Definition card for: Mount Painter Volcanics

State(s): ACT, NSW,
Status of unit: Formal
Usage: Redefined
Stratno: 24403
Replaces: Mount Painter Porphyry

Reserved by: Abell, R.S. (after Opik 1958) on: 29-OCT-90
Approved: Y
Approved by: Brakel, A. on: 26-OCT-90

Parent: Douro Group

Name source: Named after Mount Painter in Canberra (Opik 1958).

Name source: (Orig. Defn) From the current Mount Painter Porphyry (Opik, 1954; 1958).

Unit history: (Orig. Defn) The present name, Mount Painter Porphyry, was given by Opik, based on a conclusion that the rock was essentially intrusive and more or less homogeneous. Recent studies both petrological and chemical have shown that the unit is essentially volcanic with several different components with slightly different petrological and chemical characteristics. No evidence has been found to suggest that the intrusive phases are other than minor. Moreover intrusive contacts have also been observed in association with other established volcanic units and not now regarded as sufficient evidence to override the petrologically volcanic feature.

Unit history: Mount Painter Porphyry (Opik 1958).

Type section locality: Area of pasture land and pine plantation bounded by the Winslade and Deakin Faults in the north and by the Molonglo River and Lake Burley Griffin in the south (Lat 35o 17'S Long 149' 03'E).

Type section locality: (Orig. Defn) Mount Painter, 6 km northwest of Canberra City, was designated by Opik as the type locality.

Description at type locality: The formation is exposed as boulders and tors of massive bluish-grey dacitic crystal tuff containing garnet with local development of agglomerate with lithic and compacted pumice. Top and bottom are not exposed.

Extent: A folded southeast - trending belt extending from Coppins Crossing towards Narrabundah and Jerrabomberra Creek. North of the Deakin Fault the formation extends from Gooromon Ponds Creek northwards to Yass.

Extent: (Orig. Defn) In belt extending from the Coppins Crossing area to Narrabundah.

Thickness range: (Orig. Defn) Unknown.

Thickness range: In Gooromon Ponds Creek area the estimate is 1000m. South of the Deakin Fault estimates >1000m (Henderson 1981).

Lithology: (Orig. Defn) Acid volcanics - dacite, tuff, minor lenses of ashstone and tuffaceous sediments.

Lithology: Dark bluish-grey dacitic crystal tuff with prominent quartz and feldspar phenocrysts and also containing lithic xenoliths, dacitic autoliths and rafts of jasperised sediment. Minor interbeds of tuffaceous sandstone and siltstone.

Depositional environment: Subaerial.

Relationships and boundaries: (Orig. Defn) The base of the unit is not exposed, owing to faults. The upper contact is poorly defined but probably interfingers with the lowest parts of the Deakin Volcanics and Yarralumla Formation. Some contacts appear to be intrusive, however petrological studies of thin sections representative of the great majority of the unit indicate a consistently volcanic texture.

Relationships and boundaries: Base not exposed owing to faults and poor exposure. In the Fairlight area the Mount Painter Volcanics may overlie the Walker Volcanics unconformably (Sufni Hakim 1985). Top is unconformable with trangressive Yarralumla Formation and Deakin Volcanics. North of Gooromon Ponds Creek there is a lower disconformable? boundary with Mount Ainslie Volcanics while the top is unconformable with the transgressive Yass Formation and Deakin Volcanics.

Age reasons: Late Wenlock. Stratigraphically based on the Wenlock age deduced for the underlying Walker Volcanics (Chatterton and Campbell 1990, Strusz 1982) and the early Ludlow age of the overlying Yarralumla and Yass Formations (Strusz 1984, Wyborn and others 1982).

Age reasons: (Orig. Defn) Late Silurian or possibly late Middle Silurian by virtue of the relation to the fossiliferous late Silurian Yarralumla Formation. Also a possible tongue of the volcanics underlie fossiliferous late Middle Silurian shale and limestone 2 km northwest of Coppins Crossing.

Correlations: Unknown.

Proposed publication: (Orig. Defn) Coppins Crossing 1:10 000 Engineering Geology Sheet.

Proposer: (Orig. Defn) Henderson G.A.M.

State(s): (Orig. Defn) NSW [?? - Check Defn card]