Download An Overview of Coastal Depositional Environments

An Overview of Coastal Depositional Environments
Southeastern Alaska
Will Vienne
Fig. 1. Map of the Northeast Gulf of Alaska region between Cross Sound and Montague Island. The wavy
line pattern depicts the extent of the larger piedmont and valley glaciers in the region. Offshore, the shelf
break generally corresponds to the 200m contour. After Molnia, 1989.
Introduction
Alaska's southeast coast is a glacially influenced, temperate climate, active
margin. These factors create unique depositional environments along the coast. This
paper attempts to review the coastal environments of this area, and define their
depositional characteristics.
Tectonic, Climatic, and Glacial Setting
Southern Alaska is a highly active region tectonically. The Pacific Plate is
subducting under the North American Plate along the Aleutian Trench at rates of 57cm/yr. The compressional forces bringing the North American and Pacific Plates
together are responsible for the formation of the Alaska, Chugach, St. Elias, and Wragell
ranges, which is the major sediment source for Southeast Alaska's coastal areas. (Molnia,
1989)
Glaciers cover Twenty percent of Southeast Alaska's coastal region. These glaciers
come in the form of tidewater glacier cliffs, land-ending glaciers, and one floating
glacier, La Perouse Glacier in the far southeast, which calves directly into the ocean.
Sediments trapped in the glaciers are deposited into fjords, mainly, by direct calving or
are brought to the fjords via streams of meltwater and rainwater. Outwash streams
contribute a smaller amount of material, generally coarse, to the coast. (Powell and
Molnia, 1988)
Two major weather systems are responsible for the climate of Southern Alaska. The
North Pacific High dominates in the summer, while the Aleutian Low dominates in the
winter. The Aleutian Low brings large storms to the region. Maximum precipitation
occurs in the winter. Maximum runoff occurs in late Autumn or early Winter due to high
rainfall and meltwater. (Hayes and Ruby), (Powell and Molnia, 1988)
Waves and Tides
Southern Alaska is considered to have a mixed energy coastline influenced heavily by
both tides and waves. Tides average 3 m and are diurnal in nature. Data from a NOAA
buoy near the Malaspina Foreland taken from 1995 to 2001 indicates that the average
wave height in that area of the Gulf of Alaska is 1.5 m, and that the average maximum
wave height is 11 m, occurring only in the winter months (NOAA, 2003). Waves are
generally higher in Winter than in Summer because of increased storm activity.
Wind generally blows from the Southeast, causing wave fronts to approach the
coast from the same direction. Also, the Northwest blowing winds are responsible for the
westward flowing Alaska Coastal Current (40cm/s). (Hayes and Ruby)
Depositional Environments
The coastline of Southeastern Alaska has three easily distinct provences: The Yakutat
Foreland to the Robinson Mountain Headlands, Controller Bay, and the Copper River.
1. Yakutat Foreland to Robinson Mountain Headlands
The Yakutat Foreland province extends 90km from the mouth of the Alsek River to
Yakutat Bay, averaging 20-25km in width. The Alsek River is the major sediment source
for this area, and is second only to the Copper River in sediment load along Southern
Alaska. Dry Bay, at the mouth of the Alsek River, has been infilling for some time. The
sediments there consist mainly of mud and pebbly mud. Tidal flats are present in Dry
Bay at low tide.
