Figure 6. Topographic profiles of the west-side-up principal strand of the Fairweather fault that delineates the western margin of the Kaknau Creek valley. Profile locations shown in map at upper right; red arrows point to 20- to 30-m tall scarp formed by principal fault strand. (a) Tectonic uplift west of the Fairweather fault forms a 20- to 25-m-tall scarp that elevates and isolates outwash plain deposits above active stream channel deposits in the Kaknau Creek valley on the east side of the fault. Secondary scarps near the top of the main fault scarp mark headscarps of large slope failures or secondary faults related to oblique slip. (b) Tectonic uplift elevates marine terraces immediately west of the 25-to-30-m tall Fairweather fault scarp at the mount of Kaknau Creek. The1958 fault rupture produced ~3 m of right-lateral slip, but no vertical displacement along this profile.
4.2 Timing of deposition and incision in lower Kaknau Creek Valley
Along the western margin of the Kaknau Creek valley, at a site informally called the Kaknau Cliff section, periglacial deposits exposed in ~25-m tall cliffs eroded by the creek (Figure 7) constrain the initial timing of Terrace B emergence and add insight into the vertical component of displacement on the Fairweather fault at Icy Point. Kaknau Creek drains the southwestern corner of the Fairweather Range, and the west side of the valley, near the mouth of Kaknau Creek, is bordered by the active Fairweather fault (Figure 4). The Kaknau Cliff section exposes rhythmically bedded silt and sand deposited in a proglacial lake and cross-bedded sand deposits formed in a proglacial delta. Outwash gravel overlying the lake sediment marks the end of lacustrine deposition (Figure 7). The elevation of the outwash surface (~50 m) at the Kaknau Cliff section, >25 m higher than the modern Kaknau Creek channel, implies that the Fairweather fault steps ~600 m to the east and forms a 25-m tall, linear escarpment toward the northwest (Figures 4 and 6). Like the modern creek channel, which grades to present sea level, the highest surface of the outwash plain (Hst), based on its elevation and slope, was graded to sea level that formed the highest surface (~46 m) of Terrace B.