Practical 4: Weathering, sediments and sedimentary rocks

Section 1: Weathering and erosion

In this section we will look at how a granite “protolith” could be weathered. In this case we are looking close to home with some examples of the Enoggera granite which outcrops on Mt Cootha. Below are models of a fresh piece of granite and a piece of weathered granite.

Granite (fresh and unweathered). Click HERE for a full-screen 3D model.

Granite (highly weathered). Click HERE for a full-screen 3D model.

Below is a gigapixel close up image of both weathered and unweathered Enoggera Granite. Here you can see clear changes in the mineralogy.

Unweathered vs weathered granite (CLICK ON THIS IMAGE)

Above is a “zoomable” gigapixel image of a piece of unweathered and weathered Enoggera Granite collected at Mt Cootha. You can switch between them by taping the button in the middle of the screen. What is the difference between the two rocks? For help with the minerals you may want to look at the model below.

Section 2: Sediment transport

In this section we will look at the composition (what they are made from) and the texture (grain size and shape) of sediments. To begin with we will look at coarse grained sediments. Look at the gravels from a Mt Cootha creek below and answer the questions in the answer sheet.

Mt Cootha creek gravels

How to use the Hjulstrom diagram

Section 3 and 4 The composition and texture of sands

Typically sands are sediments where the particles are smaller than 2mm but we can still examine the mineralogy of individual grains. When particles are smaller than 0.063mm it’s very hard to determine composition, size and shape without advanced techniques.

Below are some gigapixel examples of sands. Examine them and answer the questions in Inspera.

Mt Cootha Stream Sediment (scale = 2mm)

River sediment from the Brisbane River (mean grain size approx 0.5mm)

Beach sand (Stradbroke Island) (scale = 2mm)

Section 5 Glaciomarine sediments

Glaciomarine sediment (Southern Ocean, Antarctic margin) (scale 2mm)

Section 6. Sandstone

Below are two gigapixel images of two different sandstones from south east Queensland. Look at these images and answer the questions in the practical sheet.