L07 sedimentary rocks and processes

Learning Goal: describe how sedimentary rocks/processes fit into the rock cycle and why they are important

- contains important energy reserves (oil/gas/coal/uranium)

- building materials (cement, sand)

- placer deposits - heavy minerals concentrated in sediments (ex gold, diamonds)

- fossils - study evolution, time

- we build on sedimentary rock/sediment

- sediments make up the surface

sediment* vs sedimentary rock

- product of weathering

- physical fragments

- boulder, gravel, sand, silt, clay

or ions

sediment vs sedimentary rock*

- sediment that has undergone lithification

- form at (or near) surface of earth

- contains evidence of past environment (sediment transport, fossils)

- tells us a story

what is lithiification?

process of clastic rock being formed

how do clastic rocks form?

- start with material from physical weathering

- transported by transport agent

-deposited in sedimentary environment

- compaction (grains are compressed together)

- cementation (binds grains together)

in clastic rocks, how is eroded material transported? aka what are the transport agents?

wind

water (rivers and waves)

ice

gravity

what is the strongest transport method in clastic rocks?

gravity

what is a clast?

clast = rock broken off from earth

how are chemical precipitates formed?

start with sediment with ions in solution

some process causes them to precipitate from solution

creates chemical sedimentary rocks

what does massive mean?

a lot of things stuck together

what are evaporites?

precipitation of minerals from solution

often requires restricted basin in warm arid climate

common evaporites

halite (NaCl), gypsum, anhydrite

what are the 2 main types of sedimentary rocks?

clastic and chemical

sedimentary features

grain size

grain sorting

grain shape

what is grain size?

the size of the grains in sedimentary rock

how is grain size helpful

grain size relates to amount of energy needed to move particles

larger grains = moved by stronger forces/faster currents

what is grain sorting?

distribution of grain sizes or types

how does grain sorting help?

all same sized grains = well sorted

differently sized = poorly sorted

what is grain shape

angular vs roundness

sediment maturity

refers to clastic sediment

sediment size/sorting/roundness/composition, figuring out how far a sediment has been transported

why is sand a mature sediment?

travelled long way

well rounded

similar sized particles

what is a mature sediment

sediment that has travelled far

sediment maturity relates to ___

distance from source

mature = distal

immature = proximal

what are clues in rocks towards their origin?

grain details

maturity

sedimentary structures

fossils

what are sedimentary structures

features in sedimentary rocks

often formed at/near deposition

bedding structure

sedimentary rocks into layers

initially horizontal deposition

cross beds structure

bedding in different directions

cross bedding

what does cross bedding tell us?

deposited by wind, then current in water

tells us which direction the wind was going when the rock was being made

mud crack structures - what do they tell us?

made in arid environment

ripple marks structures

indicates currents/waves at that moment

engineering applications of sedimentary rocks

bedding planes - weak

bedding orientation influences slope stability

dissolution of carbonates/limestone can form caves, sinkholes

some sedimentary rocks are susceptible to weathering and/or abrasion