Outwash plains and medium to course grained beach-ridge plains with some spits
make up much of the coastline along the Yakutat Foreland. Marsh growth is limited
here. Tidal inlets occur, but none are permanent fixtures. (Hayes and Ruby
Averaging 120km in width, the Malaspina Foreland stretches between Yakutat
Bay and Icy Bay. Malaspina Glacier, the largest piedmont glacier in the world, covers
most of the area. Deposits from outwash streams and erosion of glacial moraine at
Sitkagi Bluffs in the south-central portion of the Malaspina Foreland are the two main
sediment sources. Beaches are composed of course grained sand up to boulders, but they
are mostly sand and gravel. (Hayes and Ruby)
This area extends ~20km west of Icy Bay. The coastline along the Robinson
Mountains Headlands is highly erosional in nature due to the retreat of Guyot Glacier in
Icy Bay, which was the major source of sediment supply to this coastline in the past. The
beach ridges here are heavily forested. (Hayes and Ruby)
The Bearing Foreland lies upon a 75km stretch between Robinson Mountains
Headlands and Cape Suckling. It is similar in form to the Malaspina Foreland in that it
contains several outwash plains and, downdrift from them, beach-ridge plains. The
beach-ridge plains here are wider and composed of finer sediments than the Malaspina
Foreland, but have courser sediments than the Yakutat beach-ridges. Grain size
decreases as one moves further downdrift from the outwash streams. The largest
development of coastal aeolian-formed dunes along the coast occurs in this region. They
extend inland several kilometers and can reach heights greater than 10m. There are no
well-developed tidal inlets or back barrier environments in this area. (Hayes and Ruby)
The beach profiles of each of these areas is relatively steep (Hayes and Ruby).
The steepness causes the beaches to be reflective in nature.
Within this region, there are several fjords, such as Icy Bay. These fjords are
estuaries with tide water glaciers. The glaciers contribute morainal bank deposits to the
estuaries through melting and calving. These deposits are generally very coarse. (Powell
and Molnia, 1988)
5. Controller Bay
Controller Bay is located on the western side of Kayak Island--a bedrock island.
There are two wave-built sand bodies found here: Okalee Spit extends 14km to the west
of Cape Suckling, and Kanak Island is a barrier island about 7km in length located west
of Okalee Spit. Longshore transport and outwash streams are the major sources of
sediment to the area.
Controller Bay has a major estuarine complex dominated by tidal currents. It has mud
and sand tidal flats of 5-10km and fresh and brackish water marshes. There are three
major tidal inlets. Large ebb-tidal deltas exist, but there are no flood-tidal deltas due to
high freshwater runoff and a tidal range greater than 3m. (Hayes and Ruby)
6. Copper River
The Copper River is the largest source of sediment to the Gulf of Alaska. Annual
sediment load at the delta equals +/- 100 * 10^6 metric tons--4.5 times less than the
Mississippi River. The subaerial delta plain is composed of marshes and the dominant
sediment type is organic mud. The delta front is mainly sand facies, while the prodelta is
composed of clayey silt to silty clay.
The Copper River supports a large tidal inlet-barrier island complex that is tide
dominated. Tidal range is 3-4m. Mean wave height is 1.2-1.4m. Moving from east to
west, downdrift offset of tidal inlets increases due to oblique wave action from the
southeast, ebb-tidal delta size increases, inlet width increases, and drumstick appearance
is more pronounced. The drumstick shape of the barrier islands is caused by the offset
nature of the tidal inlets and wave action. The large magnitude of both waves and tides in
the region places the delta in between tide and wave-dominated system. Fluvial
processes are not very influential in comparison. (Hayes and Ruby) (Powell and Molnia,
1988)
Conclusion
The southern coast of Alaska receives a high sediment load as a result of its
proximal location to uplifted regions, and high precipitation rates. This large amount of
sediment gets brought to the coastal region, and is influenced by fluvial and marine
processes to form various depositional environments.
The coast along southeast Alaska can be divided into three distinct regions: a
beach ridge system with tidewater glacier fjords, an accreting spit barrier system, and a
delta. The greatest influence to these depositional
References
Hayes MO, Ruby CH. Barriers of Pacific Alaska. In: Geology of Holocene Barrier
Island Systems. Davis RA (ed.) pg. 395-433
Molnia BF (1989) Subarctic (Temperate) Glacial-Marine Sedimentation: The Northeast
Gulf of Alaska. American Geophysical Union. Shortcourse in Geology 9:59-109
Powell RD, Molnia BF (1989) Glacimarine Sedimentary Processes, Facies and
Morphology of the South-Southeast Alaska Shelf and Fjords. Marine Geology
85:359-390