Anna K. Geology and geochronology of the middle Behrensmeyer Miocene Kipsaramon site complex, Department of Paleobiology, Muruyur Beds, Tugen Hills, Kenya National Museum of Natural History, Smithsonian The Muruyur Beds are a substantial sedimentary deposit within Institution, Washington, DC a middle Miocene sequence of mafic volcanic flows associated 20560-0121, U.S.A. E-mail: with early stages of rifting in the central Kenyan Rift Valley. They [email protected] are best represented in the Muruyur region, near Bartabwa, north of Kipsaramon, where dates range from 16·0 to 13·4 Ma. At Alan L. Deino Kipsaramon, located about 10 km south of Muruyur along the crest Berkeley Geochronology of the Tugen Hills, the upper Muruyur Beds are absent and the lower Center, 2455 Ridge Road, part can be divided into three members. Important fossil sites within Berkeley, California 94709, Member 1 are dated between 15·8 and 15·6 Ma, and within Member U.S.A. E-mail: 3 between 15·6 and 15·4 Ma. BPRP#89, in Member 1, is a bonebed [email protected] at least 2500 m2 in areal extent and up to 30 cm thick, which constitutes one of the richest concentrations of in situ fossil vertebrate Andrew Hill bones in eastern Africa. BPRP#91, at approximately the same level at Department of Anthropology, BPRP#89, is the source of a hominoid talus and other mammal and Yale University, Box 208277, bird fossils. In Member 3, BPRP#122 has produced specimens of at New Haven, Connecticut least five individuals of the hominoid Equatorius, including a partial 06520, U.S.A. E-mail: skeleton. The Muyuyur Beds were deposited near the western margin [email protected] of a lake that was formed during the early stages of faulting and volcanism in the African Rift system. The bonebed in Member 1 appears to represent the influx of fluvially transported vertebrate and John D. Kingston plant remains into a shallow portion of the lake. Elements of the fauna Department of Anthropology, as well as stable isotopes that indicate both forest and more open Emory University, 1557 environments occurred in proximity to the lake during the time of Pierce Drive, Atlanta, Georgia deposition of Member 1. 30322, U.S.A. E-mail: [email protected] Jeffrey J. Saunders Illinois State Museum, RCC 1011, East Ash Street, Springfield, Illinois 62703, U.S.A. E-mail: [email protected] Received 28 February 2001 Revision received 13 June 2001 and accepted 14 June 2001 Keywords: Kipsaramon, Miocene, Kenya, Tugen Hills, stratigraphy, Journal of Human Evolution (2002) 42, 11–38 geochronology, hominoid, doi:10.1006/jhev.2001.0519 Equatorius. Available online at http://www.idealibrary.com on Introduction in the Baringo District of Kenya. The fossil assemblages are concentrated primarily at There are a number of very rich fossil sites in two horizons within the Muruyur Beds, and the area around Kipsaramon, Tugen Hills, contain a varied vertebrate fauna, including 0047–2484/02/010011+28$35.00/0 12 . ET AL. Figure 1. (a) Overall context of the Kipsaramon site within the East African Rift system, showing other hominoid-bearing sites of middle Miocene age (solid circles). Heavy black lines indicate major faults associated with the rift system. monkeys and hominoid primates such as first described by Chapman, who also first Equatorius africanus (Hill et al., 1986, 1991; recorded fossils from the unit (Chapman, Hill & Ward, 1988; Hill, 1989, 1994, 1995, 1971; Bishop et al., 1971). Exposures of 1999; Brown et al., 1991; Ward et al., 1996, these sediments extend N–S along the sharp 1999; Kelley et al., 2002; Sherwood et al., crest of the Tugen Hills fault block, occur- 2002), as well as plant, ostracod, and mol- ring on the steep terrain of the eastern luscan remains. The Muruyur Beds were flank where the Saimo fault-scarp faces the 13 Figure 1. (b) Mapped extent of the Muruyur Beds (dark gray areas); dashed-line box shows area enlarged in C. (c) Topographic map of the portion of the Tugen Hills indicated in (b), showing locations of the Kipsaramon fossil localities. Dashed-line box shows the area enlarged in Figure 4. Baringo basin, and on the western flank Kipsaramon area and an interpretation of which slopes more gradually westward the paleoenvironmental setting of the fossil- towards the Kerio Valley [Figure 1(a), (b)]. bearing sedimentary units. We present Fossiliferous localities occur along the preliminary hypotheses concerning the ridge itself and in small patches of exposure genesis of the two main fossil assemblages, on the heavily vegetated, and now but a comprehensive treatment of their intensively farmed, west-facing hillsides taphonomy and paleoecology is beyond the [Figure 1(c)]. scope of this article. We also discuss, but do The Kipsaramon geology and faunal not resolve, several problems concerning remains are important because of the pre- correlations of different