Geology and geologic hazards of the eastern coast of the Kenai Peninsula from Kenai to English Bay, Alaska

[From the U.S. Government Printing Office, www.gpo.gov]

DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEY

,64-

STATE OF ALASKA
DEPARTMENT OF NATURAL RESOURCES

GEOLOGY AND GEOLOGIC HAZARDS OF THE EASTERN
COAST OF THE KENAI PENINSULA FROM KENAI TO
ENGLISH BAY, ALASKA

By J.R. Riehle, R.D. Reger, and C.L. Carver

QF-, September 1977
84 LAS
X45
R5
1975

S ta te. of Alaska

Department of' Natural Resources

DIVISION OF GEOLOGICAL AND GEOPHYSICAL SURVEYS

Robert LeResche, Commissioner
Ross G. Schaff, State Geologist

THIS REPORTHAS BEEN READ BY THE STATE
GEOLOGIST. IT IS A PRELIMINARY REPORT
AND HAS NOT RECEPIED OFFICIAL SURVEYS
PUBLICATIONS STATUS, AND SHOULD NOT BE
QUOTED AS SUCH. -rHE AUTHOR(S) ASSUMES
FULL IiESPONSIBILITY FOR THE CONTENTS
OF THIS REPORT.

GEOLOGY AND GEOLOGIC HAZARDS OF THE EASTERN
COAST OF THE KENAI PENINSULA FROM KENAI TO
ENGLISH BAY, ALASKA

By J.R. Riehle, R.D. Reger, and C.L. Carver

September 1977

3001 Porcupi.ne Drive
Anchorage, Alaska 99501

CONTENTS
LPa@e

Introduction . . . . . . . . . . . . . . . . . . . . . . . .

Regional perspectives . . . . . . . . . . . . . . . . . . . .. .. . . . . . ..

Seismicity . . . . . . . . . . . . . . . . . . . . . . . . . . ... .
Faults . . . . ... ... . . . . . . .. . . . . . . . . . . . . . . . .
Ground failures . . . . . . . . . . . . . . . . . . . . . . . . . . .
Land level changes . . . . . . .
Earthquake tsunamis and seiches . . . . . . . . .. . . 7
Vulcanism . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . 8

Northern section: Kenai to Fox River . . . . . . . . . . . . . . . 10

Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 10

Bed.rock * . . I . . . . . . . . . . . . . . . . . . . . . . 10
Surficial deposits . . . ... . . . . . . . . . . . . . . . . . . 11
Geologic hazards . . . . .. . . . . . . . . I . . . . . . . . . . . 11

Ground failure . . . . . . . . . . . . . . . . . . . . . . 13
Coastal erosion . . . . . . . . . . . . . . . . . . . . . . 14
Flooding . . . . .. . . . . . . . . . . . . . . . . . ... . 14

Southern section: Fox River to English Bay . . . . . . .. . . . 14

Geology . . . . . . . . . . . . .. . . . . . . . . . . ... . . . . . . 17

General setting . . . . . . . . . . . . . . . . . 17
Bedrock 18
Surficial deposits . . . . . . . . . . . . . . . . . . . . . . . 19

Geologic hazards . . . . . . . . ... . . ... . . . . . . . . . . . . 20

Mass movements . . . . . . . . . . . ... . . . . . . . . . . . . 20
Snow avalanches . . . . . . . . . . . . . . . . . ..
. . . . . . 20
Tsunamis . . . . . . . . . ... . . . . . . . . . . . . . . . . . 2.1
Unstable ground .. . . . . . . . . . . . . . . . . . . . . . . . 21.
Floods . . .
. . . . . . . . . . . . . . . . . . . . . . . . 22

Glossary ofterms .. . . . . . . . . . . . . . . . . . . . . . . ... . . . 2.3'

References cited . . . . ... . . . . . . . . . . . . . . . ... . . . . . . 27

TABLES

Tab.le I State of volcanic activity in the Cook Inlet area . . . . . . . . 10

Table'2 Summary of potential availability of various construction
material.s in geologic-materials map units, east shore of Cook
Inlet from Kenai to Fox River, Alaska . . . . . . . . . . . . . . 12

Paqe

Table 3 Maximum known.floods for four major streams entering east ern
Cook Inlet between Kenai and Fox River, Alaska .. . . . . .. . . .

Table 4. Estimated Hood discharges at various recurrence intervals
for four major streams entering eastern Cook Inlet between
Kenai and Fox River, Alaska . . . . . . . .. . . . . . . . . . . . 16

ILLUSTRATIONS

Figure I Study area along the coast from Kenai to English Bay,
Alaska ... . . . . . . . . . . . . . . . . . . . .. . . ... . . . 2

Figure 2 Locations of earthquake epicenters for the period,1899-1974 in
Cook Inletregion, an.d recurrence interval of such earthquakes .3

Figure 3 Projected maximum intensities of earthquakes in the Cook Inlet
regfon for the period 1786-1974 . . . . . . . . . . . . . . . . 4

Figure 4 Major faults of the Cook Inlet area, Alaska . . . . . . . . . . 5

Figure 5 -Volcanoes of the Cook Inlet area, Alaska . . . . . . . . . . . . 9

Plate I Geology of the coast from.Kenai to Fox Creek, east shore
of Cook,Inlet, Alaska . . . . . . . . . . . . . . . . . . . . . Attached

Plate 2 Geologi.c materials of the coast from Kenai to Fox C-reek,
east shore of Cook Inlet, Alaska . . . . . . . .. . . . . . . . . Attached

Plate I Photointerpretation map of coastal surficial geology,
Fox River to English Bay, Alaska . . . . . . . . . . . . . . . .. Attached

Plate 4 Map showing areas of coastal erosion and deposition, areas
prone to snow avalanches, and potential deposits of
construction materials . . . . . . . . . . . . . . . . . . . . . Attached

INTRODUCTION

The following report was prepared at the request of the Planning and Research
Section of the Department of Natural Resources to summarize the present knowledge
of the geology and geologic hazards along part of the eastern shore of Cook Inlet
(fig. 1). It is based primarily on information gathered during two summers by
two field parties'supplemented by a review of published information. The section
from Fox River north was investigated by R.D. Reger, assisted by J.T. Kline and
C.L. Carver during August.1976 and May 1977. The section from Fox River to English
Bay was investigated by J.R. Riehle, assisted by,K. Emmel .during May 1977. The
main objectives of the field program were (1) to verify landforms identified during
p,reliminary interpretation of aerial photographs, and (2) to determine the composi-
tion of these landforms by studying natural exposures and the walls of pits
excavated by hand. The photography upon which the maps of the northern section
are based was taken during September 11368 at a'scale of 1:24,000'. Photography
for the southern section was taken during the summers of 1961 and 1975 at-a scale
of .1:15,840.

REGIONAL PERSPECTIVES

Seismicity

Cook Inlet and the Kenai Peninsula are situated within the well-known Circur-
Pacific belt of earthquakes where seismicity is high; the Gulf of Alaska-Aleutian
Range region accounts.for about 7% of the seismic energy released annually
throughout the world (Magoon'and others, 1976). Nine great earthquakes, which
have.Richter magnitudes greater than 8.0, have occurred-in Alaska since 1899,
including the 1903 event centered in Shelikof Strait and the 1964 event centered
in Prince William Sound. No great earthquakes have. been rec orded in the Cook
Inlet region. From 1899-1974 a total of 26 earthquakes of magnitude 6.0 or greater
were triggered in t.he Cook Inlet area, including one of magnitude 7.3 (fig. 2).

The Cook Inlet-Kenai Peninsula region is included in Seismic Probability
Zone 3, an area of potential major damage from earthquakes greater than Richter
magnitude 6 (Evans and others, 1972). According to Meyers and others (1976),
maximum Modified-Mercalli intensities have historically ranged from VIII to IX
on the Kenai Peninsula (fig. 3), indicating considerable major destruction would
.have occurred even to specially designed structures. This damage (both real and
hypothetical) resulted from very large earthquakes centered near but not in the
region, as well as from smaller.earthquakes located within the -region.

Prediction of future earthquakes is, at best, uncertain, especially in Alaska
where few fault systems have been studied in detail an-d where there has been no
monitoring of fault zones. Analysis of recurrence intervals indicates that the
southern Cook Inlet area may be subjected to a 7.3-magnitude or greater earthquake
at least once every 75 years and that an earthquake of 8.0 magnitude may occur about
every 400 years (fig.. 2).

