Supplementary Table 4. Results of laser fusion 40Ar/39Ar dating of single crystals of anorthoclase from four pumice clasts in the 

Topernawi Tuff, Lonyumun Member, Nachukui Formation. 98-307A, B from Lomekwi Laga, and 98-337A, B about 1 km northeast of Topernawi Gorge, west of Lake Turkana, northern Kenya

Sample	36Ar/39Ar
x 10-4	37Ar/39Ar
x 10-3	40Ar/39Ar	K/Ca	40Ar*/39ArK
± c.v.	% 40Ar*	39Ar
10-14 mol	Calculated age
Ma ± 1 s.d.	
									
98-307A anorthoclase; J = 4.213E-4 ± 0.50%; level 3, ANU45	
	  1	8.944 ±  1.71%	8.338 ± 3.22%	5.571 ± 0.10%	63	5.279 ± 0.14%	94.8	2.10	4.008 ± 0.021	
	  2	3.628 ±  4.69%	13.563 ± 4.41%	5.442 ± 0.13%	39	5.309 ± 0.16%	97.6	1.60	4.031 ± 0.021	
	  3	32.135 ±  0.90%	13.062 ± 2.30%	6.193 ± 0.13%	40	5.217 ± 0.23%	84.3	1.94	3.961 ± 0.022	
	  4	3.608 ±  5.58%	8.415 ± 2.39%	5.323 ± 0.15%	63	5.190 ± 0.19%	97.5	1.39	3.940 ± 0.021	
	  5	17.200 ±  2.21%	10.760 ± 3.56%	5.815 ± 0.11%	49	5.281 ± 0.25%	90.8	1.13	4.009 ± 0.022	
	  6	1.734 ± 11.66%	15.281 ± 1.53%	5.254 ± 0.13%	34	5.178 ± 0.18%	98.5	1.50	3.931 ± 0.021	
	  7	4.538 ±  7.36%	10.643 ± 4.23%	5.378 ± 0.15%	49	5.218 ± 0.24%	97.0	1.77	3.961 ± 0.022	
	  8	8.653 ±  3.04%	7.566 ± 5.83%	5.515 ± 0.10%	70	5.233 ± 0.18%	94.9	1.15	3.973 ± 0.021	
	  9	13.887 ±  3.94%	6.253 ± 5.64%	5.667 ± 0.11%	84	5.230 ± 0.33%	92.3	0.78	3.971 ± 0.024	
	10	2.756 ±  6.16%	12.228 ± 1.51%	5.392 ± 0.10%	43	5.285 ± 0.14%	98.0	2.31	4.013 ± 0.021	
	                                  Arithmetic mean K/Ca (n = 10)	   53
± 16 
	Arithmetic mean age (n = 10)
Weighted mean age (n = 10)
	3.980 ± 0.034
3.986 ± 0.020	
									
98-307B anorthoclase; J = 4.241E-4 ± 0.50%; level 4, ANU45	
	  1	1.564 ± 16.25%	10.733 ± 3.66%	5.251 ± 0.11%	49	5.174 ± 0.19%	98.5	0.99	3.954 ± 0.021	
	  3	3.277 ±  8.95%	13.020 ± 2.58%	5.308 ± 0.17%	40	5.180 ± 0.24%	97.6	1.04	3.960 ± 0.022	
	  4	5.189 ±  6.24%	6.662 ± 3.88%	5.427 ± 0.13%	79	5.243 ± 0.23%	96.6	1.21	4.007 ± 0.022	
	  5	6.156 ±  5.90%	6.290 ± 5.06%	5.438 ± 0.11%	84	5.225 ± 0.24%	96.1	1.07	3.994 ± 0.022	
	  6	3.749 ±  4.34%	15.263 ± 2.93%	5.434 ± 0.15%	34	5.293 ± 0.18%	97.4	1.07	4.045 ± 0.021	
	  7	11.187 ±  3.24%	7.821 ± 2.59%	5.597 ± 0.09%	67	5.235 ± 0.23%	93.5	1.41	4.001 ± 0.022	
	  8	9.241 ±  1.93%	14.218 ± 2.60%	5.545 ± 0.13%	37	5.242 ± 0.17%	94.5	1.19	4.006 ± 0.021	
	  9	20.862 ±  0.60%	7.028 ± 6.75%	11.472 ± 0.13%	75	5.277 ± 0.76%	46.0	0.99	4.033 ± 0.037	
	Arithmetic mean K/Ca (n = 8)	58
± 20	Arithmetic mean age (n = 8)
Weighted mean age (n = 8)	4.000 ± 0.031
3.998 ± 0.020	
									
