Figure 11. (a) Map of Icy Point marine terraces showing area of lidar
topography (Witter et al., 2017b) featured in (b) and the locations of
two elevation profiles across Terraces A and B. (b) Lidar topographic
map of the southern-most nose of Icy Point. Shore-parallel beach ridges
visible on the lowest surface, Terrace A, evidence gradual, ongoing
glacial isostatic rebound following the LIA. Erosional beach scarps
delineate shorelines (B1, B2, etc.) beveled into Terrace B and elevated
by uplift during repeated earthquakes. (c) Topographic profile F–F’
plotting the lowest lidar-measured elevations (black line) along a 50-m
wide swath shown in map (a); all other elevations along the swath
plotted in gray. Numbers below the profile indicate shoreline angle
elevation in meters (NAVD 88). Beach sand below shoreline B8 dates to
3.7–7.0 ka. (d) Topographic profile G–G’ extending from the modern
shoreline to the ancient seacliff separating Terrace B and Terrace C.
Dune sand covering the highest shorelines dates to 3.7–5.8 ka.
4.6 Age of the paleo-seacliff separating Terraces B and C
We dated coastal landforms to estimate the age of the 30-to-60-m-tall
paleo-sea-cliff that separates Terrace B from the older and
topographically higher Terrace C (Figures 4 and 9). On the highest
surface at the southern end of Terrace B, at an elevation of
~56 m near the foot of the paleo-sea cliff, we excavated
a ~2-m-deep pit in an aeolian sand dune (Sand dune,
Figure 4). The parabolic shape of the dunes, with steepest aspect toward
the southwest, suggest katabatic winds blowing off glaciers to the
northeast mobilized beach sand and deposited it on emergent marine
terraces. At depths of 1.05 and 1.42 m in the pit we sampled aeolian
sand for IRSL age determinations on feldspar grains to estimate the last
time the dune sand had been exposed to light. The shallower sample has
an IRSL age of 4.18 ± 1.39 ka and the deeper sample has an IRSL age of
4.74 ± 1.10 ka (Table 1).
We also sampled beach sand from an exposure of marine terrace sediment
below Terrace B shoreline B8 at an elevation of ~35 m
near the top of a steep-sided ravine (Tocher’s Gulch, Figures 4 and 11).
The beach sand deposit was ~4 m thick above Topsy
Formation bedrock and we sampled at depths of 2.70 m and 3.89 m below
the terrace surface. The shallower sample has an IRSL age of 4.39 ± 0.66
ka and the deeper sample has an IRSL age of 5.99 ± 1.02 ka (Table 1).
From these ages and their uncertainties we infer that beach processes
began depositing sand before 5.0–7.0 ka and terrace emergence occurred
sometime after 3.7–5.0 ka.
The IRSL ages of separate landforms provide independent minimum
estimates for the time of abandonment of the highest shoreline angle
that separates Terrace B from Terrace C (Figures 4 and 12). The highest
shoreline angle was abandoned before 5.0-7.0 ka because beach sand
exposed in Tocher’s Gulch, at an elevation more than 20 m below the
highest shoreline angle, was active at this time. A second minimum age
for the emergence of the paleo-sea cliff comes from the age of aeolian
sand, 3.6–5.8 ka, which advanced over Terrace B after its highest
shoreline was abandoned. At a human occupation site near Tocher’s Gulch,
charcoal at the base of aeolian sand dated to 4.8–5.2 ka (Crowell et
al., 2013). Considering the IRSL data and the coastal landforms from
which the samples come, the minimum age of 5.0–7.0 indicated by beach
sand more closely limits the time of abandonment (Figure 8).
To estimate the age of the Terrace B shoreline angle we use a numerical
model (OxCal, v4.4.2; Bronk Ramsey, 2009; 2023) to compute the 95% age
range (probability density function) based on maximum and minimum age
constraints. Radiocarbon ages from outwash gravel graded to the highest
Terrace B shoreline provide maximum age constraints that range from 9.1
to 10.1 ka. Minimum age constraints come from younger beach deposits,
estimated to range from 5.0 to 7.0 ka, that post-date the cutting of the
Terrace B shoreline angle. Given these age constraints, the shoreline
angle at the top of Terrace B was cut between 5.5 and 9.4 ka (Figure 8).
4.7 Reconstructing the tectonic component of relative sea-level change
at Icy Point
Profoundly different Holocene RSL histories occur on either side of the
Fairweather fault. Although geomorphic features left by the LIA
sea-level highstand do not vary across the fault, tectonic features
differ markedly across the fault. Based on these differences evident
within hundreds of meters of the Fairweather fault, we show that the
different RSL histories can be explained by tectonic uplift west of the
Fairweather fault (Figure 12).
