Structural Geology, part 3, geologic structures: FAULTS

(Under construction)

A wedge or rock stands out in this otherwise continuous sequence of marly sandstones.

These are Miocene turbidites from the Northern Apennines of Italy.
The facies turns from sandy at the bottom to more shaly towards the top.

By tracing a few contact surfaces between layers (in black)
it is possible to observe where they are interrupted.

Since the layers are offset, the interruption points mark the presence of faults (and not a simple joint).
Faults are drawn in red, with half-arrows indicating the direction of motion.

The fault to the left is a normal fault: the hanging wall is moving downward with respect to the footwall.
The fault to the right is a reverse fault: the hanging wall is moving upward with respect to the footwall.

The reverse fault does not cut the shaly portion of the outcrop, creating just a little kink in its lowermost part.
The normal fault instead cuts the whole section. That means that when reverse faulting occurred,
the marly section did not exist, or had barely started to deposit.
When instead normal faulting occurred, the whole sections was already deposited, because all of the layers are cut.

As a consequence, the normal fault is younger (occurred afterwards) than the reverse fault.

In this and the next image, an attempt to reconstruct the original, uninterrupted stratigraphy is made.
Based on what we said in the previous figure, the order of events is as follows (from older to younger):

first, the sequence is deposited, without disturbance, up to the top of the sand section
then, the reverse fault breaks the sequence
the shaly part covers the broken sequence
eventually, the normal fault breaks both the sand and the shaly parts

With this knowledge, and with the help of Adobe Photoshop© and Adobe Illustrator© softwares,
at first the left sequence is matched with the respective layers along the normal fault.

Finally, the last block is matched with the previous, rendering an image of an undisturbed sequence.