Origin and age of bornhardts, Southwest Western Australia

Abstract

Most of the granitic residuals of the Wheat Belt of southwestern Western Australia are bornhardts, with some nubbins developed at the western margin and occasional poorly developed castellated forms. Their origin and age can be deduced from their structure and their relationship to a weathered (lateritic) land surface and various palaeochannels. The bornhardts are massive and most stand lower than local palaeosurface remnants. They are best interpreted as having formed by differential fracture density controlled weathering beneath the weathered land surface in pre-Eocene times. They were exposed by the stripping of the regolith beginning in the Eocene. Many are clearly stepped, indicating that their exposure took place not all at once, but episodically. A few bornhardts stand higher than the weathered land surface. They pre-date the Eocene and the stepped morphology preserved on some suggests that their crests are much older.

The convenient fiction of steady-state soil thickness

“…Uneven weathering focused in fracture zones is the first stage in development of etch surfaces and bornhardts in the Gawler Ranges, South Australia according to Campbell and Twidale (1991). Likewise, differential deep weathering is proposed as the best explanation for granitic residuals in the wheat belt of southwestern Western Australia by Twidale and Bourne (1998), and was also found in paleoweathering fronts in the Yilgarn Craton in the same region by Clarke (1994).…”

“…Regolith stripping is particularly well established in Australia (Ollier, 2001), with specific examples known from the wheat belt of southwest Western Australia (Twidale and Bourne, 1998), southcentral Australia (Quigley et al, 2007a,b), the Gawler Ranges (South Australia; Campbell and Twidale, 1991), and the Yilgarn Craton (Clarke, 1994). In Africa, tectonically-driven weathering and stripping cycles have been identified in Uganda Howard, 1998, 1999).…”

“…Ollier's (2001) review of landscape evolution in Australia noted that much of the continent shows deep weathering products hundreds of millions of years old, and complex regolith covers with modern processes working on inherited regolith materials (see also, e.g., Hill et al, 1995;Twidale and Bourne, 1998;Quigley et al, 2007b on specific sites or regions in Australia). Regolith formation and pedogenesis may also include overlapping and time-transgressive episodes, such as when an old weathering profile is dissected, resulting in formation of newer soils on the older regolith Fig.…”

Mineralogia, geoquímica e evolução da lateritização em Apuí, sudeste do Amazonas

“…Em outros locais da Amazônia, Nordeste do Brasil, Guianas, Austrália e África, o emprego de paleomagnetismo, Ar-Ar e U-Th-He (Théveniaunt & Freyssinet 2002, Colin et al 2005, Shuster et al 2005, Lima 2008, Horbe 2014) ratificam vários pulsos da lateritização ao longo dos trópicos e a idade cenozoica para as crostas lateríticas. Essas características das crostas, por serem dependentes das condições ambientais, são utilizadas no reconhecimento e na interpretação de paleosuperfícies e na evolução da paisagem (Thomas 1994, Twidale & Bourne 1998, Anand & Paine 2002.…”

“…Nessas condições desenvolvem-se crostas lateríticas ferruginosas, bauxíticas, fosfáticas, niquelíferas e manganesíferas que constituem importantes jazidas minerais (Bardossy & Aleva 1990, Costa 1990, Costa 1997, Lucas 1997, Boulangé & Carvalho 1997, Horbe & Costa 1999, Kostchoubey et al 2005, Costa et al 2005, Silva et al 2012. As crostas são importantes marcadores morfo-estratigráficos e fornecem subsídios para correlação topográfica e evolutiva da paisagem (Tardy & Roquin 1998, Twidale & Bourne 1998. Além disso, por serem produtos de condição climática intertropical, são úteis nas reconstruções paleoclimáticas, paleoambientais e nos processos que levam a sua formação ao longo do tempo geológico.…”