Download ROAD LOG GUIDE TO THE GEOLOGY OF OGDEN CANYON

ROAD LOG GUIDE TO THE GEOLOGY OF OGDEN CANYON
This field guide focuses on the geologic history of Ogden Canyon, which provides a
remarkable natural laboratory and window into past landscapes and environments. Unlike the
Grand Canyon where rock layers are relatively flat, orderly, and read like pages of a great book,
rock layers in Ogden Canyon have been tilted, faulted, and moved about, much like a teenager’s
bedroom. Deciphering this confusion to reveal past worlds is a challenge, which hopefully you
will better appreciate after touring Ogden Canyon. The route and stops are shown in Figure 1,
and the rock column in the area is illustrated in Figure 2. We begin with a brief overview of the
geologic history of the area, which can be divided into 4 main stages.
Stage 1- Formation of basement rocks. Basement rocks that comprise much of Earth’s crust here
formed about 1.7 billion years ago as volcanic islands were accreted onto the edge of the
Archean craton (basement rocks older than 2.5 billion years). These basement rocks are called
the Farmington Canyon Complex, and are exposed along the mouth of the Canyon. Intense
deformation, metamorphism, intrusion of large granitic plutons accompanied this first stage of
major mountain building. Basement rocks were slowly uplifted and eroded over the next billion
years, represented by a major unconformity.
Stage 2- Rifting and deposition of sedimentary rocks. The western edge of North America, which
long ago was part of a super continent called Rodinia, underwent rifting from about 700 to 550
million years ago, with deposition of thick sedimentary and volcanic sequences to the west,
inlcuding ancient glacial deposits. Rifting was followed by a transition to drift and development
of a passive margin (similar to the Altantic margin along eastern North America today) and
deposition of Paleozoic (~540 to 250 million year) shallow-marine sedimentary rocks.
Sedimentation continued during early Mesozoic time, but patterns became more complex as the
passive margin was disrupted. Sedimentary rocks formed an overall westward thickening prism,
which influenced subsequent development of the Sevier thrust belt.
Stage 3- Sevier orogeny. A period of major mountain building occurred during the mostly
Cretaceous Sevier orogeny, associated with rapid subduction along the western margin of North
America. In the Sevier thrust belt sedimentary rocks (with local slices of basement) were folded
and thrust eastward. Major thrusts in northern Utah and Wyoming include the Willard, Ogden,
Crawford, Absaroka, and Hogsback thrusts. The Willard thrust, which is exposed in the upper
part of the canyon, had about 60 km (40 miles) of top-to-east slip during the Early Cretaceous.
The Ogden system imbricated basement slices, with slip transferred eastward onto the frontal
1
Crawford, Absaroka, and Hogsback thrusts during the Late Cretaceous to Paleogene.
Synorogenic sediments eroded from the growing Sevier belt were deposited in eastward
migrating foredeep basins, and locally on the wedge top, providing one of the most complete
records of thrust timing in the world.
Stage 4- Extensional deformation. Initial local extension and collapse of the Sevier belt occurred
during late Eocene to Oligocene as subduction slowed and high elevations could no longer be
supported. Localized normal faulting was synchronous with explosive (andesite to rhyolite)
volcanism and sedimentation in tilted half-grabens. By the Oligocene (~20 million years) much
of Sevier and Laramide topography was eroded or buried. Regional crustal extension occurred
during Miocene to Recent times, forming the Basin and Range Province. Regional normal
faulting produced regularly spaced horsts and grabens, synchronous with deposition of thick,
gently tilted sequences of basin fill and basalt-rhyolite volcanic activity. The Wasatch normal
fault has been active for the last 10 million years, including multiple urface rupturing events in
the last 10,000 years. The crust continues to extend at ~1 mm/yr (recorded by GPS).
MILEAGE
Interval
0.0
DESCRIPTION
Cumu
lative
0.0
Site 1- Rainbow Gardens. Begin tour at the Rainbow Gardens trailhead in the southwest
corner of the parking lot (GPS 41.2354°N, 111.9303°W). Figure 3 gives an overview of the
area.
Overview. The views from the trailhead provide a great introduction to the rich geologic
history of the Ogden area. The oldest rocks in the area, gneiss and granite of the Farmington
Canyon Complex, are visible to the east near the mouth of Ogden Canyon. These
metamorphic and igneous rocks formed in the roots of an ancient mountain system 1.7 billion
years ago, as the continental crust of much of North America was being assembled. Overlying
the Farmington Canyon Complex are Cambrian to Mississippian age (540 million to 330
million year old) sedimentary rocks exposed eastward and up the canyon. These sedimentary
rocks were initially laid down as horizontal layers of sand, mud, and precipitated minerals
(mostly calcite) in shallow oceans that flooded the margin of the continent, and then buried
and turned into sandstone, shale, and limestone. Subsequently (about 100 million years ago),
these layers were tilted and thrust eastward along the Ogden thrust fault to form part of the
Sevier mountain belt. The Sevier belt developed as the crust was compressed and shortened
within a great mountain system that extended from Alaska to Mexico. The current mountain
front before you is the result of movement along the Wasatch normal fault over the last 10
million years, which lies along the margin of the Basin and Range Province where the crust is
extending. Shorelines carved by Lake Bonneville about 15,000 years ago are visible along the
lower mountain slopes, and sediments laid down in this lake cover much of the valley floor to
the west. As Lake Bonneville dropped from its high stand to leave the modern Great Salt
2
Lake, the Ogden River cut down into the soft lake deposits to form steep slopes that are prone
to landslides, including the Rainbow landslide to the south. The Rainbow landslide
experienced several pulses of failure in 1987, 1988, 1992, and 1994, during which times about
100,000 tons (100 million kg) of material moved down slope and the main head scarp
retreated 70 feet (20 m) southward, requiring relocation of two houses. More details on the
geologic history of the Ogden area can be found in a geologic map and report by Yonkee and
Lowe (2004).
