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.