1. Grain Size:
* Conglomerate: Contains rounded clasts larger than 2 mm.
* Breccia: Contains angular clasts larger than 2 mm.
* Sandstone: Contains clasts between 2 mm and 1/16 mm.
* Siltstone: Contains clasts between 1/16 mm and 1/256 mm.
* Mudstone/Shale: Contains clasts smaller than 1/256 mm.
2. Composition:
* Quartz sandstone: Dominated by quartz grains.
* Arkose sandstone: Contains significant feldspar grains (usually derived from granite).
* Greywacke sandstone: Contains a mixture of quartz, feldspar, and rock fragments, often with a muddy matrix.
* Lithic sandstone: Dominated by rock fragments.
3. Cement:
* Calcareous cement: Cementation by calcite.
* Siliceous cement: Cementation by silica.
* Ferruginous cement: Cementation by iron oxides.
* Clayey cement: Cementation by clay minerals.
4. Texture:
* Well-sorted: Grains are similar in size.
* Poorly-sorted: Grains vary significantly in size.
* Rounded: Grains have smooth, rounded edges.
* Angular: Grains have sharp, angular edges.
* Matrix-supported: Grains are surrounded by a fine-grained matrix.
* Clast-supported: Grains are in direct contact with each other.
5. Other Factors:
* Fossils: Presence of fossils can be helpful in identifying the environment of deposition.
* Color: Color can indicate the mineral composition or weathering processes.
* Structure: Bedding planes, cross-bedding, and ripple marks can reveal information about the depositional environment.
Here's a table summarizing the key differences:
| Rock Type | Grain Size | Composition | Cement | Texture |
|---|---|---|---|---|
| Conglomerate | > 2 mm | Various | Calcareous, siliceous, ferruginous, clayey | Well-sorted, rounded, clast-supported |
| Breccia | > 2 mm | Various | Calcareous, siliceous, ferruginous, clayey | Poorly-sorted, angular, clast-supported |
| Sandstone | 2 mm - 1/16 mm | Quartz, feldspar, rock fragments | Calcareous, siliceous, ferruginous, clayey | Well-sorted, rounded, clast-supported |
| Siltstone | 1/16 mm - 1/256 mm | Quartz, feldspar, clay minerals | Calcareous, siliceous, ferruginous, clayey | Well-sorted, rounded, clast-supported |
| Mudstone/Shale | < 1/256 mm | Clay minerals, quartz, feldspar | Calcareous, siliceous, ferruginous, clayey | Poorly-sorted, angular, matrix-supported |
Important note: There are many variations within each category, and some rocks can be difficult to classify definitively. It's important to consider multiple factors when identifying clastic sedimentary rocks.
