The Prospect dolerite intrusion, or Prospect intrusion, is a

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

picrite Picrite basalt or picrobasalt is a variety of high-magnesium olivine basalt that is very rich in the mineral olivine. It is dark with yellow-green olivine phenocrysts (20-50%) and black to dark brown pyroxene, mostly augite. The olivine-rich ...

dolerite Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grain ...

laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apart ...

that is situated in

Sydney Sydney is the capital city of the States and territories of Australia, state of New South Wales and the List of cities in Australia by population, most populous city in Australia. Located on Australia's east coast, the metropolis surrounds Syd ...

New South Wales New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. Lying in the heart of

Cumberland Plain The Cumberland Plain, also known as Cumberland Basin, is a relatively flat region lying to the west of Sydney CBD in New South Wales, Australia. An IBRA biogeographic region, Cumberland Basin is the preferred physiographic and geological term ...

, in the suburb of

Pemulwuy Pemulwuy ( /pɛməlwɔɪ/ ''PEM-əl-woy''; 1750 – 2 June 1802) was a Bidjigal warrior of the Dharug, an Aboriginal Australian people from New South Wales. One of the most famous Aboriginal resistance fighters in the colonial era, he is n ...

(previously Greystanes), the

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

is Sydney's largest body of

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

, rising to a height of above sea level. The site is formed by an intrusion of dolerite rock into

Ashfield Shale Ashfield Shale is part of the Wianamatta group of sedimentary rocks in the Sydney Basin. It lies directly on contemporaneously eroded Hawkesbury sandstone or the Mittagong formation. These rock types were formed in the Triassic Period. Name ...

. At least seven different rock types occur in the intrusion. The site was formed from around 200 million years ago when volcanic material (hot

magmatic Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

fluids) from the Earth's

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...

moved upwards and then sideways, which produced many different

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s in the upper part. Formerly known as Prospect Quarry, the eroded residue of the volcanic core became a

quarry A quarry is a type of open-pit mining, open-pit mine in which dimension stone, rock (geology), rock, construction aggregate, riprap, sand, gravel, or slate is excavated from the ground. The operation of quarries is regulated in some juri ...

of the

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

plug that was carried out from 1820s until the late 2000s, where it contributed most of the

crushed rock Crushed stone or angular rock is a form of construction aggregate, typically produced by mining a suitable rock deposit and breaking the removed rock down to the desired size using crushers. It is distinct from naturally occurring gravel, which ...

that were used for building construction and roads in the Sydney area. The intrusion comprises: prospect dolerite and prospect teschenite, in addition to an abundance of coarse grained

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

and its

prehnite Prehnite is an inosilicate of calcium and aluminium with the formula: Ca2Al(AlSi3O10)(OH)2 with limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most often forms as stalactitic ...

specimens.

Igneous activity

Formation

In the

Sydney Basin The Sydney Basin is an Interim Biogeographic Regionalisation for Australia, interim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast of New South Wales, Australia and with some of its ...

, igneous activity took place in the Early Jurassic activity that resulted in the shaping of the Prospect dolerite intrusion – This unambiguously points that the site had a volcanic origin. The eroded residue of the volcanic core forms the site, which was battered down over millions of years to a small extrusion in the relatively flat lands of western Sydney. Consequent to the

volcanic activity Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

, which determined the shape of the Prospect dolerite intrusion many million years ago,

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

then undermined the main mass of volcanic material and caused it to fall in on itself and create a shallow, dish-shaped formation with shrinkage cracks that established in the intrusion, as shown by the salient cooled edges of basalt formed by high heat difference between the comparatively cool (and likely wet) sediments and the magma. This allowed hot

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

tic fluids, which are around , to relocate via developing

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic c ...

and depositing

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

and other subsidiary minerals that are found in the upper part of the intrusion, with

analcime Analcime (; ) or analcite is a white, gray, or colorless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is NaAlSi2O6 · H2O. Minor amounts of potassium and calcium ...

dolerite (teschenite) and picrite shaping the bulk of the intrusion site.