Wasatch Normal Fault. The Ogden area lies in the Intermountain Seismic Belt, a region of
historic seismic activity that extends from northwestern Montana to southwestern Utah
(Figure 4B). This belt contains major normal faults capable of generating large-magnitude
earthquakes, including the Wasatch fault, which is of particular concern because of its size
and proximity to urban areas. The Wasatch normal fault is 240 miles (400 km) long,
extending from near Malad, ID, to Fayette, UT, and consists of multiple segments that have
each produced earthquakes (i.e. only part of the fault breaks during a single earthquake). The
Ogden area lies along the Weber segment, which extends 40 miles (60 km) north-south from
Pleasant View to near North Salt Lake. A fault scarp (a steep slope where the ground surface
has been offset) marks the trace of Wasatch normal fault along the mountain front to the east
(Figure 4A, this view can be seen on the optional hike described below). By digging trenches
across the fault scarp, mapping offset soil layers in the trench walls, and dating soil layers,
geologists have determined the history of past large earthquakes in the area (Figure 4C; this
type of study is called paleoseismology and is important for forecasting the chances of large
earthquakes in the future). The Weber segment has experienced at least 4 major earthquakes
over the last 6,000 years, including one about 500 years ago (Figure 4D). Each of these major
earthquakes produced 3 to 10 feet (1-3 m) of ground offset, with estimated magnitudes of 7 to
7.5. Based on data from multiple trenches along the Wasatch Front, the average time interval
between major earthquakes for the combined five central segments of the Wasatch fault zone
is about 300 years (from paper by Pechmann and Arabasz, 1995). Thus the chance of a largemagnitude earthquake striking the greater Salt Lake-Ogden-Provo metropolitan area in the
next 50 years is about 15 percent. Hazards from such a major earthquake include surface
rupture to produce a fault scarp, widespread ground shaking, liquefaction (where soil layers
loose strength and become liquid like due to shaking), seismically-induced landslides and
rockfall, and flooding. Economic loss from a major earthquake along the Wasatch Front could
exceed 20 billion dollars with more than 5,000 deaths.
The Wasatch normal fault lies along the eastern margin of the Basin and Range Province,
which continues westward across Nevada. Within this province, the Earth’s crust is extending
and thinning as hot mantle material rises beneath, which also gives rise to local volcanic
activity (Figure 5). Extension results in slip across normal faults that uplift ranges (also called
horsts) and down drop basins (also called grabens) where thick deposits of sediment
accumulate. The hot buoyant mantle beneath helps keep the region at an overall high
elevation. Basin and Range extension in the Ogden area started about 10 million years ago. By
looking at cooling ages of minerals, geologists estimate that the average rate of uplift and
erosion in the Wasatch Range has been about 1.3 ft per 1000 years (0.4 mm/year) over this
time (from paper by Naeser and others, 1983). The basin beneath the Ogden area contains up
to 10,000 feet (3,000 m) of sediments, giving an average rate of down dropping and
sedimentation of 1 ft per 1000 years (0.3 mm/year). These rates may seem slow in the short
3
term, but over millions of years high mountain ranges and deep basins are built by thousands
of large earthquakes (the total vertical offset across the Wasatch fault in the area is an
astounding 23,000 feet [7,000 m]). The crust is currently extending at a rate of 1.6 mm/year
across the Wasatch fault, based on detailed GPS (Global Positioning System) surveys (from
paper by Chang and others, 2006), building up stress on the fault system. Like bending a stick
that stores energy until it snaps, one day the Weber segment will rupture again. The question
is not if another major earthquake will occur, but when. More details about the Wasatch
normal fault and Utah earthquakes can be found in the circular “Putting Down Roots in
Earthquake Country” available through the Utah Seismic Safety Commission; available at
http://ussc.utah.gov/putting_down_roots.html.
Lake Bonneville. Lake Bonneville, which began rising about 28,000 years ago when the
climate was colder and glaciers covered the high peaks of the Wasatch Range, has left its
signature across the area (Figure 6A). The lake reached its maximum extent about 15,000
years ago, carving the high bench visible along the mountain front to the east, called the
Bonneville shoreline, which is at an elevation of 5200 feet in the Ogden area. At this time a
freshwater lake the size of Lake Michigan stretched from SE Idaho to central Utah and
westward to the Nevada border, drowning the west desert under 1000 feet (300 m) of water.
