Geologie Austrálie (Australian geology)

GENERAL - The geology of different parts of Australia has, naturally. been studied with varying degrees of thoroughness. The great area to be covered, the difficulties to be encountered, and the limited time so far available, are obvious. Instead of attempting. therefore, to present in bold outline a general picture of Australian Geology, it is proposed to give authoritative, independent sketches of the geology of each State, notwithstanding that this will necessarily involve some degree of repetition.

A knowledge of the main features of Australian physical geography will be assumed, and references thereto consequently reduced to a minimum.

2. GEOLOGY OF NEW SOUTH WALES¹ - In physical configuration New South Wales may be divided into three regions viz - (1) The narrow coastal plain on the east; (2) the Great Dividing Range and its associated table-lands; and (3) the western plains. These will first be individually referred to.

(i.) The Main Dividinq Range. The main dividing range or table-land of New South Wales is composed for the main part of Paloeozoic sediments, together with granitic and other igneous rocks; that portion of it, however, which is situated to the westward of Maitland. Sydney. and Wollongong, is capped, with Mesozoic strata. viz., the Hawkesbury series, forming the covering of the principal coal basin.

(ii) The Coastal Plains. The coastal plains, which extend from the eastern foothills of the Dividing Range to the ocean, and which vary in width from a mile or two up to 150 miles, contain two coal-bearing basins, the chief of which extends from the neighbourhood of Maitland on the north to the Shoalhaven River on the south. This coal basin consists of the Permo-Carboniferous coal measures overlaid by the Hawkesbury (Triassic) series. The second coal-field referred to is that known as the Clarence and Richmond field. It is composed of Triassic or Trias-Jura rocks, and so far as at present known it contains no coal seams of commercial value. It may, however, he underlain by the productive Permo-Carboniferous measures.

The coastal plains are also largely composed of Post-Tertiary fluviatile deposits, which form exceedingly rich agricultura1 areas. A considerable area between the Richmond and the Tweed Rivers is occupied by basalt, the decomposition of which has produced a rich soil eminently suitable for agriculture and dairy farming.

(iii.) The Great Western Plains. The great western plains, which extend from the western foothills of the great tableland, are underlain by granitic rocks and sediments of Paloeozoic, Mesozoic, and early Tertiary age. The most northerly portion is Mesozoic (Triassic), and forms the artesian water-basin. South of this is a Paloeozoic belt stretching westerly from the great tableland to the South Australian border. During the Mesozoic era this belt formed a mountain range, whose direction was at right angles to the main divide; but this range was subsequently planed down by denudation, and its surface is now level with the surrounding country. To the south of this, along the Lower Darling and the Murray, is a large area of early Tertiary marine beds (Eocene), while the remainder of the Riverina district (up the Murray. Murrumbidgee, and Lachlan Rivers) is underlain for the most part by granitic, Silurian, and Devonian rocks.

The surface of the western plains is covered by Post-Tertiary deposits, flood loams, etc., except in isolated places where the remains of the older formations still rise above their surface.

(iv.) Classification of the Sedimentary Rocks of New South Wales. In the following classification some indication of the economic significance of the different members of each series is given:

CAINOZOIC.	Post-Tertiary.	Recent; auriferous and stanniferous soils and alluvial deposits in the beds of existing rivers Pleistocene; alluvial leads containing gold,tin and gem-stones
	Tertiary.	Pliocene; alluvial leads, frequently covered by basalt, and containing gold,tin and gen-stones. Miocene; quartzite with plant remains at Dalton, near Gunning.Eocene; marine limestone and calcareous sandstone of the Lower Darling; plant beds of the New England district.
MESOZOIC.	Cretaceous.	Upper Cretaceous (desert sandstone); contains deposits of precious opal. Middle Cretaceous; auriferous alluvial leads at Mount Brown. Lower Cretaceous (Rolling Downs formation of Queensland).
	Jurassic.	Talbragar fish bearing shales.
	Triassic.	The Ipswich coal measures and the Clarence coal measures.	Form the base of the artesian water-bearing basin. These measures contain thin coal seams, not at present worked in New South Wales.
		Hawkesbury series.	Wiannamatta shales (contain fire-clays).Hawkesbury sandstones (building stone).Narrabeen shale.
PALAEOZOIC	Permo- Carboniferous	1. Upper or Newcastlecoal measures 2. Dempsey series 3. Middle or Tomago coal measures 4. Upper marine series 5. Greta coal measures 6. Lower marine series	The productive coal seams of New South Wales occur in these measures
	Carboniferous	Rhacopteris beds and associated marine beds Gympie claystones (of Queensland)	All the metalliferous lodes and reefs occur in these forations, or in such igneus rocks as granites, quartz-porphyries, felsites, diorites, etc
	Devonian	Upper Devonian. Lower Devonian.
	Silurian	Limestones and slates at Yass, Molong Wellington, Quidong, etc
	Ordovician	Slates and tuffs at Mandurama, Cadia, Tomingley, Berridale, and in the countries of Auckland and Wellesley, on the Victorian border
	Cambrian	Limestones, schists and glacial beds of Terrawingee

