- Email: [email protected]
39Ar dating of Chemeron Formation strata encompassing the site of hominid KNM-BC 1, Tugen Hills, Kenya
Alan L. Deino Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, U.S.A. E-mail: [email protected]
Andrew Hill Department of Anthropology, Yale University, Box 208277, New Haven, Connecticut 06520, U.S.A. E-mail: [email protected] Received 11 September 2001 and accepted 17 September 2001 Keywords: 40Ar/39Ar dating, Chemeron Formation, BPRP#2, Homo, Pliocene, Tugen Hills, Kenya.
40
Ar/39Ar dating of Chemeron Formation strata encompassing the site of hominid KNM-BC 1, Tugen Hills, Kenya
A fossil hominid temporal bone (KNM-BC 1) from surface exposures at Baringo Paleontological Research Project site BPRP#2 in the Chemeron Formation outcropping in a tributary drainage of the Kapthurin River west of Lake Baringo, Kenya has been attributed to Homo sp. indet. K-feldspar phenocrysts from lapilli tuffs bracketing the inferred fossiliferous horizon yield single-crystal 40 Ar/39Ar ages of 2·4560·006 and 2·3930·013 Ma. These age determinations are supported by stratigraphically consistent ages on higher tuff horizons and from nearby sections. In addition, new 40 Ar/39Ar ages on tuffaceous units near the base and top of the formation along the Kapthurin River yield 3·190·03 and 1·600·05 Ma respectively. The base of the formation along the Kapthurin River is thus 0·5 Ma younger than the uppermost Chemeron Formation strata exposed at Tabarin, 23 km to the north-northwest. The upper half of the formation along the Kapthurin River was deposited at an average rate of 11 cm/ka, compared to 21–23 cm/ka at Tabarin. 2002 Academic Press
Journal of Human Evolution (2002) 42, 141–151 doi:10.1006/jhev.2001.0522 Available online at http://www.idealibrary.com on
Introduction
Geologic setting
A fossil hominid temporal bone (KNM-BC 1) from Baringo Paleontological Research Project site BPRP#2 (EAGRU site JM85) in the Chemeron Formation southeast of Lake Baringo, Kenya originally described as family Hominidae gen. et sp. indet. (Martyn, 1967; Tobias, 1967), received the more definitive taxonomic assignment of Homo sp. indet. (Hill et al., 1992; Sherwood et al., 1996, 2002). In Hill et al. (1992) we assigned an age of 2·430·02 Ma to a tuff lying several meters below the probable fossiliferous horizon. Additional 40Ar/39Ar radiometric ages are reported here from the site area, which provide a more comprehensive record of the age of these sediments. We also report on age determinations from tuffs near the base and top of the Chemeron Formation in the Kapthurin River valley.
The Tugen Hills, a structural horst within the Kenya Rift west of Lake Baringo, Kenya, expose on their eastern foothills discontinuous outcrops of a sequence of terrigineous and lacustrine sediments and tuffs of Plio– Pleistocene age. These beds originally constituted a portion of Gregory’s Kamasia sediments (Gregory, 1896, 1921), subsequently studied by Fuchs (1934, 1950). Much later McCall et al. (1967) recognized that the Kamasia sediments were two distinct lithological formations separated by an angular unconformity and, as they thought, a lava. The lower unit was informally called the Chemeron beds; the upper, the Kapthurin beds. The Chemeron beds referred to the sequence of sediments lying on a basalt, now termed the Kaparaina Basalts Formation, and exposed by a series of rivers, such as the Kapthurin River and
0047–2484/02/010141+11$35.00/0
2002 Academic Press
142
. . .
Chemeron River itself, that drain from the Tugen Hills eastward into Lake Baringo. Martyn (1967, 1969) formalized these names and added to the Chemeron Formation exposures an apparently separate basin centered on Kipcherere about 10 km farther west. These also lay on Kaparaina Basalts. Pickford (1975; see also Pickford et al., 1983) also attributed rocks farther north to the Chemeron Formation, some of which had originally been placed in the Kaperyon Formation by Chapman (1971). In the Kapthurin River basin, the top of the Chemeron Formation occurs at an angular unconformity with the overlying Kapthurin Formation. A few meters of sediment and tuff are found in the Chemeron Formation immediately below this contact, which are in turn underlain by a prominent stratigraphic marker unit in the area, the Ndau Trachymugearite mafic flow (Hill et al., 1985, 1986). Previous K–Ar dates on this and another similar lava gave 1·57 and 2·13 Ma (Chapman & Brook, 1978; Hill et al., 1985). The Chemeron Formation in the Kapthurin River area disconformably overlies basaltic lavas and minor intercalated sediments of the Kaparaina Basalts Formation. K–Ar and 40Ar/39Ar ages for this formation in the Tugen Hills typically range from 5·7–5·1 Ma (Chapman & Brook, 1978; Hill et al., 1985; Deino et al., 2002). BPRP#2 is located within an eastwarddipping structural block containing strata from the middle of the Chemeron Formation sequence as exposed in the Kapthurin River drainage. The sequence is dominated by fine-grain terrigineous and lacustrine sediments, primarily mudstone and siltstone, with diatomite, sandstone, and conglomerate intercalations. In addition, tuffaceous material is found as reworked shard and pumice components in sands, as dispersed lapilli and ash in diatomite or mudstone, or as discreet beds of lapilli or vitric tuff.
KENYA
N
Baringo
Site K002
Section ‘A’ Section ‘B’
Kapthurin River 0
0.5 km
Figure 1. Location of fossil locality BPRP#2, 40Ar/39Ar dating sample localities, and section lines.
A distinctive, resistant 4-m-thick bedded lapilli and block tuff, informally called the ‘‘Lower Tuffs’’ by Martyn (1967), supports a prominent dip-slope throughout the BPRP#2 area. The tuff was deposited in a lacustrine environment based on the occurrence of ripple marks and laminated, horizontal bedding within the tuff, and its stratigraphic association: the basal portion grades downward into diatomite, and the upper portion is overlain by fine-grained siltstone containing fish bones. The tuff sequence is found throughout the Kapthurin River drainage and beyond with similar sedimentological attributes and thickness, suggesting deposition in a relatively broad lake basin. Deposition of the tuff probably occurred rapidly as the result of a single eruptive sequence. We include in the uppermost part of the tuff 0·5 m of tuffaceous silts containing lapilli and blocks of pumice similar to those of the underlying laminated tuff; the admixture of silt apparently records waning of this eruption, or reworking. An age for the upper part of this tuff of 2·430·02 Ma by the single-crystal 40Ar/ 39 Ar dating method was reported in Hill et al. (1992). The location of these sections and of the 40Ar/39Ar sample localities are given in Figure 1. Stratigraphic columns are shown in Figure 2 for two measured sections near BPRP#2.
Section ‘A’
Section ‘B’ 30
40 25
35 20
30 AH90-1 2.353 ± 0.006 Ma
15
25 10
20 5
15
KAP-3 2.393 ± 0.013 Ma
KAP-2 2.367 ± 0.016 Ma
KAP-6 2.410 ± 0.022 Ma
0 KNM-BC1? 10
‘Lower Tuffs’
5
ALD86-1, ALD86-1B RD-6
2.456 ± 0.006 Ma
0 Figure 2. Stratigraphic sections measured in the Chemeron Formation in the vicinity of site BPRP#2. Thickness given in meters.
143
Section ‘‘A’’ was measured through site BPRP#2, while section ‘‘B’’ is located about 200 m south at the confluence of the tributary leading to BPRP#2 and the main Kapthurin River channel. These sections occur along strike and dip 21–27 eastward. Correlation between sections is based on lithologic comparison and structural placement. Tracing of strata from Section A to near the site of Section B indicates that this sequence lies stratigraphically above the Lower Tuffs. Several distinctive coarse clastic units, tuff beds, and a distinctive red claystone can be recognized in both sections. KNM-BC 1 was found on the surface above the ‘‘Lower Tuffs,’’ on the dip-slope west of the tributary to the Kapthurin River adjacent to the BPRP#2 site (Martyn, 1967). For a full account of the circumstances of its discovery and recognition see Hill (1999). Based on the lithology of the ‘‘lithic grit’’ adhering to the fossil, Martyn (1967) traced its probable origin to a sandy horizon upslope from the point of discovery, stratigraphically about 8 (2·4 m) above the Lower Tuffs. Subsequent workers endorsed this conclusion, and there remains no further doubt about the provenance of this fossil (Hill, 1999). In the area of Section ‘‘A,’’ this unit is probably represented by a half-meter-thick coarse sand 2·5–3·0 m above the top of the Lower Tuffs. Dating sample KAP-3 was obtained on the east slope of the gully directly across from BPRP#2; it lies stratigraphically above the inferred fossiliferous horizon by about 4 m. By projection of bedding attitudes, this tuffaceous horizon also lies stratigraphically above the youngest Chemeron Formation sediments in the BPRP#2 site area on the west slope of the gully, and thus provides a firm upper age limit for the fossil. Another lapilli tuff 13·1 m above KAP-3 in this same section was also dated (AH90-1). As a final check on the chronostratigraphy, two lapilli tuffs were dated from section B in correlative sediments (KAP-2, KAP-6).
. . .
