Genesis of pisoliths and of the Weipa Bauxite deposit, northern Australia

Consideration of the Late Cretaceous and Cenozoic geological, landscape and climatic history of western Cape York suggests that an environment suitable for the development of bauxite has prevailed for the past 100 Ma. Uplift of the western Cape exposed flat-lying Cretaceous Rolling Downs Group glauconitic and feldspathic sandstones to weathering under a climate that was seasonally wet, establishing a deep lateritic profile. The rising highlands along the eastern Cape allowed Lower Cretaceous sediments and Paleozoic rocks to be eroded by west-flowing rivers, moving weathered Rolling Downs Group materials westwards. Some coarser components may have been deposited on the lower-gradient coastal plains, while finer components would have moved farther seawards. By the Paleogene quartzose rocks were exposed along the highland spine. Their weathering and erosion products, together with eroded material from the already deeply weathered Rolling Downs Group rocks, were deposited as sands and clays in broad fans forming the Neogene Bulimba Formation over established lateritic profiles on the Cretaceous rocks. This continued until the Weipa Plateau was isolated by river capture and incision sometime during the Neogene. Continued weathering since the mid-Neogene saw the formation of lateritic profiles on the fan sediments inset into the weathered Cretaceous rocks. The Weipa bauxite deposit has similarities to other northern Australian bauxites, such as that at Gove, in having clear evidence for transport of the pisolithic bauxite. Formation, erosion, fragmentation and cortication of pisoliths have been ongoing over this time. In places, ironstone fragments, pisoliths and ooliths have accumulated in the lower parts of the landscape as unconformable transported bauxite over the weathering profiles. Elsewhere the lower parts of the pisolithic bauxite remain in situ over the weathering profile of either the Rolling Downs Group or the Bulimba Formation, though much bioturbated. The boundary between the upper parts of the lateritic profile (the Mottled Zone) and the pisolithic Bauxite may be erosional (physically unconformable), karstic (chemically unconformable) or conformable through a Transition Zone lying at the level of the wet season water-table demonstrating that processes that produced the lateritic profiles, the pisoliths and their redistribution are, in places, ongoing. The bauxite is at the same time and the same place some millions of years old and a few months old. Redistribution of pisoliths continues on the Weipa Plateau as they are eroded from its margins and redeposited in lower parts of the landscape.