Bruin Bay Fault, Contact Point

Bruin Bay Fault, Contact Point

by | May 1, 2019

Contact Point is a headland at the entrance to Bruin Bay, about 20 miles (32 km) west of Augustine Island, on the west shore of Cook Inlet, Alaska. It was named in 1926 by U.S. Geological Survey for a fault contact exposed between two geologic formations.

The Bruin Bay Fault is a complex of steeply dipping faults that extend roughly 308 miles (498 km) along the western region of the Cook Inlet Basin from Becharof Lake on the Alaskan Peninsula north to the Castle-Mountain Lake fault system. Plate tectonics has played a major role in the formation of these rocks on the Alaska Peninsula. The fault complex separates Triassic (237–201 million years ago) and Jurassic (164–145 million years ago) volcanic rocks from Mesozoic rocks that belong to the Peninsular terrane. Evidence from paleomagnetism, fossil assemblages, and depositional environments suggests that the Peninsular terrane rocks formed originally in tropical waters and were transported across the proto-Pacific Ocean to their present location by the end of the Mesozoic (66 million years ago).

The Bruin Bay Fault is exposed 4.5 miles (7 km) southwest of Contact Point and the deformation and uplift of these rocks is still ongoing today. Differential rates of weathering and erosion of the hanging rock and footwall rocks, as well as two volcanic intrusions, clearly exposes the fault plane. Read more here and here. Explore more of Contact Point here:

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