Post-Permian Geology of the Gunnedah Basin

Triassic Period – the big gravel blanket

The end of the Permian Period was marked by an increase in compressive stresses that in some areas forced the Permian sediments upwards creating an eroded surface.  This surface was then buried in the Triassic Period by coarse gravels and sand (the Digby Formation) carried by rivers from the rapidly rising New England area.  As the highlands were eroded, the rivers became less energetic and finer sands and silts were deposited (the Napperby Formation).


Jurassic – volcanoes and intrusions

The start of the Jurassic Period was marked by a world-wide adjustment of forces driving the earth’s plates. This led to changes in crustal stresses which ultimately caused the breakup of the supercontinents of Pangaea and Gondwana. In the Gunnedah Basin, and other parts of eastern Australia, this period was marked by a prolonged period of volcanic activity together with uplift and erosion of the Triassic sediments.


The lavas of the Garrawilla Volcanics covered extensive areas and in places magma did not reach the surface but squeezed into (intruded) the Permian sediments, including coal seams.  The effect on the coals ranges from cindering, where all gas is lost, to mild heating which may be beneficial in increasing the coal's gas holding capacity. Methane can also be generated during the heating, but carbon dioxide may also have been introduced with the magmas, displacing methane from the coal seams in some areas . 


From then until now

Cessation of volcanic activity led to a relatively inactive period geologically, with accumulation of sediments in wide valleys.  Thick sandstones were deposited from braided streams and form the Pilliga Sandstone, intake beds of the Great Australian Basin.  Within PELs 1 and 12 most Pilliga Sandstone only remains as eroded remnants and, as such, they no longer contribute water to the artesian aquifer.


Another phase of volcanic activity occurred during the Tertiary Period and formed the Liverpool Ranges, Mt Kaputar and the Warrumbungles.  During and since the Tertiary Period the region has undergone weathering and erosion and filling of the valleys with alluvial sediment to form the agriculturally important Liverpool Plains. 

Stratigraphy of PELs 1 & 12