Visualizing structures

An introduction to the geometry of geological structures in the field

Goals

• To familiarize you with the 3-dimensionality of geological structures in the field

• Provide you with simple rules to use in geologic mapping of structures

3D spaces to deal with

• Topographic

• Depositional/intrusive

• Deformational

They all intersect

Shaded

Non-shaded

Any structures will intersect an irregular topographyTry picturing how will a horizontal layer cut this relief

Horizontal plane - follows topo contours

• Let us now put the real topography aside and step into the realm of “primary” geologic structures - layering of sedimentary units, intrusions, viewed in an idealized 3D framework

Complicating the issue a little bit…

Unconformity, what is it?

Possible scenario: Deposition, erosion, deformation, new deposition

The way this would look in a canyon near Page, AZ

Igneous intrusions

Structures on block diagrams

• Let us know try to drape structures onto topography and come up with some simple rules for the way they will plot on maps.

To keep things simple for now, I will deal mainly with planes and not with lines.

So what can a plane be? A bed, a fault, a dike, a sill an unconformity, anything that is planar.

The simplest scenario: a horizontal bed

Horizontal beds (a’la water line)

Some shallow tilt of a plane

Rule of v’s

An example

• So then what is a geological map?

An overlay of geologic structures (e.g. like the planes shown here on the 2D topographic map). Let’s take a look.

QuickTime™ and aPhoto - JPEG decompressor

are needed to see this picture.

Tucson Mts.

Cross-sections

Remember

• The 3 spaces you deal with: topo, primary structures and deformational (secondary) structures);

• Rule of v’s, overall topography/structure intersections of any planes

• A structure looks very different depending from where you look at it!