The lake extended up Ogden Canyon into Ogden Valley and where you are now standing
would have been deep underwater. Gravels were deposited along shorelines where waves
battered the lake edge, and thin layers of silt and sand were laid down offshore on the muddy
lake bottom to the west (Figure 6B). Lake Bonneville was dammed by a landslide along its
outlet at the north end of Cache Valley. About 14,500 years ago the lake topped this earthen
dam, which catastrophically failed. Over a matter of weeks the lake level dropped by 360 feet
(110 m) until the outlet reached solid bedrock; during this time a torrent larger than the
Amazon River surged north carrying boulders the size of houses and carving huge channels.
The lake then stabilized, and waves carved a lower bench visible on the mountain side to the
east, called the Provo shoreline (Figure 6C). The lake receded from Ogden Valley and the
Ogden River formed a delta near the canyon mouth; gravel and sand from this delta cap the
hillside to the south. As the climate warmed, the lake continued to drop, progressively leaving
a series of shorelines toward the west. As the lake dropped, the Ogden River cut down into
older sediments, leaving behind progressively lower terraces capped by river gravels. By
10,000 year ago the lake was largely gone, leaving the Great Salt Lake. The Ogden River
carved down to near its current level. By studying the sediments and landforms left by Lake
Bonneville, we can imagine a changing landscape, from ancient shorelines where wooly
mammoths roamed along a vast lake beneath glacier-capped peaks, to the modern salt flats
where humans see how fast they can make machines travel. More details about the record of
Lake Bonneville can be found in the field guide by Oviatt and others (1997).
Optional Hike
For those interested in closer views of Lake Bonneville deposits and the Wasatch fault, a short
hike is described below. As for any hike in the area, stay on trails, respect private property, be
prepared for changing weather conditions, be careful of steep slopes, and be aware of poison
ivy and rattle snakes.
• Head south on the Rainbow Loop trail about 200 feet (60 m) to the trail junction.
• Take the right fork onto Birdsong trail and head west for about 0.3 miles (0.5 km). This
4
•
•
•
•
•
trail segment crosses the northwest part of the Rainbow landslide and then climbs
through thin layers of silt and sand deposited in deep waters of Lake Bonneville when it
was near its highest level (Figure 6C).
The trail reaches a terrace with excellent views of the surrounding area (GPS 41.2360°N,
111.9332°W). The terrace is capped by gravels containing rounded cobbles and boulders
of granite, quartzite, and limestone, which were deposited by the Ogden River about
12,000 years ago as Lake Bonneville receded. The river continued to down cut to its
current level below you as water levels dropped to leave the Great Salt Lake.
After enjoying the view, return east on the Birdsong trail to the junction.
At the junction turn right and continue east on the Bonneville Shoreline trail for about
0.3 miles (0.5 km). This trail segment crosses the northeast part of the Rainbow
landslide, which has hummocky topography and is covered by willows, typical of
landslide areas that develop in wet areas. The Rainbow landslide has three parts: an
upper zone with steep slopes where sand and gravel collapsed due to undercutting by
springs (visible at top of hill to south); a middle zone where sand and silt were eroded;
and a lower zone with gentler slopes where collapsed and eroded sediments were
deposited as earth flows (where you are).
The trail climbs several switchbacks to reach a flatter area where the main Wasatch fault
scarp is visible looking east (Figure 4A, GPS 41.2339°N, 111.9273°W). About 300 feet
(90 m) to the south along the trail, a second smaller fault scarp becomes apparent and
two sinkholes (circular depressions) are found along this second scarp. The origin of
these unusual sinkholes is uncertain; they may have formed as shallow groundwater
moved along the fault and removed soft sediments below, causing the ground above to
collapse.
After viewing Wasatch fault scarp, return west on the Bonneville Shoreline trail to the
parking lot.
End of optional hike.
0.0
0.0
Leave Rainbow parking lot and turn right onto Valley Drive.
0.1
0.1
Turn right and head east on Utah Highway 39 toward Ogden Canyon.
0.1
0.2
Cross the Wasatch normal fault. A hot springs emerges from fractured bedrock along the fault
south of the Ogden River.
0.2
0.4
Site 2- Ogden Canyon Mouth Turnout. Turnout is on the south side of the highway near the
mouth of Ogden Canyon (GPS 41.2367°N, 111.9229°W).
Carving of Ogden Canyon. The Wasatch Range is a geologically young mountain system,
with a steep front and high relief (elevations vary from over 9,500 feet on top of Mount Ogden
to under 4,500 feet here near the mouth of Ogden Canyon). Yet 10 million years ago the
landscape was likely much different, where remnants of the previous mountain range (the
Sevier belt) had been largely eroded and partly buried beneath a cover of Oligocene (40 to 20
million year old) sedimentary and volcanic deposits (these deposits are preserved in Ogden
Valley where they are called the Norwood Tuff). Across this subdued landscape, streams
flowed into shallow lakes. As the crust began to extend to form the Basin and Range Province,
5
the ancestral Ogden River likely flowed across gentle hills that marked the birth of the modern
Wasatch Range. The river cut easily through the softer Oligocene deposits. As the Range
continued to uplift, the river cut down into the underlying bedrock, while side streams and
landslides eroded the canyon walls and mountain front (Figure 7A). Although it is hard to
imagine a river cutting through solid rock, geologic time provides opportunity. On average,
the Wasatch Range was uplifted 1.3 ft per 1000 years (0.4 mm/year) over the last few millions
of years, giving time for the Ogden River (and the Weber River to the south) to grind and
erode away the canyon bottom, bit by bit. By climbing up the canyon walls, one encounters
different layers of rock; each layer can be matched across the sides of the canyon and you can
imagine the river cutting through each layer sometime in the past, progressively excavating
the magnificent view we have today.