In Icy Strait, 30 km east of the Fairweather fault, RSL change
fluctuated from 4.1 ±1 m below present sea level at 6.9–7.2 ka to its
present elevation, including the Little Ice Age high stand that
registered ~4 m above present for the several centuries
ending by 1750–1800 CE (Mann and Streveler, 2008) (Figure 12). Although
Mann and Streveler (2008) assume that tectonic effects were unimportant
in the RSL history in Icy Strait east of the Fairweather fault, in fact
their reconstruction incorporates all of the varied factors affecting
RSL: eustacy, isostacy, tectonism, local, and other processes (Shennan
et al., 2012). We adopt their assumptions and utilize their Icy Strait
RSL reconstruction 30 km northeast along the open-ocean coast from Icy
Strait to the east side of the Fairweather fault at Palma Bay (Figure
1). Several observations support this extension: 1) relatively low
thermochron ages for rock samples at Palma Bay sites east of the
Fairweather fault indicate low rock uplift (Lease et al., 2021): 2) the
absence of marine terraces along the coast of Palma Bay; 3) low
seismicity and the lack of mapped convergent structures east of the
Fairweather fault at the latitude of Icy Point; and 4) geodetic data
that indicate that fault-normal strain can be accommodated by reverse
faults west of the Fairweather fault.
In contrast to the coastline east of the Fairweather fault, RSL at Icy
Point has fallen 42.3 ±2.1 m during the Holocene based on the elevation
of the Terrace B shoreline angle relative to the modern shoreline
(Figure 12; Table 4). We reconstruct RSL change at Icy Point by
comparing the heights of the Terrace B shoreline angle with its modern
equivalent. The height of the Terrace B shoreline angle was estimated by
subtracting the thickness of terrace sediment (3 ±1 m), approximated
from field exposures and pits, from the elevation of the inner edge of
terrace B at the base of the sea cliff leading up to Terrace C. We
determined the mean elevation (47.3 ±1.7 m) of the Terrace B inner edge
by tabulating the elevation of the sharpest inflection at the base of
the sea cliff in 25 lidar profiles (Table 4). The elevation of the
modern shoreline angle (2.0 ±0.8 m, NAVD 88 datum) was determined from
measurements of 50 lidar profiles along the inner edge of the shore
platform. We subtract the Terrace B shoreline angle from the modern
shoreline angle (Table 4) to estimate the change in RSL at Icy Point
(-42.3 ±2.1 m) since the original formation of Terrace B between
5.5–9.4 ka.
To evaluate the contribution of tectonic processes to RSL changes at Icy
Point, we assess the difference between Holocene RSL change on either
side of the Fairweather fault (Figure 12), including all factors that
affect RSL change (Shennan et al., 2012; Shennan, 2015). We assume that,
within errors of the methods we use and those of Mann and Streveler
(2008), the contributions of eustacy, isostacy, and local tidal effects
to RSL change do not vary directly east and west of the Fairweather
fault at Icy Point. However, the tectonic contribution to RSL change
varies substantially across the fault. Therefore, taking the difference
in Holocene RSL change across the fault results in a measure of the
tectonic contribution of RSL.
To assess the difference in RSL change across the Fairweather fault, we
first reconstruct Holocene RSL curves for Icy Point and Palma Bay
(Figure 12). We estimate that since the formation of the Terrace B
shoreline angle at Icy Point between 5.5–9.4 ka, RSL has fallen
dramatically, equaling -42.3 ±2.1 m (Table 4); whereas, based on
reconstructions along Icy Strait, including Palma Bay directly east of
the Fairweather fault, RSL has risen +4.1 +1/-0.75 m since 6.9–7.2 ka
(Mann and Streveler, 2008). The difference in RSL change across the
Fairweather fault indicates that the tectonic contribution to RSL change
at Icy Point is -46.4 ±2.4 m (Table 4, Figure 12). The rate of RSL fall
at Icy Point, -6.8 ± 2.2 m/ky, is the quotient of the tectonic
contribution to RSL change divided by the age of the Terrace B shoreline
angle (see Figure 4). We attribute this substantial rate of RSL fall at
Icy Point to tectonic rock uplift since the formation of Terrace B at
the rate of 6.8 ± 2.2 m/ky (4.6 to 9.0 mm/yr).