lower lahar pucangan

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The Sangiran Formation Lower Lahar (Central Java, Indonesia):

Landscape Development Preceding Homo Erectus Immigration

E. Arthur Bettis IIIa*, Yahdi Zaimb, Roy R. Larickc, Russell L. Ciochonc, Sumintod, Yan Rizalb,

Mark Reagana, Matthew Heizlere

aDepartment of Geoscience, The University of Iowa, Iowa City, Iowa, 52242-4529, USA

bDepartment of Geology, Institute of Technology Bandung, Jalan Ganesha, no. 10, Bandung

40132, Indonesia

cDepartment of Anthropology, The University of Iowa, Iowa City, Iowa, 52242-1322, USA

dGeological Research and Development Centre, Jalan Dr. Junjunan no. 236, Bandung 40174,

Indonesia

eNew Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico, 87801-4796,

USA

*address for correspondence and proofs

Abstract

The Sangiran Dome is the primary stratigraphic window for the Solo Basin, a coastal feature on the

Plio-Pleistocene Sundan 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. Fossil molluscs indicate

that swamps or shallow lakes lay between the edifice and the current Dome area. 40Ar/39Ar analyses

on hornblende separates from six pumice lenses suggest that the LLU was deposited as early as

1.900.02 Ma. The LLU event transformed late Pliocene near-shore marine environments into

estuarine and paludal settings. Shortly thereafter, glacioeustatic sea level decreases completed the


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local transition to fully terrestrial environments that attracted Homo erectus to southernmost

Sunda.

Keywords: Solo Basin, Sundan subcontinent, lahar, 40Ar/39Ar step heating,Homo erectus

1. Introduction

The Sangiran Dome is situated in Indonesias Central Java Province, 12-20 km north of the city of

Solo (Figure 1). The Sangiran Dome offers a stratigraphic window into the Solo Basin, a

prominent Plio-Pleistocene feature on the southern coast of the former Sundan subcontinent. For

more than a century, Dome localities have yielded early PleistoceneHomo erectus and other

important vertebrate fossils. The feature was declared a World Heritage site by the 20th World

Heritage Committee in December 1996 (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; OSullivan et al., 2001).

This paper presents new geological data on the Lower Lahar unit (LLU), a mass flow deposit

and event that had a quick and major role in transforming the Solo Basin littoral toward a number

of terrestrial habitats that attractedHomo 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 the southern Sunda attractive to

Homo erectus.

2. Geology of the Sangiran Dome

Within the Sangiran Dome,Homo erectus fossils are found in a long sequence of deposits that


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range from lacustrine-paludal in the upper Sangiran Formation, to riverine in the overlying lower

and middle Bapang Formation (Figure 1). During the late Middle Pleistocene, well afterHomo

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 Domes center . These are the

marine limestones, siltstones, mudstones and muddy sandstones of the Puren (Kalibeng) Formation

(Figures 1, 2 and 3a). The fauna includes an abundance ofAnadara sp. marine molluscs, and rare

freshwater forms such as Corbicula sp. These forms suggest deposition in a shallow marine

environment.

The overlying Sangiran (Pucangan) 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 paludal 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 two-thirds of the black clay yields fossils of terrestrial vertebrates well

adapted to paludal conditions (Aimi and Aziz, 1985). Homo erectus fossils occur in the black

clays upper reaches 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 (Kabuh) Formation (Watanabe and

Kadar, 1985). The Bapang comprises fine to very coarse tuffaceous sandstones with lenses of

pumiceous conglomerate intercalated in silts and black silty clay. Cross bedding, parallel bedding

and cut-and-fill structures are characteristic of the sandstone and conglomerates. The lower and

middle reaches of the Bapang hold the majority ofHomo erectus fossils. The Pohjajar (Notoporo)

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


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diamictons, the Upper and Uppermost Lahars. Hominin fossils are not found in the upper reaches

of the Bapang or in any part of the Pohjajar Formation. The Domes most recent sediments, a

fluvial sequence consisting of alternating tuffaceous sandstone, conglomerate and fine-grained

sediments, overlies the Pohjajar.

