Pucangan and Kabuh formations in Sangiran area contain plenty of vertebrate fossils including hominid fossils. In the context of stratigraphical position, the former is older than the latter formation. On behalf of their research in Sangiran and also to avoid the confusion, Japanese scientist preferred in naming Sangiran and Bapang instead of Pucangan and Kabuh formations.
According to some scholars like van Es, von Koenigswald, Sartono and Indonesia-Japan joint research team, the Sangiran formation is divided into two parts namely the Lower Lahar at the basal and the Black Clay at the upper part.
The Lower lahar can harmoniously overlies the Puren (Kalibeng) formation, but, it is mostly placed discordantly over the Puren (Kalibeng) formation. The Lower Lahar consists of light gray unstratified andesitic tuff with rounded to subrounded fragments of pebble (2-64 mm) and cobble (64-256 mm) sizes. The fragments are mainly andesite but a small amount of claystone, sandstone, mollusks and corals can be present as well.
The Black Clay mainly composes of clay and silty clay inserted by sandstone, peat and tuff. There are more than forty tuff layers can be found in the upper part of Sangiran formation but only twelve tuff layers (T0 to T11) are useful for stratigraphical markers. Amongst them, T10 is the thickest (5-20 cm) and the others are generally thin. Furthermore, the Blac Clay can be divided in two parts that are the lower brackish and marine and the upper lacustrine parts where S-1 (Shell 1) layer as their boundary; 30 cm thick, contains of yellowish gray silt with varieties of mollusks.
The Bapang formation conformably overlies the Sangiran formation and in turn underlies discordantly the Notopuro formation. The Bapang formation consists mainly of clay, silt, sand, iron sand and gravel of fresh-water origin. The lower part of this formation is called Grenzbank bed or zone while tuff layers are normally found in the upper part.
Grenzbank (means border bed) was introduced by von Koenigswald for calcareous bed whose thickness up to 1 m, marking the boundary between Sangiran and Bapang formations. According to Sudijono, the Grenzbank zone is a pebbly (2-64 mm) sand bed with intercalation of hard calcareous sand and pebbles; often clay or silt intercalations are also present. Based on petrographicaly analysis, this hard calcareous bed consists of two different groups of litology, namely the calclithite or terrigenous carbonate and the calcareous conglomerate. The former particularly occupied the northern half and the later is distributed in the southern half of the distribution of the Grenzbank zone in Sangiran.
Although it is difficult to trace the tuff layers throughout the area of Sangiran, however, there are mainly three tuff layers can be recognized in the upper part of Bapang formation namely the Lower, Middle and Upper Tuff layers. Amongst them, only the Middle Tuff layer is most widely traceable. Based on these three tuff layers, the Bapang formation is subdivided into four parts, namely the Lowermost, Lower, Middle and Upper parts.
The Lower Tuff layer is white, its thickness up to 100 cm. It consists of fine-grained volcanic ash. The Middle Tuff layer is yellowish white and up to 110 cm thick. It consists of tuffaceous sandy silt with thin intercalations of medium-grained sand. The Upper Tuff layer consists of pink tuff and tuffaceous silt, up to 90 cm thick.
Hominid fossils in Sangiran mainly come from the uppermost part of the Sangiran formation and the lower and middle part of Bapang formation. The older hominid fossils are grouped as G/S (Grenzbank/Sangiran) assemblage and the younger one as Bapang AG (Bapang Above Grenzbank) assemblage. Based on the fission tract dating analysis, the range of age of layer bearing hominid fossils in Sangiran were between 0.78 and 1.16 million years ago.
Amongst the hominid fossils considered came from the uppermost horizon of Sangiran formation (approximately above T10) were S1b (Pb), S4 (PIV), S6 (Ma), S9 (Pc), S15a (Pd) and S22 (Pf). While, S8 (Mb) and S21 (Pe) probably came from the GB zone. S2 (PII), S3 (PIII), S12 (PVII), S17 (PVIII) and probably Skull IX (Tjg-1993.05) have originally come from the layer between the Lower Tuff and the Middle Tuff and S10 (PV) was reported from the layer between the Middle Tuff and the Upper Tuff. All the hominid fossils from Bapang formation were grouped as Bapang AG (Bapang Above Grenzbank) assemblage.
In the context of faunal assemblage, the G/S and Bapang AG assemblages are considered as members of Trinil H.K. (Haupt Knochenschicht= main fossiliferous layer) and Kedung Brubus faunas respectively.
Arif, J., M.R. Schurr, and R. Kapid. 2008. Evolutionary molar size reduction in Sangiran (Javanese) Early-Middle Pleistocene Homo erectus as an effect of paleo-climatic change. Journal of Indian Ocean Archaeology, in press.
Itihara, M., Sudijono, D.Kadar, T.Shibasaki, H.Kumai, S.Yoshikawa, F.Aziz, T.Soeradi, Wikarno, A.P.Kadar, F.Hasibuan, and Y.Kagemori. 1985. Geology and stratigraphy of the Sangiran area. In: Quaternary geology of the hominid fossils bearing formations in Java (ed: Naotune Watanabe and Darwin Kadar), Geological Research and Development Centre, Special publication no.4, pp.11-43.
Sudijono. 1985. The Grenzbank, a key marker bed. In: Quaternary geology of the hominid fossils bearing formations in Java (ed: Naotune Watanabe and Darwin Kadar), Geological Research and Development Centre, Special publication no.4, pp. 135-138.