Definition card for: Canberra Formation
Status of unit: Formal
Replaces: Canberra Group
Reserved by: Abell, R.S. (after Henderson, G.A.M., after Opik 1958) on: 29-OCT-90
Approved by: Brakel, A. on: 26-OCT-90
Name source: From the current Canberra Group.
Type section locality: A fully representative type section is not available and much of the formation is not exposed. The basal part is well exposed at State Circle and Capital Circle, and a 300 m section near the top has been continuously cored in stratigraphic drillhole 145 (Henderson, 1978).
Extent: The unit (Strusz and Henderson, 1971) extends from Gungahlin through Canberra City to Fyshwick.
Thickness range: At least 1000 m. Full thickness not known, owing to faults of indeterminable displacement.
Lithology: Sandstone, siltstone, shale, mudstone, limestone, tuff (coarse and fine and pyroclastics?), ashstone, dacite (flows or pyroclastics?).
Relationships and boundaries: Overlies Black Mountain Sandstone, State Circle Shale and Pittman Formation unconformably. Overlain conformably by Ainslie Volcanics
Identifying features: Reason for nomenclature change: The original Canberra Group was defined by Opik (1954; 1958) as comprising three formations - the Turner Mudstone, the Riverside Formation and the City Hill Shale in ascending order. The Canberra Group was redefined in 1971 for the first edition of the 1:50 000 geological map and explanatory notes to include also the Camp Hill Sandstone (underlying the Turner Mudstone) and the St.Johns Church Beds (overlying the City Hill Shale). Subsequent mapping, drilling and palaeontological studies have now reached a stage where it can be argued that the lithological and faunal distinctions between the five formations now comprising the Canberra Group are insufficient to justify the existence of all these units as separate formations. It is therefore proposed that the name Canberra Group be changed to Canberra Formation as being more in keeping with the concept of a formation in the stratigraphic nomenclature guide. Further extension of Canberra Formation: Recent mapping has shown that the Fairbairn Group (Opik, 1954; 1958) is a lateral equivalent of the upper part of the proposed Canberra Formation, and that it contains no significant lithological or faunal distinctions from the Canberra Formation. The lateral equivalence of the Canberra Formation and Fairbairn Group is supported by the observation from recent mapping that both units are conformably overlain by the Ainslie Volcanics. Opik divided the Fairbairn Group into four formations - the Molonglo Ford Hornfels, the Molonglo Sandstone, the Molonglo River Formation and the Mahon Formation. It is proposed that these formations be dispensed with as formal units, and that the Fairbairn Group be discorded as a separate entity and become part of the Canberra Formation. Members of Canberra Formation: Two rock units in the proposed Canberra Formation are regarded as being sufficiently distinctive to be retained as members. They are the Camp Hill Sandstone and the City Hill Shale. A third unit, the Narrabundah Ashstone (Opik, 1954; 1958) was originally regarded as belonging to the Ainslie Volcanics. Mapping at Fyshwick has recently shown that the ashstone is contained within the proposed Canberra Formation. It is therefore proposed that the Ashstone also be designated a member.
Age reasons: Middle to late Silurian from corals, brachiopods, trilobites, conodonts etc.
References: 78/028; B032; 79/04153;