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 topography Try 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.

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!