This long history of down cutting by the Ogden River was complicated by the rise of
Lake Bonneville starting about 28,000 years ago. Gravels, locally cemented by calcite, mantle
the canyon wall along the south side of the highway (Figure 7). These gravels are part of a
400 ft (120 m) thick sequence of young sediments capped by sand that was laid down as the
Ogden River dumped its load into the rising lake. The base of the sequence contains large
boulders of granite, quartzite, and limestone eroded from the walls of the canyon and farther
upstream, similar to gravels found along the modern river channel. As the lake rose further,
the river channel was drowned and finer grained sands were deposited. At its highest level,
Lake Bonneville extended up the canyon and into Ogden Valley. As the water level dropped
to leave the modern Great Salt Lake, most of these young sediments were washed away, once
again exposing the layers of bedrock along the canyon walls.
Farmington Canyon Complex- Remnants of an Ancient Mountain Belt. The oldest rocks
in Ogden Canyon consist of gneiss, granite, and pegmatite of the Farmington Canyon
Complex, exposed in steep slopes north and south of the highway (Figure 8, close up views
can be seen along a trail on the north side of the river). The Farmington Canyon Complex is
estimated to be at least 1.7 billion years old, with some parts possibly containing material 2.6
billion years old, based on radiometric dating of minerals (Hedge and others, 1983). The
Farmington Canyon Complex represents the “basement” (old igneous and metamorphic rocks
that make up the middle to lower continental crust), above which younger sedimentary rocks
were deposited. Regionally, the Farmington Canyon Complex contains: mica-rich schist and
quartz-rich gneiss that started out as layers of mud- and sand-rich sediments; amphibolite that
started out as dikes and lava flows; and several types of gneiss that started out as a mixture of
sedimentary rocks and igneous intrusions (excellent exposures of schist, amphibolite, and
several types of gneiss can be viewed along the Beuss Canyon to Mount Ogden trail south of
Ogden Canyon). These rocks were intensely deformed and metamorphosed about 1.8 to 1.7
billion years ago, simultaneous with intrusion of large magma chambers that crystallized to
form granite in the roots of a massive mountain system. High-grade metamorphism occurred
at temperatures of 600 to 800 degrees C (hot enough that rocks started to melt) and depths of
12 to 18 miles (20 to 30 km) within the crust. Intense deformation produced strong gneissic
banding and tight folds. Pegmatite dikes were intruded during and after the most intense
deformation. At this time the area probably resembled a combination of the Andes and
Himalayan Mountains, with crust more than 40 miles (60 km) thick and mountains several
miles (kms) high stretching for several thousand miles (kms). This massive mountain system
formed as fragments of volcanic arcs collided and assembled to form the continental crust of
6
much of ancestral North America. For more details on the Farmington Canyon Complex, see
the proferssional paper by Bryant (1984).
Following this time of profound mountain building, the crust slowly cooled and the
massive mountain system was slowly eroded over the next billion years. During this long span
of time, ancestral North America collided with other continental masses to form the
supercontinent of Rodinia. The exact shape of this supercontinent is debated; models indicate
that Siberia, Australia, and part of Antarctica may have been our neighbors to the northwest to
southwest (for one model of Rodinia see paper by Karlstrom and others, 2001).
0.4
0.8
The contact between gneiss and granite of the Farmington Canyon Complex (1.7 billion years
old) and the overlying base of the Cambrian Tintic Quartzite (about 540 million years old)
represents a great unconformity (a surface across which there is a significant gap in geologic
time). Metamorphic and igneous rocks of the Farmington Canyon Complex that had formed
deep in the roots of a great mountain system were now at the Earth’s surface as overlying
rocks were slowly eroded away over the span of a billion years. What exactly did the area
look like over this time span? No rock layers remain from this time, like missing chapters in a
novel, so we are left with only a partial record and must look elsewhere for clues. The base of
the Tintic Quartzite contains a mixture of sandstone and conglomerate deposited as an ocean
transgressed onto the western margin of North America. The sedimentary rocks of the Tintic
were deposited as horizontal layers, but were tilted to steep dips during younger episodes of
deformation.
0.3
1.1
Site 3- Perry Camp Turnout. Turnout is on the south side of the highway just past Perry
Camp sign (GPS 41.2382°N, 111.9099°W).
Tintic Quartzite, Ophir Shale, and Maxfield Limestone. The Lower Cambrian Tintic
Quartzite is well exposed along roadcuts to the west (Figure 9A). The overlying Ophir Shale
and Maxfield Limestone are visible on the steep slopes to the east. These Cambrian age (540
to 490 million year old) rocks accumulated in shallow marine environments along a subsiding
continental margin following rifting (extension and thinning of the continental crust, which
may lead to formation a new ocean basin between rifted pieces of continent) of western North
America and breakup of the supercontinent Rodinia. The section of sedimentary rocks along
Ogden Canyon is transitional between a thinner section exposed farther east, and a much
thicker section exposed to the west. The Tintic Quartzite consists mostly of well-cemented
sandstone composed largely of quartz grains (quartzite), with some beds of quartz pebbles and
thin layers of shale. Sandstone typically forms beds about 1 to 3 feet (0.3-1 m) thick that are
laterally continuous and display internal cross beds (Figure 9A). These sandstone layers were
likely deposited along a shallow, sandy ocean bottom, where storm waves reworked sand,
which was being delivered from river systems that drained the eroding continent interior to the
east. Imagine floating in a boat, waves tossing sand on the shallow ocean bottom, a shoreline
visible in the distance.
