Recognizing and Describing Deformed Rocks

A geologist must be able to visualize rock structures in 3D.
Commonly, structures are only exposed in a few outcrops located far apart.
Need careful examination of the rocks, measuring their orientation, and plotting these data on a map.

What to Look For:
1) Stratification:

Sedimentary rocks originally deposited in continuous horizontal (or near-horizontal) strata (layers).
If the strata are not broken and are still horizontal, they are not deformed.
If the strata are broken and are not horizontal, they have been deformed.

2) Contacts:
Contacts = surfaces separating adjacent bodies of rock of different types or ages.
Understanding the nature of the contact is important, because the contact records an “event”.
Depositional Contacts: separate older bodies from younger sedimentary rocks deposited upon them. In a conformable (continuously deposited) sequence of sedimentary rocks, the contacts represent bedding planes.
Ancient erosional surfaces = unconformable contacts.
Intrusive Contact – separates intrusive igneous rocks from the rock that has been intruded. These can be concordant (e.g., a sill) or discordant (e.g., a dike) with the structure of the country rocks.
Fault Contact – rocks are separated by a fault. However, faults are normally expressed as zones, rather than a plane, in which the rocks are brecciated and pulverized such that the fault orientation may be difficult to recognize.