Supplementary Table 5. Results of laser fusion 40Ar/39Ar dating of single crystals of anorthoclase from three pumice clasts 
from the Topernawi Tuff, Lonyumun Member, Nachukui Formation. Lomekwi Laga, middle branch, west of Lake Turkana, northern Kenya, at 3˚52.9’N, 35˚44.68’E

Sample	36Ar/39Ar
x 10-4	37Ar/39Ar
x 10-3	40Ar/39Ar	K/Ca	40Ar*/39ArK
	% 40Ar*	39Ar
10-14 mol	Calculated age
Ma ± 1 s.d.	
									
93-1066 anorthoclase; J = 4.532E-4 ± 0.40%; level 11, K428	
	   1	12.259 ±  4.72%	10.173 ± 2.99%	5.224 ± 0.19%	52	4.843 ± 0.41%	92.7	0.91	3.955 ± 0.023	
	   2	12.557 ±  2.35%	15.901 ± 1.59%	5.303 ± 0.31%	33	4.913 ± 0.38%	92.7	1.25	4.012 ± 0.022	
	   3	 3.615 ±  5.29%	10.635 ± 2.11%	5.019 ± 0.17%	49	4.893 ± 0.21%	97.5	1.26	3.996 ± 0.018	
	   4	 4.411 ± 11.55%	6.925 ± 1.74%	5.032 ± 0.17%	76	4.882 ± 0.36%	97.0	0.73	3.987 ± 0.021	
	   5	11.526 ±  5.00%	12.609 ± 2.97%	5.239 ± 0.59%	42	4.880 ± 0.73%	93.1	1.16	3.985 ± 0.033	
	   6	 5.716 ±  6.81%	10.515 ± 5.14%	5.077 ± 0.32%	50	4.889 ± 0.41%	96.3	0.82	3.993 ± 0.023	
	   7	 4.340 ±  8.24%	7.106 ± 9.22%	5.034 ± 0.25%	74	4.887 ± 0.34%	97.1	0.90	3.991 ± 0.021	
	   8	11.491 ±  2.41%	12.715 ± 5.11%	5.240 ± 0.34%	41	4.881 ± 0.40%	93.2	0.47	3.986 ± 0.022	
	Arithmetic mean K/Ca (n = 8)	52 ± 15	Arithmetic mean age (n = 8)
Weighted mean age (n =  8)	3.988 ± 0.016
3.991 ± 0.017	
									
93-1067A anorthoclase; J = 4.398E-4 ± 0.30%; level 12, G432	
	   1	 9.736 ±  6.36%	8.456 ± 2.97%	5.345 ± 0.22%	62	5.038 ± 0.43%	94.3	0.48	3.994 ± 0.021	
	   2	14.558 ±  5.55%	13.319 ± 2.99%	5.478 ± 0.27%	40	5.029 ± 0.56%	91.8	0.52	3.986 ± 0.025	
	   3	32.487 ±  1.55%	10.219 ± 2.09%	6.043 ± 0.37%	52	5.064 ± 0.53%	83.8	0.73	4.014 ± 0.025	
	   4	 3.244 ± 10.46%	11.454 ± 4.46%	5.176 ± 0.26%	46	5.061 ± 0.33%	97.8	0.67	4.012 ± 0.018	
	   5	10.944 ±  5.81%	7.046 ± 5.02%	5.383 ± 0.45%	75	5.041 ± 0.61%	93.6	0.58	3.995 ± 0.027	
	   6	 9.708 ±  3.42%	6.454 ± 4.49%	5.323 ± 0.44%	82	5.017 ± 0.51%	94.2	0.63	3.976 ± 0.023	
	Arithmetic mean K/Ca (n =  6)	59 ± 17	Arithmetic mean age (n = 6)
Weighted mean age (n = 6)	3.996 ± 0.015
3.999 ± 0.014	
									
93-1067B anorthoclase; J = 4.530E-4 ± 0.40%; level 9, K428	
	  1	8.786 ±  7.96%	19.679 ± 3.14%	5.131 ± 0.25%	27	4.854 ± 0.50%	94.6	0.75	3.963 ± 0.025	
	  2	7.082 ±  6.31%	7.860 ± 6.20%	5.074 ± 0.24%	67	4.845 ± 0.37%	95.5	0.72	3.956 ± 0.022	
	  3	6.115 ±  9.63%	8.699 ± 7.85%	5.070 ± 0.30%	61	4.870 ± 0.47%	96.1	0.68	3.976 ± 0.025	
	  4	9.182 ± 12.35%	34.122 ± 5.05%	5.139 ± 0.25%	15	4.851 ± 0.74%	94.4	0.37	3.960 ± 0.033	
	  5	6.529 ±  8.75%	9.085 ± 4.81%	5.086 ± 0.23%	58	4.874 ± 0.42%	95.8	0.83	3.979 ± 0.023	
	  6	6.569 ± 10.02%	6.440 ± 9.26%	5.058 ± 0.26%	82	4.845 ± 0.48%	95.8	0.47	3.956 ± 0.025	
	  7	5.929 ±  8.10%	11.233 ± 4.40%	5.093 ± 0.19%	47	4.899 ± 0.35%	96.2	0.78	4.000 ± 0.021	
	  8	4.300 ±  9.76%	9.636 ± 2.70%	5.019 ± 0.28%	55	4.872 ± 0.39%	97.1	0.69	3.978 ± 0.022	
	  9	12.731 ±  5.21%	8.666 ± 4.94%	5.257 ± 0.25%	61	4.861 ± 0.49%	92.5	0.70	3.969 ± 0.025	
	10	4.877 ± 13.38%	15.877 ± 3.38%	5.065 ± 0.25%	33	4.903 ± 0.47%	96.8	0.64	4.003 ± 0.025	
	Arithmetic mean K/Ca (n = 10)	51 ± 20	Arithmetic mean age (n = 10)
Weighted mean age (n = 10)	3.974 ± 0.017
3.976 ± 0.017	

l = 5.543 x 10-10 a-1		Sensitivity ~2.5 (± ~15%) x 10-17 mol/mV		
(36Ar/37Ar)Ca = 3.50 x 10-4	(39Ar/37Ar)Ca = 7.86 x 10-4		(40Ar/39Ar)K = 0.020
40Ar* – radiogenic 40Ar	   39ArK – K-derived 39Ar	         
Mass of alkali feldspar used in each analysis in range: 93-1066 – 0.9 to 2.4 mg; 93-1067A – 1.0 to 1.2 mg; 
93-1067B – 1.2 to 2.0 mg
Irradiated for 24 hours in X33 or X34, HIFAR nuclear reactor
Fluence monitor: 92-176 sanidine from Fish Canyon Tuff; K-Ar reference age utilized: 28.1 Ma
All errors are one standard deviation. Uncertainty on the 40Ar*/39ArK ratio is the analytical error. This error is combined quadratically with the estimated error of J to yield the quoted error for each age