Bretwood Higman, PhD
Andrew Mattox
This is the first version of this report.
It is still under revision.
There are no more recent drafts.
The area around Lake Iliamna and Kamishak Bay may hold important clues about southern Alaska tectonics. Also, seismic hazard assessment in this region is urgent in light of recent interest in developing large mines and associated permanent tailings repositories - particularly the Pebble Mine. Building on fieldwork conducted in the summer of 2009, we will clarify possible evidence of recent activity on two major faults in the region, the Lake Clark and Bruin Bay faults. Additionally we will begin mapping extensive and well-preserved terraces along Lake Iliamna and Kamishak Bay that may provide a more general picture of active tectonics in the area.
Our goal is to collect five distinct datasets during 16 days of fieldwork in July 2010, three near the north and west shores of Lake Iliamna, and two along the Kamishak coast. Each of these is covered in detail in its own section below:
Yellow lines are planned profiles, while the black dot marks a proposed trench location. Gray dots mark 2009 trenches (one is very near the 2010 trench, and may be covered by the black dot.)
In 2009 we dug two trenches, each about 4 m long and a little less than 2 m deep, across a linear step (or 'scarp') in a tundra plain we called the 'Braid-scarp'. These trenches revealed possible disruption from earthquakes, but in compiling our data we found that the trenches were too small to rule out the possibility that the scarp and underlying disrupted sediment resulted from river erosion. In this field season we will dig a larger hole near to one of these previous trenches, in hopes of conclusively eliminating either the river-erosion or fault-offset hypotheses.
Several faint lines that appear similar to the Braid-scarp on aerial photos, are visible 5 kilometers east of the Braid-scarp. They are not as linear as the Braid-scarp, and are nearly parallel to ancient river channels visible in the tundra, so we initially assumed that they are fluvial scarps. However, even if this is the case they may provide a useful comparison to the Braid-scarp, and their location is consistent with splaying of the Lake Clark fault. We will at a minimum ground-truth these features and survey across them with a mapping grade GPS.
Additionally we may dig a shallow trench across the "Slope-disruption," another feature parallel to and nearly co-linear with the Braid-scarp. At this location about a meter of loess covers the feature, possibly preserving clear evidence of deformation at the lower contact of the loess where it overlies sandy cobbly gravel.
There are clear beach-ridge terraces left by former lakeshores along much of the coast of Lake Iliamna. There are around four distinct terraces, each formed during a period of constant lake-level. Lake level dropped repeatedly in Lake Iliamna's history, leading to the stranding of old beach-ridge plains as terraces, and the formation of new beach features. These periods of lake-level drop presumably corresponded to periods of more rapid incision of the lake outlet.
Tom Evans and Lise Bernhardt will survey a series of profiles across these terraces on the north and west side of the lake using a GPS with submeter accuracy. If the beach-ridge plains are sufficiently consistent in elevation, this may reveal subtle tectonic tilting or offset in these originally horizontal terraces. Profiles will extend all the way from the modern beach, providing a modern analog for the raised beaches.
A prominent raised-beach terrace is perched about 15 m above the contemporary beach along Kamishak Bay. Like the Lake Iliamna terraces, this provides a marker for horizontal at some point in the past, so deformation in this terrace provides evidence for active tectonics. Also this terrace was likely carved by the ocean, and so may record tectonic uplift.
To estimate the age of the Kamishak terrace, we will dig pits looking for clear tephra sequences both on the terrace and in uplands well above them. If these two two surfaces have similar tephra sequences, that suggests the terrace emerged from sea level shortly after the end of the last glaciation, while a shorter tephra sequence on the terrace is evidence that the terrace is younger, likely last occupied after sea level stabilized about 6000 years ago. Additionally these pits may reveal sand layers left by tsunamis along this coast.
| Dates | People | Activity |
|---|---|---|
| July 6 | Hig (Bretwood Higman), Andrew Mattox, Mike Borden | Fly Homer to Braid-scarp and join Tom Evans and Lise Bernhardt (walking to field site from other fieldwork) |
| July 6-9 | Hig, Tom, and Andrew | Dig major Braid-scarp trench, begin documenting trench (interspersed witd digging). |
| July 10-11 | Hig, Tom, Andrew | Trench documentation, plus some additional digging. Documentation should be 'complete' at the end of this, needing only clarification and some detail. |
| July 12 | Rich Koehler (AK DGGS) | Arrives in field, examines trench and our documentation. The goal of this is to engage the state geological survey in our work and to provide an independent point of view from our interpretation of the trench. |
| July 12 | Hig | Aerial photography of trench and nearby sites, additional documentation in consultation with the trench auditor |
| July 13 | Hig, Tom, Andrew, Mike, Lise, Rich | Final documentation. |
| July 13 (late in day) | Hig, Andrew | Flight to Bruin Bay. |
| July 13 (late in day) | Rich, Mike | Flight home (same flight as Hig and Andrew) |
| July 13-17 | Tom, Lise | Survey of terraces between Braid-scarp and Iguigig. Tom and Lise will continue from here to do their own separate geological fieldwork. |
| July 14-21 | Hig, Andrew | Kamishak terrace survey and tephra pits |
| July 22 | Hig, Andrew | Flight home |
House symbols mark possible campsites, while pins mark field sites.