From Farmers to Cowbells - The Borden Formation EarthCache
From Farmers to Cowbells - The Borden Formation
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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!
The Borden Formation represents a major series of rock in Kentucky.
The Mississippian Age formation is a proven economic treasure and
includes treasured Agates and Geodes in some locations. The Borden
Formation conformably overlies black shale of the Devonian age and
is conformably overlain by limestone of Late Mississippian age. The
Borden ranges from about 220 to 450 feet in thickness. The
formation is chiefly of Early Mississippian age, but it locally
includes some beds of Late Mississippian. The Nancy Member is a
nonresistant gray clay-like silt-like shale. The Cowbell Member is
siltstone. The Farmers Member is made up of alternating beds of
light-gray, very fine grained sandstone and gray shale. A
persistent bed of shale at the base of the formation is known as
the Henley Bed.
Farmers Member is thickest at the northern end of the outcrop belt
along the Ohio River in Lewis County, and thin to pinch-outs
southward and westward. Abrupt eastward pinch-outs of these beds
along the Ohio River in northeastern Lewis County may represent a
proximal margin of the turbidite sequence. This sequence of
siltstone and shale becomes coarser grained upward to the Nancy
Member at the base as it progresses up to the siltstone of the
Cowbell Member. The transition from finer to coarser grained is
gradational over the entire sequence, and in most cases boundaries
between members can be only approximately located. The lower
contact with black shale or sandstone of the Farmers Member of the
Borden Formation is generally sharp. Fine-grained detrital rocks of
the Borden Formation and equivalent units are widespread in the
east-central United States. In Kentucky they crop out from the
northeastern corner of the State, along the Ohio River where the
formation is about 500 ft thick.
The principal outcrop area of the Farmers Member of the Borden
Formation is in Rowan and Lewis Counties in northeastern Kentucky.
Here the Borden delta-marine shelf stood higher than the basin
plain to the southwest. The succession of rock here is a super
sequence composed of two depositional sequences which correlate
with global sea-level cycles. The low stand system tract on the
basin floor consists of green shale and mounds created during sea
transgression. The high stand tract consists of marine quartz
sandstone and shale, pre-tidal carbonates and high-energy grain
stone banks with a deep ramp and slope comprised of clastic units
associated with a drainage system off the Borden shelf. A third
sequence was created in a transgressing sea level depositional
environment of planer sand bodies.
The formation was created in an deltaic zone, described as a Borden
deltaic front or shelf. Lateral and vertical facies changes
characterize the fine-grained rocks of the Borden Formation, as
shown in the columnar sections of the stratigraphic diagram below.
In ascending order, the northeastern most portion of the Borden is
composed of sandstone and shale of the Farmers Member, overlain by
the siltstone (Cowbell Member) and the shaley clay like silt of the
Nancy Member. Uplift on the Waverly Arch during deposition of the
Lower Mississippian Borden Formation was also suggested as having
caused (1) cessation of deeper water sedimentation by diversion of
the distributary system, (2) local exposure of the Cowbell Member
to erosion, and (3) formation of a platform upon which succeeding
shallow-water sediments were deposited. Nancy Member was
interpreted to be a pro-delta deposit formed as the Borden Delta
Complex prograded across Kentucky. This unit is mainly deposited in
a general aerobic environmental setting of a ocean basin in which
bottom waters were anaerobic lower waters and middle and upper
waters were aerobic. The Cowbell Member represents a series of
delta front sands and silts composed of sub-environments associated
with the advancing Borden Delta Complex or Shelf. The Farmers
Member is more clastic in nature and represents uplift of shelf and
distributary channeling as a more transitional type
environment.
In sequences 1 and 2, bed planes are graded on the shelf itself and
prograded to the southwest into the basin. Sequences 3 and 4 system
tracts are relatively layer-cake, reflecting loss of deposition
during the basin filling. They prograde more to the west.
Borden Members played a huge role in the industrialization of the
area. It is estimated that 70 percent of the petroleum and 40
percent of the natural gas produced in Kentucky have been from
Mississippian rocks. Both petroleum and natural gas has been
harvested from the Borden Formation. Member shale units are used,
or have been used, for the manufacture of face brick, drain,
quarry, and roofing tile, and lightweight aggregate. Concretionary
iron ores from the Borden were mined on a limited scale in the
1800's for smelting at small local furnaces. Rocks of Mississippian
age are the most important sources of construction stone in
Kentucky. Almost 70 percent of the active limestone quarries and
mines are in Mississippian-age strata. At one time, Kentucky
supported an active building-stone industry, the largest production
coming from limestone, sandstones or siltstone units.
One problem for construction using clay shale of the Borden
Formation is that it becomes plastic when wet and makes unstable
foundations for roads and various structures constructed on them.
Overly steep slopes and fill containing quantities of these shales
are subject to slump and slide. This can pose problems for
stretches of highway and for residences unknowingly built on top of
the strata.
The Borden Formation is a fossil mecca. Thirty-four crinoid
species, including four new species, are reported from the Nada
Member of the Borden Formation in eastern Kentucky. Fossils are
most commonly found in sedimentary rocks. Sedimentary rocks result
from the consolidation of loose sediment that has accumulated in
layers. Almost all of Kentucky's rocks at the surface (but below
the soil) are of sedimentary origin, and almost all bear fossils.
Consequently, Kentucky is an excellent place to collect
fossils.
During burial, the sediment grains became cemented together and
they became limestone. Shale and sandstones began as deposits of
non-limey mud and sand, respectively. Deposits of mud and sand
formed in seas and on land. Cementation of the muds and sands
transformed the sediment into rocks. Plant and animal remains
trapped in the original deposit became fossils.