Faults

The Cook Inlet basin is bounded by*the Castle Mountain fault on the.north
and northwest, the Bruin Bay fault on the we,st,* and the Bord er Ranges fault on
the southeast (fig. 4). The disrupt.ion of ra-diocarbon-dated deposits across the
Castle Mountain fault in the Susitna River valley demonstrates that fault movement
occurred between 225 and 1860 years ago (Detterman and others, 1974, 1976a).
Although they could not document offset: of Holocene or Quaternary deposits across

Anchorage

Kenai
Skilak Lake

Tustamena Lake
Seward
Ninilchik

Homer

English Bay

0 50 100 Km

Figure 1. Study area along the coast from Kenai to
English Bay, Alaska.

-2-

4'7.3

7.0

6.0
610 3.2 5 6.250
?

*0.75

46.7

*6.0

C 0 0 K
.600 46.5 6.1

6.2 E

I N LE T
6.C 6.80
6.50 0, 151 1@0'

20.

Augustine is, p

7 0 0
5 .4 >
U

Ln
Q)

ru-

E
L" D.
1 0 15'2 0 U
1153
a) Ln
-0
ru
154 -_7 >- 01.5 71.0 7,5 8.-
Figure 2. Locations of earthquake epicenters for the period 1899-1974 in Cook
Inlet region, and reCL.jrrence, interval of such earHiquikes.
Magnitudes are given where known. Modified from Bureau of Land
*2
5
6*2 50

.75
6. 77

Management (1976) with data from Heyers (1976). In defining the
recurrence interval, earthquakes of unknown maonitudes are arbitrarily
allocated equally to magnitude classes 6.0 and-6.25.

Anchorage

Kenai Peninsula

Mod. Mercalli Intensities

00
IX
0 less than
0 1 X to X
Q
X
to X1
X I to XII
9'reater than XII

Figure 3. Projected maximum'intensities of earthquakes in the Cook Inlet
region for the period 1786-1974; Maximum intensities in the Cook
Inlet region range f.rom Vill to IX; MM IX is defined as "....damage
.considerable in specially des-igned structures; well-designed frame
structures thrown out of plumb; great insubstantial buildings,
with partial collapse. Buildings shifted off foundations. Ground
cracked conspicuously. Underground pipes broken." From Meyers
and others, 1976. (Note: Near-fault horizontal ground motions
during earthquakes have been characte.rized for design earthquakes
along the TAPS corridor. Subject to simplifying assumptions and
to uncertainties arising from limited data, the maximum horizontal
accelerations are 1.2 and 1.05 times gravity (that is, 1.2 g an'd
1.05 9) for earthquakes of magnitude 8.0 and 7.0, respectively,.
The levels of horizontal acceleration which would be exceeded 10
times during an earthquake are .7 g and .55 g for magnitude 8.0
and 7.0 events, respectively. The times between first and last
accelerations equal to or greater than .05 g may strongly influence
the extent of damage; values are 60 seconds and 25 seconds for 8.0
and 7.0 events. Peak horizontal acceleration values and duration
values decrease with distance from the.fault source, at least
beyond a minimum distance which is greater for larger carthqtiakes.
Page, R. A., Boore, D. M., Joyner, W. B., and Coulter, H. W., 19-12,
Ground motion values for use in the seismic design of the frans-
Alaska Pipeline System: U.S. Geol. Survey Circ. 672.)

-4-

Anto

Anchora

XN
:Y.
j-\+k 0000...::-
40 : ::k
X).

Kenai

4@
Kasilo

Zone of extensive
ground breakage

Ninilchik

P
Anchor. Point

mer

Seldovia

English Bay-

'CO
I0,

0 50 100 Km

Figure 4. Major faults of the Cook Inlet area, Alaska. Concealed faults
are dotted and their positions are inferred from geophysical
data, well data, and projection of structural elements. Modified
@ /00\k

from Foster and Karlstrom (1967), Beikman (1974), Tysdal 097@),
and Hackett (1977). Offshore faults and scarps from Bouma and
Hampton (1976).

the-Bruin Bay fault,.Detterman and others.0976b) concluded that this major
structur .al discontinuity is an extension of the Castle Mountain fault system
and, as such, must be considered potentially active. Cla,rk (1972) and Tysdal.
(1976) di'd not report evidence of Quaternary or Holocene activity along the
complex trace of the Border Ranges-Eagle River fault system in or north of the
study area. Gedney and Van Wormer (1974) demonstrated no obvious correlation of
small seismic events recorded in 1972 to known faults, such as the Border Ranges
fault, in the Kenai Mountains of the southern,Kenai Peninsula.

Within the Cook Inlet b :)us small, dominantly northeast- and
asin numer4
northwest-trending faults offset beds of the Tertiar-y-Kenai Group but not late
Quaternary deposits, indicating that they are post-Tertiary in age but are not
now active (pl. 1). Large subsurface faults, such as the Homer and Sterling
faults, are inferred from indirect evidence and apparently are not active (fig.
Local subsurface faults associated with actively growing folds in the Kenai Group
may be active (Kelly, 1963; Plafker and Rubin, 1967). Although such Iota] activity
is not considered capable of generatin( thquakes, it may produce signilr-
g major ear
icant effects in the immedi .ate vicinity (Tysdal, 1976). An analysis of limited
microe.arthquake data from events in.the Kenai Lowland between 197@ and 1974-sh.ows
no correlation with known surface or, subsurface faults (Tysdal, 1976).

Offshore faults and escarpments on the bottom of Cook Inlet have been mapped
by Bouma and Hampton (1976), in particular near to the Barren Islands, at the
entrance to Kachemak Bay, and near the center of Cook Inlet (fig. 4). The fatilts
are called "small scale". Data on the nature and recency of movement along these
features are preliminary and limited; any of the features could prove to be active
faults.

Ground Failures

Numerous forms of ground failure, some catastrophic and others subtle, have
occurred during past'earthquakes in Alaska and similar responses must be antici-
pated during future seismic events. In 1958 a large rockslide released shortly
after a Richter magnitude 7.0 earthquake hit the restricted head of Lituya Bay
and generated a wave as high as 500 m (Miller, 1960). The Turnagain Heights
slide in the Bootlegger Cove Clay caused extensive damage and some loss of life
in Anchorage in 1964. The Bootlegger Cove Clay is composed of saturated marine
clays with interbedded sand and sil-t lenses (Hansen, 1965; Schmoll and others,.
1972). Liquefaction of the sand-silt lenses, or of the clay due to repeated
shear stresses imposed by seismic shaking of long duration, occurred at depth
during the earthquake, causing surface gravel and sand layers to fail due to loss
of support. With the possible exception of deposits cropping out along the bluff
of the coast between the mouth of the Kenai River and Cohoe (Karlstrom, 1958),
thick deposits similar to the Bootlegger Cove.Clay have not been recognized in
the study area.

Subaerial and submarine sliding of' delta fronts and glacial, moraines occurred
near Valdez and Seward during the Good Friday earthquake (Coulter and Migliaccio,
1966; Lemke,' 1967). Small rockfalls, slumps, and slides in weakly consolidated
Tertiary bedrock and till and outwash deposits exposed in steep cliffs and bluffs
are fairly common along the coast of Cook Inlet. Although these areas should be
considered prone to sudden mass movements, especially in the event of a large
earthquake, such failures are apparently more a direct result of proximity to
a free face than to the sensitive nature of the materials. Site-specific engineering
-evaluations, including subsurface data collection, are recommended before,final
decisions are made regarding bearing capacity and stability of foundation soils.

Widespread fracturing, fissuring, consolidation, and sand fountaining occurred
in surficial deposits during the 1964 earthquake (Coulter and Migliaccio, 1966-
Foster and Karlstrom, 1967; Lemke, 1967; Plafker and others, 1969). Some frac@uring
was primarily caused by progressive failure adjacent toblock glides or rotational
slides, but other fracturing may have been the result of movement along concealed
bedrock faults. Fissures associated with sand fountaining, a surface manifestation
of liquefaction, on alluvial fans and floodplains may have originated.from ground-
motion amplification in thick, saturated, unconsolidated sediments (Coulter and
Migliaccio, 1966) and by vertical and horizontal compaction of fine-grained
unconso,lidated sediments (Hansen, 1965;,Lemke, 1967).

