Chapter 8

SedimentaryRocks Part2

Types of sedimentary rock

Sedimentary structures

Types of sedimentary

rocks

• Sediment originates from mechanical and/or chemical weathering

• Rock types are based on the source of the material

• Clastic rocks – transported sediment as solid particles

• Chemical/Biochemical rocks – sediment that was once in solution

Sedimentaryrocks

Chapter 8 Sedimentary rocks

Clastic sedimentary rocks

Size of clasts (particles)

gravel bigger than 2 mm conglomerate (1/10 inch) or breccia

sand sand-sized (1/16-2) sandstone

silt, clay very fine-grained (<1/16) shale, mudstone,siltstone

common name size Detrital rock

See Table 8.3

conglomerate sandstone siltstone shale

claysiltsandgravel

sediments

sedimentaryrocks

Chapter 8 Sedimentary rocks

Clastic sedimentary rocks

Siltstone, Mudstone & Shale silt & clay-sized particles (clay, silt),

particles too small to identify w/ eye over 1/2 of all sedimentary rocks deposited in quiet (slow moving) water

- deep ocean & continental slope- lakes- floodplains (siltstone)

Shale w/plant remains

shale beds often underlay groundwater conduits

raw material for brick, tile, pottery, china

shale+limestone= Portland cement

shale may contains fossils, oily organics, breaks along bedding planes

Chapter 8 Sedimentary rocks

Clastic sedimentary rocks

Sandstone

composed of sand grains

2nd most abundant sedimentary rock

deposited by moderate currents: - rivers & deltas - beaches - wind (sand dunes)

mostly quartz (strong & chemically stable, but may also contain volcanic rock)

See Fig. 8.15 for major groups of sandstone

Chapter 8 Sedimentary rocks Clastic sedimentary rocks

Sandstone

sorting

degree of similarity in particle size

Chapter 8 Sedimentary rocksClastic sedimentary rocks

Sandstone

shapedegree of roundedness

Chapter 8 Sedimentary rocks

Clastic sedimentary rocks

Conglomerate

composed mostly of gravel pebbles to boulders

poorly sorted

deposited by strong, turbulent currents: - big flooding rivers - steep streams (near mountains) - glaciers

Chapter 8 Sedimentary rocks

Clastic sedimentary rocks

Breccia

conglomerate with angular grains

didn’t travel far

Chapter 8 Sedimentary rocks

Chemical sedimentary rocks

derived from material carried in solution to lakes/seas

2 types of precipitation - chemical - biochemical

precipitation from solution to form “chemical sediments”

See Table 8.4

Chapter 6 Sedimentary rocks

Chemical sedimentary rocks

Limestone

composed primarily of calcite (calcium carbonate CaCO3)

10% of all sedimentary rocks (by volume) most abundant chemical sedimentary rock

Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) Inorganic limestones include travertine (caves) and oolitic limestone (tropical beach environment) Dolostone: dolomite=CaMg(CO3)2 Typically formed secondarily from limestone

Chapter 6 Sedimentary rocks

Chemical sedimentary rocks

coquina (rock of shell fragments): bioclastic

Organic rock of biochemical origin

Chapter 6 Sedimentary rocks

Chemical sedimentary rocks

Fossiliferous limestone

Coral reef Limestone cliff(Guadalupe MountainsNatl Park, TX)

• How to build a carbonate platform

See Figure story 8.16

• How to build an atoll

See Box 8.1

Darwin’s theory on atolls (1831)

Chapter 8 Sedimentary rocks

Chemical sedimentary rocks

Common chemical sedimentary rocks

•ChertMade of microcrystalline silica (SiO2)

Varieties include flint and jasper (banded form is called agate)

Agate

Chapter 8 Sedimentary rocks

Chemical sedimentary rocks

Evaporites water evaporates and triggers the deposition of salts

sequence of precipitation: calcite (calcium carbonate), gypsum (calcium sulfate), rock salt (halite, NaCl); bitter salts (potassium and magnesium salts)

(a) calcite

Chapter 8 Sedimentary rocks

Chemical sedimentary rocks

Evaporites water evaporates and triggers the deposition of salts

ancient seawater basins evaporated and deposited saltSalt flats, Utah Death valley, Calif.

sequence of precipitation: carbonate, gypsum (calcium sulfate), rock salt (halite, NaCl); bitter salts (potassium and magnesium salts)

Chapter 8 Sedimentary rocks

Chemical sedimentary rocks

Evaporites- the story of the Mediterranean

Fig. 8.19

During the Miocene, lower sea level almost shut down the inflow of Atlantic surface water into the Mediterranean over the Strait of Gibraltar. Evaporation removed vast quantities of water and left behind evaporative sediments that underlay the sediments in the Mediterranean today.

Chapter 8 Sedimentary rocks

Chemical sedimentary rocks

Coal buried and compacted plant material different kinds of coal, depending on formation process

Sedimentary structures (Physical features)

1. Layers (bedding, or “strata”)1. Layers (bedding, or “strata”)

2. Cross-bedding2. Cross-bedding

3. Graded beds3. Graded beds

4. Ripple marks4. Ripple marks

5. Mud cracks5. Mud cracks

Sedimentary structures (Physical features)

1. Layers (bedding, or “strata”)

each layer is unique

deposited horizontally

separated by bedding planes

Sedimentary structures (Physical features)

2. Cross-bedding tilted beddingtilted bedding commonly ancient sand dunescommonly ancient sand dunes

river deltas, flow channelsriver deltas, flow channels

Sedimentary structures (Physical features)

3. Graded beds

particles within a layer gradually change:

coarse at bottom fine at top

rapid deposition from water w/ varying sed. sizes

Colorado River

Sedimentary structures (Physical features)

4. Ripple marks

wavy surfaces in sand: current ripple marks

tell direction of current becausethey are perpendicular to flow

See Fig. 8.8

Sedimentary structures (Physical features)

5. Mud cracks sediment alternatively wet/dry

shallow lakes, desert basins

flood plains

Some MC questions…

Which of the following sets of processes is written in order of increasing temperature?

A. sedimentation, metamorphism, diagenesisB. diagenesis, sedimentation, metamorphismC. sedimentation, diagenesis, metamorphismD. metamorphism, diagenesis, sedimentation

Some MC questions…

What type of sediments are accumulations of solid fragments produced by weathering?

A. biochemical sedimentsB. chemical sedimentsC. clastic sedimentsD. all of the above

Some MC questions…

Which of the following statements about transportation of sediment is false?

A. Smaller particles settle faster than larger particles.B. As a current slows, the largest particles start to settle.C. Faster currents carry larger particles than slower currents.D. Rivers and ocean currents move much more material than do air currents.

Some MC questions…

 In what type of environment did the ripples depicted above most likely form?

A. beach (waves)B. desert (wind)C. alluvial (stream)D. delta (river + tides)

Some MC questions…

Which of the following minerals is least likely to occur in a marine evaporite environment?

A. calciteB. gypsumC. halite D. quartz