Part IV. Weathering, soils, sediments, and sedimentary rocks (Chapter 6 in lecture textbook)

Sedimentary Rocks

1. Clastic (detrital): fragments of older rocks. Here is a video that reviews Clastic rocks.

2. Chemical: precipitates from evaporated seawater. Here is a video review of Chemical rocks.

3. Biochemical (bioclastic): fragments of skeletal matter. Here is a review of Biochemical rocks.

 

4. Interpreting clastic sedimentary Rocks:

a. With greater distance of transport (down a river system or more time on a beach):

• Particles become smaller. Here is a a good video.
• Quartz and clay minerals become more abundant
• Particle sizes become more uniform
• Grains become more round

Click on the diagram on the left to see a figure that shows the relationship between a mountainous source area, a simple river system, a lake, and the clastic sediments deposited.

Characteristics of Sedimentary Rocks:

1. Clastic sediments and rocks:

a. Solid products of weathering (mud, sand, and gravel)

• Fragments of: quartz, potassium feldspar, muscovite, older rock fragments are common,  and there are few others.

b. New Minerals: Clay minerals

i. Microscopic fragments and crystals (crystal structure is like micas, but microscopic).

ii. Aluminum-rich silicate minerals that are stable at the Earth’s surface.

iii. Solid products of weathering

iv. Clay minerals have an electrical charge.

 

2.) Chemical Precipitates (chemical sedimentary rocks)

a. Precipitate from water and have crystalline texture.
b. Examples:

• Gypsum (gyprock)
• Halite (Rock salt)
• Chert (Microcrystalline quartz)
• Some dolomite: magnesium + limestone (calcite) over time can make dolomite (more resistant to acids than calcite

3.) Biochemical Sedimentary Rocks

a.) Organisms involved. Direct or indirect: Precipitation of minerals.
b.) Minerals = calcite. Sometimes silica (opal)
c.) Rocks = limestones, but if made of the mineral dolomite then the rock is called dolostone.

• these sediments are born, not made. Fossils are usually present.

d. Important for global carbon cycle - climate.

Sedimentary Rock Links:

• Here is a link to a site by Lynn S. Fichter with a good explanation of sedimentary rocks.
• This site from Geology.com has some good photos of sedimentary rocks.

Sediments and Sedimentary Rocks: Study Questions

1. What are the three main types of sedimentary rocks?

2. How do siliciclastic sediments change as they are transported away form their original source area?

Go to: Weathering and Soils.