| Geology
of Lake Meredith
by Eric
Butler
Intern,
National Park Service
Geographica
> Geology of Lake Meredith |
Sedimentary processes dominate the geologic history of the
Texas Panhandle. The present surface rests on thousands of feet of
sediment deposited over hundreds of millions of years. In order to
deal with these lengthy time spans, geologists divide earth
history into various named units, such as Eras and Periods, just
as historians have divided human history into Centuries. Most of
the rocks exposed at, and underlying Lake Meredith belong to the
Permian Period, a time 250-286 million years ago, just before the
rise of the dinosaurs. The rocks beneath Lake Meredith record over
1 billion years of earth history, but the thousands of feet of
sediment currently left over represent only a fraction of this
time. Frequent periods of erosion and non-deposition leave their
mark only as the boundaries between existing rock units. This
section will describe in more detail the geologic history of these
units, with emphasis on those exposed at Lake Meredith.
Subsurface
The deepest rocks in the Texas Panhandle are ancient igneous
and metamorphic rocks over 1 billion years old, referred to by
geologists as the Precambrian basement. Beginning 500 million
years ago in the Ordovician Period, repeated cycles of sea level
change deposited sequences of sand, shale, and marine limestones
and dolomites over this basement. Around 300 million years ago,
tectonic movements began to uplift these sediments and the
underlying basement into a NW-SE trending mountain range (now
referred to as the Amarillo Uplift). Sediments eroded from these
mountains began to fill the plains to the north and south, and
eventually the mountains were buried. The Wichita Mountains of
Oklahoma are an exposed portion of this range. The thick
sedimentary sequences on the northeast and southwest sides of the
Amarillo Uplift compose the Anadarko and Palo Duro basins,
respectively, and are a major source of fossil fuels.
Permian
During the Permian Period, the ocean gradually withdrew,
leaving behind a vast floodplain. River systems deposited thick
sequences of silt and sand, along with tidal and shallow marine
sediments along the coastline. Isolated lakes and trapped pockets
of ocean evaporated in the arid environment, leaving behind
deposits of salt and gypsum. These sequences form the majority of
the rocks exposed at Lake Meredith, consisting of the Alibates and
Whitehorse Formations.
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Whitehorse Formation
The Whitehorse Formation is part of a thick sequence of red
mud, silt, and sand that extends below the 200 feet exposed at
Lake Meredith. These deposits probably formed in shallow-water,
life-rich environments such as rivers, lakes, deltas, and shallow
seas. Many microorganisms give off a great deal of oxygen, which
reacts with iron in the water and sediment to produce a bright red
color (a process similar to rusting). In a few places, narrow
layers of grey can been seen within the red beds; it is believed
that these represent floods or other events that temporarily
killed off the organisms and halted the oxidation of the sediment.
The Whitehorse also contains layers of gypsum and sandstone.
Gypsum is an evaporite mineral formed as a body of water dries up,
leaving behind minerals such as gypsum and halite (salt). These
evaporite beds are even thicker and more common beneath the
surface, and indicate that sea level was changing constantly
during this time, in cycles of flooding and evaporation. The
layers of sandstone probably represent especially low sea levels.
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Alibates Formation
The Alibates Formation is defined by two layers of white
dolomite separated by red silts of varying thickness. Dolomite (or
dolostone) is a rock very similar to limestone that is formed in
shallow, mineral-rich seas, commonly in arid environments.
The upper layer of dolomite and red silt are often
missing, having been eroded away. The lower, thicker layer is very
resistant to erosion and forms the capstone on bluffs around the
park, while the softer red beds below erode away into steep slopes
littered by dolomite boulders. In a few places red silts and sands
of the Dewey Lake Formation can be found above the Alibates
Formation.
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The "Permian Red" of a Whitehorse
Formation and
lower Alibates Formation of dolomite on top. This formation is
near the mouth of Devils Canyon
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The lower dolomite is locally replaced into chert, a unique
rock composed of millions of microscopic quartz crystals, which
are composed of silica and oxygen. As mineral-rich water
percolated slowly through the dolomite, the silica it contained
remained behind as chert. This process took place long after the
dolomite was formed and buried. Where cracks or holes in the rock
existed, larger quartz crystals formed, and many beautiful crystal
structures can be found. The many colors visible in Alibates flint
come from small impurities such as iron and magnesium. Chert is
very hard, and breaks with a sharp edge, making it a very useful
material for making tools. When worked by humans, it is commonly
referred to as flint. This unit is known by various names
depending on the source consulted.
Triassic
During the Triassic Period, 250-210 million years ago, rivers
and lakes deposited a sequence of yellow, maroon, and tan mud and
shale (a very fine-grained, platy rock). In Texas, these rocks are
referred to as the Dockum Group and are thought to be equivalent
in age and environment to the Chinle Formation of Utah, New
Mexico, and Arizona, which is famous for petrified wood and
fossils. Such fossils have been found in Dockum Group rocks,
though not in the abundances typical of the Chinle. At Lake
Meredith, these rocks have mostly eroded away, and outcrop mainly
in isolated portions of the southwestern end of the park.
Tertiary
Following the Triassic, there is a 200 million year gap in the
rock record where no sediment was deposited (or any that did has
been eroded away). The next unit present at Lake Meredith is the
Ogallala Formation, a tan-white sandstone that can be seen
overlying the dolomite ridges throughout the park and forming
rounded hills away from the river. This unit formed in the late
Tertiary Period, between 12-2 million years ago as rivers spread
sediment eroded from the newly uplifted Rocky Mountains across the
western plains. The base of the Ogallala is defined by a layer of
river-rounded cobbles up to baseball size, while the majority of
the sandstone is composed of small, rounded quartz grains with
larger pebbles mixed in. The location and elevation of the
Ogallala varies throughout the park as the sandstone follows the
surface topography present millions of years ago. The Ogallala
extends as far north as South Dakota and is a major source of
water for agriculture and settlement in the high plains.
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Chimneys
Chimneys are a unique and little-known feature around
Lake Meredith. Thick salt and gypsum layers beneath the
surface dissolve, leaving holes into which the overlying
rock collapses. This results in a deep tube-like hole filled
with various sediments depending on how and when the rock
collapsed. In a few places, erosion has stripped away the
surrounding rock, leaving the filled tube standing above the
ground like a chimney (see above). In most cases, however,
the chimneys have remained underground, exposed only in road
cuts and bluffs. The sediment and debris filling the
chimneys offer many clues about their formation. Two large
and very different chimneys are easily accessible and
visible to park visitors.
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This
article is part of the
Lake Meredith National Recreation Area
and Alibates Flint Quarries National Monument
web sites and are in the public domain.
Publishing courtesy of PanhandleNation.com, written by
Eric Butler and is an excerpt from A Visitors Guide to Lake
Meredith National Recreation Area
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