by Alex Gugel , all rights reserved
Guadalupe MountainsPermian Reef Geology Guide |
Marker 28
At this highpoint of the trail, you can find evidence that
sea levels eventually rose and the reef front moved toward
the basin center. The shallow lagoon that would have normally been behind the reef returned. Because evaporation
rates were very high in the hot arid environment, the seawater had a high concentration of minerals. Mineral rich
water permeated the limestone here, and replaced some of
its calcium with magnesium, thus forming dolomite. Note
that the reef has been eroded away here, but would have
occupied a position a few hundred yards over the edge of
the escarpment.
Guadalupe Mountains
National Park
National Park Service
U.S. Department
of the Interior
The Permian Reef
Trail Guide
At this point you are 2,000 feet above the desert floor. You
have encountered rocks formed on the seafloor and have
seen evidence of the organisms that lived there. You have
seen the forces of gravity and wave action working against
the reef’s upward growth, and you have found fossils of
organisms frozen in the same position they were in millions of years ago. The clues found in the rocks preserved
in Guadalupe Mountains National Park tell an unexpected
story of life and death hidden in a remote corner of the
Southwest.
Lagoonal deposits
Reef
Forereef
14
View from McKittrick Canyon Contact Station
Marker 27
The massive cliff at the top of the northeast wall of
McKittrick Canyon is a fossil reef that formed approximately 260-270 million years ago. The Capitan Reef grew
from the remains of billions of marine animals and plants
cemented together by calcium carbonate.
You can find another clue that sea levels dropped at this
stop. Sheet cracks and teepee structures (tent-like folds in
the rocks) are evident along the trail here. Teepee structures may have formed by the expansion of hardening rock
between softer layers of unconsolidated sediment. Today,
teepee structures are seen in areas around the Persian Gulf
in peritidal areas where sediments would be alternately
submerged and exposed to the air during tidal cycles.
Limy sands and muds were deposited in warm, quiet
lagoons behind the reef. These “backreef” layers form
horizontal rock beds visible to the left of the reef cliff. A
narrow shoal, perhaps consisting of a line of small islands
when sea level was low, restricted water circulation between the shallow lagoons and those above and oceanward of the reef. This allowed rapid evaporation of lagoon
waters often leading to hypersaline conditions.
Fragments of the growing reef edge often broke off and
rolled down slope into deep water, mixing with shells,
sand, and other sediments to create thick “forereef” deposits. Generations of reef creatures then grew seaward on
top of the remains of the old.
Sheet crack filled by sediments
Water at the base of the forereef deposits was over 1500
feet deep. Fine windblown sand and floating organic
debris sometimes crossed the shoal and barrier into the
deep, cold water of the Delaware Basin. The organic
debris eventually became the source of vast petroleum
deposits of the Permian Basin of West Texas.
How to Use This Guide...
The photos on the following pages were taken near the
numbered markers, but not necessarily right next to
them, so some searching may be involved.
Each stop highlights an aspect of the reef’s story or a
fossil from the reef community that will give the user a
broad understanding of the Capitan Reef.
Fossils are rare, non-renewable resources. Please do not
damage or take fossils.
Numbered markers not featured in this guide are described in a technical work, written by professional
geologists, called Guide to the Permian Reef Trail,
McKittrick Canyon, Guadadalupe Mountains National Park, West Texas. This book is available for loan at the
McKittrick Canyon Contact Station or for purchase at
the Headquarters Visitor Center (Pine Springs).
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Marker 24
Marker 1
Stromatolites, structures formed by communities of algae,
grew only in intertidal areas and they are featured at this
stop. Stromatolites are characterized by alternating layers
of algae and minerals that formed in mounds. They are
an indication that sea levels were somewhat lower for a
period of time.
After crossing a rocky wash, you will notice beds of
rounded rocks cemented together. These rocks pre-date
the current downcutting cycle of the stream in McKittrick
Canyon. Approximately 40,000 years ago during the
Last Ice Age, floodwaters carried rocks to this location.
Geologically speaking, the Pleistocene Epoch (11,500 to
1.8 million years ago) was not that long ago. However, the
climate was very different than today: rainfall was higher
and temperatures were cooler. Rushing streams transported the rocks eroded from the high country. Constant
tumbling along the streambed rounded these rocks, and
calcium cement in the water bound them together to form
a sedimentary rock called conglomerate. Uplift of the
mountains and erosion by the stream has exposed them.
Stromatolites can be seen on the trail. Look
for light gray stromatolites and darker gray
columnar deposits.
Stromatolites
Conglomerates
Stromatolites (cross-section) in rocks next to
trail
12
1
Marker 2
Marker 22
The limestone layers here are thin and fine-grained, because mud that settled on the ocean basin was relatively
undisturbed by strong currents. In these layers and rising
out of them you will see numerous orange chert nodules. They are made of silica and are more resistant to
erosion. Iron present in these nodules gives them their
orange color. These nodules may have formed from silica
found within sponges when mud layers were buried and
compacted. Notice also that this limestone is very dark
in color. The dark color results from organic carbon and
hydrocarbons trapped in small pores. Cold deep water
here prevented completed oxidation (rotting) of organic
remains of small animals. This type of limestone is a good
source of petroleum. In the past, American Indians used
chert to make tools and arrowheads.
