A geologic fault is a fracture or zone of fractures between two
blocks of rock. Faults allow the blocks to move relative to each
other. This movement may occur rapidly, in the form of an
earthquake - or may occur slowly, in the form of creep. Faults may
range in length from a few feet to thousands of miles. Most faults
produce repeated displacements over geologic time.
This site will introduce you to a local fault, known as the
Eagle River Thrust Fault, which runs from the vicinity of
Eagle River to the Kenai Peninsula crossing Turnagain Arm at Falls
Creek near Indian.
This is an EarthCache - without
container or logbook - where you'll have to observe, document
through photographs, and report your findings via email to earn
credit for this cache. Logs which do not conform to these
requirements will be deleted, as this cache requires a demonstrated
acquisition of knowledge about the geologic forces at work. Read
all the requirements for completing this cache before you visit the
fault area, and equip yourself with the necessary tools (including
a camera).
The creation and behavior of faults, in both an individual small
fault and within the greater fault zones which define the tectonic
plates, is controlled by the relative motion of rocks on either
side of the fault surface. Because of friction and the rigidity of
the rock, the rocks cannot simply glide or flow past each other.
Rather, stress builds up in rocks and when it reaches a level that
exceeds the strain threshold, the accumulated potential energy is
released as strain which is focused into a plane along which
relative motion is accommodated — the fault. Strain is both
accumulative and instantaneous depending on the deformation of the
rock; the ductile (pliable) lower crust and mantle accumulates
deformation gradually via shearing whereas the brittle upper crust
reacts by fracture, or instantaneous stress release to cause motion
along the fault. A fault in ductile rocks can also release
instantaneously when the strain rate is too great.
The energy released by instantaneous strain release is the cause
of earthquakes, a common phenomenon along transform boundaries.
During an earthquake, the rock on one side of a fault suddenly
slips with respect to the other. The fault surface can be
horizontal or vertical or some arbitrary angle in between. Earth
scientists use the angle of the fault with respect to the surface
(known as the dip) and the direction of slip along the fault to
classify faults. Faults which move along the direction of the dip
plane are dip-slip faults and described as either normal
or reverse, depending on their motion. The dip of a reverse
fault is relatively steep, greater than 45°. 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. Faults which move horizontally are known as
strike-slip faults and are classified as either
right-lateral or left-lateral. Faults which show both
dip-slip and strike-slip motion are known as oblique-slip
faults. See great animations of the types of faults.
The coordinates take you to the vicinity of the Eagle River Thrust
Fault. On your way from Anchorage or parts South, observe the
structure of the rock walls along the highway. As you travel from
Anchorage you are observing rock that belongs to the McHugh Complex
of the Chugach Terrane. As you travel North from Girdwood, you are
observing rock from the Valdez Group of the Chugach Terrane. The
Eagle River Thrust Fault separates the two. The simplest definition
of a terrane is a series of related rock formations or an
area having a preponderance of a particular rock or rock groups. As
you drive East from Falls Creek, you can observe more of the fault
for the next mile.
Park at the coordinates and walk to the East about 600 feet
until you reach a rock wall at the waypoint coordinates.
CAUTION: THIS IS A VERY BUSY HIGHWAY! Walk along
the gravel shoulder. Also be careful for falling rock. Do not cross
the highway!
Observe the structure and characteristics of the rock. Note the
small patches of distinctive brown rock with striations. These are
known as slickensides. The fault movement gouges the rock
causing striations in the direction of movement. Slickensides feel
smooth in the direction of movement and rough and hackly opposite
the fault movement.
To demonstrate your acquisition of
knowledge about the fault, please answer these questions briefly in
an email to me through the geocaching.com website.
Post a photo in your log of yourself at the
parking area or the footwall, but don't post your answers in
your log. Report in the online log how many were in
your party during your visit. Logs not conforming to these requirements will be
deleted.
1. Describe the appearance of the rock at the waypoint in terms
of texture, form and any other characteristics.
2. In what direction does it appear the thrust occurred?
3. How does this rock formation differ from that at McHugh or
Beluga Point?
4. What direction was the rock movement as indicated by the
slickensides?
For extra credit, tell me what kind of rock the slickensides is.
The above information was compiled
from the following sources:
Cathy Connor & Daniel O’Haire, “Roadside Geology of Alaska,”
1988 Missoula, MT
Timothy M Kusky, Dwight C. Bradley and Peter Haeussler,
Progressive Deformation of the Chugach, accretionary complex,
Alaska, during a paleogene ridge-trench encounter.
Susan M Karl, Nancy R Vaughn, Timothy J. Ryherd, Ed., “1997 Guide
to the Geology of the Kenai Peninsula, Alaska, 1997 Anchorage,
AK
Susan Bartsch-Winkler and Henry Schmoll, “Guide to Late
Pleistocene and Holocene Deposits of Turnagain Arm”,1984 Anchorage,
AK
Dwight Bradley and Marti Miller, “Field Guide to South-Central
Alaska’s Accretionary Complex, Anchorage to Seward, 2006 Anchorage,
AK
L.M. Dilley & T. Dilley, "Guidebook to Geology of Anchorage,
Alaska", 2000 (1st edition) Anchorage, AK
The Geological Society of America (GSA)
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