98-337A anorthoclase; J = 4.258E-4  0.40%; level 6, ANU45	
       1	8.098 ±  2.83%	11.605 ± 1.80%	5.528 ± 0.11%	45	5.251 ± 0.17%	95.0	2.07	4.029 ± 0.018	
       2	1.856 ± 10.84%	7.181 ± 3.72%	5.281 ± 0.12%	73	5.188 ± 0.17%	98.2	1.60	3.981 ± 0.017	
       3	2.620 ±  5.28%	9.785 ± 3.60%	5.355 ± 0.11%	54	5.240 ± 0.14%	97.8	1.74	4.020 ± 0.017	
       4	10.996 ±  2.80%	8.180 ± 4.07%	5.564 ± 0.15%	64	5.201 ± 0.24%	93.5	1.33	3.991 ± 0.019	
       5	8.140 ±  1.96%	13.240 ± 3.47%	5.489 ± 0.13%	40	5.210 ± 0.16%	94.9	1.90	3.998 ± 0.017	
       6	9.383 ±  1.50%	10.851 ± 3.27%	5.519 ± 0.10%	49	5.204 ± 0.14%	94.3	2.12	3.993 ± 0.017	
       7	23.332 ±  2.20%	9.728 ± 4.20%	5.957 ± 0.13%	54	5.230 ± 0.33%	87.8	0.80	4.013 ± 0.021	
       8	7.143 ±  5.01%	8.573 ± 5.72%	5.420 ± 0.17%	61	5.171 ± 0.27%	95.4	1.04	3.967 ± 0.019	
       9	96.108 ±  0.59%	8.693 ± 2.12%	8.116 ± 0.20%	61	5.238 ± 0.44%	64.5	1.22	4.019 ± 0.024	
     10	48.871 ±  0.61%	7.054 ± 4.40%	6.692 ± 0.11%	75	5.209 ± 0.22%	77.8 	1.18	3.997 ± 0.018	
                                                         Arithmetic mean K/Ca (n = 10)	58
± 11 	Arithmetic mean age (n = 10)
Weighted mean  age (n = 10)	4.001 ± 0.019
4.001 ± 0.016	
									
98-337B anorthoclase; J = 4.249E-4  0.40%; level 7, ANU45	
       1	4.076 ±  3.75%	6.884 ± 1.77%	5.358 ± 0.15%	78	5.194 ± 0.18%	96.9	2.26	3.978 ± 0.017	
       3	4.539 ±  9.27%	10.753 ± 2.45%	5.342 ± 0.13%	49	5.165 ± 0.27%	96.7	1.42	3.955 ± 0.019	
       4	4.698 ±  4.99%	10.329 ± 6.58%	5.334 ± 0.13%	51	5.153 ± 0.19%	96.6	1.32	3.946 ± 0.017	
       5	4.403 ±  6.25%	9.122 ± 6.32%	5.374 ± 0.12%	58	5.200 ± 0.20%	96.8	1.36	3.983 ± 0.018	
       6	4.736 ±  5.31%	6.236 ± 2.98%	5.417 ± 0.14%	84	5.234 ± 0.20%	96.6	1.08	4.008 ± 0.018	
       8	6.435 ±  4.03%	10.540 ± 3.85%	5.405 ± 0.17%	50	5.172 ± 0.23%	95.7	1.30	3.961 ± 0.018	
                                                       Arithmetic mean K/Ca (n = 6)	    61
± 15	Arithmetic mean age (n = 6)
Weighted mean age (n = 6)	3.972 ± 0.022
3.973 ± 0.016	

 = 5.543 x 10-10 a-1    	Sensitivity 2.4 (± ~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 = 0.029 to 0.044 depending on position
40Ar* – radiogenic 40Ar           39ArK – K-derived 39Ar               Irradiated for 24 hours in X33 or X34, HIFAR reactor
Mass of alkali feldspar used in each analysis in range: 98-307A – 1.4 to 4.1 mg; 98-307B – 1.0 to 1.8 mg; 
98-337A – 1.6 to 5.7 mg; 98-337B – 1.7 to 4.1 mg
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