Evolution of landscapes in northern South Australia in relation to the distribution and formation of silcretes

Abstract

Geomorphological features on the southwestern margins of the Lake Eyre Basin provide a basis for interpreting the evolution of old landscapes containing pedogenic and groundwater silcretes. and thick bleached and weathered profiles. Recurrent sequences of cut-and-fill and duricrust formation have been identified and related to major sea level changes and tectonic movements in the Lake Eyre Basin. An extensive high pediment had formed around a basement inlier by the late Eocene. An armour of pedogenic silcrete developed on this pediment under alternating dry and wet climates during the late Eocene and Oligocene. The characteristic kaolinite + opal + alunite + gypsum assemblage of the bleached profiles formed in acid saline groundwaters during the Miocene at a time of regional low water table and arid climate. Groundwater silcretes formed in the bleached profile in response to dissection of the high pediment. They are related to a period of high groundwater tables. humid climate. and gradual sinking of the Lake Eyre basin. In the meantime, widespread low pediments (glacis) formed in Lake Eyre catchment, possibly in the early Pliocene. There is a good correspondence between a geomorphic approach, taking in to account the relationship between weathering features and palaeolandscapes, and the information provided from stratigraphic studies in the region.

Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist

“…A source of acidity is often invoked to explain this deep weathering (Senior & Mabbutt 1979;Thiry & Milnes 1991;Thiry et al 1995Thiry et al , 2007Simon-Coinçon et al 1996;Horton 2002;Newberry 2005). Geochemical modelling has shown that weathering of fresh shales containing pyrite by a sulfaterich brine produces sulfuric acid and explains the formation of kaolinite, alunite, gypsum and amorphous silica (or common opal) observed in Cenozoic paleosurface (Thiry et al 1995).…”

“…For instance, in bleached Cretaceous and Cenozoic lithologies of the GAB, common opal can make up to 60% of the rock (e.g. Thiry et al 1995Thiry et al , 2007Simon-Coinçon et al 1996). However, a low pH is not suitable for the formation of precious opal (Iler 1979).…”

Sedimentology and taphonomy of the upper Karoo-equivalent Mpandi Formation in the Tuli Basin of Zimbabwe, with a new 40Ar/39Ar age for the Tuli basalts

“…Formation of the Mpandi silcrete at depth due to fluctuations of the Eh and pH of a silica-saturated water table is inconsistent with general morphological characteristics and petrographic data. Subsurface silcretes tend to occur in lenses or pods, typically show a simple micromorphology, and are often associated with well-developed illite or kaolinite profiles (Nash et al, 1994;Simon-Coincon et al, 1996;Ullyott et al, 1998). The Mpandi silcrete, however, is a laterally extensive sheet that is not associated with any significant clay development and is characterized by complex void fills of length fast chalcedony, microquartz, and macroquartz.…”

Desert Crusts and Rock Coatings

“…Development is thought to have been relatively rapid, with each lens taking 30 000 years to form (Thiry, Ayrault and Grisoni, 1988). Similar processes have been proposed for groundwater silcrete formation in Australia Simon-Coinçon et al, 1996;Lee and Gilkes, 2005). Carbonate-hosted groundwater silcretes may have derived their silica from overlying sands and soils, with silicification proceeding by infilling of karst voids and carbonate replacement (Ribet and Thiry, 1990;Thiry and Ben Brahim, 1997).…”