Methods

Lahars fall within a range of volcaniclastic mass flow deposits that include large surges, far-traveled

pyroclastic flows, concentrated grain fluid mixtures, and hyperconcentrated stream flows. A

number of volcaniclastic mass flows have recently been studied in several parts of the world,

including the Merapi cone located just west of the Sangiran Dome in Central Java. We focus on

the LLU, recording 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 9

localities that represented the variability we observed (Figure 1). At 6 localities (Pondok, Pagarejo,

Ngampan, Pablengan a, Bukuran, and Cengklik a), sediment samples were dry-sieved on-site to

separate large-size (>1.5 cm diameter) and intermediate-size (0.5 to 1.5 cm diameter) clasts from

the matrix (Zaim et al., 1999). Oriented block samples were cut from the upper and lower portions

of the LLU at five localities (Pondok, Puren, Pablengan a, Pablengan b, and Cengklik b). Polished

slabs and thin sections were prepared from these blocks. Pebble-size pumice clasts for bulk-sample

hornblende 40Ar/39Ar analysis were carried out on samples from 6 localities (Pagarejo, Ngappon,

Pablengan a, Pablengan b, Cengklik a and Cengklik b). 40Ar/39Ar analytical methods and age

calculation details are provided in Table 3.


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3. The Lower Lahar Unit

The LLU is a matrix-supported diamicton with a medium to coarse volcanogenic sand matrix that

supports rounded to angular polymictic clasts (Figure 3). Deposit thickness ranges from 6 m 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, Figure 2). Matrix mean grain size is in the fine sand range, except

at locality 1, where it is very fine sand. The LLU matrix grain-size distribution is fine-skewed in the

southern part of the outcrop belt; it shifts to coarse to very coarse-skewed northward; and then to

nearly symmetrical at the Puren and Pablengan b localities (Table 2 and Figure 4). The matrix is

very poorly sorted with about 40-50% clay and silt (Table 2). Sand-sized angular volcanogenic

crystal fragments of plagioclase, augite, magnetite, hornblende, and orthopyroxene make up about

15-30% of the matrix. The remaining matrix consists of sand-sized angular to subrounded mafic to

felsic volcanic rock fragments and rounded siltstone and mudstone.

Pebble- to cobble-sized clasts (4-64 mm) are primarily pyroxene and pyroxene-hornblende

andesite with subordinate pumice, tuffaceous clay, sandstone, vesicular basalt, limestone, and

altered volcanic rocks. These constitute from 15-5 weight percent of the LLU (Table 1). The

andesite clasts range from gray, reddish gray, and brown, to shades of yellow and green (Figure 3),

reflecting pre-transport weathering. Cobble-size clasts are most common in the lowest 2 m of the

unit, but pebbles tend to be uniformly distributed through the LLU. Locally derived siltstone,

mudstone and limestone clasts and mollusc shells are most abundant in the lower 1-2 m of the LLU.

Clast content decreases slightly upward, except for large (3-8 m) undeformed blocks of siltstone

and sandstone that occur in the upper few meters at several localities.

Shells of the freshwater molluscs Corbicula sp., Sulcospira (Tereba) sp., Bellamya sp., and

Unio sp. are found incorporated into the LLU mostly in the southwest outcrop area. A few shells

of the marine molluscAnadara sp. appear at the Pondok and Cengklik b sections (Figures 1 and 2).

The freshwater fossils are often unbroken and undeformed (Figure 3b), while the marine fossils are


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highly fragmented. Rare vertebrate fossil fragments have also been found in the LLU (Zaim et al.,

2002).

Long axes (C-axis) of clasts in the lowest 0.5-1 m of the LLU are orientated roughly parallel to

the base of the unit, while clasts show no preferred orientation through the remainder of the LLU.

With the exception of its lower 1-1.5 m, the LLU is massive. The lower part has weakly expressed

graded bedding, and preferred orientation of fine and medium sand grains around some pebbles and

cobbles (Figure 3a and c). In the lower portion of the LLU at Pablengan b, one end of a large (12

X 25 m) block of weakly cemented sandstone is deformed into a recumbent fold, indicating the

presence of shearing conditions during transport. Taken together, sedimentary features expressed

in the lower 1-1.5 m of the LLU are consistent with transport by laminar flow in a thin basal layer

(Sparks, 1996).

