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).