144 Table 1a
40 Ar/39Ar analytical data, Chemeron Formation, Middle Kapthurin River, Baringo Basin, Kenya
Lab ID#
Ca/K
36
Ar/39Ar
40
Ar*/39Ar
40 Ar (mol1014)
%40Ar*
Age (Ma) 1
Sample AH90-1: Lapilli tuff (pumice to 2 cm), with detrital matrix of sub-angular class of volcanic rock and lesser feldspar. ‘‘A,’’ ‘‘B,’’ and ‘‘C’’ are individual lapilli. Sample AH90-1/A: 6551-01 0·013 6551-02 0·044 6551-03 0·019 6551-04 0·043 6551-05 0·033 6551-06 0·065 6551-07 0·046 6551-08 0·012 6551-09 0·058 6558-01 0·037 6558-02 0·046 6558-03 0·040 6558-04 0·047 6558-06 0·022 6558-07 0·013 6558-08 0·012 6558-09 0·037 6558-10 0·012 6558-11 0·045 6558-12 0·012
0·00005 0·00004 0·00004 0·00006 0·00006 0·00008 0·00095 0·00016 0·00008 0·00057 0·00022 0·00016 0·00010 0·00004 0·00007 0·00005 0·00006 0·00009 0·00023 0·00006
0·824 0·816 0·815 0·811 0·815 0·809 0·809 0·795 0·803 0·808 0·809 0·809 0·819 0·800 0·806 0·821 0·810 0·812 0·820 0·811
1·1 1·5 0·6 0·7 0·7 0·6 0·6 0·4 0·9 0·4 0·3 0·2 0·5 1·0 1·5 1·4 0·9 0·7 1·8 0·8
97·9 98·2 98·2 97·4 97·4 96·6 96·6 94·1 97·0 82·5 92·3 94·0 96·1 98·3 97·2 97·8 97·5 96·4 91·9 97·3 Wtd. Ave.=
2·400·02 2·380·02 2·370·03 2·360·02 2·370·02 2·360·02 2·360·02 2·310·02 2·340·02 2·360·03 2·360·03 2·360·03 2·390·03 2·340·02 2·350·02 2·400·02 2·360·02 2·370·02 2·390·02 2·370·02 2·3610·009
Ar*): 0·045
0·00109
0·798
2·7
71·2
2·320·02
Sample AH90-1/B 6552-01 0·019 6552-02 0·019 6552-03 0·015 6552-04 0·066 6552-05 0·070 6552-06 0·068 6553-01 0·065 6553-03 0·013 6553-04 0·060
0·00004 0·00007 0·00004 0·00039 0·00048 0·00041 0·00054 0·00006 0·00006
0·811 0·803 0·805 0·809 0·801 0·805 0·795 0·812 0·803
1·2 1·1 1·4 1·4 2·3 0·7 1·2 0·4 1·0
98·0 96·9 98·1 87·3 84·7 86·8 83·2 97·3 97·6 Wtd Ave.=
2·370·02 2·350·02 2·350·02 2·370·02 2·340·02 2·350·03 2·320·02 2·380·03 2·350·02 2·3470·009
Ar*): 0·061
0·00092
0·798
1·1
74·5
2·340·02
Sample AH90-1/C 6555-01 0·016 6555-02 0·016 6555-03 0·013 6555-04 0·016
0·00007 0·00003 0·00003 0·00007
0·804 0·804 0·818 0·800
1·0 2·2 1·3 0·7
Omitted (<80% 6558-05
Omitted (<80% 6553-02
40
40
96·9 98·5 98·4 97·2 Wtd. Ave.= Overall weighted mean of samples AH90-1/A, B, and C=
Sample KAP-2; Lapilli (<1 cm) from laminated 1444-01 0·0007 0·00013 1444-02 0·0030 0·00004 1444-03 0·0063 0·00041 1444-04 0·0025 0·00007
thin white tuff and sandstone interbeds 2–15 cm 0·968 1·5 96·0 0·964 1·7 98·6 0·982 1·2 89·0 0·959 2·0 97·7
2·350·02 2·350·02 2·390·02 2·340·02 2·3510·012 2·3530·006 total thickness. 2·360·02 2·350·01 2·400·02 2·340·01
Table 1a
145
Continued
Lab ID#
Ca/K
Sample KAP-2; Lapilli (<1 cm) 1444-05 0·0019 1444-06 0·0111 1444-08 0·0004 Omitted (<80% 1444-07
40
Ar*): 0·0006
36
Ar/39Ar
40
from laminated 0·00002 0·00037 0·00009
0·00207
Ar*/39Ar
40 Ar (mol1014)
%40Ar*
Age (Ma) 1
thin white tuff and sandstone interbeds 2–15 cm total thickness. 0·971 1·2 99·2 2·370·02 0·988 1·4 90·0 2·410·02 0·986 1·2 97·4 2·410·02 Wtd. Ave.= 2·3670·016 0·986
2·6
61·7
2·410·02
Sample KAP-3: Fragile sparse lapilli up to 4 cm in diameter dispersed in a 5 cm red mudstone bed, directly opposite JM85 location. 1447-01 0·036 0·00006 0·984 4·0 98·2 2·400·01 1447-02 0·023 0·00006 0·973 2·3 98·3 2·380·01 1447-03 0·031 0·00008 0·980 4·0 97·8 2·390·01 1447-04 0·039 0·00020 0·994 3·0 94·5 2·430·01 1448-01 0·038 0·00008 0·978 1·7 97·7 2·390·01 1448-02 0·029 0·00001 0·991 2·1 99·6 2·420·01 1448-03 0·031 0·00005 0·989 2·3 98·6 2·410·01 1448-04 0·021 0·00005 0·981 1·6 98·7 2·400·01 1448-05 0·039 0·00009 0·978 2·7 97·4 2·390·01 Wtd. Ave.= 2·3930·013 Omitted (Greater than two standard deviations beyond mean): 1447-05 0·037 0·00004 1·023 2·5 99·0 2·500·01 Sample KAP-6: 15-cm pumice block from top of a white diatomaceous siltstone with sparse dispersed lapili. 4701C-01 0·021 0·00012 0·630 1·1 91·8 2·430·03 4701C-02 0·031 0·00022 0·616 0·6 87·9 2·370·05 4701C-05 0·003 0·00036 0·639 2·2 83·6 2·460·02 4701C-06 0·040 0·00017 0·626 1·6 90·2 2·410·02 4701C-07 0·013 0·00022 0·616 1·7 87·7 2·370·02 Wtd. Ave.= 2·4100·022 Omitted (<80% 40Ar*): 4701C-03 0·035 0·00171 0·626 2·9 54·4 2·400·03 4701C-04 0·062 0·00059 0·630 1·2 76·5 2·420·03 4701C-08 0·154 0·00707 0·568 0·7 21·3 2·180·20 Sample ALD86-1B: Pumice block from near top of the ‘‘Lower Tuffs.’’ 7387/2-01 0·053 0·00007 1·008 5·1 7387/2-02 0·051 0·00004 1·008 8·4 7387/2-03 0·057 0·00007 1·003 6·8 7387/2-04 0·020 0·00010 1·003 3·2 7389/2-01 0·050 0·00013 0·997 3·9 7389/2-02 0·021 0·00005 1·004 4·0 7389/2-03 0·055 0·00005 1·004 6·7 7389/2-04 0·021 0·00014 1·003 7·2 7389/2-05 0·049 0·00023 1·002 5·9
98·1 99·0 98·3 97·2 96·4 98·5 98·7 96·2 93·8 Wtd. Ave.=
2·450·01 2·450·01 2·440·01 2·440·01 2·440·01 2·450·01 2·450·01 2·450·01 2·450·01 2·4510·008
Sample ALD86-5: Vitric fine-grained tuff approximately 0·6 m thick at top of Chemeron Formation, just above Ndau Trachymugearite along the south bank of the Kapthurin River. 292-02 0·112 0·00007 0·329 0·9 95·4 1·650·08 292-03 0·050 0·00009 0·307 0·8 92·3 1·540·08 292-04 0·082 0·00008 0·322 0·8 94·1 1·610·08 Wtd. Ave.= 1·600·05 Omitted (<80% 40Ar*): 292-01 0·107 0·00065 0·324 1·2 63·4 1·620·10
. . .