The amount of shaley layers increases toward the top of the Tintic Quartzite, transitioning
into the overlying Middle Cambrian Ophir Shale (Figure 9B). The Ophir Shale formed as thin
layers of mud were deposited in deeper, quieter waters. This transition records a rise in sea
level as the shoreline shifted to the east. Fossils are extremely rare in the Tintic Quartzite;
only a few trace fossils (evidence of an organism’s behavior) have been found, but trace
7
fossils become common in the overlying Ophir Shale, along with a few trilobite fragments in
Ogden Canyon (spectacular trilobite fossils are found elsewhere in Utah, including the world
famous Wheeler Shale in the House Range, see www.trilobites.info/Utah.htm). Many of these
organisms were bottom feeders with soft bodies and left no hard fossils, but their feeding trails
are common. Prior to 580 million years ago most living organisms were made of single cells,
some of which formed colonies. The Cambrian represents a time of rapid diversification of
multicellular marine animals (the “Cambrian Explosion”), during which most modern phyla
appeared. Why was diversification so fast in the Cambrian? Various models have been
proposed including an increase in oxygen in the atmosphere that allowed larger animals to
develop, prior “Snowball Earth” glacial episodes (see site 6) that caused mass extinctions after
which organisms diversified to fill open niches, and development of eyes resulting in an “arms
race” between predators and prey.
The overlying Maxfield Limestone consists of interlayered shale, shaley limestone, and
limestone (Figure 10A), recording a transition as calcite was precipitated from ocean waters
and input of mud from the continent became more limited. The Upper Cambrian Nounan and
St. Charles Formations, which are exposed farther east, consist almost entirely of limestone
and dolostone, marking development of a large carbonate bank as much of North America
was covered by a shallow sea and input of sand and mud was reduced (a modern analogy is
the carbonate bank of the Bahamas). Imagine your boat now drifting across a vast tropical
ocean that covered much of North America, with carbonate shoals locally emerging. At this
time, North America (including Utah) straddled the area near the equator. Since then, the
North American continent has slowly drifted northwards as the dance of tectonic plates
formed the supercontinent of Pangea and then rifted it apart. Past locations of continents over
time can be viewed on a series of paleogeographic maps at www.scotese.com/precambr.htm.
0.4
1.5
Site 4- Coldwater Canyon Trailhead. Park at the trailhead to Coldwater Canyon on the
south side of the highway (GPS 41.2401°N, 111.9047°W). The restored kiln near the trailhead
was originally built in the late 1800’s and used to make lime (needed for cement) by baking
limestone quarried nearby.
Ogden Thrust and the Sevier Mountain Belt. Beds of the Tintic Quartzite, Ophir Shale,
Maxfield Limestone (exposed in the quarry west of the trailhead, Figure 10A), and Nounan
Formation (exposed along the trail to the east), originally deposited as horizontal layers,
currently dip to the east, are fractured and folded, and are repeated across several large faults
(Figure 10A). About 100 million years ago these horizontal layers were tilted and thrust
eastwards as the Sevier mountain belt grew. Unlike the Grand Canyon where rock layers are
still nearly horizontal and intact (like neatly filed pages), rocks in Ogden Canyon have been
folded and faulted, presenting a challenge to deciphering the history (like trying to organize a
messy office where papers have been thrown about).
The large fault visible here is the lower branch of the Ogden thrust, which dips to the east
and places the Lower Cambrian Tintic Quartzite over the Upper Cambrian Nounan Formation
(Figure 10B). The fault thus places older rock layers over younger rock layers, which is
typical of thrust faults. To the south, the lower branch cuts downward and places the
Farmington Canyon Complex over the Tintic Quartzite near Malans Peak. The Ogden thrust
also includes an upper branch that places another repeat of the Tintic Quartzite over the Ophir
Shale to the east. The Ogden fault system thrust rocks more than 12 miles (20 km) toward the
8
east; thus rocks you now see up the canyon originally started out far to the west and were
thrust into their current positions about 100 million years ago. The Ogden thrust currently dips
east due to tilting on the eastern limb of the Wasatch anticlinorium (a very large, complex fold
with an overall domal shape and older rocks in the core); the thrust system initially dipped to
the west. Regionally, the Ogden thrust and an underlying basal thrust stacked up multiple
slices of basement rocks, leading to development of the Wasatch anticlinorium. Uplift and
erosion of the anticlinorium occurred simultaneously with slip on thrusts faults and deposition
of thick conglomerates to the east, which provide one of the most complete records of
thrust timing in the world. For more information on the growth of the Sevier mountain belt
in northern Utah, see papers by DeCelles (1994) and Yonkee and others (1997).
Turn back onto Highway 39 heading east.
0.4
1.9
Cross lower branch of the Ogden thrust fault. The Lower Cambrian Tintic Quartzite is
exposed in the roadcut to the east above the lower fault branch.
