al., 1985, 1990), proximal pyroclastic units in the Guraghe, Munesa, and Wagebeta areas are temporally and chemically similar to the Moiti Tuff in the Turkana Basin and to the Sidi H a k o m a - T u l u Bor Tufts in the Turkana and Afar Basins. However, the occurrence of contemporaneous silicic eruptions throughout the Ethiopian and Afar Rifts during the Pliocene indicate that more than one source region was active during this time period, and that detailed dating and compositional studies are required in order to fully document the regional temporal and chemical patterns of silicic volcanism in Ethiopia, and to make inferences about the origin of distal tephra deposits in the Turkana and Afar Basins.
ACKNOWLEDGEMENTS Field work for this project was accomplishedunder the aegis of the Ethiopian Institute of Geological Surveys and the Department of Geothermal Energy (Ethiopian Ministry of Mines and Energy). Principle financialsupport for this research was provided by National Science Foundation (Grant No. EAR83-06386) to J.L. Aronson and R.C. Walter. We are grateful to Grant Heiken for his comments on the manuscript. A portion of this work by the first author (GWG) was done under the auspices of the U.S. Department of Energy, Earth and Environmental Sciences Division of the Los Alamos National Laboratory. Typing and editorial work on initial drafts of the manuscript by Barbara Hahn and drafting by Anthony Garcia are greatly appreciated. We wish to thank J. Desmond Clark and Tim White for their continued encouragement and support of this project.
REFERENCES Brown, F.H. (1982). Tulu Bor Tuff at Koobi Fora correlated with the Sidi Hakoma Tuff at Hadar. Nature, 300, 631-633. Deino, A., Tauxe, L., Monaghan, M. and Drake, R. (1990). 4°Ar/ 39At age calibration of the Litho- and Paleomagneticstratigraphies of the Ngorora formation, Kenya. Journal of Geology, 98, 567587.
76
G. WoldeGabriel et al.
REFERENCES Di Paola, G.M. (1972). The Ethiopian Rift Valley (between 7000' and 8040, lat., north). Bulletin Volcanologique, 35, 517-560. EIGS (1985). Geothermal exploration project, Ethiopian Lakes District Rift. Ethiopian Institute of Geological Surveys Technical Report, 1.1-7.1. Feibel, C.S., Brown, F.H. and McDougall, I. (1989). Stratigraphic context of fossil hominids from the Omo group deposits: Northern Turkana Basin, Kenya and Ethiopia. American Journal of Physical Anthropology, 78, 595--622. Haileab, B. (1988). Characterization of tephra from the Shungura Formation, southwestern Ethiopia. University of Utah, M.S. Thesis. Hall, C.M., Walter, R.C., Westgate, J.A. and York, D. (1984). Geochronology, stratigraphy and geochemistry of Cindery Tuff in the Pliocene hominid-bearing sediments of the Middle Awash, Ethiopia. Nature, 308, 26-31. Hart, W.K., Walter, R.C. and WoldeGabriel, G. (1992). Tephra Sources and Correlations in Ethiopia: Application of elemental and neodymium isotope data. Quaternary International, 13/14, 7786. Kazmin, V. and Behre, S.M. (1978). Geology and development of the Nazret area, northern Ethiopian Rift Sheet NC37-15. Addis Ababa, Ethiopia, Ethiopian Mapping Agency. Kazmin, V., Behre, S.M., Nicoletti, M. and Petrucciani, C. (1980). Evolution of the northern part of the Ethiopian Rift. Academia Nazionale dei Lincei, 47, 275-292. Kunz, K., Kreuzer, H. and P, M. (1975). K-Ar determination of the trap basalts of the southeastern part of the Afar Rift. Afar Depression of Ethiopia, 370-374. Laury, R.L. and Albritton, C.C. (1975). Geology of the Middle Stone Age archaeological sites in the Main Ethiopian Rift. Geological Society of America Bulletin, 86, 999-1011. McDougall, I. (1985). K-Ar and 4°Ar/39Ar dating of the hominidbearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya. Geological Society of America Bulletin, 96, 159-175. McDowell, F.W. (1983). K-Ar Dating: Incomplete extraction of radiogenic argon from alkali feldspar. Chemical Geology (Isotope Geoscience), 1, 119-126. Merla, G., Abatte, E., Azzaroli, A., Burni, P., Fazzouli, M., Sagri, M. and Tacconi, P. (1979). Comments and a geological map of Ethiopia and Somalia, Scale 1:2,000,000. Frienze, Italy, Consiglio Nazionale della Recerche. Mohr, P.A., Mitchell, J.G. and Raynolds, R.G.H. (1980). Quaternary volcanism and faulting at Oia caldera, central Ethiopian Rift. Bulletin Volcanologique, 43, 173--189.