Land Level Changes

The Kenai Mountains and the-eastern lowlands we-re overridden by glacial
ice as recently as about 10,000 to 12,000 years ago (Karlstrom, 1964). The earth's
crust was depressed beneath these expanded glaciers until deglaciation allowed
crustal rebound to reestablish gravitational (isostatic) equilibrium. Such -
rebound probably accounts for c-urrent uplift rates of nearly 3.96 cm/yr at Juneau
(Hicks and Shofnos, 1965) Active isostatic. rebound has not@been reported in the
lower Cook Inlet area, but the effects of this process may be masked by other
processes such as eustatic sea-level changes or regional tectonics.

The entire study region subsided relative to sea level during the 1964
earthquake. Plafker (1969) showed isobase contours of re-lative subsidence that
trend approximately N30E through Kachemak Bay; a 1.22-m subsidence contour passes
through the mouth of Tutka Bay. Subsidence was greater to the east (1.83 m at the
to Port Dick) and less to the west (0..61 m at Anchor Point). In summarizing
available evidence, Plafker (1969) concluded that Kachemak Bay and the west side
of the southern Kenai Peninsula are part of a larger area that has tectonically
subsided at least since the most recent deglaciation. The available evidence
indicates that parts of the study area have undergone about 6.1 m of submergence
relative to sea level during the past 2,000 to 3,000 years, including about 1.22 m
of regional subsidence in the 1964 earthquake. Presumably such net subsidence is
a reflection of crustal strain release during earthquakes. If so, it is likely
that the study area will continue to undergo net subsidence during future large
earthquakes.

Earthquake Tsunamis and Seiches

Tsunamis are gravitational sea waves generated by any large-scale, short
duration disturbance of the ocean floor, including shallow submarine fault movements,
submarine landslides, subsidence,, and volcanic eruptions. The 1964 tsunamis that
affected Cook Inlet were not generated by submarine faulting or sliding within the
basin, but were triggered about 161 km seaward of the Barren Islands near the edge
of the continental'shelf. Damage in the Cook Inlet area was not widespread, although
the largest of several seawaves at Seldovia was 7.9 m .(above mean lower low water?)
(Plafker and others, 1969) and the Homer Spit was inundated by a 1.1 m highwave
(Waller, 1966). The lackof tsunami production in Cook Inlet during the 1964
earthquake may be due to the absence,of nearshore, glacially oversteepened bedrock
slopes, to the apparent absence of larcle failures of delta deposits, or to the low
magnitude and short d.ura,tion of seismic shaking.

Outside Cook Inlet severe damage was sustained in the Kodiak Islands where
tsunami runups were as high as 12.2 m above the existing tide levels; a t Kodiak-
City the highest wave coincided with and crested 7 m above the high-tide level

-7-

(Pla,fker and.Kachadoorian, 1966; Kachadoorian and Plafker, 1967). Submarine
landsliding of parts of a glacial end moraine in Port Valdez probably produced
waves from 10.7 to 51.8 m high (Plafker and others, 1969). Submarine failure of
fan.delta deposits along the Seward waterfront and at the head of Resurrection
Bay during.the Good Friday earthquake triggered local tsunamis that caused con-
siderable damage.(Lemke, 1967).

Seiches are standing waves that travel back and forth in an enclosed or
semi-enclosed.basin with a period dependent o.n.the depth and size of the water
body. Seiches may be caused by changes in atmospheric pressure aided by winds,
tidal currents,.and earthquakes. They were reported on Tustumena, Skilak, and
Lakes and on Bradley River at the head of Kachemak Bay during the 1964
earthquake (Waller, 1966; McCulloch, 1966;.McGarr and Vorhis, 1968@.. In restricted
and shallow portions of Kenai Lake seiche waves.reached heights of 6 to 9 m
(McCulloch, 1966).

Vulcanism

line of recently active volcanoes extends up the western side of Cook Inlet
from Mt. Douglas to Hayes Volcano (fige 5). This group of six.volcanoes is the
northern segment of a belt of volcanoes studding'the Aleutian arc (Coates, 1950).
The volcanoes, are predominantly andesite, a composition that characteristically
produces relatively violent eruptions. They are classified as quiescent to active
and poteptially eruptive (table,l). Four eruptive episodes have occurred during
the past 25 years and numerous buried ash layers record a history of many major
eruptions.

The potential local hazards associated with violent volcanic eruptions on
Cook Inlet include severe blast effects, nu6es ardentes, pyroclastic-flows, lava
flows, volcanic mudflows (lahars), turbulent clouds of ash and hot gases, lightning
discharges, corrosive rains, flash and,outburst floods, earthquakes, and tsunamis.
Because the volcanoes are situated along the-,western.side of Cook Inlet, the eastern
shore will be indirectly affected by volcanic eruptions. Special concern must be
devoted to the effects of tsunamis on developme'rits-along the coa st from Kenai to
English Bay. The 1883 eruption of Augustine Volcano produced a large mudflow and
nu&es ardentes which hit the inlet and generated a sea wave 8 to 9 in high.' This
sea wave struck Port Graham 85 km to the east 25 minutes later (Dall, 1884, in
'Kienle and Forbes, 1977). Fortunately,the incident occurred at low tide or
significant damage and perhaps loss of life could have resulted. Another tsunami
may have been generated by the 1901 eruption of Augustine.Volcano (Cox and Parraras-
Carayannis, 1976). Augustine Volcano is particularly dangerous because it is
surrounded by marine waters. Krakatoa, a small island similarly situated in the
Dutch East Indies, also erupted in 188', but with a series of extremely violent
.3
explosions produced by the vaporization of sea water entering the magma chamber
through cracks in the walls. About 36,000 people living on the lowlands of
neighboring Java and Sumatra drowned during inundations by tsunamis generated by
the explosions (Bolt and others, 1975). -The shores and flats bordering Cook Inlet
are potentially exposed to similar inundations produced during explosive eruptions
of Augustine Volcano. Becuase of this potential danger, Augustine Volcano has
been continuously monitored since 1970 by.the Geophysical Institute of the University
.of Alaska,.Fairbanks (Kienle and Forbes,i]977).

Other, less dangerous, indirect hazards of vulca,nism are ashfalls and acid
rains. Ash from the 1912 eruption of tit. Katmai covered parts of Kodiak Island
to depths of more than 0.4:m and corrosive rains fell on Seward and Cordova,

-8-

A Hayes Volcano

A Mt. Spurr
Volcano

Anchorag

Kenai
Redoubt A
Volcano
Kasilof
Cohoe

lliamnaA Ninilchik
Volcano

Happy Valley

Anchor Point

Homer

Seldovia

A Augustine
10 Volcano English Bay Port Graho

Mt. Douglas A
Volcano 0 50 100 Krn

Figure 5. Volcanoes of the Cook Inlet area, Alaska. Modified from Evans
and others (1972),and Miller and Smith (1976)..

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Table 1. State of Volcanic Activity in the Cook Inlet Area (Wilcox, 1959;
Evans and others, 1972; Miller and Smith, 1976).

Volcano Last Eruption Present State

Mt. Douglas None in historic time Quiescent

@Augustine .1976 Active and pot.enti.ally
eruptive

Iliamna 1867 Active but quiescent

Redoubt 1966 Active and potentially
eruptive

Mt. Spurr 1953 Active but quiescent

Hayes None In historic time Quiescent

painfully burning people, damaging vegetation, and corroding exposed metal
(Wilcox, 1959). The 1953 eruption of Mt. Spurr deposited ash on the outskirts
of Anchorage that damaged air:craft and its removal required considerable expendi-
ture of money and labor... Airborne measurements of effluents from Augustine Volcano
in February 1976 indicate the ash was erupted at a rate of about lo5 kg/s during
brief intervals and the ash cloud attained a height of 5.3 km (Hobbs and others,
1977). Depending on wind conditions, ash clouds could force cessation or diversion
of aircraft flights during eruptions, even as far away as the eastern shore of
Cook Inlet.