On July 14, 2000, agate was officially designated as Kentucky's
state rock. The Kentucky Geological Survey was not consulted prior
to this designation, which is unfortunate, because although
beautiful agates are found in Kentucky, agate is scientifically a
variety of the mineral quartz, and not a rock. Minerals are the
building blocks of rocks. Rocks are composed of many minerals and
are formed through sedimentary, igneous, or metamorphic processes,
which, strictly speaking, agate is not. Now Kentucky has a state
rock that is really a mineral, and a state mineral (coal) that is
really a rock!
One of the unique inclusions of some Borden Members in Kentucky is
known as Agate. It’s appearance is almost jeweled. It has delicate
and varying shades of color arranged in layers. In the typical
occurrence the bands are irregular, curved, or in concentric
patterns. Agate is used as an ornamental material or in
semi-precious jewelry. The color banding is usually related to
chemical impurities; for example, iron gives a red or orange color
and manganese or calcium give black or blue colors. Kentucky Agates
are derived from the Renfro-Borden Formation of Early Mississippian
age and can be collected along some river drainages where the
Borden is exposed to weathering. Many of these agates are displayed
at rock shows across the United States.
Another brilliant inclusion of this formation is geodes. Quartz is
the hardest, most resistant mineral found in abundance in Kentucky.
It is the main constituent in sandstones and geodes, and also
occurs as vein quartz. Crystals usually consist of six-sided
hexagonal prisms capped by pyramids on one or both ends. Quartz
crystals are found in geodes that occur in several different rock
types, particularly limestone. Some Borden Formation Members in
Kentucky include quartz geodes, often found in valleys and stream
beds downslope from the Formation. Many geodes contain precious
minerals, including amethyst (another variety of quartz).
Since 1964, a road cut in central Kentucky has been producing
thousands of quartz geodes containing a variety of minerals
including extraordinary specimens of millerite, jamborite and
unusual forms of pyrite. The Halls Gap geode locality is located
along U.S. Highway 27 near Stanford, Kentucky, near the community
of Halls Gap. It lies near the top of an escarpment known as
Muldraugh Hill. In the development of the improved highway, road
engineers uncovered what is probably the best millerite geode
horizon known in the world. Geodes from this road cut have been a
favorite among Midwest collectors for years. The geode quartz rock
is marked by a thin seam of dark green glauconitic shale which is
persistent throughout the Midwest. It was deposited over a vast
area, extending from Ohio to Alabama and from Virginia to Kansas.
It may record a period of extremely slow deposition caused by a
sudden rise in sea level. Concretions are commonly misunderstood
geologic structures. Often mistaken for fossil eggs, turtle shells,
or bones, they are actually not fossils at all but a very common
geologic phenomenon in all types of sedimentary rock; including
sandstone which is made up of compacted sand grains, shale which is
made up of compacted mud, siltstone which is made up of a fine
grained silt, and limestone which is made up of calcium carbonate
precipitated by many marine invertebrates. Concretions form as
minerals within a rock segregate and begin to precipitate within
cracks and cavities, or as a sediment builds up in successive
layers around a nucleus such as a shell or pebble.
The Cowbell Member of the Borden Formation is identified by
submarine fans that thin and thicken and are visible easily it’s
units. Upward thickening sandstone that include the presence of
fans represents a deep water environment when formed. The sediments
which are wide-spread thin uniform units of silt and sand thicken
downward to overlying black shale. Cowbell includes evidence of
organisms that manifested themselves into the rock by
burrowing.
Silty-Shale overlaid with sandstone such as the Nancy Member
represents an upward movement section toward the shore with finer
sediments coursing upward. In this unit you do not see continuity
of bedding with siltstones and shallowing sequences or evidence of
cut-and-fill and vertical and horizontal burrowing.
Upper most Mississippian inter-bedding with shale and sometimes
obvious channel fill incisions are sequenced by shallowing upward
dolomite type limestone. The presence of burrowing organisms
indicates deeper water formation followed by a period of lowering
of sea level. The Farmer Member of Borden is such a unit of
deepening shale created in a more open marine environment. The
channel fills are a chocolate color with a uniform cut-and-fill
type bedding plane visible where lime mud is mixed with chert. You
will notice a progressive stepwise down cutting into the top of the
Farmers Member where channel cut and fill has occurred. A sub-sea
channel progressively truncates underlying beds of Farmers Member
of Borden Formation here and is clearly visible.
Travel to Mile Marker 7.7 to 8.1 along the AA Highway in Lewis
county to view an exposure of the Borden Formation. Focus on the
Northwest Side of the outcrop. Take a picture that clearly shows
the three member units of the Borden Formation here. Post the
picture with you log. Email the answers to the questions below. Now
you know what a Farmer and a Cowbell have to do with rocks!
1. If the total rock exposure for the Borden Formation here is 270
feet from top to bottom- how much of each member is clearly
identified?
a. Cowbell 90 feet/ Nancy 120 feet/ Farmers 60 feet
b. Cowbell 60 feet/ Nancy 120 feet/ Farmers 90 feet
c. Cowbell 30 feet/ Nancy 100 feet/ Farmers 140 feet
2. Shoot an elevation from the bottom of the formation.
Kentucky has a rich heritage that includes world-class geologic
features. Certain of these features have attracted scientific
attention from around the globe. Some of the geologic features are
used as examples in science text books and some as “best in class”
examples for geology for the world. The AA Highway through
northeastern Kentucky is included to be one of the best up close
learning centers in the country.
Many Kentuckians who pass these features, some on a daily basis,
do not know how valued they are in the world of science and
geology. The AA designation as one of the distinguished geologic
sites can bring pride to local citizens as it provides a new
destination for tourists to come to Kentucky. Now buckle up and
head on back down the road to another geologic wonder!
Additional Hints
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Treasures
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