Hello, I am back...haven't posted a question for a long time...

1. http://www.earthsci.org/teacher/basicgeol/geotim/geohist.gif

"The Eocene rocks are older than the 57 m.y. old dike and younger than the 36 m.y. old dike that cuts through them."

http://www.earthsci.org/teacher/basicgeol/geotim/geotim.html

WHY? I don't get the quoted part from the web site above, can someone explain?



2) For the law of crossingcutting relationships, it states that an intrustion or a fault it younger than the layers it cuts through. However, does this law also implies that the fault or intrusion is older than all the layers that it doesn't cut through? (This is the main thing that drew me into confusion)

So, say if a fault is cut off below an unconformity, how can you use this law alone to tell the relative age of the fault and of the rock layers that were deposited above the unconformity?

I would say that the fault is younger than all layers below the unconformity, but what else can I say based on the above record? How about the age of the layers above the unconformity in relation to the age of the fault, can I really tell?

I really need some explanations to clear up these concepts! Thanks! :)

You are absolutely correct, the authors of that web site got it backwards. It should read:

"The Eocene rocks are younger than the 57 m.y. old dike and older than the 36 m.y. old dike that cuts through them."

As to your question about the fault/unconformity- you are once again correct. Rocks below the unconformity are older than the fault, rocks above the unconformity are younger than the fault- i.e. the fault movement occured before the unconformity.

For all the rock layers the a fault or intrusion doesn't cut through, would all these layers always be younger than the fault or intrusion?

not necessarily-

An example might be a thrust fault displacing older rocks on top of younger rocks- the fault may or may not cut through the older rock layers.

Um...so for the question that I have posted:
"A fault is cut off below an unconformity, use the law of crosscutting relationships to determine the relative age of the fault and of the rock layers that were deposited above the unconformity?"

How do you know or how can you tell that the "rocks above the unconformity are younger than the fault"? Can you explain a bit, please? Is this applying the law of crosscuttin relationships?

Um...so for the question that I have posted:
"A fault is cut off below an unconformity, use the law of crosscutting relationships to determine the relative age of the fault and of the rock layers that were deposited above the unconformity?"

How do you know or how can you tell that the "rocks above the unconformity are younger than the fault"? Can you explain a bit, please? Is this applying the law of crosscuttin relationships?

If the fault is cut by an unconformity (often written as u/c) then the fault is older than the u/c. The rocks above the u/c have to be younger than it, hence they are younger than the fault. This is applying the laws of cross-cutting relationships.

Few caveats:
o Assumes faults are infinite lines of displacment and are used only once. This is not true as faults have a "start" and an "end" and are often reactivated.
o Assumes the fault is cut by the u/c, not stopped by it. The fault could have moved after the youngest rocks, but not had sufficient displacement to "breach" the u/c

Jon

"the fault is cut by an unconformity"

Doesn't "cut" mean separating into 2 pieces? For example, if a fault cut through rock layers A,B, and C, these layers are cut into 2 separate parts.

The fault in my question is not really cut by an unconformity...would the law of crosscutting relationships still work here, then?

Better wording might be:
"the fault is truncated by an unconformity", meaning the fault ends at the unconformity surface- which indicates the fault occured before the unconformity.
The law of crosscutting relationships still applies