U.S. Department of the Interior

U.S. Geological Survey

Quaternary Uplift History of Wingham Island, 

South-Central Alaska

Professional Paper 1760–B

Studies by the U.S. Geological Survey in Alaska, 2007
This page intentionally left blank
Quaternary Uplift History of Wingham Island, 

South-Central Alaska

By James B. Chapman, Peter J. Haeussler, and Terry L. Pavlis

Professional Paper 1760–B

U.S. Department of the Interior

U.S. Geological Survey 

Studies by the U.S. Geological Survey in Alaska, 2007
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2009

This report and any updates to it are available online at:

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Although this report is in the public domain, it may contain copyrighted materials that are noted in the text. Permission

to reproduce those items must be secured from the individual copyright owners.

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, February 18, 2009

Text edited by Tracey Suzuki 

Layout and design by Stephen L. Scott

Suggested citation:

Chapman, J.B., Haeussler, P.J., and Pavlis, T.L., 2009, Quaternary uplift history of Wingham Island, south-central Alaska, 

in Haeussler, P.J., and Galloway, J.P., eds., Studies by the U.S. Geological Survey in Alaska, 2007: U.S. Geological Survey 

Professional Paper 1760-B, 13 p. [http://pubs.usgs.gov/1760/b/].

FRONT COVER 

Photograph of Wingham Island taken from Suckling Hills looking west-southwest. (Photograph by James B. Chapman.)
Contents

Abstract ------------------------------------------------------------------------------------------------------1

Introduction---------------------------------------------------------------------------------------------------1

Geologic Setting----------------------------------------------------------------------------------------------1

Glacial History ------------------------------------------------------------------------------------------2

Methods and Results ----------------------------------------------------------------------------------------5

Stratigraphy-------------------------------------------------------------------------------------------- 5

Sea-Level History --------------------------------------------------------------------------------------7

Isostatic Uplift ------------------------------------------------------------------------------------------7

Discussion and Conclusions------------------------------------------------------------------------------ 10

Acknowlegments------------------------------------------------------------------------------------------- 11

References Cited------------------------------------------------------------------------------------------- 11

Figures

1. Overview map of the Wingham Island area showing major faults, proposed 

 glacial extent during the Last Glacial Maximum (LGM), and ice flow direction 

 of major glaciers---------------------------------------------------------------------------------2

 2. Geologic map of southern Wingham Island showing bedrock and 

 surficial units, modified from Plafker (1974)--------------------------------------------------3

 3. Photographs from Wingham Island --------------------------------------------------------------4

 4. The radiocarbon age from section D is plotted with eustatic sea-level data 

 in respect to the present, as presented in Lambeck and others (2002)-------------------6

 5. Spatial correlation of measured sections A-E and units 1-5 in the Wingham 

 Island deposits-----------------------------------------------------------------------------------6

 6. Asthenospheric “flow” model for isostatic adjustment showing an increasing 

 cumulative surface displacement and decreasing uplift rate as a function 

 of time since instantaneous removal of the glacial load (at t=0) --------------------------9

 7. The relative sea-level (RSL) curves predicted by the two isostatic (“flow” and 

 “flexure”) models combined with eustatic sea-level data from figure 4 --------------- 10

Tables

 1. Radiocarbon dates for selected samples--------------------------------------------------------7

 2. Variables used in equations -----------------------------------------------------------------------9

III
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1

Abstract 

Marine terraces cut into Pleistocene deposits on 

Wingham Island in the Gulf of Alaska provide new 

constraints on the position of sea level, ice thickness 

and total glacioisostatic rebound at the end of the Last 

Glacial Maximum. A radiocarbon age of 13.9 ± 0.15 

ka on the most prominent terrace is coincident with the 

end of meltwater pulse 1A, possibly suggestive of a link 

between changes in relative sea level and terrace formation. 

Isostatic modeling suggests a local ice thickness of 600 

to 700 m with high (~10 cm/yr) initial rates of postglacial 

rebound. In addition to the unique ties to meltwater 

pulse 1A, the timing of emergence for Wingham Island 

following the Last Glacial Maximum has implications 

for the early migration of humans into North America.

Introduction

The coastal St. Elias Mountains in southern Alaska 

contain the largest glaciers and most extensive ice fields 

on the North American continent. However, the glacial 

extent and timing of retreat following the Last Glacial 

Maximum (LGM) are poorly constrained (Hamilton, 1994; 

Kaufman and Manley, 2004). Additionally, the sea-level 

response and isostatic effect from collapse of these glacial 

systems at the end of the LGM are relatively unexplored. 

In this paper, we describe unconsolidated sedimentary 

deposits at Wingham Island of probable Pleistocene age, into 

which are cut a series of marine terraces. New radiocarbon 

ages suggest a temporal correlation between incision of 

the highest terrace and meltwater pulse 1A (MWP1A), 

characterized by rapid eustatic sea level rise. We apply 

two models of glacioisostatic rebound to compare local 

uplift with eustatic sea-level in order to asses whether 

terrace incision at Wingham Island is compatible with a 

lull in relative sea-level (RSL) fall during MWP1A. We 

also provide new estimates of ice thickness and total 

isostatic uplift and comment on local rates of ice retreat. 

Finally, we speculate on the relationship between the 

timing of glacial retreat, paleogeography, and early human 

migration into the Americas by means of a coastal route.

