Ordovician ;Pittman Formation The Pittman Formation was described originally by Öpik in 1958 who named it after the Pittman Valley, southeast of
Aranda, Australian Capital Territory. It is entirely of Ordovician age and about 800 metres thick near Canberra, but at
Captains Flat it is over 1,200 metres thick. The lower levels are
greywacke, exposed east of
Queanbeyan and north are very thick and heavily overturned and thrusted. At Etheridge Creek, the type locality is a repeating pattern of
sandstone, micaceous sandy
shale,
mudstone, black
argillaceous and radiolarian
chert. In the sandstone beds there are occurrences of graded
bedding, clay pellets, and current bedding. Fossils of
graptolites,
radiolarians,
conodonts, and occasionally
brachiopods and
sponges are found. The geological formation east of Queanbeyan used to be known as the Muriarra Formation. This alternates between sandstone with a high quartz content and mica, and phyllite. Radiolarian chert is found in the central section. The railway forms the border between Queanbeyan in NSW and
Oaks Estate in the ACT. West of the Queanbeyan railway station is a cutting where folding has overturned the beds, with axes dipping to the east at 50 degrees. 300 metres of thickness is exposed in this cutting. Llanvirnian age (Pygodus serrus conodont zone). Fossils found include Phyllograptus anna, Trigonograptus ensiformus, Pterograptus, Didymograptus, Isograptus, Hallograptus from the Darriwillian age. Near the top of the formation are fossils Dicellograptus sextans, D divaricatus, D salopiensis, which are Gisbornian. Chemical analysis reveals low concentrations of
sodium,
calcium and
strontium because of the low
feldspar content. The Ordovician
turbidites, consisting of
greywacke, feldspathic sandstone, micaceous siltstone, micaceous shale, chert, and
phyllite, are very similar in all parts of the
Tasman Orogen, including
New Zealand and the
Transantarctic Mountains. Detrital
zircons from the turbidites have been
isotopically dated with age ranges of 0.46 and 0.60 Gya, 1.0 and 1.2 Gya, and at ~1.8 Gya, ~2.2 Gya and ~2.7 Gya. These do not match the age of zircons from the interior of the
Australian Shield, so the source of the sediments is from another continent. The other evidence from the Ordovician sediments are chemical composition indicates granitic source, with the absence of feldspar. Secondly the fine grained nature shows the sediments have been transported a long way from their ultimate, source, and could have been second cycle, derived from sedimentary rocks. At the time these sediments were deposited the location was at least 3,000 km from the Australian continent. Underneath there is no Proterozoic continental basement, instead these sediments are lying on top of oceanic crust. ;Acton Shale Acton Shale is a grey to black thinly laminated siliceous
shale containing
graptolites. It is generally leached, and frequently silicified. The colour of the beds alternates between grey and black, but where weathered, it changes to whitish grey. The graptolites appear on the bedding planes as black films. In Canberra, the Acton Shale appears in several outcrops in Acton, in two bands through Aranda, through Bruce near the Calvary Hospital, on the Bruce Ridge behind Lyneham. Also another band starts under the University of Canberra in Belconnen, and heads north east through Lawson and Giralang and folded and faulted into several bands in Crace on Gungahlin Hill. Another band is found on the west side of Queanbeyan, extending north to Dundee, and south around the east side of Jerrabomberra Hill. Acton shale is only preserved in the cores of synclines, being eroded from uplifted parts.
Brachiopods,
conodonts and
sponges
fossils are rarely found. The beds are up to 60 metres thick and appear high in the Pittman Formation. The age range is Gisbornian to Bolindian of the
Ordovician period. Lower beds contains fossils like Dicranograptus nicholsoni. Upper beds contain Climacograptus bicornbis, Chastatus, C tubuliferis, Dicellograptus elegans, and Dicranograptus hians which are late Eastonian in age. The sediments making the Acton Shale were deposited in the ocean in a reducing environment starved of oxygen, and lacking fresh sediment.