Morton, W.H. and Rex, D.C. (1975). Geochronology of the Addis Ababa region. Bulletin of the Geophysical Observatory, Addis Ababa, 15, 156. Morton, W.H., Mitchell, J.G. and Mohr, P. (1979). Rift-ward younging of volcanic units in the Addis Ababa region, Ethiopian Rift Valley. Nature, 280, 284-288. Samson, S.D. and Alexander, E.C. (1987). Calibration of the interlaboratory 4°Ar-39Ar dating standard, MMhb-1. Chemical Geology (Isotope Geoscience Section), 66, 27-34. Sarna-Wojcicki, A.M., Meyer, C.E., Roth, P.H. and Brown, F.H. (1985). Ages of tuff beds at East African early hominid sites and sediments in the Gulf of Aden. Nature, 313, 306-308. Steiger, R.H. and Jager, E. (1977). Subcommission on geochronology: Convention on the use of decay constants in geo and -cosmochronology. Earth and Planetary Science Letters, 36, 359362. UNDP (1973). Geology, Geochemistry, and Hydrology of the Hot Springs of the East African Rift-system within Ethiopia. United Nations, New York. Walter, R.C. (1981). The volcanic history of the Hadar Early Man Site and the surrounding Afar region of Ethiopia. Case Western Reserve University, Ph.D. Dissertation. Walter, R.C. and Aronson, J.L. (1982). Revisions of K/Ar ages for the Hadar hominid site, Ethiopia. Nature, 296, 122-127. Walter, R.C. and Aronson, J.L. (submitted). Age and Source of the Sidi Hakoma Tuff, Hadar Formation, Ethiopia. Geology. Walter, R.C. and Hart, W.K. (1988). Nd isotopes as stratigraphic, provenance, and petrogenetic indicators for rhyolitic tephras. Geological Society of America Abstracts with Programs, 20, A113. Walter, R.C., Westgate, J.A., WoldeGabriel, G., Aronson, J.L. and Hart, W.K. (1985). Geochemical and Temporal Patterns of Felsic Volcanism in Ethiopia. Geological Society of America Abstracts, 17, 743. Walter, R.C., WoldeGabriel, G., Aronson, J.L. and Hart, W.K. (1990). Tephrochronology and source of SHT and Tulu Bor tephras. INQUA Inter-Congress Committee on Tephrochronology
Field Conference and Workshop on Tephrochronology, Abstracts, 49. WoldeGabriel, G. (1987). Volcanotectonic history of the central sector of the main Ethiopian Rift: A geochemical and petrological approach. Case Western Reserve University, Ph.D. Dissertation, WoldeGabriel, G. and Aronson, J.L. (1987). The Chow Bahir Rift: A 'failed' rift in southern Ethiopia. Geology, 15, 430-433. WoldcGabriel, G., Aronson, J.L. and Walter, R.C. (1990). Geology, geochronology, and rift basin development in the central sector of the Main Ethiopian Rift. Geological Society of America Bulletin, 102, 439-458.
1040-6182/92 $15.00 1992 INQUA/Pergamon Press Ltd
GEOCHRONOLOGY AND DISTRIBUTION O F S I L I C I C V O L C A N I C R O C K S O F P L I O PLEISTOCENE AGE F R O M THE CENTRAL SECTOR OF THE MAIN ETHIOPIAN RIFT
Giday W o l d e G a b r i e l , * R o b e r t C. W a l t e r , t James L. Aronson$ and William K. Hart§ *Earth Environmental Sciences Division, EES-1/D462, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. tGeochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, CA 94709, U.S.A. SDepartment of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, U.S.A. §Department of Geology, Miami University, Oxford, OH 45056, U.S.A.
Plio-Pleistocene silicic volcanoes, calderas, and their eruptive products are common throughout the Main Ethiopian Rift (MER). A compilation of K - A r and ~)Ar/39Ar dating results for the central sector of the M E R demonstrate that the most voluminous and widespread ignimbrites of this region are Pliocene in age, between 4.2 and 3.5 Ma. These units occur in and around: (1) The Awasa and Wagebeta calderas, (2) the Guraghe and Munesa escarpments, where a major buried caldera is believed to occur in the adjacent rift floor, (3) the Addis Ababa-Nazret area, and (4) the foothills and shoulders of the northern sector of the Main Ethiopian Rift. The older units are overlain by less voluminous and, in most cases, rift-bound late Pliocene and Pleistocene pyroclastic rocks. The Pliocene ignimbrites in this area are likely to be the near-source equivalents of widespread ash-fall units associated with fossil hominid deposits of the Turkana and Afar sedimentary basins.
INTRODUCTION The purpose of this report is to document the source, distribution, and age of major Plio-Pleistocene silicic tephras of the central sector of the Main Ethiopian Rift (MER), located between Lake Awasa in the south and Lake Zway in the north (Fig. 1). This study area encompasses the rocks exposed in the rift valley itself, as well as rocks exposed along the escarpments of the rift valley and on the highland regions adjacent to the right margins. The volcanic rocks in the vicinity of the central MER range in age from Miocene to Quaternary. Late Miocene rocks of the Guraghe Mountains consist of basaltic flows (ca. 800 m thick) interbedded with ash-fall deposits, welded ignimbrites, and rhyolitic lavas. This interbedded basalt-rhyolite sequence is overlain by densely welded, crystal-rich Early to Middle Pliocene rhyolitic ignimbrites that may have erupted from calderas within the M E R graben (WoldeGabriel et al., 1990). Proximal silicic deposits throughout the central sector of the MER have been arranged into time-stratigraphic units based on field relationships and K-Ar dating by WoldeGabriel et al., 1990. In this paper, we present new dating results that refine the ages of the silicic pyroclastic rocks from the central sector of the MER. These results can be compared with dates for distal tephra deposits in the Turkana Basin, the Afar Basin, and DSDP cores in the Gulf of Aden and Indian Ocean, for which several chemical correlations have been made (Brown, 1982; Sarna-Wojcicki et al., 1985). For example, the prominent Tulu Bor Tuff from the Koobi Fora Formation of East Turkana, Tuff-B and Tuff U-10 from the Shungura and Usno Formations of Omo Basin, have been chemically correlated with the Sidi Hakoma Tuff 69
(SHT) of the Hadar Formation in the west-central Afar (Brown, 1982; Walter, 1981). Sarna-Wojcicki et al. (1985) recognized a tephra layer in DSDP cores from the Gulf of Aden that matched the SHT-Tulu Bor glass chemistry. The Turkana and Afar Basins are approximately 900 km apart, with the central MER located approximately midway (Fig. 1, inset map).