0.2
2.1
View to the north up Johnson Draw, where the upper branch of the Ogden thrust fault places
Lower Cambrian Tintic Quartzite over the Middle Cambrian Ophir Shale. The Tintic
Quartzite and Ophir Shale are thus repeated three times across the lower and upper branches
of the Ogden thrust going up the canyon.
0.7
2.8
A small landslide is developed in the steep slopes on the south side of the highway. Rockfall
presents another hazard along the canyon.
0.3
3.1
Cross bridge over the Ogden River. The core of a tight fold is exposed in the cliff face
southeast of the bridge. Shale and limestone in this area display extreme variations in
thickness due to tight folding and faulting. Here, the Ophir Shale is thickened by folds. The
Maxfield Limestone is exposed in higher slopes north of the highway.
0.7
3.8
Dolostones of the Upper Cambrian St. Charles and Nounan Formations are exposed in the
steep slopes and cliffs north and south of the highway.
0.3
4.1
Ordovician to Devonian (490 to 360 million year old) strata are relatively thin along Ogden
Canyon, and are exposed in the overlying slopes, although minor faulting here has locally
omitted some layers obscuring relations. These strata were deposited across a shallow
carbonate bank.
0.4
4.5
Interlayered limestone, dolostone, shale, and sandstone of the Devonian Beirdneau Formation
are exposed at the base of a quarry on the north side of the road. The influx of sand may
provide subtle clues to the development and erosion of a mountain belt far to the west in
central Nevada, called the Antler belt, which marked the initial disruption of the western
continental margin.
0.4
4.9
Cross bridge over Ogden River. Minor folds are developed in thin-bedded shale and limestone
of the Mississippian Deseret Formation within the steep limb of the large-scale “Z” fold.
Fossiliferous limestone of the underlying Mississippian Gardison Formation forms the lower
9
part of the cliffs to the west (Figure 11). Interlayered sandstone and limestone of the overlying
Mississippian Humbug Formation form the ledges to the east.
0.3
5.2
Site 5- Wheeler Creek Trailhead. Park along turnout to Wheeler Creek trailhead on south
side of highway (GPS 41.2534°N, 111.8446°W).
Z Fold and Sevier Mountain Belt. The large “Z” fold visible to the north is a dramatic
structure that formed within the Sevier Mountain belt (Figure 11). This fold developed during
shortening and shear beneath the Willard thrust fault. This major thrust moved rocks above
the fault more than 40 miles (60 km) to the east. At first glance it is hard to imagine how solid
rock can be bent into such large folds, but geologic time provides part of the answer. Like
slowly bending a large piece of oak to form the hull of a wooden ship, over the course of
millions of years solid rock can be slowly bent. If the bending becomes too fast, however, the
rock may break to form a fault. Also, when the Z fold formed, the rocks before you were
buried several miles (kms) deep within the Sevier Mountains, allowing the rocks to more
easily bend at the higher pressures and temperatures. Dramatic views of the Z fold and
additional exposures of Mississippian rocks can be seen by hiking up the Wheeler Creek trail.
The trail crosses the Willard thrust about 1.5 mile (2.5 km) to the southwest, and then enters
the Snow Basin area.
Turn back onto Highway 39 heading east.
0.2
5.4
Turn left on Utah Highway 158 and cross Pineview dam. Pineview Reservoir and canal
system was completed in 1937 to enhance irrigation, as well as provide hydroelectric power
and flood control protection. This is an earthfill-type dam constructed with 15,500 cubic yards
(11,800 m³) of soil and rock materials, and the reservoir has a storage capacity of capacity
110,150 acre-feet (135,870,000 m³) of water. Recently, the dam was enhanced to minimize
chances of failure during a major earthquake. Numerous large rocks were placed across the
west face to add mass and reduce potential for liquefaction of the earthfill. The cement wall
along the road was added as a splash guard to stop a seiche (earthquake-induced water wave)
from overflowing and washing away part of the earth fill.
0.5
5.9
Site 6- Pineview Reservoir. Park in turnout on south side of highway along Pineview
Reservoir (GPS 41.2587°N, 111.8352°W).
Willard Thrust and Sevier Mountain Belt. The Willard thrust, one of the largest faults in
the Sevier mountain belt, is exposed in the roadcut (Figure 12A). The thrust currently dips to
the east in this area, reflecting tilting on the eastern limb of the Wasatch anticlinorium; the
fault initially dipped gently west. Sandstone and limestone of the Mississippian Humbug
Formation are exposed in the western part of the roadcut below the thrust. These rocks are
highly fractured near the thrust. The main fault core crosses through the central part of the
roadcut, but has been covered by weathered debris washing down the hill slope. Samples dug
out from the fault core reveal very finely ground up rock (fault gouge) and finely
recrystallized, highly strained rock (mylonite). By looking at these fault rocks with a
microscope, one sees evidence for a complex mixture of brittle fracturing, plastic flow, and
chemical alteration along a weak fault zone. Slate, pebbly sandstone, and diamictite of the
10
Neoproterozoic (~720 to 650 million year old) Perry Canyon Formation are exposed above
the thrust in the eastern part of the roadcut. These rocks have a strong foliation (planar fabric
produced by slow “flowage” of rock). Pebbles have been stretched, flattened, and sheared into
oval shapes, with long to short ratios reaching 5:1 near the thrust (Figure 12B). The Perry
Canyon Formation marks the base of a 6- to 10-mile (10 to 16 km) thick section of
sedimentary rocks (well exposed across large parts of northern Utah and SE Idaho) that was
originally deposited far to the west along the continental margin and then thrust eastward
about 40 miles (60 km) along the Willard fault (Figure 12C). Imagine the energy required to
move a thrust sheet more than 6 miles (10 km) thick covering 10,000’s of square miles (km2)
over a distance of 40 miles (60 km)! Two factors help explain how such a massive sheet of
rock could be moved so far: geologic time and a weak fault zone. Radiometric dating of mica
here indicates that deformation occurred from about 120 to 100 million years ago. As the
Willard thrust sheet was emplaced, high mountains were created to the west. Rivers cut into
and eroded the mountains, then flowed eastward and deposited up to 10,000 feet (3,000 m) of
sediments in basins across parts of eastern Utah and Wyoming. Note the Willard thrust fault
formed during growth of the Sevier mountain belt, and has since been inactive.
