Geological Survey

Goulburn Geological Mapping Project

Goulburn Mapping Project

Project status

Geological mapping has been completed for the six 1:100 000 map sheets - Boorowa, Crookwell, Yass, Gunning, Goulburn and Taralga, and provisional map sheets are available. The Goulburn 1:250 000 map sheet is currently being compiled. The Goulburn 1:250 000 GIS CD ROM is currently being assembled.

Geology

The Goulburn 1:250 000 map sheet area lies in central eastern New South Wales in the eastern Lachlan Fold Belt. The area consists of Palaeozoic age sequences (Ordovician to Permian) overlain in part by Cainozoic age basalts and sediments.

The Ordovician rocks broadly occupy the central part of the map sheet and consist of Adaminaby Group turbidites, Bendoc Group black shale and the Kenyu Formation, a mafic volcanic sequence.

Silurian and Devonian sedimentary and volcanic sequences, including the Douro Group, Black Range Group, Campbells Group, Mount Fairy Group and Bindook Group flank the Ordovician units to the east and west. Extensive Silurian-Devonian granite batholiths (Wyangala Batholith and Wologorong Batholith) meridionally intrude Ordovician and Silurian sequences in the central part of the map sheet area.

There are over 580 metallic mineral occurrences known in the Goulburn 1:250 000 map sheet area. In the past, the area has produced significant amounts of gold and base metals, particularly from Tuena, Peelwood and Currawang. Tungsten has been mined at Rye Park and the Phoenix mine, near Frogmore. The Goulburn map sheet also hosts important industrial mineral resources. For example, coarse aggregate is extracted from volcanic units near Marulan; sand is currently being mined at the northern end of Lake George; iron oxide at Breadalbane; and limestone at Galong.

Stratigraphy

The stratigraphy of the Ordovician Adaminaby Group and the Late Ordovician Bendoc Group and Kenyu Formation is now more clearly understood. The Mundoonen Sandstone conformably overlies the Late Ordovician Bendoc Group and may represent continued sedimentation into the early Silurian.

The delineation of the Ordovician stratigraphy has been greatly aided by the recognition of age diagnostic conodont faunas in cherts of the Adaminaby Group. New graptolite occurrences and re-examination of old collections constrains the age of the Bendoc Group to the Late Ordovician. Conodonts from limestone and chert provide a Late Ordovician age for the Kenyu Formation.

Angular unconformities at the base of mid to late Silurian units reflect the widespread early Silurian Benambran Orogeny in the Goulburn map sheet area. The Hawkins Volcanics unconformably overlies both the Kenyu Formation and Mundoonen Sandstone, and the Mount Fairy Group and Bungonia Group unconformably overlie the Adaminaby Group.

A complex volcanic and clastic stratigraphy is now recognised in the late Silurian to Early Devonian felsic to mafic Mount Fairy Group and Campbells Group, which includes the Kangaloolah Volcanics and Wet Lagoon Volcanics. These units were deposited in, and on, the eastern margin of a marine basin extending from the Hill End Trough in the north to the Goulburn Basin in the south. A previously unmapped, late Silurian to Early Devonian volcanic centre has been identified west of Taralga. The Ordovician and Silurian volcanic units are prospective for Au-Cu-Pb-Zn-Ag and barite mineralisation. Minor deformation during the Early Devonian Bowning event produced a low angle unconformity in the west at Bowning and in the east near Goulburn and Bungonia-Windellama.

SHRIMP U-Pb dating and diagnostic fossils from above and below the unconformity constrain the break to the late Lochkovian stage of the Early Devonian. Newly recognised volcanic and clastic units belonging to the Bindook Volcanic Complex and Black Range Group have been recognised above the unconformity.

The effects of the mid-Devonian Tabberabberan Orogeny are reflected by an angular unconformity between the Late Devonian Hervey and Lambie groups and older units. Younger stratigraphic units in the Goulburn map sheet area include the overlapping Permian rocks of the Sydney Basin, and alluvial gravel and alkali basalt along Cainozoic river channels.