Above the basal zone the LLU is massive, matrix-supported and contains undeformed and

unbroken fragile material such as freshwater mollusc shells. These properties indicate a lack of

turbulence during transport (Johnson, 1970; Fisher and Schmincke, 1984), and suggest that the

bulk of the LLU was transported to the Solo Basin as a nonturbulent plug riding on a basal zone

undergoing laminar flow. The large, undeformed sandstone and siltstone blocks in the upper part

of the LLU were probably suspended in the upper part of the plug by a combination of high density

(buoyancy) and high strength of the matrix. Small, low-density materials, especially epiclastic

pumice, that are preferentially oriented around the large blocks, may have accumulated in a thin

water-rich zone surrounding the blocks as they floated along with the mass flow.

The LLUs basal contact with the Puren Formation ranges from erosive to nonerosive, and in

some instances is fluted. Flute amplitudes range from 5-10 cm and wavelengths from 1.5-2 m.

Minor deformation, usually compaction, is present in the upper few centimeters of the Puren

Formation. These features indicate that, at least locally, deep scouring or significant deformation of

the sea floor did not take place during the mass flow event that emplaced the LLU.


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Two lines of evidence shed light on the nature of the LLU upper surface following deposition.

First, the black clay/LLU contact is conformable. Second, LLU surface sediments are neither

winnowed nor sorted. Both observations indicate that the LLU surface sediments were either laid

down below wave base, or that the receiving water body was not subject to waves or currents.

In total, the LLU shows an absence of bedding, poor sorting and few indications of turbulence

prior to deposition of short-traveled clasts. This suite of features indicates that the mass flow was

probably cohesive. Unlike the many historic lahars at nearby Merapi volcano (Newhall et al.,

2000), the flow that deposited the LLU did not transform into 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 shallow marine environments in the Sangiran area. Considering the distance to the

nearest source volcanoes (>20 km), the lahar that deposited the LLU was a very large magnitude

event or closely spaced sequence of events. The small area of outcrop represented by the Sangiran

Dome (ca. 2.25 km2) precludes a detailed discussion of lateral variations in the rheology of the mass

flow that deposited the LLU.

In summary, the LLU is heterolithologic and contains clasts of altered and unaltered andesites,

vesicular basalt, pumice, sandstone and mudstone. The low percentage of pumice clasts, absence

of evidence such as bread crusts for rapidly cooled clasts, and the variety of andesites in various

stages of weathering all suggest that the mass flow from which the LLU was deposited was not

associated with an eruption. Many of the pebble and cobble size clasts are moderately rounded to

well rounded and appear to have been derived from a relatively high-energy fluvial environment

such as a mountain stream. These clasts indicate that the mass flow originated along the flanks of a

stratovolcano at some distance from the Sangiran area. The abundance of silicic clasts may suggest

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


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lavas in Central Java where fragments of the pre-Cenozoic oceanic trench crop out (Neuman van

Padang, 1951; Whiteford, 1975).

4. Geochronology

Two 40Ar/39Ar step-heating procedures are currently used to date the volcaniclastics of the

Sangiran Dome. In France, the common procedure analyzes single hornblende grains derived from

fine-grained matrix (Falgures, 1998; Falgures et al., 1998; Smah et al., 2000). In the United

States, the common procedure isolates small bulk hornblende samples from epiclastic pumice

(Swisher, 1997; Swisher et al., 1994; Swisher and Curtis, 1998; Widiasmoro, 1998; Swisher, 1999,

Larick et al., 2001). The two methods have been applied throughout the dome sedimentary

sequence with consistently divergent results (Larick et al., 2000). The divergence also holds for

published dates from the LLU. Thus 40Ar/39Ar analysis of a lahar matrix hornblende grain from

Cengklik a produced an age of 1.77 0.08 Ma; hornblende grain from Pablengan a yielded 1.66

0.04 Ma (Smah et al., 2000). Alternatively, one bulk sample 40Ar/39Ar analysis for the LLU

yielded a plateau age of 2.08 Ma (Swisher, 1999).

