Australia: The Land Where Time Began

A biography of the Australian continent 

The Bulldog Shale

The authors1 suggest that the Bulldog Shale from South Australia, that outcrop along the northern margin of the Carnarvon Basin, can typify marine rocks from the Aptian of Australia. A series of mudstones and sandstones that are finely laminated make up this unit, deposition taking place under shallow marine conditions during a phase of marine transgression. The bottom waters are indicated to have been poorly oxygenated by dark layers that are organically rich and microplankton that are of low diversity. A characteristic of the Bulldog Shales is that weathering has severely leached and bleached them leaving them white. The area around Andamooka and Coober Pedy in South Australia are the best places to see the results of this weathering. At these locations bleaching may reach depths of about 40-50 m, incorporating opal deposits that are extensive. The opal at Andamooka and Coober Pedy often occurs along cracks or faults or in cavities formed by fossils. Soft tissue structures have also been found that have been replaced by opal. Shale horizons that contain widespread gypsum and carbonate limestone concretions that are fossiliferous dominate the Bulldog Shale where bleaching has not occurred. An age of early Aptian to early Albian is inferred by macroinvertebrate and fossil plankton assemblages.

The authors1 suggest there is evidence, in the form of ice-rafted boulders, glendonites, that are calcite pseudomorphs of the mineral ikaite that forms only between -1o and 6o C, dense growth rings suggesting a climate that was seasonal with cool to very cold winters that possibly involved freezing.

Sources & Further reading

  1. Kear, B.P. & Hamilton-Bruce, R.J., 2011, Dinosaurs in Australia, Mesozoic life from the southern continent, CSIRO Publishing.
 
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated  15/12/2011



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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading