Palaeogeography, Palaeoclimatology, Palaeoecology 206 (2004) 115 – 131 www.elsevier.com/locate/palaeo
Landscape development preceding Homo erectus immigration into Central Java, Indonesia: the Sangiran Formation Lower Lahar E. Arthur Bettis III a,*, Yahdi Zaim b, Roy R. Larick c, Russell L. Ciochon c, Suminto d, Yan Rizal b, Mark Reagan a, Matthew Heizler e b
a Department of Geoscience, The University of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242-4529, USA Department of Geology, Institute of Technology Bandung, Jalan Ganesha, no. 10, Bandung 40132, Indonesia c Department of Anthropology, The University of Iowa, Iowa City, IA 52242-1322, USA d Geological Research and Development Centre, Jalan Dr. Junjunan no. 236, Bandung 40174, Indonesia e New Mexico Bureau of Geology and Mineral Resources, Socorro, NM, 87801-4796, USA
Received 15 January 2003; received in revised form 13 October 2003; accepted 6 January 2004
Abstract The Sangiran Dome is the primary stratigraphic window for the Solo Basin, a coastal feature on the Pliocene – Pleistocene Sunda subcontinent south margin. In the Dome, the Lower Lahar unit (LLU) is a lahar-type debris flow overlying near-shore marine sediments. The event that emplaced the LLU likely originated from sector collapse on a neighboring volcanic edifice. Freshwater mollusc fossils in the LLU indicate that swamps or shallow lakes lay between the edifice and the current Dome area. 40 Ar/39Ar analyses on hornblende separates from six pumice lenses suggest that the LLU was deposited as early as 1.90 F 0.02 Ma. The event resulting in deposition of the LLU transformed late Pliocene near-shore marine environments into estuarine and marsh settings. Shortly thereafter, glacioeustatic sea level falls completed the local transition to fully terrestrial environments that attracted Homo erectus to southernmost Sunda in the Early Pleistocene. D 2004 Elsevier B.V. All rights reserved. Keywords: Homo erectus; solo basin; Sunda; Lahar;
40
Ar/39Ar step-heating
1. Introduction The Sangiran Dome is situated in Indonesia’s Central Java Province, 12– 20 km north of the city of Solo (Fig. 1). It offers a stratigraphic window into the Solo Basin, a prominent Pliocene – Pleistocene feature on the southern coast of the former Sunda subcontinent. For more than a century, Dome localities have yielded Early Pleistocene Homo erectus and * Corresponding author. Fax: +1-319-335-1821. E-mail address:
[email protected] (Y. Bettis). 0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.01.016
other important vertebrate fossils, and in December 1996 the area was declared a World Heritage site by the 20th World Heritage Committee (Widianto et al., 1996). The Dome has a long history of colonial, national and international research focusing on stratigraphy and paleontology. In recent years, archaeological and paleoanthropological attention has turned to the ecological conditions under which Plio-Pleistocene terrestrial fauna dispersed across the emergent subcontinent to inhabit its southernmost reaches (Huffman, 1999; Larick et al., 2000; O’Sullivan et al., 2001).
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Fig. 1. Location of the Sangiran Dome in Central Java, Indonesia. Study localities and the location of mud volcanoes in the Dome’s interior are shown on the left.
This paper presents new geological data on the Lower Lahar unit (LLU), a mass flow deposit that had a major role in transforming the Solo Basin littoral toward a number of terrestrial habitats that attracted Homo erectus and other large terrestrial mammals to the Solo Basin. We describe the sedimentology, stratigraphy, and age of the LLU as the first step in understanding the nature of environmental changes that made southern Sunda attractive to Homo. erectus.
2. Geology of the Sangiran Dome Within the Sangiran Dome, Homo erectus fossils are found in a long succession of deposits that range from lacustrine-marshy in the upper Sangiran Formation, to
riverine in the overlying lower and middle Bapang Formation (Fig. 1). During the late Middle Pleistocene, well after Homo erectus disappeared from the basin, a series of mud volcanoes domed the area presently known as Sangiran. Consequently, several tributaries of the Solo River have dissected the Dome, formed hilly topography, and exhumed Late Pliocene and Pleistocene deposits relating to Solo Basin infilling. The oldest exposed sediments ring four mud volcanoes at the Dome’s center. These are the marine limestones, siltstones, mudstones, and muddy sandstones of the Puren Formation (Figs. 1, 2, and 3A). Associated fauna includes an abundance of Anadara sp. marine molluscs, and rare freshwater forms such as Corbicula sp. These forms suggest deposition in a near-shore shallow marine environment.