STRATIGRAPHY A regional stratigraphy for the central sector of the MER has been provided by DiPaola (1972), and studies on specific segments of the central sector of the MER have been made by other investigators, who have defined the major stratigraphic units in the region (Laury and Albritton, 1975; UNDP, 1973). Subsequent detailed fieldwork and geochronologic studies further refined the stratigraphy of the central MER and documented the eruptive history of individual volcanic centers within the region (EIGS, 1985; Mohr et al., 1980; WoldeGabriel et al., 1990). Stratigraphic nomenclature and K-Ar ages for the pyroclastic units in the MER are given in Table 1. The oldest rhyolitic units in the region are silicic centers of Mid-Oligocene to Late Miocene age (31 Ma and 10 Ma) and several tuffaceous units ranging from Mid- to Late Miocene age (ca. 17-8 Ma). These silicic tuffs are intercalated with basaltic flows along the rift escarpments and the shoulders of the central sector of the MER (WoldeGabriel et al., 1990). The Miocene silicic rocks are small in volume when compared with the voluminous basaltic flows. The Plio-Pleistocene units of the central MER are attributed to three time-stratigraphic groups: (1) The Butajira Ignimbrites (4.2-3 Ma), (2) the Chilalo
70
G. WoldeGabriel et al.
3oina
otl . \
42°
Ida Ale Gabilima
~yelu
Bora At Dire Dawa "---~Qa"~.
I ~,fdem
_....Harar~
~ ~/llb. ~ f / ~ O" ~ FantaleW'~umbi / Ambo- Lineament __ Yerer Zuqua~lt'"-'~ . o B ° s e t i , ~ °Id°yi " <0 N a z m t /
GURAGHEZB°raO f
8°
/'
"
~'~fL_.
Ziway
~
A/~O_qil~/ QChilalo~&~ ,, /:~:~" MUNES Wagebeta/ #t"~, G~ • ~ C ab~ - ;'~C~ike(~ H~unkuIo /.N L. Awasa.j ~.,/t /~lc" Kaka kfuul°C)<~ \~'~r~ Damot ~,-~ •,, ~-"~s~'Shashemene ,¢.,<%v /
~
/AoDUgeU~lam
.~
~/~W~ ver
abiShebe,
"~ ] ~
6°
BOUNDARYFAULTS •
[D..[
0 I
100 km
RHYOLITECENTERS CaIderasin mostcases
I
I
-
e~
Q I
TRACHYTECENTERS I
FIG. 1. Distribution of Plio-Pieistocene silicic centers in the central to northern Main Ethiopian Rift and the southern Afar Rift. Many of these centers are associated with one or more caldera features. Major rift-bound calderas are shown by dotted circles. Thick stratigraphic sections investigated for the dispersal of Plio-Pleistocene pyroclastic rocks are: (1) Awash River Gorge; (2) Wabi Shebele Gorge; (3) Ambo fault scarp; and (4) Omo River Gorge (inset map). Broken lines encircling rift-bound lakes indicate existing (Awasa) or inferred calderas.
Trachytes (3-1.6 Ma), and (3) the Wonji Group (< 1.6 Ma), which crop out along the rift escarpments, fault scarps within the rift, and caldera walls (WoldeGabriel et al., 1990). Several prominent sections exposing these units are briefly summarized below. The voluminous Butajira Ignimbrites (4.2-3 Ma), the focus of this study, erupted from the Wagebeta caldera complex of the western rift shoulder just along the eastern slopes of the Omo River, the Awasa Caldera on the rift floor and from inferred calderas buried beneath the ShallaLangano-Abiata Lakes of the rift floor between Chabi and Aluto (Fig. 1, and inset map). Analytical Methods Analytical methods for the K-Ar dating study discussed here are summarized by WoldeGabriel et al. (1990). New laser-fusion 4°Ar/39Ar results were obtained from samples irradiated for 0.5 hr at 8 MW at
the Omega West research reactor of the Los Alamos National Laboratory, which has a fast neutron fluence of 5.7 × 10]3 n cm -1 sec -1. Cadmium shielding was used to reduce the thermal neutron production of 4°Ar. After irradiation, the samples were transferred to a copper holder and loaded onto the extraction line for overnight bakeout at ca. 200°C. Single grains of glass (Munesa Vitrophyre) and single alkali feldspar grains (Butajira Ignimbrites) were fused with an 8 W argonion laser. The volume of the feldspar grains were < 1 x 10-3 cm3, corresponding to a sample mass of < 2.5 mg per grain. Abundances of the Ar isotopes were measured on a MAP-215 noble gas mass spectrometer fitted with a Johnston electron multiplier operating at a gain of about 30,000. The analyses and data collection are fully automated, with one analysis lasting about 20-30 min. Typical system blank volumes of 4°Ar, 39Ar, 3BAr, 37Ar and 36Ar, which were automatically measured
Plio-Pleistocene Silicic Rocks
71
TABLE 1. Stratigraphy of the Central Sector of the Main Ethiopian Rift Aluto Area (Rift floor) Geothermal Well LA-3 (EIGS, 1985) (WoldeGabriel et al., 1990)
Ethiopian Rift Valley 7°-8°40 lat. north and adjacent plateau (Di Paola, 1972)
Central sector of main Ethiopian Rift (WoldeGabrielet al., 1990)
Awasa Caldera: Eastern wall (WoldeGabrielet al., 1990)
Alluvium and lacustrine sediments (recent)
Silicic and mafic rocks of the Wonji Group (~< 1.6 Ma)
Silicicand mafic rocks of the Wonji Group (<~ 1.6 Ma)
Silicic, trachytic and basaltic units of the Wonji Group (~< 1.6 Ma)
Chilalo trachytes and silicic rocks (Late Pliocene, 3-1.6 Ma)
Chilalo trachytes (Trachytictuff) (1.85 Ma)
Chilalo trachyte (Late Pliocene)
Butajira ignimbrites (Pliocene, 4.2-3 Ma) Guraghe basalts plus silicics (Miocene, 11-5 Ma) Shebele trachytes (MidMiocene, 17-12Ma) KeUabasalts (Oligocene, 31-29 Ma) Pre-Tertiary units (Mesozoic and Precambrian)
Butajira ignimbrite (3.55-3.69 Ma) Guraghe basalt and silicicflows (8.7-8.8) (Late Miocene)
Butajira ignimbrite (Early to Mid-Pliocene)
Alkaline and peralkaline silicic flows (Holocene) Trachyte flows and basaltic tuff (Recent Pleistocene) Alkaline and peralkaline silicic rocks (Early Pleistocene-- Late Pliocene) Alkaline and peralkaline silicic rocks (Pliocene) Basalts and ignimbritesplateau trap series (Pliocene-Early Eocene)
every 3 samples, were 4, 2, 0.08, 0.3 and 0.3 x 10 -iv mol, respectively. Sanidine from the Fish Canyon Tuff was used as the neutron fluence monitor, which has a reference age of 27.84 Ma (Deino et al., 1990). Errors for individual analyses (lo) reflect errors in J as well as in the determination of Ar isotopic ratios, which in turn propagate errors in discrimination (D) and errors in Ar beam intensities from sample and blank (Deino et al., 1990). The weighted mean age and uhcertainty, which are the preferred age and error estimates (Table 2), are computed using an inverse variance weighting factor, which uses deviations about a weighted mean to determine the weighted uncertainty (Deino et al., 1990; Samson and Alexander, 1987). The Ca/K ratio of the sample, and an additional tool for compositional correlation, is derived by dividing the measured 37Arca/ 39ARK ratio by 0.51 (Table 2).
