The increasing aridity of inland Australia has caused most of the drainage systems to become internal, ending in salt lakes. The valleys have become choked with sand and clay, through which groundwater flows downgrade to the lakes. Evaporation of the water transforms this alluvium to calcrete, and/or to gypsite in the lower parts of the drainageways and around the margins of the lakes. Typically, this diagenesis proceeds (towards the surface) from sand and clay rt arrow mottled calcrete rt arrow massive calcrete rt arrow brecciated calcrete rt arrow pisolitic calcrete rt arrow laminar calcrete, but because of fluctuating water-tables this succession is not everywhere the same. Because the scanty rainfall is extremely variable in time, space and amount, each individual drainageway has its own diagenetic history, which may even differ in different parts of the same drainage system. Much of the calcrete, together with any carnotite it may contain, has undergone repeated solution and reprecipitation. Thus, no single model relating carnotite concentrations to the nature of the calcrete can be generally applied. Detailed descriptions of the various calcrete/gypcrete lithofacies, both above and below the water- table, are given. -H.R.B.
|Number of pages||22|
|Journal||Journal of Sedimentary Petrology|
|Publication status||Published - 1982|