Index Introduction Geologic and Stratigraphic Framework Stratigraphic Relationship Acknowledgments References

Introduction

The ‘Upper Member’ of the Eocene Pondaung Formation (Aung, 1999) in the Pondaung area of Myanmar consists of fluvio-deltaic sediments (Soe et al., 2002) and bears many vertebrate fossils (e.g. Pilgrim and Cotter, 1916; Colbert, 1938; Tsubamoto et al., 2000a, b, 2002a, 2003). In particular, primate fossils from this member have attracted the attention of anthropologists and primatologists, because they have been discussed in association with the anthropoid origins (Pilgrim, 1927; Colbert, 1937, 1938; Maw et al., 1979; Ciochon et al., 1985, 2001; Ciochon and Holroyd, 1994; Jaeger et al., 1998, 1999; Chaimanee et al., 2000; Takai et al., 2000, 2001, 2003, 2004; Gebo et al., 2002; Gunnell et al., 2002; Shigehara et al., 2002; Egi et al., 2004).

In the present paper, we report on the detailed stratigraphy of the ‘Upper Member’ of the Pondaung Formation in the Paukkaung area (Figure 1). In the Paukkaung area, there are several Eocene vertebrate fossil localities, which are named with Pk serial numbers (Tsubamoto et al. 2000a). Collections of a few primate fossils from the Pk1 locality have previously been reported (Ciochon et al., 2001). More recently, the Myanmar–Japan Joint Pondaung Fossil Expedition Team found primate fossils from the Pk2, Pk3, and Pk5 localities (Takai et al., 2000, 2003, 2004; Gebo et al., 2002; Gunnell et al., 2002; Egi et al., 2004; Kay et al., 2004). Despite these discoveries, detailed stratigraphic relationships among these localities of the Paukkaung area have not yet been reported. We provide data on the stratigraphic relationships of the primatefossil-bearing beds of Pk1, Pk2, Pk3, and Pk5.

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Figure 1. Map of Myanmar showing the several vertebrate fossil localities of the Eocene Pondaung Formation.

Geologic and Stratigraphic Framework

The Paukkaung area is located in the western part of central Myanmar (Figure 1). Geologically, it is located in the eastern limb of the Salin sub-basin of the Inner-Burman Tertiary basin, which consists of Paleogene to Neogene deposits (Bender, 1983). It occupies the central part of a local anticline known as the Thadut anticline (Htut and Ngwe, 1974). The Paleogene and older deposits of the Inner-Burman Tertiary basin underwent tectonic deformation with dominant folding and faulting due to the Himalayan orogeny.

The depositional furrow that is known in the Inner-Burman Tertiary basin between the over-rising Indo-Burman ranges and the Sino-Burman ranges is comprised of Eocene deposits more than 10,000 m in thickness, containing various sedimentary environments and lithologies. The Eocene deposits of the Inner-Burman Tertiary basin are composed, in ascending order, of the Laungshe, Tilin, Tabyin, Pondaung, and Yaw Formations (Figure 2) (Stamp, 1922; Bender, 1983; Holroyd and Ciochon, 1994).

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Figure 2. General Eocene stratigraphy of the Inner Myanmar Tertiary basin in central Myanmar (after Bender, 1983; Aung, 1999).

The whole sequence of units that yielded Eocene vertebrate fossils was originally defined as the ‘Pondaung Sandstones’ by Cotter (1914). It conformably overlies and partially interfingers with the shallow marine claystones of the middle Eocene Tabyin Formation (Tabyin clay), and is conformably overlain by the shallow marine shale of the upper Eocene Yaw Formation (Yaw shale) (Figure 2) (Bender, 1983; Aung, 1999). Khin and Win (1969) renamed it the ‘Pondaung Formation’. The Burma National Committee for International Geological Correlation Program approved this change of name in 1978.

On the other hand, Aung (1999) has recently redefined the Pondaung Formation as a freshwater part of the ‘Pondaung Sandstones’. The Pondaung Formation (sensu Aung, 1999) is about 2000 m in thickness and is subdivided into the ‘Lower’ and ‘Upper’ Members.

The ‘Lower Member’ consists of brown to greenish grey, medium-grained to pebbly, often cross-bedded sandstone with variegated mudstone interbeds and subordinate polymitic conglomerates. It is about 1500 m in thickness at the type section, which is located near Padaukkon village. The predominant sandstones are mostly lithic sandstone (litharenite), composed of 25% or more unstable constituents in which rock fragments exceed feldspar (Lwin, 1999). The clast-supported polymictic conglomerates (~20 m in thickness) are composed mainly of pebble- to cobble-sized clasts, which are derived from intrusive and extrusive igneous rocks, serpentinites, schistosed and gneissosed rocks, cherts, flints, and some reworked clastic and carbonate rocks. In the middle portion of the member, medium-bedded, whitish, and tuffaceous sandstone overlays the carbonized plant-bearing sandstone sequence. Silicified fossil woods occur in abundance in medium- to coarse-grained sandstones of the member (Soe et al., 2002; Aung, 2004).