Results Samples selected for this study are discussed with respect to their geographic location and geologic context. For example, most of the Pliocene ignimbrites are well exposed on the faulted, uplifted escarpment walls that form the western and eastern margins of the MER. The Pleistocene ignimbrites, on the other hand, are predominantly exposed within the rift valley itself. The composition of the Pliocene ignimbrites on the margins is substantially and characteristically different from the Pleistocene rhyolites in the rift valley (Walter et al., 1985), suggesting that changing tectonic conditions may have influenced the composition of silicic magma generation during the evolution of the MER.
Western Rift Margin at Guraghe The Guraghe Mountains are exposed on the highlands west of the M E R escarpment (Fig. 1). Riftward, step-faulted blocks are composed of Pan-African crystalline basement rocks, Mesozoic sedimentary rocks, Oligocene basalts and voluminous Miocene basalts and interbedded silicic units that are unconformably overlain by the widespread Butajira Ignimbrites (WoldeGabriel et al., 1990). At the Guraghe Escarpment, the Butajira Ignimbrites are dominantly represented by a single 200-250 m thick unit composed of a welded, crystal-rich, ash-flow tuff. This unit, termed the Crystal Tuff, overlies several thin (ca. 5 m), crystal-poor, moderately welded, lithic-rich and localized ignimbrite sheets. The Crystal Tuff is overlain by moderately welded pumiceous and crystal-rich flows. Conventional K - A r ages on several samples from the western escarpment at Guraghe are between 4.2 and 3.5 Ma (WoldeGabriel et al., 1990). New single-crystal laserfusion (SCLF) 4°Ar/39Ar data on sanidine from the dominant Crystal Tuff collected from the Guraghe escarpment are consistent with the K - A r data, and show that the Crystal Tuff erupted about 3.5 Ma (Table 2). Three sanidine separates from the topmost units collected along the Guraghe escarpment yielded K - A r ages of 2.6 and 2.72 Ma and belong to the late Pliocene Chilalo Trachyte units (WoldeGabriel et al., 1990). Eastern E s c a r p m e n t at Munesa The Munesa escarpment is located 75 km across the rift valley, opposite the Guraghe escarpment (Fig. 1). The Munesa escarpment is composed of a moderately welded crystal-rich tuff, petrographically and chemi-
G. WoldeGabriel et al.
72
TABLE 2. Radiometric Ages on Whole Rocks and Feldspars from the Central Sector of the Main Ethiopian Rift I. K/Ar data K:O (wt%) 4°Ar* (10-~1 tool/g)
Sample
Location
Rock Type
ET-1 ET23~ ET23:~ ET-21:~ ET-9:~ BT-755 W83-1 W83-43B
Addis Ababa Munesa Munesa Munesa Guraghe Guraghe Bora ~it Asebot
Tuff Tuff K-feldspar K-feldspar K-feldspar K-feldspar Obsidian Obsidian
Sample
L o c a t i o n Material
89MK-1B BT-87B BT-87B ET-9 BT-75 BT-87A
Guraghe Sanidine M u n e s a Anorthoclase
0.0006 0.0439
9.6111 10.3865
83 97
Guraghe Sanidine G u r a g h e Sanidine M u n e s a Sanidine
0.0049 0.0053 0.0006
1.3436 10.5536 1.3656
97 95 95
ET-21 89W6-1B ET-25 84N-1
Munesa Awasa Guraghe Nazret
0.0028 0.0020 0.0036 0.026
1.3768 10.4513 1.4033 1.4255
95 96 92 90
Sanidine Sanidine Sanidine Anorthoclase
4.55 2.0275 4.60 2.0639 6.68 3.1276 6.72 3.2413 5.95 3.0126 6.45 3.2708 5.67 3.7104 4.57 3.4464 1I. 4°Ar/39Ar single crystal laser-fusion age data Ca/K 4°Ar*/39Ar 4°Ar* (%) 1
4°Ar* (%) 87 78 60 86 79 92 51 84 N
Age (Ma)
1.885 x 10-4 1.83 x 10 4
5 4
3.27 + 0.03 3.40 + 0.07
1.42 X 10 -3 1.83 x 10 4 1.42 X 10 -3
6 5 6
3.44 + 0.02 3.49 + 0.03 3.51 + 0.03
6 1 6 4
3.51 + 0.02 3.55 + 0.08 3.58 + 0.01 3.63 _+ 0.02
1.42 1.89 1.415 1.415
x x × x
10 3 10-4 10 3 10-3
Age (Ma)f 3.10 + 3.1 + 3.25 + 3.35 + 3.5 + 3.50 + 4.53 + 5.23 +
0.14 0.15 0.15 0.15 0.16 0.15 0.23 0.24
K/Ar§ (Ma)
2.83 + 0.17 2.95 + 0.2 WR 3.5 + 0.16 3.5 + 0.15 3.51 + 0.17 3.53 -+ O.2 WR 3.35 + 0.15 3.6 + 0.2 3.32 _+0.0611 3.11 + 0.06¶
*Radiogenic argon. fDetermined from decay constants and isotopic abundance of 4°K according to Steiger and J/iger (1977). ~:Crystal tuff. §K/Ar data (WoldeGabriel et al., 1990). IIFeldspar separate (Morton et al., 1979). ¶Whole rock without feldspars (Morton et al., 1979). N Number of feldspars analyzed.
