CHAPTER 6-3SEDIMENTARY ROCKS

What is sediment?

Loose fragments of rock, minerals, and organic material that result from natural processes (including the breakdown of rock)

Characteristics of sedimentary rocks are determined by:

• The source of the sediment• The way the sediment was

moved• The conditions under which the

sediment was deposited

Processes

• Compaction- the process in which sediment is squeezed and the size of the pore space between the sediment particles (porosity) is decreased by the weight and pressure of overlying layers

• Cementation- the process in which sediments are glued together by minerals that are deposited in water.

Chemical Sedimentary Rocks• Minerals made up of ions such as Ca, K, and Cl

can dissolve in water.• This type of rock forms when dissolved

minerals precipitate out of water• Minerals precipitate due to evaporation

(when water evaporates, the minerals that were dissolved in the water are left behind increasing the concentration of the minerals)

• The minerals left behind form rocks called EVAPORITES.

• Examples are halite (or rock salt) and gypsum.

Organic Sedimentary Rock

• Rock that forms from the remains of living things

• Examples are coal, coquina and fossiliferous limestone

• INTERESTING NOTE: Chalk is made up of the shells of tiny one celled marine organisms that settle to the ocean floor.

Clastic Sedimentary Rock

• Made up of fragments of rock that are carried away from their source by water, wind, or ice

• Over time these fragments become cemented or compacted into solid rock

• They are classified by the size of their sediments

Clastic continued

• Conglomerate- rock composed of round fragments from 2 mm to boulders in size

• Breccia- rock composed of sharp angular fragments

• Sandstone- rock composed of sand grain size sediment cemented together

• Shale- rock composed of clay sized particles cemented and compacted together.

Sediments are transported by:

•Water•Wind• Ice •Effects of gravity

Sorting

• The tendency for currents of air or water to separate sediments according to size

• Well sorted- all grains are roughly the same size and shape

• Poorly sorted- consists of grains that are many different sizes.

Angularity• As sediment is transported from its source to

where it is deposited, it collides with other sediments and other objects.

• These collisions cause the sediment to change size and shape.

• When particles first break they are angular and uneven…..

• As they travel farther from source they become rounded and smooth.

• The farther it travels the finer the sediment becomes.

The place in which setting is deposited is called the DEPOSITIONAL ENVIRONMENT

Examples are rivers, deltas, beaches, and oceans

Characteristics of CLASTIC sediments• Stratification- layering of sedimentary rock– This occurs when the conditions of sediment

deposition change• Beds- stratified layers• Massive beds- beds that have no internal

structures• Cross-beds- sedimentary rock characterized by

slanting layers• Graded bedding- when various sizes and shapes

of sediment settle to different layers. (largest on bottom; smallest grain on top)

• Reverse grading- smallest grains on bottom; largest on top.

RIPPLE MARKSCaused by the action of wind or water on sand. Shows that the sediment was once part of a beach or river bed.

Mud CracksForm when muddy deposits dry and shrink

FOSSILSThe remains or traces of plants and animals may be preserved in sedimentary rock

CONCRETIONSLumps of rocks on sedimentary rocks that have a composition different than that of the main rock body.

GEODESGroundwater sometimes deposits dissolved minerals inside cavities of sedimentary rock where they crystallize.