The ‘Upper Member’ consists of variegated claystones and light brown-to-buff-colored sandstones (Aung, 1999), and has been interpreted, at least in the Pondaung area, as fluvio-deltaic deposits (Soe et al., 2002). It contains many terrestrial mammalian and other vertebrate fossils. Although there are many fossil-bearing horizons, all mammalian fossils have been recovered from the lower half of the ‘Upper Member’ (Aung, 2004). The ‘Upper Member’ is about 500 m in thickness at the type section, which is located close to the entrance of Konywa Village. The lower boundary of this member is demarcated at the base of a unit consisting of a sandstone bed and a ferruginous clay bed bearing leaf fossils and ironstones (Aung, 1999). This horizon contains the lowermost occurrence of vertebrate remains in several places. There are at least three lateritic layers in the topmost part of the Pondaung Formation in the Paukkaung area (Aung, 1999). The age of the member is now considered to be the late middle Eocene (=Bartonian) on the basis of mammalian faunal correlation (Pilgrim and Cotter, 1916; Pilgrim, 1925, 1928; Colbert, 1938; Holroyd and Ciochon, 1994; Ducrocq, 1999), nannoplankton assemblage (Mon, 1999), biostratigraphic relationships with adjacent marine units (Bender, 1983; Holroyd and Ciochon, 1994), and fission-track dating on the tuff bed (37.2±1.3 Ma: Tsubamoto et al., 2002b).

Stratigraphic Relationship

The Paukkaung area is located about 17 km away from the type section of the ‘Upper Member’ of the Pondaung Formation. In the Paukkaung area, localities Pk1, 2, 3, and 5 have yielded primate fossils.

At the Pk1 locality (Sabapon-Taung or Sabapondaung Kyitchaung) (Figure 1, Figure 3), the fission-track zircon age of the ‘Upper Member’ was reported (Tsubamoto et al., 2002b), and primate postcranial fossils (NMMP 20) were discovered (Ciochon et al., 2001). The tuff bed is stratigraphically just 1 m above the horizon bearing the primate postcranial fossils (Figure 3) (Tsubamoto et al., 2002b).

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Figure 3. Columnar sections of the ‘Upper Member’ of the Pondaung Formation at the Pk1 and Pk5 localities. Abbreviations: VF.S, very fine-grained sand; F.S, fine-grained sand; M.S, medium-grained sand.

In November 2002, at the Pk5 locality (Ayoedawpon Taung Kyitchaung) (Figure 1, Figure 3), we found an upper molar of Pondaungia (NMMP-KU 1557). The Pk5 locality is approximately 700 m away from the Pk1 locality. We also discovered a tuff bed at the Pk5 locality. This tuff bed is about 3.2 m above the horizon where NMMP-KU 1557 was found. Although the Pk1 locality has been relatively more eroded than the Pk5 locality, both stand above the same medium-grained and buff-colored sandstone horizon and have the same stratigraphic sequences (Figure 3). This is confirmed by tracking the basal sandstones of the localities from Pk1 to Pk5. The tuff beds at Pk1 and Pk5 probably consist of the same single layer on the basis of its color, grain size, and stratigraphic position.

The Pk2 and Pk3 localities are about 1200 m and 1400 m away from the Pk5 locality, respectively (Figure 1) (Tsubamoto et al., 2000a). At the Pk2 locality, we found part of a skull (NMMP-KU 0229) and maxilla (NMMP-KU 0228) of Amphipithecus, and calcaneus (NMMP-KU 0961) and mandibles (NMMP-KU 1203) of eosimiids (Takai et al., 2000, 2004; Gebo et al., 2002), and French paleontologists found a mandible of Pondaungia (Gunnell et al., 2002). In November 2001, at the Pk3 locality, we found a mandible of Amphipithecus (NMMP-KU 1125). These localities are located above the basal sandstone horizon of Pk1 and Pk5, which was confirmed by tracking the basal sandstone of the localities and by the presence of a tuff bed at Pk2 and Pk3.

In conclusion, our stratigraphic work in the Paukkaung area has revealed that the primatefossil-bearing beds at the Pk1, Pk2, Pk3, and Pk5 localities are located nearly at the same stratigraphic level. Because these horizons are located just below or above a tuff bed that has been age-calibrated (Tsubamoto et al., 2002b), an age date of approximately 37.2 Ma (late middle Eocene) is confirmed for the Pondaung primates from the Paukkaung area.

Acknowledgments

We are grateful to Brigadier General Than Tun and Major Bo Bo (Office of the Chief of Military Intelligence, Ministry of Defence, Myanmar) for their sincere guidance and help in the field. We thank Dr U Thein for instructive suggestions and critical reading of the manuscript. We are grateful to Professor Tin Thein, Professor Khin Maung Myint, Soe Thura Tun, and Aung Naing Soe for their assistance in field work, and for helpful discussions. Thanks are also due to Professor Nobuo Shigehara and Dr Masanaru Takai (Primate Research Institute, Kyoto University) for their discussion and financial support. This research was supported by the Overseas Scientific Research Funds of the Ministry of Education, Culture, Sports, Science and Technology (No. 14405019 to N. Shigehara) and by the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (No. 04836 to T. Tsubamoto and No. 04748 to N. Egi).