Indonesian Journal on Geoscience Vol. 5 No. 3 December 2018: 221-234 INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources Journal homepage: hp://ijog.geologi.esdm.go.id ISSN 2355-9314, e-ISSN 2355-9306 The Magnetostratigraphy and the Age of So’a Basin Fossil-Bearing Sequence, Flores, Indonesia Dida Yurnaldi, Ruly Setiawan, and Emma Yan Patriani Centre for Geological Survey, Geological Agency Jln. Diponegoro no 57 Bandung - 40122 Corresponding author: [email protected] Manuscript received: November, 07, 2017; revised: January, 08, 2018; approved: June, 05, 2018; available online: November, 19, 2018 Abstract - Three fossil-bearing intervals have been recognized in the Pleistocene So’a Basin, with the upper one holding important evidence of hominin fossils. The sequence also contains numerous in situ stone artifacts and fossils of other vertebrate taxa. Therefore, multiple dating techniques are crucial to secure the age of the fossil and artifact- bearing layers, especially the one with the hominin remains. This paper deals with the palaeomagnetic dating of the So’a Basin sequence to assist other dating methods that have been applied, and to refine the chronostratigraphy of the area. Palaeomagnetic sampling was conducted in four sections along a west to east transect. Four magnetozones can be recognized, consisting of two reverse and two normal polarity zones. By using the available radiometric ages as a guide and comparing the So’a Basin magnetostratigraphy with the Standard Geomagnetic Polarity Time Scale (GPTS), it became clear that both reverse magnetozones are part of the Matuyama Chron, while the normal magnetozones are the Jaramillo subchron and the Brunhes Chron. These palaeomagnetic dating results support the available radiometric dates and refine the age of the fossil-bearing deposits of the So’a Basin. Keywords: Palaeomagnetic, Homo floresiensis, hominin, dating, polarity, rock magnetics © IJOG - 2018. All right reserved How to cite this article: Yurnaldi, D., Setiawan, R., and Patriyani, E.Y., 2018. The Magnetostratigraphy and the Age of So’a Basin Fossil-Bearing Sequence, Flores, Indonesia. Indonesian Journal on Geoscience, 5 (3), p.221-234. DOI: 10.17014/ijog.5.3.221-234 Introduction fossils, which are estimated to be c. 700,000 years old (Brumm et al., 2016). In order to refine the Recently, a mandible fragment and six hominin chronostratigraphy of the So’a Basin sequence teeth were recovered from one of the fossil-bearing and to allow basin-wide cross correlation, palaeo- strata in the So’a Basin, Flores, Indonesia. These magnetic sampling and analysis were carried out specimens are relatively similar to the holo-type on four stratigraphic sections. The combination of Homo floresiensisIJOG from Liang Bua, except the of palaeomagnetic dating and other radiometric size which is slightly smaller and the mandibular dating techniques has widely been used and first molar that has more primitive characteristics proven to be successful for dating archaeo- and than the Liang Bua specimens (van den Bergh et palaeontological sites around the world, for al., 2016a; see also Brown and Maeda, 2009). example: the Koobi Fora site (Lepre and Kent, Multiple dating methods were applied to 2010), Sterkfontein Cave (Herries et al., 2010), determine the age of these important hominin and the Melka Kunture site (Tamrat et al., 2014) Accredited by: - LIPI, valid August 2016 - August 2021 Indexed by: Scopus - RISTEKDIKTI, valid May 2016 - May 2021 221 Indonesian Journal on Geoscience, Vol. 5 No. 3 December 2018: 221-234 in Africa, the Ceprano- Fontana Ranuccio site up to 5° to the south (Hartono, 1961; O’Sullivan (Muttoni et al., 2009) and the Poiana Cireşului et al., 2001; Suminto et al., 2009). The Ola Bula site in Europe (Zeeden et al., 2009), and the Ni- Formation unconformably overlies the Ola Kile hewan Basin (Zuo et al., 2011; Ao et al., 2013) Formation and fills much of the basin with an up in Asia. In Indonesia, palaeomagnetic dating was to 100 m thick sequence. It consists of relatively used to date the Sangiran site in Java (Hyodo et undisturbed and horizontally bedded volcanic al., 1993; 2011) and The Talipu site in Sulawesi and sedimentary deposits, which started to in- (van den Bergh et al., 2016b). Magnetostratigra- fill the basin during the Late Pliocene or Early phy was applied to better constrain of the numeric Pleistocene (Murouka et al., 2002; Suminto et ages and to complement other dating methods, al., 2009). The Ola Bula Formation is divided into including Fission-Track, Argon-Argon, Electron three lithological members, from old to young: Spin Resonance, and optical dating techniques. the Tuff Member, the Sandstone Member, and the Limestone Member (Hartono, 1961; van den Geological, Palaeontological, and Geochrono- Bergh, 1999; and Suminto et al., 