The Perry Canyon Formation provides an intriguing record of ancient glaciation and
volcanic activity. Diamictite in the region contains blocks of granite up to 10 feet (3 m) in
size, plucked by massive glaciers, as well as fine-grained matrix ground up by moving ice.
These ancient glacial deposits are particularly well exposed on Fremont Island, UT, and near
Pocatello, ID, where interlayered volcanic tuff has been dated to be 670 million years old.
Similar glacial are found on other continents, suggesting that the Earth may have been largely
covered by ice during parts of Neoproterozoic time; this idea is referred to as the “Snowball
Earth” model (Figure 13). Equally surprising, layers of limestone, which were rapidly
deposited in warm waters, immediately overlie diamictite at many locations. How could the
Earth become covered by glaciers, then suddenly warm? According to the Snowball Earth
model, carbon dioxide (a greenhouse gas) became depleted in the atmosphere, resulting in
colder temperatures and accumulation of ice across parts of the supercontinent Rodinia. As the
ice expanded, more sunlight was reflected and temperatures grew even colder, which caused
more ice to form, and so on until ice covered the Earth. With ice covering the land and oceans,
carbon dioxide emitted from volcanic eruptions slowly built up in the atmosphere, until the
strengthening greenhouse effect started melting the ice. As the ice melted, more sunlight was
absorbed, resulting in still warmer temperatures, and so on until the ice was quickly gone. The
built up carbon dioxide then combined with dissolved calcium in the oceans to form
limestone. The Perry Canyon Formation also contains ancient basalt lava flows that record an
early phase of rifting from about 720 to 670 million years ago. A second phase of rifting and
eruption of more basalt lava flows occurred about 600 to 550 million years ago, finally
separating continental fragments that drifted away, leaving a subsiding continental margin
along western North America.
Continue east on Highway 158.
0.8
6.7
The Pineview-Skyline trailhead is located along the highway. The Perry Canyon Formation is
exposed along the Skyline trail. Ogden Valley opens up before you.
1.9
8.6
The Oligocene (40 to 20 million year old) Norwood Tuff is exposed in the roadcut. The tuff
11
includes thick layers of volcanic ash deposited into a lake, and locally contains plant fossils.
The tuff underlies part of the valley and is prone to landslides.
0.4
9.0
Cross the North Fork of the Ogden River. The North Arm trailhead is located to the southeast.
The trail crosses riparian areas, with a branch continuing southward where the Norwood Tuff
is exposed above the lake shores.
0.2
9.2
Continue straight east onto 2200 North at the Y junction in the highway.
0.6
9.8
Turn right onto 5500 East at the stop sign, and head south.
0.3
10.1
Continue straight south onto Old Highway at the bend in the road, and proceed to parking lot.
0.2
10.3
Site 7. Ogden Valley Overview. Park at the end of the road (GPS 41.2912°N, 111.8152°W),
and then walk along a trail to the south to reach an overlook of Ogden Valley.
Ogden Valley lies within a down dropped and tilted block (called a half graben) that is
bounded on the east by the East Ogden Valley normal fault and partly filled with east-tilted
Norwood Tuff (Figure 13). The East Ogden Valley normal fault connects with the Morgan
normal fault to the south, and both valleys likely formed over a complex history, beginning
with Oligocene (40 to 20 million year old) collapse of the Sevier mountain belt, followed by
younger Basin and Range extension. Lake Bonneville extended into Ogden Valley during its
high stand, but the lake then retreated from the valley. Sediments that cover parts of the valley
floor accumulated during the high stand, and were partly reworked by streams that drained the
valley following retreat of the lake. The Snowbasin ski area (site of the 2002 Olympics
downhill ski racing events) can be seen to the southwest. Cirques near the range crest provide
evidence of glaciation during the recent ice age (Figure 14). Glacial deposits along the
Wasatch Range provide a record of two glacial episodes, one peaking about 100,000 years ago
and the most recent peaking about 20,000 years ago; these stages can be related to subtle
changes in Earth’s orbit around the sun.
The panorama about you is but one frame in a geologic movie spanning billions of years.