In attempting to resolve the discrepancy, Smah et al. (2000) refer to a pumice hornblende bulk

sample 40Ar/39Ar analysis of questionable provenance. Swisher et al. (1994) associated a bulk

sample 40Ar/39Ar age of 1.66 0.04 Ma with an important Homo erectus fossil findspot

stratigraphically above the LLU in the Lower Pucangan formation (c.f. Sangiran Formation;

Swisher et al., 1994: 1120). Smah et al. (2000) observe that Swisher et al.s determination

matches the single grain40

Ar/39

Ar age from the LLU at Pablengan a. They conclude that Swisher et

al. mistakenly sampled the LLU. This explanation, nevertheless, does not cover the bulk sample

40Ar/39Ar age of 2.08 Ma (Swisher 1999).

We continue to use the laser step-heating age spectrum method on small bulk samples of

pumice hornblende. Dome pumice epiclasts have a relatively uniform mineralogy throughout the


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sedimentary sequence, suggesting that they erupted from a single volcanic center (Larick et al.,

2001). LLU pumice clasts generally show 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 3, 4 and 5).

Age spectrum diagrams for each sample are shown in Figures 5 and 6. For samples LL-1 (Cengklik

a), LL-2 (Ngampon) and LL01-4 (Cengklik b), 3 to 4 replicate runs were preformed to evaluate

sample heterogeneity (Tables 4, 5; Figs. 5a-f, 6a-d). Inspection of the amphibole separates under a

a binocular microscope reveals the hornblendes to be poikiolic with significant plagioclase

inclusions and also distinct color populations in some instances. Sample LL-1 provided 3 replicate

analyses of a green hornblende. LL-2 gave 2 replicates of a green amphibole and one of a brown

hornblende. Samples LL-3 and LL-4 each yielded one hornblende (LL-3 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. 5, 6; Table 5). Most spectra record a flat segment that contains between 2 to 6 steps and 50

to 100% of the 39Ar released. Plateau ages range between 1.880.02 Ma (LL-2-a Ngampon)) to

2.750.08 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 4). As demonstrated by LL-1 and LL01-4, replicate plateau ages do not agree at the 2 sigma

error level (Table 5) 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. 5, 6; Table 4). Radiogenic yields are typically low for initial steps,

but rise to values as high as about 80% for the high temperature heating steps (Table 4; Figs. 5, 6).

These high radiogenic yields contribute to age results that are quite precise.


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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 mostly likely cause

for that 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 1.880.02 and 1.920.03 Ma from LL-2-a and LL-2-b (Ngampon), respectively (Figs. 5a,

5b) allow the interpretation that the LLU is not older than 1.900.02 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 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 (Fig. 5f). 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 is degassing during the intermediate part of the age

spectrum. Another possibility is that the hornblende has experienced argon loss during alteration or

oxidation. These explanations are not entirely satisfactory, however based on all other data this

sample must be considered anomalous and inaccurate.


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In summary, six pumice lenses within the LLU yielded hornblende variable with respect to

color and plagioclase inclusions. While the age spectra showed varying degrees of complexity and

the replicate runs did not always produce consistent apparent ages, most recorded a flat segment

that contained between 2 to 6 steps and 50 to 100% of the 39Ar released. Plateau ages ranged

between 1.880.02 to 2.750.08 Ma. 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 1.900.02 Ma for the entire deposit.

The LLU pumice hornblende 40Ar/39Ar plateau ages are consistent with those reported for the

Bapang Formation, which overlies the black clay. The 40Ar/39Ar plateau age for the lowest Bapang

pumice hornblende is 1.510.08 Ma (Larick et al., 2001). This oldest Bapang age and the estimated

maximum LLU age (1.900.02 Ma) provide a chronological bracket for the intervening Sangiran

Formation black clay. These data suggest that the black clay accumulated in a period approaching

400,000 years.Homo erectus appeared in the Sangiran dome toward the end of this period.