Erosion

At the next stage of its geological development, which lasted over 60 million years, was the gradual

of the overlying layers of

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

by the motion of rainwater, which ultimately revealed the edges of the

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

and metamorphic rocks of the Prospect intrusion. The intrusion has been an inadvertent outcome of tense continental crust breaking all the way down to the

during development of the rift divergence zone that occurred before the breakup of the Australian and Antarctic continents in the Eocene epoch. The fractures acted as pathway for basaltic magma from the mantle area and a few of these would have been feeder dikes for the intrusion, whereby the magma ascended to an area of density Isostatic equilibrium, equilibrium inside the surface rocks. The site's heat transformed the nature of the encompassing rock to produce many type of minerals, which were mainly coarse-grained picrite, alongside

-dolerite. The picrite did not extend to the surface, though it Types of volcanic eruptions, thrusted the surface rocks upwards to create a dome.

Geological description

The Prospect Intrusion is one of many Mesozoic intrusions that were dispositioned into the

depositional area at the conjunction of the Triassic fluvial Sydney sandstone and the lacustrine

which underlies the intrusion. The site is above ground level and above sea level, and is long and . The dolerite intrusion is a circular teschenite encroachment that has intruded the Triassic Wianamatta shale, with the intrusion's external, elliptic shaped portion featuring dissentious impinging. The intrusion site has a sill (geology), sill that is roughly thick and is covered by to of shale. The dome is called a doleritic

. A regular laccolith's intruded material features a rather flat lower surface and a bell-shaped upper surface, where it is more like half a lens in shape. Though because of the descent of the volcanic material, combined with consequent surface erosion, the laccolith maintained a concave set, concave upper surface until present, due to quarrying altering its shape, owing to its description as "caldera-like". The intrusion has been Stratum, stratified, but that depends on how its parts distinguished when it cooled down.

Minerals

The hill contains a slender, chilled margin of fine-grained basalt with most of the mass of the intrusion being made up of picrite, dolerite, and high level intrusives. The picrite is a farinaceous-grained rock dominated by olivine and is made up of two-thirds of the lower constituent of the intrusion, with the upper third of dolerite also containing other mafic minerals. The shales above and below the intrusion had very reduced levels of metamorphism and exhibited superficial modification to a fine quartz hornfels with nearly no growth in grain size. The copious amount of

in the rocks at the intrusion's top shows significant reservation of magmatic water during crystallization. If Hydrothermal vent, venting of the hydrothermal fluids into the encompassing sediments had happened, the diverse range of unique rock types and related late-stage minerals wouldn't have formed. Volcaniclastic sediments were rare and volcanic rocks on the site are predominantly mafic. In the 1960s, a small amount of gold was observed in a sample test, but since then no more has been discovered. The site also has had a sweeping collection of other minerals, such as: * Albite-anorthite series * Analcime * Apatite * Apophyllite (KF) * Aragonite * Augite * Basanite * Baryte * Biotite * Chabazite-Ca * Calcite * Chalcedony * Chlorite Group * Feldspar group * Gold * Halotrichite * Heulandite * Ilmenite * Laumontite * Leucite * Marcasite * Montmorillonite * Natrolite * Nephelinite * Olivine * Opal * Pectolite * Phillipsite * Pickeringite * Plagioclase * Pyrite * Pyroxene * Rhombohedra * Quartz * Rhyolite * Siderite * Smectite group * Syenite * Trachybasalt * Trachyandesite * Trachyte * Tuff

History

Discovery, 1788–91

The area of Prospect Reservoir is an area of known Aboriginal Australian, Aboriginal occupation, with favorable camping locations along the Eastern Creek, New South Wales, Eastern Creek and Prospect Creek (New South Wales), Prospect Creek catchments, and in elevated landscapes to the south. The area was History of Australia (1788–1850), settled by Europeans by 1789. On 26 April 1788, an exploration party heading west led by Arthur Phillip, Governor Phillip, climbed the site, which was known as Prospect Hill (New South Wales), Prospect Hill or "Bellevue". An account by Phillip states that the exploration party saw from Prospect Hill, "for the first time since we landed Carmathen [sic] Hills (Blue Mountains (New South Wales), Blue Mountains) as likewise the hills to the southward". On 18 July 1791 Phillip placed a number of men on the eastern and southern slopes of Prospect Hill, as the soils weathered from the basalt cap were richer than the sandstone derived soils of the

. The site provided a point from which distances could be meaningfully calculated, and became a major reference point for other early explorers. While there is no documentary evidence of Watkin Tench having named Prospect Hill, there is no doubt that it is in fact the hill that was shortly afterwards known by that name. In view of Tench's literary allusions to John Milton, Milton's ''Paradise Lost'', it seems highly probable that the experience of climbing it reminded him of the "goodly prospect of some forein land first-seen" by Milton's scout and that it was indeed Tench who first named it.