Granite emplacement

Numerous plutons of S-, I- and A-type affinities have been recognised from mapping in the early Silurian to Early Devonian Young, Wyangala, Wologorong and Marulan batholiths. New co-magmatic intrusions have also been mapped in the Early Devonian Mountain Creek Volcanics and Bindook Group. The intrusions have been grouped into suites and supersuites where possible and assessed for metallogenic potential.

Thrust faults crosscutting plutons of the Wyangala Batholith have exhumed migmatites, abundant metamorphic xenoliths and kilometre-scale, andalusite-cordierite bearing, contact aureoles. The western or northern margins of several plutons have narrow contact aureoles and miarolitic cavities suggesting shallower levels of emplacement. Granite shapes and emplacement mechanisms for plutons of the Wyangala Batholith are being investigated using gravity modelling and field relationships. Interestingly, the Ordovician-Silurian contact is a favoured level of emplacement for particular plutons in the Wyangala and Wologorong batholiths.

Structural geology

Initial thrust stacking of the Adaminaby Group and the Bendoc Group shale sequences in the early Silurian probably controlled the location of subsequent extensional and contractional structures.

The regional geophysical data highlight the previously poorly recognised geological structures on the Goulburn map sheet area. The character of the major NNW-SSE trending Frogmore Fault Zone was inadequately known prior to the data collection. However, after data collection, it was more clearly discernible on the magnetic and radiometric geophysical images. Identification of the fault zone has led to several key observations of its nature. For example it:

has an easterly dip, intense kinking, quartz veining, scaly cleavage with S-C fabrics, and acted as a conduit for silica and iron bearing fluids
forms part of a high strain structural zone separating areas of no foliation to the west, and well developed, predominantly west-dipping foliation; east-verging folds; and east-directed thrusts to the east
has a long history of movement that includes a large strike slip component
links the Columbine Mountain-Woodstock faults to the north with the Sullivans and Queanbeyan faults to the south
partly marks the western margin of the Hill End Trough and Goulburn Basin between mid-late Silurian shallow marine and subaerial volcanic rocks of the Douro Group to the west and deeper marine clastic rocks to the east.

Surficial geology and landscape

The surficial geology is also a focus of the Goulburn mapping project. Three major surficial units of residuum, colluvium and alluvium were mapped, as well as minor lacustrine and aeolian deposits, and silica and iron cemented duricrusts. Older weathering profiles from the Permian-Carboniferous are also preserved along the eastern part of the Goulburn map sheet area beneath rocks of the Sydney Basin.

Landscape inversion is common in the study area. Basalts and silcrete accumulated in palaeo-valley floors which protected these areas from erosion, so that they now form hill tops. Dramatic east-facing fault scarps are a feature of the landscape, the most obvious of which is the Lake George Fault. This fault scarp resulted from Neogene fault movement. Similarly, the Cullarin Fault, Grabben Gullen Fault, Binda Thrust and Blackmans Creek Thrust are westward-dipping Palaeozoic thrusts that are still active. Their movements affect stream drainage courses and surficial deposition and erosion.

Mapping of numerous stacked basalt flows that exhibit both normal and reverse magnetic polarity, in combination with previous K/Ar age dating, indicate multiple volcanic outpourings in the Eocene, Oligocene and Miocene. Eruption centres for basalts in the Crookwell-Taralga area are now better constrained, and of possible economic significance for diamond and gemstone exploration.

The new mapping of the Goulburn surficial and basement geology provides an invaluable information resource for minerals exploration, land management, as well as broader environmental issues such as soil fertility, salinity, water catchment quality and groundwater, agricultural and bioregion studies.

Further studies

Databases on whole- rock chemistry and assays, U-Pb zircon absolute age dates of granites and volcanic rocks, geophysical properties, and petrographic studies will be used to provide a better four-dimensional evolutionary history of the Goulburn map sheet area.

Prior to, and concurrent with the mapping, metallic and industrial mineral occurrences in the MetIndEx database have been updated. Specific deposits are currently being investigated using new lead and sulphur isotope datasets, to develop new ideas on the timing and controls of mineralisation.

For more information on the mapping project please contact the Geological Mapping Team.

(Last updated on 24/02/2009)