Late Early Silurian ;State Circle Shale The State Circle Shale was named by Öpik in 1958. It is named after the street where it was described. Its age is
Llandoverian. Its
lithology is shale, mudstone, siltstone and minor sandstone. In the type location there is about 60 metres of non-calcareous sandy shale and dark grey shale with beds of fine-grained sandstone. Between Kings Avenue and Commonwealth Avenue, there is a good outcrop on State Circle, consisting of buff-coloured laminated siltstone and shale with fine sandstone beds contorted by slumping. Its top is an unconformity, with Camp Hill Sandstone lying on top. There is probably up to 200 metres thickness of this shale. The shale was deposited in the deep sea as turbidites. It can be found in Yarralumla,
Parkes,
Acton, north and south of
Black Mountain, and from
Lawson, to
Crace and
Ngunnawal. ;Black Mountain Sandstone Black Mountain Sandstone is deposited on top of State Circle Shale conformably. It is made up from thick beds of grey
quartz sandstone mostly, but has some beds included of
siltstone and grey
shale. The grain size is fine to medium. It was originally named by Öpik after the mountain—
Black Mountain where it is found. Originally it was believed to be Ordovician, but is actually from the
Silurian period, late
Llandoverian epoch. Some of the slopes of Black mountain are covered in
fanglomerate. The deposition was in a marine proximal turbidite fan, with the turbidity current flowing to the east. There are no fossils, but there is some sedimentary structure including plane, cross or convolute laminations, load casts, slump units and flute moulds. ;Tidbinbilla Quartzite Tidbinbilla Quartzite has been modified by granite intrusions close by. It consists of medium grained sandstone, partly silicified and changed to quartzite. Belts of silstone and sandstone are included becoming more frequent at the top. The exposure is 300 metres thick. Low down there is a 2-metre thick bed of ashstone across a broad area that can be used as a marker bed.
Late Middle Silurian ;Canberra Formation In the 1840s
fossils of
brachiopods and
trilobites from the
Silurian period were discovered at Woolshed Creek near
Duntroon. At the time these were the oldest fossils discovered in Australia, though this record has now been far surpassed. These fossils were from the Canberra Formation. In the past these rocks were known as the Canberra Group with components of Turner Shale, Riverside Formation, and City Hill Shale. Canberra Formation can be found in the east part of South Canberra in
Fyshwick,
Kingston,
Barton and
Parkes. It is also found through North Canberra, excluding Campbell and Russell. It occurs through most of Gungahlin apart from Crace and Nicholls. The beds extend north in a wide band to 35 deg 03S near Bald Hill. Narrabundah Ashstone is a member of the Canberra formation In addition to the ashstone the Canberra Formation has layers of green-grey to reddish dacite, also tuff, quartz andesite, but mostly it is calcareous shale, limestone or sandstone. Much is deeply weathered, which has posed difficulties for building foundations. ;Walker Volcanics Walker Volcanics appear as purple or greenish-grey dacitic ignimbrite. These volcanics contain chloritised cordierite and some have garnet. They are Wenlock age. They occur in southern Belconnen including Macquarie, Weetangera, Hawker, Page, Scullin, Higgins, Holt and the Pinnacle. ;Hawkins Volcanics Hawkins Volcanics is a green-grey dacite or dacitic tuff. These volcanics contain chloritised cordierite, and some have garnet. Their age is Ludlow to Wenlock. The volcanics occur in northern Belconnen, in
Dunlop,
Fraser,
Spence, Mount Rogers,
Flynn,
Melba and
Hall. Maximum thickness is 1770 m. ;Ainslie Volcanics Named after
Mount Ainslie, Australian Capital Territory where their type location occurs, the Ainslie Volcanics are composed of
Dacitic ignimbrite and minor volcaniclastic and argillaceous
sediments. The lithology is bluish grey dacitic tuff, which can be massive or foliated, also dacitic agglomerate and shale. These volcanics contain chloritised
cordierite and some have red
almandine garnet.
