Chronostratigraphy of the Miocene–Pliocene Sagantole Formation, Middle Awash Valley, Afar rift, Ethiopia Paul R. Renne* Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 and Department of Geology and Geophysics, University of California, Berkeley, California 94720 Giday WoldeGabriel Los Alamos National Laboratory, Los Alamos, New Mexico 87545 William K. Hart Department of Geology, Miami University, Oxford, Ohio 45056 Grant Heiken Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Tim D. White Laboratory for Human Evolutionary Studies, Museum of Vertebrate Zoology and Department of Integrative Biology, University of California, Berkeley, California 94720 ABSTRACT 1999). The sediment deposited in these basins is largely volcaniclastic in ori- gin and is interbedded with mafic lava and compositionally bimodal tephra The Sagantole Formation comprises more than 200 m of lacustrine, that are products of intermittent volcanism related to the Afar plume. The alluvial, and volcaniclastic sediments, plus compositionally bimodal Afar and the Main Ethiopian rift, covering an area of ~3.5 × 105 km2,began tephras and basaltic lavas, exposed in a domelike horst named the to form at least by 25 Ma (Moore and Davidson, 1978; Berhe et al., 1987; Central Awash Complex in the southwestern Afar rift of Ethiopia. The WoldeGabriel et al., 1990, 1991, 1992; Ebinger et al., 1993; Yemane et al., Sagantole Formation is widely known for abundant vertebrate faunas, 1999), and thick accumulations of sediment, lava, and tephra have been sub- including the 4.4 Ma primitive hominid Ardipithecus ramidus. New sequently deposited into the subsiding rift basins. In the Middle Awash re- lithostratigraphic data are used to subdivide the Sagantole Formation gion of the southern Afar rift, faulting, uplift, differential subsidence, and ero- into the Kuseralee, Gawto, Haradaso, Aramis, Beidareem, Adgantole, sion have produced windows into one such rift-basin succession. and Belohdelie Members, in ascending order. The members are de- The domelike structure of the Central Awash Complex (Kalb et al., 1982) fined on the basis of lithologic differences and laterally continuous comprises rift-bound Neogene basaltic and silicic lava and tephra as well as bounding tephras. volcaniclastic fluvial and lacustrine deposits. This structure rises just west of 40Ar/39Ar dating of 12 intercalated volcanic units firmly establishes the modern Awash River, some 20 km east of the western escarpment bound- the age of the Sagantole Formation to be 5.6 to 3.9 Ma, significantly ing the southern Afar rift. The sedimentary strata have produced an extensive older than previous proposals based on erroneous correlations. Mag- vertebrate fauna, including fossil remains of the earliest known Pliocene ho- netostratigraphic data reveal eight paleomagnetic polarity zones, minid Ardipithecus ramidus (White et al., 1994, 1995; WoldeGabriel et al., which can be correlated unambiguously with the Thvera, Sidufjall, 1994, 1995). Nunivak, and Cochiti Subchrons of the Gilbert Chron. Thus, by refer- The north-northeast–trending rift axis of the Main Ethiopian rift projects ence to the geomagnetic polarity time scale, seven additional chrono- east of the modern Awash River and is at a higher elevation compared with logical datums can be placed in the Sagantole Formation. With a total the western side of the rift floor where the Central Awash Complex is lo- of 19 such datums, the age resolution anywhere in the Sagantole cated. The region surrounding the Central Awash Complex is dominated by Formation is better than ±100 k.y., making this the best-dated the flat terrain of the rift floor except for low, gravel-capped mesas, hog- Miocene–Pliocene succession in Africa. backs, and fault-controlled, undulating hills. The Central Awash Complex structural dome, which rises about 200 m above the surrounding rift floor, is INTRODUCTION radially dissected by drainages that are generally controlled by rift-parallel and transverse normal faults (Fig. 1). Strata are exposed in all these The Ethiopian rift system comprises the Afar Depression in the northeast, drainages, but the overall succession is best revealed in the Amba, Urugus, the centrally placed Main Ethiopian rift, and the 300-km-wide rifted zone lo- Sagantole, and Aramis catchments (Fig. 1). The volcaniclastic sediments of cated in the southwest of the country. The system is characterized by large, the Central Awash Complex are differentially armored from erosion by rapidly subsiding depocenters, some of which are well known for important basaltic lavas and by strongly cemented sandstone and conglomerate beds. paleontological and archaeological resources, including much of the fossil Previously published information (Haileab and Brown, 1992; Kalb, 1993, record of the Pliocene-Pleistocene hominids (e.g., WoldeGabriel et al., 1995) on the chronostratigraphic framework for the Central Awash Complex relied on lithologic similarity, coarse biochronology, and broad structural in- *E-mail: [email protected]. ference without radioisotopic, geochemical, or magnetostratigraphic data. As Data Repository item 9951 contains additional material related to this article. GSA Bulletin; June 1999; v. 111; no. 6; p. 869–885; 9 figures; 1 table. 