NORTHERN SECTION: KENAI TO FOX RIVER

The northern section is a strip approximately 158 km long and I to 3 km
wide (pl. 1). It includes parts of 9 quadrangles: Kenai A-4, A-5, B-4, C-4,' and
Seldovia C-4, C-5, D-3, D-4, and D-5. The 'area includes the settlements of Kenai,
Kasilof, Cohoe, Clam Gulch, Ninilchik, Happy Valley, Anchor Point., and Homer.

Geo I-ogy

Bedrock

The Kenai Group is a sequence of middle to late Tertiary fine-gra-ined
sedimentary rocks comprising the only bedrock along the coast from Kenai to Fox
River. The units' crop out almost continuously in the precipitous wavecut cliffs
and steep walls of stream canyons from Fritz Creek northeast to Fox River and
from Happy Creek north to about 11 kin south of the Kasilof River mouth (pl. 1)
Less continous exposures occur from Fritz Creek north to the Anchor River and in
steep walls of the valleys of Anchor River, Happy Creek, Deep Creek, and Ninilchi.k
River.

The sequence consists of at least 1500 m of moderately to weakly indurated,
locally conglomeratic sandstone with iriterbedded.siltstone., claystone, and coal.

The proportion of sandstone increases northward and the number and thickness of
coal beds and the quality of the coal decreases. The coal is ranked as lignitic
to subbituminous.. Coal beds are generally 'Ienticular, ranging in thickness from
I to 2 m; at.least 30 different beds are recognized (Barnes and.Cobb, 1959).
Many former coal beds have been burned, baking the adjacent rocks a distinctive
b.rick orange'color..

The Kenai Group occupies the southern part of the large structure comprising
the Cook Inlet basin. Superimposed on the general basin structure is a series of
broad, gentle folds striking generally no'rthea.st. The limbs of these folds
generally dip less than 100, except in the vicinity of faults.where dips up to
200 have been measured (Barnes and Cobb, 1959). Numerous high-angle faults,offset
bedding from a few cm to nearly 25 m. Seismic investigations by the USGS indicate
that stratigraphic and structural relationships continue offshore where anticlines
and.syncl.ines have wavelengths of 8 to 12 km (Magoon and.others, 1976).

Surficial Deposits

Bedrock is overlain by a widespread complex of unconsolidated deposi,ts laid
down, as a direct or indirect consequence of glaciation or by subsequent stream.
activity (Karlstrom, 1964) (pl. 1). Tlhe.mos.t extensive deposits are glacial till
and organic swamp deposits. Outwash sand and gravel (coalesced 'fan deltas) are
exposed in the coastal bluffs from Cohoe north to Kenai (Karlstrom, 1958). out-
wash and meltwater channel fills locally bury or are inset into glacial drift
south of Cohoe, particularly near major stream valleys. Stream floodplains are
underlain by granular alluvium and are bounded by discontinuous fluvial terraces.,
Sand and gravel beaches separate wavecut cliffs and bluffs from inlet waters
along the entire coast and tidal flats are extensive between Homer Spit and Fox
River. Alluvial and colluvial-alluvial fans are especially well developed in the
vicinity of Homer where they are derived from rapid erosion of the Kenai Group
exposed in the glacially steepened slopes north and northeast of town. Various
forms of colluvium (including a large rockfall southeast of Bluff Point near
Homer), slumps, and mudflows are scattered along steep coastal bluffs and on the
walls of stream valleys throughout the study area. Wind-blown silt (loess) forms
a blanket 0.1 to 0.4 m thick almost everywhere except on beaches and tidal flats.
The thickness of the surficial deposits varies.from less than 1 m to more than
50 m.

No samples'were collected for granulometric analyses, but estimates of the
grain-size composition of each landform were made in the field. These field
observations are the bases for derivin(
.3 the geologic materials map'(pl. 2) and
the summary of the potential availability of various construction materials '
.(table 2). Precise evaluations of the economic viability of specific gravel and
sand deposits will require more detailed studies of the physical properties of
each deposit, including area] extent, thickness of overburden, thermal state of
the groun, and depth to the water table, waswell as consideration of such
economic factors as general.availability of gravel and sand in the area, haul
distance from source to jobsite, type of transportation available, and demand
for the product.

Geologic Hazards

In addition to the seismically related and volcanic hazards discussed in the
section onregional perspectives, major geologic hazards in the norther *n section
may result from (1) ground failure, (2.) coastal erosion, and (3) flooding.

Table 2. Summary of potential availability of various construction materials in
geologic-materials map units, east shore of Cook Inlet from Kenai to Fox River,, Alaska,

Probability of locating 2ood sources of
Gravel Mixed coarse-
Map and and fine-grained Crushed Riprap Building Incorporated
Unit Composition sand Sand material Clay aggregate armor rock stone geologic units
I Sand and Good, Good Poor Nil Good Nil Nil Floodplain deposits, terrace
gravel alluvium;'fills in abandoned
meltwater channels; kame-esker
complexes; outwash alluvium;
beach deposits.
11 Chiefly -moderate Moderate Poor Ni V Moderate Nil Nil Granular alluvial fans; col-
gravel and luvial-alluvial fans.
sand

III Chiefly Moderate Good Poor Poor Nil Nil Nil Tidal@-flat deposits.
sand

IV Chiefly silt Poor Poor Nil Moderate Nil Nil Nil Lake deposits.
and clay

V Chiefly Poor Poor Good Poor Poor Nil Nil Glacial till; colluvium;
mxed coarse- landslides; mudflow deposits;
and fine- alluvial fans.
grained
naterial

V; Chiefly Nil Nil Nil Poor to Nil Nil Nil Swamp deposits.
peat nil

VII Chiefly Nii Moderate Nil Moderate 'Nil Nil Nil Kenai Group.
weakly.in- to good
durated.
sandstone
with inter-
bedded
siltstone
and clay-
stone
*The imprecise terms "cood," "moderate," "poor," and "nil" are purposely used to indicate the re,-_@,ive probability of locating good deposits of
each construction material in the various map units. No definite values are assigned to each term, but they may indicate a probability of 80%
or more for "good," 30% to 80% for "moderate," less than 30% for "poor.," and essentially zero chance for "n I I

Ground Fa i I ure

The largest slope fa-ilure along the coast in the northern section is the
4.7-km long rockfall below Bluff Point near Homer (pl. 1). This massive fall
of weakly to moderately consolidated Tertiary sandstone and siltstone occurred
more than 1555 + 135 years ago, according to radiocarbon dating of-alluvial-
colluvial, deposits partially capping the feature. The considerable degree of
post-failure modification indicates that. the fall may have happened shortly after
deglaciation of the area at the end of the Skilak Stade of the Napt6wne
Glaciation before 9,000 years ago and perhaps 'as early asabout 12,000 years
ago (Karlstrom, 1964). Most of the body of the rockfall appears to,be stabilized,
although the fan deposits on top of the southern 60 to 400 m of the rockfall have
been rotated northward by secondary slumping i.n response to wave erosion at the
toe of the sea cliff or perhaps seismic shaking during the past 1160 + 120 years.
During the 1964 earthquake-fissures as deep as 6 m developed parallel-to the
230-m high headwall as far as 30 in back from the edge (Waller, 1966). These
cracks pose a serious slope stability hazard if surface or near-surface water
enters them. Other potential rockfall hazards are promontories extending out
from precipitous bluffs and cliffs weathered in the Kenai Group north and north-
east of Homer (pl. I).

In addition to the southern portion of the Bluff Point rockfall, numerous
small- to.moderate-sized.slumps are present along the coastal bluff and in steep
canyons; one of the largest single slumps is at the mouth of McNeil Canyon (pl..
These rotational failures have occurred in both the Tertiary bedrock and the
overlying glacial till. The times of failure are unknown but most are fresh.
The triggering mechanism in most cases is probably undercutting of the toe of the
slope by wave and stream erosion. An other potential cause is seismic shaking,
which induces liquification in some saturated soils, but Waller (1966) and Foster
and Karlstrom (1967) documented surprisingly few slumps resulting from the 1964
earthquake in spite of the high intensity and long duration of the shaking, the
extensive high bluffs, and the unconsolidated to weakly consolidated character
of the bedrock and overlying deposits.

