Sedimentary Rocks are rocks formed from sediment or organic deposits
Clastic - broken fragments of pre-existing rocks or minerals
Chemical - sediments which precipitate from solution
Biologic - sediment produced by biologic activity
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 |
Characteristics of the individual clasts
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
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
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
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
| 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• |
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
• Fluvial Environments - Streams
• Alluvial Fans
• Dunes
• Glacial
• Swamps (Swamp Example 1, Swamp Example 2)
• Lakes
• Deltas
• Beaches - Barrier Islands
• Tidal Flats - Sahbkas
• Shelf - Platform
• Reef
• Deep Marine
• Restricted Marine
Structures which develop from the deposition of sediment 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 the bedding, cross-bedding and other sedimentary structures.
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•
Fossils - remains of ancient life - strong indicators of ancient environments
Given environments of deposition can result in more than one type of sediment or structure being deposited.
Sedimentary Facies are packages of rocks - based on the total field aspect of the rock layers
Based on:
- spatial distribution
- lithology
- sedimentary structures
- environment of deposition
- change from one facies to another
- reflects a change in environment
Sea level rise
Sand is deposited on the beach
Silt is deposited just off shore
Calcite is deposited further off shore
Resulting facies sequence is:
• Limestone
• Shale
• Sandstone
Sea level fall
Calcite is deposited further off shore
Silt is deposited just off shore
Sand is deposited on the beach
Resulting facies sequence is:
• Sandstone
• Shale
• Limestone
Sedimentary Rocks Study Guide
