Geological mapping of Boulia 4-mile Sheet, western Queensland, was carried out in 1957 by the Georgina Geological Party (leader J.N. Casey). Two formations of Ordovician sediments have been recognized - the Ninmaroo Formation and the Swift Beds. They constitute the most easterly known exposures of fossiliferous Ordovician in northern Australia.
Both formations are lower Ordovician in age. The Ninmaroo Formation is essentially Tremadocian, although its uppermost part may be early Arenigian; the Swift beds are early, but not initial, Arenigian. Late Arenigian and younger Ordovician fossils are not known to occur in situ in the area, but a pebble containing younger Ordovician fossils has been collected from a conglomerate that is considered to be Mesozoic (Cretaceous) in age.
The Ninmaroo Formation rests with apparent conformity on late Upper Cambrian limestone (Chatsworth Limestone) in the Black Mountain section; the Swift Beds are unconformable on the Ninmaroo Formation at Digby Peaks, but no angular discordance can be detected between the two formations in the area west of Black Mountain.
The fossils of the Ninmaroo Formation are well preserved. Those of the Swift Beds are mostly too fragmentary for determireation, but comparatively well-preserved material has been secured from this formation at Digby Peaks by Mr.D.J. Taylor, palaeontologist to Frome-Broken Hill Pty.Ltd. The fossils include a graptolite, and, in order to present as complete a picture as possible of the Ordovician faunas of the Boulia area, advance notice of this first discovery of Ordovician graptolites in Queensland is incorporated in this report.
Few Ordovician fossils are described from northern Australia, and none at all from Queensland. The fossil identifications are therefore tentative, and the generic names here cited may be changed when the fossils are systematically studied.