Jasper is found on low hills on the north side of the
Molonglo River. The thickness is at least 700 metres. The magma was formed by melting an
aluminium-rich
pelitic sediment. The eruption came from a volcano into shallow sea water. The deposits built above sea level as they progressed. The underlying sediments now make up the Canberra Formation. Between Hall and Nanima Hill on Spring Range, the Mount Painter Volcanics lie unconformably on top. These rocks date from the Late
Wenlockian epoch of the
Silurian period. They were formed about the same time as the Walker Volcanics and Paddys River Volcanics. The Ainslie Volcanics occur on Mount Ainslie,
Mount Majura, and in a band extending from Bonshaw and
Harman north to the east of Woolshed Creek, through Majura and at least to Gooroo Hill and Old Joe on the NSW border. On Mount Ainslie the sequence starts with dacitic tuff, banded dacitic tuff, massive dacitic tuff, fifty metres of agglomeratic tuff, massive dacitic tuff, fifty metres of ashstone and topped with massive dacitic tuff. The rocks on top of the eastern ridge is altered. ;Mount Painter Volcanics This was named after Mount Painter in Canberra by
Öpik, who called it a porphyry. The description is a massive dark bluish-grey dacitic crystal tuff containing garnet and chloritised cordierite. There is a local appearance of agglomerate and pumice. There are prominent quartz and feldspar phenocrysts. Xenoliths include
jasperised sediments and there are some beds of tuffaceous siltstone and sandstone. It was mostly deposited from the air without water. Mount Painter Volcanics overlie the Walker Volcanics unconformably. The top is also an unconformity with Yarralumla Formation and Deakin Volcanics and Yass Formation. The age is of Late
Wenlock epoch or early Late
Silurian. These rocks form a belt from Coppins Crossing towards Narrabundah and Jerrabomberra Creek. ;Yarralumla Formation In an intervening phase in volcanism predominantly sediments were deposited. Yarralumla Formation was named after the suburb of
Yarralumla by Öpik in 1958. It consists mainly of mudstone which may be cemented by lime, or originally derived from tuff. There are some beds inserted of quartz sandstone or limestone. The bottom of the formation is on top of the Mount Painter Volcanics. The top of the formation grades up into pyroclastics from the Deakin Volcanics. Deposition occurred in a shallow marine environment with a delta. The sea level was relative higher during this depositional phase compared with the earlier and later subaerial volcanic deposit periods. Its age is early Ludlow. This has been determined by shelly marine fossils, and a tonalite intrusion southwest of Red Hill with age determined as 417±8 Mya. Outcrops occur on Red Hill (the hill) and throughout
Deakin, Australian Capital Territory and in southern Yarralumla, and also
Hughes, Australian Capital Territory. The formation extends from
Red Hill and
Woden in the South to
Yarralumla and
Lake Burley Griffin in the north. The formation is evidence of the last major period when eastern Australia was still covered by shallow seas. It shows fossil evidence of
trilobites,
coral and primitive
crinoids. Another band stretches from
Lyons, Australian Capital Territory north north west towards the
Molonglo River. There may also be a patch in
Symonston, Australian Capital Territory. Exposures can be seen at the Deakin Anticline—with pale brown siltstone; and also at the Yarralumla brickworks with olive-green calcareous mudstone. ;Yass Subgroup Dated early Ludlow to Wenlock. This may outcrop in Belconnen and Florey. The composition is calcareous and tuffaceous
shale,
sandstone,
ashstone and
limestone. These volcanics lack
cordierite or
garnet. ;Deakin Volcanics The
Deakin Volcanics can be seen in the road cutting along the
Tuggeranong Parkway between Hindmarsh Drive and Cotter Road. The visible base shows a weathered repeating sequence of
interbedded rhyodacitic ignimbrite,
sandstone,
siltstone and red and yellow
shale. Southwards the section passes up into a massive and partly banded rhyodacitic ignimbrite. The bottom of the Deakin Volcanics is exposed on nearby Heysen Street. At least 400m thickness of beds are exposed in the Parkway cutting. The
Tuggeranong area has the thickest deposits. Rock types in the Deakin Volcanics are rhyodacitic ignimbrite,
lava (Mugga Mugga Porphyry Member),