869 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/6/869/3383204/i0016-7606-111-6-869.pdf by guest on 27 September 2021 Figure 1. Location map based on composite aerial photography showing measured sections, sample localities (numbered), streams, and Afar villages. Locations of magnetostratigraphic sections shown by white lines between Gawto and Adgantole drainages. 870 Geological Society of America Bulletin, June 1999 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/6/869/3383204/i0016-7606-111-6-869.pdf by guest on 27 September 2021 SAGANTOLE FORMATION, MIDDLE AWASH VALLEY,AFAR RIFT, ETHIOPIA Figure 2. Lithostratigraphy of the Sagantole Formation showing stratigraphic levels of the six members defined herein. Locations of named and unnamed tephras are indicated by arrows. Stratigraphy is based on measured sections shown in Figure 1. a result, previous age estimates and correlations have proven inaccurate. Fur- the ages of recent and anticipated future paleontological discoveries as well thermore, the stratigraphic nomenclature used previously has proven difficult as long-range tephra correlations. To this end, this paper presents new to apply and in some instances misleading. We have focused, therefore, on 40Ar/39Ar, paleomagnetic, and stratigraphic data. The new data substantiate the abundant, laterally extensive tephra of the Central Awash Complex to the antiquity of the volcanic and sedimentary units of the Central Awash construct a chronostratigraphic framework. Chemical analyses of these Complex and firmly place Ardipithecus ramidus at 4.4 Ma. A further goal of tephras, from hundreds of samples collected throughout the Central Awash this paper is to compare the ages of geomagnetic polarity transitions derived Complex since 1992, are ongoing. from 40Ar/39Ar dating with ages predicted by various polarity-reversal time Where possible we have adopted earlier stratigraphic nomenclature. For scales. Our results are consistent with the time scale based on Milankovitch tephra significant to geochronologic or correlation purposes, we have fol- cycles (e.g., Hilgen, 1991; Lourens et al., 1996). lowed prior convention in assigning them Afar names for animals, e.g., the Daam Aatu Basaltic Tuff (WoldeGabriel et al., 1994, 1995). LITHOSTRATIGRAPHY OF THE SAGANTOLE FORMATION Although research in the Middle Awash paleoanthropological study area is ongoing, a basic chronostratigraphic framework can now be established The various Neogene and Quaternary deposits of the Middle Awash region for the Central Awash Complex. Such a framework is useful for constraining and environs were grouped broadly into the Awash Group and subdivided into Geological Society of America Bulletin, June 1999 871 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/6/869/3383204/i0016-7606-111-6-869.pdf by guest on 27 September 2021 RENNE ET AL. different formations and members on the basis of biochronology (Kalb, lava flows, stratigraphically below the Haradaso Member of the Sagantole 1993). The Sagantole Formation contains an extensive record of pyroclastic Formation. The Kuseralee Member is therefore defined herein as the lowest volcanism, primarily represented by distal tephra layers, interbedded with la- member of the Sagantole Formation and removed from the currently more custrine, lake-margin, flood-plain, and fluvial sediment. Kalb et al. (1982) in- poorly defined Adu-Asa Formation of the western rift margin. formally subdivided the Sagantole Formation into the Haradaso and Aramis Over 20 m of dark olive-gray gypsiferous siltstone and claystone and in- members and the “Beearyada” and “Kalaloo” beds. Following five seasons of terbedded bentonite layers and light gray sandstones are exposed in the field work (1992–1996), we have subdivided the formation into more pre- southwestern Central Awash Complex between Kuseralee and Amba, cisely defined members that are temporally discrete, laterally continuous, where they underlie the basalt flows of the Gawto Member. The base of the lithostratigraphically distinct, and readily recognizable in the field. The Kuseralee Member is obscured by sand in the modern Amba drainage. A Sagantole Formation comprises