The Prospect dolerite intrusion, or Prospect intrusion, is a Jurassic picrite or dolerite laccolith that is situated in Sydney, New South Wales, Australia.Igneous intrusions by the Australian Museum. 13 November 2018. Retrieved 1 November 2021. Lying in the heart of Cumberland Plain, in the suburb of Pemulwuy (previously Greystanes), the igneous intrusion is Sydney's largest body of igneous rock, rising to a height of above sea level. The site is formed by an intrusion of dolerite rock into Ashfield Shale and the Hawkesbury sandstone. 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 fluids) from the Earth's upper mantle moved upwards and then sideways, which produced many different minerals in the upper part. Formerly known as Prospect Quarry, the eroded residue of the volcanic core became a quarry of the basalt plug that was carried out from 1820s until the late 2000s, where it contributed most of the crushed rock 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 picrite, olivine and its prehnite specimens.NSW Government Architect's Office, Prospect Hill Heritage Landscape Study and Plan, NSW Department of Commerce, 2008, p 31.
Igneous activity Formation
In the Sydney Basin, 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.Johnson R. W. (1989). Volcano distribution and classification. In: Johnson R. W., Knutson J. And Taylor S. R. eds. Intraplate Volcanism: In Eastern Australia and New Zealand, pp. 7 11. Cambridge University Press, Cambridge, UK. 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, which determined the shape of the Prospect dolerite intrusion many million years ago, erosion 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.Jones, I., and Verdel, C. (2015). Basalt distribution and volume estimates of Cenozoic volcanism in the Bowen Basin region of eastern Australia: Implications for a waning mantle plume. Australian Journal of Earth Sciences, 62(2), 255-263.
This allowed hot magmatic fluids, which are around , to relocate via developing pegmatite and depositing prehnite, calcite and other subsidiary minerals that are found in the upper part of the intrusion, with analcime dolerite (teschenite) and picrite shaping the bulk of the intrusion site.Branagan, D.F., and Packham, G.H., 2000. Field Geology of New South Wales. 3rd Edition. New South Wales Department of Mineral Resources, Sydney.
Erosion At the next stage of its geological development, which lasted over 60 million years, was the gradual erosion of the overlying layers of sedimentary rock by the motion of rainwater, which ultimately revealed the edges of the volcanic and metamorphic rocks of the Prospect intrusion.Conybeare Morrison, Prospect Hill Conservation Management Plan, Holroyd City Council, 2005 The intrusion has been an inadvertent outcome of tense continental crust breaking all the way down to the upper mantle 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 equilibrium inside the surface rocks.Jones, I., Verdel, C., Crossingham, T., and Vasconcelos, P. (2017). Animated reconstructions of the Late Cretaceous to Cenozoic northward migration of Australia, and implications for the generation of east Australian mafic magmatism. Geosphere, 13(2), 460-481. The site's heat transformed the nature of the encompassing rock to produce many type of minerals, which were mainly coarse-grained picrite, alongside olivine-dolerite. The picrite did not extend to the surface, though it 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 Sydney Basin depositional area at the conjunction of the Triassic fluvial Sydney sandstone and the lacustrine Ashfield Shale which underlies the intrusion. The quarry site is one the several Mesozoic intrusions positioned into the Sydney Basin sequence at the intersection of the Triassic fluvial Sydney sandstone and overlying shallow-marine Ashfield Shale.William M.L. & Carr P.F., Isotope systematics of secondary minerals from the Prospect Intrusion, New South Wales, Australian Journal of Earth Sciences (2005) Volume 52, Issue 6, 2005 H.G. Wilshire described the Prospect quarry site as "an annular teschenite intrusion which has invaded the Triassic Wianamatta shale. The outer oval shaped part of the intrusion has discordant contacts and rises about 75 feet above the central part which is occupied by a sill approximately 250 feet thick and which is overlain by 20 to 70 feet of shale".Wilshire, HG, The Prospect Alkaline Diabase-Picrite Intrusion New South Wales, Australia, Journal of Petrology (1967) 8 (1): pp 97-163
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 that is roughly thick and is covered by to of shale.
The dome is called a doleritic laccolith. 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 upper surface until present, due to quarrying altering its shape, owing to its description as "caldera-like". The intrusion has been stratified, but that depends on how its parts distinguished when it cooled down.Wilshire: Wilshire, H.G., The Prospect alkaline diabase-picrite intrusion, New South Wales, Australia in Journal of Petrology, Vol. 8 (1), pp 97-163, 1967.
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.Prospect, New South Wales By Keith Compton, Mindat.org, Hudson Institute of Mineralogy 1993-2021. Retrieved 1 November 2021.
The copious amount of analcime in the rocks at the intrusion's top shows significant reservation of magmatic water during crystallization. If 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.HODGE-SMITH, T. (1943) Mineralogical notes No. VI. Records of the Australian Museum, 21 (4), 251. 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.England, B. M. (1994) Minerals of the Prospect Intrusion, New South Wales, Australia. Mineralogical Record 25, pp.185-194.
The site also has had a sweeping collection of other minerals, such as:CLARK, B. (1976) The Prospect Intrusion. Mineralogical News, 12, 8-11.
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 occupation, with favorable camping locations along the Eastern Creek and Prospect Creek catchments, and in elevated landscapes to the south. The area was settled by Europeans by 1789. On 26 April 1788, an exploration party heading west led by Governor Phillip, climbed the site, which was known as 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) 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 Cumberland Plain.
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 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:Geological Observations on the Volcanic Islands, visited during the Voyage of H.M.S. Beagle (1844)
"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:Vol 10 of the United States Exploring Expedition during the years 1838, 1839, 1840, 1841, 1842 under the command of Charles Wilkes, USN-Geology by James D Dana pp 495-501
"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