Deformed Structures

Rocks that show evidence of deformation that has modified the original bedding and structure of the lithology are not uncommon. The deformation may have resulted from either soft-sediment deformation or brittle deformation.

The most common soft-sediment structures are called mudflows. They are the result of originally deposted sediment being resuspended and moved downslope in a convoluted mass. The result is a mixture of grain sizes showing swirled masses of material, like a marble cake.

Mudflows are classified according to their grain size:

  • 017 Shale mudflow
  • 018 Sandy shale mudflow
  • 019 Sandstone mudflow
Example of a sandy shale mudflow in core (018).
Example of a sandy shale mudflow in core (018).

The second type of soft-sediment structure is called a slump. Slumped deposits result from the collapse of a stream bank into the adjacent channel. The unconsolidated sediment rotates along a glide plane until the mass stabilizes. Original bedding usually stays intact, but may be deformed by the movement, especially at the toe of the slide. In core, slumps are identified by inclined bedding, usually within heterogeneous lithologies, that is overlain and underlain by normally bedded rocks. Bed dip may increase downward across many feet in core through a slump. Slumps normally have a slickensided surface at the base of the deposit, as well as throughout the rotated mass. Shales in slumps are commonly slickensided. Slickensides are also abundant toward the toe of a slump mass where beds are commonly highly deformed. Small brittle deformation and small offset faults are also common in core through slumps. To learn more about slumps in mine roofs see paleoslumps.

Slumps in core are classified by their dominant grain size:

  • 013 Shale slump
  • 014 Sandy shale slump
  • 015 Sandstone slump
Example of a sandy shale slump in core (014).
Example of a sandy shale slump in core (014).

Brittle deformation occurs when lithified rocks are crushed or folded as a result of fault movement. Brittle deformation consists of generally sharp offsets and blocky deformation, whereas soft-sediment deformation consists of folding and flowing. In some cases, however, distinguishing brittle from soft-sediment deformation may be difficult, and geologic context may be required to evaluate this lithology. Small, brittle offsets are observable in core. Large-scale brittle deformation like folding and fault offset are more difficult to discern.

When tectonic faults are crossed in core, the cored interval commonly exhibits a fault gouge, formed by movement and crushing of rock masses. Fault gouge may show (1) brecciation similar to the breccias and pseudo-breccias formed in flint clays, but near faults, and occuring in non-clay rock, (2) pebbly appearance like a conglomerate, but associated with deformed rocks and common slickensides, or (3) have structures resembling the soft-sediment structures described above. Core through strata near faults may also have consistently higher bed dips than core through the same strata away from a fault. If tens of feet of core show similar moderate to high dip, there may be a nearby fault.

  • 010 Faulted rock
Example of a fault gouge in core (010). This specimen was taken from a core that was known to have intersected a fault plane.
Example of a fault gouge in core (010). This specimen was taken from a core that was known to have intersected a fault plane.

To learn more about faulting in mine roofs go to Faulting in discussion of discontinuities and mine roofs at this website.


Last Modified on 2023-01-05
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