GSV Report 111 - Craigie 1:100 000 map area geological report
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Product description:Download The downloadable version of this report is supplied in PDF format and is a large file (20.9MB). Abstract Ordovician strata in the Craigie 1:100 000 map area, as for elsewhere in the southeastern Lachlan Fold Belt, are divided into the lower Adaminaby Group (Pinnak Sandstone) and the upper Bendoc Group (Warbisco Shale). The Pinnak Sandstone consists of turbidites with monotonous quartz-rich sandstone and mudstone, in addition to minor chert and black shale. Most of the unit lacks fossils apart from rare Darriwilian graptolites near the top. The Warbisco Shale is dominated by black siliceous shale and mudstone with graptolites of late Gisbornian to Eastonian age. The Ordovician succession was deposited as part of a huge submarine fan in a deep oceanic setting and derived from a mountain chain bordering the Gondwana continent to the west (using present day co-ordinates). The change from abundant terrigenous detritus in the Early Ordovician to a sand-starved succession in the Late Ordovician was probably due to rising sea levels and lowering elevations of the mountain chain over time. Granitic rocks cover a large part of the Craigie 1:100 000 map area and belong to the Early Devonian Bega Batholith. The plutons are all biotite granites and hornblende granodiorites and tonalites of I-type affinity, with the possible exception of the granodiorites within the Kuark Metamorphic Complex which lack hornblende and contain some muscovite and are therefore of indeterminate affinity. The Ordovician succession and Early Devonian granites are un(non)conformably overlain by the Upper Devonian Combyingbar Formation which is a thick succession of red sandstone and mudstone and minor coarse-grained sandstone and conglomerate. These rocks were deposited on a vast floodplain predominantly by a meandering river system transporting a large sediment load towards the east. A thin veneer of Tertiary sand and gravel covers the Palaeozoic rocks in the southeastern part of the Craigie 1:100 000 map area. The Palaeozoic rocks of the Craigie 1:100 000 map area have been affected by two major episodes of deformation: a Silurian to Middle Devonian event and an Early Carboniferous event. The first episode occurred during one or more of the Silurian to Middle Devonian deformations (Benambran, Bowning, Tabberabberan) that cannot be distinguished in the map area. Southeast of the Combienbar Fault, it involved deformation of the Ordovician succession with upright tight to close folding, cleavage development and accompanying faulting. Northwest of the Combienbar Fault, two steeply dipping cleavages or schistosities (S1, S2) were formed. Structural trends are mainly to the northeast in the western part of the area and to the north and northeast in the eastern part of the area. Regional vergence is towards the east with west-over-east thrust faulting. Some granites were deformed and contain a steeply dipping north-trending foliation and a down-dip lineation that formed during cooling in the Early Devonian. Major shear zones developed with northerly and northeasterly orientations in response to continued east-west shortening. The northeasterly set includes the Fiddlers Green Shear Zone which consists of mylonitic rocks and has undergone predominantly vertical movement involving southeast-side-up and northwest-side-down. The north-trending Cann Valley Shear Zone has undergone mainly dextral strike-slip movement. The Carboniferous deformation was associated with east-west compression and formed a northeast-trending zone of deformation, neighbouring the Combienbar Fault. This zone had dextral shear with associated upright, close to open folding of Upper Devonian strata that formed the en echelon north-trending Combienbar and Buldah synclines. On the Genoa River, Late Devonian strata were gently tilted to the southwest with local formation of monoclines and reactivation of the north-trending Rockton Fault. The area has received relatively little attention from mineral exploration companies, reflecting the small number of historical mining ventures and the inaccessibility of the area. Most exploration work to date has focussed on porphyry copper and hydrothermal gold deposits. This report also includes a geological interpretation of aeromagnetic and radiometric data. Bibliographic reference Simpson, C.J., Fergusson C. L., and Oranskaia, A., 1997. Craigie 1:100 000 map area geological report. Geological Survey of Victoria Report 111. Related products:
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