Silurian Fault EarthCache
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This series of earth caches is based on the publication “Roadside
Geology Along the Alexandria to Ashland (AA) Highway.” The road
logs were published by the Kentucky Geological Survey to give the
public an appreciation of the geologic world around them.
Many geologists have referred to the AA Highway as a “treasure
trove” and “an outdoor classroom” in which to study diverse and
significant geologic features. Buckle your seat belts and head back
in time. Each cache in this series will stop at a unique geologic
formation and will seek answers to some basic questions that should
be easy to calculate. Sizeable pull off areas are available at each
stop in the series. Geology students frequent the locations
routinely. The calculations can be made from your car even, making
it handicap accessible!
During the Silurian Period some 417 to 443 million years ago, the
Earth underwent considerable changes that had important
repercussions for the environment and life within it. The Silurian
witnessed a relative stabilization of the earth's general climate,
ending the previous pattern of erratic climatic fluctuations. One
result of these changes was the melting of large glacial
formations. This contributed to a substantial rise in the levels of
the major seas. Coral reefs made their first appearance during this
time, and the Silurian was also a remarkable time in the evolution
of fishes. It is also in the Silurian that we find the first clear
evidence of life on land. Perhaps most striking of all biological
events in the Silurian was the evolution of vascular plants. A
Silurian sea floor here shows numerous corals. Crawling among the
coral are various kinds of gastropods.
During the Silurian , the Earth witnessed many changes in the way
in which landmasses were distributed around the globe. At that
time, the continents were distributed very differently than they
are today. There was no major volcanic activity during the
Silurian; however, the period is marked by major orogenic
(mountain-building) events in eastern North America and in
northwestern Europe, resulting in the formation of the mountain
chains there.
The Silurian Period strata is much less widely distributed than the
Devonian and Ordovician Period strata in Kentucky. Geologists say
this and other corroborative facts imply a widespread emergence of
land occurred 440 million years ago. As a result of this emergence,
the stratigraphic break between the Ordovician and the Silurian is
one of the greatest in the whole Paleozoic Era group. Silurian rock
is comprised of limestone, dolomite, sandstone and shale. In the
interior the thickness of the system is less than 1000 ft. in many
places, but in and near the Appalachian Mountains its thickness is
much greater, more than five times as great.
Economically, petroleum and natural gas have been the most
important resources obtained from the Silurian in Kentucky.
Carbonate rocks of Silurian age have been the reservoirs of two of
the largest oil pools in the State, the Big Sinking Pool in eastern
Kentucky and the Greensburg Pool in south-central Kentucky. The
Louisville Limestone and the Laurel Dolomite, which crop out on the
west side of the Cincinnati arch have been sources of stone for
aggregate, building stone, and agricultural limestone. Shale of the
Crab Orchard Formation offer possibility for structural clay
products. Hematite iron ore occurs in the upper part of the
Brassfield Formation.
The Silurian rocks crop out in Kentucky in narrow belts on the
east and west flanks of the Cincinnati arch and in small isolated
areas mostly in south-central Kentucky The upper contact is a
regional erosional unconformity, and on the crest of the Cincinnati
arch the entire Silurian section has been removed. The large
variation in thickness is caused by erosion along this
unconformity. The original thickness of the Silurian strata is not
known, as the upper part has been everywhere removed. The total
thickness of the Silurian within this outcrop belt ranges from 0 to
about 180 ft, as a result of erosion.
Three formations were recognized in the eastern outcrop belt in
Kentucky- (1) Brassfield Dolomite (Lower Silurian), (2) Crab
Orchard Formation (Lower and Middle Silurian), and (3) Bisher
Dolomite (Middle Silurian). On the State geologic map, the two
lower formations are combined as a single map unit (Scb), forming a
sequence with dolomite at the base, clay shale at the top, and a
thin gradational zone of interbedded dolomite and shale in
between.
Travel to Mile Marker 18.1 on the AA Highway in Lewis County where
an outcrop of rare Silurian Rock can be seen. Here the Bisher
Dolomite contacts the Crab Orchard Formation of Shale.
The Silurian Shale is subject to water erosion and faulting and
eventually landslide. Construction along hillsides underlain with
shale requires additional foundation support to prevent failure and
slide. Foundation instability provides slope failure. The picture
below is on old Ky 10 on Crab Orchard Formation fill. When
saturated with water the fill lost strength and slid downward.
A fault is present in the center of this road cut. All slopes
containing shale are subject to faulting and eventual slide. Slope
stability increases as water moves in and causes erosion and
movement of the material.
Gravity is the main driving force in the eventual fault or break by
adding weight and lubricating the weathering shale. Clay is known
to swell. Heat can also cause shale to shrink. Eventually this can
cause a heave to occur which prompts the fault and then downward
slide.
It would be helpful to bring a measuring device such as a tape
measure with you to this cache. You will need to make calculations
regarding the fault and estimates regarding the rock sequences
exposed.
Email the answers and take a picture of the fault. Include it with
your log of your visit to the Silurian Period in Kentucky.
1. How much Bisher Dolomite and Crab Orchard Formation are exposed
here if the exposure from bottom to top is roughly 150 feet?
a/ 75 feet of Bisher/75 feet of Crab Orchard
b/ 50 feet of Bisher/100 feet of Crab Orchard
c/ 30 feet of Bisher/120 feet of Crab Orchard
2. Measure the horizontal width of the base of the fault.
a/ 10 feet
b/ 20 feet
c/ 30 feet
3. What is the elevation of the rock exposure here?
The visit here reminds us that we live in a land of perpetual
change. This landslide is still moving and filling. The forces of
movement are never ending which makes geology such an exciting
study with new discoveries of change around every corner. In
Geology, the only constant thing is change!Now buckle up. It’s time
to head on down the AA Highway for another geocaching geologic
adventure!
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