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Stratigraphy of the Koobi Fora Formation (Pliocene and Pleistocene) in the Loiyangalani Region of Northern Kenya

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Stratigraphy of the Koobi Fora Formation (Pliocene and Pleistocene) in the Loiyangalani Region of Northern Kenya Available online at www.sciencedirect.com Journal of African Earth Sciences 51 (2008) 277–297 www.elsevier.com/locate/jafrearsci Stratigraphy of the Koobi Fora Formation (Pliocene and Pleistocene) in the Loiyangalani region of northern Kenya Patrick N. Gathogo a, Francis H. Brown a,*, Ian McDougall b a Department of Geology and Geophysics, University of Utah, 135 South 1460 E, Room 209, Salt Lake City, UT 84112, USA b Research School of Earth Sciences, The Australian National University, Mills Road, Acton, Canberra, ACT 2601, Australia Received 30 October 2007; received in revised form 17 January 2008; accepted 29 January 2008 Available online 20 February 2008 Abstract Tuffaceous marker beds in the Loiyangalani region were previously identified as correlative with similar beds in the Shungura and Koobi Fora Formations of southwestern Ethiopia and northwestern Kenya. Associated sedimentary strata correlate with the Koobi Fora Formation, and include intercalated basalt flows. In the Loiyangalani region the Koobi Fora Formation has a composite thickness of at least 257 m, and records one time interval for which no strata are known in the Koobi Fora region. Four members of the Koobi Fora Formation (Lonyumun, Tulu Bor, Burgi, and KBS) are recognized in the region, and a composite member (Moiti–Lokochot) pro- posed because the Lokochot Tuff, the formal boundary between the Moiti and Lokochot Members has not been identified. This extends the known geographic area of the Koobi Fora Formation 100 km southward. These strata, together with others, were previously termed ‘Loiyangalani formation (or Formation)’. Deposition in the Loiyangalani region began at about the same time as in the Koobi Fora and Nachukui Formations. Newly named basalts are Kankam (3.2–3.3 Ma), Lenderit (2.02 to 2.18 ± 0.02 Ma), and Balo (1.79 ± 0.02 Ma). We also report K/Ar ages on basalts from Mt. Kulal (2.04 ± 0.02 to 2.40 ± 0.03 Ma) east of the region and from Serima Gorge (0.77 ± 0.01 Ma) south of the study area. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Kenya; Turkana Basin; Omo Group; Koobi Fora Formation; Stratigraphy; Loiyangalani 1. Introduction mation and intercalated basalts in the Loiyangalani region southeast of Lake Turkana (Figs. 1 and 2). The Koobi Fora Formation northeast of Lake Turkana Sedimentary and volcanic sequences of the Loiyangalani in northern Kenya is renowned for its prehistoric sites and region discussed in this study were previously mapped and comparatively complete stratigraphic Pliocene and Pleisto- assigned pre-Pliocene ages by Ochieng’ et al. (1988), Wil- cene sections. Detailed accounts of the stratigraphic record kinson (1988), and Rop (1990). Preliminary visits to the of the formation are available for the Ileret, Karari, Koobi region, however, showed that the area preserves tuffaceous Fora, and Allia Bay regions (Brown and Feibel, 1986, marker beds that lie within the Omo Group (de Heinzelin, 1991; Gathogo and Brown, 2006), and the mammalian fos- 1983), a sedimentary sequence that crops out elsewhere in sil record is described by Leakey and Leakey (1978), Harris the Omo–Turkana Basin (southwestern Ethiopia and (1983, 1991), Wood (1991), and Isaac and Isaac (1997).In northwestern Kenya). Potassium–argon dates on basalts this study we describe newly recognized sections of the for- from the Loiyangalani region are of Pliocene age. Motiva- tion for this study, therefore, commenced from the need to * clarify inconsistencies about the geology of the region. Corresponding author. Tel.: +1 801 581 8767; fax: +1 801 581 7065. Potassium–argon ages reported here were obtained at E-mail addresses: [email protected] (P.N. Gathogo), [email protected] (F.H. Brown), [email protected] (I. the Research School of Earth Sciences, the Australian McDougall). National University, using methods described in McDou- 1464-343X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2008.01.010 278 P.N. Gathogo et al. / Journal of African Earth Sciences 51 (2008) 277–297 Fig. 1. Map of the Loiyangalani region with contours at 100 m intervals taken from the 90-m USGS digital topographic dataset. Locations of dated samples are shown as filled circles with the sample number. Locations of measured sections are shown as short lines identified by the last two digits of the section number (of the form PNG-LY08) given in italics. gall and Feibel (1999). Electron microprobe analyses on the (2004) for the Pliocene–Pleistocene boundary at 1.81 Ma, glass phase of tuffs were obtained at the University of Utah and the Pliocene–Miocene boundary at 5.33 Ma. following procedures described by Nash (1992), and using standards reported in Brown et al. (2006). Trace element 1.1. Previous studies analyses on the glass phase of tuffs were obtained by X-ray fluorescence at the University of Utah following pro- Hitherto the geology