5. Discussion

A lahar is a rapidly flowing mixture of rock debris and water (other than normal stream flow) from

a volcano. As an event, a lahar results from one or more discrete processes, such as debris flow

and hyperconcentrated stream flow. The basal contact of laharic sediments with underlying

deposits can be erosive or nonerosive, depending on local topography, nature of the debris flow,

and sediment characteristics (Fisher and Schmincke, 1984; Lavigne et al., 2000; Rodolfo et al.,

1998). Stratigraphical, sedimentological, and lithic characteristics of the LLU are consistent with

its deposition by a lahar or a sequence of lahars closely spaced in time.

The sedimentology of the LLU may be compared with a number of recently studied mass flow

volcanic deposits. For example, the LLUs primary features are apparent sedimentary cohesiveness


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throughout most of the unit and a basal zone transported by laminar flow. Johnson (1970) and

Hampton (1972) argue that in some conditions, lahar matrix is strong enough to produce a rigid

plug in which shear stress is below the yield threshold throughout. Such a plug can ride on a basal

zone of laminar flow within which the shear stress is greater than the yield threshold.

Consequently, clast rotation, abrasion, and deformation are minimized in the plug and fragile

materials, such as shells may be transported without much damage. The well-preserved mollusc

shells in the LLU indicate this kind of sediment cohesiveness during the LLU event.

Recently, Iverson and Vallance (2001) argued that concentrated grain fluid mixtures, such as

lahars, do not behave as simple materials with fixed rheologies. They argue that the rheology of

these materials evolves as mixture agitation, grain-concentration and fluid-pressure change during

the initiation, transit and deposition of the flow. LLU grain size and sorting characteristics are very

similar to cohesive debris flows at Cascade volcanoes. At Cascade, cohesive debris flows often

begin as debris avalanches, transform quickly to debris flows, and remain as such to their termini

(Scott et al., 1995). The relatively small outcrop area and an absence of subsurface information on

the LLU beyond the Sangiran Dome do not allow us at this time to investigate the nature of lateral

variations in rheology of the flow(s) that emplaced the LLU.

Heterolithologic lahars are most often generated by collapse of crater walls or avalanching of

rain-soaked debris covering steep volcanic slopes (Fisher and Schmincke, 1984). Historic lahars at

southeastern Asia volcanoes, such as Mount Pinatubo in the Philippines (Rodolfo et al., 1998) and

at the neighboring Merapi volcano in Central Java (Lavigne et al., 2000), are triggered primarily by

intense rainfall during the rainy season. More frequent and larger magnitude lahars occur during

and immediately following eruptive phases of these volcanoes, when abundant loose volcanic

materials are present high on the volcano flanks.

Large surge deposits, as well as very large and far-traveled pyroclastic flows and lahars are

associated with massive collapse of statovolcanoes (for example, at Mount St. Helens). At


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Cascade volcanoes, large cohesive debris flows are associated primarily with older volcanic

edifices, where hydrothermal alteration is intense (Scott et al., 1995). The LLUs sedimentary

cohesiveness implicates debris flow from a relatively old volcanic edifice with significant

hydrothermal alteration. As the LLU event was apparently not associated with eruption, it

probably was generated by sector collapse of a large stratovolcano or volcanic complex.

Based on the Merapi mass flows, Newhall et al. (2000) offer Holocene examples of lahar-

induced landscape transformations, 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 LLUs 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 meters. It is probably during subsequent early Pleistocene glacioeustatic

sea level decreases that LLU-derived 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 outcrops dominate the raw material menu. Andesite lahar clasts are naturally oriented

towards the production of bolas, which are common and opportunistic tools. More complex

tools appearing on andesite cobbles include large retouched flakes and cleavers (Smah et al.,

1999). Clasts of andesite are present in the LLU, but at the time of the firstHomo erectus

immigration into the Sangiran area the LLU was buried by many meters of Sangiran Formation

paludal 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.


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6. Conclusions

The LLUs stratigraphic and lithological characteristics suggest deposition as a heterolithological

yet 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. 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. The LLU

event significantly decreased the depth of shallow near-shore environments in the Sangiran area and

set the stage for full local terrestrial emergence during early Pleistocene glacial episodes. The

evolution of post-LLU estuarine and paludal environments in the Sangiran area produced terrestrial

settings that attractedHomo 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 has been provided

by the University of Iowa Center for Global and Regional Environmental Research, the University

of Iowa Central Investment Fund for Research Enhancement, the University of Iowa Office of the

Vice-President for Research and the Office of the Dean of the College of Liberal Arts, and the

Human Evolution Research Fund at the University of Iowa Foundation. Johan Arif and Sujatmiko

aided in fieldwork. Computer graphics and other digital images were produced by Rubn Uribe

and Shirley Taylor, and updated by K. Lindsay Eaves-Johnson. K. Lindsay Eaves-Johnson proof-

read and copy-edited the manuscript.


