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We will learn to measure and report on a map structural elements such as bedding surfaces, form lines, fols axis, fold axial plane etc. We will learn to construct cross sections and a basic geological map.

Assumed knowledges include concepts such as strike-dip-dip direction, plunge plunge-direction, cleavage and bedding, fold classification, fold axis, fold axial surface, fault and fault classification.

Participants should be familiar with stereonet projection.

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Equipment-

Notebook, map clipboard with waterproof covering.
Tracing and graphic paper, a pin (for the stereonet).
Erasers, scale and protractor.
HB and B pencils preferably 0.5 mm in size, coloured pencils and a sharpener (prefer a knife to sharp your pencils).
A few waterproof 0.1 pigment liner, black, blue, red, green. Allow the write neatly on your map and your fieldbook.
Geological hammer, hand lens.
Compass, choose one with a clinometer (not essential as we will provide them).
Sample bag, and basic first-aid kit, water bottles (at least 2litres), camera.

In case you get lost (extremely unlikely) bring matches, a torch and a whistle.

Equipment can be bought from Prospectors Supplies at Unit 4, 195 Prospect Road, Seven Hills (ph: 9838 7899).

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Lachlan Orogen-

From the Late Ordovician (450 Ma) to Eary Carboniferous (350 Ma) the geological history of the Australian eastern margin was that of a convergent margin. Deformation, metamorphism and magmatism impacted over 1200 km across strike from NSW, to VIC, to Tasmania, and the Eastern part of SA. This roughly East-West,100 Myr long, accretion history is known as the Lachlan Orogen.

The oldest rocks in the Lachlan Orogen are mafic volcanics of Cambrian age overlain by pelagic cherts and rare limestones, possibly an oceanic crust. Much of the Lachlan Fold Beld however is characterised by a ~2000m thick Ordovician turbiditic succession. Silurian to Middle Devonian rocks are also abundant and include deep and shallow marine sediments as well as subaerial successions including silisic igneous rocks and mafic volcanics.

Convergence was driven by a number of subduction zones, some with opposite vergence. For some the Lachlan Orogen results from a globally convergent progressive deformation over 100 Myr, punctuated by short lived extensional deformation stages related to slab roll-back, slab detachment, or laterial extrusion. For others the Lachlan Orogen is better described by successive orogenic events separated by extension. From the older to the younger:

Lower Ordovician to Early Silurian -> The Benambran Orogeny produced East-West compression.
Upper Silurian -> Extension producing basin filled with silisic volcanics and marine sediments.

Silurian/Devonian boundary -> Bindian Orogeny (also known as Bowning Orogeny) produced orogen parallel displacement along strike slip faults than locally accommodated the emplacement of I-type granites.
Early Devonian -> Extension producing rift basins.

Middle Devonian -> Tabberabberan Orogeny -> inversion of Early Devonian rift basins.
Upper Devonian -> Extension producing continental basins.

Early Carboniferous -> Kanimblan Orogeny had little impact, mainly recorded on the Eartern part of the Lachlan Orogen.

Progressive displacement and subduction of oceanic lithosphere can results and a highly volatile intraplate state of stress. Therefore there is no need to invoke switches in plate-boundary stresses to explain switches in continental stresses.

The Taemas-Cavan region is located in the Eastern segment of the Lachlan Orogen. It is your job to decipher the geology of the region and to find out for yourself where this piece fits in the Lachlan Orogen puzzles.

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Stratigraphic Column-

We will map Mt Erin close to Hume Park on Fred Patmore farm. The geology of Mt Erin includes the entire stratigraphy of the Taemas stage and the red shale of the Marjurgong stage. Locally you will be able to found some rhyiolites and granites.


Fine stuff shales

Crinoidal limestone: current bedding, prominent calcarenite to calcirudite composed of well-sorted crinoid ossicles.

Warroo limestone: recessive (shaly), thin bedded, very fossiliferous
with zone of Metriophyllum

Receptaculites limestone: thin bedded very fossiliferous with zone of R. australis above Xystriphyllum.



Bloomfield limestone: recessive (shaly).


Currajong limestone: Massive, bedded, with zone of Breviphyllum above zone of Syringopora.

Yassensis limestone: Richly fossiliferous with Brachio. (spinella yassensis), tabulate corals, cephalopods, arthrodires
Red shales, sandstones and minor quartzites. Cross-beds, ripple marks, thin bands with gasteropods and lingula.

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Cross-Section

Cross-section here.

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Formline Mapping

Here is an outcrop in the Good Hope resort. One can see a number of limestones beds (from the Yassensis limestone). These beds are folded into a synclinal. On the ground the beds rotate making near concentric formlines.

View looking East (close to Mt Erin) showing a number of formlines. The massive beds on the middle of the picture are from the Receptaculite limestones. The more recessive limestones underneath are from the Bloomfield limestones. Further North (on the left) the massive beds where the tree stands is the Currajong limestones.

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Sketches

Sketches here.

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Synformal fold closure with a weak axial planar fracture cleavage in crinoidal limestones.

Extensional fractures and stylolites in Yassensis limestones (at Good Hope Resort).

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Lachlan Orogen
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