Geologic Setting

Wingham Island is located at the easternmost extent of 

the Aleutian subduction zone, along the edge of the Yakutat 

microplate, which is presently being subducted beneath North 

America (fig. 1). A local splay-fault system on the Aleutian 

megathrust, the Kayak Island Zone (Plafker and others, 

1994), surfaces 10 to 15 km southeast of Wingham Island 

and is currently uplifting Kayak Island and the Suckling Hills 

(fig. 1). Tilting of an upland surface in the Suckling Hills 

(Chapman and Vorkink, 2006) and backlimb rotation within 

the Kayak Island Zone, as imaged in a high-resolution seismic 

line (Worthington and others, 2008), suggest that significant 

tectonic uplift occurred since the LGM. This deformation is 

highly concentrated near the Kayak Island zone and drops to 

undetectable levels 5 to 10 km to the northwest. The entire 

region is uplifted during megathrust events, including 1 to 3 

m of uplift during the last event in 1964 (Plafker, 1969), but 

paleoseismic studies suggest that coseismic slip is recovered 

within a few centuries (Shennan and others, 1999), well within 

the proposed 500 to 900 yr recurrence interval (Cohen and 

Freymueller, 2005; Hamilton and Shennan, 2005a, 2005b). On 

the basis of these studies, we make the simplifying assumption 

that nearly complete interseismic relaxation occurs before the 

next earthquake, such that Wingham Island has experienced 

negligible net vertical tectonic uplift (<5 m) since the LGM.

The majority of Wingham Island is surrounded by steep 

sea cliffs cut into Tertiary sedimentary and volcanic rocks of 

the Orca Group, part of the Cenozoic accretionary complex 

rimming southern Alaska (Plafker and others, 1994). At 

the southern tip of the island, the Orca Group is thrust over 

sedimentary rocks of the Yakutat microplate, locally forming 

a suture zone (fig. 2). Sedimentary units consist of syntectonic 

glaciomarine mudstones and diamicts of the Pliocene and 

Quaternary Uplift History of Wingham Island, 

South-Central Alaska

By James B. Chapman1, Peter J. Haeussler2, Terry L. Pavlis1

1

Department of Geological Sciences, Geology 405, 500 West University 

Boulevard, Univeristy of Texas at El Paso, El Paso, TX 79968. 2

U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508.

Studies by the U.S. Geological Survey in Alaska, 2007

Edited by Peter J. Haeussler and John P. Galloway

U.S. Geological Survey Professional Paper 1760–B
2 Quaternary Uplift History of Wingham Island, South-Central Alaska

younger Yakataga Fm. (Eyles and others, 1991); marine shales 

and siltstones of the ~Oligocene Poul Creek Fm. (Plafker, 

1987); and shallow marine sandstones of the Redwood Fm. 

(K. Ridgway, oral comm., July 2007), which is laterally 

equivalent with the basal Yakataga Fm. (Plafker and others, 

1987). There is a large topographic escarpment (~80 m) at the 

boundary between the Redwood Fm. and Poul Creek Fm. that 

likely records an erosional event that preferentially eroded the 

friable shales within the Poul Creek Fm. Buttressed against 

this bedrock escarpment are up to 55 m of unconsolidated 

marine to terrestrial deposits (fig. 3A), previously estimated 

to be Quaternary in age (Miller, 1961; Plafker, 1974). The 

deposits are sheltered within a small cove and are protected 

from the direction of the Pacific storm track by Kayak Island 

(figs. 1 and 2), which may explain their preservation.

Associated with these deposits are four terraces (t3 – t0, fig. 

2). Terrace t3 is a depositional terrace at ~40 m above mean sea 

level that gently slopes to the south of the island and is cut into 

by erosional terrace t2 at ~20 m. Terrace t1 consists of beach 

and dune deposits uplifted ~ 2 m during the 1964 Alaska 

earthquake (Plafker, 1969). We mapped the upper limit of t1

by using an aerial photograph taken in 1952 (fig. 2). The 1964 

Alaska earthquake also exposed terrace t0, a marine terrace cut 

into bedrock that was uplifted into the intertidal zone (figs. 3A

and B; Plafker, 1969). Because coseismic slip is recovered, 

t0 was likely formed during several earthquake cycles.

Glacial History

During the LGM, glacial systems within the St. Elias 

Mountains were part of the Cordilleran Ice Sheet that stretched 

from central Alaska to Puget Sound and abutted the Laurentide 

Ice Sheet to the east (Clague and James, 2002; Booth and 

Copper River

Kayak Island

Bering

Glaicer

Stellar lobe

20 km

Wingham Island Suckling Hills

Don Miller

Hills

Ragged

Mountain

9840 ± 50 9980 ± 75

 10160 ± 120

Controller Bay

Gulf of Alaska

Martin

River

Fig. 2

proposed extent of LGM

Kayak Island Zone

Katalla

Bay

9840 ± 50

Ice-flow direction during LGM

Oakalee Spit

C age (yrs. B.P.) 14

Cape Suckling

Q

Q

Thrust fault LGM extent

9510 ± 475

 13900 ± 150

Glacier

14430

10730

145° W 144° W

60°10' N

60° N

60°20' N

Alaska

this figure

Figure 1. Overview map of the Wingham Island area showing major faults, proposed glacial extent during the Last Glacial Maximum (LGM), and 

ice flow direction of major glaciers. Orange stars are radiocarbon dates that represent minimum ages for glacial retreat as reported in Huesser 

