Solving mammalian riddles Chapter 3. The palaeoenvironmental scene based on data mining Palaeoenvironments of Borneo Geology and palaeogeography of Borneo Wilson and Moss (1999) provided a detailed overview of the geology of Borneo, and their information was used in the palaeoenvironmental reconstructions in this work, alongside the information provided by Hall (1998). A land connection between southern Borneo and mainland Southeast Asia is inferred to have existed during the Eocene and Oligocene (Pupilli, 1973 in Wilson & Moss 1999). During the Late Cretaceous–Early Tertiary, a large river (possibly the ancestral Chao Phraya/Mekong River) ran across the length of Sundaland, from its Eurasian source areas to the central and northern ‘Borneo fan’ area, with a delta in the Natuna Island region (Moss & Chambers 1999). The central ranges of Borneo were uplifted towards the end of the Oligocene, and the erosion of these areas supplied sediments eastwards towards the Makassar Straits (Moss et al, 1998 in Wilson & Moss 1999). Figure 3.10. Main geological features of Borneo (after Steinshouer et al. 1997); for legend refer to Fig. 3.5. 80 An important aspect of the geology of Borneo was brought to light by the work of Haile et al. (1977), who obtained palaeomagnetic data from Cretaceous igneous rocks in West Kalimantan. They stated that, although Borneo had remained approximately in the same paleolatitude since the late Cretaceous, it had rotated counter-clockwise about 50º acting as a unit with the Malay Peninsula. Similarly, Nishimura & Suparka (1997) stated that Borneo, the Celebes Sea basin and the western arm of Sulawesi rotated 50º counterclockwise during the early Miocene (20–17 Mya). This rotation continued until some 10 Mya (Hall 1998), although Lumadyo et al. (1990 in Lee & Lawver 1994) had stated that the stable western part of Kalimantan has not been rotated since the Eocene. The rotation history of Borneo and Western Sulawesi is therefore still highly contentious (for an overview see Wilson & Moss 1999), and data reported by Morley (2000a) suggest that significant rotation of Borneo cannot be accommodated by Tertiary structures onshore and in the Gulf of Thailand. Pieters et al. (1987) provided an overview of the early Tertiary development of Borneo. In Late Eocene times the deposition of terrestrial to shallow marine sediments began over a large part of the island of Borneo around a central highland formed by an emergent orogenic complex (the Kuching Arch). During the Paleogene the Kuching Arch comprised island and shallow water areas, which separated the more rapidly subsiding portions of the Sarawak and Kutai Basins (Rose & Hartono 1978). The continental basement of the Schwaner Mountains in Southwest Kalimantan probably persisted as a highland area, connected by land to SE Sumatra and the Malay Peninsula. The Meratus Mountains (Fig. 3.10) and the Barito region were still submerged (Pieters et al. 1987), and the Paternoster Block, offshore of present-day SE Kalimantan, was only partly emergent during the Early Paleogene, being later transgressed during the Oligocene (Rose & Hartono 1978). In Early Oligocene times a westward transgression of the sea reached the upper Mahakam River and the Barito Shelf. A long and narrow arm of this sea extended westward almost across to the present-day west coast of Kalimantan, confined between the tectonically active emergent orogenic complex to the north and the persisting highland basement to the south. Continuing deformation and uplift along the southern margin of the orogen led to the demise of this central Borneo Basin and by Early Miocene times the basin had disappeared altogether according to Rose and Hartono (1978). In Middle Miocene 81 Solving mammalian riddles Chapter 3. The palaeoenvironmental scene based on data mining times, the Kutai Basin became separated from the Ketungau-Melawi Basin by a high area near the present-day Müller Mountains, after which both basins slowly filled up (Ott 1987). This resulted in volcanic activity, which formed a high plateau of tuffs and other volcanic material (Molengraaff & Weber 1920, p. 467). This was then also the time when the northern part of Borneo (present-day Sarawak, north East Kalimantan, Sabah, and Brunei) became connected to the southern part (consisting of the Schwaner Mountains). Interestingly, considering that the entire Kapuas and Mahakam River valleys are low-lying (< 100 m a.s.l. even in the upper reaches), it is not impossible that, during extreme sea level highstands, the two river systems became connected again, thereby effectively separating northern Borneo from the rest of Sundaland. Between 17 and 11 Mya, sedimentation rates rose everywhere in SE Asia, spectacularly so in the Sarawak and Sabah Basins, north of Borneo. Rates of deposition of carbonates and shallow-water detrital material changed from 0.17 to 0.4 mm/year. During the Pliocene, this rate