Sedimentary Rocks

Question 1

what are sedimentary rocks?

Answer 1

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

Question 2

what are the two types of sedimentary rock?

Answer 2

clastic and non-clastic

Question 3

what are clastic sedimentary rocks?

Answer 3

clastic sedimentary rocks are rocks that were deposited as clastic sediments (often broken down rocks) before

Question 4

what are non-clastic sedimentary rocks?

Answer 4

non-clastic sedimentary rocks are often carbonates and are formed by chemical and biological processes

Question 5

what are clastic sediments?

Answer 5

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]

Question 6

what are the differences in sedimentary rock caused by?

Answer 6

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)

Question 7

what is often the transportation medium of sediments that form sedimentary rock?

Answer 7

water - turbulent flow is more efficient compared to laminar flow at picking up particles from the channel bed and keeping them moving

Question 8

generally what does a faster flow mean in terms of particle transportation?

Answer 8

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

Question 9

where are clastic sedimentary rocks usually deposited?

Answer 9

on sea and ocean floors

Question 10

where are non-clastic sedimentary rocks usually deposited?

Answer 10

these are deposited in shallow seas, and are made

from the carbonate shells of marine organisms, which dissolve in deeper water

Question 11

give some examples of clastic sedimentary rocks?

Answer 11

• sandstone
• siltstone
• clay
• shale/mudstone

Question 12

give some examples of non-clastic sedimentary rocks?

Answer 12

• limestone
• gypsum
• halite
• chert
• chalk

Question 13

some info on limestone?

Answer 13

• 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

Question 14

some info on sandstone?

Answer 14

• 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

Question 15

some info on clay?

Answer 15

• sedimentary rock
• clastic
• very fine grained and structureless (pieces are too small to see the structure)
• UCS 1-20MPa
• water does not flow easily

Question 16

what is bedding?

Answer 16

layers of sediment within a sedimentary rock are called strata or bedding, the plane separating the layers are called bedding planes

Question 17

what is graded bedding?

Answer 17

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)

Question 18

how are sedimentary rocks often described and classified by?

Answer 18

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

Question 19

what do ripple marks indicate?

Answer 19

indicate agitation by water or wind, they can be used to orientate a rock and also to calculate palaeo currents and wind directions

Question 20

what is cross bedding?

Answer 20

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

Question 21

what is a cyclothem?

Answer 21

cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, often interbedded with coal seams

Question 22

what is lithification?

Answer 22

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

Question 23

for something to be a soil what does it require (in engineering terms)?

Answer 23

not to be cemented

Question 24

what is something unique to sedimentary rocks?

Answer 24

they are the only type of rock that may contain fossils, or evidence of past life

Question 25

what are 3 types of limestone?

Answer 25

• freshwater shelly limestone
• marine shelly limestone
• oolitic limestone