The Petrified Forest EarthCache
Hidden : 4/10/2013
Difficulty:
2 out of 5
Terrain:
2 out of 5

Size: Size: other (other)

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Geocache Description:

Welcome to 'The Petrified Forest' located along the southern headland of Swansea in an area known as 'Reid's Mistake'.



At the published coordinates, you will find yourself standing upon a rock platform that was once a densely populated area of forest. Access to this earthcache is from 'Reid's Mistake Reserve', located at the end of Lambton Parade, and by following the path or beach from the car park to the rock platform. Please note that the best time to visit this location is during low tide.


Geology

Petrified wood (from the Greek root petro meaning "rock" or "stone") is the name given to a special type of fossilised remains of terrestrial vegetation. It is the result of a tree or tree-like plants having completely transitioned to stone by the process of permineralisation. All the organic materials have been replaced with minerals, while retaining the original structure of the stem tissue. The petrifaction process occurs underground, when wood becomes buried under sediment and is initially preserved due to a lack of oxygen which inhibits aerobic decomposition. Petrification occurs through two main processes: Permineralisation and Replacement.


Permineralisation

Permineralisation occurs when groundwater containing dissolved minerals (most commonly quartz, calcite, pyrite, siderite (iron carbonate), and apatite (calcium phosphate) ), fills pore spaces and cavities of specimens, particularly bone, shell or wood. The pores of the organisms' tissues are filled when these minerals precipitate out of the water. Two common types of permineralization are 'silicification' and 'pyritisation'.


Silicification

Silicification is the process in which organic matter becomes saturated with silica. A common source of silica is volcanic material. Silicification most often occurs in two environments-either the specimen is buried in sediments of deltas and floodplains or organisms are buried in volcanic ash. Water must be present for silicification to occur because it reduces the amount of oxygen present and therefore reduces the deterioration of the organism by fungi, maintains organism shape, and allows for the transportation and deposition of silica. In wood samples, as the process proceeds, cellulose and lignin, two components of wood, are degraded and replaced with silica. The specimen is transformed to stone (a process called lithification) as water is lost. For silicification to occur, the geothermic conditions must include a neutral to slightly acidic pH and a temperature and pressure similar to shallow-depth sedimentary environments; under these conditions, silicification can occur in 50,000 years or less.

Pyritisation

Pyritization is a process similar to silicification, but instead involves the deposition of iron and sulfur in the pores and cavities of an organism. Pyritization can result in both solid fossils as well as preserved soft tissues. In marine environments, pyritization occurs when organisms are buried in sediments containing a high concentration of iron sulfides. Organisms release sulfide, which reacts with dissolved iron in the surrounding water, when they decay. This reaction between iron and sulfides forms pyrite. Carbonate shell material of the organism is then replaced with pyrite due to a higher concentration of pyrite and a lower concentration of carbonate in the surrounding water. Pyritization occurs to a lesser extent in plants in clay environments.


Replacement

Replacement, the second process involved in petrification, occurs when water containing dissolved minerals dissolves the original solid material of an organism, which is then replaced by minerals. This can take place extremely slowly, replicating the microscopic structure of the organism. The slower the rate of the process, the better defined the microscopic structure will be. The minerals commonly involved in replacement are calcite, silica, pyrite, and hematite. It is rare to find organisms preserved by replacement alone (as opposed to in combination with permineralization).




The Petrified Forest

The petrified forest located at Reid's Mistake was once a group of Glossopteris trees that were fossilised approximately 250 million years ago by a volcanic eruption believed to have occurred some 20kms away. The radiating blast from the volcano caused a shockwave which snapped off the trees, and tilted the stumps aligning the trunks east west. This volcanic eruption also produced a fast moving current of hot gas and rock (known as a pyroclastic flow or pyroclastic density current), which resulted in the ancient forest being buried under a layer of volcanic ash. Over time, the volcanic ash consolidated and lithified to form 'tuff'. The fossilised stumps throughout the rock platform appear to have been preserved in their original growth position, some of which have been 'coalified'. Scattered throughout the rock platform are many example of leaf fossils embedded within the fine laminated tuff, which indicates 'ash-fall'.




To log this earthcache, please email me the answers to the following questions!

1. Located at the published coordinates is one of the larger examples of a petrified tree stump. What is the diameter of it?

2. Name the main element which can be found embedded within the fossilised stump?

3. In looking at the formations of the fossilised tree stumps, from which direction do you think the pyroclastic flow originated from?

4. Located at WP2 is another interesting formation. What is the formation known as, and what do you think is the most likely cause this formation?

5. What is the strike (bearing) of the formation found at WP2?



Please feel free to log your visit online before receiving confirmation of your answers from me, as I will contact you if there is a problem with any of them. If your email is not received within a timely manner of your online log...then your log will be deleted.



References:

Geo-Log 2010 - Hunter Valley Amateur Geological Society

Petrification. (2013). http://en.wikipedia.org/wiki/Petrifaction

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