Site 7

Scree Slope of Metamorphic Rock

Welcome to site 7! The GPS coordinates for this site are 0236357 6278646. This field location is made up of rocks that are not in-situ. A scree slope, as you can see here, is a collection of rock fragments that have accumulated over time at the base of steep slopes. This forms a talus deposit. Using the image below, make a quick sketch of this deposit in your field note books and think about how this scree may have formed.

This is the same rock type as those at site 3. However, the talus deposit is sourced from a separate unit in the regional stratigraphy. Hint: look at the map in the unit of study booklet of the Hartley region.

Scree controls

At site 3, you learnt about some of the different planar features that can be observed within a rock. The first planar feature you identified wasn't an important control for the rock at site three. However, the fracturing of the rock at this location has acted as a significant control in the appearance of the rock fragments in the scree.

Question 1: Given that you know the fractures are important in the erosive processes that created the rock fragments in the scree, can you describe how fractures interact to form this talus deposit?

Examine the model of a hand sample from site 7 in Pedestal3D below. Click on the < button to hide the side menu and give you more space to view the sample. Choose 'High' instead of 'Medium' from the drop down menu to maximise the quality of the sample image.

Task: Please choose an informative view direction in the Pedestal3D model and make a sketch in your field note book. You may zoom into interesting features and make multiple sketches rather than sketching the whole hand sample if needed. Please use the tools in Pedestal3D to measure grain size etc. for your sketches. It is important to recognise the difference between a fresh and weathered face of a sample. Sometimes weathering can conceal important features and the fresh face gives the best view, however the opposite can also be true. Study both faces in these samples to see which gives you the best view of the rock's microstructures.

Rock observations

Another planar feature you learnt about at site 3 can also be seen in the rocks of site seven. Looking at the rock in all the images, you can see alternating layers of different coloured minerals making fine lines of darker and lighter bands.

The layers can also be observed in thin section under a microscope, showing alternating bands of minerals and layers that are truncated.

Question 2: What is the term used to describe these alternating fine grained bands?

Classifying a rock

We use the mineral assemblage of metamorphic rocks to classify them. Use the close up image (Figure C) to identify the different minerals making up this rock.

There are three different coloured minerals that you can observe.

The first mineral is the dominant mineral and appears colourless to white or almost grey.

The second mineral is much less prominent and is also white in colour. Based on the chemical composition of this mineral, it can also appear pink, due to iron staining that happens during weathering.

The third mineral is speckled throughout the rock and is quite dark in appearance.

Question 3: Can you name these dominant minerals within the rock?

Determining a protolith

The rocks found here at site 7 are the same rock types as those found at site 3, a contact metamorphic rock.

Question 4: Given that you have already studied this rock, name this metamorphic rock type?

You have already identified the mineral assemblage of this rock which is also very useful for identifying the protolith of a rock. Another tool that can be used to identify a protolith is any planar feature the rock may present. See if you can identify any prominent planar features in the images of rocks from site 7. The features within this rock are distinctly sedimentary forming from sedimentary processes in a marine environment.

Question 5: Based on your interpretations, what is the protolith of this rock?

Question 6: What specific features in this metamorphic rock including the composition and planar characteristics led you to infer the protolith?

Previous sightings of this rock type

Look at the images below. Do these rocks look familiar?

Question 7: What field sites did you see these same rock types?

Observing the same rock type at two field sites can be due to two main reasons.

Question 8: Can you think of the two different ways these rocks are repeated at sites 3 and 7?

To help you along with this question think about the following:

Is it the same rock type as the other sites or are there slight differences in the mineral assemblage or other features?
If the rock is a part of the same deposit, what might have caused it to appear in multiple locations with changes in surface geology in between? i.e. how are rock units repeated due to structural patterns formed during deformation of rocks?

Only two stops to go. Site 8 is at the bottom of the hill over there -->

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