Figure 9. Marine terrace elevation profiles; locations shown on index map at lower left. (a) Elevation profile H–H’ trending northwest showing relative elevation of Terraces D, C, B and A. The riser between Terrace C and B is a paleo sea cliff that marks an interval of relative sea level stasis, when riser height was increased by continued freshening of the base of the riser through coastal erosion. (b) Elevation profile I–I’ trending ~N70°E, perpendicular across the long axes of Terraces D, C, B and A. The highest Kaknau Creek outwash plain east of Terrace D grades downstream to the highest surface of marine Terrace B. Red letter F marks active strand of Fairweather fault. (c) Elevation profile J–J’ trending ~N70°E perpendicular across the long axes of Terraces C, B and A. (d) Elevation profile K–K’ trending ~N20W across Terraces B and A. Arrows in all profiles indicate the direction and length over which terrace slopes were measured on lidar topography (Witter et al., 2017b).
4.4 Little Ice Age coastal geomorphology reflects glacial rebound
Mann and Streveler (2008) reconstructed post-LGM relative sea-level change in Icy Strait and documented a high-sea stand during the LIA and subsequent RSL fall accompanying glacial rebound following 1750 CE. The geomorphology of Terrace A at Icy Point and the 14C ages of coastal landforms backing Palma Bay are consistent with the Icy Strait relative sea-level history. On Terrace A, west of the Fairweather fault, the age (400–30 cal yr BP, Table 2) of a bivalve shell collected within beach sand (Figure 10) implies emergence of the terrace surface within the past few centuries, but broad date uncertainties preclude its use in the age model (Figure 8). The highest beach ridge along the landward edge of Terrace A was formed by the LIA high stand and reaches elevations of 12–14 m (Figures 4 and 10) (Mann and Streveler, 2008).
East of the fault a 12–14 m high, barrier beach ridge fronts the Kaknau Creek valley along the northern shore of Palma Bay. The crests of the beach ridge east of the Fairweather fault has a similar elevation as beach ridge crests backing Terrace A (Figure 10) and both landforms emerged after 1700 CE, based on spruce forest ages and soil development (Mann and Streveler, 2008). We infer that these beach ridges on either side of the Fairweather fault were constructed by ocean waves near the end of the LIA when RSL was ~4 m higher than present. The concordance of the beach ridge elevations on either side of Kaknau Creek (Figure 4) indicate that little or no vertical displacement has occurred on the Fairweather fault since the marine transgression that occurred at the height of the LIA, ca. 1750 CE.