• Fossil- and opal-bearing rocks in the Lightning Ridge area are confined to the Lower Cretaceous Griman Creek Formation, situated in the Surat Basin that extends over parts of south-eastern Queensland and northern New South Wales. Together with the neighbouring Eromanga Basin, these form the majority of the present day Great Artesian Basin (GAB). The Griman Creek Formation is composed of thinly laminated and interbedded fine- to medium-grained sandstones, siltstones and mudstones, with carbonate cements, intraformational conglomerate beds and coal deposits (Burger, 1980; Green et al., 1997). Within the Griman Creek Formation, opal and fossils occur within interbedded siltstone and mudstone layers, often referred to as the Finch clay facies (Byrnes, 1977). Palynological evidence indicates that the Griman Creek Formation is associated with the Coptospora paradoxa Zone and correlates to the middle Albian (Burger, 1980). Apatite fission-track analyses on grains derived from core samples of the Queensland extent of the Griman Creek Formation indicate an upper age boundary of approximately 107 Mya (Raza, Hill & Korsch, 2009).