146 Table 1a
Continued
Lab ID#
Ca/K
36
Ar/39Ar
40
Ar*/39Ar
40 Ar (mol1014)
%40Ar*
Age (Ma) 1
Sample AD/RD-3: Vitric fine-grained tuff 0·1 m thick, with 5–10% lapilli to 2 cm, 19·3 m above base of Chemeron Formation where it overlies Kaparaina Basalt along the south bank of the Kapthurin R. 279-01 0·050 0·00006 0·652 1·9 97·9 3·210·05 279-02 0·051 0·00014 0·638 3·1 94·4 3·140·04 279-03 0·053 0·00002 0·647 1·8 99·3 3·180·06 279-04 0·081 0·00015 0·641 1·7 93·9 3·160·07 279-06 0·022 0·00016 0·647 1·0 93·2 3·180·05 279-07 0·040 0·00011 0·657 1·6 95·6 3·230·04 Wtd. Ave.= 3·190·03 Omitted (<80% 40Ar*): 279-05 0·019 0·00935 0·510 4·2 15·6 2·510·22 Notes: Errors in age quoted for individual runs are 1 analytical uncertainty. Weighted averages are calculated using the inverse variance as the weighting factor (Taylor, 1982), while errors in the weighted averges are 1 standard error of the mean (Samson and Alexander, 1987) and incorporate error in J, the neutron flux irradiation parameter. Ca/K is calculated from 37Ar/39Ar using a multiplier of 1·96. 40Ar* refers to radiogenic argon. ‘‘Moles 40 Ar’’ refers to the estimated total moles of 40Ar released during fusion based on spectrometer sensitivity considerations. =5·54310 10 y 1. Table 1b
40
Ar/39Ar irradiation facilities and parameters
Lab ID#
Reactor
Irrad. time (hr)
Monitor Mineral
J 10 3
(36Ar/37Ar)Ca 10 4
(39Ar/37Ar)Ca 10 4
(40Ar/39Ar)K 10 2
279 292 1444 1447 1448 4701 6551 6552 6553 6555 6558 7387
B B L L L L* D D D D D O
10 10 2 2 2 3 7 7 7 7 7 14
MMhb-1 MMhb-1 FCT FCT FCT PF FCT FCT FCT FCT FCT FCT
2·7260·02 2·7720·02 1·3520·007 1·3520·007 1·3520·007 2·1280·010 1·6120·005 1·6180·005 1·6230·005 1·6190·005 1·6150·005 1·3580·004
2·590·06 2·590·06 2·580·06 2·580·06 2·580·06 2·580·06 2·640·02 2·640·02 2·640·02 2·640·02 2·640·02 2·640·02
9·00·3 9·00·3 6·70·3 6·70·3 6·70·3 6·70·3 6·70·4 6·70·4 6·70·4 6·70·4 6·70·4 6·70·4
0·860·07 0·860·07 0·080·07 0·080·07 0·080·07 2·080·13 0·450·50 0·450·50 0·450·50 0·450·50 0·450·50 0·070·03
Reactors: ‘‘B’’=Central thimble facility of the UCB TRIGA reactor in Berkeley, California, operated at 1 MW; ‘‘L’’=Hydraulic rabbit facility of the Los Alamos Omega West reactor operated at 5 MW; ‘‘D’’=Central thimble facility of the USGS TRIGA reactor in Denver, Colorado, operated at 1 MW; ‘‘O’’=CLICIT Dummy fuel element position in the Oregon State University TRIGA reactor, operated at 360 KW. *Indicates no Cd shielding; all others are Cd shielded. J=neutron fluence parameter (36Ar/37Ar)Ca (39Ar/37Ar)Ca are isotopic ratios derived from irradiation of Ca in the sample (40Ar/39Ar)K is an isotopic ratio derived from irradiation of K in the sample. Monitor minerals: MMhb-1=McClure Mountain Complex hornblende, reference age 523·1 Ma (Renne et al., 1988); FCT=Fish Canyon Tuff sanidine, reference age 28·02 Ma (Renne et al., 1988); PF=sanidine from ash-flow tuff of the Pahranagat Formation, reference age 22·7850·009 determined against FCT at 28·02 Ma (Best et al., 1995, modified for Renne et al., 1998).
40
Ar/39Ar dating K-feldspar phenocrysts extracted from five pumice tuff samples were analyzed by the
single-crystal, laser-fusion, 40Ar/39Ar dating method. Pumice were gently crushed and sieved between the 12 to 28 mesh size
fraction. Feldspar crystals were removed and treated with dilute HCl, HF, and distilled water in an ultrasonic bath to remove adhered matrix, and then hand-selected to obtain pristine, inclusion-free material. Irradiation facilities and parameters are listed in Table 1B. After a sufficient interval to permit radiological cooling to acceptable levels following irradiation, samples were loaded in a ultra-high-vacuum sample chamber and baked at 300C overnight. Grains were first illuminated with a lowpower, diffuse laser beam (Ar-ion or Nd-YAG) to desorb surficial atmospheric argon, which was immediately pumped away. The feldspars were then fused with a focused laser beam at high power (8–35 Watts). Gases released by this step were scrubbed of reactive chemical species by exposure to SAES getters for 3–5 min, then analyzed for five Ar isotopes on a MAP 215 low-blank mass spectrometer. Further details of analytical procedures are provided in Deino & Potts (1990). Results of these analyses are presented in Table 1. Radiogenic 40Ar content (40Ar*) ranged from 15·6 to 99·6%, with the great majority of grains yielding >90% 40Ar*. The relatively high atmospheric yields of some grains could arise from a number of factors, including alteration, fluid or glass inclusions, incomplete fusion, and relatively small grain size. Those few analyses with 40 Ar* less than an arbitrary 80% cut-off were culled from the data set to mitigate the influence of these factors. Elimination of these analyses affects the weighted mean ages by less than 1%, as runs with low proportions of radiogenic argon tend to have higher uncertainties due to the large atmospheric corrections required, and so are typically not influential in a weighted mean calculation. One other analysis (1447-05) from sample KAP-3 was omitted because of an anomalously old age that fell two standard deviations beyond the mean. This may
Relative probability
650 600 550 500 450 400 350 300 250 200 150 100 50 0
147
Sample AH90-1
Overall A B C 26
2.
28
2.
30
2.
32
2.
34
2.
36
2.
38
2.
40
2.
42
2.
44
2.
Age (Ma)
Figure 3. Age-probability spectra for lapilli A, B, and C from sample AH90-1. The vertical axis of the plot is a relative probability measure of obtaining a particular age for given sample, based on the sum of the gaussian errors of the individual single-crystal analyses. The location of the weighted mean and one-sigma standard error of the weighted mean are also shown for each lapillus, and for the overall distribution.
represent an analytical artefact or a geologic contaminant. Elimination of this run changed the weighted mean age for this sample by less than 0·5%. As a check on analytical reproducibility and age variability between pumice lapilli, anorthoclase from three lapilli from the same tuff at a single locality (AH90-1) were extracted and analyzed separately (Table 1). The age population systematics are illustrated in Figure 3 using age-probability diagrams. These diagrams are essentially the sum of the gaussian errors of the individual singlecrystal analysis (Deino & Potts, 1992). They are similar to conventional histograms that show the frequency of occurrence of a specified age against time, with the difference that the age-probability spectrum specifically accommodates the variable analytical uncertainties inherent in argon-isotopic age data sets. The age-probability diagram shows that lapilli A and B have unimodal age distributions with n of 20 and 9, respectively. C, however, is bimodal, though this is probably due to insufficient replicate analyses since
. . .
148 Table 2
Population statistics for separate age analyses of three lapilli from sample AH90-1
Lapillus
n
Weighted mean age (Ma)
1 S.E.M. (Ma)*
Median age
Principal mode
A B C
20 10 4
2·361 2·347 2·351
0·009 0·009 0·012
2·357 2·349 2·348
2·357 2·349 2·348
Weighted mean of A, B, and C: 34 2·353
0·006
2·356
2·349
*Standard error of the mean, incorporating uncertainty in J, the neutron flux parameter.
only four anorthoclase grains were available for dating. Although there is some indication in A and possibly C of skewing toward older ages, the effect on the weighted mean age is negligible since the median and mode, fairly robust measures of central tendency with small or skewed populations, both fall within a few tenths of a percent of the weighted-mean age (Table 2). The maximum difference between lapilli mean ages is 0·01 Ma, or 0·4%. This is within stated errors, and none of the lapilli mean ages could be differentiated using a Student’s t-test at the 95% confidence level. The unimodal nature of populations A and B: the agreement in age of all samples; concordance of the mean, median, and mode; and reproducibility on the order of a few tenths of a percent attest to the general analytical reliability of the dating results. As a check on the age we obtained previously for the ‘‘Lower Tuffs’’ (Hill et al., 1992), we re-separated and analyzed phenocrysts from one of the original samples, pumice block ALD86-1 (here identified as ALD86-1B). The overall weighted mean age reported by Hill et al. (1992) for the ‘‘Lower Tuffs’’ was 2·450·01 Ma, or 2·430·02 Ma by the isochron approach (ages are adjusted for a revised age of the standard, see Renne et al., 1988). In the interim since these early analyses were performed, much of the analytical methodology has changed; most
importantly, the new samples were irradiated in a different reactor, calibrated with different monitor minerals, and measured on a different mass spectrometer and extraction line, using considerably modified gasextraction and data-reduction techniques. Nevertheless, the new analyses yielded a very consistent age population with a weighted mean age of 2·4510·008 Ma, within analytical error of the earlier result. The final weighted mean age of the prior and current results is 2·4560·006 Ma (uncertainty is 1 standard error of the weighted mean; Taylor, 1982). Lapilli tuff sample KAP-3 provides a younger age limit for the stratigraphic interval from which the fossil must have been derived, yielding 2·3930·013 Ma (Table 1 and Figure 2). A t-test indicates that this age is significantly different from that of the marker tuff (ALD86-1B) at the 95% confidence level. The mean sedimentation rate between these tuffs is 122 cm/ka (uncertainty calculated as the square root of the sum of the squares of the errors of the dating results), based on a measured stratigraphic thickness of 8·0 m over the 63 kyr time interval (Figure 4). This interval consists dominantly of lacustrine mudstones and siltstones and lacks any sign of a hiatus. A lapilli tuff (sample AH90-1) 12·9 m above KAP-3 in section ‘‘A’’ yielded a stratigraphically consistent age of 2·353 0·006 Ma. An overall sedimentation rate
10
AH90-1
2 0 –2 –4
ka m/
4
c 15
6
± 32
Distance from reference horizon (m)
8
KAP-2 12 cm ± 1 /k 1 a
KAP-3
KAP-6
–6 –8
12
Probable KNM-BC1 Horizon
–10 –12
±2
cm
/k
a
ALD86-1, ALD86-1B, and RD-6
Interpolated fossil age
–14
34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. Age (Ma)
Figure 4. 40Ar/39Ar ages plotted as a function of stratigraphic height, showing mean sedimentation rates. The base of a correlative conglomerate sequence found in section ‘‘A’’ at 20·7 m and in section ‘‘B’’ at 10·0 m was chosen as a stratigraphic reference horizon for comparing the two sections on this diagram.
