In Australia generally numerous biogeographic and geomorphic studies have indicated that Tertiary climates varied considerably, and recent theories of continental drift may provide a partial explanation (e.g. Griffiths 1971, Rowen and Axelrod 1972, Walker 1973). However, few of these studies have been able to date accurately the events or to assess sharply changes they have described.
Geomorphological evidence of Tertiary climates in the East Kimberleys has been obtained mainly from the study of weathering crusts. Laterite-capped remnants of a deeply weathered landsurface, particularly in areas immediately adjacent to the study area, leave little doubt that it was developed quite widely over tropical Australia now lying within the Northern Territory and Western Australia.
Laterites in the Northern Territory (Wright 1963, Hays 1967, and Williams 1969b) and the West Kimberleys (Wright 1968) are believed to
range from late Cretaceous to Plio-Pleistocene in age, and some laterite-capped landsurfaces have developed within the profile of an earlier land surface (Hays 1967).
Remnants of laterite surfaces survive in the southern margins of the East Kimberley Region. In the southwestern margin and just beyond (i.e. within the area covered by the Mount Ramsay sheet of the 1:250,000 geological series) the laterites show a variable
relationship with other Cainozoic deposits (Traves 1955, Roberts,
Halligan, and Playford 1968) and with the relief. For example, laterite is unconformably overlain in places by the Low ford Beds (Tertiary (?) limestone, calcareous siltstone, silicified limestone, and chalcedony); elsewhere laterite overlies the Lawford Beds. This is compatible with one period of lateritisation interrupted by a transgression, or may indicate two periods of laterite formation. In some areas the laterite surfaces form low plains (e.g. between the Laura and Mary Rivers 25 km ESE of Margaret River station); in others they form hills rising up to 30 m above the surrounding plains (e.g. 2.5 km ESE of Louisa Downs station where the Lawford Beds overlie the laterite); in other areas the present land surface appears to lie entirely below the former
weathering front (e.g. between Moola Bulla and Mount Amherst stations). In the southeastern margin (e.g. near Nicholson station, and in the main Ord River Valley SSW of Ord River station) the laterite surfaces often lie only a few metres above the modern floodplain,
indicating only very limited incision. On basalt the laterite forms low mesas and buttes up to 8 m high. Undissected laterites are found in nearby areas on the Sturt Plateau (Paterson 1970), in inland parts of the Fitzroy Basin (Guppy and others 1958), and in the Great Sandy Desert
(Veevers and Wells 1961). The lack of dissection may be attributed to distance from the coast, lower elevation, and the drier climate in such inland positions.
The age of these land surfaces is hard to determine in the absence of datable material. Indeed the remnants of the East Kimberleys could belong to more than one phase or cycle of deep weathering,
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evidence for which is found in nearby areas of the West Kimberleys and the Northern Territory.
Paterson (1970) describes a surface of possible Mesozoic age in the East Kimberleys which may be correlated with a surface in the Vest Kimberleys. However, he ascribes the major period of deep lateritic weathering to the early to mid-Tertiary. The weathering profile could have been stripped towards the end of the Tertiary following the incision of drainage accompanying uplift and climatic desiccation (Paterson 1970). Van Andel and Veevers (1967) indicate that the relative rise in the land between the Miocene and Pleistocene was
about 400 m , and evidence for considerable incision is found, particularly near the coast (e.g. at Carlton Gorge). Climatic desiccation is
implied by the fact that laterite does not appear to be forming at present, except locally in poorly drained areas (Stewart 1970),
despite the favourable if not necessary circumstance of high temperatures, and it is concluded that there is now insufficient precipitation for
its development. Traves (1955) was unable to specify the age of profiles south of Hall’s Creek, but believed that they formed
contemporaneously with laterite in the rest of Australia. However, there is evidence of regional variation in the beginning and ending of
laterisation in Australia. Hays (cited in Stephens 1971) has concluded from all the evidence:
"laterization started late in the Cretaceous on the north side of the continent, and that laterite and analogous deposits appear to age towards the centre of Australia from the north, east and west,"
Mabbutt (1965a) concluded that lateritic weathering ceased in Central Australia earlier than in the Northern Territory.
The interpretation of these surfaces in paleoclimatic terms is far from clear, since agreement has not been reached on the mode of formation. Stephens (1971) concludes that laterite formation is
nas so d a t e d predominantly with the regime of vertical and lateral movement of groundwater and the consequent geochemical partition of the dissolved iron, aluminium and silica hy
different physical and chemical reactions with the atmospheric environment above, the significant and direct effect of
climate being the control of the rates of physical and chemical reactions by temperature,"
In the East Kimberleys laterite is not forming at present, and since temperatures are high it is concluded that there is insufficient precipitation for its development. We may therefore infer that when laterites were formed on the southern margins of the East‘Kimberleys, probably in the early to mid-Tertiary, that the climate was appreciably wetter than at present. Furthermore, it is reasonable to assume that
the present wetter northern areas were also favourable to the
development of deep weathering profiles, since a rainfall gradient in the same direction and with the same sense is likely to have obtained in the Tertiary.
Two other deposits indicate wetter climate in these northern areas during the Tertiary. First, the lacustrine sediments of the White Mountain Formation found to the north of Ord River station are
thought to be of Tertiary age. They are rich in silica and similar to other deposits in the Barkly Tableland: they are probably related to the period of laterisation (Traves 1955). Second, Tertiary alluvium on the Sturt plateau described by Traves (1955) was deposited in swamps and probably also developed contemporaneously with the formation of laterite on higher and better-drained topography.