Importance of Sedimentary Rocks

1. Economically important - Coal, Limestone

2. Contains resources - Coal, crude oil, natural gas, water, salt, iron ore
 

Sediment is a collective name for loose, solid particles originating from weathering and erosion of pre-existing rocks and chemical precipitations from solution, including secretion by organisms in water.
 

Unconsolidated - means that the grains are separate

Processes that effect the character of sediments:

Transportation - The rounding or abrasion causing removal of sharp edges/corners as in Sand/gravels in rivers glaciers, and waves

Sorting - sediments are selected based on size
 

Deposition - Settling of materials or accumulation of chemical/organic sed.
 

Depositional Environments- each is marked by characteristic physical, chemical and biological conditions that may be due to source, mode of transportation and the environment.

Deep sea flow-

Desert valley-

River channel-

Coral reef-

Lake bottom-

Sand dune-
 

Lithification - A group of processes that transform sediment into sedimentary rocks

Compaction - pore space  between  grains is 40-50% original volume 

Cementation - cements - silica (from weathering of feldspar), calcite (CaCO₃), dissolution of limestone. Others include oxides, hydroxides [FeO(OH) ], and  Limonite.

Crystallization-
 
 
 

Types of Sedimentary Rocks

1. Clastic 

2. (Chemical) Organic

3. Organic Chemical

4. Bioclastic

Note:   Sedimentary rocks may also be classified as Clastic and Biochemical. Also note that some rocks can belong to several catergories.

1. Clastic (Detrital) Sedimentary Rocks -  are formed from fragments of pre-existing rocks

a. Sedimentary Breccia - is a coarse grained sedimentary rock formed by cementation of coarse, angular fragments of rubbles

b. Conglomerate - did not travel far but further than those particles that make up breccia.

c. Sandstone - shows variety in mineral composition, degree of sorting, and degree of rounding.

i. Quartz sandstone (sst). is made up of  > 90% is Quartz

ii. Arkose sandstone (sst). is made up of > 25% feldspar (i.e. feldspar with lithic sandstone rock) fragment rich with a lot of matrix

iii. Graywacke 'gray-whacking'  is made up of > 15% of the rocks vol. consisting of fine grained matrix usually deposited by turbidity currents. This type of rock may sometime be called dirty sandstone.
 

Fine-grained rocks

d. Shale - has fissibility laminations made from silt/clay deposited at lake bottoms, deltas, and quiet water.

e. Siltstone

f. Mudstone - similar to shale but is not fossil

Note: 70% of all clastic sedimentary rocks fall under the fine grain
 

2. Carbonate Rocks

Limestone - usually calcite CaCO_{3
Types of carbonate rocks}

a. Coquina - formed from conked shells

b. Chalk - light colored porous very fine grained

c. Oolitic limestone - sound size oolites (or ooids) small sphere of calcite inorganically precipitated in warm, shallow sea.

Note: Recrystallization can take place replacing clastic texture & fossils of rock with calcite

d. Dolomite or dolostone-- Mg² + 2CaCO₃ CaMg(CO₃)₂+ Ca²
 

(Other Sedimentary Rocks)

Chert - a hard, compact, fine-grained sedimentary rock formed entirely by silica. Principal forms - Irregular lumpy and layered. Note: the origin is usually doubtful - organic, inorganic

Flint - black in color

Jasper - red/brown
 

Evaporites - Rocks formed from crystals that precipitate during evaporation

Rock gypsum - CaSO₄ * 2H₂O

Rock salt - halite (NaCl)

Borates, K salts and magnesium salts
 

Coal -
Peat,
Coal from plant remains  

 

Chemical

a. Inorganic..calcite (limestone)

Halite

Dolomite

Chert

b. Organic... Calcite

Chert

Coal

Calcite CaCO₃,
Dolomite CaMg(CO₃)₂Carbonates

Gypsum CaSO₄.2H₂ O,

 

Biochemical (Organic/Bioclastic)

Calcium carbonate (shells)...limestones...chalk, coquina, etc are also carbonate rocks

Altered microscopic shells of silica dioxide (SiO₂), chert, coal from plants .

Note: Some rocks were listed more than once in different categories...this will depend on the classification used. If in doubt, check with your instructor or consult your textbook.

 
 
 
 
 

Sedimentary Structures

Features found in sedimentary rocks

1. Bedding - contains visible layers. The Law of original horizontality is especially important for water laid deposits.

a. Horizontal bedding plane is a surface separating two layers of rock. This is caused by changes in size/composition or pause during deposition

b. Cross-bedding

c. Graded bedding

2. Mud Crack - polygonal cracks that form in mud when the sediment shrinks as it dries.

3. Ripple Marks - small, nearly parallel ridges and troughs formed in sand by using H₂O or wind.

Symmetric ripple marks are caused by waves.

4. Fossils - trace or remains of plants/animals
    bioturbation  is the mixing of sediments by animals (usually lake/river/ocean bottom dwellers.
 

Interpretation of Sedimentary Rocks

I Rock Unit....

1. Size

2. Shape

3. Composition

4. Sedimentary structure
 

II. Source area - rock type distance and direction to the source area. Structures could help eg paleocurrents.
 

2. Environment of Depositions

a. Glacial environments e.g. till - unsorted mix of unweathered boulder, cobbles, pebbles, silt, sand, clay

b. Alluvial fan - coarse cross bedded lens like channel deposit

c. River channel and flood plain

d. Delta, sub environments

e. Lake - thin bedded shale - fossil?

f. Beach, barrier island, dune

g. Lagoon - semi enclosed, quiet body of water between barrier island and mainland. Fine grained dark shale cut by tidal channels of coarse sand, containing fossils (oysters)

h. Shallow marine environments, tidal flats of fine grained, sandstone, silt with ripple marks.

i. Reefs - massive limestone

j. Deep marine environments-shale, graywacke, siltstone, graded bedding & current ripple