Soft Sediment Faulting on the Kenai River – An outcrop analogue for UHR3D Seismic
By
Brian Brookshire, Applied Science Group, NCS SubSea
I recently participated in a charity fishing tournament on the Kenai River near Sterling, Alaska. We didn’t have any trouble catching fish, and almost, maybe for a split second, had a contender for the largest rainbow trout (the target fish of the tournament). However, to me, more exciting than the world class rainbow fishing was the scenery. The water is a silt-saturated, cloudy, aquamarine – oddly similar to the look they are going for in the waterways of Disney World, but this is the real thing. There are bald eagles everywhere, sitting in trees, flying by, and devouring salmon washed up on the shore. The trees along the bank are mostly small fir trees (Black Spruce, I believe), but there are at least a few aspens as evidenced by the bright yellow fall colors. Though, beyond all of this, the thing that most intrigued me was the interesting geology evident in some of the river bank bluffs. While we were on one of our runs back up-river, after having drifting downstream fishing, I noticed a particularly interesting section of bluff. I started snapping pictures with my cell phone, and our guide “coach” asked: “You want to go over there and get a closer look?” Of course I did, and given that the only other fisherman in the boat was Pat, a co-worker of mine who had no objections, we broke off the fishing for a couple of minutes to go take a closer look.


Figure 1. – Satellite image of the Kenai River meandering through the Nikishka Lowland. The blue arrow signifies the position of the bluff, and the red arrow identifies a glacial moraine.


 
The Kenai River flows out from Skilak Lake and meanders through a section of the Kenai Peninsula called the Nikishka Lowlands. The geology of the region is very much a product of numerous Pleistocene glacial advances and retreats (Karlstrom 1964). Very obvious in the available satellite imagery are numerous end, lateral and interlobate moraines (Figure 1). The stratigraphy, as revealed in river and sea bluffs, is dominated by glacial till, glaciolacustrine and glaciofluvial deposits (Karlstrom 1964). Such is the case with the river bluff that we investigated during our fishing trip (Figure 2). The river bluff, roughly 17 meters in vertical relief above the river bed, is comprised of three somewhat distinct units (Figure 3). Unit 1 appears to be stratified and potentially contorted sand. Due to the undercutting of the bank, and the varying state of failure, it is difficult to describe subtle aspects of this unit. Unit 2 appears to be interbedded layers of semi-brittle sandy silt and silty sand. This unit is fragmented via numerous normal faults (perhaps of seismic origin) that, in some cases, extend into the underlying and overlying units (Hamilton and Shennan 2005, Vanneste, Meghraoui, and Camelbeeck 1999). The uppermost unit, unit 3, appears to be stratified, gravelly, sandy silt. Here again, it is difficult to clearly discern subtle aspects of this unit due to slope failure and overhanging trees and moss, but the color appears to be predominantly grey (silt with a higher organic content) and the increase in gravel concentration is obvious.

While looking at the bluff, and then later in the office looking at the pictures I took, my mind kept running in the same direction. Given the scale of the geology, what of these feature would we likely be able to resolve if we were imaging a similar sequence via ultrahigh-resolution (marine) seismic methods (UHR3D)? Of course, the good geoscientist in me says: we just don’t know. In a marine environment it would depend on seafloor type, lithology, water content, etc., etc. – all the things that affect acoustic impedance and attenuation. However, conceptually, we can make a couple of reasonable conclusions. Conclusion 1: assuming that unit 2 (around 2-3 meters thick) is of significant contrast in term of acoustic impedance, we would be able to image this unit, at the very least, as a pair of reflectors (top and bottom of unit). In order to accomplish this, we would only need a dominant frequency of about 150Hz. Conclusion 2: given the apparent horizontal resolution of very small fault blocks (~30 meters) in the SAFE-BAND dataset from the GoM, we should be able to discern, at least, the fault along which the largest offset has occurred (just to the right of center of the photograph). Now, these are not earth shattering conclusions, but by virtue of this comparison, I am now able to draw a mental image as to the rough scale of feature that it is possible to image via UHR3D methods. Now for the challenge, how many faults can you find in this picture? Enjoy!



Figure 2. – Uninterpreted image of the bluff along the Kenai River. (Click for a hi-resolution version)





Figure 3. – Interpreted image of the bluff along the Kenai River. The green lines represent fault planes as interpreted by the author. The red gradation identifies the three general stratigraphic units identified in the cross section. (Click for a hi-resolution version)

References

Hamilton, S., and I. Shennan. 2005, Late Holocene great earthquakes and relative sea-level change at Kenai, southern Alaska. Journal of Quaternary Science, 20, no. 2,95-111.
Karlstrom, T. N. 1964, Quaternary geology of the Kenai lowland and glacial history of the Cook Inlet region, Alaska. US Govt. Print. Off.
Vanneste, K., M. Meghraoui, and T. Camelbeeck. 1999, Late Quaternary earthquake-related soft-sediment deformation along the Belgian portion of the Feldbiss Fault, Lower Rhine Graben system. Tectonophysics, 309, no. 1,57-79.