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 5- 7cm/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