This Earth Cache is designed to bring you to another great place on the Western Australian Southern Coast and hopefully make a simple to understand a small bit of our past and the land that we live on, especially around our rugged coast line.  This can be a very dangerous coast so all precautions must be taken for your own safety, remember KING WAVES can kill!  You can get  close, (about 3-400 metres) to GZ with a conventional 2 wheel drive vehicle, however you can cross the Thomas River with a four wheel drive and even head all the way along the beach to Mt Yokinup.  It's about a 25 Kms drive and conditions can vary so caution is required.

Yokinup Bay is situated in the Cape Arid National Park, so you may need to pay a day entry, or get a yearly park pass, then you can visit all the great national parks for the one low fee!

At GZ you will find some interesting colours and layout of rocks, I believe the black Basalt or Dolerite you see was caused by volcanic activity and the quartz running off at nearly 90 degrees has some orange coloration.  The major rocks in this section of the south coast are migmatites typical of those that have suffered intense wrenching and metamorphism.  Migmatite is a rock formed when a metamorphic rock is partially melted.  This creates a mixture of unmelted metamorphic rock and melted and recrystallised igneous rock.  (Ref: Geology of Westerrn Australia's National Parks 4th Edition, Peter Lane,)

Geology:-

The rivers in the area are of very recent origins geologically.  During the last major glaciation of the Pleistocene period (the last 2 million years), and only 20,000 years ago, the sea level was more than 100 metres lower than it is now. For some kilometres beyond the present coastline there was dry land, across which the rivers flowed to the sea through river valleys, the location of which is indicated by submarine canyons along the edge of the continental shelf.  With the melting ice caps the sea rose to its present level about 6,000 years ago flooding the lands, and for some time after sea levels may have been about 2 metres higher than it is now.  

The greater part of this area lies within the Albany-Fraser geological Province, off the south eastern edge of the ancient Yilgarn Block. The basement rocks are Proterozoic granite rocks (Migmatites, more than 1200 million years old), but near the coast these are covered by the Eocene Pallinup Siltstone (40 million years olds).

Thomas River is in a deep valley between the high Pleistocene limestone dunes, through which the rivers maintained a channel.  It is located along a probable fault structure, which may explain the deep incision of the river valley and the sighting of some Basalt seams running through the granite around the  GZ vacinity.  At GZ you will notice an interesting blend of quartz which appears to have been fractured or cracked at some stage.  You can also observe the differing rate of erosion between the harder quartzite and the surrounding granite.

Faults are fractures in Earth's crust where rocks on either side of the crack have slid past each other.

Sometimes the cracks are tiny, as thin as hair, with barely noticeable movement between the rock layers. But faults can also be hundreds of miles long, such as the San Andreas Fault in California and the Anatolian Fault in Turkey, both of which are visible from space.

Three types of faults:

There are three kinds of faults: strike-slip, normal and thrust (reverse) faults, said Nicholas van der Elst, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York. Each type is the outcome of different forces pushing or pulling on the crust, causing rocks to slide up, down or past each other.

Strike-slip faults indicate rocks are sliding past each other horizontally, with little to no vertical movement. Both the San Andreas and Anatolian Faults are strike-slip.

Normal faults create space. Two blocks of crust pull apart, stretching the crust into a valley. The Basin and Range Province in North America and the East African Rift Zone are two well-known regions where normal faults are spreading apart Earth's crust.

Reverse faults, also called thrust faults, slide one block of crust on top of another. These faults are commonly found in collisions zones, where tectonic plates push up mountain ranges such as the Himalayas and the Rocky Mountains.

Strike-slip faults are usually vertical, while normal and reverse faults are often at an angle to the surface of the Earth. The different styles of faulting can also combine in a single event, with one fault moving in both a vertical and strike-slip motion during an earthquake.

All faults are related to the movement of Earth's tectonic plates. The biggest faults mark the boundary between two plates. Seen from above, these appear as broad zones of deformation, with many faults braided together. "Plate boundaries are always growing and changing, so these faults develop kinks and bends as they slide past each other, which generates more faults," van der Elst said.

Individual fault lines are usually narrower than their length or depth. Most earthquakes strike less than 50 miles (80 kilometers) below the Earth’s surface. The deepest earthquakes occur on reverse faults at about 375 miles (600 km) below the surface. Below these depths, rocks are probably too warm for faults to generate enough friction to create earthquakes, van der Elst said.  (Ref: https://www.livescience.com/37052-types-of-faults.html)  

Faults are normally quite large in size and area, whereas fractures are simply cracks in the earths surface where there is minimal movement or disruption to the earths crust.  

When can a fracture be considered a fault?

When rocks break in response to stress, the resulting break is called a fracture. If rocks on one side of the break shift relative to rocks on the other side, then the fracture is a fault.

To log this Earth Cache you will need to answer the questions below:-

1.  Can you identify the fault or crack at GZ, and say what you believe it to be?

2. Tell me what you see and how far it has moved?

3. A photo with you and your GPS device at GZ.  Please upload this on the page with your "found it" log

You may log this cache after your visit however you should send the answers and the photo to me within say 10 days of logging the cache, via my profile, failure to do so may result in me deleting your log.  Enjoy your visit to Cape Arid and  Bay.  Happy fishing, swimming or camping and caching.  

Congratulations to JasenJanine for a great FTF