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The overlying Sangiran Formation has two members; the LLU is the lower and the ‘‘black clay’’ is the upper. The black clay includes dark-colored siltstones and mudstones that accumulated in shallow marine, brackish-water and marsh environments. Thin water-laid tuff layers occur throughout the black clay to form about 3% of its total thickness (Yoshikawa and Suminto, 1985). The upper twothirds of the black clay yields fossils of terrestrial vertebrates well adapted to marshy and lake-margin environments (Aimi and Aziz, 1985). Homo erectus fossils occur in the upper part of the black clay as one component in the fully terrestrial and endemic island-type fauna known as Ci Saat (de Vos et al., 1994; Larick et al., 2000). Above the black clay are fluvial deposits of the Bapang Formation (Watanabe and Kadar, 1985; Larick et al., 2000). The Bapang comprises a series of upward-fining fluvial cycles consisting of fine to very coarse tuffaceous sandstones with lenses of pumiceous conglomerate that grade upward to silts and silty clay in which paleosols are developed. Trough-cross bedding, parallel bedding and shallow cut-and-fill structures are characteristic of the sandstones and conglomerates. The lower and middle parts of the Bapang hold the majority of Homo erectus fossils. The Pohjajar Formation lies above the Bapang, and is also fluvial, but with a higher proportion of fine-grained volcanic sediments. These occur as aerial tuffs, fluvially reworked ash fall, and two lahar-formed diamictons, the Upper and Uppermost Lahars. Hominid fossils are not found in the upper reaches of the Bapang or in any part of the Pohjajar Formation in the Dome. The Dome’s most recent deposits, a fluvial ensemble of alternating tuffaceous sandstone, conglomerate and fine-grained sediments, unconformably overlies the Pohjajar and in many places cuts into the underlying Bapang and Sangiran Formation deposits.
3. Methods Lahars fall within a range of volcaniclastic mass flow deposits that include large surges, far-traveled, water-rich pyroclastic flows, concentrated grain fluid mixtures, and hyperconcentrated stream flows. Vol-
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caniclastic mass flows have recently been studied in several parts of the world, including the Merapi volcano located just west of the Sangiran Dome in Central Java. We focus on the LLU, recording
Table 1 40 Ar/39Ar analytical methods and age calculation procedures for the dated pumice hornblendes Sample preparation and irradiation
Mineral separates obtained by standard magnetic, heavy liquid and hand-picking techniques. Samples irradiated for 1 (NM-132) or 2 h (NM-155) in the D-3 position of the Texas A&M reactor along with neutron flux monitor Fish Canyon Tuff sanidine (FC-1) with an assigned age of 27.84 Ma (Deino and Potts, 1990), relative to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987). Instrumentation Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system. 50 W CO2 laser furnace: Samples analyzed by step-heating with defocused laser beam, each step 3 min. Reactive gases removed during a 20-min reaction with 2 SAES GP-50 getters, one operated at f 450 jC and one at 20 jC. Gas also exposed to a W filament operated at f 2000 jC and a cold finger operated at 140 jC. Analytical Electron multiplier sensitivity averaged 0.70 or parameters 1.25 10 16 mol/pA for NM-132 and NM-155 samples, respectively. Total laser system blanks plus backgrounds were assigned to be: 930, 3.4, 1.3, 3.3, and 5.7 10 18 moles at masses 40, 39, 38, 37, and 36, respectively. J-factors determined to a precision of F 0.1% by CO2 laser-fusion of four single crystals from each of six radial positions around the irradiation tray. Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows: (40Ar/39Ar)K = 0.0002 F 0.0003; (36Ar/37Ar)Ca = 0.000280 F 0.000005; and (39Ar/37Ar)Ca = 0.00072 F 0.00002. Age Total gas ages and errors calculated by isotopic calculations recombination of gas derived from all heating steps. Plateau ages calculated for the indicated steps by weighting each step by the inverse of the variance. Plateau age errors calculated using the method of Taylor (1982). MSWD values are calculated for each plateau age. If the MSWD is above 1, the plateau age error is multiplied by the square root of the MSWD. Decay constants and isotopic abundances after Steiger and Ja¨ger (1977). All errors reported at F 1r.
118 E. A. Bettis III et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 206 (2004) 115–131 Fig. 2. Logs of selected LLU stratigraphic sections in the Sangiran Dome. The sections depicted cover the range of observed thickness of the LLU in its outcrop belt. The location of sample blocks that were made into polished slabs is indicated.
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sedimentary characteristics that have proven useful for interpreting modern and late Cenozoic volcaniclastic mass flow deposits at Merapi (Newhall et al., 2000, Lavigne et al., 2000), Mount Pinatubo in the Philippines (Rodolfo et al., 1998), and in the Cascade Range of the United States (Scott et al., 1995). We measured, photographed, and described all known LLU outcrops and sampled the LLU at nine localities that represented the variability we observed (Fig. 1). At six localities (Pondok, Pagarejo, Ngampon, Pablengan a, Bukuran, and Cengklik a), sediment samples were dry-sieved on-site to separate large-sized (>1.5 cm diameter) and intermediate-sized (0.5 – 1.5 cm diameter) clasts from the matrix (Zaim et al., 1999). Oriented block samples were cut from the upper and lower LLU sectors at five localities (Pondok, Puren, Pablengan a, Pablengan b, and Cengklik b). Polished slabs and thin sections were prepared from these blocks. Bulksample hornblende 40Ar/39Ar analyses were carried out on pebble-sized pumice clasts from six localities (Pagarejo, Ngampon, Pablengan a, Pablengan b, Cengklik a and Cengklik b). Details of the 40Ar/39Ar analytical methods and age calculations are provided in Table 1.