(1960), Sirkin and Tuthill (1987), Fleisher and others (1999), and this study. All ages were calibrated to calendar years B.P. Q, Quaternary sediments.
3

others, 2004). The glacial history of the St. Elias Mountains 

remains one of the least well-defined in the Cordillera 

due to poor exposure and a lack of preservation of glacial 

geomorphic features (Hamilton, 1994). Early workers in the 

area believed that many glaciers, including the Bering and 

Martin River glaciers were never more extensive than their 

present position (Miller, 1958; Reid, 1970), but evidence has 

since accumulated that documents large glacial advances in 

the LGM. Maximum extent is constrained by bathymetric 

surveys of sea valleys (Carlson and others, 1990; Muller and 

Fleisher, 1995; Green and others, 2007) and glacial erratics 

or till deposits at Cape Suckling, the entrance to Katalla Bay 

near the Don Miller Hills, and the southern tip of Ragged 

Mountain (fig. 1) (Fleisher and others, 1999; Muller and 

Fleisher, 1995). We slightly expanded the maximum glacial 

extent of Kaufman and Manley (2004) to reflect what we 

interpret as glacially influenced topography on Wingham and 

Kayak Islands and large (3-5 m diameter) glacial erratics on 

terrace t0, although they may also be ice-rafted dropstones 

(fig. 3B). Fleisher and others (1999) identified Quaternary 

deposits at 165 m elevation on the flanks of the Suckling 

Hills that they interpreted as glacially dammed lacustrine 

sediment, suggesting a minimum ice thickness of ~165 

m, although tectonic uplift has not been considered; the 

deposits may be related to a post-LGM advance. No other 

published estimates of ice thickness exist for this area.

The glacial maximum and beginning of ice retreat for the 

Cordilleran Ice Sheet (14-16 ka) lagged a few thousand years 

behind that of the Laurentide Ice Sheet (18-20 ka; Clague 

and James, 2002). Mann and Hamilton (1995) suggest the 

glacial complex in Cook Inlet broke up prior to 16.5 to 16 

ka, and deglaciation was underway by 14.5 ka on the Kenai 

Peninsula (Rymer and Sims, 1982), by 16 to 15 ka in the 

Queen Charlotte Islands (Blaise and others, 1990), before 

13.7 ka just west of Icy Bay (Denton, 1974), and as early as 

14.5 ka in the Katalla Bay area (Sirkin and Tuthill, 1987). 

Fleisher and others (1999) report younger (~10 ka) ages for 

basal peat layers overlying till deposits in the Katalla area 

(fig. 1), although those ages may represent a significant 

delay in peat formation (Gorham and others, 2007), or post100

120

20

?

?

?

1952

suture zone

modern

1952

QTy

Tr

Tpc

To

Qb

t1

t2

t3

t0

A B CDE

500 m

1952

! B

contour interval: 20m

Measured section

Thrust fault

Depositional contact

1952 high-water mark

!

!

!

! !

59°59'30" N

144°23' W 144°22' W

60° N

Tpc

Tr

To

Poul Creek Fm.

Redwood Fm.

Orca Group

incomplete section

faulted out

QTy

Yakataga Fm.

t3

Qb

t1

t2

Beach

deposits

terrace 1

terrace 2

terrace 3

laterally equivalent

Figure 2. Geologic map of southern Wingham Island showing bedrock and surficial units, modified 

from Plafker (1974). Terrace t3 is a depositional terrace that forms a buttress unconformity against a 

bedrock escarpment. Terraces t2 and t1 are incised into the deposits of t3 and terrace t0 is a bedrock 

strath terrace. The upper extent of t1 was mapped using an aerial photograph taken in 1952. The position of the thrust fault between sections B and C is uncertain and could occur anywhere within the 

small cove.

Geologic Setting
4 Quaternary Uplift History of Wingham Island, South-Central Alaska

terrace t0

Buttress

unconformity

Section A

Section B

Tr

QTy

A B

C D

E F

Unit 1

silt

sand

silt

sand

mud-injection

Unit 2

Unit 3

Unit 3

Orca Group

QTy

?

Figure 3. Photographs from Wingham Island. A, A northeast looking aerial view of the buttress unconformity and unconsolidated deposits. 

B, The largest glacial erratic or dropstone found on terrace t0

. C-G from section B, and H from section D. C, Granular conglomerate and shell 

hash within unit 1 and a small reverse fault oriented 019/57 (strike dip following right-hand rule). D, Alternating silt and sand layers in unit 2. 

E, Low amplitude, long wavelength cross-stratification in unit 3. P. Haeussler for scale is pointing at silt rip-up clasts at the base of the unit. 

F, J. Chapman for scale is pointing at a mud-drape in unit 3, ~2 m above photograph E. Mud-injections cross-cut unit 3 and in some cases 

have offset associated with them. G, Unit 4 containing prominent peat layers, of which the uppermost was sampled for radiocarbon dating. 

Unit 5 conformably overlies unit 4. H, Fine lamina of heavy minerals on foresets within unit 5.
5

LGM glacial readvances. Since the LGM, there have been 

at least two to three ice readvances, including a Younger 

Dryas event (Muller and Fleisher, 1995). At a minimum, 

the available data suggests that nearby glaciers had reached 

their present position by 9.6 to 9.3 ka (Hamilton, 1994). 

