Sedimentary Rocks Flashcards
what are sedimentary rocks?
sedimentary rock are rocks made up of deposits of sediment (often small bits of rock), they are built up in layers, they are formed by the compactation and cementation of sediments
what are the two types of sedimentary rock?
clastic and non-clastic
what are clastic sedimentary rocks?
clastic sedimentary rocks are rocks that were deposited as clastic sediments (often broken down rocks) before
what are non-clastic sedimentary rocks?
non-clastic sedimentary rocks are often carbonates and are formed by chemical and biological processes
what are clastic sediments?
clastic sediments are accumulations of clastic particles derived from the weathering of pre-existing rocks, they are all rounded by physical abrasion during transportation (usually by water) they are often dominated by quartz because it is hard and does not weather [in simple terms they are fragments of rock that have been broken down as a result of weathering]
what are the differences in sedimentary rock caused by?
sedimentary rocks reflect the environment in which they were deposited, the differences are caused by differences in geological processes and environmental conditions like:
- location (valley, coast, ocean, river bed etc)
- transport medium (water, ice, air etc)
- transport agent (river, sea, waves etc)
- climate (tropical, arctic etc)
- plate tectonic setting (continental, oceanic etc)
what is often the transportation medium of sediments that form sedimentary rock?
water - turbulent flow is more efficient compared to laminar flow at picking up particles from the channel bed and keeping them moving
generally what does a faster flow mean in terms of particle transportation?
generally, the faster the flow, the larger the particle size it can start moving, and the larger the particle size it can keep moving, particles will settle when they enter channels that are too slow and therefore get deposited
where are clastic sedimentary rocks usually deposited?
on sea and ocean floors
where are non-clastic sedimentary rocks usually deposited?
these are deposited in shallow seas, and are made
from the carbonate shells of marine organisms, which dissolve in deeper water
give some examples of clastic sedimentary rocks?
- sandstone
- siltstone
- clay
- shale/mudstone
give some examples of non-clastic sedimentary rocks?
- limestone
- gypsum
- halite
- chert
- chalk
some info on limestone?
- sedimentary rock
- non-clastic
- either organic or chemical carbonate
- calcite 95% (contains high level of calcium carbonate and will react with strong acid like HCl)
- usually light in colour like marine shelly limestone and oolitic limestone (but can be dark like freshwater shelly limestone)
- soluble in rainwater
- can form in massive coral reefs
- older limestone is more completely recrystalized and so is stronger: UCS 20-100MPa
some info on sandstone?
- sedimentary rock
- clastic
- sand grains mostly of quartz
- older sandstones tend to be better cemented and stronger, clay cements are notably weak whilst quartz cements are generally strong
- they form productive aquifers
- UCS 20-80MPa
some info on clay?
- sedimentary rock
- clastic
- very fine grained and structureless (pieces are too small to see the structure)
- UCS 1-20MPa
- water does not flow easily
what is bedding?
layers of sediment within a sedimentary rock are called strata or bedding, the plane separating the layers are called bedding planes
what is graded bedding?
graded bedding is the term used to describe a systematic change in grain size within a bed from coarse grains at the base to fine grains at the top (often seen in normal bedding)
how are sedimentary rocks often described and classified by?
they are classified by grain size and shape - for example angular, subangular, very angular, rounded, subrounded and very rounded are all terms to describe the shape of rock grains and particles
what do ripple marks indicate?
indicate agitation by water or wind, they can be used to orientate a rock and also to calculate palaeo currents and wind directions
what is cross bedding?
cross-bedding refers to near horizontal units that are internally composed of inclined layers, the tilting is caused by the formation of structures such as a dune system or ripples
what is a cyclothem?
cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, often interbedded with coal seams
what is lithification?
the turning of sediment into sedimentary rock - compression is the dominant factor, but precipitation of minerals which stick the particles together can be very important in determining the strength of the rock
for something to be a soil what does it require (in engineering terms)?
not to be cemented
what is something unique to sedimentary rocks?
they are the only type of rock that may contain fossils, or evidence of past life
what are 3 types of limestone?
- freshwater shelly limestone
- marine shelly limestone
- oolitic limestone
