Field Trip Day 2 - Turnagain Arm Area
Overview of Trip
There are several objectives of the Turnagain Arm trip. Various features associated
with a subduction zone are located along the Seward highway. Rock exposed in outcrops
features metamorphosed igneous rock with faulting and numerous sills and dikes.
Results of the 1964 earthquake are seen where the lower land mass has had the ocean
eroding covering soil. Depositional features resulting from strong bore tide effects
are seen. Effects of glaciers are noted in bedrock outcrops, glacial gravel deposits
and in several glaciers located along the field trip route.
The most prominent geologic feature in the Anchorage area is the presence of a subduction
zone. The Pacific plate is colliding with the Alaska plate getting pushed down into
the mantle. The trench of the subduction zone is east of Anchorage in the Gulf of
Alaska. As you approach Anchorage, the Valdez formation rises out of the ocean.
Close to Anchorage, the rock changes to the McHugh formation across the Eagle River
fault. The Border Ranges fault separates the Chugach mountains from Cook Inlet.
A large forearc basin is present - the Cook Inlet. Sediments in this basin are 150
million years old. Across the inlet, west of Anchorage, volcanoes are present.
The Chugach terrane is exposed in road cuts and along the beach beside the Seward
highway. A terrane is an area with similar geology, usually near faults and separate
from other areas with unique geology. Two types of rocks are present - a narrow
belt of melange and a wider belt of impure sandstone. These belts are separated
by a fault called the Eagle River fault and truncated on the west by the Border
ranges fault.
As you look south towards the Kenai peninsula, an obvious geologic feature is the
abrupt end of the mountains and the start of much flatter topography. This is due
to a fault called the Border Ranges fault. This fault was thought to be inactive,
but a couple of quakes were detected 2 years ago. The active fault that causes most
of the small quakes felt in the Anchorage area is the Castle Mountain fault. It
is located north of anchorage in the Talkeetna mountain area. The Eagle River fault
is a less obvious fault. It separates the McHugh and Valdez formations.
The McHugh formation is known of as a melange. Melange is a French word meaning
"the putting together of diverse things". Rocks in this formation are a jumble of
rocks and pieces of rocks from a variety of sources and mixed together my movements
of the earth’s crust. Metamorphosed basalt is common. As the Pacific plate is subducted,
lighter material riding on the top of the plate is scrapped of and piles up in front
of the Arctic plate, forming a structural high known of as the Chugach Mountains.
Paleomagnetism studies indicate these and rocks from the Valdez formation were formed
in the Cretaceous (about 65 million years ago) and transported to Alaska from 20
degrees south of their current location. Rocks located west of the Border Ranges
fault originated 20 north of their present location. This indicates a southerly
moving plate to the north and a north moving plate from the south, colliding in
the Anchorage area.
The Valdez formation is a collection of deepwater sandstone and siltstone deposits
and is referred to as a flysch. Flysch is a Swiss term that refers to a collection
of sediments shed by a mountain range as it is uplifted and eroded, usually from
a volcanic source. These sediments were deposited on the ocean floor as turbidites.
These were not subducted but scrapped off and piled up as the plates collided. Depositional
features such as current ripples, laminar bedding, cross-bedding and graded bedding
are seen in rock outcrops.
Stop 1 - Potter Station House
Location: Chugach State Park headquarters at Potter Marsh Train Station Depot. From
the parking lot by the train, walk east away from the park headquarters building,
along the railroad tracks. Cross the bridge and walk down to the ocean.
Description: A hard rock outcrop is located along the gravelly beach as opposed
to the gravelly deposits found throughout the Anchorage bowl area. The rock is black
and dark green colored with occasional white streaks through it. Surface is very
rough and fine grained. Layers are present, but have folded and the rock has a wavy
look. Height of the rock outcrop is about 10-15 feet. Walking along the beach, a
cave like depression about 15’ deep, 5’ high and 10’ wide is carved out of the rock.
About 50’ further down the beach, a sea arch is present.
Interpretation: This rock is referred to as the McHugh formation. It is thought
to be composed of basalt deposited on the ocean floor in pillow deposits. It also
has other material mixed in - whatever was scrapped off as the Pacific place is
subducted under the Arctic plate.
The rock at this stop appears to be composed of basalt from a "spreading zone" -
the ocean floor. As the basalt was extruded onto the ocean floor, it cooled as it
contacted the water. The magma would find a weak spot in the newly formed crust
and break through, forming another "pillow". The pillows here are hard to pick out
- they have been metamorphosed. Outcrops south of Seward at Resurrection Bay are
much better preserved. The quick cooling of the basalt causes lots of cracking and
breccia is common. This rock appears to be metamorphosed. The layers are no longer
flat, but folded. The green colored rock is Greenstone, formed when basalt contains
the mineral chlorite. Chlorite is formed when basalt is metamorphosed. Chlorite
is also present in sedimentary rocks, but is formed due to soil building processes.