Glaciers may have also retreated well beyond their present 

position in the early Holocene (7-13 ka) when the terminus 

of the Bering Glacier was located ~30 km further inland 

and marine conditions prevailed (Pasch and Foster, 2006; 

Muller and Fleisher, 1995). Together, these ages provide an 

incomplete record of ice retreat that our data helps to expand.

Contemporaneous with local ice retreat was eustatic 

sea-level rise that began ~19.5 ka with the melting of global 

ice sheets (fig. 4; Lambeck and others, 2002; Lambeck and 

Chapell, 2001). Total sea-level rise is estimated to be 120 to 

135 m (Clark and Mix, 2002) and was punctuated by periods 

of moderate sea-level rise, like the Younger Dryas, and 

relatively rapid sea-level rise, like MWP1A (fig. 4; Fairbanks, 

1989; Bard and others, 1990; 1996). MWP1A corresponds 

to a sudden increase in RSL rise around 14 ka, although the 

exact magnitude and timing is debated. Some researchers 

suggest a rapid rise (4.5-6 cm/yr) during a short interval 

(500-1,000 yrs; Blanchon and Shaw, 1995; Bard and others, 

1996; Hanebuth and others, 2000) while others prefer more 

conservative estimates of 1 to 2 cm/yr averaged over much 

longer periods (~3,500 yrs; Lambeck and others, 2002). 

Methods and Results

Stratigraphy

We collected data from Wingham Island for three 

days during the 2007 field season as part of the St. Elias 

Erosion and Tectonics Project (STEEP). Quaternary 

sedimentary deposits first noted by Miller (1961) were 

surveyed by using a tape measure and linked to extreme 

high tide (driftwood line) by using a laser range finder. 

On the basis of repeated measurements and correlation to 

U.S. Geological Survey (USGS) topographic maps, we 

estimate a vertical error of <5 m. We examined silt samples 

(stars in fig. 5) for compositional and biological variation 

to help determine depositional environment. We sampled 

for radiocarbon throughout the stratigraphic section and 

report all 14C ages in calendar years before present.

We divided the deposits into 5 units, which are presented 

in Sections A-E (figs. 2 and 5). The lowermost unit, unit 1, 

consists of <5 m of tan to light brown, coarse sand to subrounded granular conglomerate with shell hash (fig. 3C). 

Disarticulated shells within the hash are concave down, a 

hydrodynamically stable position in high-energy depositional 

environments (Messina and Labarbera, 2004). There are also 

discontinuous layers of charcoal and woody material, which 

are beyond the limit of the radiocarbon method (reported 

as >48 ka, table 1). We interpret unit 1 as a beach-to-surf 

zone facies. Bedding in unit 1 is oriented 275/04 (strike/dip, 

following the right-hand rule), roughly consistent with all 

other units and in the down-dip direction of regional faults 

and structural grain (fig 2). Small, narrow, thrust faults with 

offset <30 to 40 cm (fig. 3C), are present throughout the 

measured sections and locally offset layers within the unit. We 

did not perform an extensive survey of faults within the area, 

although two fault sets oriented 019/57 and 189/46 appear to 

be dominant. In most sections, the base of unit 1 is covered 

by slope wash and mass-wasting deposits, however, in section 

A, bluish-gray silt to sand beds with scattered shell fragments 

appear with increasing frequency toward the base of unit 1, 

suggestive of an unrecognized silt unit older than unit 1.

Unit 1 is separated from unit 2 by a sharp planar 

contact, parallel to stratification with a significant grain 

G H

Unit 4

Unit 5

peat

~50 cm

Unit 5

Figure 3. Continued.

Methods and Results
6 Quaternary Uplift History of Wingham Island, South-Central Alaska

size and color change. Unit 2 consists of 3 to 4 m of 

regularly alternating, bluish-gray mud or silt draped over 

medium to fine sands with typical thicknesses of 15 to 25 

cm (fig. 3D). The sands are well-sorted, clean, and devoid 

of any sedimentary structures, while the silts are clean, 

homogeneous, and lack any biogenic material (for example, 

diatoms, pollen, and organic detritus). We interpret unit 2 as 

a low-energy, shallow or protected marine facies. The lack 

of biologic activity may be related to high sedimentation 

rates, or an unfavorable environment for growth. Unit 2 

thickens significantly toward the north in section A, where 

it contains abundant shell fossils and is interbedded with 

coarser sands, possibly correlative with the overlying unit 3.

Unit 2 has a transitional contact with unit 3 characterized 

by increasing sand content and a change in color. We chose a 

prominent layer with mud rip-up clasts incorporated into an 

overlying sand layer as the boundary between the units (fig. 

3E). Unit 3 consists of 9-10 m of tan to light reddish brown 

coarse sands in shallowly to moderate dipping cross-beds 

with wavelengths of several meters. Mud drapes (fig. 3F), 

organic-rich layers, concentrations of heavy minerals, and, 

rarely, very coarse sand to granular layers were all present 

within the cross-stratification. The presence of heavy mineral 

sands and low truncation angles to foresets suggests cross 

bedding in unit 3 is related to wave-base bedforms (White 

and Williams, 1967). We interpret unit 3 as a shallow marine, 

meltwater pulse 1A

initial melting

Younger Dryas

13,900 ±150

glaciers near

present positions

15 10 -150 20

-100

-50

Time (years B.P.)

