West Cook Inlet Geology: Volcanoes, a Major Fault and Jurassic Fossils

Ammonite fossils are in the bedrock layers of the park's coastal areas. Photo: NPS/M. Ruga.

Ammonites, spiral-shelled sea creatures, lie in the rocks along Lake Clark National Park and Preserve's coast, remnants of an ocean that covered the area in the Jurassic. The coast shows a powerful, dynamic geology carved by volcanoes, earthquakes, coastal uplift and glaciation. Two stratovolcanoes, Redoubt and Iliamna, are the tallest peaks in the park, and a major fault, the Bruin Bay fault, bisects Chinitna and Tuxedni Bays.

The rock record

The bedrock from Chinitna Bay to Tuxedni Bay formed in the Jurassic. It is split into four units: the Talkeetna Formation, the Tuxedni Group, the Chinitna Formation and the Naknek Formation. NPS calls them one of the most complete and thickest successions of Jurassic bedded rock in the United States. They come from the ancient Talkeetna arc, a volcanic arc that formed in the proto-Pacific ocean.

• Talkeetna Formation: the preserved volcanic carapace of the arc, mostly volcanic rock formed under the sea in the Early Jurassic, with rare land deposits that include tree stumps in growth position.
• Tuxedni Group: Middle Jurassic marine rocks lying over the Talkeetna, with fossils of bivalves, ammonites and belemnites.
• Chinitna Formation: Late Jurassic marine sediments with ammonites throughout.
• Naknek Formation: Late Jurassic marine rock marking renewed mountain building, with fewer fossils than the layers below.

The Bruin Bay fault runs parallel to Cook Inlet, just south of Iliamna and Redoubt. It separates older volcanic and plutonic rocks in the northwest from younger sedimentary rocks in the southeast, and is thought to have helped expose the Talkeetna arc, whose eroded sediment piled up in the Cook Inlet forearc basin. NPS asks visitors to leave fossils where they are; removing them from the park is illegal.

Plates, volcanoes and ice

Subduction beneath Redoubt and Iliamna Volcanoes. USGS graphic.

The Pacific plate is thrust beneath the North American plate along the Alaska-Aleutian subduction zone at 2 to 3 inches a year, reaching depths of as much as 60 miles beneath the Chigmit Mountains. Fluids released from the descending plate help produce the magma that feeds the volcanoes of the Aleutian arc, the northern part of the Ring of Fire. The 1964 Great Alaska earthquake along this zone, magnitude 9.2, was the world's second largest in recorded history.

Hydrothermally altered Iliamna, with dirty glaciers and bright yellow, orange, red and gray rocks. Photo: Matt Loewen, Alaska Volcano Observatory/U.S. Geological Survey.

• Iliamna has active fumaroles on its upper east side. Hot gas and meltwater from ice and snow have altered its rock into clays, which hold water, fail more easily and feed frequent debris avalanches and flows off the volcano.
• Redoubt erupted in 1902, 1966, 1989–90 and 2009; lake cores from Bear Lake, 22 km southeast, indicate more than 100 eruptions in the last 10,000 years. The 2009 eruption had 19 major ash-producing explosions and left a lava dome that still steams.
• Glaciers. Ten glaciers radiate from Iliamna. Tuxedni Glacier is the longest, running 16 miles to about 3 miles from Tuxedni Bay; Red Glacier is second. During the Last Glacial Maximum nearly all of the Cook Inlet basin was filled with ice, and U-shaped valleys show that glaciers once reached sea level. Active volcanism, tides and landslides have since removed or buried many of the glacial markers.

A shifting landscape

The land on both sides of Cook Inlet keeps moving. After the 1964 earthquake the mantle flows toward the Aleutian trench and slightly upward, a viscoelastic relaxation that NPS compares to a guitar string slowly losing its tune, and Cook Inlet moves trenchward by about 15 mm a year. Isostatic rebound, the rise of land once weighed down by ice, adds to this: on the Kenai Peninsula, 10 to 25 cm of uplift was seen between 1965 and 2000. Monitoring plots show sediment leaving the shore at Chinitna Bay and Spring Point and building up near Red River and Slope Mountain.

Iliamna Volcano stands between Chinitna and Tuxedni Bays at 10,016 feet above sea level. Photo: NPS/D. Liles.

NPS sums up the coast as a setting established 200 million years ago and now meeting a young, active landscape shaped by uplift, volcanic activity, glaciation, coastal erosion and river deposition.

Sources

• National Park Service, Lake Clark National Park and Preserve, West Cook Inlet Geology; rewritten in hubnx's own words. The page's map modified from a journal figure and its dates, which the page gives in more than one way, are left out where they conflict.