calculated for the interval between these tuff horizons is 3215 cm/ka, more than twice that obtained for the lower part of the section, but within the large uncertainties of these determinations. Two tuffs dated at meter level 5 and 10 in section ‘‘B’’ yielded ages of 2·4100·022 (KAP-6) and 2·3670·016 Ma (KAP-2), stratigraphically consistent with the age determinations from section ‘‘A’’ if the lithostratigraphic correlations are correct (Figure 2). A sedimentation rate calculated for the stratigraphic interval between these tuffs is 1211 cm/ka, equal in magnitude to the rate calculated for the lower part of section ‘‘A.’’ In addition to our analysis of tuffs near the hominid temporal level, we have obtained radiometric age constraints for the top and bottom of the Chemeron Formation in the Kapthurin River valley. The top of the formation is 1·600·05 Ma, based on a single-crystal 40Ar/39Ar age on a fine-grained tuff at the top of 2 m of sedimentary
149
rocks conformably overlying the Ndau Trachymugerite. This thin sequence is disconformably overlain by the Kapthurin Formation. The approximately 90-m-thick section between the hominid temporal level and the top of the formation was deposited in 0·8 Ma, indicating a mean sedimentation rate of 11 cm/ka. The base of the Chemeron Formation along the Kapthurin River must be older than 3·190·03 Ma, as constrained by a 40 Ar/39Ar age of pumice from a fine-grained tuff occurring in a section of brown mudstones 19 m above the contact with weathered basalt of the Kaparaina Basalts Formation (sample AD/RD-3). A sedimentation rate for the lower part of the formation along the Kapthurin River cannot be estimated due to structural complications and incomplete exposure. However, by analogy to the sedimentation rate determined for the upper part of the section, the undated 19 m interval may be roughly 0·15 kyr in duration. Thus Chemeron Formation deposition in this region may have begun at about 3·3 Ma. Discussion KNM-BC 1 was found on a dip slope comprising the western wall of a small gully cut by a tributary of the Kapthurin River. The dip slope is underlain by a prominent tuffaceous marker unit with an age of 2·4560·006 Ma. A tuff on the opposite side of the gully stratigraphically above the projection of the dip slope yielded an age of 2·3930·013 Ma. These ages are statistically distinguishable at the 95% confidence level, and constrain the fossil to a 63 ka interval. Further, if attribution of the fossil to a sandstone bed about 2·4 m above the marker tuff is correct, then its interpolated age is about 2·42 Ma (Figure 4). Study of nearby sections in the same stratigraphic interval provides additional verification of this general age assignment.
150
. . .
The KNM-BC 1 fossiliferous horizon lies approximately in the middle of the Chemeron Formation in the Kapthurin River valley, encompassing the interval 3·3– 1·6 Ma. This sequence is 0·5 Ma younger than the 5·3–3·8 Ma Chemeron Formation preserved at Tabarin, 23 km to the northnorthwest (Deino et al., 2002). Geologic reconnaissance has identified outcrops of the Chemeron Formation at Kipcherere and elsewhere that apparently bridge this interval. Overall sedimentation rates measured for the upper part of the formation along the Kapthurin River (11 cm/ka) are about half that determined for Tabarin (21–23 cm/ka). One possible explanation is that sedimentation is occurring in a wedge-shaped trough, the deeper part of which is closest (e.g., Tabarin) to the main north–south Saimo fault. Another explanation may be that these results reflect waning tectonism by 2·4–1·6 Ma. Several other specimens potentially attributable to the genus Homo in the 2·4– 2·0 Ma range have been reported in the last decade or so. Kimbel et al. (1996, 1997) provide reasonable evidence of Homo at 2·33 Ma, from Hadar, Ethiopia. Schrenk et al. (1993) and Bromage et al. (1995) describe a mandible from Malawi dated faunally at 2·4 Ma, but their estimate appears to be based on a misunderstanding of data in Hill et al. (1992; see Hill, 1995, 1999), and the specimen could be much younger. Suwa (1988, 1990) and Suwa et al. (1996) report isolated teeth from the Shungura Formation, Omo, Ethiopia, that could plausibly belong to Homo dated between 2·4 Ma and 2·0 Ma. Yet the age assignment of 2·456–2·393 Ma established herein for KNM-BC 1 makes it the oldest securely dated specimen attributed to genus Homo (Sherwood et al., 2001). The lower date of 3·2 Ma provides a younger age limit for the type specimens of Paracolobus chemeroni and Theropithecus
baringensis, both of which come from site BPRP#97 (EAGRU JM90/91) at the base of Chemeron Formation in the Kapthurin River. Acknowledgements This research is part of the work of the Baringo Paleontological Research Project (BPRP), based at Yale University, and carried out in collaboration with the National Museums of Kenya. We thank the Government of the Republic of Kenya for permission to carry out research in Kenya (Permit OP/13/001/C 1391/issued to AH), and permission to excavate from the Minister for Home Affairs and National Heritage. BPRP has been supported by grants to AH from NSF (most recently SBR-9208903), the Louise H. and David S. Ingalls Foundation, the Louise Brown Foundation, Clayton Stephenson, and Yale University. The radiometric work was largely supported by National Science Foundation grants BNS-9106787, BNS8919236 to ALD. We further thank Sally McBrearty and John Kingston for assistance in the field. References Best, M. G., Christiansen, E. H., Deino, A. L., Gromme´, C. S. & Tingey, D. G. (1995). Correlation and emplacement of a large, zoned, discontinuously exposed ash-flow sheet: 40Ar/39Ar chronology, paleomagnetism, and petrology of the Pahranagat Formation, Nevada. J. geophys. Res. 100, 24593– 24609. Bishop, W. W. (1972). Stratigraphic succession ‘‘versus’’ calibration in East Africa. In (W. W. Bishop & J. A. Miller, Eds) Calibration of Hominoid Evolution, pp. 219–246. Edinburgh: Scottish Academic Press. Bromage, T. G., Schrenk, F. & Zonneveld, F. W. (1995). Paleoanthropology of the Malawi Rift: an early hominid mandible from the Chiwoindo Beds, northern Malawi. J. hum. Evol. 28, 71–108. Chapman, G. R. (1971). The geological evolution of the northern Kamasia Hills, Baringo District, Kenya. Ph.D. Dissertation, London University. Chapman, G. R. & Brook, M. (1978). Chronostratigraphy of the Baringo Basin, Kenya. In (W. W. Bishop, Ed.) Geological Background to Fossil Man. pp. 207–223. London: Geological Society of London: Scottish Academic Press.
Deino, A. & Potts, R. (1990). Single-crystal 40Ar/39Ar dating of the Olorgesailie Formation, Southern Kenya Rift. J. geophys. Res. 95(B6), 8453–8470. Deino, A. & Potts, R. (1992). Age probability spectra for examination of single-crystal 40Ar/39Ar dating results: example from Olorgesailie, southern Kenya Rift. Quatern. Int. 7/8, 81–89. Deino, A. L., Tauxe, L., Monaghan, M. & Hill, A. (2002). 40Ar/39Ar geochronology and paleomagnetic stratigraphy of the Lukeino and lower Chemeron Formations at Tabarin and Kapcheberek, Tugen Hills, Kenya. J. hum. Evol. 1/2, 117–140. Fuchs, V. (1934). The geological work of the Cambridge Expedition to the East African Lakes, 1930–31. Geol. Mag. 71, 97–112. Fuchs, V. (1950). Pleistocene events in the Baringo Basin. Geol. Mag. 87, 149–174. Gregory, J. W. (1896). The Great Rift Valley. London: John Murray. Gregory, J. W. (1921). The Rift Valleys and Geology of East Africa. London: Seeley Service. Hill, A. (1985). Early hominid from Baringo, Kenya. Nature 315, 222–224. Hill, A. (1994). Late Miocene and early Pliocene Hominoids from Africa. In (R. S. Corruccini & R. L. Ciochon, Eds) Integrative Paths to the Past: Paleoanthropological Advances in Honor of F. Clark Howell, pp. 123–145. Engelwood Cliffs, NJ: Prentice Hall. Hill, A. (1995). Faunal and environmental change in the Neogene of east Africa: evidence from the Tugen Hills sequence, Baringo District, Kenya. In (E. S. Vrba, G. H. Denton, T. C. Partridge & L. H. Burckle, Eds) Paleoclimate and Evolution, with Emphasis on Human Origins. pp. 178–93. New Haven and London: Yale University Press. Hill, A. (1999). The Baringo Basin, Kenya: from Bill Bishop to BPRP. In (P. Andrews & P. Banham, Eds) Late Cenozoic Environments and Hominid Evolution: a Tribute to Bill Bishop, pp. 85–97. London: Geological Society of London. Hill, A. & Ward, S. (1988). Origin of the Hominidae: the record of African large hominoid evolution between 14 My and 4 My. Yearb. phys. Anthrop. 31, 49–83. Hill, A., Drake, R., Tauxe, L., Monaghan, M., Barry, J. C., Behrensmeyer, A. K., Curtis, G., Fine Jacobs, B., Jacobs, L., Johnson, N. & Pilbeam, D. (1985). Neogene palaeontology and geochronology of the Baringo Basin, Kenya. J. hum. Evol. 14, 759–773. Hill, A., Curtis, G. & Drake, R. (1986). Sedimentary stratigraphy of the Tugen Hills, Baringo, Kenya. In (L. E. Frostick, R. W. Renaut, I. Reid & J.-J. Tiercelin, Eds) Sedimentation in the African Rifts, pp. 285–295. Oxford: Blackwells & Geological Society of London Special Publications 25. Hill, A., Ward, S., Deino, A., Curtis, G. & Drake, R. (1992). Earliest Homo. Nature 335, 719–722. Kimbel, W. H., Walter, R. C., Johanson, D. C., Reed, K. E., Aronson, J. L., Assefa, Z., Marean, C. W., Eck, G. G., Bobe, R., Hovers, E., Rak, Y., Vondra, C., Yemane, T., York, D., Chen, Y., Evensen, N. M. & Smith, P. E. (1996). Late Pliocene Homo and Oldowan tools from the Hadar Formation
