Largest to Smallest
1. Supergroup: This is the largest formal unit, encompassing multiple groups of related rocks. It's often used for very large, regional-scale units of rock.
2. Group: A group consists of multiple formations that share some common characteristics or depositional history.
3. Formation: This is the basic building block of the sedimentary rock record. A formation is a mappable body of rock that has distinct characteristics (mineralogy, texture, fossils) that set it apart from rocks above and below.
4. Member: A member is a distinctive part of a formation, usually with a specific lithology (rock type) or a unique fossil assemblage.
5. Bed: The smallest formal unit, a bed is a single layer of rock that is distinct from the layers above and below. Beds can be very thin or quite thick.
Important Note: While these are the standard units, there are other informal terms used to describe sedimentary rock units, such as:
* Lithosome: A body of rock with a specific lithology (rock type), regardless of its thickness or extent.
* Facies: A rock unit that represents a specific depositional environment, characterized by its lithology, fossils, and sedimentary structures.
Let me know if you'd like more detail on any of these terms!
