Earth Systems 3209 – Unit 3

The Rock Cycle

Why study sedimentary rocks? Economic use, fossils and earths history.

5% of Earths crust is sedimentary rock.

75% of Earths surface is sedimentary rock.

Sedimentary Processes Formed from sediments of pre-existing rocks

(i.e. metamorphic, igneous or sedimentary). Created by the processes of WEATHERING

(i.e. break-up) and EROSION (i.e. removal and transportation).Weathering (Two Types: Chemical and

Mechanical) Erosion (Agents: water, ice, and wind)

Aided by gravity, sediments travel to bodies of water where they are deposited (i.e. come to rest). Over geological time, sediments transform into sedimentary rock.

Sedimentary Processes Sediments are formed into rock by the process of

LITHIFICATION, which includes two sub-processes:Compaction: pressure from the burial of sediments

solidifies material into rock. This results in reduced porosity and permeability. (Note: effective with small grain sizes)

Cementation: minerals precipitate out (i.e. crystallize) of circulating ground water, which cements sediments together (SiO2, CaCO3, FeO3)

Cementation and Compaction

Can anyone give examples of sedimentary rocks?

Sediment Particles Cement

CoalBreccia

Sandstone Conglomerate

Shale

Rock Salt Gypsum

Siltstone

Dolomite

Limestone Coquina

Chaulk

CLASSES of Sedimentary Rocks Clastic (also called detrital):

(e.g., shale, siltstone, sandstone, conglomerate, breccia)Formed from mechanical weatheringClassified based on particle size (and particle shape)

Chemical: Evaporites (e.g., rock salt, gypsum, sylvite)Precipitates (e.g., compact limestone, travertine, dolomite,

chert)Chemical weathering – dissolving of chemicals

Biochemical (e.g., coquina, chert, coal, coral limestone, chaulk)Accumulation of living material – i.e. organic remains

Clastic Characteristics: Clastic rocks are classified based on grain size (e.g.,

shale - clay, siltstone - silt, sandstone - sand) with the exception of Breccia (angular gravel particles), and conglomerate (rounded gravel particles), which are classified based on grain shape.

Grain Shape (rounded, angular) Sorting (well sorted, poorly sorted) DOES NOT APPLY TO

CHEMICAL OR BIOCHEMICAL, BUT IT DOES APPLY TO CLASTICS!

Cementation (e.g., siliceous, calcareous, or iron oxide)

Porosity (pore space - % of open space or holes) Permeability (communication or interconnectiveness

between pore space – Darcy’s)

What can we tell from grain size/shape?

Grain size decreases with decreasing current velocity (i.e. decreasing energy). In general, larger grain sizes are closer to the source of weathering. Smaller grain sizes are further from the source of weathering.

The roundness of grains increases with increasing distance from the source of weathering. Angular grains have not traveled far from the source.

High Velocity Low Velocity Breccia, Conglomerate Sandstone Siltstone Shale

HORRIZONTAL SORTING (COARSE TO FINE FROM LEFT TO RIGHT)

Clastic Depositional Environments AND Rocks Fluvial (Rivers/streams) – Conglomerate, Breccia,

Sandstone, Siltstone, Shale Lagoonal/Bays – Siltstone, Shale Beaches – Conglomerate, Sandstone Deep Marine – Involves turbidites (Conglomerate,

Sandstone, Siltstone, Shale), but is dominated by chemical sedimentary rocks.

Shallow Marine – Conglomerate, Sandstone, Siltstone, Shale

FluvialBeach

Fluvial Shallow Marine

Turbidites Deep marine sedimentary rocks resulting from

underwater avalanches. Characterised by graded bedding - fining upwards sequences of conglomerate at the bottom, followed by sandstone, siltstone and shale.

Chemical Sedimentary Rocks Chemical:

1) Evaporites (e.g., halite, gypsum, sylvite)

NOTE THAT THE ABOVE ARE THE RESULT OF THE EVAPORATION OF WATER.

2) Precipitates (e.g., compact limestone, travertine, dolomite, nodular chert)

DUE TO CHANGES IN ENVIRONMENTAL CONIDITONS (E.G., TEMPERATURE CHANGE, CHEMICAL CHANGE, CONCENTRATION CHANGE).

Chemical weathering – dissolving of chemicals

Chemical Sedimentary rocks

Chemical Depositional Environments:

1. Shallow Marine (e.g., gypsum, halite, sylvite, limestone and dolomite)

2. Deep Marine (e.g., limestone, dolomite and nodular chert)

3. Cave - Stalactites and stalagmites

Stalactites are icicle-like pendants that hang from the ceiling. Water seeps through cracks in the ceiling of the cave.

Stalagmites form from the floor.

The precipitated limestone that makes up stalactites and stalagmites is called travertine.

Stalactites and Stalagmites

Biochemical Sedimentary Rocks Coquina – forms from the build-up, compaction, and cementation of shells from dead organisms (mostly on beaches).

Chaulk – forms from the accumulation, compaction, and cementation of microscopic marine organisms such as formaminifera (i.e. deep marine)

Chert – forms from the accumulation of microscopic marine organisms, such as radiolaria and diatoms, that form a very hard rock consisting of microcrystalline silica. Examples of chert include jasper, flint, and agate (i.e. deep marine)

Coral limestone – accumulations of coral can be compacted and cemented into coral limestone (i.e. shallow marine)

Coal – formed form the accumulation of plant material, which is buried and chemically altered over millions of years.

What are corals?

Corals are examples of organisms that are capable of creating large quantities of marine limestone from their shells and external skeletons which are composed of calcium carbonate.

The Formation of Coal - Swamps There are 4 stages of coal formation:

1) Peat – earliest stage of plant accumulation. Contains large amounts of volatiles. (PEAT IS NOT A ROCK). (VOLATILES – E.G., CARBON DIOXIDE, WATER).

2) Lignite – decreased levels of volatiles. It is classified as a sedimentary rock.

3) Bituminous – higher grade of coal than lignite. It is classified as a sedimentary rock.

4) Anthracite – final stage of coal formation. It is classified as a metamorphic rock due to heat, pressure, and hot chemical fluids. It burns cleaner than other stages of coal due to less volatiles present. Small amount available world-wide.Increasing

Grade

Four Stages of Coal Formation

Peat Lignite Bituminous Anthracite

Biochemical Sedimentary Rocks Depositional Environments- Swamp – Coal- Shallow Marine – Coquina, Limestone (Coral)- Beach – Coquina- Deep Marine – Chaulk, Chert

- Think about the concept of systems. How do biochemical sedimentary rocks and coral represent a dynamic interaction of the spheres?

Core Lab 3 – Sedimentary Rocks

Review:

Concept Map for rocks

Next class – Metamorphic Rocks