4. The Lower Lahar unit The LLU varies in thickness from 6 to about 19 m in the northern part of the outcrop ring, to 27 m at the southernmost exposure along the Cemoro River (Cengklik b, Fig. 2). It is a matrix-supported diamicton with a medium to coarse, very poorly sorted volcanogenic sand matrix that supports rounded to angular polymictic clasts (Fig. 3; Zaim et al., 1999). Subangular to rounded pebble- to cobble-sized clasts constitute from 15 to 5 wt% of the LLU and consist primarily of pyroxene and pyroxene –hornblende andesite with subordinate pumice, tuffaceous clay, sandstone, vesicular basalt, limestone, and altered volcanic rocks. The LLU consists of three distinct zones. Cobble-sized clasts, and locally derived siltstone, mudstone and limestone clasts and mollusc shells are most abundant in the lower 1– 2 m of the deposit. This zone has weakly expressed graded bedding and preferred orientation of fine and medium sand grains around some pebbles and cobbles (Fig. 3C) and long
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axes of pebble- and cobble-sized clasts orientated roughly parallel to the base of the unit. These sedimentary features suggest transport by laminar flow (Sparks, 1996). Above the thin basal zone the LLU is massive, matrix-supported, has clasts with no preferred orientation, and contains undeformed and unbroken fragile material such as freshwater mollusc shells. These properties indicate a lack of turbulence during transport and suggest that the bulk of the LLU was transported to the Solo Basin as a viscous debris flow deposit with a rigid plug riding on a thin basal zone undergoing laminar flow. Large (3– 8 m) undeformed blocks of siltstone and sandstone occur in a 1 – 2 m thick upper zone at several localities. These were probably suspended in the upper part of the plug by a combination of high density (buoyancy) and high strength of the matrix. The marine limestone and mudstone beneath the LLU show little evidence of significant erosion and only minor deformation, suggesting that the mass flow event that emplaced the LLU did not scour or deform the sea floor sediments. The conformable contact between the overlying black clay and the LLU in combination with an absence of winnowing or sorting of the LLU surface sediments indicates that the LLU was probably deposited below wave base. Unlike the many historic lahars at nearby Merapi volcano, the flow that deposited the LLU apparently did not become a muddy streamflow or ‘‘banjir’’ as it moved away from the volcanic edifice where it was initiated, but instead remained cohesive as it passed into a shallow marine environment. Considering the distance to the nearest source volcanoes (>20 km), the mass flow that deposited the LLU was a very large magnitude event or closely spaced sequence of events. The low percentage of pumice clasts, absence of evidence for rapidly cooled clasts, such as ‘‘bread crusts’’, and the variety of andesite clasts in various stages of weathering all suggest that the mass flow was not associated with an eruption. The abundance of silicic clasts may indicate that the lahar originated in the silica-rich volcanic area southeast of the Sangiran Dome rather than in the lower silica content volcanic area to the south and southwest (Ninkovich et al., 1982). This interpretation is subject to debate, however, since there is a great variation in the silica content of lavas
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Fig. 3. Images of selected polished slabs of the LLU. Refer to the text for a discussion of features indicated on the images. Images were made by scanning polished slabs on a flat bed scanner.
in Central Java (Neuman van Padang, 1951; Whiteford, 1975).
5.
40
Ar/39Ar ages
We used the laser step-heating age spectrum method on small bulk samples of pumice hornblende to obtain 40Ar/39Ar ages. Dome pumice epiclasts have a relatively uniform mineralogy, suggesting that they erupted from a single volcanic center (Larick et al., 2001). LLU pumice clasts generally have 5 – 15% plagioclase, 2 – 5% hornblende, 1 – 2% opaque oxides, 0 – 1% augite, and < 1% apatite phenocrysts. Pumice hornblende is usually pleochroic olive green or brown-green to yellow,
although some is dark red-brown due to oxidation. Crystals in some pumice were apparently shattered by eruption. We analyzed hornblende separates from six pumice lenses within the LLU (Tables 1 and 2). Age spectrum diagrams for each sample are shown in Figs. 4 and 5. For samples LL-1 (Cengklik a), LL-2 (Ngampon), and LL01-4 (Cengklik b), three to four replicate runs were performed to evaluate sample heterogeneity (Tables 2 and 3; Figs. 4a– f and 5a –g). The hornblendes are poikilic with significant plagioclase inclusions and have distinct color populations in some instances. Sample LL-1 provided three replicate analyses of a green hornblende. LL-2 gave two replicates of a green amphibole and one of a brown hornblende. Samples LL-3 and LL-4 each yielded one hornblende (LL-3
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Fig. 4. Age spectra, K/Ca, and radiogenic yield diagrams for hornblende samples LL-1 (a – c) and LL-2 (d – f). Replicate analysis of LL-1 does not yield consistent results and suggests multiple age populations within the bulk mineral separates. LL-2-a and LL-2-b combine to provide a maximum age of 1.90 F 0.02 Ma for the LLU. LL-2-c yields an anomalously young apparent age and is considered inaccurate.
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Fig. 5. Age spectra, K/Ca, and radiogenic yield diagrams for LL01-4 (a – d), LL-3-a (e), LL-4-a, (f), and LL01-9-a (g). Like LL-1, LL01-4 has replicate ages that do not agree at 2r and indicate a heterogeneous age population of hornblende crystals. All apparent ages are interpreted as maximum emplacement ages for the LLU.
