Physical Geology 101
Sedimentary Rocks


Sedimentary Rocks

Sedimentary Rocks are rocks formed from sediment or organic deposits

Classification of Sediment

Clastic - broken fragments of pre-existing rocks or minerals
Chemical - sediments which precipitate from solution
Biologic - sediment produced by biologic activity

Clastic Sedimentary Rocks

Clastic sediments are distinguished by size

Sediment Clast Size Rock Type
clay < 1/256 mm Claystone*
silt 1/256 - 1/16 mm Siltstone*
* - mudstone, shale
sand 1/16 - 2 mm Sandstone
gravel > 2 mm Conglomerate
Breccia

Clast Characteristics

Individual clast characteristics
Roundness
- well rounded - indicates long distance of transportation
- angular - indicates short distance of transportation
Sphericity
- elongated - one dimension is longer than the other
- equidimensional - all dimensions are roughly equal

Clast Composition

Sandstones are classified based on the clast size and clast composition
Quartz Sandstone - quartz sand grains
Lithic Sandstone - rock fragment sand grains
Arkose - sandstone or conglomerate composed dominantly of feldspar grains typically has a dark reddish color

Chemical Sedimentary Rocks

Based on chemical/mineral composition
Formed by chemical precipitation from an aqueous solution
Limestone - composed of calcite
Dolostone - composed of dolomite
Chert - composed of cryptocrystalline quartz
Evaporites - formed from the evaporation of an aqueous solution
- Rock Salt - halite
- Rock Gypsum - gypsum

Biologic Sedimentary Rocks

Formed from biologic precipitation or accumulation of organic material
Limestone - coral, shells, skeletons, calcite
Coquina - composed entirely of shell or skeleton fragments
Peat - plant fragments, loosely compacted
Coal - plant fragments, densely compacted

Sedimentary Rocks

Clastic Chemical Biologic
Claystone Limestone Limestone
Siltstone Dolostone Fossiliferous Ls
Shale Chert Chert
Mudstone Rock Salt Coquina
Sandstone Rock Gypsum Chalk
Conglomerate   Coal
Breccia    
Arkose    

Formation of Sedimentary Rocks

Sedimentary Cycle

Environments of Deposition

The environment or setting where sediments are deposited
The type of environment of deposition will influence the types of sediments produced and deposited
Present day environments and sediments
Continental - land based environments
Transitional - located on continental margins - strong interaction with water
Marine - oceanic conditions prevail

Continental Environments

• Fluvial Environments - Streams
Alluvial Fans
Dunes
Glacial
• Swamps (Swamp Example 1, Swamp Example 2)
Lakes

Transitional Environments

Deltas
Beaches - Barrier Islands
• Tidal Flats - Sahbkas

Marine Environments

• Shelf - Platform
Reef
• Deep Marine
• Restricted Marine

Sedimentary Structures

Structures which develop from sediment deposition due to currents or environmental conditions.
Sorting - Measure of the range of grain sizes present
- Well sorted - all grains are of similar size
- Poorly sorted - wide range of grain sizes present
Bedding - Sediment deposited in layers
Cross-bedding - sediment deposited in inclined layers - results from a change in current direction.
Ripplemarks - small dune-forms, (another example of ripplemarks in stone)
Paleocurrents - ancient currents - current direction is determined by sedimentary structures.
Flutes - scour marks on stream bed.
Mudcracks - fine grained sediment shrinks when it dries, producing cracks
- indicates sediment that was wet at one time but then dried.
Mudcracks Example 1
Mudcracks Example 2
Salt casts
Rain casts
Fossils - remains of ancient life - strong indicators of ancient environments

Sedimentary Facies

Packages of rocks
Environments of deposition produce more than one type of rock.
Total field aspect of rock layers:
- spatial distribution
- lithology
- sedimentary structures
- environment of deposition

Facies Sequence

- change from one facies to another
- reflects a change in environment

Transgressive Facies Sequence


Sand is deposited on the beach
Silt is deposited just off shore
Calcite is deposited further off shore

Sea level rise
Resulting facies sequence is:
•   Limestone (on top)
•   Shale
•   Sandstone (on bottom)

Regressive Facies Sequence

Sea level fall
Resulting facies sequence is:
•   Sandstone (on top)
•   Shale
•   Limestone (on bottom)