Geology 101 - Sort of...

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Geology 101 - Sort of...

Postby jon_12091 » Fri Sep 11, 2009 6:29 am

Given some of the recent discussion regarding the geologists intellectual tool box I thought I would jot down some of the more basic stuff for the interest of the less geologically aware.

This is being done from memory at the moment so it may not be perfect....

This concerns only sedimentary rocks as I'm no expert on the other two, igneous and metamorphic. Sedimentary rocks also account for approximately 70% of the rock record. When geologists ascribe to a particular bed of sedimentary rock an environment of deposition, i.e. the conditions under which it was laid down or deposited, they aren't doing so on a whim or even just one or two pieces of evidence (usually). The following is list of the characteristics geologists use to make this determination (in no particular order):

Rock Fabric - Most sedimentary rocks are made up of particles, though some are precipitated, the descibption of these include the size any component particles, their texture or shape and the statistical distribution of sizes or 'sorting'. The sizes of individual particles and their distribution are in strong part controlled by the energy of the environment they were depositied in. Boulder as can be imagined require considerable energy to move or some assistance from gravity. The very finest and lightest particles, clays, require virtually zero energy to be depositied.

Structures - Sedimentary rocks are seldom massive homogeneous block of sand and often contain patterns or structures. These are key evidence in determining the rocks environment of deposition, for example preserved ripple structures can provide information about direction of flow and the energy of the flow that laid them down. Cross bedding is a very common structure in sandstones and the angle at which these structures dip can be used to determine whether it was formed in an underwater setting or was a product of wind.

Setting - The context within which a bed of sedimentary rock lies is important so no bed is considered alone and is placed in the context of its neighbours above and below. When describing a large sequence of beds a geologist will group those beds of similar characteristics into 'facies'.

Composition - Except in the case of chemical preciptates such as gysum the composition of is little used in determining the environment of deposition but can provide alot of information about the nature of the sediment being supplied and where it came from. Chemical analysis methods can of clay-rich rocks can be used to find the amount of boron present which is an indicator of the salinity of the water the sediment was laid down in.

Fossils - Essentially these come in three different flavours, macro, micro and trace. Starting with the often overlooked trace fossils these most commonly are the humble burrowings of relatively simply creatures like worms, but can include can include complex dinosaur trackways. These can be helpful in determining the environment, as different types of burrow belong to different creatures living in different habitats, and can even provide information about how firm the sediment in which they developed was. Macro fossils are the physical remains of large creatures, such as their shell or bones, or plants.

Micro fossils are the remains of microscopic animals such as foraminifera and coccoliths. These are useful indicators of environment and they are known in sufficient detail that they can provide accurate information for the relative dating and correlation of rocks. While most micro fossils require a microscope some are sufficiently large to resolved with a hand lens or the naked eye. The remains of coccoliths, in their billions, are the building block of chalk and other micro fossils can be a significant component in some limestones.

Macro fossils are those that can be seen with the naked eye.





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