Pride of Australia

Pride of Australia / Red Emperor a prized and unique specimen (source from http://www.opalsdownunder.com.au)

Lightning Ridge lies in a large geological feature called the Surat Basin, which is part of the vast Great Australian Basin. The Great Australian Basin covers 1.7 million square kilometres of eastern Australia.

It was formed during the Jurassic to Cretaceous, when dinosaurs walked the Earth. About 140 million years ago, the basin contained a large inland sea which accumulated sediments that later hosted the formation of precious opal. The sedimentary host rocks are essentially horizontal because they were deposited on the seafloor and have been little deformed since.

The rocks which host the opal at Lightning Ridge were deposited in shallow water near the edge of the basin, probably in an estuary.

Overlying the Cretaceous sedimentary rocks are sandstones and conglomerates that were deposited by streams and rivers in the Tertiary period, about 15 to 5 million years ago. Many of these younger rocks have hardened due to weathering processes to form silcrete and are often quarried for road materials.

Opal occurs at Lightning Ridge either as irregular nodules ('nobbies') or in seams and thin layers within vertical or horizontal joint planes, in a characteristic soft, grey to buff-coloured claystone that hardens and whitens on drying. This claystone is commonly called 'opal dirt'. In some parts of the Lightning Ridge field, a thin, hard, silicified band, known as the 'steel band', is found immediately above the topmost opal dirt lens.

Opal is irregularly distributed, but it is normally found within, and close to the top of, the various opal dirt lenses. In some places it may extend into the steel band, where that is present.

Most of the levels of opal dirt occur between 6 and 18 metres below the surface in the local fields, but are not necessarily horizontal or persistent. In the newer fields, shaft depths of up to 30 metres are common.

Opalised shells, wood, fish, bird, mammal and reptilian bones of Cretaceous age are sometimes found in parts of the field, but are not as abundant as at White Cliffs.

For further information
Lightning Ridge Office
Phone:
+61 (0)2 6820 5200
Fax:
+61 (0)2 6829 0825
lightningridge.office@planning.nsw.gov.au
Postal:
NSW Department of Planning, Industry and Environment, Division of Resources and Geoscience, PO Box 314 Lightning Ridge NSW 2834
Office:
Lot 60 Morilla Street Lightning Ridge NSW 2834 Map