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Figure Captions

Figure 1. Location of the Sangiran Dome in Central Java, Indonesia. Study localities and the

location of mud volcanoes in the Domes interior are shown on the left.

Figure 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.

Figure 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.

Figure 4. Cumulative weight percent diagrams and histograms depicting grain-size distribution of

the LLU matrix. Grain-size distribution was determined by sieving LLU samples in the field. The

sample localities are shown on Figure 1. Grain-size statistics are given in Table 2.

Figure 5. 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 do not yield consistent results and suggest multiple age

populations within the bulk mineral separates. LL-2-a and LL-2-b combine to provide a maximum

age of 1.900.02 Ma for the LLU. LL-2-c yields an anomalously young apparent age and is

considered inaccurate.

Figure 6. Age spectra, K/Ca and radiogenic yield diagrams for LL01-4 (a-d), LL-3-a (e), LL-4-a,

(f) and LL01-9 (g). Like LL-1, LL01-4 has replicate ages that do not agree at 2 sigma and indicate

a heterogeneous age population of hornblende crystals. All apparent ages are interpreted as


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maximum emplacement ages for the LLU.


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Table 1. Grain-size classes of the LLU, including the coarse fraction, from numbered localitiesshown on Figure 1.

_________________________________________________________________________

SITE FRACTION DIAMETER (mm) %_________________________________________________________________________

Pagarejo Pebble >2.0 2.46Very Coarse Sand 2.0 1.0 4.17Coarse Sand 1.0 - 0.5 9.12Medium Sand 0.50 - 0.25 14.57Fine Sand 0.25 - 0.12 14.96Very Fine Sand 0.12 - 0.06 10.19Mud 2.0 3.80Very Coarse Sand 2.0 1.0 8.88Coarse Sand 1.0 - 0.5 11.92Medium Sand 0.50 - 0.25 15.54Fine Sand 0.25 - 0.12 15.82

Very Fine Sand 0.12 - 0.06 10.10Mud 2.0 11.73Very Coarse Sand 2.0 1.0 4.33Coarse Sand 1.0 - 0.5 8.16Medium Sand 0.50 - 0.25 12.67Fine Sand 0.25 - 0.12 14.73Very Fine Sand 0.12 - 0.06 10.36Mud 2.0 12.50Very Coarse Sand 2.0 1.0 7.00Coarse Sand 1.0 - 0.5 10.29

Medium Sand 0.50 - 0.25 14.06Fine Sand 0.25 - 0.12 13.70Very Fine Sand 0.12 - 0.06 9.29Mud 2.0 5.21Very Coarse Sand 2.0 1.0 6.27Coarse Sand 1.0 - 0.5 11.27Medium Sand 0.50 - 0.25 17.30Fine Sand 0.25 - 0.12 16.98Very Fine Sand 0.12 - 0.06 10.29Mud 2.0 9.06Very Coarse Sand 2.0 1.0 6.70Coarse Sand 1.0 - 0.5 10.73Medium Sand 0.50 - 0.25 14.76Fine Sand 0.25 - 0.12 14.47Very Fine Sand 0.12 - 0.06 9.20Mud


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Table 2. Grain-size statistics for the LLU matrix.