2009 - Figure 2). logical Context of The So’a Basin Three main fossil-bearing intervals have been The So’a Basin is located in the central recognized within the Ola Bula sequence. The part of Flores, almost entirely surrounded by oldest fossiliferous layers are developed within mountains and active volcanoes (Figure 1). It the Tuff Member, which has its most complete is drained by the Ae Sissa River, which empties development near the Tangi Talo site in the the basin through a deeply incised gorge to the central and deepest part of the basin. The fossil northeast, and forms a delta plain on the north fauna from this level consists of a dwarfed extinct coast (O’Sullivan et al., 2001; Suminto et al., elephant (Stegodon sondaari), a giant tortoise, 2009). The basin basement consists of the Ola and Varanuskomodoensis, but the site has not Kile Formation comprising massive and resis- yielded any stone artifact (Sondaar et al., 1994; tant andesitic breccias interbedded with minor Aziz et al., 2009). Two fossil intervals higher up tuffaceous siltstones, sandstones, and lava flows. in the sequence lie within the Sandstone Member It had been tilted by tectonic activity which dips and can be found at the Mata Menge site. Both 120o53’51” E 121o22’36” E So’a Basin < 400 m 32’58” S 32’58” S o o 8 8 Keli Esu s a is S e Welas Caldera 2 A 400 - 1000 m 4 Kelilambo So’a < < < < 1000 - 2000 m 1 3 > 2000 m Bajawa Boawae Kelindora Ambulobo Active volcano Inactive volcano Inirie Town Sawu Sea < Excavation/Trench 1 = Mata Menge 3 = Tangi Talo 0 km 10 20 2 = Wolo Sege 4 = Gero 58’56” S 58’56” S IJOG N o o 8 8 120o53’51” E 121o22’36” E INDONESIA Flores Komodo 8 River Liang Bua Ruteng Bajawa Ende Maumere 19 Soa Basin Boundary of 0 km 100 So’a Basin sequence 120 122 124 10 Figure 1. The four site locations of the So’a Basin that were sampled for this study (map modified after van den Bergh, 2010). 222 The Magnetostratigraphy and the Age of So’a Basin Fossil-Bearing Sequence, Flores, Indonesia (D. Yurnaldi et al.) Stratigra- Vertebrate fauna Age Lithology phic Unit & stone tools ime (Ma) T Recent Holocene volcanics & Tuffaceous sands and silts; lava flows alluvium gravels, sands, and silts. Hiatus 0.65 Thin-bedded micritic ftreshwater limestone and tuffaceous silts; minor tuffaceous sands. Member Limestone Tuffaceous sands and silts; debris flows; sheet flow deposit; conglomerates; Stegodon florensis minor white tuffs; Varanus komodoensis increasingly fine-grained in upper part. Hooijeromys nusatenggara Ola Bula Fm Small crocodile Sandstone Member 0.80 Large crocodile Stone artefacts 0.88 Early to Middle Pleistocene White and pink pumice tuffs and Stegodon sondari tuffaceous silts and sands; Varanus komodoensis Giant tortoise f Member minor fluvial channels. uf Small crocodile 0.94 T Hiatus 1.8 Andesitic breccia + minor tuffaceous Fm. silts and sands. Ola Kile Late Pliocene Figure 2. Generalized lithostratigraphy of the So’a Basin (after Suminto et al., 1999). levels contain the remains of a larger Stegodon recognized throughout the basin, including at the (Stegodon florensis), a giant rat (Hooijeromys Tangi Talo site, where it occurs at ~20 m above nusatenggara), Komodo dragon (Varanus komo- the Tangi Talo fossil bearing layer. Therefore, the doensis), unidentified crocodilian remains, and Tangi Talo fossil bearing layer could be much stone artifacts (Sondaar et al., 1994; van den Bergh older than ~0.9 Ma. 1999; van den Bergh et al., 2009; Brumm et al., In 2016, the Mata Menge two fossil-bearing 2016). In addition, the upper level has yielded the layers have been dated as well. Both fossil intervals remains of a small-sized hominin, and thought to occur above the WSI, indicating that they must be be the ancestral form of Homo floresiensis (van younger than ~1.02 Ma. Ages of the lower fossil den Bergh et al., 2016). interval lie between ~0.88 Ma and ~0.8 Ma, based Dates have come from multiple methods. Pre- on the fission-track dating of two sampled levels, viously, the Tangi TaloIJOG fossil assemblage was es- one directly below and one associated with the timated to be 0.9 Ma old based on palaeomagnetic fossil bearing layer (O’Sullivan et al., 2001). The and fission track dating (Sondaar et al., 1994; upper fossil-bearing interval, containing hominin Morwood et al., 1998). More recently, 40Ar/39Ar remains, has an age estimate based on its strati- dating by Brumm et al. (2010) of a widespread graphic position, because no direct dating associ- ignimbrite marker bed yielded an age of ~1.02 ated with this sedimentary layer. The hominin layer ma. This Wolo Sege ignimbrite (WSI) can be occurs 12.5 m above the level that yielded the F-T 223 Indonesian Journal on Geoscience, Vol. 5 No. 3 December 2018: 221-234 date of ~0.8 Ma and 13.5 m below a tephra layer a magnetic stirrer for SEM-EDX purposes.