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Field Trip Info
Day One: Structure and Depositional Environment of Ordovician sediments at Potato Point, NSW Australia.

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Objectives

  • To construct a stratigraphic section of Ordovician rocks at Potato Point
  • To log Bouma sequences across a 30 metre to determine true stratigraphic thickness.
  • To Analyse well-exposed sedimentary structures to help complete the above two tasks.
  • To Determine and analyse palaeo-current directions that existed during deposition of these rocks to help determine palaeogeography.

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Introduction

Ordovician rocks of the Lachlan Orogen preserve a huge deposit of continental margin sandstones and quartzites with dimensions reching 1.2 million km2. Their large volume appears to have controlled a significant portion of the Lachlan Orogen during its construction in the Silurian to Carboniferous. As such, they are also important for understanding orogenic processes at convergent margins. Potato Point, located south of Moruya, offers a well exposed section of these rocks and a wide variety of sedimenatry structures. The section is deformed but the degree of tectonic disruption is such that depositional structures are well preserved.

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Exercice

Each participant must turn in finished structural and sedimentary sections anotated with sketches and comments. Using flute casts and cross-beds, determine the direction and sense of palaeocurrent (steronet will be provided). You will be assessed on 2 aspects:

  • the quality and accuracy of your cross section,
  • the accurary of the paleocurrent analysis.

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Turbidites and the Bouma sequence

Turbidites result from under-water slope instability (gravitational sliding often related to earthquake) along continental margins. The unconsolidated sediments get dispersed in sea water to form a suspension: a mixture between sea-water and particles. The density of the suspension is much large that sea-water and therefore is pulled by gravity along the continental slope. Turbiditic currents can be powerfull and they may travel hundred of kilometers at high speed.

Turbiditic deposits know as turbidites shows very coherent internal organisation. This internal organisation result from the competition between the size fo the particle in the suspension, and the hydrodynamic of the turbiditic currents. This organisation vary along the current from the proximal position (close to the source region) to the more distal position (far away from the source region), but also across the current from the central part (through turbidites) to the side of the current (contourites).

The Bouma sequence describes various facies related to a single turbiditic current. A the bottom the Layer A consists of coarse sandstones massive or with graded bedding. The Layer B above corresponds to the deposition of finer sandstone mixed with clay particle at a time where the turbiditic current is laminar. This layer is characterised by very fine lamination. The layer C is characterised by cross-lamination. The grain size is comparable to that of layer B, but the current is slower and allow the development of cross-beds. Layer D above consists of very fine sediments with parallel lamination. The layer E is the layer that deposits in between two turbiditic currents, it is made of pelitic sediments.

This picture shows a Bouma sequence for A to D. It is 1.5 m height. Note the charge figure at the base of the sequence.

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Layer D

Fine-grained Pelites in layer D.

Contact layer C (bottom) - layer D top.

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Layer C

Typical cross-beds in layer C

From bottom to top one can see: Layer A, layer B, and layer C above the hammer.

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Layer B

Typical layer B, well layered fine grained sandstones.

Contact between layer A (bottom) and layer B (unless it is already layer C) on top.

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Layer A

Gradded bedding (younging up) in sandstones form layer A.

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Structures

Some flute casts at the bottom of layer A. Flutes in the geologic context are lobate depressions a few cm long, formed by the action of turbulent of water flowing over unconsolidated sediments. Flute have and abrupt upcurrent end where the depth of the mark tends to be the greatest, and a larger smooth end that merge smoothly with the bedding surface. The direction of the current is parallel to the long axis of the flute. The sediment that fills the flute constitutes the cast. These photograph are of casts on the bottom of a bed of sandstone that was deposited upon the bed containing the grooves. The steep, upcurrent ends of the flute casts are on the right side of each photograph, indicating that the current direction was from the bottom right edge of the photograph to the upper left edge.

Charge figures at the base of layer A:

Nice fold hinge.

Details on the fold hinge.

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More Pictures

The group at work...

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Objectives
Introduction
Excercice
Turbidites and the Bouma sequence
Layer D
Layer C
Layer B
Layer A
Structures
More Pictures
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