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Table 2 40 Ar/39Ar isotopic data for the laser step-heated bulk hornblende samples K/Ca
40 Ar* (%)
39 Ar (%)
Age (Ma)
F 1r (Ma)
LL-1-a green hornblende, wt.=14.70 mg, J=0.0001109, NM-132, Lab#=51889-01 A 15 973.3 4.364 1997.2 0.125 B 20 64.10 6.585 45.21 0.615 C 28 22.54 8.143 30.31 3.66 D 32 14.90 8.208 11.45 3.44 E 36 13.47 8.281 8.403 2.66 F 45 14.57 8.751 13.24 6.01 Total gas age n=6 16.5 Plateau MSWD=1.40 n=2 steps E – F 8.7
0.12 0.077 0.063 0.062 0.062 0.058 0.062 0.059
39.4 80.0 63.3 81.8 86.7 78.1
0.8 4.5 26.6 47.5 63.6 100.0
75.4 10.28 2.867 2.452 2.348 2.290 3.32 2.30
3.1 0.22 0.040 0.035 0.043 0.022 0.02 0.02
LL-1-b green hornblende, wt. =15.92 mg, J=0.0001109, NM-132, Lab#=51889-02 A 15 1535.4 8.430 4918.4 0.181 B 20 32.54 5.606 38.99 0.312 C 28 15.07 8.304 18.30 3.62 D 32 13.27 8.692 11.82 4.20 E 36 12.33 8.100 10.16 5.55 F 45 12.76 8.375 11.15 4.87 Total gas age n=6 18.7 Plateau MSWD=2.80 n=4 steps C – F 18.2
0.061 0.091 0.061 0.059 0.063 0.061 0.062 0.061
5.4 66.0 68.7 79.1 81.1 79.6
16.6 4.31 2.081 2.111 2.011 2.042 2.23 2.05
1.9 0.37 0.035 0.028 0.023 0.028 0.03 0.02
LL-1-c green hornblende, wt.=10.52 mg, J=0.0001109, NM-132, Lab#=51889-03 A 15 1247.5 6.991 3934.4 0.135 B 20 28.56 6.816 50.29 0.540 C 28 17.35 8.966 20.54 2.71 D 32 21.27 8.776 34.05 2.62 E 36 30.91 8.528 69.29 3.51 F 45 14.40 8.834 13.46 2.05 Total gas age n=6 11.6 Plateau MSWD=3.76 n=4 steps C – F 10.9
0.073 0.075 0.057 0.058 0.060 0.058 0.059 0.058
6.8 50.0 69.3 56.1 36.0 77.5
17.1 2.87 2.419 2.401 2.240 2.245 2.52 2.34
2.0 0.21 0.045 0.050 0.049 0.060 0.04 0.05
LL-2-a green hornblende, wt.=18.94 mg, J=0.0001107, NM-132, Lab#=51890-01 A 15 531.5 5.600 1707.4 0.235 B 20 47.44 6.199 77.32 0.271 C 28 16.61 10.13 26.36 1.87 D 32 14.68 10.53 19.38 2.38 E 36 13.09 10.68 15.46 3.10 F 45 11.81 10.73 11.71 8.48 Total gas age n=6 16.3 Plateau MSWD=1.95 n=4 steps C – F 15.8
0.091 0.082 0.050 0.048 0.048 0.048 0.049 0.048
5.2 52.9 58.1 66.9 71.9 78.2
5.49 5.03 1.942 1.975 1.893 1.859 2.00 1.88
0.91 0.44 0.065 0.052 0.041 0.017 0.02 0.02
LL-2-b green hornblende, wt. =13.75 A 15 380.0 B 20 19.77 C 28 14.41 D 32 13.80 E 36 12.65 F 45 14.16 Total gas age Plateau MSWD=2.05
0.087 0.055 0.049 0.048 0.048 0.048 0.049 0.048
11.9 65.1 66.1 71.5 73.9 78.7
9.08 2.59 1.917 1.985 1.882 2.242 2.12 1.92
0.83 0.19 0.040 0.039 0.034 0.090 0.03 0.03
ID
Power (W)
40
Ar/39Ar
37
Ar/39Ar
36 Ar/39Ar ( 10 3)
39
ArK ( 10
16
mol)
mg, J=0.0001107, NM-132, Lab#=51890-02 5.878 1134.1 0.212 9.263 25.93 0.598 10.51 19.46 3.10 10.62 16.28 3.24 10.74 14.17 3.74 10.62 13.16 1.32 n=6 12.2 n=3 steps C – E 10.1
52.5
1.0 2.6 22.0 44.4 74.0 100.0 97.4
1.2 5.8 29.3 52.0 82.3 100.0 94.2
1.4 3.1 14.5 29.1 48.1 100.0 96.9
1.7 6.6 32.0 58.5 89.2 100.0 82.5
(continued on next page)
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Table 2 (continued) K/Ca
40 Ar* (%)
39 Ar (%)
Age (Ma)
F 1r (Ma)
0.063 0.029 0.028 0.030 0.046 0.032
26.8 44.3 20.0 36.2 43.9
3.4 25.7 57.2 91.4 100.0
19.4 1.97 1.12 1.401 2.89 2.18
1.1 0.12 0.11 0.086 0.32 0.07
LL-3-a green hornblende, wt.=4.99 mg, J=0.0001111, NM-132, Lab#=51893-01 A 15 1469.8 2.632 4897.1 0.126 B 25 30.49 10.23 64.65 0.638 C 32 33.96 10.41 69.77 0.468 D 38 69.39 13.58 171.3 0.237 E 45 79.97 67.49 184.5 0.090 Total gas age n=5 1.56 Plateau MSWD=1.20 n=3 steps A – C 1.23
0.19 0.050 0.049 0.038 0.008 0.057 0.064
1.6 40.1 41.8 28.7 38.8
8.1 49.0 79.1 94.3 100.0
4.6 2.47 2.87 4.02 6.5 3.20 2.62
2.2 0.18 0.25 0.50 1.3 0.20 0.16
