Sedimentary Rocks

Question 1

why are they important?

Answer 1

• Economically important: oil/gas/coal, building materials (cement, sand)
• Placer deposits -> heavy minerals concentrated in sediments (ie. Gold, diamonds)
• Important deposits like potash, rock salt, and gypsum
• Paleoenvironmental information (info about past environmental conditions)
• Contain fossils to help us study evolution and time

Question 2

what is a sedimentary rock?

Answer 2

• Products of weathering (clastic), the accumulated remains of organisms (biochemical), or precipitation of ions in solution (chemical)
• Account for 5% of earth’s outer 16km but cover 75% of earth’s surface

Question 3

3 types of sedimentary rocks

Answer 3

1. clastic/detrital
2. biochemical
3. chemical

Question 4

clastic/detrital sedimentary rocks

Answer 4

• Weathering -> produces fragments
• Transportation -> Water, wind
• Deposition -> marine or terrestrial
• Compaction and cementation: After deposition, there’s lots of pore spaces between the grains, but then compaction happens, reducing pore spaces. Then cementation happens, binding the grains together (ie. With quartz)
• Ex. Mudstone (shale), siltstone, sandstone, conglomerate

Question 5

biochemical sedimentary rocks

Answer 5

• Form from precipitation of material produced by organisms

- Ex. Limestone, chalk, coal, carbonate reefs

Question 6

chemical sedimentary rocks

Answer 6

• Weathering (chemical) -> ions in solution
• Transportation -> as dissolved ions and ionic groups in water
• Precipitation by evaporation: evaporates like rock salt (halite)
• Ex. Potash industry in SK; Mediterranean, Rock salt

Question 7

examples of clastic rocks

Answer 7

• Mudstone/shale: fine-grained, made up of silica and clays
• Siltstone: coarser than mud but finer than sand
• Sandstone: composed of sand-sized particles
• Conglomerate: large rounded clasts (like beach pebbles) held in a finer matrix

Question 8

examples of biochemical rocks

Answer 8

• Limestone: composed of calcium carbonate
• Chalk: composed of calcium carbonate from coccolithophores (microscopic fossils)
• Coal: compressed plant remains

Question 9

example of chemical rocks

Answer 9

Rock salt: formed from evaporation of water and precipitation of salt (halite – sodium chloride)

Question 10

sedimentary environments

Answer 10

As sediments are deposited, they record information about the environment such as water, depth, flow directions, locations of bodies of water, etc.

Question 11

uniformitarianism

Answer 11

• “the present is the key to the past”
• Observe environments where sediment is deposited today
• Search rocks for clues to their origin: Grain compisition; Grain size, sorting, shape; Sedimentary structures; Fossils

Question 12

sedimentary structures

Answer 12

• Arrangement of sedimentary rocks into layers
• Graded bedding
• Flame structures
• Cross bedding
• Mud cracks
• Ripple marks
• River channels

Question 13

graded bedding

Answer 13

• sedimentary structure
• Goes from coarse to fine -> fine is at the top, coarse is at the bottom. Tells you which way up the rock is and which direction in time

Question 14

flame structures

Answer 14

• sedimentary structure
• Soft, goopy sediment deposited, then a wet sand deposited on top -> goopy sediment squishes up (picture smacking your hand into a bowl of custard)
• Turbidite flows are where graded bedding and flame structures are common -> Like an avalanche of sediment underwater

Question 15

cross-bedding

Answer 15

• sedimentary structure
• Indicates the direction of movement of the current
• Lines that slope down to the right indicate current was moving to right (and vice versa)
• Different layers of sedimentary rock can have cross-bedding in different directions
• Ex. Navahoe sandstone

Question 16

mud cracks

Answer 16

• sedimentary structure
• Common in arid environments with seasonal precipitation
• Polygonal pattern from dried-up mud

Question 17

ripple marks

Answer 17

• sedimentary structure
• Indicate currents and tides
• Ex. On beaches

Question 18

fossils (and their 2 types)

Answer 18

• Remains or traces of ancient plants and animals
• Marine (eg. Shells, shark teeth, microfossils)
• Non-marine (eg. Flowering plants, ferns, dinosaurs)
• “trace fossils” are tracks and trails, not the original organisms (ex. Footprints)
• Fossils can give us clues about environments (Ex. Fossils of well-preserved leaves tell us that the environment was low-energy/quiet because the fossilized leaves have been preserved so well (haven’t been broken down like they would in a high-energy environment))