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(Kadar Hadar Member), Ethiopia. J. hum. Evol. 31, 549–561. Kimbel, W. H., Johanson, D. C. & Rak, Y. (1997). Systematic assessment of a maxilla of Homo from Hadar, Ethiopia. Am. J. Phys. Anthropol. 103, 235–62. Martyn, J. E. (1967). Pleistocene deposits and new fossil localities in Kenya. Nature 215, 476–480. Martyn, J. E. (1969). The geological history of the country between Lake Baringo and the Kerio River, Baringo District, Kenya. Ph.D. Disseration, London University. McCall, G. J. H., Baker, B. H. & Walsh, J. (1967). Late Tertiary and Quaternary sediments of the Kenya Rift Valley. In (W. W. Bishop & J. D. Clark, Eds) Background to Evolution in Africa, pp. 191–220. Chicago: Chicago University Press. Pickford, M. (1975). Stratigraphy and palaeoecology of five late Cainozoic formations in the Kenya Rift Valley. Ph.D. Dissertation, London University. Pickford, M., Johanson, D. C., Lovejoy, O., White, T. & Aronson, J. L. (1983). A hominoid humeral fragment from the Pliocene of Kenya. Am. J. phys. Anthrop. 60, 337–346. Renne, P. R., Swisher, C. C., Deino, A. L., Karner, D. B., Owens, T. L. & DePaolo, D. J. (1998). Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117–152. Samson, S. D. & Alexander, E. C. Jr (1987). Calibration of the interlaboratory 40Ar/39Ar dating standard, MMhb-1. Chem. Geol. Isot. Geosci. 6, 27–34. Schrenk, F., Bromage, T. G., Betzler, C. G., Ring, U. & Juwayeyi, Y. M. (1993). Oldest Homo and Pliocene biogeography of the Malawi Rift. Nature 365, 833–836. Sherwood, R. J., Ward, S. & Hill, A. (1996). Mandibular fossa anatomy of the Chemeron temporal bone (KNM-BC 1). Am. J. Phys. Anthropol. Supplement 22, 214–15. Sherwood, R. J., Ward, S. C. & Hill, A. (2002). The taxonomic status of the Chemeron temporal (KNM-BC 1). J. hum. Evol. 1/2, 153–184. Suwa, G. (1988). Evolution of the ‘‘robust’’ australopithecines in the Omo succession: evidence from mandibular premolar morphology. In (F. E. Grine, Ed.) Evolutionary History of the ‘‘Robust’’ Australopithecines, pp. 199–222. New York: Aldine de Gruyter. Suwa, G. (1990). A comparative analysis of hominid dental remains from the Shungura and Usno Formations, Omo Valley Ethiopia. Ph.D. Dissertation, University of California, Berkeley. Suwa, G., White, T. D. & Howell, F. C. (1996). Mandibular postcanine dentition from the Shungura Formation, Ethiopia: crown morphology, taxonomic allocations, and Plio-Pleistocene hominid evolution. Am. J. phys. Anthrop. 101, 247–282. Taylor, J. R. (1982). An Introduction to Error Analysis. Mill Valley, CA: University Science Books. Tobias, P. V. (1967). Pleistocene deposits and new fossil localities in Kenya. Nature 215, 478–480. Ward, S. & Hill, A. (1987). Pliocene hominid partial mandible from Tabarin, Baringo, Kenya. Am. J. phys. Anthrop. 72, 21–37.
Andrew Hill Department of Anthropology, Yale University, Box 208277, New Haven, Connecticut 06520, U.S.A. E-mail: [email protected] Received 11 September 2001 and accepted 17 September 2001 Keywords: 40Ar/39Ar dating, Chemeron Formation, BPRP#2, Homo, Pliocene, Tugen Hills, Kenya.
40
Ar/39Ar dating of Chemeron Formation strata encompassing the site of hominid KNM-BC 1, Tugen Hills, Kenya
A fossil hominid temporal bone (KNM-BC 1) from surface exposures at Baringo Paleontological Research Project site BPRP#2 in the Chemeron Formation outcropping in a tributary drainage of the Kapthurin River west of Lake Baringo, Kenya has been attributed to Homo sp. indet. K-feldspar phenocrysts from lapilli tuffs bracketing the inferred fossiliferous horizon yield single-crystal 40 Ar/39Ar ages of 2·4560·006 and 2·3930·013 Ma. These age determinations are supported by stratigraphically consistent ages on higher tuff horizons and from nearby sections. In addition, new 40 Ar/39Ar ages on tuffaceous units near the base and top of the formation along the Kapthurin River yield 3·190·03 and 1·600·05 Ma respectively. The base of the formation along the Kapthurin River is thus 0·5 Ma younger than the uppermost Chemeron Formation strata exposed at Tabarin, 23 km to the north-northwest. The upper half of the formation along the Kapthurin River was deposited at an average rate of 11 cm/ka, compared to 21–23 cm/ka at Tabarin. 2002 Academic Press
Journal of Human Evolution (2002) 42, 141–151 doi:10.1006/jhev.2001.0522 Available online at http://www.idealibrary.com on
Introduction
Geologic setting
A fossil hominid temporal bone (KNM-BC 1) from Baringo Paleontological Research Project site BPRP#2 (EAGRU site JM85) in the Chemeron Formation southeast of Lake Baringo, Kenya originally described as family Hominidae gen. et sp. indet. (Martyn, 1967; Tobias, 1967), received the more definitive taxonomic assignment of Homo sp. indet. (Hill et al., 1992; Sherwood et al., 1996, 2002). In Hill et al. (1992) we assigned an age of 2·430·02 Ma to a tuff lying several meters below the probable fossiliferous horizon. Additional 40Ar/39Ar radiometric ages are reported here from the site area, which provide a more comprehensive record of the age of these sediments. We also report on age determinations from tuffs near the base and top of the Chemeron Formation in the Kapthurin River valley.
The Tugen Hills, a structural horst within the Kenya Rift west of Lake Baringo, Kenya, expose on their eastern foothills discontinuous outcrops of a sequence of terrigineous and lacustrine sediments and tuffs of Plio– Pleistocene age. These beds originally constituted a portion of Gregory’s Kamasia sediments (Gregory, 1896, 1921), subsequently studied by Fuchs (1934, 1950). Much later McCall et al. (1967) recognized that the Kamasia sediments were two distinct lithological formations separated by an angular unconformity and, as they thought, a lava. The lower unit was informally called the Chemeron beds; the upper, the Kapthurin beds. The Chemeron beds referred to the sequence of sediments lying on a basalt, now termed the Kaparaina Basalts Formation, and exposed by a series of rivers, such as the Kapthurin River and
0047–2484/02/010141+11$35.00/0
2002 Academic Press
142
. . .
Chemeron River itself, that drain from the Tugen Hills eastward into Lake Baringo. Martyn (1967, 1969) formalized these names and added to the Chemeron Formation exposures an apparently separate basin centered on Kipcherere about 10 km farther west. These also lay on Kaparaina Basalts. Pickford (1975; see also Pickford et al., 1983) also attributed rocks farther north to the Chemeron Formation, some of which had originally been placed in the Kaperyon Formation by Chapman (1971). In the Kapthurin River basin, the top of the Chemeron Formation occurs at an angular unconformity with the overlying Kapthurin Formation. A few meters of sediment and tuff are found in the Chemeron Formation immediately below this contact, which are in turn underlain by a prominent stratigraphic marker unit in the area, the Ndau Trachymugearite mafic flow (Hill et al., 1985, 1986). Previous K–Ar dates on this and another similar lava gave 1·57 and 2·13 Ma (Chapman & Brook, 1978; Hill et al., 1985). The Chemeron Formation in the Kapthurin River area disconformably overlies basaltic lavas and minor intercalated sediments of the Kaparaina Basalts Formation. K–Ar and 40Ar/39Ar ages for this formation in the Tugen Hills typically range from 5·7–5·1 Ma (Chapman & Brook, 1978; Hill et al., 1985; Deino et al., 2002). BPRP#2 is located within an eastwarddipping structural block containing strata from the middle of the Chemeron Formation sequence as exposed in the Kapthurin River drainage. The sequence is dominated by fine-grain terrigineous and lacustrine sediments, primarily mudstone and siltstone, with diatomite, sandstone, and conglomerate intercalations. In addition, tuffaceous material is found as reworked shard and pumice components in sands, as dispersed lapilli and ash in diatomite or mudstone, or as discreet beds of lapilli or vitric tuff.
KENYA
N
Baringo
Site K002
Section ‘A’ Section ‘B’
Kapthurin River 0
0.5 km
Figure 1. Location of fossil locality BPRP#2, 40Ar/39Ar dating sample localities, and section lines.
A distinctive, resistant 4-m-thick bedded lapilli and block tuff, informally called the ‘‘Lower Tuffs’’ by Martyn (1967), supports a prominent dip-slope throughout the BPRP#2 area. The tuff was deposited in a lacustrine environment based on the occurrence of ripple marks and laminated, horizontal bedding within the tuff, and its stratigraphic association: the basal portion grades downward into diatomite, and the upper portion is overlain by fine-grained siltstone containing fish bones. The tuff sequence is found throughout the Kapthurin River drainage and beyond with similar sedimentological attributes and thickness, suggesting deposition in a relatively broad lake basin. Deposition of the tuff probably occurred rapidly as the result of a single eruptive sequence. We include in the uppermost part of the tuff 0·5 m of tuffaceous silts containing lapilli and blocks of pumice similar to those of the underlying laminated tuff; the admixture of silt apparently records waning of this eruption, or reworking. An age for the upper part of this tuff of 2·430·02 Ma by the single-crystal 40Ar/ 39 Ar dating method was reported in Hill et al. (1992). The location of these sections and of the 40Ar/39Ar sample localities are given in Figure 1. Stratigraphic columns are shown in Figure 2 for two measured sections near BPRP#2.