of the Loiyangalani region has cedures described by Cerling and Brown (1982). For our been based on studies conducted at reconnaissance scale discussion, we accept the age estimates of Gradstein et al. by Ochieng’ et al. (1988), Wilkinson (1988), and Rop P.N. Gathogo et al. / Journal of African Earth Sciences 51 (2008) 277–297 279 Fig. 2. Geological map of the Loiyangalani region. See Fig. 1 for sample locations and section locations. (1990). Their published geologic maps (scale 1:250,000) outcrop areas in the region. Metamorphic rocks regarded show Neogene sedimentary and volcanic rocks as the dom- as being of late Proterozoic or early Phanerozoic age (Ochi- inant exposures, comprising nearly equal proportions of eng’ et al., 1988) are also present, but they are exposed over 280 P.N. Gathogo et al. / Journal of African Earth Sciences 51 (2008) 277–297 Composite Column Named markers Koobi Fora Fm. Members Balo Basalt KBS 250 KBS Tuff Member upper Lenderit Basalt 200 Burgi Member middle Kokiselei Tuff 150 lower Lokalalei Tuff Kankam Basalt Tulu Bor Member Thickness(m) Tulu Bor Tuff 100 Moiti-Lokochot Member Moiti Tuff 50 Gombe Group basalts Lonyumun Member Key to lithology Basalt flow Volcanic ash Sediments(claystone, mudstone, and sandstone) 0 Fig. 3. Composite section of the Koobi Fora Formation in the Loiyangalani region. a very limited area. These earlier studies assigned conflict- that the ‘‘base of the Loiyangalani formation appears to be ing geologic ages to rock sequences in the region, and no contemporaneous with the Kajong Formation (early to standard stratigraphic scheme for the rock units was mid Miocene; Savage and Williamson (1978)), being simi- defined or used. A synopsis of these three studies is pre- lar in lithologic composition and stratigraphic position sented below to demonstrate the nature of stratigraphic (overlying metamorphic rocks).” Wilkinson (1988) maps problems that exist. deposits exposed in the northern part of the region as Ochieng’ et al. (1988) map sedimentary rocks in the cen- ‘‘[s]ub-volcanic sediments (‘Turkana Grits’)” that ‘‘includes tral and southern parts of the Loiyangalani region as the Sera Iltomia and Kajong Formations of previous work- ‘‘Loiyangalani formation”, an informal stratigraphic unit ers”, and he tentatively assigns these deposits to the Creta- to which they tentatively assign a Miocene age. They note ceous System. Rop (1990, Fig. 15) mapped all sedimentary P.N. Gathogo et al. / Journal of African Earth Sciences 51 (2008) 277–297 281 rocks exposed in the northern and southern parts of the Five members that constitute the Pliocene sequence of region as one stratigraphic unit, the ‘‘Kajong Formation.” the formation have been identified, most outcrops of which Assignment of sedimentary strata in the region to pre- occur in the central and western parts of the region. These Miocene by Rop (1990) is based on his observations that Pliocene deposits are locally underlain by metamorphic these strata are overlain by ‘‘Loiyangalani Formation”, rocks of presumed late Proterozoic and/or early Phanero- and he notes that ‘‘recent K–Ar dates in the base of the zoic age, and are disconformably overlain by the Holocene Loiyangalani Formation yield a date of 18.4 ± 0.5 m.y. Galana Boi Formation of Owen and Renaut (1986).In (Fitch and Miller Associates, Cambridge, unpub. data).” thickness and exposure Pliocene basalts dominate over sed- Virtually all volcanic rocks in the Loiyangalani region imentary strata in the northern, eastern, and southern parts are mapped as plateau basalts and assigned a Pliocene of the region. Reconnaissance work and satellite imagery age in the northern part of the region by Wilkinson suggest that these basalts also extend north and east (1988), but a Miocene age in the central and southern parts beyond the region of study. of the region by Ochieng’ et al. (1988). In addition Ochieng’ Strata of the Lonyumun, Tulu Bor, Burgi, and KBS et al. (1988) mapped the two northern hills at El Molo Bay members in the Loiyangalani region are identified and (islands during high lake levels) as Pliocene and Pleistocene described under their corresponding formal names below. volcanic breccia, and two igneous rock outcrops immedi- The remaining two Pliocene members, Moiti and Loko- ately northeast and southeast of the bay as dolerite plugs chot, cannot be differentiated as separate formal units and sheets. Rop (1990) mapped all plateau basalts in the because the base of the Lokochot Tuff, the formal bound- region as Miocene. A revised geological map of the region ary between the two members, has not been identified in (Fig. 2) is based on work reported here and a summary of the Loiyangalani region. Thus, using conventions set by the stratigraphy is shown in Fig. 3. the International Stratigraphic Code (Salvador, 1994) for naming of units with stratigraphic uncertainty, we provi- 2. Koobi Fora Formation and Omo Group deposits sionally adopt the name Moiti–Lokochot Member to refer to undifferentiated strata of the Moiti and Lokochot Mem- Brown and Feibel (1986) formally defined the Koobi bers in the Loiyangalani region.
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