cally equivalent to the Crystal Tuff exposed at Guraghe (WoldeGabriel, 1987). At Munesa, the Crystal Tuff is more than 300 m thick and forms a single flow unit capped by a thin layer (ca. l m) of densely welded and partially weathered vitric tuff and a basalt flow. Northward the Crystal Tuff is overlain by Pleistocene silicic tuffs of the Wonji G r o u p ( < 1.6 Ma). Several rift-oriented horsts west of Munesa are entirely composed of the Crystal Tuff. New SCLF 4°Ar/39Ar analyses on sanidines from two samples of the Crystal Tuff at Munesa, BT-87A and ET-21, yield ages of 3.51 + 0.03 and 3.51 + 0.02 Ma (Table 2). These samples yielded K - A r dates of 3.53 and 3.35 Ma on whole-rock and alkali feldspar, respectively (Table 2) (WoldeGabriel et al., 1990). The spuriously young K - A r date measured on ET-21 is attributed to incomplete extraction of radiogenic 4°Ar from the sanidine, a problem that can occur in high-K volcanic feldspars (McDowell, 1983). Samples from the graben floor adjacent to the faulted horsts of Crystal Tuff are slightly younger, with apparent ages ranging from 3.48 to 3.19 Ma (WoldeGabriel et al., 1990). Alkali Feldspar and whole-rock K - A r ages of 2.83 Ma and 2.95 Ma were obtained on a glassy ignimbrite, termed the Munesa Vitrophyre, stratigraphically above the Crystal Tuff at Munesa (WoldeGabriel et al., 1990). Laser microprobe 4°Ar/39Ar dating of individual handpicked glass fragments from this unit yield a date of 2.88 + 0.3 Ma, in close agreement with K - A r results
(Table 2). Hand-picked anorthoclase grains from the same sample, however, yield a significantly older SCLF 4°mr/39Ar date of 3.406 + 0.007 Ma (Table 2). The younger anorthoclase K - A r age is likely due to incomplete extraction of 4°Ar, as has been noted for sample ET-21 (Table 2). The discordance between glass dates and SCLF anorthoclase dates may be caused by open-system chemical exchange in the glass, such that K enrichment and/or Ar depletion during diagenesis would clearly lower the measured K - A r age. Therefore, the SCLF anorthoclase age of 3.406 + 0.007, is considered a more reliable estimate for the age of the vitrophyre due to the consistency and quality of the measurements, and the known dependability of alkali feldspar compared to glass as a 4°Ar/39Ar chronomoter (Walter et al., 1990). Eastern R i f t E s c a r p m e n t at A wasa The number and magnitude of fault displacements along the eastern escarpment decreases southward, such that the faults are barely visible in the Shashemene area (Fig. 1). Here, the eastern wall of the Awasa caldera merges with the eastern rift margin, where more than 500 m of Late Miocene (9.7 Ma), Pliocene (3.7-1.9 Ma), and Pleistocene (1.6-0.5 Ma) volcanic rocks are exposed (WoldeGabriel et al., 1990). The section is dominantly composed of Plio-Pleistocene pyroclastic rocks. The Pliocene units are composed of crystal-rich, densely welded tuffs, which are exposed in
Plio-Pleistocene Silicic Rocks an isolated fault block that may be part of the original caldera wall. The basal Pliocene unit in this section, which occurs unconformably above a Late Miocene (9.7 Ma) welded tuff, contains abundant fiamme and minor amounts of basaltic lithic clasts. This basal unit is overlain by a series of cliff-forming, intensely welded turfs (> 75 m thick). A sample from the top of this fault block (89W6-1B) is strongly welded and lithic-free. This isolated fault block does not contain the late Pliocene trachytic and Pleistocene silicic tephras that crop out along the main rift wall just a few kilometers (> 5 km) to the north and east of the block. The Pleistocene rocks of the Awasa-Corbetti calderas belong to the Wonji Group (< 1.6 Ma). The southern wall of the Awasa caldera is less prominent and consists of basalts (1.6 Ma) capped by a crystal-rich welded tuff (1.1 Ma). Along the western wall, a major fault exposes more than 250 m of welded and nonwelded tephra. A welded tuff from the middle of the western section yielded a K-Ar date of 1.27 Ma (WoldeGabriel et al., 1990). The Corbetti caldera walls and the two resurgent domes within the caldera consist of several layers of pyroclastic flows and aphyric and crystal-rich obsidian units. Several samples from sections within the nested Awasa caldera yield K-Ar results ranging from 3.69 to 0.021 Ma (WoldeGabriel et al., 1990). A new SCLF 4°Ar/39Ar date on one of the welded turfs above the 3.69 Ma fiamme-rich welded tuff yielded an age of 3.55 Ma which is within the age range of the Crystal Tuff of the Guraghe and the Munesa sections of the northern part of the central sector. Apparent