Other common types. of ground failure include.slides, flows, surface raveling
on oversteepened slopes, subsidence, and mudflows. Several small slides and
..earthflows were triggered by the 1964 earthquake along the coast and on the steep
walls. of stream valleys (Waller, 1966; Foster and Karlstrom, 1967). Two submarine.
slides occurred near the tip of the Horner Spit; one dropped the seawall of the
,small boat harbor beneath the waters of Kachemak Bay. The Homer.Spit subsided
up'to.1.8 m and ground cracking occurred during the Good Friday earthquake in
response to tectonic movement, compaction, and lateral spreading. At the same
time differential compaction or removal of underlying materials by groundwater
eruption caused extens.ive cracking of the ground in a linear belt from Kasilof
to Chickaloon Bay (fig. 4). Foster and Karlstrom (1967) concluded that this zone
of ground breakage may be related to movement along a subsurface fault system,
but Ty-sdal (1976) disagreed, pointing out the lack of discernible aftershock
activity within the belt and the lack of corroborating aeromagnetic and gravity
data. Ancient co-11apse pits and linear troughs record previous episodes of
ground cracking and groundwater eruption in the same area and similar responses
.should be expected during major earthquakes in the future. Mudflows are a serious
hazard during rainy seasons in the steep canyons cut into the Kenai Group and in
the area of the mouths of these canyons.

Coastal Erosion

Most of the coast from Kenai to Fox River is receding at various, presently
unquantified rates in response to wave attack. This erosion has resulted in loss
of property and the destruction of buildings or facilities constructed near the
edge of the coastal bluff. Erosion is a serious problem, particularly where the
bank is composed of peat or unconsolidated granular deposits as in the area of
Miller's Landing east of Homer. The present cycle of erosion apparently began
after the 1964.earthquake when the western Kenai Peninsula subsided relative to
sea level (Stanley, 1968). Retreat wil 'I continue until beaches are sufficiently.
built up to protect banks from wave attack.

Flooding

Surface waters have been monitored on the Kenai Peninsula for only a very
short period of time. As of 1970 complete.data were collected by the USGS only
on the Kenai River, Ninilchik River, and Anchor River and crest gages were
maintained on Kasilof River, Diamond Creek, and Fritz Creek (Childers, 1970).
Floodcrest discharges for maximum known floods vary from 650 to 30,000 cfs (table
3). Flood discharges have been calculated for various recurrence intervals up
to 25 years for the Kenai, Kasilof, and Anchor Rivers.and up to IO.years for
Ninilchik River (table 4). These calculations incorporate such factors as area
of -the drainage basin, swamps, and lakes, mean annual precipitation, and
precipitation intensity.

The Kenai and Kasilof Rivers are subject to periodic catastrophic flooding
resulting from the abrupt release of waters impounded in glacier-dammed lakes
(Post and Mayo,' 1971) During these events the volume of water suddenly released
may be many times greater than nonoutburst floods. At these times flooding may
result in serious damage and loss of life, especially if the water level fs already
high or the stream is frozen or partially frozen. For example, during a period of
very cold weather in January 1969 the drainage of an unnamed lake held in by
Skilak Glacier produced a large ice jam in the Kenai River that caused inundat,ion
of low areas and considerable ice-shove damage (U.S. Army Corps of Engineers,
1973). Prediction of outburst events is often difficult because the number and
size of ice-dammed lakes vary greatly each year and because many lakes are partially
or completely subglacial; outbursts are not simply related to seasonal changes.

Property damage from inundation, channel erosion, and deposition should. be
expected on all floodplains and some of the lower terraces in the study area
(pl. 1). Maps of areas subject to inundation by 25- and 50-year floods along the
lower Kenai River have been prepared by the U.S. Army Corps of Engineers (1967;
1973).

SOUTHERN SECTION: FOX RIVER TO ENGLISH BAY

The southern section comprises a coastal strip which extends inland generally
to bedrock ridgelines, and from Sheep Creek and Fox Riveron the north to English
Bay on the south (p]. 3).. Topographic base maps are parts of the Seldovia B-4,
B-5, B-6, C-3, c-4, C-5,and D-3 quadrangles. Established communities in the southern
section are Seldovia, Port Graham, and English Bay.

Table 3. Maximum known floods for four majo.r streams entering eastern Cook Inlet
between Kenai and Fox River, Alaska (Childers, 1970).

USGS Years of Gage height Discharge
station n-o-. Local ity record- Date (ft.) (cfs)

2663 Kenai R at Soldotna 4. 10@15-69 12.20 30,000

2420 Ka;silof R near Kasilof 20 .09-14-57 7.90. 12,30.0

.2416 Ninilchik R at Ninilchik 6 06-02-64 4.46 650

2400 Anchor R at Anchor Point 14 05-08-63 6.36. 3,030

Table 4. Estimated flood discharges at various recurrence intervals for four major streams entering eastern Cook Inlet
between Kenai and Fox River, Alaska. Based on all records available as of September 30, 1968 (Childers., 1970),l

.USGS Drainag Period of record Flood discharges for various recurrence intervals (cfs)@
station no. Locality area (mil) (years/dates) Q2 Q5 Q10 Q25
2663 Kenai R at Soldotna 2,010 ](1965-1966) 25,500 32,000
36,7oo 4o,6oo

2420 Kasilof R near Kasilof 738 20(194871968) 8,130 9,550 10,500 11,800
2416 Ninilchik R at Ninilchik 131 6(1962-1968) 470 565 622 ------3

24oo Anchor R at Anchor Point 226 140952-1966) 1,88o 2,36o 2,680 3,080

lValues for the Kenai River at Soldotna are taken from a tentative Deak discharge f r e qu.ency curve based on only ord and correlation
g one year of rec
of drainage basin relationships with the Cooper Landing station on the Kenai River (U.S. Army Corps of Engineers, 1967)
O\
2Recurrence interval is defined as the average interval of time within which the given' f1cod discharge will be exceeded once (Childers, 1970). For
..example, Q10 is the peak discharne for a 10-year average- recurrence interval and there is a 10% chance that the annual flood in a given year will
exceed QI 0. For a 5-year average-recurrence interval, there is a 20% chance that the annual flood will exceed Q 5 in a given year. Standard e'r-
of Q2, Q5., QIO, -b
and Q25 are 80%, 80@, 81,, and 72%, respectively.
3The record is too short for prediction of the 25-year flood.

Geology

General Setting

Kachemak Bay is a glacial fiord, eroded along the contact of relatively soft
Tertiary rocks (sedimentary) which underlie the Kenai Lowlands and-relatively
hard pre-Tertiary rocks (sedimentary and metamorphic) which underlie the Kenai
..Mountains. Glacial ice occupied the bay at least as recently as 10,060 B.C.
Northwest-trending valleys enter Kachemak Bay from the Kenai Mountains; most
(such as Sadie Cove and Tutka Bay) have such attributes of classical fiords as..
a steep-walled, U-shaped profileand deep marine waters. Other bays have
shallower water (for example, Seldovia Bay) and some are drowned only at the
mouth (for example, English Bay), but all are glacially eroded. Glaciers still
remain at the heads of the major valleys from Tutka Bay to the north edge of the
southern section.

The absolute a-ges of the most recent major glacial advance(s) in the southern'
section are not known. Karlstrom (1964, p. 20) suggests by i'mplic*ation that the
Homer Spit may have formed at an ice terminus between 10,000 and 7,500 B.C.;
if so, it is reasonable to expect that Alpine glaciers in the major valleys of
the southern section would also have been in advanced positions at about that,
time. However, Karlstrom proposes several relatively minor glacial advances
during the peri'od after 7,500 B.C. for the northern Kenai Peninsula, and most
glaciers occupying major valleys in the southern section probably advanced to
some,extent at-similar intervals. None of the existing glaciers are presently
advancing.

The coastline of the southern section is, overall, one of submergence.
Glacial erosion produced steep-walled, U-shaped tributary valleys; these valleys
were inundated by rising sealevel following recession of the valley glaciers. In
addition to sealevel,rise, Plafker (1969) concludes that Kachemak Bay and the
west side of the southern Kenai Peninsula are part of a larger area which.has
experienced regional (tectonic) subsidence at least since the most recent de-
glaciation. Evidence of subsidence includes the classic submergent morphology
(drowned cirques) on the outer (southeastern) coastline of the Kenai Peninsula,
as well as drowning middens in several Kachemak Bay sites. The middens are
described by De Laguna (1934) from Kas'itsna Bay, Ismailof Island, and Yukon
Island; she concludes that submergence at the Yukon Island site was 4;2 m since
initial occupation of the site (De Laguna, 1934, p. 28). Pieces of caribou antlers
were dated from each of 2 different levels at this site (Ra.iney and Ralph, 1959)
and the ages are presented by Plafker (1969, p. 1-54) to give an implied average
rate of subsidence at the site over the past 2700 years of 2.24 m per* 1000 years
(datum is 1965 sea level).. However, in a preface to her second edition in 1975,
De Laguna (p.. ix) emphasizes that the samples used for radiocarbon analysis were
,contaminated. Consequently, any attempt to quantify the average rate of net
subsidence of the southern section for the past few thousand years is attended
by uncertainty.