outcrops of the cision of the dating, the quality of the verte- Muruyur Beds outside of the Kipsaramon area brate record, and the geographic position of and the stratigraphic position of the unit in this record between the hominoid-producing the regional sequence of volcanic formations. sites of Maboko and Fort Ternan in the Winam Gulf in western Kenya and the Turkana Basin and Samburu areas of Geological overview northern and central Kenya. This paper pro- The fossiliferous deposits of the Kipsaramon vides the first comprehensive geological site complex occur near the base of the long and geochronological framework of the Neogene volcanic-sedimentary sequence 14 . ET AL. Figure 2. General stratigraphic section of the Tugen Hills succession, showing the positions of the sedimentary and volcanic units and hominoid sites. The Kipsaramon localities occur in the Muruyur Beds, which interfinger with phonolite flows near the base of the succession. forming the regional topography of the Rift present in the Tugen Hills, is Group II, also and the Tugen Hills. Correlation and radio- known as the Tugen Hills Group. It is com- metric dating of strata throughout the region posed mainly of flood phonolites, and in the have revealed a relatively continuous record Tugen Hills consists of the following for- of geology and faunas for this time interval, mations: the Sidekh Phonolites Formation extending from over 16 Ma to the present (Chapman, 1971; originally the Saimo (Figure 2). Various aspects of structure and Phonolites Formation of Martyn, 1969), the the large-scale stratigraphy are described in Noroyan Formation, the Tiim Phonolites Bishop et al. (1971), King & Chapman Formation, the sedimentary Ngorora For- (1972), King (1978), Chapman et al. (1978) mation, and the Ewalel Phonolites. and Hill et al. (1986). Neogene formations The Sidekh Phonolites Formation con- in the northern Rift, at least south of about tains a number of sedimentary members, 130N, can be divided into five groups. The largely unfossiliferous. In the north the Group that concerns us here, the lowest formation totals 1200 m in thickness, 15 comprising 10–12 phonolite flows and two lying the Ngorora Formation at Kabasero of sedimentary intercalations. Further south it 12·3 Ma. A re-dating of this sample using is about 700 m thick, and includes 5–6 flows 40Ar/39Ar step-heating resulted in a date of and up to 80 m of interbedded sediment. 13·240·13 Ma (Deino et al., 1990, cor- The Sidekh Phonolites represent some of rected for revised age of the Fish Canyon the earliest volcanics associated with the Tuff standard, Renne et al., 1998). developing rift valley in this part of East Africa. Geology of the Muruyur Beds The lavas of the overlying Tiim Phonolites Formation are about 1050 m The first allusion to the Muruyur Beds thick in the center of the Tugen Hills area, (spelled Muruywr) is in Martyn (1969) who thinning to about 300 m by 1N. There are anticipated Chapman’s later description of between 11 and 15 flows in the type area, the unit (Chapman, 1971) in suggesting that around Tiim mountain about 12 km south these beds are lateral equivalents of sedi- of Kipsaramon. The Formation was divided ments within the Tiim Phonolites near by Chapman (1971) into two members, the Kipcherere in the neighboring area to the lower Atimet Trachyphonolite Member, south. In formally naming the unit, and an upper Kamuiton Phonolites Chapman (1971) gave a detailed account of Member. The Muruyur Beds were described their stratigraphy and lithologies. He by Chapman as a sedimentary intercalation described the beds as ranging in thickness within the Kamuiton Phonolites Member. from less than 10 ft (3 m) near Kamelon to The Sidekh Phonolites Formation is 900 ft (274 m) in the Muruyur section itself probably at least partially equivalent to the [Figure 1(b)], and estimated that 500–600 ft Elgeyo Formation, the Chof Phonolites (152–183 m) of this discrepancy was due to Formation, and the sedimentary Tambach contemporaneous movement along a single Formation of the Elgeyo Escarpment, which fault, leading to differential sedimentation. forms the present western boundary of the Apart from the occurrence of fish fossils Rift in this region (Lippard, 1973; Chapman in various parts of the section, he noted et al., 1978). The Tiim Phonolites For- one fossil site (2/304) that produced a mation is, on the basis of radiometric deter- crocodilian vertebra. The age of the unit was minations (Chapman & Brook, 1978; Hill constrained by virtue of its stratigraphic et al., 1985; Deino et al., 1990), generally placement within the Tiim Phonolite equivalent to the Uasin Gishu Phonolite Formation. Formation of the Elgeyo. The first mention of the unit other than in K/Ar determinations reported in unpublished dissertations was in Bishop Chapman & Brook (1978) give dates for the et al.