Figure 7. In the lower Kaknau Creek valley, periglacial deposits are
preserved in a 25-m-tall cliff that exposes the southwestern bank of the
creek that we informally call the Kaknau Cliff section (photo credit:
USGS). The Kaknau Cliff section occurs on the uplifted, west side of the
Fairweather fault. The deposits include proglacial lake and delta
deposits. Radiocarbon ages (Table 1) chronicle a proglacial lake that
was extant at ~11,350 yr BP and persisted until no later
than ~9,600 yr BP. An intervening interval of
progradation of clastic deltaic sediment interrupted the fine-grained
lake sedimentation. The onset of progradational fluvial outwash gravels,
at least by 9,600 yr BP, marks the end of lake sedimentation. The
outwash gravels (~50 m elevation) grade to the highest
surface (~46 m elevation) of Terrace B.
The Kaknau Cliff section implies that a lake, which we informally name
Kaknau Lake, was dammed behind a terminal moraine formed by ice that
advanced down the Kaknau Valley. The lake filled the entire valley and
extended across the Fairweather fault. This interpretation is supported
by the presence of recessional moraines on Terrace C and radiocarbon
ages that indicate the lake formed prior to ~11.4 ka and
persisted until no later than ~9.8–10.6 ka (Table 2)
(Figure 8). This time frame is consistent with the inference that the
lake was dammed behind a terminal moraine formed by ice that advanced
down the Kaknau Valley during the Younger Dryas period, 12.9-11.7 ka
(Brauer et al., 2008; Rasmussen et al., 2006).
The outwash unit that caps the Kaknau Lake deposits (unit Hst) grades
downstream to the highest surface of Terrace B (Figure 4) and, when
active, filled the entire Kaknau Creek valley. The age of the outwash
provides a maximum-limiting estimate for when the paleo-sea cliff
separating Terraces B and C was cut and abandoned. Abandonment occurred
after 9.6–10.1 ka, the age of stumps rooted in the deposits of Kaknau
Lake and buried by outwash gravel (Figures 7 and 8). Deposition of
outwash gravels continued at least until 9.5–9.7 ka, based on the death
age of a younger stump buried by outwash (sample #17IP-15-S1, Table 2).
Outwash deposition ceased by 5.0–7.0 ka because the outwash gravel
predates lower shoreline deposits on Terrace B (discussed below).
The exposure at Kaknau Cliff is the product of lateral stream erosion
into a fault scarp. The cliff exposure is not a product of fluvial
incision. The exposure is a consequence of lateral erosion by Kaknau
Creek into uplifted deposits. The deposits are graded to an uplifted
marine platform that only occurs on the west side of the Fairweather
fault. Uplift must be at least 25 m because that is the relief of the
cliff face.