Australia: The Land Where Time Began

A biography of the Australian continent 

Birdrong Sandstone

The Birdrong Sandstone and the Nanutarra Formation are the oldest known marine sequences from the Lower Cretaceous of Australia that contain fossils. They are seen as weathered cliffs where they are exposed in the Murchison River region of the northwest of Western Australia, in the Carnarvon Basin. Both vertebrate and invertebrate fossils have been recovered from these units, though the fossil assemblages from the Eromanga Basin and the Carpentaria Basin of the Lower Cretaceous, that included the Bulldog Shale of South Australia and the Wallumbilla Formation of northern New South Wales-southern Queensland that contained much richer fossil assemblages.

The authors¹ suggest that the Birdrong Sandstone (as well as the Bulldog Shales) from South Australia, that outcrop along the southern 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 Birdrong Sandstone shared with 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 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 Soft tissue structures have also been found that have been replaced by opal.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

Sources & Further reading

1. Kear, B.P. & Hamilton-Bruce, R.J., 2011, Dinosaurs in Australia, Mesozoic life from the southern continent, CSIRO Publishing.