Eustatic sea-level (m)

 sea-level data from

Lambeck and others, 2002

Figure 4. The radiocarbon age from section D (green vertical line) is 

plotted with eustatic sea-level data (pink dots), in respect to the present, 

as presented in Lambeck and others (2002). Relatively rapid sea-level rise 

associated with meltwater pulse 1A occurred just prior to the minimum age 

estimate for abandonment of terrace t2 at ~13.9 ka.

>48,000

section C

section B

section D section E

section A

Covered

*

*

*

*

* * * *

* 13,900

± 150

Peat

Pebble to cobble

conglomerate

Cross-bedded sand

Interlayered silt / sand

Cross-bedded sand

to gravel conglomerate

with shell hash

Mud injection 8

16

24

32

40

48

56

Elevation (m)

Covered

Covered

Covered

Covered

sealevel

* Microscopic analysis

13,900

± 150 Years B.P. (calibrated)

Unit 1 Unit 5 Unit 3

Unit 4

>48,000

20

30

40

50

60

b

c

d e

covered

vertical exaggeration = 4

a

sea-level

10

Elevation (m)

bedrock

100m

terrace 2

?

stream channel

buttress unconformity

peat

marine terrace (t0)

Unit 1

Unit 2 Unit 3

Unit 4

Unit 5

Unit 2

Figure 5. Spatial correlation of measured sections A-E and units 1-5 in the Wingham Island deposits.
7

nearshore facies. Multiple generations of mud injections cut 

through unit 3, in some instances with 20 to 30 cm offset 

(fig. 3F). We suggest these are seismites related to the 

faults within unit 1 and with a sediment source in unit 2. 

Unit 4 consists of ~5 m of alternating light to medium 

gray silts and sands with thin peat layers (5-20 cm; fig. 

3G) that were too old for radiocarbon dating (>48 ka; 

table 1). Unit 4 is in sharp depositional contact with the 

underlying unit 3. Silts within unit 4 contain abundant 

organic fragments, although no biological material is 

present apart from a few bisaccate pollen grains (likely 

Pinus), which could be contamination. In section A, the 

sands and silts are replaced with coarser sands, granular 

conglomerate and shell hash similar to that in unit 1. This 

transition represents a lateral facies change towards the 

buttress unconformity. The top of unit 4 contains welllaminated and regularly alternating thin sand and silt 

layers that we interpret as rhythmites, which, along with 

the presence of peat, suggests a tidal to estuarine facies.

Conformably overlying unit 4 is >8 m of medium, 

well-sorted, tan to light brown, cross-stratified sand of 

unit 5. Cross-beds are at a high angle to bedding, climb 

considerably toward the buttress unconformity forming 

a ~20 m high sand pile-up, and have very fine laminae of 

heavy minerals on forsets (fig. 3H), all suggestive of aeolian 

processes (White and Williams, 1967). We interpret unit 

5 as a beach ridge or dune complex. Channelized lenses 

of rounded pebble to cobble conglomerate are locally 

present in the uppermost section of unit 5 (section B, fig. 

5), which we interpret as immature fluvial deposits. 

Gravel to pebble conglomerate is also present at 

the top of Sections D and E, where they are in erosional 

contact with unit 4 (fig. 5). We interpret these occurrences 

as lag deposits formed following the incision that created 

terrace t2. These lag deposits may have been deposited 

synchronously with the channelized conglomerates in unit 

5. Overlying all the measured sections are thick (0.5-3 m) 

layers of peat, whose deposition post-dates the formation 

of terrace t2. A sample from the base of these peat layers 

in section D yielded a radiocarbon date of 13,900 ± 150 

yr B.P. (table 1). This is a minimum age estimate for the 

formation of terrace t2, which along with the stratigraphic 

relationship, constrains the position of sea level at this time.

Sea-Level History

Based on our interpretation of the origin of terrace t2, 

RSL was ~20 m higher than present at 13.9 ka. During this 

time period, RSL was affected by the competing effects of 

global eustatic sea-level rise and local isostatic rebound 

following glacial retreat at the end of the LGM. Eustatic sealevel data suggest that sea level has risen 80 to 105 m since 

13.9 ka (Lambeck and others, 2002; fig. 4), which, like the age 

estimate, is a minimum. Because the t2 surface is ~20 m above 

modern sea level, we calculate the minimum total surface 

displacement since 13.9 ka as 100 to 125 m. Furthermore, 

we suggest that MWP1A may have resulted in a brief period 

where eustatic sea-level rise kept pace with isostatic uplift, 

providing the necessary time for the incision of terrace t2.

Isostatic Uplift

To assess the magnitude and rate of isostatic uplift we 

apply a 2-dimensional (2D) analytical model of a single, 

rectangular ice load. Two-dimensional approximations 

are routinely applied for large ice sheets within a stable 

continental interior (Greve, 2001). However, in a tectonically 

active and glacially dynamic area like southern Alaska, 

this may not be valid. We emphasize that the models 

presented below are intended to illustrate the general 

effects of isostatic rebound, but we also attempted to 

model the crustal response as accurately as possible. 

The response of the lithosphere following removal 

of a glacial load is related to asthenospheric flow and 

to the elastic strength of the lithosphere. In the case 

of instantaneous removal of the glacial load (herein 

referred to as the “flow” model), isostatic uplift is 

related to the viscosity of the Earth’s mantle by

 , (1)

where w(t) is surface displacement remaining until equilibrium 

with respect to time (t), wmax is the total surface displacement, 

and tr is the characteristic relaxation time defined as

Sample Laboratory No. Material 13C Conventional 14C Age Calibrated 14C Age 

WingRC18 Beta-239246 peat -26.9 ‰ 12,040 ± 60 B.P. 14,050-13,750 B.P. 