(v) Cambrian System. The oldest sedimentary rocks of New South Wales are probably those forming the Barrier Ranges in the far west. No organic remains have yet been found in them, and their geological age has been a matter of speculation for many years. Quite recently Mr. Mawson, of Adelaide, has stated that he has traced the Lower Cambrian beds of South Australia to Terrawingee, north of Broken Hill, and he also considers that the metamorphic rocks of Broken Hill may be of pro-Cambrian age. These statements have not yet been confirmed by the New South Wales Geological Survey, though it is quite possible they are correct.

The rocks at Broken Hill consist of a laminated series of crystalline gneisses, quartzites, micaceous and hornblendic schists, and garnet sandstones. Broken Hill itself is a low range in which these rocks have been folded into an anticline. The great Broken Hill lode occupies the saddle-shaped cavity caused by the folding of the strata as stated, but the saddle lode is now of larger dimensions than the original cavity, owing to the gradual replacement (metasomatism) of the country rock forming the walls by ores of lead, silver, and zinc.

To the north of Broken Hill the metamorphic rocks just described give place - in an unbroken series - to less altered slates and schists. traversed by tin-bearing dykes of coarse pegmatite, as at Euriowie, while at Terrawingee there are massive beds of blue Limestone (and, according to Mr. Mawson, glacial till), which apparently belong to the same series.

(vi.) Ordovician System. At the Lyndhurst goldfields, near Mandurama, occurs a series of banded sedimentary rocks, consisting of indurated bluish grey claystones alternating with highly altered volcanic tufts. The claystones contain Trilobites (agnostidoe), Brachiopods (obohella), Pteropods (hyolithes). Graptolites (diplograptus, dicellograptus, chimacograptus, etc.). amid remains of Radiolaria. The tuff beds, which vary from the thickness of paper up to 20 feet, contain bunches and impregnations of auriferous sulphides, and are worked for gold.

The series of banded rocks has been intruded by sills and dykes of hornblende. andesite, etc., which are apparently offshoots from a large body of hornblendic granite. The intrusions appear to have occurred while the sediments were still in a plastic condition, for the tufts have been so forced into the claystones as to give the former the appearance of being intrusive.

Dark blue claystones and slates containing similar Graptolites also occur at Tomningley, Cadia. Berridale, and on the Victorian border - counties of Auckland and Wellesley. At Tomingley the slates are intersected by auriferous quartz reefs.

(vii) Silurian System. Silurian rocks cover a large area of New South Wales, but the locality where they can be most satisfactorily studied is between Yass and the Murrumbidgee River. There they consist of a considerable thickness of slates, sandstones, and limestones, with numerous characteristic fossils, such as Trilobites, Corals, Echinoderms, Brachiopoda. and Mollusca.

The celebrated auriferous reefs at Hill End, Tambaroora, and Hargraves occur in Silurian rocks, consisting of slates with interhedded volcanic tufts, the latter being fossiliferous at Hill End. The Silurian rocks have been intruded, altered, and disturbed by granites, felspar porphyries, etc.

(viii.) The Devonian System. The Silurian slates and limestones to the south of Yass are succeeded by a belt of lavas (rhyolites, etc.) arid tufis, which separate them from a newer series of blue limestones, quartzites, and slates containing fossils of Lower Devonian affinities. At Wellington also the junction can be seen between Silurian and Lower Devonian rocks. At Tamworth, rocks of the same age as the Carboniferous of Europe are underlain by a series of banded claystone and volcanic tufts, with occasional beds of limestone and intrusive sills of granite. The claystones contain numerous Radiolarian remains, while in the tuffs is found the plant Lepidodendron. australe, and the limestones contain an abundant fossil fauna, including corals, which enable these beds to be correlated with the Upper Devonian of Queensland. A good section of Upper Devonian quartzites and shales containing Lepidodendron australe and numerous marine fossils can also be seen at Mount Lambie, near Rydal.