tuff, tuffaceous shale and minor quartz sandstones and volcanic
breccia-units show reddish brown alteration. The unit is of Early Ludlovian age. The rock on Mount Rob Roy and Pemberton Hill used to be known as Tuggeranong Granite, but is actually ignimbrite. The ignimbrite forms escarpments, with the lower-lying land being underlain by tuff. The Deakin Volcanics can be found south of the Deakin Fault between Belconnen and
Charnwood and
MacGregor. It also is found through
Weston Creek north of
Chapman and
Fisher, through
Woden apart from Curtin, and across to
Hume, and south through
Tuggeranong. The different colours found in the rocks are due to weathering, the red being from
hematite, and the green from clay minerals such as
celadonite. The pink crystals are potash feldspar. The Mugga Mugga Porphyry is a lava flow. It is blue or mauve grey in a mass. The rock is veined with
calcite, light green
epidote, and deep red
hematite. The
phenocrysts are
quartz, grey
plagioclase, pink potash
feldspar in lesser amounts and flakes of
biotite. It is dated at 414±9 Mya. The Federal Golf Course
Tonalite introduces some veins and
saccharoidal galena. ;Laidlaw Volcanics The Laidlaw volcanics are a
rhydacitic ignimbrite in the form of pale to dark grey rhyodacitic to dacitic crystal
tuff. They are the top layer of
Volcanics in the ACT. The Laidlaw Volcanics occur in a band along the
Murrumbidgee River to the south west of Canberra, and also north west of
Belconnen. They exist in the suburbs of
Latham, Chapman,
Kambah, and Greenway. The volcanics are of
Ludlovian age 420.7±2.2 m.y. and have been used to set an absolute date for the
Ludlow epoch. They are up to 850 metres thick. Some
shale and water deposited tuff occur to the west of Pine Island and Point Hut Crossing. North west of Mount Stromlo, on Uriarra Road, there is an occurrence of
limestone over a few square kilometres. ;Porphyrys
Porphyrytic
intrusions occur throughout the area. Middle
Silurian intrusions could be
volcanic necks. A coarse green grey
rhyodacitic intrusive outcrops over one square kilometre west of
Holt in the Walker Volcanic sediments. It is likely to be the
magma chamber for the uppermost eruption of Walker Volcanics. A coarse green grey rhyodacitic intrusive with white prominent feldspar crystals appears between the arms of Lake Ginninderra, McKellar, Evatt, Nine Elms, east Spence, and Nichols. This covers a few square kilometres. A small outcrop of the same material occurs in Cook and Jamison Center. This rock matches the upper layer of the Hawkins Volcanics. East and north east of Watson is an intrusion of grey and cream dacite. This may correspond to the lower layer of the Ainslie Volcanics. Late Silurian to early
Devonian intrusions could also be volcanic necks. If so they have been responsible for the Laidlaw Volcanics. They are all unnamed. The first three intrude Laidlaw Volcanics. A coarse rhyodacitic intrusive outcrops to the north west of Charnwood intrudes Laidlaw Volcanics, with about half a square kilometre of surface area. A line of outcrops of coarse pink-brown rhyodacitic porphyry outcrops on the north side of Canberra on Forster Hill, MCquoids Hill and Neighbour Hill, and on East side of
Mount Taylor. A distorted band of coarse grey rhyodacitic porphyry occurs along the
Murrumbidgee River from 1 to 4 kilometres south of Point Hut Crossing. Another pink and green rhyolite porphyry occurs in Holder, Weston and Lyons. It intrudes Deakin Volcanics. ;Glebe Farm Adamellite A coarse porphyritic micro-adamellite was intruded sometime from late Silurian to early Devonian. It intrudes the Hawkins Volcanics in Belconnen in
Flynn, Melba,
McKellar, the Town Center and Bruce. ;Sutton Granite Also known as the
Greenwood Granite, the Sutton Granite is a medium grained
granite intruding the Pittman formation in the hills to the north east of
Canberra Airport. The outcrop covers 4 square kilometres in the ACT. It was intruded during the late
Silurian and is dated at 410+4Mya. The colour is pale grey with mineral white
feldspar, milky grey
quartz, black
biotite, and with
hornblende rich
xenoliths. Minor minerals are apatite and zircon. The surrounding Pittman formation rocks have been
metamorphosed in a
contact aureole to
hornfels and spotted
schist. A magnetic high matches the location indicating the presence of magnetite. ;Adaminaby Beds The Adaminaby Beds were formed from quartz