Parts of the coastline are presently building out by construction of fan-
deltas at the mouths of major rivers (McKeon Flats, Greywingk Creek, Mallard
Bayj and at the heads of fiords); in such areas the net rate of sedimentation
has been greater than the rate of submergence, although seaward portions of the
fan-deltas are periodically flooded because of regional subsidence. Nearly
everywhere else in the southern section, the coastline consists of seacliffs cut
by wave erosion in bedrock. Seastacks and seacaves attest to the dominance. of
wave'erosion over deposition (for example, at the entrance to Seldovia Bay).

Beach materials in the southern section are mainly pebbles and cobbles,
commonly mixed with sand; waves have constructed spits and tombolos of these
materials.along the outer coastline (for example, Seldovia Bay). The implied
direction of net littoral drift along the outer coastline is from the southwest.
Barrier spits* and beaches offshore of some fan-deltas provide protection from
waves (for example, China Poot Bay). Barrier beaches at some small lagoons (for
example, Selenie Lagoon and English Bay) consist in part of rounded boulders as
much as 1 to 2 feet in diameter; probably these boulders are the wave-eroded
remnants of glacial drift deposits (bay-mouth moraines?). No data exist on the
sources of littoral-drift sediments along the outer coastline, but the association
of wider beaches with seacliffs cut in soft Tertiary rocks suggests that much of
the material is locally derived by mass wasting of material comprising the sea-
cliffs. Intertidal' and beach deposits from Neptune Bay to.the head of Kachemak
Bay probably are derived chiefly.from sed.iments transported by the larger rivers,
which consist in part of glacial meltwater.

Bed-rock

'Bedrock'is widely exposed in the :southern section and consists mainly of
slightly metamorphosed sedimentary and volcanic rocks, crystalline metamorphic
rocks, and minor amounts of intrusive igneous rocks. Lithologies are chiefly
slate, greywacke, volcanic flows (basaits),, and volcanic tuffs.and breccias, with
lesser amounts of chert, limestone, schist, and marble. Sedimentary ages range
from Triassic to Cretaceous; t 'he crystalline metamorphic rocks were metamorphoseu
in Late Triassic. or Early Jurassic time. Deformation ranges in degree from slight
(for example, sedimentary rocks exposed at the southwest headland, Seldovia Bay
entrance) to intense (schists, and some interla'yered cherts, tuffs, and flows).
(See Forbes and Lanphere, 1973-, Beikman, 197.4.)

In addition to pre-Tertiary rocks, Tertiary rocks are exposed locally along
the outer coastline southwest of Tutka Bay., The Tertiary rocks consist of weakly
to moderately indurated sil.tstones, sandstones, conglomerates, and local, relatively
..minor coal beds. Faults with apparent maximum displacements on-the. order of a few
meters were seen cutting the Tertiary rocks near Seldovia Point; no such faults
were observed to clearly offset the overlying glacial deposits, and the faults
are tentatively considered to be inactive. Where observed, bedding planes in
.Tertiary rocks were w-ithin a few degrees of horizontal.

Sources of large riprap (armor riprap) are limited in the study area.
Tertiary sedimentary rocksare relatively poorly indurated and susceptible to
-Tertiary rock are either deformed, or pervasively fractured
weathering. Pre s I
and jointed, or are susceptible to parting along bedding planes. Consequently,
identification of specific rocks suitable for use as armor ri'rap requires detailed
P
site inspections. Field time during the present study was not adequate for
comprehensive inspections. In general, armor riprap has been obtained from the
headland at the entrance to Sadie Cove (meta-volcanic rock) and from Grey Cliff
point near Seldovia (impure marble). The Sadie Cove tidewater site, and a tide-
water site at Halibut Cove (igneous initrusive) were identified as potential
riprap sources by State of Alaska, Department of Highways personnei'during-a field
investigation in 1968. The Grey Cliff site was briefly inspected,by U.S. Army
Corps of Engineers' personnel,in.1965. According to Mr. Glen Greeley, Corps of
Engineers (personal communication, August 1977) , no other feasible tidewater sources
were identified by Corps'. personnel during a.coastal reconnaissance in the southern
section.

Surficial Deposits

The major surficial deposits recognized in the southern section are:
1) glacial drift,,comprising till and stratified drift (outwash); the drift is
generally thin, less than 2 or 3 m thick, except along the outer coastline in
the southern part of the map area and along the southeast side of Sheep Creek;
2) marine deposits, mainly s.and-shingle beaches and deposits of intertidal flats
and deltas; 3) alluvial fan and floodplain deposits of active rivers and glacial
outwash deposits; 4) a large rotational block landslide southeast of Sheep Creek,
as well as numerous smaller slides, falls, and debris flows in bedrock and un@-
consolidated deposits,near unconfined steep surfaces elsewhere in the southern
section.

Relatively thin deposits of 'drift are included in Unit Qd; thicker drift
deposits which have been observed in, bank or roadcut exposures are shown as
Unit Qdo (P]. 3). Glacial outwash deposits which are morphologically separable
from drift are shown as Unit Qo, which includes valley train.depos'its in valleys
with apparently underfit streams, vegetated terraces above present valley flood-
plains, and floodplains of active streams which presently drain glaciers. Narrow
channels with no apparent streamflow and no obvious drainage area are shown on.
plate 3 by a wide arrow. These channels are inferred to have been meltwater
drainages which eroded at former glacier margins; little or no fill is visible
on air photos, although the channels may locally contain minor amounts of sand,
g rave I and bou 1 de rs.

Marine deposits are shown on plate 3 as deltas (Qid), intert.idal flats (Qif),,
and beaches (Qib).- Each of the three types of deposits is subdivided as to
presence (example: Qidv) or absence (example: Qida) of.veget 'ative cover. Vegetative
cover does not imply that a deposit is no longer within reach of seawater; beaches
maybe overwashed only infrequently by storm waves at highest tides and so support
vegetation, and salt-tolerant plants May grow in tidal marshes in the upper part
of the intertidal zone. The "a".and "v" distinction for marine deposits should
be-used only as 6 general guide to.relative frequency of marineAnundation.

Large alluvial fans,with relatively low surface gradients are shown on plate
by the symbol Qaf. Smaller fans with steeper gradients are represented by Qff@
and floodplains are shown by the symbols Qal and (for.the larger rivers presently
draining glaciers) Qo.

Deposits resulting from mass movements are shown by the symbols Qct (talus
deposits) and Qcl (landslide deposits). Talus deposits originate by indivi-dual
rock falls, that is, "one piece at a time". Land.slides involve mass movements
of larger amounts of material; a variety of slides,,falls, and flows will break
apart during movement, producing a heterogeneous and nonsorted deposit. Other
slides occur by rotational (slump) or 'lateral (transiatory) movement of material
along a well-defined plane of failure, and little or no disruption of the moving
mass occurs. Several landslides tentatively classified as slumps are recognized
in the.southern section. These slides are shown by symbols '.'Qc*l/x", where "x'@
is-given as the map symbol for the original deposit involved in the landslide.
Other landslides in the southern section are.shown only by the symbol Qcl; -these
landslides are mainly rockfalls or rockslides (including the large rockfall/slide
on the terminus of Greywingk Glacier), and smaller deposits tentatively classified.
as debris flows or earthflows.