Section ‘A’
Section ‘B’ 30
40 25
35 20
30 AH90-1 2.353 ± 0.006 Ma
15
25 10
20 5
15
KAP-3 2.393 ± 0.013 Ma
KAP-2 2.367 ± 0.016 Ma
KAP-6 2.410 ± 0.022 Ma
0 KNM-BC1? 10
‘Lower Tuffs’
5
ALD86-1, ALD86-1B RD-6
2.456 ± 0.006 Ma
0 Figure 2. Stratigraphic sections measured in the Chemeron Formation in the vicinity of site BPRP#2. Thickness given in meters.
143
Section ‘‘A’’ was measured through site BPRP#2, while section ‘‘B’’ is located about 200 m south at the confluence of the tributary leading to BPRP#2 and the main Kapthurin River channel. These sections occur along strike and dip 21–27 eastward. Correlation between sections is based on lithologic comparison and structural placement. Tracing of strata from Section A to near the site of Section B indicates that this sequence lies stratigraphically above the Lower Tuffs. Several distinctive coarse clastic units, tuff beds, and a distinctive red claystone can be recognized in both sections. KNM-BC 1 was found on the surface above the ‘‘Lower Tuffs,’’ on the dip-slope west of the tributary to the Kapthurin River adjacent to the BPRP#2 site (Martyn, 1967). For a full account of the circumstances of its discovery and recognition see Hill (1999). Based on the lithology of the ‘‘lithic grit’’ adhering to the fossil, Martyn (1967) traced its probable origin to a sandy horizon upslope from the point of discovery, stratigraphically about 8 (2·4 m) above the Lower Tuffs. Subsequent workers endorsed this conclusion, and there remains no further doubt about the provenance of this fossil (Hill, 1999). In the area of Section ‘‘A,’’ this unit is probably represented by a half-meter-thick coarse sand 2·5–3·0 m above the top of the Lower Tuffs. Dating sample KAP-3 was obtained on the east slope of the gully directly across from BPRP#2; it lies stratigraphically above the inferred fossiliferous horizon by about 4 m. By projection of bedding attitudes, this tuffaceous horizon also lies stratigraphically above the youngest Chemeron Formation sediments in the BPRP#2 site area on the west slope of the gully, and thus provides a firm upper age limit for the fossil. Another lapilli tuff 13·1 m above KAP-3 in this same section was also dated (AH90-1). As a final check on the chronostratigraphy, two lapilli tuffs were dated from section B in correlative sediments (KAP-2, KAP-6).
. . .
144 Table 1a
40 Ar/39Ar analytical data, Chemeron Formation, Middle Kapthurin River, Baringo Basin, Kenya
Lab ID#
Ca/K
36
Ar/39Ar
40
Ar*/39Ar
40 Ar (mol1014)
%40Ar*
Age (Ma) 1
Sample AH90-1: Lapilli tuff (pumice to 2 cm), with detrital matrix of sub-angular class of volcanic rock and lesser feldspar. ‘‘A,’’ ‘‘B,’’ and ‘‘C’’ are individual lapilli. Sample AH90-1/A: 6551-01 0·013 6551-02 0·044 6551-03 0·019 6551-04 0·043 6551-05 0·033 6551-06 0·065 6551-07 0·046 6551-08 0·012 6551-09 0·058 6558-01 0·037 6558-02 0·046 6558-03 0·040 6558-04 0·047 6558-06 0·022 6558-07 0·013 6558-08 0·012 6558-09 0·037 6558-10 0·012 6558-11 0·045 6558-12 0·012
0·00005 0·00004 0·00004 0·00006 0·00006 0·00008 0·00095 0·00016 0·00008 0·00057 0·00022 0·00016 0·00010 0·00004 0·00007 0·00005 0·00006 0·00009 0·00023 0·00006
0·824 0·816 0·815 0·811 0·815 0·809 0·809 0·795 0·803 0·808 0·809 0·809 0·819 0·800 0·806 0·821 0·810 0·812 0·820 0·811
1·1 1·5 0·6 0·7 0·7 0·6 0·6 0·4 0·9 0·4 0·3 0·2 0·5 1·0 1·5 1·4 0·9 0·7 1·8 0·8
97·9 98·2 98·2 97·4 97·4 96·6 96·6 94·1 97·0 82·5 92·3 94·0 96·1 98·3 97·2 97·8 97·5 96·4 91·9 97·3 Wtd. Ave.=
2·400·02 2·380·02 2·370·03 2·360·02 2·370·02 2·360·02 2·360·02 2·310·02 2·340·02 2·360·03 2·360·03 2·360·03 2·390·03 2·340·02 2·350·02 2·400·02 2·360·02 2·370·02 2·390·02 2·370·02 2·3610·009
Ar*): 0·045
0·00109
0·798
2·7
71·2
2·320·02
Sample AH90-1/B 6552-01 0·019 6552-02 0·019 6552-03 0·015 6552-04 0·066 6552-05 0·070 6552-06 0·068 6553-01 0·065 6553-03 0·013 6553-04 0·060
0·00004 0·00007 0·00004 0·00039 0·00048 0·00041 0·00054 0·00006 0·00006
0·811 0·803 0·805 0·809 0·801 0·805 0·795 0·812 0·803
1·2 1·1 1·4 1·4 2·3 0·7 1·2 0·4 1·0
98·0 96·9 98·1 87·3 84·7 86·8 83·2 97·3 97·6 Wtd Ave.=
2·370·02 2·350·02 2·350·02 2·370·02 2·340·02 2·350·03 2·320·02 2·380·03 2·350·02 2·3470·009
Ar*): 0·061
0·00092
0·798
1·1
74·5
2·340·02
Sample AH90-1/C 6555-01 0·016 6555-02 0·016 6555-03 0·013 6555-04 0·016
0·00007 0·00003 0·00003 0·00007
0·804 0·804 0·818 0·800
1·0 2·2 1·3 0·7
Omitted (<80% 6558-05
Omitted (<80% 6553-02
40
40
96·9 98·5 98·4 97·2 Wtd. Ave.= Overall weighted mean of samples AH90-1/A, B, and C=
Sample KAP-2; Lapilli (<1 cm) from laminated 1444-01 0·0007 0·00013 1444-02 0·0030 0·00004 1444-03 0·0063 0·00041 1444-04 0·0025 0·00007
thin white tuff and sandstone interbeds 2–15 cm 0·968 1·5 96·0 0·964 1·7 98·6 0·982 1·2 89·0 0·959 2·0 97·7
2·350·02 2·350·02 2·390·02 2·340·02 2·3510·012 2·3530·006 total thickness. 2·360·02 2·350·01 2·400·02 2·340·01
Table 1a
145
Continued
Lab ID#
Ca/K
Sample KAP-2; Lapilli (<1 cm) 1444-05 0·0019 1444-06 0·0111 1444-08 0·0004 Omitted (<80% 1444-07
40
Ar*): 0·0006
36
Ar/39Ar
40
from laminated 0·00002 0·00037 0·00009
0·00207
Ar*/39Ar
40 Ar (mol1014)
%40Ar*
Age (Ma) 1
thin white tuff and sandstone interbeds 2–15 cm total thickness. 0·971 1·2 99·2 2·370·02 0·988 1·4 90·0 2·410·02 0·986 1·2 97·4 2·410·02 Wtd. Ave.= 2·3670·016 0·986
2·6
61·7
2·410·02
Sample KAP-3: Fragile sparse lapilli up to 4 cm in diameter dispersed in a 5 cm red mudstone bed, directly opposite JM85 location. 1447-01 0·036 0·00006 0·984 4·0 98·2 2·400·01 1447-02 0·023 0·00006 0·973 2·3 98·3 2·380·01 1447-03 0·031 0·00008 0·980 4·0 97·8 2·390·01 1447-04 0·039 0·00020 0·994 3·0 94·5 2·430·01 1448-01 0·038 0·00008 0·978 1·7 97·7 2·390·01 1448-02 0·029 0·00001 0·991 2·1 99·6 2·420·01 1448-03 0·031 0·00005 0·989 2·3 98·6 2·410·01 1448-04 0·021 0·00005 0·981 1·6 98·7 2·400·01 1448-05 0·039 0·00009 0·978 2·7 97·4 2·390·01 Wtd. Ave.= 2·3930·013 Omitted (Greater than two standard deviations beyond mean): 1447-05 0·037 0·00004 1·023 2·5 99·0 2·500·01 Sample KAP-6: 15-cm pumice block from top of a white diatomaceous siltstone with sparse dispersed lapili. 4701C-01 0·021 0·00012 0·630 1·1 91·8 2·430·03 4701C-02 0·031 0·00022 0·616 0·6 87·9 2·370·05 4701C-05 0·003 0·00036 0·639 2·2 83·6 2·460·02 4701C-06 0·040 0·00017 0·626 1·6 90·2 2·410·02 4701C-07 0·013 0·00022 0·616 1·7 87·7 2·370·02 Wtd. Ave.= 2·4100·022 Omitted (<80% 40Ar*): 4701C-03 0·035 0·00171 0·626 2·9 54·4 2·400·03 4701C-04 0·062 0·00059 0·630 1·2 76·5 2·420·03 4701C-08 0·154 0·00707 0·568 0·7 21·3 2·180·20 Sample ALD86-1B: Pumice block from near top of the ‘‘Lower Tuffs.’’ 7387/2-01 0·053 0·00007 1·008 5·1 7387/2-02 0·051 0·00004 1·008 8·4 7387/2-03 0·057 0·00007 1·003 6·8 7387/2-04 0·020 0·00010 1·003 3·2 7389/2-01 0·050 0·00013 0·997 3·9 7389/2-02 0·021 0·00005 1·004 4·0 7389/2-03 0·055 0·00005 1·004 6·7 7389/2-04 0·021 0·00014 1·003 7·2 7389/2-05 0·049 0·00023 1·002 5·9
98·1 99·0 98·3 97·2 96·4 98·5 98·7 96·2 93·8 Wtd. Ave.=
2·450·01 2·450·01 2·440·01 2·440·01 2·440·01 2·450·01 2·450·01 2·450·01 2·450·01 2·4510·008
Sample ALD86-5: Vitric fine-grained tuff approximately 0·6 m thick at top of Chemeron Formation, just above Ndau Trachymugearite along the south bank of the Kapthurin River. 292-02 0·112 0·00007 0·329 0·9 95·4 1·650·08 292-03 0·050 0·00009 0·307 0·8 92·3 1·540·08 292-04 0·082 0·00008 0·322 0·8 94·1 1·610·08 Wtd. Ave.= 1·600·05 Omitted (<80% 40Ar*): 292-01 0·107 0·00065 0·324 1·2 63·4 1·620·10
. . .
