Paleomagnetism of Miocene East African Rift sediments and the calibration of the geomagnetic reversal time scale.
Journal of Geophysical Research. 90:4639-4646., Washington, DC, United States (USA): American Geophysical Union, Washington, DC 10.1029/JB090iB06p04639 Abstract
Paleomagnetic stratigraphy and K-Ar age determinations are reported for the type section of the middle Miocene Ngorora Formation, found in the Kenya rift valley. The magnetostratigraphy is well correlated to the geomagnetic reversal time scale (GRTS) and spans from the lower part of Chron C5 (9) to Chron C5AB-r (14). K-Ar dates were determined for euhedral sanidines, handpicked from seven tuff horizons within the Ngorora Formation and the underlying Turn phonolite flow. These dates can therefore be tied directly to the GRTS. The eight dates fall into three discrete groups averaging 12.5 + or - 0.22 Ma (mean and standard deviation of results from four tuffs), 11.6 + or - 0.06 Ma (mean and standard deviation from three tuffs), and 10.16 + or - 0.38 (average of three analyses from one tuff). We interpret the age groups as resulting from three successive eruptive episodes, the stratigraphic positions of which are well constrained. In spite of episodic supply at the eruptive source, sediment accumulation is continuous at the resolution of the GRTS. This suggests that accumulation is controlled by basin subsidence rather than sediment supply. Sanidine dates support an age for the older boundary of marine magnetic anomaly 5 of about 10 Ma, as opposed to 11.12 Ma, suggested by the most recent results from Icelandic basaltic lava flows.