AR-40/AR-39 AGE CALIBRATION OF THE LITHOMAGNETIC AND PALEOMAGNETIC STRATIGRAPHIES OF THE NGORORA FORMATION, KENYA

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. Magnetochronologic age estimates are younger than the isotopic ages by an average of 0.18 Ma, but much of the discrepancy 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 ~25 cm/1000 yr to ~5 cm/1000 yr toward the top of the section. A premolar tentatively identified as Proconsul sp. indet. found in the Ngorora Formation near the village of Bartabwa has been dated at ~12.42 Ma, representing perhaps the last known occurrence of this genus in the fossil record. -from Authors