chemical differences between the 3.55 Ma tuff at Awasa and the Crystal Tuff (WoldeGabriel, 1987) suggest that the two units either erupted contemporaneously from two separate centers or that the chemical variations are due to eruption from a chemically zoned magma system. Western Rift Escarpment at the Wagebeta Calderas A major east-west lineament projects through the southern part of the central sector of the rift, incorporating the Wagebeta calderas of the western rift shoulder north of Sodo and the nested Awasa-Corbetti calderas (Fig. 1) (Merla et al., 1979). Three or more calderas of the Wagebeta caldera complex and associated silicic domes are aligned along this feature, which dissects a narrow volcanic plateau between the rift floor to the east and the deep canyon of the Omo River about 15 km to the west (Fig. 1). The Wagebeta calderas are smaller and older than the Awasa and Corbetti calderas. The main Wagebeta caldera, situated between two smaller east-west aligned calderas, is a well-defined, steep-walled, circular depression at least 5 km in diameter. Feldspar separates from the welded tuff from the eastern caldera wall yielded a K-Ar age of 4.2 Ma, and a rhyolite dome ca. 2 km south of Wagebeta is dated to 3.6 Ma (WoldeGabriel et al., 1990). The southern part of the caldera complex is characterized by rhyolite domes, trachytic lavas, and silicic tephras
73
exposed by faulting and streams that flow into the spectacular and deep (ca. 1.5 km) Omo River Canyon. Discussion The Crystal Tuff of the Butajira Ignimbrite sequence is well exposed at the Guraghe (200--250 m thick) and Munesa (250-300 m thick) escarpments. At Guraghe and Munesa, the Crystal Tuff is correlated on the basis of age, chemistry, and petrography. We suspect that the rift floor adjacent to Guraghe and Munesa, roughly 75 km apart, is part of a major rift-bound caldera now mostly buried beneath younger volcanic and volcaniclastic deposits of the central MER. A conservative estimate of the volume of Crystal Tuff erupted is more than 1100 km 3 (WoldeGabriel et al., 1990), suggesting that the evacuation of such a large volume of magma from beneath the M E R may have induced a phase of rifting 3.5 Ma ago, subsequently destroying the caldera itself. Four sections outside the central sector of the MER have been thoroughly investigated to document the areal distribution of the Crystal Tuff: (1) The Wabi Shebele River Gorge (Fig. 1 and inset map) 50 to 60 km southeast of Munesa contains neither the Crystal Tuff nor any of the other Butajira Ignim-. brites. Instead, the time period of the Butajira Ignimbrites (4.2-3.1 Ma) is represented by an unconformity, where Mid-Miocene (17 to 16 Ma) trachyte and silicic lavas and tephras are overlain by Late Pliocene basalts (2.86 to 2.82) (WoldeGabriel et al., 1990). (2) The Ambo fault escarpment (Fig. 1) about 150 km northwest of the Guraghe section consists of Oligocene basalts capped by lateritized soils. (3) The Omo River Gorge in the southern part of the central sector west of Sodo (Fig. 1, inset map) is more than 1.2 km deep and exposes volcanic rocks ranging in age from 17 Ma to 2 Ma (WoldeGabriel and Aronson, 1987). There, a 30 m thick vitric ash occurs below a 3.97 Ma basalt. The vitric tuff section is 50-60 km south of the Wagebeta caldera complex (Fig. 1), but none of the rocks so far examined from Wagebeta is chemically similar to the > 3.97 Ma vitric tuff in the Omo Gorge. (4) In the northern sector of the Main Ethiopian Rift, the Awash River Gorge (Fig. 1, inset map), cuts through Pliocene basalts (5.6-2.0 Ma) with no tuffaceous units in the time range of the Butajira Ignimbrites (WoldeGabriel, 1987). The absence of the Crystal Tuff in the deep Omo, Wabi Shebeli and Awash gorges suggests that the Butajira Ignimbrites were confined to a pre-existing structural depression, perhaps a precursor to the present-day rift graben (WoldeGabriel et al., 1990). However, a 30-50 m thick crystal-rich tuff occurs on top of the Guraghe Mountains at an elevation of greater than 3400 m a.s.l. (BT-119; Table 3) (WoldeGabriel, 1987), which is identical in age and petrography to the Crystal Tuff on the Munesa and Guraghe escarpments. The absence of the Crystal Tuff in adjacent areas may be due to erosion. Although it is somewhat surprising that the Crystal
74
G. WoldeGabriel et aL
o
=~
:
<~.-
&
,-H m ,-H ,-,.-; 4
.E
~.N ¢.,,1 c,,l ~
o
m
(D t--- ~o
+1 +1 ~
,.~< < ?
~'-~£ .~,~
k~
@
E
+1 +1 +1 +1 +1 o +1 +1 +1 +1 +1 ~ ~ + l + l ~ m ~ ~
~ m
.~
. ~
e-, e.,
o ca
~
a4
+t
+~ +~
o
+1 +1
eg~
e~
o
@
.o
0 e~
e"-
~g
=.= ,
O.ID
~'~<~
+1
+1 +1
ee~
+1
o~o.
g.