To the west you see the Wasatch Range, with the Willard thrust and Ogden thrust system that
expose Precambrian to Mississippian rocks. But this is only part of the story. To the southeast
lies Durst Mountain and upper Weber Canyon, where Pennsylvanian to Cretaceous rocks are
exposed, revealing more of the fantastic geologic history of the area. Imagine being strapped
to a time machine that transports you over the eons, through phantasmagorical scenes.
Scene 1 (1.7 billion years ago). You stand atop a massive mountain system, like a
combination of the Andes and Himalayan Mountains, with peaks perhaps 20,000 feet (6,000
m) high stretching for thousands of miles (kms). Fragments of volcanic arcs are colliding and
assembling to form ancestral North America. Beneath you the crust has grown to more than
40 miles (60 km) thick, magma crystallizes to form granite, rocks transform from the extreme
heat and intense deformation to become schist and gneiss. You are witnessing the birth of the
Farmington Canyon Complex- the basement upon which following scenes will be overlayed.
Scene 2 (1.0 billion years ago). The massive mountain system is now dead, being slowly
eroded away so that only low hills remain. The landscape is barren; no plants have evolved at
12
this time. The supercontinent of Rodinia has formed and you sit far inland. Rocks of this time
interval are not preserved in the Ogden area, only the great unconformity remains.
Scene 3 (700 million years ago). Rodinia is starting to rift apart. Lava flows and volcanoes
mark the edge of a narrow ocean basin starting to open to the west. There is a chill in the air,
glaciers approach, and soon the land and sea are buried beneath an ice world; ground up rock
and boulders plucked by the moving ice accumulate to form diamictite of the Perry Canyon
Formation. The volcanoes continue to erupt through the icy cover, spewing their gases
(including carbon dioxide) into the atmosphere. Eventually the temperature starts to warm in
the atmospheric greenhouse, and the glaciers rapidly retreat to reveal a stark land and a
growing sea to the west.
Scene 4 (500 million years ago). Rifting is complete and an ocean extends west beyond the
new continental margin. You sit in a boat, waves tossing the sandy ocean bottom, a shoreline
visible nearby. As sea level rises, a shallow ocean floods the continent and the shoreline shifts
east. Trilobites and other strange creatures from the Cambrian explosion feed along the muddy
ocean bottom. As the shoreline moves still farther away, a carbonate bank forms, like the
Bahamas, as you bask under an equatorial sun. The layers of sand, mud, and carbonate are
buried and turned to rock of the Tintic Quartzite, Ophir Shale, and Maxfield Limestone.
As time moves relentlessly forward to the end of the Paleozoic, sea level falls and rises, the
shoreline shifts back and forth, and more layers of limestone and sandstone accumulate. Then
251 million years ago, over 80% of living genera go extinct, the greatest mass extinction in
the geologic record.
Scene 5 (225 million years ago). You sit along a hot coastal plain covered by mudflats, a
searing sun overhead. The shoreline has shifted west. Mud and sand slowly accumulate,
stained by hematite and buried to form redbeds (now exposed as the Woodside Shale in upper
Weber Canyon). Life has rapidly recovered from the mass extinction. Dinosaurs are on the
rise. As time passes, the climate moistens and forests grow (like the Petrified Forest in
northern Arizona). Then the climate dries and sand dunes migrate across a vast area, forming
the “great sand pile” (now exposed as the Navajo Sandstone in Zion National Park and the
Nuggett Sandstone in upper Weber Canyon).
Scene 6 (100 million years ago). The Sevier mountain belt is growing as the ancestral oceanic
Pacific plate rapidly subducts along the western margin of North America, much like what is
occurring today in the Andes. Volcanoes erupt to the west in the Sierra Nevada arc, spreading
layers of ash across the landscape. A high plateau builds in Nevada. Gargantuan sheets of rock
are thrust eastward (the Willard thrust and Ogden thrust system), forming a dramatic mountain
front. Rivers carve into the mountains, emerging from steep canyons to drop their loads of
gravel, sand, and mud along a coastal plain and into an interior seaway that extends from
Alaska to the Gulf of Mexico. Dinosaurs roam among subtropical forests and swamps; small
mammals scurry in the night.
Scene 7 (30 million years ago). The Sevier mountains have stopped growing as subduction
slows along the western margin. Compression that had formed the mountains wanes; the high
elevations can no longer be supported and the mountain system starts to collapse, forming the
East Ogden Canyon normal fault. Explosive volcanism across parts of Utah generates layers
of ash, which along with mud and sand washed by rivers from the eroding landscape, are
deposited in a shallow lake to form the Norwood Tuff. With time much of older Sevier
mountains have been eroded, collapsed, and buried, leaving a gentle landscape across which
the ancestral Ogden River gently flows.
13
Scene 8 (5 million years ago). The modern Wasatch Range is rising as the crust extends and
the Basin and Range Province forms. A large earthquake on the Wasatch normal fault lifts the
range a few feet higher, as the Ogden River excavates a rugged canyon. The Yellowstone hot
spot to the north has a catastrophic eruption, spreading volcanic ash across western North
America and upward into the stratosphere, temporarilly dimming the sun.
Scene 9 (15 thousand years ago). Lake Bonneville extends into the valley. A lake trout rises,
as wooly mammoths forage wind swept grasses and saber-toothed cats stalk the shoreline.
Glaciers carve cirques into the mountains. A large earthquake shakes the area as the Wasatch
Range rises yet a few feet higher.
You blink and the scene is today.
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28