It’s Waukesha’s
Fault!
This is an EarthCache! To log an EarthCache as a find there are
special tasks which must be undertaken. To log this cache you will
need to email me the answers to the questions listed. Failure to
email me through my profile may result in the deletion of your log.
Do not take photos of the quarry as there is no safe and
legal parking which allows a view of the quarry; I will delete the
log of anyone trespassing on, or posting a photo from the quarry
owned property unless they can prove they were given permission to
be there.
Geologic faults are
rare in Wisconsin but one exists right here in Waukesha county. In
Wisconsin, this “Waukesha Fault” extends approximately
80 miles from the Illinois border to approximately 10 miles north
of Milwaukee where the fault meets Lake Michigan. The only
significant surface exposure of this fault is about 24 feet long
and located at the Waukesha Stone and Lime Quarry just a few feet
east of ground Zero (NO TRESPASSING...not even in the employee
parking area).
So what is a fault? A fault or fault line is a planar (flat)
fracture in rock in which the rock on one side of the fracture has
moved with respect to the rock on the other side. The sites of
these fractures are often the locations of earthquakes due to the
friction created (and subsequently rapidly released) when the two
surfaces move independently of one another. The term “fault
zone” is often used to describe a fault as faults do not
usually consist of a single, clean fracture.
So how do we recognize a fault? Unlike the mountain states, the
bedrock formations in Wisconsin are relatively flat lying which
makes faults difficult to see. Bedrock outcroppings provide the
best verification of layer offset, but much of Wisconsin lies under
glacial deposit making viewing of the bedrock difficult.
There are three general categories of faults which are
determined by a fault’s “slip”. A
“slip” is defined as the relative movement of
geological features on either side of a fault plane. The sense of
slip basically defines the type of fault.
• Normal faults form when the hanging wall drops down. The
forces that create normal faults are pulling the sides apart, or
extensional.
• Reverse or “thrust” faults form when the hanging
wall moves up. The forces creating reverse faults are
compressional, pushing the sides together. A thrust fault has the
same sense of motion as a reverse fault, but with the dip of the
fault plane at less than 45°. Thrust faults typically form
ramps, flats and fault-bend (hanging wall and foot wall)
folds.
Together, normal and reverse faults are called dip-slip faults,
because the movement on them occurs along the dip
direction—either down or up, respectively.
• Strike-slip faults have walls that move sideways, not up
or down. That is, the slip occurs along the strike, not up or down
the dip. In these faults the fault plane is usually vertical, so
there is no hanging wall or footwall. The forces creating these
faults are lateral or horizontal, carrying the sides past each
other.
A fault which has a component of dip-slip and a
component of strike-slip is termed an oblique-slip fault. Nearly
all faults will have some component of both dip-slip and
strike-slip, so defining a fault as oblique requires both dip and
strike components to be measurable and significant.
So, what type of fault is here beneath your feet? Some studies
show the Waukesha Fault has a maximum vertical displacement to a
depth of about 600m and dipping at about 10° to 20° to the
southeast. Based on the visual evidence at the Waukesha quarry and
well tapings taken along the fault, the Waukesha Fault appears to
be a normal fault (as pictured below)…
…however, recent gravity surveys suggest
that this fault may be a Listric fault. Listric faults can be
defined as being a type of curved normal fault in which the fault
surface is concave upwards; its dip decreases with depth.
 |
Looking at the gravity data (on
the left) which serves to help identify underground features,
you’ll notice that the fault runs in a line past Waukesha to
Port Washington. One might wonder if this is a reason why there are
so many mining operations to the east of this line in Waukesha,
Sussex, Lannon, Menomonee Falls, Germantown, Cedarburg, Grafton and
in Port Washington. Do you think this is a coincidence? |
Logging Requirements:
Email me the answers to the following
questions.
1. Looking at the sectional diagram above
(showing the Precambrian bedrock & fault location), why do you
think there are so many quarries east of the fault?
2. From the page text, tell me what kind of fault
you’d have if the fault was both “Normal” and
“Strike-slip”.
3. Click on the image of the mined wall taken at the
Waukesha Stone and Lime Quarry* (below):
a. Are the stone layers laid predominately
horizontally?
b. Are there obvious signs of a fault in the image?
4.Elevation changes give us an idea of the movement of a
fault, the elevation of the quarry at ground level is 710 feet.
Now, as something must be measured in all EarthCache listings as
proof of your visit, take an elevation reading at the posted
coordinates and answer the following:
a. How many feet higher are you here than the quarry at
ground level?
b. Based on the cache page as well as your observations, do
you think the ridge you are on was created by the fault or by some
other process?
*The quarry is private property and thus the only
significant fault exposure is out of view and off limits to the
general public. Do not enter the quarry property.
Resources:
http://geology.com/news/images/listric-fault.gif
http://www.uwex.edu/wgnhs/pdfs/geoscipdf/gs16sver5.pdf
https://pantherfile.uwm.edu/dyanna/www/ILSG2007.html
http://wri.wisc.edu/Downloads/Projects/Final_WR03R003.pdf
http://www.wgwa.org/articles/faultsinwi.pdf
http://www.uwex.edu/wgnhs/pdfs/geoscipdf/gs16front.pdf
http://earthquake.usgs.gov/earthquakes/states/wisconsin/history.php
http://www.ukescc.co.uk/subjects/engineering/engineering.html
http://geology.about.com/library/bl/blnutshell_fault-type.htm