The basalt indicates a deep marine environment.
There are numerous thin chert veins present. These veins spread through the rock
like a sinuous spiderweb. These veins were formed when microscopic animals in the
water column died and their shells fell to the sea floor. These ended up on top
of the pillow basalts and were covered with more basalt. Ribbon chert forms between
the pillows and inter-pillow chert is present as the thicker layers. The deposition
of remains is controlled by the saturation of the material in the water column.
As the remains of these animals fall through the water column, the temperature,
salinity and pressure are changing and a depth is reached where the calcium based
material, then the silica shells go into solution and are not deposited on the ocean
floor. These depths are know as the CCD and SCD - calcium compensation depth and
silica compensation depth.
A sea cave has formed along the beach Rock been eroded out of the bedrock. The roof
of the cave is composed of numerous hanging stalactite like formations. These were
not formed from groundwater deposition of calcium, but are remnant pillows. A sea
arch is also present - two slabs of rock have fallen against each other as their
support has been eroded away by the ocean.
Stop 2 - Road Quarry
Location: Mile 113.5, approx. 1 mile past Potter Marsh Train Station Depot. Park
in the parking lot located to the left of the highway.
Description: A large cliff, 100’-150’ high, is present. Large boulders have broke
off the solid rock and are scattered around the area. Lots of bends and folds are
observed in the rock. The rock is black and green with occasional thin chert streaks.
A white, softer powdery material is present along several of the fault surfaces.
Several faults are seen, one large one that cuts the rock cliff from the upper right
to lower left. Exposed surfaces along the fault plane have a slickenside texture
- smooth in one direction, rough on the opposite.
Interpretation: This appears to be McHugh formation rock - metamorphosed basalt.
The jointing makes the presence of faults easy to see and to determine the direction
of movement. This is a reverse fault - the hanging wall has ridden up over the footwall.
This is determined by feeling exposed rock along the fault plane - it is smoother
when running your hand in the opposite direction of movement. Also, jointing in
the rock make it easy t line up rock that may have been adjacent. Jointing is formed
when rock is brought to the surface and weight is unloaded. Any bedding features
have been destroyed by metamorphic processes.
This being a reverse fault fits in with the regional picture. As a subduction zone,
rock masses are being pushed together. This results in compressive forces and any
faults should be the result of this compression - reverse faults.
The white material is calcium carbonate deposited due to ground water movement.
The fault surface provides a permeable path for groundwater to percolate through.
Materials in solution or moved by groundwater can be deposited along the fault.
Stop 3 - Bedrock Cliff
Location: Bedrock cliff along road by gate - after waterfall turnout. Approximately
mile 105.
Description: Nicely layered beds of sandstone and mudstone. Lighter bands of sandstone
grade upward into darker bands of mudstone. Numerous depositional features observed
- current ripples, laminar and graded bedding. Occasional ripple clasts noted. Lots
of faulting and cracks associated with mountain building process.
Interpretation: This is rock of the Valdez formation. Somewhere between stop #2
and this stop, we crossed over the Eagle River fault. This fault is gradual and
does not have an abrupt interface. Instead, there is a zone of several hundred feet
of ripped up material. No exposures exist along this fault accessible from the road.
The depositional environment for these sediments is thought to be deepwater turbidity
currents. As plumes of sediment travel through the ocean, the heavier sand falls
out of the water first, followed by the smaller sized particles. Between plumes,
there is a rain of mud from the water column. Eventually, another sediment plume
travels along the ocean floor, possible set off by an earthquake or a submarine
slump, depositing another layer of sand and starting a new grading upward sequence.
Current ripples preserved in the rock give an indication of direction of current
movement - the steeper side of the ripples pointing downstream. Ripped up pieces
of mud can be seen embedded in the sand, redeposited after being ripped out of their
position of the ocean floor. Paleomagnetism studies indicate these were formed around
40 north, somewhere off the Oregon coast about 65 million years ago.
Folding, faulting and joints are evident. The folding and faulting is a result of
the metamorphic process, compressing the layers together. The joints form when the
rock is lifted and weight removed.
Stop 4 - Bird Point Railroad Quarry
Location: At west end of new road. Turn off Seward highway when it starts to climb
away from the ocean. Turn right into the Railroads quarry.
