Australia: The Land Where Time Began

A biography of the Australian continent 

The Bulldog Shale

The Nanutarra Formation and Birdrong Sandstone record the initial onset of shallow marine transgressive conditions in northwestern Australia during the middle-late Neocomian (Hauterivian-Barremian).  These rock units are lateral equivalents that crop out along the northeastern and southern margins of the Carnarvon Basin respectively. They are comprised mostly of silty sandstones that grade upwards to fine claystone layers in the uppermost part of the sequence. It is common to find glauconite beds and evidence has been found in both beds of extensive bioturbation (disturbance of sediments by burrowing organisms). It is thought that the palaeoenvironmental setting was shallow coastal marine, with deposition over the seafloor of irregular topography composed of basement rocks from the Palaeozoic. Estimates of age that were derived from macrofossils, marine microplankton, fossil spores and pollen indicate that deposition began in the northwestern Carnarvon Basin during the early Hauterivian, and then proceeding southeastwards through the Barremian.

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 Soft tissue structures have also been found that have been replaced by opal.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 -1^o and 6^o 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.