7
0 II
7
,..D
+1
0 •
e~
=-6
~
II
.~
+1 +1 + 1 ~ +1 '~
+1~+1
0
+1
Ca E e~ o
o
~
~=~
~
~
.1
e~ <
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
]
I
I
I
I
Plio-Pleistocene Silicic Rocks Tuff is not found in the adjacent plateaus, a geothermal well drilled into the Quaternary caldera of Aluto within the central M E R axis (Fig. 1) bottomed at a depth of 2,143 m (ca. - 2 5 0 m b.s.1.) into a > 500 m thick crystalrich tuff that is petrographically similar to the Crystal Tuff on the adjacent rift escarpments (WoldeGabriel et al., 1990). If this correlation is correct, the difference in elevation between the Crystal Tuff on the escarpments and in the graben suggest a vertical displacement along the rift margins of over 3,000 m. Furthermore, the great thickness of the Crystal Tuff at > 2,000 m depth in the rift floor may represent the confined 'fallback' accumulation of the pyroclastic eruption within the caldera itself. Sedimentary sequences in the west-central Afar Basin of northeastern Ethiopia and the Turkana Basin of southwestern Ethiopia and northern Kenya contain numerous tuffaceous units in the temporal range of the Butajira Ignimbrites and other Pliocene tephras in the vicinity of the M E R (Bro~vn, 1982; Haileab, 1988; Hart et al., in press; McDougall, 1985; Walter and Hart, 1988). The sedimentary basins are fed by the Awash and the O m o Rivers, respectively, and share a narrow drainage divide in the Guraghe Mountains along the western escarpment of the Main Ethiopian Rift (Fig. 1, inset map) (Brown, 1982). The glass chemistry of the 30 m thick vitric tuff in the Omo Gorge, for example, is chemically similar to the Moiti Tuff of the Koobi Fora Formation (Turkana Basin) and to a vitric tuff, VT-1, at the base of the Pliocene section in the Middle Awash deposits (Afar Basin) (Hart et al. in press). The Moiti Tuff is dated to < 4.1 Ma (McDougall, 1985), which is consistent with the estimated age of the vitric tuff in the Omo Gorge. In addition, the Munesa vitrophyre is temporally and chemically equivalent to the SHT-Tulu Bor Tufts in the Hadar Formation and Koobi Fora Formations, respectively (Brown, 1982; Walter and Aronson, submitted; Walter et al., 1990). These examples indicate that the M E R region has played a significant role as the source of tephra deposits in the Awash and Turkana Basins. Several Miocene to Pliocene volcanoes are aligned along the southern margin of the Afar depression. According to Kazmin and Behre (1978), these centers, such as Bora'at and the Gara Adi, Afdem, Asebot, Gumbi and Woldoyi (Fig. 1), formed along the western border fault of an ancestral rift graben that runs parallel and adjacent to the present-day eastern escarpment. Two obsidian samples from Bora'at and Asebot yield K - A r dates of 4.53 and 5.23 Ma, respectively (Table 2), and an age of 6.1 Ma has been reported from the Gara Gumbi silicic center (Kunz et al., 1975). The Nazret Group, mostly composed of pyroclastic rocks, is confined to the eastern part of the northern sector of the Main Ethiopian Rift, and is dated to between 9 and 3 Ma (Kazmin and Behre, 1978; Kazmin et al., 1980). In the vicinity of Addis Ababa, along the western edge of the northern M E R system, several tephra layers occur near trachytic and silicic centers that range from 5.3 to 3.1 Ma (Morton et al., 1979). K - A r dates from the
75
silicic flows of Mount Yerer, about 25 km southeast of Addis Ababa, are between 3.6 and 3.3 Ma (Morton et al., 1979; Morton and Rex, 1975). Morton et al. (1979), described porphyritic obsidians flows collected along the Addis A b a b a - N a z r e t road, southwest of Mount Yerer yielding K - A r dates of 3.11 and 3.32 Ma on a feldspar-free matrix and feldspar separates, respectively. SCLF 4°Ar/39Ar analyses on anorthoclase grains from a porphyritic obsidian recently collected from this locality yields a mean date of 3.63 Ma (Table 2). Silicic rocks near Mount Yerer, south of Addis Ababa, are similar in age to the Crystal Tuff of the Butajira Ignimbrite and the densely welded vitrophyre that occurs in the Munesa section above the Crystal Tuff. CONCLUSION Plio-Pleistocene silicic centers and their eruptive products are widespread throughout the rift systems of Ethiopia, but the central sector of the Main Ethiopian Rift is still regarded as a main source area for many widespread silicic pyroclastic units found in the fossiliferous deposits of the Turkana and Afar ~asins. As shown in this paper and elsewhere (Hart et al. in press; Walter et al., 1985, 1990), proximal pyroclastic units in the Guraghe, Munesa, and Wagebeta areas are temporally and chemically similar to the Moiti Tuff in the Turkana Basin and to the Sidi H a k o m a - T u l u Bor Tufts in the Turkana and Afar Basins. However, the occurrence of contemporaneous silicic eruptions throughout the Ethiopian and Afar Rifts during the Pliocene indicate that more than one source region was active during this time period, and that detailed dating and compositional studies are required in order to fully document the regional temporal and chemical patterns of silicic volcanism in Ethiopia, and to make inferences about the origin of distal tephra deposits in the Turkana and Afar Basins.
ACKNOWLEDGEMENTS Field work for this project was accomplishedunder the aegis of the Ethiopian Institute of Geological Surveys and the Department of Geothermal Energy (Ethiopian Ministry of Mines and Energy). Principle financialsupport for this research was provided by National Science Foundation (Grant No. EAR83-06386) to J.L. Aronson and R.C. Walter. We are grateful to Grant Heiken for his comments on the manuscript. A portion of this work by the first author (GWG) was done under the auspices of the U.S. Department of Energy, Earth and Environmental Sciences Division of the Los Alamos National Laboratory. Typing and editorial work on initial drafts of the manuscript by Barbara Hahn and drafting by Anthony Garcia are greatly appreciated. We wish to thank J. Desmond Clark and Tim White for their continued encouragement and support of this project.
REFERENCES Brown, F.H. (1982). Tulu Bor Tuff at Koobi Fora correlated with the Sidi Hakoma Tuff at Hadar. Nature, 300, 631-633. Deino, A., Tauxe, L., Monaghan, M. and Drake, R. (1990). 4°Ar/ 39At age calibration of the Litho- and Paleomagneticstratigraphies of the Ngorora formation, Kenya. Journal of Geology, 98, 567587.
