Shales and Sandy Shales with Limestone Nodules (2XX and 4XX)

If a rock has light-colored limestone masses or pebble-like nodules set in a matrix of fine-grained material that does not effervesce, then it is classified as a shale or sandy shale with limestone nodules. If the matrix material is smooth to the touch, it is a shale. If it is distinctly gritty or has visible sandy material, then it’s a sandy shale. The first digit refers to the major category and can be categorized as:
- 2XX Shale with limestone nodules
- 4XX Sandy shale with limestone nodules
The color of the matrix material is used to define the second digit, using the same criteria as that for shales and sandy shales:
- X2X Dark gray
- X3X Light gray/light gray green
- X4X Green
- X5X Red and green
- X6X Red
For the last digit, look for the presence of root structures or fossil shells, similar to the classification used for fine-grained siliclastic rocks.
The above properties combine to form to the six categories of this class:
- 2X4 Massive shales with limestone nodules
- 2X7 Fireclays with limestone nodules
- 2X9 Fossiliferous shales with limestone nodules
- 4X4 Massive sandy shales with limestone nodules
- 4X7 Sandy fireclays with limestone nodules
- 4X9 Fossiliferous shales with limestone nodules
- Back to Limestone