The Devonian system is characterised by the prevalence of grey and red quartzites md grits, and vary large areas of the southern half of the State arc covered by these rocks.

(ix.) The Carboniferous System. A considerable area of the coastal plain and tableland north of Newcastle is occupied by bluish claystones and tufts, with occasional belts of limestones, corresponding in age with the Lower Carboniferous rocks of Europe. Near Port Stephens they contain interbedded deposits of Magnetite, which, however, contains a considerable percentage of Titanium, whereby its value as an iron ore is reduced. At Copeland and several other goldfields the claystones are intersected by gold-bearing reefs. The plant Lepidodendron australe is fairly common in Lower Carboniferous rocks as well as in the Upper Devonian.

In the neighbourhood of Stroud is an area of shales, sandstones, and cherts containing abundant impressions of Rhacopteris, and these beds have been classified as Upper Carboniferous. No workable seams of coal have been found in the Carboniferous system, though in the Rhacopteris series near Stroud several very inferior seams with numerous bands are known.

(x.) The Permo-Carboniferous System. The productive coal measures of New South Wales contain fossil remains, shewing affinities to both the Permian and Carboniferous systems of Europe, hence the composite name which has been given to them. The measures are about 15,000 feet in thickness and have been classified as follows: -

(a) Upper or Newcastle Coal Measures, containing an aggregate of about 100 feet of coal.

(b) Dempsey Series: freshwater beds containing no productive coal. This series thins out completely in certain directions.

(c) Middle, or Tomago, or East Maitland Coal Measures, containing an aggregate of about 40 feet of coal.

(d) Upper Marine Series; sandstones and shales specially characterised by the predominance of the brachiopod Productus brachythoetus. At Braniton traces of glacial action have been found in these beds.

(e) Lower, or Greta Coal Measures, containing front 20 to 40 feet of coal.

(f) Lower Marine Series; sandstones and shales: specially characterised by the molluse Eurydesma cordata. Glaciated boulders and erratics have been found in these beds at Lochinvar.

The three coal-bearing series contain numerous plant remains, including Glossopteris, Gangamopteris, Phyllotheca, Nagqerathopsis, etc., while the Lower and Upper Marine series are characterised by an abundant fauna. The Permo-Carboniferous coal basin occupies an area of about 25,000 square miles extending to the north, west, and south of Sydney. and is the storehouse of one of the States most valuable assets. ln several collieries near West Maitland very fine seams of coal of 20 feet and upwards are being worked. A narrow isolated deposit of the Permo-Carboniferous system extends from near Inverell to the Queensland border. It contains a fine seam of coal (27 feet thick in places), which probably belongs to the Greta series. These measures lie unconformably upon altered claystones of Lower Carboniferous age, and have been intruded by granite which has tilted the coal seam to an angle of about 40 degrees.

(xi.) The Triassic System. The Permo-Carboniferous coal basin is overlain in most places by a thickness of over 1000 feet of shales and thick-bedded sandstones There is no apparent stratigraphical unconformity between these beds and the underlying coal measures, nevertheless there is a very decided break in the fossil life, and the fauna and flora of the newer beds have been correlated with the Triassic system of Europe. These shales and sandstones have been named the Hawkesbury series, and have been subdivided as follows in descending order: -

(a) Wiannamatta Shales. Blue, red, and grey shales, with occasional beds of sandstone. These shales are used for the manufacture of bricks and tiles, and some have the qualities of fireclay.

(b) Hawkesbury Sandstones. Thick-bedded greyish- white freestones, used commonly about Sydney for building purposes.

(c) Narrabeen Shales. Beds of chocolate-coloured shales and greenish tuffs varying from a foot or so to about 1800 feet in thickness. These shales form a very definite and persistent horizon.

The Clarence River coal basin is composed of rocks closely resembling the Hawkesbury series, and they are regarded as contemporaneous, thus the -

(d) Upper Clarence shales may be the equivalents of the Wiannamitta shales.
(e) Clarence sandstones may be the equivalents of the Hawkesbury sandstones.
(f) Lower Clarence shalesmay be the equivalents of the Narrabeen shales.

It should be noted, however, that while the Clarence River series contains the fossil plants Toeniopteris daintreei and Thinnfeldia odontopteroides, the first-named has never been found in the Hawkesbury series, though Thinnfeldia is common in these rocks. It is possible, therefore, that the Clarence series may be newer than the Hawkesbury.

