Mudstone and the Dunham classificaiton
What questions do we need to ask to figure out if the sample is mudstone?
- Is the drill cutting calcium carbonate?
- Is the depositional texture recognizable?
- Were the original components not bound during deposition?
- Dose it contain mud?
- Is it mud-supported?
- Is there less than 10% grains?
Let’s look at the definitions.
Mud, what is it? When you talk about mud you are talking about grain size. Specifically, mud is composed of silt-sized particles that are between 0.031 mm to 0.004 mm in diameter, and clay-sized particles which are less than 0.004 mm. Note how it says silt and clay “sized” particles not silt and clay “minerals”.
The differences between Mudrock (Clay mineral) and Mudstone (Calcium Carbonate).
When describing drill cuttings, it can be easy to get terminology mixed up especially when you are dealing with two different rock types that have some similarities in the naming process.
1.Mudrock, which contains mostly clay minerals, quartz, and feldspars made up of clay “sized” particles. If we see a silicate-based rock that is made predominantly of mud sized particles of clay minerals, quartz, and feldspars we are going to call it shale or clay.
2. Mudstone, Is the Dunham classifications name for a type of limestone, a mudstone is defined as a carbonate rock that is mud-supported with the mud “sized” particles being predominately calcium carbonate and containing less than 10% grains. When we see a lime-based rock that is made predominantly of mud sized particles of carbonate material we are going to call it Mudstone.
There is some crossover between these terms so we will all use this method to identify mudstone for consistency.