Description: We toured 3 areas at this stop. The first area was the quarry. The
rocks have a very plate like cleavage and occasional small traces of mica could
be seen. These rocks were highly folded and broken up.
The second area was a mud flat next to the railroad tracks. The old railroad bed
could be seen, large posts sticking out of the mud bank. A light gray, very muddy
material was present out to the ocean. The mud surface was firm when walking on
it, but would liquefy when any object was shaken against it.
The last area was a rock outcrop next to mudflats leading to the ocean. The bedrock
was layered with lighter colored rock interspaced with darker layers. It has a very
smooth surface with numerous scratches and striations. Beds are tipped at a 90 degree
angle. Several large blocks have been removed from the underlying bedrock.
Interpretation: The first area was composed of phyllite, a metamorphic rock next
up the scale of metamorphism from slate. Phyllite is composed of shale and mudstone
with mica and chlorite minerals. It is a very foliated rock. Phyllite is formed
at low/intermediate temperatures with directed pressure. This deposit is one of
the most metamorphosed rocks in the Turnagain Arm area. The railroad used the material
for roadbed, but the quarry has not been active for years.
The second area indicated the amount of subsidence from the 1964 earthquake. The
old railroad bed had to be abandoned and a new one constructed when the old bed
sank ~ 10’. Old timbers are being covered by mud and being washed out of the mud
bank by the ocean.
Mud along the beach exhibits thixotropic properties. When vibrated, water in the
pore space of the mud is forced out and the solid materials collapse. People occasionally
walk or drive out on the mud flats and get stuck. To free them, water hosed are
used to force water back into the pore space.
The last area exposes bedrock that has been scoured by a glacier. The very smooth
surfaces result at the glacier grinds over the top of the rock. The large blocks
missing from the rocks are the result of plucking. Area of the bedrock fracture,
possibly from freeze/thaw cycles. The glacier can freeze to these blocks and pull
them out of their place in the underlying bedrock.
Graded beds help determine which way is up. Coarsening upward sequences can be observed.
Also, these is an abrupt change from the mudstone to the coarser sandstone, while
it is a more graded progression from sandstone to mudstone. The lighter colored
sandstone is overlaid by the darker mudstone. These beds are tilted towards Anchorage.
Two scales of ripples are observed in the tidal flats. A larger scale set of ripples
exist parallel to the tide movement. Smaller, much more common ripples are perpendicular
to the large ripples. Turnagain Arm has a powerful tidal bore that sweeps into the
inlet. This bore tide can set up the large scale ripples that resist being eroded
out by the retreating tide. The smaller ripples are from the outgoing tide and are
perpendicular to water movement. This is an indication of the force of the tide
in cook inlet, that the bore tide ripples are so large as to resist erosion by the
outgoing tide.
The third area is special because of the exposed bedrock. The ocean would normally
quickly erode these rocks. They are exposed because of the 1964 earthquake. When
the land subsided 8’-10’, it allowed the ocean to remove the mud cover, but has
not yet weather the rock away. Freeze/thaw cycles and plant/lichen growth will erode
this away over time. Lichen are noted on much of the rock. They are an early colonizing
plant and use a mild acid to get the nutrients they need from the rock.
Stop 5 - 20 Mile Glacier Overlook
Location: Turnout by Twentymile River
Description: Old buildings to the east are sitting in the tidal flats. The Twentymile
river is discharging into Turnagain Arm through the mud flats.
Interpretation: This area subsided 8’ during the 1964 earthquake. The town of Portage
was lowered into the tidal range and was inundated with water at high tide. Property
in the area was bought out by the state. The road was covered at high tide and a
higher road has been built. Since 1964, the land has built back up and is at the
approximate level is was before the earthquake. This is due to an influx of sediment
from the rivers draining the surrounding basins and from the bore tide carrying
in material from the inlet. After the earthquake, the land surface level was below
"base level" and deposition of sediment occurred. If it has been lifted above this
base level, erosion would have taken place.
Old surfaces - grass and soil - are found in soil borings in the area. The oil 1964
surface has been
preserved by the deposition of mud. Surfaces from previous subsidence events can
bees seen in these borings. These surfaces can be used to determine an earthquake
recurrence interval. The USGS and State of Alaska have done studies that indicate
every 100 to 400 to 700 years, there is a major subsidence event. Numerous assumptions
are used for this evaluation - that Portage is effected by all earthquakes, all
the deposits are preserved in the geologic record.
