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The tectonic evolution of the Rocky Cape geanticline in northwest Tasmania

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Gee, Richard Dennis (1967) The tectonic evolution of the Rocky Cape geanticline in northwest Tasmania. PhD thesis, University of Tasmania.

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Abstract

This thesis is a cross-sectional study of the
Rocky Cape Geanticline, one of the major tectonic
elements of Tasmania. The Rocky Cape Geanticline consists
of deformed, unfossiliferous, and dominantly unmetamorphosed
sedimentary rock of presumed Proterozoic age. The internal
structure is due to the Penguin Orogeny of late Proterozoic
age. Two distinct lithological assemblages are recognised,
corresponding to different basins of deposition. These
basins have a northeast southwest trend, parallel to the
axis of the Rocky Cape Geanticline, and flank the older
Precambrian nucleus Which lies to the east.
To the west of the axis of the geanticline is
an unstable shelf sequence about 20,000 feet thick,
consisting of Shale, Oiltstone, orthoquartzite and minor
dolomite. This comprises the Rocky Cape Group; a revised
definition is given for the Rocky Cape Group, and new
names are given to the Constituent formations. First
record of sedimentation indicates a starved euxinic
environment. This was followed by a stable, Shallowwater,
frees-circulating environment in which unusually
large thicknesses of cross-bedded orthoquartzite
accumulated. Palaeocurrent directions from cross-bedding
have diametrically opposed, bimodal or polymodal patterns,
which are interpreted as the expression of transverse
dispersal currents. These may reflect a gently oscillating
palaeoslope. Further to the west the Rocky (Cape Group
is overlain, with a low-angle unconformity, by the
Smithton Dolomite which has a basal conglomerate.
East of the axis of the geanticline, a flysch-
type sequence at least 15,000 feet thick accumulated.
This is called the Burnie Formation, and is considered to
be younger than the Rocky Cape Group. It consists of slate,
quartz wacke and minor pillow lava. Axially directed
turbidity currents were an important agent of sedimentation, although, large-scale festoon cross-bedding Indicates that traction movement by bottom currents was significant.
These currents are considered to be Independent of the
turbidity currents.
A variety of sedimentary deformation structures
occur. Most are post-depositional, and can be further
classified into those due either to-down-Slope mass
movement, (block slide, slump sheet, and fluxoturbidite);
or to small vertical, horizontal, or toroidal movements,
(load cast, psendonddule, diapiric contortion, ball-andpillow
structure and convolute lamination). Liquefaction
is considered to be an important process In the development
of the latter category. The imposed tectonic cleavage
is Commonly-controlled by the pre-existing sedimentary
deformation, but crosscutting relationships also occur,
and assist in the distinction between sedimentary and
tectonic Structures.
Major tectonic Movements during the Penguin
Orogeny were to the southeast, so that the Burnie Formation
was squashed between the older basement to the east and
the Rocky Cape Group to the west. Albite dolerite (Cooee
Dolerite) in the form of dykes, sills and sheets were
intruded during the early stages of folding. The two
main sedimentary assemblages are now separated by the
Keith Metamorphics, a belt about five miles wide,
composed of pelitic schist, basic schist and amphibolite.
The western boundary is gradational across, strike, and
displays cataclastic textures. The Keith Metamorphics
was derived in part from the surrounding sediments and
partly from basic igneous rock, believed to be equivalent
to the Cooee Dolerite. This belt is interpreted as a
high-angle Shear zone. Correlation with similar schists
along the axis of the Rocky Cape Geanticline indicates
that this is a fundamental structure, and it is here
termed the Arthur Lineament.
The structure in the Rocky Cape Group is simple,
consisting of a series of folds trending parallel to the
major axis. The folds are tight and asymmetrical near the
shear zone, and become more open and symmetrical to the
west. This folding affects the Smithton Dolomite.
Detailed structural analysis of the Burnie
Formation reveals a complex picture of progressive .
deformation involving five phases, all on approximately
the same axis.. The first and third generations were the
major events in the structural evolution. The major
structure consists of two overturned synclines separated
by a flat belt and an anticlinal hinge. The overall
vergence is to the southeast, toward the older Precambrian
nucleus.
Mesoscopic folds and associated Structures of
one phase tend to form in zones which are spatially
segregated from zones of other generations. Thus,
the first generation folds are Mostly confined to the
overturned limbs of the major synclines, and the third
generation folds are confined to the intervening fiat
belt. Smaller scale segregation is demonstrated by the
occurrence of the later phases in zonee of planar
bedding between clusters of.first generation folds. The
area of overlap of these zones is generally small, So
that examples of refolded structures are not abundant,
and a false impression is given of rapid changes in
tectonic style of the one fold generation. The segregation
of fold phases is most strongly expressed where the
bedding remained kinematically active, a feature which
suggests that there is a mechanical limitation to
Superposed folding.
In the first generation mesoscopic folds, the
geometry of the arenite beds approximately fits that Of
flattened concentric folds. At Sulphur Creek, the
mesoscopic folds have a marked asymmetry, shown by
common-limb thinning of the fold couplet, a "half-fan" cleavage, and offset carinate structures. These features
may be explained by an obliquity between the initial
axial plane and the plane of flattening. The flattening
stage of fold development was followed by a disharmonic
stage involving variable lateral shortening, and the
formation of "slip-off" structures, oblique shear joints
and boudinage. Cleavage formed continuously throughout
the sequence of development of the P1 minor structures,
although the mairpariod of formation was during the
flattening stage. The change in behaviour of the
arenite beds during folding from initially competent, to
incompetent, and back to competent, may be due to the
role of internal pore water during deformation.
The evolution of the Rocky Cape Geanticline
fits into a pattern of pxoterozoic Lower Palaeozoic
orogenesis, in which the axis of sedimentation progressively
shifted toward the older nucleus, and the tempo of
- sedimentation and structural deformation increased to
a climax in the Upper Cambrian.

Item Type: Thesis (PhD)
Keywords: Geology, Structural, Geology
Copyright Holders: The Author
Copyright Information:

Copyright 1967 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.) - University of Tasmania, 1968. Includes bibliography

Date Deposited: 08 Dec 2014 23:56
Last Modified: 11 Mar 2016 05:56
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