LL-4-a brown hornblende, wt.=9.69 mg, J=0.0001112, NM-132, Lab#=51894-01 A 15 486.1 14.78 1481.7 0.115 B 25 25.53 10.61 46.41 1.04 C 32 19.20 10.39 20.03 2.86 D 38 18.82 10.19 23.99 1.87 E 45 22.49 9.634 31.18 0.949 Total gas age n=5 6.83 Plateau MSWD=6.42 n=4 steps B – E 6.72
0.035 0.048 0.049 0.050 0.053 0.050 0.050
10.2 49.7 73.6 66.8 62.6
10.0 2.56 2.854 2.538 2.84 2.84 2.75
1.3 0.12 0.043 0.066 0.13 0.04 0.08
LL01-4-a hornblende, wt. =15.50 mg, A 12 190.3 B 17 49.37 C 22 26.36 D 29 29.74 E 35 15.68 F 45 25.47 Total gas age Plateau MSWD=2.99
0.066 0.043 0.043 0.040 0.043 0.048 0.048 0.045
4.1 16.5 26.9 24.9 28.8 25.8
2.99 3.10 2.704 2.83 1.72 2.50 2.75 2.60
0.40 0.14 0.086 0.44 0.35 0.12 0.10 0.12
J=0.0002095, NM-155, Lab#=53409-02 11.25 154.7 1.08 13.84 83.38 4.44 12.46 74.21 7.92 12.78 50.82 5.78 13.61 37.92 0.676 15.83 92.25 0.231 n=6 20.1 n=5 steps B – F 19.0
0.045 0.037 0.041 0.040 0.037 0.032 0.040 0.039
19.6 19.5 22.7 32.8 33.3 17.4
94.6
4.16 2.18 2.334 2.59 1.92 2.1 2.45 2.39
0.49 0.14 0.089 0.10 0.67 2.0 0.07 0.08
LL01-4-c hornblende, wt. =14.98 mg, J=0.0002095, NM-155, Lab#=53409-03 A 12 141.1 4.512 456.1 3.14 B 17 30.09 13.53 85.75 3.92 C 22 23.20 13.68 62.29 7.51 D 29 21.34 12.83 56.62 5.53 E 35 20.52 13.30 54.85 1.63
0.11 0.038 0.037 0.040 0.038
4.7 19.5 25.5 26.5 26.4
14.0 31.5 65.0 89.7 97.0
2.54 2.24 2.258 2.16 2.06
0.33 0.16 0.086 0.11 0.30
ID
Power (W)
40
Ar/39Ar
37
Ar/39Ar
36 Ar/39Ar ( 10 3)
LL-2-c brown hornblende, wt.=8.49 mg, J=0.0001112, A 15 361.4 8.131 B 25 21.89 17.61 C 32 27.69 17.94 D 38 19.06 16.96 E 45 32.65 11.09 Total gas age n=5 No plateau
LL01-4-b hornblende, wt.=14.35 mg, A 12 55.72 B 17 29.20 C 22 27.02 D 29 20.76 E 35 15.10 F 45 31.42 Total gas age Plateau MSWD=1.80
39
ArK ( 10
16
mol)
NM-132, Lab#=51892-01 897.9 0.141 46.18 0.934 80.00 1.32 45.89 1.43 65.12 0.362 4.18
J=0.0002095, NM-155, Lab#=53409-01 7.685 619.5 4.00 11.99 142.9 5.72 11.83 68.50 7.97 12.68 79.07 1.18 11.90 41.10 1.43 10.55 66.93 5.07 n=6 25.4 n=4 steps C – F 15.7
79.1
1.7 16.9 58.8 86.1 100.0 98.3
15.8 38.3 69.7 74.4 80.0 100.0 61.7
5.4 27.5 66.8 95.5 98.9 100.0
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Table 2 (continued) ID
Power (W)
40
Ar/39Ar
37
Ar/39Ar
36 Ar/39Ar ( 10 3)
39
ArK ( 10
16
K/Ca
40 Ar* (%)
39 Ar (%)
100.0
mol)
LL01-4-c hornblende, wt. =14.98 mg, F 45 26.38 Total gas age Plateau MSWD=0.33
J=0.0002095, NM-155, Lab#=53409-03 16.76 74.15 0.674 n=6 22.4 n=6 steps A – F 22.4
0.030 0.048 0.048
22.2
LL01-4-d hornblende, wt. =14.60 mg, A 12 109.7 B 17 32.92 C 22 22.11 D 29 14.82 E 35 17.57 F 45 23.47 Total gas age Plateau MSWD=0.07
J=0.0002095, NM-155, Lab#=53409-04 7.891 340.5 4.19 12.40 92.61 6.26 12.15 55.42 9.51 12.93 31.40 3.78 12.78 39.80 0.761 14.32 61.78 0.388 n=6 24.9 n=5 steps B – F 20.7
0.065 0.041 0.042 0.039 0.040 0.036 0.045 0.041
8.9 20.0 30.5 44.6 39.1 27.2
LL01-9-a hornblende, wt. =13.37 mg, A 12 137.3 B 17 18.84 C 22 13.89 D 29 26.96 E 35 12.80 F 45 11.38 Total gas age Plateau MSWD=1.03
J=0.0002058, NM-155, Lab#=53395-02 8.982 435.8 1.68 12.96 45.48 2.93 12.93 30.06 4.67 12.93 72.47 4.59 12.81 27.55 4.16 12.54 22.66 1.27 n=6 19.3 n=6 steps A – F 19.3
0.057 0.039 0.039 0.039 0.040 0.041 0.041 0.041
6.8 34.4 43.8 24.5 44.7 50.3
100.0
16.8 42.0 80.2 95.4 98.4 100.0 83.2
8.7 23.9 48.1 71.9 93.4 100.0 100.0
Age (Ma)
F 1r (Ma)
2.24 2.25 2.23
0.70 0.08 0.06
3.70 2.51 2.567 2.52 2.62 2.4 2.73 2.54
0.25 0.11 0.079 0.14 0.62 1.2 0.08 0.06
3.47 2.42 2.28 2.48 2.14 2.14 2.41 2.35
0.60 0.30 0.19 0.20 0.21 0.68 0.12 0.15
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. Individual analyses show analytical error only; plateau and total gas age errors also include error in J. Analyses in italics are excluded from plateau age calculations.