There are large amounts of glacial silt being carried into Turnagain Arm by the
Twentymile River and other rivers in the area. This material consists mostly of
beat up Valdez formation fragments - sand and silt. Studies of sediment in the Arm
indicate igneous rock is present in recent deposits. The nearest source for this
type of material is the Talkeetna’s and Alaska Range. This is thought to be carried
in by the powerful bore tides from Cook Inlet.
This area is heavily effected by glaciers. U shaped valleys, cirques and hanging
glaciers can be seen through out the area. Well boring indicate the sediment is
at least 900’ deep. Sea level was 400’ lower than present day, allowing glaciers
to scour out deep depressions into the bedrock. Across the inlet, well rounded peaks
and sharp, pointy peaks are observed. Glaciers that envelope mountains tend to round
off the top, while pointy peaks result to glaciers not making it to the top but
instead scrapping away the sides of the mountains.
Stop 6 - Portage Lake
Location: By Portage Glacier Visitor Center
Description: Portage glacier has retreated up valley and a large lake has formed.
Few ice burgs were present. Hanging glaciers, cirques and U shaped valleys are seen.
Two types of ice were observed on the beach - ice with numerous bubbles trapped
in it and a more solid looking ice that was bubble free. Rocks on the beach were
gray and well rounded of all sizes.
Interpretation: Portage glacier has retreated almost all the way back to the mountains
at the head of Portage lake. From 1800 - 1900, the glacier retreated ~1 km. From
1900 - 2000, the glacier has retreated an additional 5 km, a much greater rate of
retreat. This is due to the glacier pulling back to the lake. Portage lake is 600’
deep and sits 91’ above sea level. The initial retreat started due to a general
warming of the earth's climate. Once the lake was exposed, rate of retreat greatly
increased. Presence of the lake allows water to undercut the glacier and remove
large blocks of ice. Also, the lake lifts the glacier and induces calving. the rate
of retreat will remain rapid until it reaches bedrock. For Portage glacier to advance
again, it will need to push a sediment package in front of it. Water draining off
the glacier will deposit material in front of the glacier, allowing it to advance
onto the new moraine.
Glaciers are formed when snow falls on it’s surface and is packed down by more snow.
This traps a lot of air and results in bubbly ice. The bubbles can be under pressure
and this ice is used in drinks to impart an bubbling effect as the bubbles melt
out and pop without the pressure. In parts of the glacier, the ice melts and allows
the air to escape. The pressure of the glacier forces crystallization of the ice
as it refreezes and these structures can be observed in melting ice. The average
age of the ice in Portage glacier is 70 years old. Age is calculated by knowing
the length and velocity of ice movement on the glacier.
Moraine deposits indicate there may have been as many as six glacial advances in
the Cook Inlet basin. Turnagain Arm had several lobes from the Bird Creek and Potter
Creek area. These moraines are poorly preserved due to erosion and later glaciers
reworking them.
The rocks along the beach are graywacke, sandstone mixed with mud. The rocks are
well rounded, indicating transport some distance from their source. Striations are
common.
Stop 7 - Bedrock Outcrop
Location: Mile 86.8, on old highway 1/4 mile west of Chugach National Forest sign.
Description: A light gray dike has intruded into the darker surrounding rock. Small
crystals can be seen using a hand lens.
Interpretation: Studies of this dike indicate it is 53 million years old. The surrounding
Valdez formation is 65 million years old. This is a porphyritic igneous rock. There
was a large magma chamber that cooled slowly, allowing crystals to form. These early
forming crystals settled out of the magma and left quartz and other miscellaneous
minerals. As pressure built up, the magma shot up and formed the dike. Quartz is
among the last minerals to crystallize. This quartz, along with the other non-crystallized
minerals fills cracks in the rocks. In this area, gold is found with the quartz
in these dikes and sills.
Lode and placer mining has occurred throughout the Turnagain Arm area. Lode mining
involves removing the gold bearing rock from the bedrock and crushing and removing
the gold. Placer mining is where streams that these deposits drain to are sifted
through looking for gold. Gold, being heavier than the rock, settles into the stream
beds. Suction dredging is being used to vacuum out small bits of gold trapped in
cracks and crevices in the bedrock underlying these streams.
References
Chernicoff,S., Fox,C. and Venkatakrishnan,R., 1997, Essentials of Geology, Worth
Publishers
Conner,C. and O’Hare,D., 1988/1993, Roadside Geology of Alaska, Mountain Press Publishing
company
Chugach Gem and Mineral Society, 1986, Alaska ... A Guidebook for Rockhounds, Glacier
House publications
Zimmerman, J., 1993, A Naturalists Guide to Chugach State Park, A.T. Publishing
and Printing