146 Table 1a
Continued
Lab ID#
Ca/K
36
Ar/39Ar
40
Ar*/39Ar
40 Ar (mol1014)
%40Ar*
Age (Ma) 1
Sample AD/RD-3: Vitric fine-grained tuff 0·1 m thick, with 5–10% lapilli to 2 cm, 19·3 m above base of Chemeron Formation where it overlies Kaparaina Basalt along the south bank of the Kapthurin R. 279-01 0·050 0·00006 0·652 1·9 97·9 3·210·05 279-02 0·051 0·00014 0·638 3·1 94·4 3·140·04 279-03 0·053 0·00002 0·647 1·8 99·3 3·180·06 279-04 0·081 0·00015 0·641 1·7 93·9 3·160·07 279-06 0·022 0·00016 0·647 1·0 93·2 3·180·05 279-07 0·040 0·00011 0·657 1·6 95·6 3·230·04 Wtd. Ave.= 3·190·03 Omitted (<80% 40Ar*): 279-05 0·019 0·00935 0·510 4·2 15·6 2·510·22 Notes: Errors in age quoted for individual runs are 1 analytical uncertainty. Weighted averages are calculated using the inverse variance as the weighting factor (Taylor, 1982), while errors in the weighted averges are 1 standard error of the mean (Samson and Alexander, 1987) and incorporate error in J, the neutron flux irradiation parameter. Ca/K is calculated from 37Ar/39Ar using a multiplier of 1·96. 40Ar* refers to radiogenic argon. ‘‘Moles 40 Ar’’ refers to the estimated total moles of 40Ar released during fusion based on spectrometer sensitivity considerations. =5·54310 10 y 1. Table 1b
40
Ar/39Ar irradiation facilities and parameters
Lab ID#
Reactor
Irrad. time (hr)
Monitor Mineral
J 10 3
(36Ar/37Ar)Ca 10 4
(39Ar/37Ar)Ca 10 4
(40Ar/39Ar)K 10 2
279 292 1444 1447 1448 4701 6551 6552 6553 6555 6558 7387
B B L L L L* D D D D D O
10 10 2 2 2 3 7 7 7 7 7 14
MMhb-1 MMhb-1 FCT FCT FCT PF FCT FCT FCT FCT FCT FCT
2·7260·02 2·7720·02 1·3520·007 1·3520·007 1·3520·007 2·1280·010 1·6120·005 1·6180·005 1·6230·005 1·6190·005 1·6150·005 1·3580·004
2·590·06 2·590·06 2·580·06 2·580·06 2·580·06 2·580·06 2·640·02 2·640·02 2·640·02 2·640·02 2·640·02 2·640·02
9·00·3 9·00·3 6·70·3 6·70·3 6·70·3 6·70·3 6·70·4 6·70·4 6·70·4 6·70·4 6·70·4 6·70·4
0·860·07 0·860·07 0·080·07 0·080·07 0·080·07 2·080·13 0·450·50 0·450·50 0·450·50 0·450·50 0·450·50 0·070·03
Reactors: ‘‘B’’=Central thimble facility of the UCB TRIGA reactor in Berkeley, California, operated at 1 MW; ‘‘L’’=Hydraulic rabbit facility of the Los Alamos Omega West reactor operated at 5 MW; ‘‘D’’=Central thimble facility of the USGS TRIGA reactor in Denver, Colorado, operated at 1 MW; ‘‘O’’=CLICIT Dummy fuel element position in the Oregon State University TRIGA reactor, operated at 360 KW. *Indicates no Cd shielding; all others are Cd shielded. J=neutron fluence parameter (36Ar/37Ar)Ca (39Ar/37Ar)Ca are isotopic ratios derived from irradiation of Ca in the sample (40Ar/39Ar)K is an isotopic ratio derived from irradiation of K in the sample. Monitor minerals: MMhb-1=McClure Mountain Complex hornblende, reference age 523·1 Ma (Renne et al., 1988); FCT=Fish Canyon Tuff sanidine, reference age 28·02 Ma (Renne et al., 1988); PF=sanidine from ash-flow tuff of the Pahranagat Formation, reference age 22·7850·009 determined against FCT at 28·02 Ma (Best et al., 1995, modified for Renne et al., 1998).
40
Ar/39Ar dating K-feldspar phenocrysts extracted from five pumice tuff samples were analyzed by the
single-crystal, laser-fusion, 40Ar/39Ar dating method. Pumice were gently crushed and sieved between the 12 to 28 mesh size
fraction. Feldspar crystals were removed and treated with dilute HCl, HF, and distilled water in an ultrasonic bath to remove adhered matrix, and then hand-selected to obtain pristine, inclusion-free material. Irradiation facilities and parameters are listed in Table 1B. After a sufficient interval to permit radiological cooling to acceptable levels following irradiation, samples were loaded in a ultra-high-vacuum sample chamber and baked at 300C overnight. Grains were first illuminated with a lowpower, diffuse laser beam (Ar-ion or Nd-YAG) to desorb surficial atmospheric argon, which was immediately pumped away. The feldspars were then fused with a focused laser beam at high power (8–35 Watts). Gases released by this step were scrubbed of reactive chemical species by exposure to SAES getters for 3–5 min, then analyzed for five Ar isotopes on a MAP 215 low-blank mass spectrometer. Further details of analytical procedures are provided in Deino & Potts (1990). Results of these analyses are presented in Table 1. Radiogenic 40Ar content (40Ar*) ranged from 15·6 to 99·6%, with the great majority of grains yielding >90% 40Ar*. The relatively high atmospheric yields of some grains could arise from a number of factors, including alteration, fluid or glass inclusions, incomplete fusion, and relatively small grain size. Those few analyses with 40 Ar* less than an arbitrary 80% cut-off were culled from the data set to mitigate the influence of these factors. Elimination of these analyses affects the weighted mean ages by less than 1%, as runs with low proportions of radiogenic argon tend to have higher uncertainties due to the large atmospheric corrections required, and so are typically not influential in a weighted mean calculation. One other analysis (1447-05) from sample KAP-3 was omitted because of an anomalously old age that fell two standard deviations beyond the mean. This may
Relative probability
650 600 550 500 450 400 350 300 250 200 150 100 50 0
147
Sample AH90-1
Overall A B C 26
2.
28
2.
30
2.
32
2.
34
2.
36
2.
38
2.
40
2.
42
2.
44
2.
Age (Ma)
Figure 3. Age-probability spectra for lapilli A, B, and C from sample AH90-1. The vertical axis of the plot is a relative probability measure of obtaining a particular age for given sample, based on the sum of the gaussian errors of the individual single-crystal analyses. The location of the weighted mean and one-sigma standard error of the weighted mean are also shown for each lapillus, and for the overall distribution.
represent an analytical artefact or a geologic contaminant. Elimination of this run changed the weighted mean age for this sample by less than 0·5%. As a check on analytical reproducibility and age variability between pumice lapilli, anorthoclase from three lapilli from the same tuff at a single locality (AH90-1) were extracted and analyzed separately (Table 1). The age population systematics are illustrated in Figure 3 using age-probability diagrams. These diagrams are essentially the sum of the gaussian errors of the individual singlecrystal analysis (Deino & Potts, 1992). They are similar to conventional histograms that show the frequency of occurrence of a specified age against time, with the difference that the age-probability spectrum specifically accommodates the variable analytical uncertainties inherent in argon-isotopic age data sets. The age-probability diagram shows that lapilli A and B have unimodal age distributions with n of 20 and 9, respectively. C, however, is bimodal, though this is probably due to insufficient replicate analyses since
. . .
