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Field Trip Info
Day Two: Analysis of Superposed Folds and Fabrics at Bermagui, NSW Australia.

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Objectives

  • To reconstruct the macroscopic structure of a polydeformed area.
  • To recognise and record different fold styles and their related fabrics.
  • To work out the vergence of folding events using cleavage-bedding relationships (3rd year only).
  • To determine the sequence of events.

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Introduction

The beachheads located south of Bermagui (opposite an industrial park located on Bunga road south of Bermagui) contain excellent exposures of Ordovician psammitic turbidites that were poly-deformed between the Late Ordovician and Early Silurian. Paul Williams performed earlier structural analysis and did recognised a few fabrics. There is discussion about the significance of one the earlier fabric. For Williams this fabric is related to the compaction of the sediment and it is therefore a primary structure, whereas Powell and Rickard claim that this fabric is oblique of the bedding and therefore is a tectonic fabric...

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Exercice

You will complete the following tasks at about four-five sites. All of the tasks will be turned in before you leave the outcrop.

  • Sketch in detail the geometric relationship between all fabrics (including the bedding). Measure the strike-dip-dip direction of each planar features, and plunge-plunge direction of any linear features (intersecting lineations between S0-S1, S1-S2, ...Sn-Sn+1). Plot these on a stereonet.
  • Look for overprinting relationships between these fabrics, and use these relationships to determine the sequence of deformation.
  • Combine the information and sketches you collected to construct a serie of 3D block diagrams showing the sequential development of all structural features from the bedding plane to the last generation of folds and cleavages.

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Analysis of poly-deformed area

Try to identify first the latest deformation event. Look for a cleavage that show no sign of being refolded or alternatively a folded surface with an axial planar surface that show no sign of being refolded.

Once you have identified, recorded (strike-dip-dip direction), and described (slaty, spaced, fracture, metamorphic mineral etc) the last cleavage, let's called it Sn, try to identify the folded surface for which Sn is axial planar. Since the area is poly-deformed it is expected that this folded surface is a former cleavage Sn-1.

Once you have found the folded surface measure the strike-dip-dip direction of the axial planar surface of this fold (Fn), check that it is parallel to Sn. Measure the plunge and plunge direction of the Fn fold axis. If there is an Sn cleavage, try to located the intersecting lineation between Sn and Sn-1, this intersection should have a plunge-plunge direction similar to the plunge plunge-direction of the Fn fold axis.

Look now for the folded surface for which Sn-1 is axial planar. At that stage itcould be either and early tectonic fabric Sn-2, or a bedding S0.

If this folded surface is a tectonic fabric then it must be axial planar of a folded surface which need to be identified. If it is a bedding surface then the Sn-2 corresponds to S0. In that case since Sn-2=S0 then n=2 which is the number of phase of deformation that have affected your area.

It is critical that to clearly identified the nature in each cleavage. Sn can correspond to a slaty cleavage in pelitic beds and to a fracture in sandstones. Be aware of cleavage refraction across rheological boundaries, cleavage refraction that significantly re-orient the cleavage.

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Site 1

General picture of Site 1.

Detail A. In all pictures S0 is indicated by a yellow lines, in red is the last fabric, in blue is an early fabric.

Detail B.

Detail C.

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Site 2

Detail of site B.

Very locally one can notice a fabric older that the bleu fabric and a earlier phase of folding.

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Site 3

That face is parallel to F1 axis.

Open fold trending N140.

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Site 4

At that site S1 is clearly and consistently oblique on S0. the last fabric (in red) is still upright.

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More Pictures

Progressive cleavage refraction at Bermagui.

Uprigth fold at Narooma neat the caravan park...

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Objectives
Introduction
Excercice
Analysis of poly-deformed area
Site 1
Site 2
Site 3
Site 4
More Pictures
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