WingRC03 Beta-242794 peat ----- >48,000 B.P. ----- 

WingRC01 Beta wood ----- >48,000 B.P. ----- 

Table 1. Radiocarbon dates for selected samples.

[Accelerator Mass Spectrometry 14C ages were obtained by Beta Analytic and calibrated with IntCal04. Calibrated ages are reported to two standard deviations]

w t w e t tr ( ) max

( / ) = −

Methods and Results
8 Quaternary Uplift History of Wingham Island, South-Central Alaska

 

, (2)

where μ is the upper mantle viscosity, ρm is the density of 

the upper mantle, g is the gravitational constant, and λ is 

the wavelength of the glacial load (table 2; Turcotte and 

Schubert, 2002). The surface displacement, w(t), represents 

the amount of isostatic uplift remaining until equilibrium and 

decreases exponentially such that w(t=0) = wmax. The uplift 

rate is the derivative of equation 1 with respect to time. 

We chose an average upper mantle viscosity of 

2×1019 Pa•s, based on glacial-rebound studies in Glacier Bay 

that suggest viscosities ranging from 3.7×1018 to 5×1019 Pa•s. 

(Larsen and others 2003; 2004; 2005). This is consistent 

with values elsewhere along the southeastern Alaska and 

British Columbia margin (Clague and James, 2002), but it 

is low compared to other regions like the Canadian Shield 

and Northern Europe that have mantle viscosities of 1020 

to 1022 Pa•s (Mitrovica and Peltier, 1992; Lambeck and 

others, 1990). The wavelength of the glacial load was 

based on the distance from the glacial divide in the St. Elias 

Mountains to the LGM extent offshore (~150 km; table 2). 

Previous to this study, there were no explicit estimates 

of the total surface displacement in the study area as a 

result of glacial loading in the LGM. If we make the 

simplifying assumption that net tectonic uplift is negligible, 

glacioisostatic adjustment is equal to the total crustal 

displacement (wmax), calculated in the previous section as 

100 to 125 m (table 2). Because isostatic equilibrium is 

achieved relatively quickly (<5 k.y.; Larsen and others, 

2005), we can estimate total ice thickness (h) at the 

height of the LGM by using Archimedes’ principle 

 

 , (3)

where ρi is density of ice, suggesting an ice thickness of 

350 to 450 m. Although the Wingham Island deposits 

are presently above sea-level, they are surrounded by an 

erosional LGM surface that is now underwater and depressed 

by the overlying weight of the water and young sediments 

so that an ice thickness of 350 to 450 is an underestimate. 

Expanding equation 3 to include these factors yields

 , (4) 

where ρw and ρs are the density of water and sediment, 

respectively, tw is the average water depth, and ts is 

the thickness of post-LGM sediments as determined 

from Jaeger and others (1998) and Worthington and 

others (2008; table 2). This suggests an upper limit 

for ice thickness of 600 to 700 m, which brackets the 

estimated ice thickness along the British Columbia coast 

during the LGM (Hetherington and others, 2004). 

Combining total surface displacement, wmax, in equations 

1 and 2, we calculate isostatic uplift as a function of time 

following deglaciation (fig. 6A). Postglacial rebound 

is initially rapid, with rates exceeding 10 cm/yr for the 

first few hundred years, and remains above 1 cm/yr for 

nearly 3,000 yr (fig. 6A). Similarly high rates of initial 

postglacial rebound have been estimated for Puget Sound 

(~40 cm/yr) (Dethier and others, 1995), along the British 

Columbia coast (>10 cm/yr) (Clague and James, 2002), 

and Glacier Bay where modern uplift rates are 3 to 4 cm/

yr in response to the end of the last pulse of the Little Ice 

Age about 100 years ago (Larsen and others, 2003; 2005). 

Because the “flow” model assumes instantaneous 

ice removal, which may be a poor assumption, we also 

calculated isostatic rebound in response to a retreating ice 

sheet by modeling the elastic flexure of the lithosphere 

overlying a fluid half-space (herein referred to as the 

“flexure” model). Assuming the foredeep in front of the 

retreating ice sheet is filled with water of density ρw, the 

surface displacement, w(x) with respect to the horizontal 

position (x) from the center of the load, is given by

 

 , (5A)

when x < λ, and by

 , (5B)

when x > λ, (Angevine and others, 1990). In equations 5A

and 5B, α is the flexural parameter, which is given by

 

 , (6)

where Te is the effective elastic thickness of the lithosphere, E

is Young’s modulus, and ν is Poisson’s ratio (table 2). We use 

an elastic thickness of 15 km based on refraction and seismic 

inversion studies near Wingham Island that image the Moho 

(Eberhart-Phillips and others, 2006; Brocher and others, 1994). 