Sand and gravel deposits potentially suitable forcommercial extraction occur
in a variety of ways: as marine deposits along beaches and deltas, as river deposits
in channels,.floodplains, and large alluvial fans, and as glacial outwash and
stratified drift.r The largest deposits are in beaches and in floodplains and
alluvial fans of the larger rivers. Exploitation of such large deposits will be
hampered by seawater and shallow groundwater. Lesser volumes are available in
glacial outwash deposits, and in stratified drift which occurs locallywithin
glacial drift. Sand and gravel deposits are shown on plate 4; in delineating the
deposits, no attempt wasmade to classify them by potential feasibility. Feasibility
.depends on many factors, including physical characteristics of the sand-and
gravel and suitability for particular uses, size of the deposit, ease of extraction,
and distance from the site of intended use. Consequently, plate 4 should be used
onl.y as a general guide for future feasibilitystudies once specific needs have
been identified.

Geologic Hazards

The main geologic hazards in the southern section, as identified during the.
course of the present study, are outlined below.

Mass Movements

Rockfalls are rapid movements of 'large rock masses on very steep slopes
essentially by failing; deposits of two rockfalls are tentatively identified along
the north shore of Halibut Cove and on the Greywingk'Glacier near the terminus
(Qcl, Pl. 3). (The Greywingk Glacier rockfall apparently involved.some lateral
movement onto the glacier, and so may be more properly termed a combination fall/
slide.) Debris f1ows and earthflows are slow to moderately rapid movements of
"slurries". The slurry results from the addition of water to soil, weathered
bedrock, and/or unconsolidated surficial deposits. Motion may-be started in a
variety of ways; commonly, slumping of saturated surficial deposits or rapid
erosion during high runoff are generating mechanisms. Deposits of debris'flows
or earthflows are tentatively identified at the foot of slopes covered by glacial
till and outwash at the northeast corner of Kachemak Bay and nearby along Sheep
Creek (Qcl, Pl. 3). Also, many alluvia.1 fans (especially small steep fags, unit
Qff on plate 3) probably consist in part of poorly sorted debris flow or mudflow
de-posits.. Rotational landslides@(slumps) are characterized by ground failure
along one (or more) well defined planes, so that commonly a visible scarp results
as the slide block moves down and later-ally. Rotational landslides are tentatively
iden.tified on the slopes southeast of Sheep Creek near Kachemak Bay (local stream
erosion.could cause renewed local or massive slumping; Qcl/Qdo, pl.. 3), and in
several localities along seacliffs on-Cook Inlet southwest of Tutka Bay (glacial
till, outwash, and Tertiary sedimentary rocks adjacent to steep seacliff5; unit
Qcl Tx, pl. 3). Earthquakes commonly trigger a variety of mass movement.

In addition to the preceding deposits, rocks falling individually from moderate
to very steep slopes will accumulate as, talus (commonly cone-shaped deposits).
Those.talus deposits large enough to be mapped at a scale of P to I mile are
shown on plate 3 by the symbol ''Qct".

Snow Avalanches.

The forms of snow avalanches,.and the various conditions of weather.and
topography which can give rise to avalanches, are numerous. The size may range
from small local events to large.events involving tens of thousands of tons of

snow; velocity of snow movement ranges from slow creep to true avalanches trave'lling
from several miles per hour to as much as three hundred km per hour.

Avalanche prediction is obviously a complex subject, and it is not the purpose
of the present study to attempt such prediction for the'southern section. I.nstead,
those areas which are known or thought to.-have experienced snow avalanching are
delineated on plate.4. Identification of such areas is based primaril.y on
vegetation "scars" on or adjacent to slopes of moderate to steep gradients.
Limitations inherent to this method are as follows: 1) areas may.experien*ce
avalanches only at highly infrequent intervals, so'that no discerni-ble scar
exists at present; 2) scars may be due.to other processes, such as mud-flows or
rockfalls. (although avalanches often are channelled down pre-existing runoff
channels and talus chutes); 3) on steeper slopes or above local treeline, no
vegetation exists to precisely delimit areas subject to.avalanching. Bare slopes
in excess of 450, for example, are often not Prone to massive avalanches because
snow will slide off more or less continuously during accumulation.

In view of the aforementioned limitations, plate 4 should be used only asa
general guide to areas of snow avalanching. The vulnerability of particular
sites to potential avalanches should be assessed on a site-specific basis by
qualified.personnel; in particular, unusually large avalanches may occur at very
infrequent intervals at some sites. Runout zones from such avalanches may extend
well beyond the edge of the avalanche zones shown on plate 4.

Tsunamis

The largest recorded tsunami' to affect the study area occurred in 1883, had
a height (relative to tide level at the time of the tsunami) estimated to have
been from 8 to 9 m, and was generated by an eruption of Augustine Volcano. The
1883 tsunami travelled from Augustine Volcano to Port Graham in about 25 minutes;
ideally, warning could be given today to co astal communities in time for people
to move to high.ground in the event of another tsunami-generating eruption.

The probability of even higher tsunamis being generated within Cook Inlet
is difficult to assess. Furthermore, given a tsunami of specified magnitude, the
prediction of runup heights at specific sites along such an irregular coastline
as in the southern section is difficult. For the purposes of this report, it i,,s
suggested only that persons carrying out coastal activities keep in mind the
fact that a tsunami 8 to 9 m (25 to 30 feet) high had been recorded at Port
Graham, that the generating mechanism of that tsunami is still active, and that
the tsunami could have arrived at the peak of an extreme high tide.

Unstable Ground

Fissuring, fracturing, and settlement of'unconsolidated surficial deposits
occurred widely in south-central Alaska during the great earthquake of 1964.
Areas especially prone to such failures; included thick alluvial deposits.in river
vall'eys and on-fan-deltas.and areas underlain by fine-grained, saturated sediments
which may have liquefied during seismic shaking.

No ground failures in 1964 have been reported from the southern section.
However, dead trees along the seaward margins of most large alluvial fan-delt.as
were probably kill ed by relatively higher seawater levels after the earthquake.
The surface,gradients of the fan-deltas are low and seawater incursion could be
due solely to regional subsidence; local subsidence of fan-deltas by compaction,
however,-could have occurred there as well.

_21-

Extensive deposit.s of fine-grained saturated sediments were not observed in
the study area (except for marine deposits of intertidal flats and deltas),
although such deposits may. occur beneath the larger floodplains and alluvial
fans. Glacial drift is generally thin (unit Qd,'p). 3) but in'places exceeds
several meters in th-ickness (unit Qdo, Pl...3); the drift is mainly till but it
includes stratified sand and gravel which could be prone to liquefaction.

In general, the areas of potentially most unsta-ble ground in the coastal
..strip from the Fox River to English Bay are probably the alluvial fan-deltas.
Such areas elsewhere during the great earthquake of 1964 were prone to settlement
by compaction of sediments, to ground fissuring by liquefaction at depth, and to'
submarine landsliding'. No such.ground.failures were reported from the present
study area, nor was evidence for ground failures found during this study, but
failures could occur in future large earthquakes. In addition, proposed develop-
ment in areas of drift (units Qd, Qdo, Pl. 3) and outwash (unit Qo) should be
preceded by a detailed site-specific evaluation of the bearing capacity of the
surficial deposits, including subsurface (borehole) data..

Floods

Identification of specific areas which would be flooded during 50- and 100-year
floods along parti,cula-r streams requires detailed data on stream discharge and
topography. Hydrographic data are available for several streams in the vicinity
of Seldovia and for the Bradley River, but the topographic bases are inadequate
to define precisely the limits of 50- and 100-year flood areas. Consequently,
areas.shown on plate 3 as "Qal" are probably the best approximation to areas of
potential river flooding available at present. Sand and gravel deposits adjacent
to the Greywin 'gk River, Fox River, Sheep Creek, and the river discharging into
Neptune Bay are shown on plate 3 as outwash (unit Qo), because these rivers
presently head at glaciers. Are'as.shown on Qo along these rivers should a] , so
be included in the list of areas potentially subject to flooding. (The flood of
record on the Bradley River for the period 1954-55 and 190-70 occurred on.
Octobe@ 1-4, 1969; gage height at peak was 9.13 feet (Childers, 1970).)

