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.