green, LL-4 brown). Hornblendes from LL01-4 and LL01-9 were very dark green to black. The age spectra show varying degrees of complexity and replicate runs do not always produce consistent apparent ages (Figs. 4 and 5; Table 2). Most spectra record a flat segment that contains between two and six steps and 50 – 100% of the 39Ar released. Plateau ages range between 1.88 F 0.02 Ma (LL-2-a-Ngampon) and 2.75 F 0.08 Ma (LL-4-a-Cengklik b). Total gas ages are typically older than plateau ages as the initial steps of the spectra record anomalously older apparent ages relative to the plateau segments (Table 3). As demonstrated by LL-1 and LL01-4, replicate plateau ages do not agree at the 2r error level (Table 2) and indicate scatter above what can be explained by analytical error alone. K/Ca spectra are generally flat except for initial steps that record slightly higher values compared to the majority of the sample (Figs. 4 and 5; Table 2). Radiogenic yields are typically low for initial steps, but rise to values as high as about 80% for the high temperature heating steps (Table 2;
Figs. 4 and 5). These high radiogenic yields contribute to age results that are quite precise. As the hornblende separates are heterogeneous in mineralogy and age, the plateau age spread must be interpreted with caution. Considering that mixed age populations is the most likely cause for the lack of replicate analysis reproducibility, we suggest that the youngest plateau ages probably reflect a maximum age for the deposit. Thus the analytically indistinguishable plateau ages of 1.88 F 0.02 and 1.92 F 0.03 Ma from LL-2-a and LL-2-b (Ngampon), respectively (Fig. 4d,e), indicate that the LLU is not older than 1.90 F 0.02 Ma. It is probable that older apparent ages result from pumice that was recycled from older tephra sequences and their overall flat age spectra represents homogenization of the age populations during the step-heating of the bulk samples. The spread in apparent plateau ages does not appear to be caused by excess argon contamination. Isochron analysis was conducted on all of the samples and does not reveal apparent ages that are
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Table 3 Compilation of Lower Lahar dating results % 39Ar in plateau
Locality
Sample
Weight (mg)
Plateau age (F 1r Ma)
Plateau MSWD
No. of steps on plateau
Cengklik a
LL-1-a (green) LL-1-b (green) LL-1-c (green) LL-2-a (green) LL-2-b (green) LL-2-c (brown) LL-3-a (green) LL-4-a (brown) LL01-4-a (black) LL01-4-b (black) LL01-4-c (black) LL01-4-d (black) LL01-9-a (black)
14.7 15.92 10.52 18.94 13.75 8.49 4.99 9.69 15.50 14.35 14.98 14.60 13.37
2.30 F 0.02 2.05 F 0.02 2.34 F 0.05 1.88 F 0.02 1.92 F 0.03 – 2.62 F 0.16 2.75 F 0.08 2.60 F 0.12 2.39 F 0.08 2.23 F 0.06 2.54 F 0.06 2.35 F 0.15
1.40 2.80 3.76 1.95 2.05
2 4 4 4 3
(E – F) (C – F) (C – F) (C – F) (C – E)
52.5 97.4 94.2 96.9 82.5
1.20 6.42 2.99 1.80 0.33 0.07 1.03
3 4 4 5 6 6 6
(A – C) (B – E) (C – F) (B – F) (A – F) (A – F) (A – F)
79.1 98.3 61.7 94.6 100 100 100
Ngampon
Pagerejo Pablengan a Cengklik b
Pablengan b
significantly different than the plateau ages. The isochron plots are not provided because they suffer from too few data points for each individual sample, the isochron arrays can be visualized by inspection of the radiogenic yield and age spectra plots (note that indication of excess argon which manifests as a correlation between old apparent age and low apparent radiogenic yields does not exist) and, finally, isochron analysis did not supplant the plateau ages in the overall interpretation of the LLU age. The brown hornblende extracted from LL-2-c yielded an overall saddle-shaped age spectrum with a minimum age of 1.12 Ma (Table 2; Fig. 4f). It was noted during mineral separation that this brown amphibole appeared altered relative to the other amphibole separates and had significant matrix material adhering to the crystals. A possible explanation for the apparent young age is that the matrix material has experienced argon loss and degassed during the intermediate part of the age spectrum. Another possibility is that the hornblende has experienced argon loss during alteration or oxidation. Although these explanations are not entirely satisfactory’ based on a comparison with other data, this sample is considered anomalous and its age inaccurate. In summary, six pumice lenses within the LLU yielded hornblende variable with respect to color and plagioclase inclusions. Age spectra showed varying degrees of complexity and the replicate runs did not always produce consistent apparent ages (Tables 2 and 3). Nevertheless, most spectra recorded a flat
Total gas age (F 1r Ma) 3.32 F 0.02 2.23 F 0.03 2.52 F 0.04 2.00 F 0.02 2.12 F 0.03 2.18 F 0.07 3.20 F 0.20 2.84 F 0.04 2.75 F 0.10 2.45 F 0.07 2.25 F 0.08 2.54 F 0.06 2.41 F 0.12
segment containing between two and six steps and 50 –100% of the 39Ar released. Plateau ages ranged between 1.88 F 0.02 and 2.75 F 0.08 Ma (Table 3). As the spread in apparent plateau ages seems not to result from excess argon contamination, the older apparent ages probably indicate pumice recycled from older tephra sequences. We therefore suggest that averaging the two youngest plateau ages provides a maximum age of 1.90 F 0.02 Ma for the LLU.
