Figure 8. Modeled timing of Kauknau Lake drainage, the age of the Terrace B shoreline angle, and the deaths of sub-fossil stumps 33 and 34 at Stump Slough (Figure 10). Radiocarbon (n = 15) and luminescence (n = 4) ages inform the model (Witter and Bender, 2021). Red probability density functions (PDFs) show model age ranges (in years before 1950 CE) that span the 95% confidence interval. Light gray PDFs show prior ages for 14C samples (Table 2) and IRSL samples (Table 1); dark gray PDFs show posterior ages. The onset of rapid glacier rebound at 150–200 years before 1950 CE (Mann and Streveler, 2008) is used as a minimum limiting constraint on the death age of stump #33. PDFs are computed using OxCal version 4.4.2 (Bronk Ramsey, 2009; 2023) and the IntCal20 dataset of Reimer et al. (2020) and reported at the 95% confidence interval. The terms Boundary, R_Date, Phase, C_Date, and D_Sequence are commands applied in OxCal model code, which is included in the Appendix.
4.3 Block uplift of Icy Point implied by marine terrace slopes
The slopes of marine terrace surfaces at Icy Point show little evidence for fault-perpendicular tectonic tilting and imply block uplift of the peninsula implicating vertical slip on the Fairweather fault. Slopes measured on lidar topography fall within the range of gradients measured on modern, undeformed shore platforms in central California. Bradley and Griggs (1976) demonstrate that modern platforms slope gently seaward and consist of two sections: an inshore platform (1.1°–2.3° slope), usually 300–600 m wide, and a slightly flatter offshore platform (0.4°–1.0°).
We measure the surface slopes of Terraces C and B from profiles of lidar topography depicted in Figure 9 and listed in Table 3. Terrace C, profile H–H’, oriented parallel to the Fairweather fault, slopes gently southeast 0.1°–0.9°. Southwest-facing slopes on Terrace C, profiles I–I’ and J–J’, oriented perpendicular to the fault, range from 0.9°–2.3°. At the nose of Icy Point, the flat upper portion of Terrace B slopes 0.8° to the southeast. Terrace C surface slopes do not necessarily reflect the slope of the underlying erosional marine platform because late Pleistocene glaciers crossed Terrace C and probably modified its surface (Witter et al., 2021). Faults, like the Finger Glacier fault, vertically displace Terraces B and C and impose local, shore-parallel surface deformation. However, all measured surface slopes fall within the range of slopes inherent to undeformed shore platforms, and tectonic tilting of Terraces B and C, if any, likely amounts to less than ~1° (Figure 9).