JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. B6, PAGES 4639-4646, MAY 10, 1985 Paleomagnetism of Miocene East African Rift Sediments and the Calibration of the Geomagnetic Reversal Time Scale L. TAUXE, 1M. MONAGHAN, 2, R. DRAKE, 3 G. CURTIS, 3 AND H. STAUDIGEL 1 Paleomagnetic stratigraphy and K-Ar age determinationsare reported for the type sectionof the middle Miocene Ngorora Formation, found in the Kenya rift valley. The magnetostratigraphy is well correlatedto the geomagnetic reversal time scale (GRTS) and spansfrom the lower part of Chron C5 (9) to Chron C5AB-r (14). K-Ar dateswere determined for euhedralsanidines, hand- pickedfrom seven tuff horizonswithin the Ngorora Formation and the underlyingTiim phonolite flow. These dates can therefore be tied directly to the GRTS. The eight dates fall into three discrete groups averaging 12.5+ 0.22 Ma (mean and standard deviation of results from four tuffs), 11.6 + 0.06 Ma (mean and standard deviationfrom three tuffs), and 10.16+ 0.38 (average of three analyses from one tuff). We interpretthe agegroups as resulting from three successive erup- tive episodes, the stratigraphic positions of which are well constrained. In spite of episodic supply at the eruptivesource, sediment accumulation is continuous at the resolution of the GRTS. This sug- gests 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 basalticlava flows. INTRODUCTION The Baringo Basin of the Kenya Rift has long been a fruitful source of Neogene vertebrate fossils, including fossil hominoids [Bishop and Chapman, 1970;Bishop et al., 1971; Bishop and Pickford,1975; Pickford,1975, 1978a, b]. Within the basin, a relatively completesedimentaryrecord spanning the last 20 Ma lies exposed in many disjunct outcrops. Temporal and stratigraphiccorrelations among the many exposures, although essential for the under- standing of the vertebrate record, are only poorly con- strained. The purpose of our research in the Baringo Basin is to establish a temporal framework based on K-Ar dating and paleomagnetictechniques. We present here a discussion of the paleomagnetic data from the type section of the Ngorora Formation as well as a discussion of the implications of our results for the calibration of the geomagnetic reversaltime scale(GRTS). GEOLOGICAL BACKGROUND The Baringo Basin lies in the Kenya Rift between the equator and IøN latitude (Figure 1). Regional mapping of the basin began as part of the East African Geological Research Unit directed by B.C. King, then of Bedford College (University of London). Early efforts focused on regional geochronology and structural aspects of the region and formed the basisfor the stratigraphic sequence as it is understood today [Martyn, 1967; McCall et al., 1967; Bishop and Chapman, 1970; King and Chapman, 1972; Bishop andPickford, 1975]. I Scripps Institution of Oceanography, La Jolla, California. 2Department of Geologyand Geophysics, Yale University, New Haven, Connecticut. 3Department of Geology and Geophysics, University of Califor- nia, Berkeley. Copyright 1985 by the American Geophysical Union. Paper number 4B5215. 0148-0227/85/004B-5215505.00 The Miocene outcrops are dominated by thick sequences of lava flow units, including the Sidekh, Noroyan, Tiim, Ewalel, Kabarnet, and Kaparaina forma- tions with interbedded sedimentary units including the Ngorora and Lukeino formations. The Ngorora Forma- tion is defined as the strata overlying the Tiim phonolite and underlying the Ewalel phonolite. The following is a discussion of results from the type sectionof the Ngorora Formation at Kabarsero. Outcrops of Ngorora are shown in Figure 1. LITHOSTRATIGRAPHY OF THE TYPE NGORORA FORMATION The Kabarsero section of the Ngorora Formation (described by Bishop and Pickford [1975]) is composed of volcaniclasticand epiclastic sediments deposited on top of a weathered Tiim phonolite flow. It is well bedded on a millimeter to meter scale but displays substantial lateral facies variation, making lithostratigraphic correlations between isolated outcrops tenuous. Nearly all of the sedi- ments are reworked pyroclastic material, depositedin lakes and streams or on floodplains. Basement-derived clasts are extremely rare, comprising less than 0.1 vol % of the exposed strata. In the type area the formation is 420-460 m thick. At the base of the section the sedimentsare poorly sorted, volcaniclastic sandy siltstonesand clays. Debris flows are common. The poorly sorted lower layers are overlain by cross-bedded tuffaceous fine sands and silts as well as an accretionary lapilli tuff. This unit is overlain by finely lam- inated clays and shales with minor channel-sand deposits. Superposed on these sediments is a sequence of clays interbeddedwith tuffaceoussilts and sands. Near the top of the section is a thick unit of low-density white shales (diatuffs). These white shales consist primarilyof altered volcanic ash but are in part diatomaceous with locally abundant fish debris. The white shale unit is punctuated by a single, poorly sorted, matrix-supported debris flow and is cappedby a pumice lapilli, medium to coarse sand- stone, known as the "buff tuff." The section between the 4639