6. Geochronological frameworks An accurate chronology is essential for examining the arrival of Homo erectus to Sunda in a context of regional and global environmental change. The initial paleontology-based chronology of the Dome’s sedimentary sequence placed the Puren and Lower Sangiran formations in the Late Pliocene, and the Bapang in the Pleistocene (van Es, 1931; von Koenigswald, 1940a,b). After World War II, Hooijer (1956, 1957) and Sartono (1961, 1969, 1970, 1975) reinterpreted the existing evidence to shift the Sangiran Formation to the Early Pleistocene and the Bapang Formation to the Middle Pleistocene. Thus began the controversy over ‘‘long’’ and ‘‘short’’ chronologies for the Dome’s sediments. In the 1970s and 1980s, a new generation of microfossil analyses emerged in support of the long chronology. Ninkovich and Burckle (1978) used
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updated ages of published diatom assemblages in relation to deep sea cores to suggested an age of 2.1 –1.9 Ma for the base of the Sangiran Formation, and Sartono et al. (1981) corroborated that determination in their independent analysis of new samples. The biostratigraphy of calcareous nanoplankton fossils in the Upper Puren and Lower Sangiran formations also supports the long chronology, lying between NN 16 and NN 18 (1.65 – 3.25 Ma; Sartono et al., 1981; Seisser et al., 1984). Fission-track ages from Dome sediments (on tephra zircons and on tektites) are highly scattered, possibly reflecting problems in standardization, annealing, or overall counting (Faure, 1986, p. 343). Magnetostratigraphic studies conducted on Dome sediments have not reached a consensus on the number or stratigraphic position of polarity intervals in the sequence (Se´mah, 1982; Shimizu et al., 1985; Yokoyama et al., 1980; Danisworo, 1992; Hyodo et al., 1993). Hyodo et al. (2002) cite fission-track ages and the reported stratigraphic occurrence of tektites in Dome sediments as strong support for their magnetostratigraphy which supports a ‘‘short’’ chronology. The age of Australasian strewn field tektites has been refined to 0.803 Ma (Hou et al., 2000) and associated microtektites are found just below the Brunhes – Matuyama boundary in undisturbed Southeast Asian seafloor sediments (Schneider et al., 1992). Controlled excavations in the Sangiran Dome during the joint Indonesian/Japanese CTA-41 project claim to have recovered two tektites in primary context in the middle Bapang Formation (Aziz et al., 1985; Itihara et al., 1985; Sudijono et al., 1985). Placed ‘‘low’’ (within the middle Bapang Formation), these two tektites continue to be cited as evidence supporting the short chronological framework (Se´mah, 1982; Langbroek and Roebroeks, 2000; Hyodo et al., 2002). Detailed documentation of these tektite finds was never provided, and despite several detailed and meticulous excavations in the lower and middle Bapang sequence at several Dome localities over the past decade, additional tektites have not been recovered (Se´mah et al., 1992; Widianto et al., 1997; Baba et al., 2000). Until additional tektites are discovered in primary context in Dome sediments, questions regarding their local stratigraphic occurrence preclude their use as key stratigraphic markers for Homo erectus in Central Java.