148 Table 2
Population statistics for separate age analyses of three lapilli from sample AH90-1
Lapillus
n
Weighted mean age (Ma)
1 S.E.M. (Ma)*
Median age
Principal mode
A B C
20 10 4
2·361 2·347 2·351
0·009 0·009 0·012
2·357 2·349 2·348
2·357 2·349 2·348
Weighted mean of A, B, and C: 34 2·353
0·006
2·356
2·349
*Standard error of the mean, incorporating uncertainty in J, the neutron flux parameter.
only four anorthoclase grains were available for dating. Although there is some indication in A and possibly C of skewing toward older ages, the effect on the weighted mean age is negligible since the median and mode, fairly robust measures of central tendency with small or skewed populations, both fall within a few tenths of a percent of the weighted-mean age (Table 2). The maximum difference between lapilli mean ages is 0·01 Ma, or 0·4%. This is within stated errors, and none of the lapilli mean ages could be differentiated using a Student’s t-test at the 95% confidence level. The unimodal nature of populations A and B: the agreement in age of all samples; concordance of the mean, median, and mode; and reproducibility on the order of a few tenths of a percent attest to the general analytical reliability of the dating results. As a check on the age we obtained previously for the ‘‘Lower Tuffs’’ (Hill et al., 1992), we re-separated and analyzed phenocrysts from one of the original samples, pumice block ALD86-1 (here identified as ALD86-1B). The overall weighted mean age reported by Hill et al. (1992) for the ‘‘Lower Tuffs’’ was 2·450·01 Ma, or 2·430·02 Ma by the isochron approach (ages are adjusted for a revised age of the standard, see Renne et al., 1988). In the interim since these early analyses were performed, much of the analytical methodology has changed; most
importantly, the new samples were irradiated in a different reactor, calibrated with different monitor minerals, and measured on a different mass spectrometer and extraction line, using considerably modified gasextraction and data-reduction techniques. Nevertheless, the new analyses yielded a very consistent age population with a weighted mean age of 2·4510·008 Ma, within analytical error of the earlier result. The final weighted mean age of the prior and current results is 2·4560·006 Ma (uncertainty is 1 standard error of the weighted mean; Taylor, 1982). Lapilli tuff sample KAP-3 provides a younger age limit for the stratigraphic interval from which the fossil must have been derived, yielding 2·3930·013 Ma (Table 1 and Figure 2). A t-test indicates that this age is significantly different from that of the marker tuff (ALD86-1B) at the 95% confidence level. The mean sedimentation rate between these tuffs is 122 cm/ka (uncertainty calculated as the square root of the sum of the squares of the errors of the dating results), based on a measured stratigraphic thickness of 8·0 m over the 63 kyr time interval (Figure 4). This interval consists dominantly of lacustrine mudstones and siltstones and lacks any sign of a hiatus. A lapilli tuff (sample AH90-1) 12·9 m above KAP-3 in section ‘‘A’’ yielded a stratigraphically consistent age of 2·353 0·006 Ma. An overall sedimentation rate
10
AH90-1
2 0 –2 –4
ka m/
4
c 15
6
± 32
Distance from reference horizon (m)
8
KAP-2 12 cm ± 1 /k 1 a
KAP-3
KAP-6
–6 –8
12
Probable KNM-BC1 Horizon
–10 –12
±2
cm
/k
a
ALD86-1, ALD86-1B, and RD-6
Interpolated fossil age
–14
34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. 2. Age (Ma)
Figure 4. 40Ar/39Ar ages plotted as a function of stratigraphic height, showing mean sedimentation rates. The base of a correlative conglomerate sequence found in section ‘‘A’’ at 20·7 m and in section ‘‘B’’ at 10·0 m was chosen as a stratigraphic reference horizon for comparing the two sections on this diagram.
calculated for the interval between these tuff horizons is 3215 cm/ka, more than twice that obtained for the lower part of the section, but within the large uncertainties of these determinations. Two tuffs dated at meter level 5 and 10 in section ‘‘B’’ yielded ages of 2·4100·022 (KAP-6) and 2·3670·016 Ma (KAP-2), stratigraphically consistent with the age determinations from section ‘‘A’’ if the lithostratigraphic correlations are correct (Figure 2). A sedimentation rate calculated for the stratigraphic interval between these tuffs is 1211 cm/ka, equal in magnitude to the rate calculated for the lower part of section ‘‘A.’’ In addition to our analysis of tuffs near the hominid temporal level, we have obtained radiometric age constraints for the top and bottom of the Chemeron Formation in the Kapthurin River valley. The top of the formation is 1·600·05 Ma, based on a single-crystal 40Ar/39Ar age on a fine-grained tuff at the top of 2 m of sedimentary
149
rocks conformably overlying the Ndau Trachymugerite. This thin sequence is disconformably overlain by the Kapthurin Formation. The approximately 90-m-thick section between the hominid temporal level and the top of the formation was deposited in 0·8 Ma, indicating a mean sedimentation rate of 11 cm/ka. The base of the Chemeron Formation along the Kapthurin River must be older than 3·190·03 Ma, as constrained by a 40 Ar/39Ar age of pumice from a fine-grained tuff occurring in a section of brown mudstones 19 m above the contact with weathered basalt of the Kaparaina Basalts Formation (sample AD/RD-3). A sedimentation rate for the lower part of the formation along the Kapthurin River cannot be estimated due to structural complications and incomplete exposure. However, by analogy to the sedimentation rate determined for the upper part of the section, the undated 19 m interval may be roughly 0·15 kyr in duration. Thus Chemeron Formation deposition in this region may have begun at about 3·3 Ma. Discussion KNM-BC 1 was found on a dip slope comprising the western wall of a small gully cut by a tributary of the Kapthurin River. The dip slope is underlain by a prominent tuffaceous marker unit with an age of 2·4560·006 Ma. A tuff on the opposite side of the gully stratigraphically above the projection of the dip slope yielded an age of 2·3930·013 Ma. These ages are statistically distinguishable at the 95% confidence level, and constrain the fossil to a 63 ka interval. Further, if attribution of the fossil to a sandstone bed about 2·4 m above the marker tuff is correct, then its interpolated age is about 2·42 Ma (Figure 4). Study of nearby sections in the same stratigraphic interval provides additional verification of this general age assignment.
150
. . .
The KNM-BC 1 fossiliferous horizon lies approximately in the middle of the Chemeron Formation in the Kapthurin River valley, encompassing the interval 3·3– 1·6 Ma. This sequence is 0·5 Ma younger than the 5·3–3·8 Ma Chemeron Formation preserved at Tabarin, 23 km to the northnorthwest (Deino et al., 2002). Geologic reconnaissance has identified outcrops of the Chemeron Formation at Kipcherere and elsewhere that apparently bridge this interval. Overall sedimentation rates measured for the upper part of the formation along the Kapthurin River (11 cm/ka) are about half that determined for Tabarin (21–23 cm/ka). One possible explanation is that sedimentation is occurring in a wedge-shaped trough, the deeper part of which is closest (e.g., Tabarin) to the main north–south Saimo fault. Another explanation may be that these results reflect waning tectonism by 2·4–1·6 Ma. Several other specimens potentially attributable to the genus Homo in the 2·4– 2·0 Ma range have been reported in the last decade or so. Kimbel et al. (1996, 1997) provide reasonable evidence of Homo at 2·33 Ma, from Hadar, Ethiopia. Schrenk et al. (1993) and Bromage et al. (1995) describe a mandible from Malawi dated faunally at 2·4 Ma, but their estimate appears to be based on a misunderstanding of data in Hill et al. (1992; see Hill, 1995, 1999), and the specimen could be much younger. Suwa (1988, 1990) and Suwa et al. (1996) report isolated teeth from the Shungura Formation, Omo, Ethiopia, that could plausibly belong to Homo dated between 2·4 Ma and 2·0 Ma. Yet the age assignment of 2·456–2·393 Ma established herein for KNM-BC 1 makes it the oldest securely dated specimen attributed to genus Homo (Sherwood et al., 2001). The lower date of 3·2 Ma provides a younger age limit for the type specimens of Paracolobus chemeroni and Theropithecus
baringensis, both of which come from site BPRP#97 (EAGRU JM90/91) at the base of Chemeron Formation in the Kapthurin River. Acknowledgements This research is part of the work of the Baringo Paleontological Research Project (BPRP), based at Yale University, and carried out in collaboration with the National Museums of Kenya. We thank the Government of the Republic of Kenya for permission to carry out research in Kenya (Permit OP/13/001/C 1391/issued to AH), and permission to excavate from the Minister for Home Affairs and National Heritage. BPRP has been supported by grants to AH from NSF (most recently SBR-9208903), the Louise H. and David S. Ingalls Foundation, the Louise Brown Foundation, Clayton Stephenson, and Yale University. The radiometric work was largely supported by National Science Foundation grants BNS-9106787, BNS8919236 to ALD. We further thank Sally McBrearty and John Kingston for assistance in the field. References Best, M. G., Christiansen, E. H., Deino, A. L., Gromme´, C. S. & Tingey, D. G. (1995). Correlation and emplacement of a large, zoned, discontinuously exposed ash-flow sheet: 40Ar/39Ar chronology, paleomagnetism, and petrology of the Pahranagat Formation, Nevada. J. geophys. Res. 100, 24593– 24609. Bishop, W. W. (1972). Stratigraphic succession ‘‘versus’’ calibration in East Africa. In (W. W. Bishop & J. A. Miller, Eds) Calibration of Hominoid Evolution, pp. 219–246. Edinburgh: Scottish Academic Press. Bromage, T. G., Schrenk, F. & Zonneveld, F. W. (1995). Paleoanthropology of the Malawi Rift: an early hominid mandible from the Chiwoindo Beds, northern Malawi. J. hum. Evol. 28, 71–108. Chapman, G. R. (1971). The geological evolution of the northern Kamasia Hills, Baringo District, Kenya. Ph.D. Dissertation, London University. Chapman, G. R. & Brook, M. (1978). Chronostratigraphy of the Baringo Basin, Kenya. In (W. W. Bishop, Ed.) Geological Background to Fossil Man. pp. 207–223. London: Geological Society of London: Scottish Academic Press.
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