Organisms not found elsewhere help identify this area as
the top of the reef. A type of algae, Collenella, that lived
only near the reef’s upper limit, is fossilized here. The reef
top would have experienced tremendous force from waves
crashing above it. The waves ground up animal skeletons
and reef rock and washed it into low areas along the reef
top. The type of sedimentary rock that resulted is called
“grainstone” and it is very common here.
Chert nodules
Collenella
2
11
Marker 21
Marker 3
The outcrops along this section of the trail are important
for understanding diagenesis. Diagenesis refers to chemical and physical changes to the sedimentary rock after its
formation. The large calcite crystals (called calcite spar)
in the rock formed when the reef was buried during the
Mesozoic Era (250-66 million years ago).
Every rock provides clues to the forces that have shaped
the land. Geologists have studied thin cross-sections of
this rock with microscopes and have found that some of
the pebbles embedded in it are made of a type of limestone called boundstone which was formed by algae,
growing around sponges. These types of organisms only
grew on the reef, so it has been moved from its place of
origin. The reef was made by billions of organisms and
grew vertically until environmental conditions could no
longer support continued upward growth. The reef community then started growing outwardly towards the ocean
basin and became unstable. The force of wave-action and
gravity tore off pieces of the reef. These pieces formed a
steep slope of debris below and in front of the reef. Some
particularly violent collapses carried debris all the way out
into the deep sea floor where the pieces were buried and
fossilized. Further up the trail near Marker 12, look for reef
boulders six to nine feet in size.
Calcite crystals
10
Boundstone from the reef
3
Marker 4
Marker 16
Trace fossils are evidence of the past activity of organisms.
At this stop the burrowing activity of a worm-like organism is preserved. In paleontology, trace fossils are named
(Planolites in this case), but the organism that left the trace
is not always known. The worm-like organisms that left
these traces were burrowing through the seafloor millions
of years ago in search of food. Only worms and a few animals could live in this cold, deep, dark, low-oxygen environment at the bottom of the sea. Now, in one of nature’s
great reversals, you can examine this record of an ancient
sea preserved above ground in the Chihuahuan Desert.
This area of the reef was sponge paradise. Numerous
types of sponge fossils can be found here as well as bryozoans. The various sponges were not restricted to specific
zones because their filter-feeding habits allowed them
to live in variety of environments. This is in contrast to
modern day reefs where coral distribution is limited by the
quantity of light available due to their symbiotic relationship with photosynthetic algae. Thus, ancient sponge reefs
could grow in sea water hundreds of feet deep. Modern
sponge reefs grow as deep as 500 feet.
Sponges
Worm burrows
Sponges
4
9
Marker 15
Marker 7
You are now at the base of the reef and will see numerous
fossils along this section of trail including ammonoids,
sponges, and bryozoans. Some of the sponges are fossilized in growth position. They grew hanging upside down
by attaching themselves to the skeletons of bryozoans that
were growing in holes in the reef. Eventually these areas
were filled in with minerals call “marine cement”. The
cement helped to preserve a moment of time in the reef’s
life.
Along the slope beneath the reef, different forces transported material off the reef. You have already seen the
force of wave action on unstable portions of the reef at
the third stop. At this stop, evidence of a submarine debris slide is preserved in the rock. Numerous fragments
of clam-like creatures called brachiopods are found here.
They lived in an area higher up and closer to the reef.
Debris Slide
Ammonoid (cross-section)
Brachiopods
Bryozoan (cross-section)
8
5
Marker 9
Marker 14
The clues yielded by rocks help us build a picture of past
environments and create a story of what occurred at a certain location. Unlike a photograph that represents one moment in time, the scene you encounter in the landscape is
more like a painting that shows several moments in time as
a progression from earlier to later stages. Sometimes there
are gaps in the rock record due to the forces of erosion.
At this stop you can see many rice-shaped grains within
the rock. These are the fossilized remains of fusulinids, a
type of single-celled organism that became extinct at the
end of the Permian Period. They were similar to amoebas
but they had a hard protective outer covering with holes in
it. This allowed them to extend part of their bodies out of
the holes to catch micro-organisms for food. Some fusulinids lived in the mud behind and on the reef. They could
move, but the great numbers evident here were transported after the organisms died in mass. Possibly, this mass
die-off was due to a change in environmental conditions.
Geologists can recognize different portions of the reef
and changes that took place within the community by the
differing organisms found there and the types of mineral
cements that filled the spaces on the reef. The youngest
portions of the reef would be found on the crest of the
ridge above if it were not for erosion or gravitational collapse. Some portions of the reef tumbled down slope and
can be found at this stop as huge boulders up to 30 feet
across.
Look for reef boulders (algal boundstone)
along this stretch of the trail
Fusulinids
Closeup (~2 inches wide)
of algal boundstone
6
7