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Two 40Ar/39Ar step-heating procedures are currently used to date the volcaniclastics of the Sangiran Dome. One procedure analyzes single hornblende grains derived from fine-grained matrix (Falgue`res, 1998; Falgue`res and Yokoyama, 2001; Se´mah et al., 2000). The other procedure, used by our group and other investigators, isolates small bulk hornblende samples from epiclastic pumice (Swisher, 1997, 1999; Swisher et al., 1994; Swisher and Curtis, 1998; Widiasmoro, 1998; Larick et al., 2001). Ironically, the two 40Ar/39Ar chronologies recapitulate the divergent microfossil-based schemes. Thus, 40Ar/39Ar analysis of a single LLU matrix hornblende grain from Cengklik a produced an age of 1.77F0.08 Ma and a hornblende grain from Pablengan a yielded an age of 1.66F0.04 Ma (Se´mah et al., 2000). Alternatively, one bulk sample 40Ar/39Ar analysis by Swisher (1999) for the LLU yielded a plateau age of 2.08 Ma. In attempting to resolve the discrepancy, Se´mah et al. (2000) question the provenience of one of Swisher’s pumice hornblende bulk40Ar/39Ar analysis samples. Swisher et al. (1994) associated a bulk sample 40 Ar/39Ar age of 1.66 F 0.04 Ma with an important Homo erectus fossil findspot stratigraphically above the LLU in the lower Pucangan Formation (cf. Sangiran Formation; Swisher et al., 1994, p. 1120). Se´mah et al. (2000) observed that Swisher et al.’s (1994) determination matches the single grain 40 Ar/39Ar age from the LLU at Pablengan a. They concluded that Swisher et al. mistakenly sampled the LLU rather than sediments stratigraphically higher in the sequence. This explanation, nevertheless, does not cover the bulk sample 40Ar/39Ar age of 2.08 Ma (Swisher, 1999), nor does it explain the older ages of LLU pumice hornblendes reported here. The LLU pumice hornblende 40Ar/39Ar plateau ages reported here are consistent with those determined by analyzing bulk sample hornblendes from the Sangiran and Bapang formations (Widiasmoro, 1998; Swisher, 1997, 1999; Larick et al., 2001). An 40 Ar/39Ar plateau age for the lowest Bapang Formation pumice of 1.51 F 0.08 Ma (Larick et al., 2001) and our estimated maximum LLU age (1.90 F 0.02 Ma) provide a chronological bracket for the intervening Sangiran Formation black clay. These data suggest that the black clay accumulated over a time interval of about 400 000 years. Homo erectus appeared in the Sangiran area toward the end of this period.
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7. Discussion Grain size and sorting characteristics of the LLU are very similar to those of cohesive debris flows at Cascade volcanoes that often begin as debris avalanches, transform quickly to debris flows, and remain as such to their termini (Scott et al., 1995). Historic heterolithic lahars at southeastern Asia volcanoes, such as Mount Pinatubo in the Philippines (Rodolfo et al., 1998) and at Merapi volcano near Sangiran in Central Java (Lavigne et al., 2000), are triggered primarily by intense rainfall on water-saturated volcanic edifices. At Cascade volcanoes, large cohesive debris flows that deposit sediments similar to the LLU are associated primarily with older volcanic edifices, where hydrothermal alteration is intense (Scott et al., 1995). The general absence of eruptive-phase indicators associated with the LLU suggest that the mass flow that deposited it was most likely generated by sector collapse of a large stratovolcano or volcanic complex. Newhall et al. (2000) offer Holocene examples of lahar-induced landscape transformations resulting from Merapi mass flows, and Scott et al. (1995) illustrate similar effects at Mount Rainier. Besides burying preexisting topography, lahars often block valleys and impound lakes in traversed drainages. The LLU’s effect on the local landscape must have included such effects in terrestrial environments up gradient of the shallow marine environment present in the Sangiran area at the time of deposition. The LLU event did not create terrestrial surfaces as it flowed into a near-shore marine or lagoonal environment in the Sangiran area. Nevertheless, it did reduce local water depth by as much as 20 m. During subsequent early Pleistocene glacioeustatic sea level fall these shallow aquatic environments emerged as fertile land surfaces (Ninkovich et al., 1982). As land bridges then connected southern Sunda with mainland Southeast Asia, Homo erectus arrived to the Solo Basin to find a variety of habitable environments. In all archaeological assemblages recovered within the Dome, lahar-derived andesite pebbles and cobbles dominate the raw material menu. Andesite lahar clasts are naturally oriented towards the production of ‘‘bolas’’, which are common tools associated with Homo erectus. More complex tools appearing on andesite cobbles include large retouched flakes and cleavers
(Se´mah et al., 2003). Clasts of andesite are present in the LLU, but at the time of the first Homo erectus immigration into the Sangiran area the LLU was buried by many meters of Sangiran Formation deposits and thus was not available as a local source of raw material for Homo erectus lithic technology. Only after the commencement of Bapang Formation coarse fluvial deposition about 1.5 Ma was a local source of lithic material present in the Sangiran area.
8. Conclusions The LLU’s stratigraphic and lithological characteristics suggest deposition as a heterolithological, cohesive debris flow. The debris flow was a lahar, or series of closely spaced lahars, that was probably triggered by sector collapse of a relatively old volcanic edifice located east or southeast of the Sangiran area. Bulksample 40Ar/39Ar pumice hornblende ages place the event as early as 1.90 Ma. This age postdates glacioeustatic sea level lowering caused by the first major continental glaciation of the late Pliocene, but predates the more frequent glacial episodes of the early Pleistocene. Emplacement of the LLU significantly decreased the depth of shallow near-shore marine environments in the Sangiran area and set the stage for terrestrial emergence during early Pleistocene glacial episodes. The evolution of post-LLU estuarine and marsh environments in the Sangiran area produced terrestrial settings that attracted Homo erectus to southern Sunda generally and the Solo Basin in particular.
Acknowledgements The Institute of Technology Bandung and the University of Iowa (UI) collaborated in this research, with assistance from the Indonesian Geological Research and Development Centre and the National Archaeological Research Centre. The Indonesian Institute of Sciences issued research permits 7450/ V3/KS/1998, 3174/V3/KS/1999 and 4301/1.3/KS/ 2001. Funding was provided by the following UI programs: Center for Global and Regional Environmental Research, Central Investment Fund for Research Enhancement, Office of the Vice-President for
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