turbidite capped with black shale. It consists mainly of fine to medium grained quartz rich sandstone and some mostly thin beds of siltstone, shale and slate. The sediments occur in a band on the west side of the ACT on Bulls Head, Mount Franklin, Mount Ginini and along the upper parts of the Cotter River. A separate outcrop extends into the ACT from the south in the area of the
Gudgenby River. Yet another outcrop is on the Bullen Range. ;Paddys River Volcanics The Paddys River Volcanics consist of dacite and tuff with some shale phyllite and limestone. They occur to the west of the Bullen range, along the lower parts of the Paddys River. They were deposited on top of the Ordovician Adaminaby Beds, and intruded by Shannons Flat Adamellite. ;Uriarra Volcanics The Uriarra Volcanics consists of
dacite lava flows and
pyroclastic deposits of
tuff. A fine
ashstone bed called the Tarpaulin Creek Ashstone Member outcrops in an approximate north–south line and acts as a marker within the volcanics. Tuff and flows above and below the ashstone member contain obvious pink
feldspar crystals. The tuff shows
bedding, and the flows have banded flow structure. The Cotter
Porphyry to the north of the Cotter Dam is actually a dacite flow. There is a
limestone lens north of Uriarra Crossing. The outcrop goes from Mountain Creek Road in the west to the Murrumbidgee river in the east. It extends a few kilometres to the north of the ACT border and south to the Winslade Fault near the Cotter River. A wedge extends to the south south west, which includes Pierces Creek.
Late Silurian to Middle Devonian ;Murrumbidgee Batholith The Murrumbidgee Batholith formed by melting different sediments to the Ordovician Pittman Formations, as the granites contain more feldspar elements Ca, Na and K. At
Cooma there is migmatite where the sediments have been partially melted in order to produce the plutonic rocks in the batholith. Variations in their composition are explained by partial mixing with melted oceanic crust. The batholith was largely crystallised before emplacement, and made space for intrusion by displacement rather than dissolution. ;Clear Range Granodiorite Clear Range Granodiorite covers 475 square kilometres from
Tharwa to
Thredbo on the west side of the Murrumbidgee batholith on
Clear Range. Clear Range ridge forms the ACT border in the south east. Described as
foliated with numerous
inclusions. The inclusions are metamorphosed sediments and are common in biotite. It contains quartz and microcline feldspar and brown biotite and also muscovite. It has more biotite and plagioclase than Shannons Flat Granodiorite. The muscovite is distinctly foliated and there is also blue quartz. Texture is fine to medium grained. Close to the Murrumbidgee Fault the texture is
mylonitic and the rock is easily weathered. ;Shannons Flat Granodiorite Shannons Flat Granodiorite is poor in xenoliths and is coarse grained. It was intruded after the clear Range Granodiorite. Tracking Station] ;Tharwa Adamellite The Tharwa Adamellite resembles Shannons Flat Adamellite, but with more strongly zoned plagioclase (An50-20) contains microcline, it is poor in xenoliths and is coarse grained. ;Booroomba Leucogranite Booroomba
Leucogranite is found at Mount Tennant and
Booroomba Rocks. The total outcrop is 52 square kilometres. It is coarsely
crystalline and pale in comparison to other
granites.
Muscovite occurs in unweathered rock. This leucogranite intrudes
Clear Range Granodiorite and
Shannons Flat Granodiorite. It is low in
iron and
calcium, and higher in
potassium than other Murrumbidgee Batholith intrusions. ;Olivine Teschenite from Red Rocks Gorge A
dyke of
teschenite occurs in the Red Rocks Gorge near Allens Creek. It is oriented northwest–southeast and heads towards
Kambah Pool Area. A similar dyke also outcrops on the west side of the Murrumbidgee River near Pine Island. The dyke is less than 0.5 metres thick. The rock is black, showing up well against the reds and browns of the Laidlaw Volcanics. The rock consists of microphenocrysts of
olivine altered to
serpentine, pinkish brown
titanaugite, brown
kaersutite amphibole, and about 15%
magnetite, embedded in a colourless
analcime groundmass. Kaersutite contains
titanium and has a formula near NaCa2(Mg4Ti)[Si6Al2O22]O(OH). The dyke is believed to be Tertiary in age. ==Geophysics==