____________________________________________________________________________________

SITE MEAN GRAIN SIZE SORTING SKEWNESS KURTOSIS(Mz) (S) (Sk) (Kg)

____________________________________________________________________________________

Pagarejo 3.4 phi 2.1 phi -0.06 phi 1.56 phivery fine sand very poor very leptokurtic

Pondok 2.6 phi 2.1 phi -0.02 phi 1.40 phifine sand very poor leptokurtic

Ngampan 2.8 phi 2.3 phi +0.003 phi 1.37 phifine sand very poor leptokurtic

Cengklik a 2.4phi 2.3 phi +0.49 phi 1.27 phi

fine sand very poor leptokurtic

Bukuran 2.7 phi 2.2 phi +0.66 phi 1.25 phifine sand very poor leptokunic

Pablengan a 2.7 phi 2.3 phi +0.39 phi 1.18 phifine sand very poor leptokurtic

____________________________________________________________________________________


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Table 3. 40Ar/39Ar analytical methods and age calculation procedures for the dated pumicehornblendes.

Sample preparation and irradiation:Mineral separates obtained by standard magnetic, heavy liquid and hand-picking techniques.

Samples irradiated for 1 (NM-132) or 2 hours (NM-155) in the D-3 position of the Texas A&Mreactor along with neutron flux monitor Fish Canyon Tuff sanidine, (FC-1) with an assigned age of27.84 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 extractionsystem. 50 W CO2 laser furnace: Samples analyzed by step-heating with defocused laser beam,each step 3 minutes. Reactive gases removed during a 20 minute reaction with 2 SAES GP-50getters, 1 operated at ~450C and 1 at 20C. Gas also exposed to a W filament operated at~2000C and a cold finger operated at -140C.

Analytical parameters:Electron multiplier sensitivity averaged 0.70 or 1.25x10-16 moles/pA for NM-132 and NM-155samples, respectively. Total laser system blanks plus backgrounds were assigned to be: 930, 3.4,1.3, 3.3, 5.7 x 10-18 moles at masses 40, 39, 38, 37, and 36, respectively. J-factors determined to aprecision of 0.1% by CO2 laser-fusion of 4 single crystals from each of 6 radial positions aroundthe irradiation tray. Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows: (

40Ar/39Ar)K = 0.00020.0003; (36Ar/37Ar)Ca =

0.0002800.000005; and (39Ar/37Ar)Ca = 0.000720.00002.

Age calculations:Total gas ages and errors calculated by isotopic recombination of gas derived from all heating

steps.Plateau ages calculated for the indicated steps by weighting each step by the inverse of thevariance.Plateau age errors calculated using the method of (Taylor, 1982). MSWD values are calculated foreach plateau ages. If the MSWD is above 1, the plateau age error is multiplied by the square rootof the MSWD. Decay constants and isotopic abundances after Steiger and Jger (1977). All errors

reported at 1.


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Table 4.40Ar/39Ar isotopic data for the laser step-heated bulk hornblende samples.

Excel table here


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Table 5. Compilation of Lower Lahar dating results.

Locality Sample Weight Plateau age Plateau # Steps %

39

Ar

Total gas

age

(mg) ( 1 Ma) MSWDon

plateau in plateau ( 1 Ma)

Cengklik a LL-1-a(green) 14.7 2.300.02 1.40- 2 (E-F) 52.5 3.320.02LL-1-b(green) 15.92 2.050.02 2.80 4 (C-F) 97.4 2.230.03LL-1-c(green) 10.52 2.340.05 3.76 4 (C-F) 94.2 2.520.04

Ngampon LL-2-a(green) 18.94 1.880.02 1.95 4 (C-F) 96.9 2.000.02LL-2-b

(green) 13.75 1.920.03 2.05 3 (C-E) 82.5 2.120.03LL-2-c(brown) 8.49 - 2.180.07

Pagerejo LL-3-a(green) 4.99 2.620.16 1.20 3 (A-C) 79.1 3.200.20

Pablengan a LL-4-a(brown) 9.69 2.750.08 6.42 4 (B-E) 98.3 2.840.04

Cengklik b LL01-4-a(black) 15.50 2.600.12 2.99 4 (C-F) 61.7 2.750.10LL01-4-b

(black) 14.35 2.390.08 1.80 5 (B-F) 94.6 2.450.07LL01-4-c(black) 14.98 2.230.06 0.33 6 (A-F) 100 2.250.08LL01-4-d(black) 14.60 2.540.06 0.07 6 (A-F) 100 2.540.06

Pablengan b LL01-9-a(black) 13.37 2.350.15 1.03 6 (A-F) 100 2.410.12

lower lahar pucangan

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