The “flexure” model is presented in figure 6B where 

the range in values of surface displacement (green shaded 

area) represents the uncertainty associated with crustal 

depression and ice thickness. Because the amount of surface 

displacement in this approach is dependent on the distance 

from the ice front, we calculated surface displacement in 

time (fig. 6B) by using the distance between Wingham 

Island and various onshore points for which there are 

timing constraints of glacial retreat. We also include the 

general ages of 15.5 ± 1 ka constraint for the beginning of 

glacial retreat and 9 ± 1 ka constraint for the completion 

of glacial retreat based on regional patterns (Clague and 

James, 2002). In the Katalla Bay area, we prefer the older 

deglaciation age of ~14.5 ka (Sirkin and Tuthill, 1987; 

t

g r

m

= 4π

ρ λ

h w m = max

ρ

ρi

i

sswwm w tt h



++  = max

w(x) = i

h

2( m − w )

2 − e

− x + λ

α

⎛

⎝

⎜

⎞

⎠

⎟

cos x + λ

α

⎛

⎝

⎜ ⎞

⎠

⎟− e

− λ + x

α

⎛

⎝

⎜

⎞

⎠

⎟

cos λ − x

α

⎛

⎝

⎜ ⎞

⎠

⎟

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥



 

w(x) = i

h

2( m − w ) e

− x − λ

α

⎛

⎝

⎜

⎞

⎠

⎟

cos x − λ

α

⎛

⎝

⎜ ⎞

⎠

⎟− e

λ − χ

α

⎛

⎝

⎜

⎞

⎠

⎟

cos

λ + x

α

⎛

⎝

⎜ ⎞

⎠

⎟

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥



 

4

(1–3  2  )

 = g

ET

m

e
9

A. Asthenospheric-flow model

2

4

6

8

10

0 12

40

80

120

160

200

Uplift rate (cm/yr)

0 1 2 3 4

Time since ice removal (kyr)

Sea-level below present (m)

uncertainty in

calculations

“modified” = includes

sediment and water depth

modified sea-level curve

modified uplift rate

sea-level curve

uplift rate

Distance

-20

0

20

40

60

80

100

120

140

modern sea-level

ice-thinning

prior to retreat

9 ka

~14 ka

>13.9 ka

15.5 ka

preferred hypotheticcal model

ice front

forebulge

B. Elastic flexure of the lithosphere

Surface displacement (m)

50 km

rapid ice

retreat and

flexural rebound

surface at 15.5 ka

surface at 14 ka

Figure 6. A, The asthenospheric “flow” model for isostatic adjustment showing an increasing cumulative surface displacement and decreasing 

uplift rate as a function of time since instantaneous removal of the glacial load (at t=0). The “modified” curves include sediment accumulation 

since the last Glacial maximum and present water depth, which we consider more accurate (equation 4). Initial uplift rates exceed 10 cm/yr and 

66 percent of isostatic rebound is completed within 2 k.y. B, The “flexure” model of isostatic rebound assuming bending of a thin elastic plate 

over an incompressible fluid half-space. The model predicts a ~5 m high forebulge in front of the glacial load. The position of the ice front at 

varying times is plotted (dashed purple vertical lines) and used to constrain the timing of isostatic uplift in response to flexure of the lithosphere 

as ice retreats (fig. 7). In general, ice retreat is very rapid following the beginning of deglaciation. In both A and B the shaded areas represent 

uncertainty associated with total crustal displacement and ice thickness.

Symbol Description Value Reference or Note 

flexural parameter ~40 km calculated 

E Young’s modulus Pa Turcotte and Schubert (2002) 

g gravitational acceleration 9.81 m/s2

h ice thickness 650 +/- 50 m calculated 

 wavelength for load 75 km distance to drainage divide 

μ viscosity Pa•s Larsen and others (2003; 2004; 2005) 

i density ice 930 kg/m3

m density upper mantle 3,300 kg/m3

s

s

density sediment 2,200 kg/m3

w density water 1,000 kg/m3

Te elastic thickness 15 km Eberhart-Phillips and others (2006) 

tr relaxation time 1,230 yr calculated 

t sediment thickness 100 m Jaeger and others (1998) 

t water depth 15 m USGS topographic maps 

Poisson’s ratio 0.25 Turcotte and Schubert (2002) 

10

w

Table 2. Variable used in equations.

Methods and Results
10 Quaternary Uplift History of Wingham Island, South-Central Alaska

fig. 1), although the young ages (~9.5-10 ka; labeled as a 

possible glacial readvance in fig. 7) are also included for 

comparison (Huesser, 1960; Fleisher and others, 1999). 

Glacial thinning is likely to occur prior to ice retreat 

resulting in restrained rebound (Benn and Evans, 1998). 

For the Laurentide ice sheet, Andrews (1973) suggested 

up to 75 percent of total rebound may be restrained, which 

we consider a maximum value. Restrained rebound in the 

Cordilleran Ice Sheet is unlikely because tidewater glaciers 

are keenly sensitive to sea level rise, which often results in 

rapid rates of retreat due to calving (Clague and James, 2002). 

The black arrows in figure 6B represent a modest 15 percent 

restrained rebound and are intended to illustrate a possible 

path of crustal displacement during glacial retreat. Although 

our error calculations remain unchanged, we use the 15 

percent restrained rebound curve for a centroid in figure 7.

Discussion and Conclusions

The two models (“flexure” and “flow”) are plotted with 

the eustatic sea-level curve in figure 7. The flexure model is 

tied to the sea-level curve based on the onshore age data, but 

the only control points for the “flow” model are the time of 

instantaneous ice removal and the age of abandonment for 

terrace t2. In general, the two models compare well because 

ice retreat was sufficiently rapid following deglaciation 

to meet the instantaneous load removal assumption in the 

“flow” model. The uplift rates for the first thousand years are 

actually greater for the “flexure” model than the predicted 

rates for the “flow” model, suggesting isostatic rebound may 

initially be limited by the viscosity of the upper mantle if both 

processes simultaneously affect rebound. After a period of 

rapid retreat, stabilized glacial systems onshore may begin 

Relative Sea Level below modern (m)

Time (years B.P.)

