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Possible evidence for active faults in Kamishak Bay

From Amakdedori Beach south at least to Shaw Island, the coast along Kamishak Bay is inscribed with a distinct terrace. This terrace is the remains of an ancient beach, now lifted over 10 meters above the modern beach, evidence of uplift since its formation. Additionally, the terrace is not perfectly horizontal, evidence of further deformation after it was uplifted.

During four days of fieldwork at this site, we dug trenches into the raised beaches of the terrace, finding that they were composed of loose gravel. Elongate beach ridges were apparent as well. These loose sediments would be easily eroded by a glacier, evidence that they were formed after the most recent glaciation of this coast.

The raised beach along Kamishak Bay forms a distinct, flat-topped terrace with a steep slope leading down to it. The flat lower portion of the terrace was once a wave-cut platform, an area of intertidal reefs and tide-pools below where waves and freezing temperatures wore away the bedrock. The steep slope is what remains of an ancient bluff along the coast. The raised beach itself is not obvious in these photos, but it forms a gently sloping surface between the terrace and ancient bluff.

Geomorphology and widespread erratic boulders provide evidence that Amakdedori was fully glaciated during the last glacial maximum. The erratics, boulders stranded by a retreating glacier, are common above the elevation of the beach terrace. The landscape has rounded 'drumlin' ridges, elongate grooves, domed hilltops, rounded 'U-shaped' valleys, and other features distinctive of recently glaciated terrain. Our Kamishak raised-beach page has details of the sedimentology and geomorphology of this terrace. Based on this evidence, the beaches on this terrace must be younger than the retreat of glaciers starting less than 20,000 years ago.

We also surveyed the height of the terrace relative to the modern beach. As we had not originally planned to do this sort of surveying, we used a crude horizon-line technique. At their highest point, near Amakdedori, raised beach ridges are 20-25 m above the modern beach plain. A raised bluff was similarly high above the modern bluff. A few kilometers further south the apparent uplift was less, just 12-17 m. We have no survey results further south, but the terrace is clearly visible from the air for another 50 kilometers to the southeast.

Locally, one fault - the Bruin Bay Fault - is mapped near the southern end of the higher terrace near Amakdedori and north of the lower terrace further south. Given that terrace elevation varies by 5 m along less than 2 km of coast, this is strong evidence for active fault motion in the time following the formation of the terrace, likely on the Bruin Bay Fault. Details of this interpretation are on our possible Bruin Bay Fault offset page.

We surveyed an ancient beach above the modern beach of Kamishak bay (using a horizon-line survey method). Its elevation is quite variable, and may be related in part to motion on the Bruin Bay Fault. Aerial observations reveal that the terrace continues at a similar elevation further south and east, but it does not continue north into Bruin Bay.

Kamishak Bay raised beach:

Along the coast of Kamishak Bay is a raised beach. This beach cut a broad terrace and steep bluff. The terrace is mapped here, along with the Bruin Bay Fault (blue) and mapped folds in the area (white). The terrace is ranked according to how well we have documented it: Black terraces have been ground-truthed, middle-gray terraces have been seen from the air, and light-gray terraces were noted only on aerial or satellite imagery.

Full-size map

The terrace itself may be evidence of an active fault in the area. The beach may have formed, and then been uplifted during one or several earthquakes. This simple explanation seems unlikely however, since sea level in the local area stabilized at its current level only 6000 years ago. To create 20 m of uplift, it would take a series of earthquakes unlikely to occur within a 6000 year time span. Alternately the terrace uplift may be more gradual, spanning the last glaciation, but the beaches may have formed quickly when this area was depressed below sea-level by glaciers more recently. We explore the possible interpretations of this terrace in our Kamishak Raised Beach page.

This terrace definitely records a change in land-level over some timescale, whether long or short. This summer we will return and survey the terraces in more detail, and search for locations where the relative age of the beach and terraces can be determined. Our summer 2010 field plan page has details.