Observation, 19th century

Quarrying of the basalt plug in the area began in the 1820s. Charles Darwin visited this site in 1835, where he states in his notebook ''Notes on the Geology of places visited during the Voyage'':

"At Prospect Hill the sandstone country is intermitted by a mass of Trappean rocks, the quarry which I saw consisted of a black Basalt(?) the structure of which was roughly prismatic. I imagine, but have no proof that this has burst through the Sandstone."

James Dwight Dana, an American geologist, arrived in Sydney in on 29 November 1839 and visited the intrusion site. In January 1840, William Branwhite Clarke, geologist and Church of England reverend, travelled with Dana to the intrusion site to study the rocks. Dana analysed material from the site and recorded his expedition in ''Vol 10 of the United States Exploring Expedition''. Dana described the rock material from Prospect as:

"A dark bluish rock, finely porphyritic, with small points (not tables) of feldspar. It occurs at Prospect Hill...A porphyritic basalt, in which the augite and feldspar are both distinct, and some of the crystals of the augite are a fourth of an inch long. It occurs at Prospect Hill...The compact black basalt changes to a compact rock, with disseminated points of feldspar; next, to a porphyritic basalt, with distinct crystals of both augite and feldspar; and next, to the feldspar rock in which Augite is almost wholly wanting."

Land development, 1900s–present

The bulk of the present CSIRO site was acquired by the Commonwealth in 1946, and a further 15 hectares was acquired in 1963, which became the primary source of roadstone for the city's expanding infrastructure until the reserves of

were exhausted. In 1998 Boral reviewed its holdings with a view to future redevelopment as its quarry neared the end of its life. As at February 2001, the southern portion of the site, located within the Boral Brickworks site has been extensively quarried. The area is under immediate threat from development, which has the potential to impact upon the significance of the site. The then Minister for Urban Affairs & Planning took over planning powers for the employment area in November 2000 and approved the Employment Precinct Plan in June 2001, approving subdivision and associated works in the northern employment lands later in June 2001. Since, parts of the employment land have been sold and further subdivided and sold. The gap (landform), gap in the ridge that had previously been created by quarrying has been lowered to the floor level of the quarry and the drainage of the area reversed from its earlier northward flow to empty into Prospect Creek, while a new road, Reconciliation Road, has been driven through the centre of the hill from Prospect Highway and across the gap to Wetherill Park. The land inside the oval-shaped site was Regrading, levelled from 2008 to 2010 and is rapidly filling with large Industrial park, industrial buildings.Holroyd 2007: Holroyd Development Control Plan 2007, (Part G: Former CSIRO Site Pemulwuy Residential Lands – Western Precinct), Holroyd City Council, 2007.

Heritage listing

As at 19 February 2001, the area where the dolerite intrusion is situated in had state significance due to its unique combination of significant landscape feature, potential archaeological site, and association with important historical phases. As a dolerite outcrop that rises to a height of , it is a unique geological and significant topographic feature providing panoramic views across the

. The intrusion site, which is situated on Prospect Hill (New South Wales), Prospect Hill, was added to the New South Wales State Heritage Register on 17 October 2003. The site is mentioned in the 'SHR Criteria C', under 'Aesthetic significance':

''Prospect Hill has aesthetic significance as Sydney's largest body of igneous rock, which rises to a height of 117 metres and provides expansive views across the Cumberland Plain. The large dolerite formation of Prospect Hill is a rare geological and landmark topographic feature, lying centrally within the Cumberland Plain.

References

Attribution

External links

Minerals of the prospect intrusion
The Free Library by Farlex.inc {{Sydney landmarks, state=collapsed Landmarks in Sydney Quarries in Australia New South Wales State Heritage Register Landforms of Sydney Extinct volcanoes Calderas of Oceania Laccoliths Volcanoes of New South Wales Geology of New South Wales Jurassic magmatism Jurassic volcanism