20 18 16 14 12 10 8

0

50

100

150

Younger-Dryas

MWP1A

viscous flow

elastic flexure

isostatic rebound

outpaces relative

sea-level rise

Rapid Isostatic Rebound

possible glacial

readvance

relative sea-level

rise keeps pace with

isostatic rebound

radiocarbon ages

and surface 

displacement

from Fig. 6.

intersection

initial

deglaciation

13,900 ± 150

Figure 7. The relative sea-level 

(RSL) curves predicted by the two 

isostatic (“flow” and “flexure”) 

models from fig. 6, combined with 

eustatic sea-level data (black 

crosses) from figure 4. The rate 

of sea-level rise associated with 

meltwater pulse 1A (MWP1A) is 

close to the rate of isostatic uplift 

in both the “flow” and “flexure” 

models. Rapid ice retreat suggests 

that initial isostatic rebound was 

largely, if not entirely, limited by 

the viscosity of the asthenosphere. 

The intersection of the “flow” and 

“flexure” models curves may represent the time after which the rate 

of isostatic rebound was limited by 

the collapse of a foredeep in front 

of the retreating ice.
11

to limit rebound (represented by the area of intersection 

in figure 7). Thus, during glacial retreat, a transition may 

occur in the dominant mode of isostatic response.

Both models show that isostatic rebound outpaced sea 

level rise during the last 11 to 15 k.y. The only period where 

the rate of sea-level rise approaches the rates of isostatic 

uplift predicted in either the “flow” or “flexure” model, was 

during MWP1A, which is consistent with the timing for the 

abandonment of terrace t2. In this scenario, the temporary 

lull in RSL fall provided the time necessary to stabilize sealevel and incise t2. A rise in RSL can also occur during the 

collapse of a glacial forebulge as documented in the Queen 

Charlotte Islands to the southeast (Luternauer and others, 

1989; Hetherington and others, 2004). In these studies, the 

existence and migration of a forebulge resulted in local uplift 

(lowering RSL) and emergence above sea level, followed 

by forebulge collapse (raising RSL) and continued eustatic 

sea level rise, such that the emergent deposits (terrestrial 

facies) are now submerged 80 to 100m below modern sea 

level. The collapse of the forebulge is one of the final stages 

in the rebound process and only small, isostatic adjustments 

from glacial unloading are expected to occur after the 

forebulge has migrated or collapsed. The “flexure” model 

predicts a small, ~5 m forebulge located 75 to 100 km from 

the ice front, suggesting ~5 m of RSL rise in response to 

forebulge collapse (fig. 6B). Because little isostatic uplift is 

expected following forebulge collapse, absolute sea level 

would need to have been close (~5 m) to its present position 

at ~13.9 ka, which is clearly not the case as documented 

by terrace t2 that is presently ~20 m above sea level. 

There is theoretical evidence that viscous flow in the 

deep mantle may influence the isostatic response at long 

wavelengths, whereas flow in the upper mantle controls 

short wavelength isostatic perturbations (Mitrovica and 

Peltier, 1991). This could suggest that the response to rapid 

retreat of glacial ice and an associated migration or collapse 

of a forebulge, controlled by upper mantle flow, may be 

superimposed on a broader, regional isostatic-uplift signal, 

controlled by deep mantle flow. In this case, the timing for 

forebulge collapse may not occur during the final stages of 

isostatic rebound. However, we have no way to distinguish 

between the effects or timing of a transient forebulge and 

more general isostatic uplift controlled by the deep mantle. 

In lieu of more data, we prefer the simpler explanation that 

the lull in RSL fall causing the incision of terrace t2 is related 

to MWP1A rather than a superimposed isostatic response. 

The timing of ice retreat and emergence of Wingham 

Island have important implications for the early migration 

of humans into North America. Our results show that by 

~13.9 ka, the Cordilleran Ice Sheet had retreated past 

Wingham Island, and rapid uplift had resulted in a emergent 

paleoshoreline. Based on the modeled rates of isostatic 

rebound, Wingham and Kayak Island were likely connected 

to the mainland by a land bridge within a thousand years 

of their emergence. While isostatic uplift exceeded eustatic 

sea level rise (until ~ 11 ka, fig. 7), RSL continued to fall, 

exposing portions of the continental shelf that may have 

been suitable for human occupation or travel. Unlike areas 

along the British Columbia coast that were resubmerged 

by ~9.5 ka (Josenhans and others, 1997), the Wingham 

Island area remained above sea level to the present. These 

conclusions are consistent with a coastal migration route 

for early humans into the Americas (Fladmark, 1979) 

and can help explain the presence of pre-Clovis artifacts, 

remains, and coprolites along the western seaboard 

(Dillehay, 1997; Dixon, 2001; Goebel and others, 2008).

Acknowledgments

This research was supported by NSF grant 

EAR0409009 to T. Pavlis. Reviews by Jose Hurtado 

and Tom Hamilton improved the paper. Ken Ridgway 

and Richard Langford provided valuable discussion 

of depositional environments. William Cornell kindly 

assisted our efforts with micropaleontological analysis. 

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Haeussler and Galloway, editors—Studies by the U.S. Geological Survey in Alaska, 2007—Professional Paper 1760–B