Taz Basin, Granite Island

Taz Basin, Granite Island

by | Sep 18, 2021

Taz Basin is a small cove on the west coast of Granite Island, one of the Chiswell Islands on the south coast of Kenai Fjords National Park, about 61 miles (98 km) east of Homer and 34 miles (55 km) south-southwest of Seward, Alaska. The cove is a remarkable natural harbor with depths of 15 fathoms (27 m) and bounded by sheer granite cliffs. The entrance is only 240 feet (73 m) wide but has a rock in the middle that effectively narrows the entrance to about 100 feet (30 m). Small vessels enter on the north side of the rock where there is a reported depth of about 2 fathoms (3.6 m). Once inside the entrance, vessels can maneuver and find shelter from storms. The local name was reported in the early 1950s by the U.S. Coast and Geodetic Survey. Granite Island is about 5.5 miles (8.9 km) long and 0.7 miles (1.1 km) wide and separated from the Harris Peninsula by Granite Passage only 0.5 miles (0.8 km) wide. Chiswell Islands are a group of mountainous uninhabited islands, sea stacks, rock spires, and reefs. The named islands include Granite, Twin, Dora, Harbor, Natoa, Beehive, Matushka, Chiswell, Lone Rock, Seal Rocks, Chat, and Cheval islands. According to Captain George Vancouver, these islands were named Chiswell Isles in 1786 by Captain Nathaniel Portlock, possibly for Richard Muilman Trench Chiswell. The Russians called them Ayaliki Islands which may have been an Aleut name. The Chiswell Islands are managed by the Alaska Maritime National Wildlife Refuge as part of a sanctuary for seabirds and marine mammals but are very much a part of the landscape in Kenai Fjords National Park.

Alaska is a collage of tectonically displaced rocks called terranes. An individual terrane is a fault-bounded package of rocks with a similar geological history that differs from surrounding terranes. Alaskan terranes have been tectonically transported from where they originally formed and accreted to the edge of North America as the Pacific Plate subducted beneath southern Alaska. The mountains of Kenai Fjords National Park formed as an accretionary terrane called the Chugach-Prince William complex. However, the bedrock of this terrane is much older and composed of metamorphosed igneous and sedimentary rock ranging in age from 359 to 56 million years ago. These rocks formed from marine sediments that accumulated in a trench located where an ancient oceanic crust subducted beneath an oceanic arc. The bedrock includes a chaotic mixture of rocks called the McHugh Complex that are remnants from both the subducting oceanic plate and the upper continental plate, and a thick package of metamorphosed sedimentary rocks called the Valdez Group, and the Orca Group that includes a portion of oceanic crust and granitic igneous intrusions called plutons that formed through partial melting of the sediments of the Valdez and Orca Groups. These plutons in the accretionary prism are called near-trench plutons because they formed near a subduction trench. The heat from the upwelling magma caused the sedimentary rocks of the accretionary prism to melt. Granite Island is a piece of one of these plutons.

Granite Island is composed of erosion-resistant granodiorite that remained after glaciers scraped away the softer sedimentary rock. During the Last Glacial Maximum, about 25,000 to 11,000 years ago, the sea level was much lower and glaciers covered the entire coastal shelf of southern Alaska. The increased ice volume resulted in severe erosion of the underlying bedrock. Glaciers carved out the deep, steep-sided fjords and sharpened the mountain’s peaks and ridges. Since the Last Glacial Maximum, the earth has warmed, causing the glaciers to melt and sea level to rise. Despite this overall trend of glacial retreat, there were short periods of glacial advance including the Little Ice Age from about 1540 to 1710, and again from 1810 to 1880 in southcentral Alaska, but these had little effect on the offshore islands. Erosion during the Last Glacial Maximum exposed the plutons as granite and granodiorite rocks that formed from 61 to 50 million years ago. These are exposed in Resurrection Bay on Hive and Rugged Islands, in the Chiswell Islands, and along McCarty Fjord. Dikes and sills are also present and are related to the plutons. They were formed by magma that escaped from the plutons through tabular weaknesses such as joints, fractures, and shear zones. Read more here and here. Explore more of Granite Island and the Chiswell Islands here:

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