Flashcards in SEDIMENTARY Deck (37)
What mode of deposition causes clay-y fine-grained sediments?
Long distance transportation and/or deposition in calm conditions
What mode of deposition tends to cause coarse grained sediments?
Shorter transportation and/or deposition in turbulent conditions
What is the difference between a cement and a matrix?
cements are later precipitated crystals, matrices are originally deposited mud
Describe an immature sedimentary rock:
•Wide range of clast types
•Wide range of grain sizes, some often large.
•Very angular fragments
Describe a mature sedimentary rock:
•Narrow range of clast types
• Narrow range of grain sizes; mostly small
what 3 factors increase with maturity?
sorting, rounding, sphericy
Describe freeze thaw weathering
Water expands by 9 per cent on
freezing, so fractures filled with water are prized further apart each time the water freezes until the fragment breaks off.
Describe how heating and cooling f rocks can cause weathering:
Heating and cooling of rocks in desert
regions causes minerals to expand and contract at different rates until the rock eventually fractures. Not proven.
Rainwater containing carbon dioxide (CO2) from the air and soil reacts with carbonates to form soluble hydrogen carbonates.
Water reacts with minerals,particularly silicate minerals like feldspars,breaking them down.
The addition of water molecules to the atomic structures of minerals
The addition of oxygen to the atomic structures of minerals or a similar chemical
How does bacteria cause weathering?
Minerals decompose much more
quickly in the presence of bacteria; the
bacteria produce an acidic microenvironment
that allows them to remove elements directly
what do silicate rocks break down in to?
–resistant heavy minerals
what is the best conditions for weathering?
warm humid climate
what sedimentary rock is quartz most common in?
why are feldspar less abundant in sedimentary rocks?
it is more easily decomposed
it has cleavage planes
what does the presence of feldspar suggest?
low amounts of chemical weathering (i.e desert)
proximity to igneous source
how are clays deposited?
suspension in water
contains quartz and feldspar
f'spar are angular and may be twinned
may be orange due to orthoclase
Dark coloured, dense and well cemented
contains rock fragments, of dark mafic igneous rocks and clays.
in what age are greywake most common?
in what environment is quartzite deposited?
–Very mature sediment: i.e. has had a lot of transport and weathering.
–Tends to be formed in a beach or desert setting
what depositional environment is arkose typical of?
–Form in dry conditions as feldspar chemically weathers in wet
–Often form close to granites from which the
f’spar is derived.
In what environment is grywake depositioned?
–Mostly deep water: deposited from turbidity currents associated with tectonism in magmatic arc.
Describe an aeolian sand
-pale fine sands
-well rounded well sorted
-wind-blasted and coated with heamotite
-may have f'spa
-large scale crossbeds
-few fossils (some bones and vertabrates)
describe the process of abrasion
-pebbles colide together becoming smaller and more rounded
-Rivers with load scour away the bed and sides of the river.
When is abrasion most effective?
at high speeds and if rock fragments are being carried