Bretwood Higman, PhD
Andrew Mattox
This is the first version of this report.
It is still under revision.
There are no more recent drafts.
Along the southern coast of Kamishak Bay an ancient beach lies stranded 10-20 m above modern high-tide. The beach notches the coastal hills, leaving a flat terrace below a steep slope. The terrace is broad and flat, and terminates up-slope in a short gentle slope below an angle-of-repose slope. In places beach plains with clear beach ridges are at roughly the same elevation as the upper edge of this terrace.
The northern portion of this terrace near Amakdedori Beach is higher, at over 20 m above the modern beach, than the terrace near Chenik head, which is closer to 15 m. We have no elevation data further south than this, although it does continue and we photographed the terrace from a plane southeast as far as Shaw Island. To the north, in Bruin bay, there is no evidence of a terrace.
Given its remarkable preservation, there is little doubt that this terrace is a raised beach. A number of possible interpretations might explain its origin and the relative sea-level drop since then. In particular:
The terrace may have formed during the previous interglacial about 120,000 years ago. In this scenario, the area is assumed to have been unglaciated in the last ice age, even though much of the surrounding area was covered by ice. The lack of glaciation in this particular area allowed the ancient beach to remain, while other areas were eroded. Gradual land-level rise since then, largely when sea level was lower during the last ice-age, perched this beach where it is now.
The terrace may have formed during the last interglacial, but then been overrun by glaciers. The basic form of the terrace remained, and was re-occupied briefly when the land was depressed by the weight of glaciers at the end of the last ice-age, creating the beach ridges and other loose sediment we observed. In this scenario, the area is assumed to have been glaciated in the last ice age, and the beach sediments on the terrace are assumed to have formed during a brief period of higher relative sea level around 10,000-20,000 years ago.
The perched beach may have formed on the shore of a large ice-marginal lake late in the last glaciation.
The terrace may have formed after sea level stabilized about 6000 years ago, and have been uplifted by one or more earthquakes since that time.
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.
Each of these interpretations has substantial problems. Our preferred interpretations are that the terrace is a re-occupied beach originally eroded during the last interglacial and recording gradual uplift (B), or that it is a recent beach that is evidence of rapid tectonic uplift (D). The re-occupied terrace hypothesis (B) is the most geologically plausible, but is contradicted by many pieces of evidence. The tectonic uplift hypothesis (D) is most consistent with the evidence, but the rapid rate of uplift is geologically improbable.
During the last interglacial, about 120,000 years ago, sea levels are thought to have been 6-8 m higher than today (Kopp et al., 2009). Uplift of 5-15 meters during that 120,000 years, a rate of about 0.1 mm/yr., would be a fairly normal rate for mostly stable mountainous terrain. However, delicate beach ridges composed of loose gravel would not have survived glaciation in the millennia following the last interglacial, so if this explanation is true, the area must have been unglaciated during the whole of that 120,000 years. This seems very unlikely, given the glacial landforms and erratics that are common just above the terrace. Also, the northern end of this terrace lies at the base of a broad low pass between the Lake Iliamna and Cook Inlet basins, both of which supported large glaciers during the last interglacial. In short, this area seems very likely to have been overrun by thick glaciers for long periods of time, which would erase the delicate beach features we observed.
Alternately, the terrace may have formed during the last interglacial, and then been overridden during the ice-age that followed. If the basic bedrock form of the terrace survived, and if this area deglaciated early after the last glacial maximum about 20,000 years ago, then it might have been exposed to beach erosion during a brief period when glacial ice further up Cook Inlet and in Lake Iliamna depressed the land surface (raising relative sea level). Hypothetically the isostatic rebound that followed ice-retreat could even have resulted in uplift that was at the same rate as sea level rise for some time. This would allow more time for the ocean to rebuild the beach on the ancient terrace.
One feature that particularly fits with this interpretation is the presence of beach ridges that lie below the highest beach ridges, but remain well above contemporary beach ridges and are separated from them by erosional banks. These may record a time following reoccupation of the terrace when relative sea level gradually dropped as isostatic uplift outpaced sea level rise.
If this is in fact the explanation, it is surprising that no evidence of the erosion of the original terrace remains. Preserved bluffs above the terrace are sharp and distinct, with no signs of glacial alteration. In contrast, raised terraces on Kodiak Island, overridden by the same Cook Inlet Glacier, are much less distinct, and the upper edge of the uplifted bluffs are rounded (Carver et al., 2008). These Kodiak terraces, in a much better position to be exposed to the ocean early in deglaciation, show no sign of re-occupation.
This scenario also requires a strong coincidence between uplift and sea level rise. Relative sea level had to remain constant for long enough to build beach ridges and erase all sign of glacial erosion.
In exposures at the top of actively eroding sea-cliffs, we found no cases where moraine lay directly on top of the terrace, as would be expected if the terrace pre-dated glaciation.
Another coincidence in this scenario is that the re-occupying water body must not have extended to Bruin Bay just to the north, where there is no evidence of beaches above modern sea level. This might be explained by a small valley glacier lying in Bruin Bay at this time.
The apparent deformation of the terrace since its formation is further evidence that it is not an old terrace recently re-occupied. In this scenario, the deformation of the terrace was gradual, occurring over the last 120,0000 years, mostly before the terrace was re-occupied by a beach. But then the recent re-occupying beach should have been mis-aligned with either the lower or the higher portion of the terrace, leaving one of them without the veneer of a beach. But both the lower and higher terraces we surveyed had sharp landforms that would not have survived glaciation.
Finally, if the terrace from the last interglacial survived mostly intact to the beginning of this interglacial, it seems like some trace should remain of another even higher terrace from the interglacial before that. Carver et al., 2008 found this to be the case in Kodiak, but in Kamishak slopes above the terrace are smooth glacial landforms with no signs of inscribed beaches.
If a lake persisted in Kamishak Bay, impounded by the Cook Inlet Glacier to the east at the end of the last glaciation, it could have led to the formation this terrace. However, this would have to be a very broad lake, with enough fetch to stimulate the formation of prominent storm ridges on its beach plains. Additionally, it would have had to be extremely stable, more so than most ice-marginal lakes. This seems unlikely given the lack of any bedrock spill-point at the appropriate elevation. The spillway from the lake would have to be over ice, which likely would vary in elevation, leaving a series of 'bathtub ring' beaches like are often seen in ice-marginal lakes, not a single prominent beach.
What appears to be a sloping beach along the base of preserved bluffs suggests that there was a substantial tidal range on the water body that formed the beach, which would certainly not be true for a lake.
The geomorphology of this terrace is consistent with Holocene uplift in a series of earthquakes recorded by the sequence of raised beach ridges. The main problem with this interpretation is that the rate of uplift would have to be so high as to be nearly absurd as a long-term average.
Both the uplifted terrace and the modern wave-cut bedrock platform are similarly broad, so if both formed after sea-level stabilized around 6000 years ago, then presumably uplift of the upper terrace began sometime in the middle of that time-range, say 2000-4000 years ago. Given that, the uplift rate to get 20 m of uplift at Amakdedori would have to be 5-10 mm/yr. As a long-term average, this is comparible to the most active mountains in the world near Mt. St. Elias and the eastern Himalayas. Given the gently rolling hills and low mountains, this area has clearly not experienced this rate of uplift over the past few million years.
It is possible that this terrace still arises from tectonic uplift, but that uplift was anomalous, and did not reflect the long-term average. If a short period of rapid uplift occurred every 100,000 years, then the long-term average uplift would be more in-line with the regional geomorphology.