There are numerous seams of coal in the Clarence Measures, but they are too thin and their quality too inferior to be of commercial value. It is very probable, however, that these Triassic rocks may be underlain by the Permo-Carboniferous Coal Measures, which may mean a considerable addition to the coal resources of the State. The Clarence Coal Measures extend through Southern Queensland to the western flanks of the tableland of New South Wales. and dip thence under the north-western plains, forming the great artesian basin.

(xii.) Jurassic System. About 20 miles north-cast of Gulgong is a small lacustrine deposit of thin-bedded yellow shales containing plants and fish remains which are considered to be Jurassic. The deposit referred to lies unconformably upon massive beds of Hawkesbury sandstone: it is of small extent and is the only known representative of the Jurassic in the State. Amongst the fossil plants are Toeniopteris daintreei, Podozamites lanceolatus, Alethupteris australis, Thinnfeldia falcata, and Baiera bidens; the fish include Leptolepis gregarius, Archoeomene robustus. Coccolepis, etc.

(xiii.) Cretaceous System. The Rolling Downs formation of Queensland, which has been classified as Lower Cretaceous, and which consists of a series of shales, limestones and sandstones, is not known to outcrop ab tile surface anywhere in New South Wales, but its characteristic fossils have been met with in wells at Yandama, in the Milparinka district, and a solid core from the Wallon bore, in the Moree district, shows that the drill penetrated about 1600 feet of Lower Cretaceous sediments there. It is possible, therefore, that these rocks underlie some considerable portion of the north-western plains.

The desert sandstones formation, which is believed to belong to the Upper Cretaceous epoch, is of very widespread occurrence over the north-western plains. There is a very marked stratigraphical unconformity between it and the Lower Cretaceous series, though there seems to be no practical distinction in regard to fossil life in the two formations. The most important fossils include - Isocrinus, Maccoyella. Pseudavicula, Belemnites, Ancycloceras. Crioceras, and Cimoliosaurus. The desert sandstone is generally horizontally bedded, and occurs as isolated hills and low ranges. Two varieties of rock are particularly noticeable, one being a greyish-white freestone, while the other is a vitreous rock of the character of porcellanite. Occasional beds of conglomerate occur, containing pebbles of quartz, agate, and chalcedony. and there is also a soft, flue-grained. siliceous rock having somewhat the appearance of kaolin. At White Cliffs, in the Wilcannia district, and at Lightning Ridge. north of Walgett, precious opal occurs in this rock, and extensive mining operations are carried on there.

(xiv.) Tertiary System. (a) Eocene. In the south-western portion of the State, along the course of the Lower Darling and Murray Rivers, there is a large area of marine calcareous sandstones. which have been classified as Eocene. In the Arumpo bore these beds have been proved to be at least 900 feet thick, the fossil Trigonia semiundulata being found at that depth.

At Tooraweenah, Warrumbungle Mountains, a lacustrine deposit, consisting of two series of shales arid sandstones, occurs, containing Eocene plant remains. The two series of beds are separated by a flow of trachytic lava, and a similar lava covers the upper beds.

In New England (at Elsmore, Emmaville, etc.) Eocene leaves are found in fluviatile deposits (tin-bearing gravels) covered by basalt.

(b) Miocene. At Dalton near Gunnning. there is a lacustrine deposit of quartzite which has been classified as Miocene, on account of the plant remains found therein -

(c) Pliocene. Deep auriferous leads at (Gulgong and Forest Reefs have been found to contain Pliocene plant remains - seeds. etc. These deposits are mostly covered by basalt. Most of the Tertiary deposits are of lacustrine or fluviatile origin, and they are important chiefly on account of the alluvial gold and tin ore, as well as diamonds, contained in then).

(xv.) Post-Tertiary. Much of the alluvial gold, tin ore, and gems has been found in Post-Tertiary soils and gravels. These are for the most part shallow, and their contents have been easily recovered by the miners.

Pleistocene surface deposits cover great areas of the western plains, and are the means of obscuring the underlying geological formations and rendering prospecting operations difficult. At Mount Kosciusko there are evidences of much glaciation during Post-Tertiary times - striated boulders are very numerous, and glaciated pavements, roches moutonnees, and terminal and lateral moraines occur in a good state of preservation.

3. Geology of Victoria.² - The State of Victoria is of irregular shape, with the narrowest part to the east. Near the eastern end the Great Dividing Range enters, running south-westerly and westerly, being on the whole most rugged and of greatest altitude as it enters Victoria, i.e.. the general height falls as it runs westerly. On the whole also its southern faces are more steep than its northern, and as the Murray River is approached the character of the country is identical with that of the western plains of New South Wales.

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