76
G. WoldeGabriel et al.
REFERENCES Di Paola, G.M. (1972). The Ethiopian Rift Valley (between 7000' and 8040, lat., north). Bulletin Volcanologique, 35, 517-560. EIGS (1985). Geothermal exploration project, Ethiopian Lakes District Rift. Ethiopian Institute of Geological Surveys Technical Report, 1.1-7.1. Feibel, C.S., Brown, F.H. and McDougall, I. (1989). Stratigraphic context of fossil hominids from the Omo group deposits: Northern Turkana Basin, Kenya and Ethiopia. American Journal of Physical Anthropology, 78, 595--622. Haileab, B. (1988). Characterization of tephra from the Shungura Formation, southwestern Ethiopia. University of Utah, M.S. Thesis. Hall, C.M., Walter, R.C., Westgate, J.A. and York, D. (1984). Geochronology, stratigraphy and geochemistry of Cindery Tuff in the Pliocene hominid-bearing sediments of the Middle Awash, Ethiopia. Nature, 308, 26-31. Hart, W.K., Walter, R.C. and WoldeGabriel, G. (1992). Tephra Sources and Correlations in Ethiopia: Application of elemental and neodymium isotope data. Quaternary International, 13/14, 7786. Kazmin, V. and Behre, S.M. (1978). Geology and development of the Nazret area, northern Ethiopian Rift Sheet NC37-15. Addis Ababa, Ethiopia, Ethiopian Mapping Agency. Kazmin, V., Behre, S.M., Nicoletti, M. and Petrucciani, C. (1980). Evolution of the northern part of the Ethiopian Rift. Academia Nazionale dei Lincei, 47, 275-292. Kunz, K., Kreuzer, H. and P, M. (1975). K-Ar determination of the trap basalts of the southeastern part of the Afar Rift. Afar Depression of Ethiopia, 370-374. Laury, R.L. and Albritton, C.C. (1975). Geology of the Middle Stone Age archaeological sites in the Main Ethiopian Rift. Geological Society of America Bulletin, 86, 999-1011. McDougall, I. (1985). K-Ar and 4°Ar/39Ar dating of the hominidbearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya. Geological Society of America Bulletin, 96, 159-175. McDowell, F.W. (1983). K-Ar Dating: Incomplete extraction of radiogenic argon from alkali feldspar. Chemical Geology (Isotope Geoscience), 1, 119-126. Merla, G., Abatte, E., Azzaroli, A., Burni, P., Fazzouli, M., Sagri, M. and Tacconi, P. (1979). Comments and a geological map of Ethiopia and Somalia, Scale 1:2,000,000. Frienze, Italy, Consiglio Nazionale della Recerche. Mohr, P.A., Mitchell, J.G. and Raynolds, R.G.H. (1980). Quaternary volcanism and faulting at Oia caldera, central Ethiopian Rift. Bulletin Volcanologique, 43, 173--189.
Morton, W.H. and Rex, D.C. (1975). Geochronology of the Addis Ababa region. Bulletin of the Geophysical Observatory, Addis Ababa, 15, 156. Morton, W.H., Mitchell, J.G. and Mohr, P. (1979). Rift-ward younging of volcanic units in the Addis Ababa region, Ethiopian Rift Valley. Nature, 280, 284-288. Samson, S.D. and Alexander, E.C. (1987). Calibration of the interlaboratory 4°Ar-39Ar dating standard, MMhb-1. Chemical Geology (Isotope Geoscience Section), 66, 27-34. Sarna-Wojcicki, A.M., Meyer, C.E., Roth, P.H. and Brown, F.H. (1985). Ages of tuff beds at East African early hominid sites and sediments in the Gulf of Aden. Nature, 313, 306-308. Steiger, R.H. and Jager, E. (1977). Subcommission on geochronology: Convention on the use of decay constants in geo and -cosmochronology. Earth and Planetary Science Letters, 36, 359362. UNDP (1973). Geology, Geochemistry, and Hydrology of the Hot Springs of the East African Rift-system within Ethiopia. United Nations, New York. Walter, R.C. (1981). The volcanic history of the Hadar Early Man Site and the surrounding Afar region of Ethiopia. Case Western Reserve University, Ph.D. Dissertation. Walter, R.C. and Aronson, J.L. (1982). Revisions of K/Ar ages for the Hadar hominid site, Ethiopia. Nature, 296, 122-127. Walter, R.C. and Aronson, J.L. (submitted). Age and Source of the Sidi Hakoma Tuff, Hadar Formation, Ethiopia. Geology. Walter, R.C. and Hart, W.K. (1988). Nd isotopes as stratigraphic, provenance, and petrogenetic indicators for rhyolitic tephras. Geological Society of America Abstracts with Programs, 20, A113. Walter, R.C., Westgate, J.A., WoldeGabriel, G., Aronson, J.L. and Hart, W.K. (1985). Geochemical and Temporal Patterns of Felsic Volcanism in Ethiopia. Geological Society of America Abstracts, 17, 743. Walter, R.C., WoldeGabriel, G., Aronson, J.L. and Hart, W.K. (1990). Tephrochronology and source of SHT and Tulu Bor tephras. INQUA Inter-Congress Committee on Tephrochronology
Field Conference and Workshop on Tephrochronology, Abstracts, 49. WoldeGabriel, G. (1987). Volcanotectonic history of the central sector of the main Ethiopian Rift: A geochemical and petrological approach. Case Western Reserve University, Ph.D. Dissertation, WoldeGabriel, G. and Aronson, J.L. (1987). The Chow Bahir Rift: A 'failed' rift in southern Ethiopia. Geology, 15, 430-433. WoldcGabriel, G., Aronson, J.L. and Walter, R.C. (1990). Geology, geochronology, and rift basin development in the central sector of the Main Ethiopian Rift. Geological Society of America Bulletin, 102, 439-458.