$$^{40}Ar/^{39}Ar$$Age Calibration of the Litho- and Paleomagnetic Stratigraphies of the Ngorora Formation, Kenya

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Abstract
Precise eruptive ages have been determined by the laser-fusion, single-crystal 40Ar/39Ar method for juvenile volcanic feldspars from reworked and contaminated volcaniclastic rocks of the middle Miocene Ngorora Formation, Kenya Rift Valley. These ages range from 13.06 Ma at the base to 10.51 Ma toward the top of the type section near Kabarsero. Correlation of the local paleomagnetic stratigraphies with the geomagnetic reversal time scale yields magnetochronologic age estimates that are younger than the isotopic ages by an average of 0.18 Ma. Much of the dicrepancy can be eliminated if an inferred change in sea-floor spreading rate occurred at 13 Ma or earlier, rather than at 10.42 Ma as previously suggested. Sedimentation rates at Kabarsero calculated from the 40Ar/39Ar results decrease from initial values of .apprx. 25 cm/1000 yr to .apprx. 5 cm/1000 yr toward the top of the section. The initial rapid sedimentation rates characterize the first 0.1 to 0.3 m.y. following emplacement of the underlying, voluminous, basin-filling Tiim Phonolites, indicating that the Baringo Basin at this time may not have existed as a rift valley created by extensional tectonics, but instead may have been a subsidence feature formed in response to removal of large volumes of magma from the lithosphere. A premolar tentatively identified as Proconsul sp. indet. found in the Ngorora Formation near the village of Bartabwa has been dated at .apprx. 12.42 Ma, representing perhaps the last known occurrence of this genus in the fossil record.