A. Relative Age Determination (relative timing):
1. Law of Superposition: In a series of sedimentary layers, the bottom layer is the oldest.
*Assumes the layers have not been overturned by plate tectonic activity.
2. Cross Cutting Relationships: younger events (faults, igneous intrusions, erosion, metamorphism) "cut" or are superimposed on something. That something had to be there to be "cross cut" or modified.
The lowermost sedimentary layer is the oldest (Law of Superposition ), so the relative timing of events shown would be deposition of layers #1-3. Then these layers are cut by a fault (Cross-cutting relationship -- the layers had to already be there to be cut). The faulting event would be #4.
See section 10.3 in your textbook (p. 279) and figures 10.1 and 10.2.
B. Interpreting Geologic Histories:
1, Use relative timing relationships (use basic information in rocks, superposition, and cross-cutting relationships) to work out general events.
2. We also use examples of processes and relationships observed in the modern world to help interpret the past.
This principle is called: uniformitarianism (sometimes also referred to actualism). This principle is summarized by: Understanding the present is key to understanding the past.
This is a simplified cartoon of the quarry at Redgranite, Wisconsin (see photos above) with relative timing of event interpretations (numbers 1-7, with #1 being the oldest event).
The baked contact metamorphic "halo" at the boundary between the granite intrusion and sedimentary rock layers 1-3 indicate that layers 1-3 were there to be baked (cross-cutting relationship). The bottom-most layer is the oldest (Law of Superposition).
There is a vertical sheet-like mafic intrusion (see photo above) cuts the granite; because it cuts the granite, it post dates the granite and because the granite baked layers 1-3, the mafic intrusion (#5) post dates layers 1-3 and the granite.
Sedimentary rock layer #6 contains inclusions of the granite and was not baked by the granite. This indicates that layer #6 post-date the granite.
We can't tell for sure that layer #6 came after the mafic intrusion because there are no cross-cutting relationships to constrain their timing. I drew the diagram with the assumption that there were inclusions of the mafic intrusion in layer #6, but as shown, there is no definitive answer.
The quarry operation cross-cut both the granite and the mafic intrusion, so we can say for sure that the quarry came last (#7).
|Back to 102 Syllabus||Back to Physical Geology Page||Eric Hiatt's Homepage|