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Geomorphology
Igneous rocks of basic character dominate the Central
Plateau. A great dolerite sheet of Jurassic caps the
Plateau and forms its resistant surface. Later, sporadic
basalt lavas of Tertiary age fill old drainage depressions
cut in the Plateau. The dolerite is far more voluminous,
but less varied in its chemical composition (approx. 1500
cu. km; silica range 52-60%) than the basalts (approx. 15
cu. km; silica range 36-53%).
Both these rocks express important events which
affected the Southern Hemisphere. The dolerite is the
vast molten response to initial fracturing of the southern
supercontinent, Gondwanaland, of which Tasmania is a small
fragment. The basalts form part of the eastern Australian
volcanic province which erupted in response to warping,
stretching and increased heat flow along the continental
margin as sea-floor spreading opened up the Tasman Sea and
Southern Ocean, beginning about 85 million years ago
A Darwin manuscript on Hobart Town
The collection of Darwin's papers in the Library of
Cambridge University includes a 22-page manuscript on
the Geology of Hobart Town. Consideration of the
manuscript suggests that Darwin's collection of fossils
described in 'Geological Observations'came from the
Bundella Mudstone at Porter Hill, the Cascades Group
near Barossa Road, Glenorchy, and the Malbina Formation
at Eaglehawk Neck. The manuscript further
demonstrates his assiduity and acuteness as a geological
observer, and suggests a considerable fund of geological
knowledge Of Van Diemen's Land in 1836
Mr Gordon Hale 1921-2004, Honorary Life Member, Royal Society of Tasmania: Obituary
Gordon Hale was born in Hobart on 19 June 1921 and died
in Hobart on 18 August 2004. During his lifetime he knew
severe hardship but gave much to theTasmanian community
through his profession as engineering geologist, through
active membership in professional societies, including the
Royal Society ofTasmania, and in war veterans' bodies. He
was widely respected
Preface
The Lake Country of Tasmania - high, flattish and
bejewelled with lakes - is unique in Tasmania and even in
Australasi
General geology
We may think of the Plateau as consisting of two main,
nearly horizontal, layers of rock resting on a basement of older,
steeply tilted rocks. The surface layer, a few hundred metres
thick, is dolerite about 165 million years old. This was injected
into marine and non-marine sedimentary rocks deposited during
a span of about 90 million years beginning about 290 million
years ago. The part of this sequence originally over the dolerite
has been removed by erosion, that beneath the dolerite is
preserved as an almost continuous band around the northern base
of the Plateau from Travellers Range to Table Mountain. The
basement consists of metamorphic, sedimentary and igneous rocks
some older than 570 million years, intruded by granite over 480
million years ago and again about 345 million years ago, and
folded into a mountain range about 370 million years ago
Obituary, Samuel Warren Carey, AO, 1911-2002
The Royal Society of Tasmania lost one of its members of long standing on 20 March 2002 when Emeritus Professor Samuel Warren Carey died in Hobart at the age of 90
A type section of the Permian system in the Hobart Area
An unbroken succession of Permian sediments
about 1430 feet thick on the northern flanks of
Mt. Nassau, about 10 miles north-west of Hobart,
Tasmania, shows the following succession of formations
from the base upwards: Bundella Mudstone;
Faulkner Group including the Geiss Conglomerate,
Rathbones Sandstone and Siltstone, Byers Sandstone,
Jarvis Siltstone, Parramore Sandstone and
Siltstone, Altamont Conglomerate and Fergusson
Siltstone; Rayner Sandstone; Cascades Group including
the Nassau Siltstone, Berriedale Limestone
and Grange Mudstone; "Woodbridge Glacial Formation";
Risdon Sandstone and finally the Ferntree
Mudstone. These range in age from Lower
Artinskian to Kungurian. These formations, except
the "Woodbridge Glacial Formation", are defined.
The Faulkner Group consists of two cyclothems,
recording two brief emergences, one soon
after the other,in a time of general submergence.
The Grange Mudstone and Berriedale Limestone are
at least partly facies variants of one another. A
notable feature is the presence of erratics, except
in the two non-marine formations in the Faulkner
Group, and this is perhaps related to the poor
sorting, and mineralogical immaturity of the sediments
and the angularity of most of the grains in
all rocks. All of these features are considered as
the result of glacio-marine deposition. The source
area included granitic, sedimentary and regionally
metamorphosed rocks
Formations close to the Permo-Triassic boundary in Tasmania
The Cygnet Coal Measures is a unit of carbonaceous shale,
coal and feldspathic sandstone in Tasmania of
Upper Permian age. It is overlain
gradationally or disconformably by the Springs
Sandstone, the lower member of which, Barnetts
Sandstone, is a thinly-bedded, fine-grained feldspathic
to arkosic sandstone, and the upper, the
Mountain Lodge Sandstone and a more thickly-bedded,
medium-grained protoquartzite. The upper part
of the Springs Sandstone is probably Otoceratan.
The Permo-Triassic boundary lies within these
gradational non-marine units and cannot yet be
fixed accurately. The base of the Mountain Lodge
Member is probably the most convenient boundary
to use as the Permo-Triassic boundary in local field
mapping
Charles Darwin's field notes on the geology of Hobart Town- a modern appraisal
A transcription of unpublished field notes made by Charles Darwin details the observations and initial deductions he made on the geology of Hobart, Tasmania, and comments thereon place his work in a modern context. The field notes enable the routes of his excursions while in Hobart Town to be inferred in considerable detail and confirm earlier ideas about the site from which important fossils were collected. They also allow some appreciation of Darwin's style of geological study. Darwin's field notes reveal that he thought of the possibility of the influence of earthquakes on Tertiary beds at Sandy Bay. Several of his ideas on the geology, shown by these notes, some of them also in his publications (e.g. the origin of a Tertiary freshwater limestone, and relative movement of the land and sea in the area), remain topics for further study. His work in the Hobart area and his understanding of the geology of Van Diemen's Land were enhanced by contact with George Frankland, Surveyor-General, who had a long-term and systematic interest in the topic. The notes formed the basis ofa geological "Memo on Hobart Town" and, later, two publications, each work having a different purpose and emphasis. His work near Hobart did not produce enough information for the construction of a stratigraphic framework ("imperfect sketches of the Geology"). The imperfections, with other considerations, may have influenced him not to publish the memo which rendered subsequent publications less clear and less useful than had the memo been published. Darwin recognised the influence of glaciation on the Permian sedimentary rocks in the area but did not publish his observations and interpretatio
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