Glacier surge and outburst flooding have not been identified in the study
area (Larry Mayo, pers. comm., 1976). However, a large rockfall in August, 1967
occurred at the no�e of Greywingk Glacier; part of the fall entered Graywingk
Lake and generated a wave which.apparently surged through the outlet of the lake
basin. Bedrock was scoured clean of surficial deposits at a channel constriction
approximately 2.4 km below the lake outlet (compare 1961 and 1975 aerial photos),
to. an elevation of about 12 m above the stream level on May 21, 1977. The site
of the rockfall was inspected briefly-, and several distinct bedrock joints or
faults with minor displacement subparallel to the present cliff face were observed.
The cliff is approximately 2000 feet above lake 1-evel, and the upper (glacially
oversteepened) part is nearly vertical in places. A down-dropped block in the
steep bedrock wall just down valley from the scar of the 1967 rockfall constitutes
a definite,hazard from similar rockfalls-in the future, and the Greywingk River
plain must be considered as.potentially subject to flooding by this mechanism.

-.22-

GLOSSARY OF TERMS

Ash,(volcanic) particles of volcanic ejecta mainly less than 4.mm diameter.
Ash often consists of congealed bits of@,fragmented lava. Fine
volcanic ash can remain i-n the earth's atmosphere for months
or years.

Avalanche (snow) a large mass mainly of snow and ice which moved rapidly down
a slope. Avalanches may move at speeds up to 300 km/hour a-nd
may be accompanied by destructivewind blasts.

Beach.- that part of a shoreline which is regularly exposed and falls under
direct wave action. Normally extend's from low tide level to
the landward side of material which is regularly (though perhaps
infrequently) acted upon by the highest waves.

Bedrock solid, relatively indurated-rock. Compare "surficial deposits''. The
base upon or against which loose deposits rest (for example,
alluvial sands and gravels, glacial deposits, beach deposits).

Cretaceous (Period) - a period of geologic time from about 130 million years to
65 million years.before present.

Delta (fan-delta) - deposits of river-borne sediments in standing water at the
mouth of the river. Fans are cone-shaped river deposits formed
at places of abrupt decrease of gradient. As used here, delta
refers to that part of a river deposit.which comes regularly
under the influence of waves..

Drift (glacial) a term referring to glacial deposits in general. Includes
".'till" (nonstratified, nonsorted rock material deposited directly
by glacial'ice) and "stratified drift" (sorted.and stratified
silts, clays, sands,.or gravels deposited in or by glacial
meltwater.).

Eustatic refers to world-wide c'hanges in sealevel., as opposed to local changes
due to a rise or fall of land.

Fan (alluvial, colluvial) - a broad, cone-.shaped deposit formed at a place of
abrupt.decrease of land surface gradient. Alluvial fans are
formed of river-borne sediments; colluvial fans include material
moved chiefly by gravity.

Fault a fracture or break in the earth's crust along which there has been
relative movement (displacement)*. Movement may be abrupt
(thereby generating earthquakes) or slow and relatively
continuous. Active faults are those along which movement can
be expected at any time; the problem in identifying a fault
as active is to determine accurately the time of most recent
movement.

Fiord a lo,ng deep marine embayment in a valley with high steep sides. Usually
refers to a glacially eroded valley with characteristic U-shaped
cross-section which has been.inundated by the sea.

Floodplain that part of a river valley which i-s covered by water.when the river
floods. Underlain by sediments (usually fine sands and silts)
deposited,during flood stages of the present river regimen,
as opposed to river deposits.underlying higher'terraces.

Holocene an epoch of geologic time extending from the end of the Pleistocene to
the present, that is, geologic time since about 10,00.0 years
before the present.

Indurated refers to rocks.which have hardened natu-rally (by heat, pressure,
and/or cementation) from an original state of relative looseness.

Intensity a subjective evaluation of the effects of an earthquake at a particular
locality. Depends on many factors, including distance to the
earthquake, magnitude, and local geologic conditions. One such
Vntensity scale is the Modified Mercalli Scale.

Intertidal flat a portion of a shoreline which is regular@ly exposed during
tidal cycles, with an essential flat surface and a surface
gradient less than that of any adjacent beach. Commonly under-
lain by silts or fine sands which are only briefly exposed to
wave action during each tidal cycle.

Isostatic in geology, a term referring to a concept which holds that relatively
large portions of the earth's crust act as though they were
floating on denser underlying material. Thus, advance of a large
glacier wi.11 depress the crust due to the added weight of the
ice; upon retreat of the ice, the crust will slowly rise.
Jurassic (Period) - a period of geologic time from about 185 million years to about
130 million years before present.,

Landslide movement of rock, surficial deposits, or soil under the influence of
gravity. Ve,locity of movement can range from slow to terminal
free-fall vel.ocity. Depending on type of material involved
and mechanism of movement, may be classified as, a variety of
types (debris flows, slumps, translatory slides, rockfalls,
etc.).

Liquefaction a process by which unconsolidated sediments are transformed to a
state of little or no shear strength, due to increase of pore
water pressures. Commonly occurs during seismic shaking of
great magnitude and/or duration, in water-saturated deposits
of silt and sand. Effects of liquefaction include fissurina
and sand fountaining at the overlying ground.surface, and lateral
jlowage of the liquified deposits and/or overlying deposits.

Littoral drift movement along a shoreline of materials which.comprise beaches,
spits, bars, etc. Origin of the movement is in waves and near--
shore currents. Direction of movement may vary with storms,
tides, seasons,.etc., but generally a single direction of movement
prevails over a period of years.

Magnitude the size of an earthquake as relates roughly to the amount of energy
released at the source. Magnitudes are determined from seismogram..
records; "Richter magnitude'' is a particular magnitude scale.

--24-

Moraine a constructional deposit of 41"'cial drift (chiefly till), usually with
.3 a
a morphology which is characteristic of the position relative
to the ice mass at the time of deposition.

Nu&e ardente French term referring to a hot, highly mobile mixture of gas and
fragmented lava which can move rapidly down even slight inclines.
A ''fiery avalanche"; a type of volcanic eruption which is
characteristic of some Aleutian volcanoes.

Outwash rock material deposited by glacial meltwater streams beyond the,glacier
ice.

@Quaternary (Period) a period of geologic time beginning about 2 mi.ilion.years.
before present.

.Recurrence interval - the frequency at which earthquakes of a-spe cified magnitude
have occurred in a specified area. In using recurrence intervals,
the presumption is that the period of recording earthquakes is.
sufficiently long that the record is a valid predictor of the
likelihood of future large earthquakes. Recurrence intervals
may not be used to predict the likely time and place of future
earthquakes.

Sea stack, sea cave - features indicative of erosion on a rocky coastline. Sea
stacks are chimneylike columns of rock isolated from an adjacent
headland or cliff,-and sea caves are niches or grottos in sea
cliffs. Both have their origin in erosion mainly by waves and.
currents.

Seiche periodic oscill-ation of a body of water, generally at least partly
enclosed.. Seiches may be generated by winds, atmospheric,pressure
changes, tidal currents, or earthquakes.

Seismic a term referring to earthquakes or earth vibrations. "Seismicity of a
region", for example, generally refers to locations and magnitudes
of recorded earthquakes in the region, to analysis of earth
stresses and fault movements thought to be responsible for the
earthquakes, and to prediction of the likelihood of future large
earthquakes in the re4i
gion.

Spit a small,projection of a shoreline or of a shoal into a body of water. As
used here, spit refers to a small, linear or slightly curved
Projection of beach deposits into the sea.

Submergence 7 inundation of some land area by the sea,. regardless o-f cause (see
11subsidence").

Subsidence,- relative fall of some land area, resulting in inundation by the sea
if the land area is at the coast.,

8urficial (unconsolidated) deposit relatively loose deposits of*transported rock
material (river deposits, glacial deposits, etc.), residual soils,
or,organic deposits at or near the earth's surface (see ."bedrock").

-25-

Tertiary (Period) a period of geologic time from about 65 mill.ion years to abol,t
2 million*years before present.

Till (glacial) unconsolidated rockmaterials. deposited directly by glaciers.
Characterized by poor sorting and absenceof marked stratification.

Tombolo a bar which connects one island to another or to the mainlan.d. As used
here, refers to a beach deposit which connects bedrock exposures
offshore ofthe beach to the mainland.

Triassic (Period) - a period of geologic time from about 230 million, years to
about 185 million years.before present.

Tsunami a seawave generated by some large, short duration disturbance of the.
ocean.floor. Tsunamis may be generated by submarine fault
movements (generally by movements involving vertical displacements
in shallow water during large.ear,thquakes)., by submarine or
subaerial landslides (including volcanic eruptions), or by
subsidence.

--26-

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-.27-

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..29-

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