increased even more to 0.69 and 0.65 mm/year for the Sarawak and Sabah Basins respectively (Metivier & Gaudemer 1999). It remains unclear whether this increase was due to the climatic optimum during the Middle Miocene, which may have led to higher rainfall and erosion, or whether increased uplift provided the sediments. Considering that the increase continued into the Pliocene, mountain building may be a more likely explanation. Adding up the solid phase volume accumulated in the Sabah and Sarawak sedimentary basins also suggests the occurrence of considerable mountain areas on Borneo. Métivier et al. (1999) provided a total estimate of 1.3 * 106 km3 of sediment accumulation in the Sabah and Sarawak Basins since 17 Mya. Under the assumption that the Bornean highlands were equally drained in a northern, southern, and eastern direction this would add up to approximately 4 * 106 km3 of upland being eroded in 17 Mya, if no further uplift occurred. The present Bornean uplands are approximately 1 * 106 km3, which would indicate that over the last 17 Mya, a mountain area with a height of several kilometres has been eroded. This was confirmed by R. Hall (pers. comm.), who remarked that Borneo shed vast amounts of sediments in the last 10 Mya, equivalent to the removal of 6 km of crust; as much as the Himalayas now but on a third of the area. Based on research on geological and topographic criteria, Thomas et al. (1999) estimated that in NW Kalimantan the groundsurface was lowered between 82 1,200 and 1,500 m since 30 Mya, giving an average denudation rate of 40–50 mm/Kya (which is considerably lower than the estimates by Metivier & Gaudemer 1999). East Borneo Zanial and Luki (1984) described the depositional cycles in the Tarakan Basin. From the latest Oligocene to early Middle Miocene sedimentation in the basins occurred in a marine environment. By the end of the Early Miocene the delta front had advanced approximately 200 km eastward of the present-day coastline (Moss et al., 1997 in Friederich et al. 1999), which brought north-east Kalimantan very close to the Sulawesi area. At the end of this period the area was uplifted, but presumably re- flooded during the Middle Miocene highstand. A major change in the sedimentation history of north-east Kalimantan occurred in early Middle Miocene, when a deltaic environment developed at the western side of the region. This delta front started to prograde eastwards during the Miocene until all sedimentary processes in this area were terminated by a Late Miocene uplift at approximately 6.6 Mya. The eastern coastline of Borneo in Early Miocene times ran approximately from the south-west corner of Kalimantan, via the area of the upper reaches of the Barito and Mahakam to just north of the Mangkalihat Peninsula (Pieters et al. 1987). During the Miocene, delta systems in the Kutai Basin prograded south-eastward filling the Kutai Basin so that by the Late Miocene, deltaic deposition had generally reached a position beyond that of today’s East Kalimantan coastline (Rose & Hartono 1978). By the end of the Miocene, the drainage system within Borneo was similar to the present day (Wilson & Moss 1999), although Smit-Sibinga (1953b) stated that the Mahakam River came into being only 2 Mya, and that this river initially flowed into the very large Kutai Lake (of which the present lakes are only remnants). In the Late Miocene (Middle Miocene according to Ott 1987) and Early Pliocene the Meratus Graben was uplifted and started to shed sediments to the west and to the east (Rose & Hartono 1978; van de Weerd et al. 1987), eventually leading to the rise of the Meratus Mountains. At that time, the Barito Basin became separated from the Kutai Basin by the Adang Flexure/Fault, which resulted from the uplift of the Meratus Mountains (Satyana et al. 1999). Miocene coals on the west and east side of the 83 Solving mammalian riddles Chapter 3. The palaeoenvironmental scene based on data mining Meratus Mountains, suggests that the basins on either side of these mountains were becoming terrestrial and that the climate was warm and wet (Friederich et al. 1999). The uplift of the Meratus Mountains continued into the Pleistocene (Satyana et al. 1999). East of the Meratus Mountains, a large island existed during the Tertiary (van Bemmelen 1970). The elevated area was surrounded by the depositional basins of SE and E Borneo, West Sulawesi, the Kangean-Madura-Rembang belt, and Bawean. Van Bemmelen named this land area the Pulau Laut centre of diastrophism. According to him this Pulau Laut centre was elevated at the end of the Pliocene, but the crest of this dome was rapidly engulfed during the Pleistocene, presumably leaving only the present-day land area of Pulau Laut. Emmet and Bally (1996) claimed that the eastern extension of the Kangean High was emergent in the Late Miocene, but it is unclear whether the Kangean High was part of this Pulau Laut centre.