Modern Clastic Depositional Systems of South-Central Alaska

49 Pages·1.581 MB·English

by  Miles O. Hayes, Jacqueline Michel(auth.)

Modem Oastic Depositional Systems
of South-Central Alaska
Anchorage to Cordova, Alaska
June 29-July 7, 1989
Field Trip Guidebook T101
Leader: Miles O. Hayes
Associate Leader: Jacqueline Michel
American Geophysical Union, Washington, D.C.
Copyright 1989 American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C. 20009
ISBN: 0-87590-599-4
Printed in the United States of America
COVER Oblique aerial view of Scott Glacier outwash fan in the summer of
1970. Distance from the bottom of the photo to the Scott Glacier at the
top is about 18 km. Note well-developed longitudinal bars the
foreground. Photograph by Jon C. Boothroyd.
Leader:
Miles O. Hayes
RPI International, Inc.
1200 Park Street
Columbia, SC 29202
Associate Leader:
Jacqueline Michel
RPI International, Inc.
1200 Park Street
Columbia, SC 29202
IGC FIELD TRip T101:
MODERN CLASTIC DEPOSITIONAL
SYSTEMS OF SOUTH-CENTRAL ALASKA
Miles O. Hayes and Jacqueline Michel
RPI International, Inc., Columbia, South Carolina
INTRODUCTION 1 July 1989
Boat trip in Kachemak Bay to examine its
The south-central coast of Alaska is an tide-dominated bayhead depositional sys
excellent location to view dynamic coastal tems as well as the fan deltas on the
processes. It is remarkable that although south shore.
the area is situated on the leading edge of a
major continental plate, the sediment produc 2 July 1989
tion is such that large depositional systems Examine the shoreline between Homer and
with a good preservation potential are pre Ninilchik via automobile.
sent. This eight-day field trip is designed
to show the participants the entire spectrum 3 July 1989
of Holocene depositional systems that occur Day off. Fly from Homer to Cordova.
on this part of Alaska's coast.
The first part of the trip will focus on a 4 July 1989*
modern forearc embayment, lower Cook Inlet Drive to the head of the Copper River del
(Fig. 1). Though erosive processes dominate ta to view the braided streams, outwash
in Cook Inlet, excellent examples of fan del plains, and dunes associated with the del
tas, macrotidal bayhead depositional systems, ta.
and depositional spits occur throughout the
area. These depositional features, plus 5 July 1989*
spectacular erosional coastlines, typically Overflight to beach-ridge plain in front
mild weather, and some of the most beautiful of Bering Glacier (via float plane). Vis
scenery on earth, should combine to provide it to outer beaches is planned.
for an enjoyable visit.
The second part of the trip will be on the 6 July 1989*
less hospitable outer coast of south-central Boat trip to Egg Island, a large, meso
Alaska, a 700-kilometer-long Holocene coastal tidal barrier island on the delta front.
plain dominated by glaciers and large coastal Inlet and beach processes and sediments
storms. During this part of the trip, we will be reviewed.
will emphasize the 3 major depositional sys
tems in .the area: (1) glacial outwash 7 July 1989
plains, (2) beach-ridge plains, and (3) the One-half day of review, discussion, lec
mixed-energy Copper River delta complex. tures, and short field trip. Fly to
A detailed discussion of the daily activ Seattle in the afternoon.
ities, as well as specific stop descriptions,
will be distributed separately. A brief This guidebook is presented in two parts
itinerary follows: to coincide with the planned field trip.
Part 1 focuses on the morphology and sedi
Date / Activity ments of lower Cook Inlet, and Part 2 is
devoted to the depositional systems of the
29 June 1989 Holocene coastal plain of the outer coast be
Arrive in Anchorage, Alaska. Introductory tween Cordova and Yakutat.
lecture at 7:30 p.m. at a location to be
designated later.
30 June 1989
Fly in small aircraft to Homer, viewing
the western side of upper Cook Inlet and
both sides of lower Cook Inlet along the
way. Visit the beaches in the vicinity of *Note: This day will be scheduled to fit the
Homer. weather conditions, which tend to be
cool, windy, and rainy at this time
of year.
T101: 1
a f A los Ie a
~ Resultant energy flux
o WaveEnergyFlux20
~ long-term sediment
transport direction
1010~
m sec
FIGURE 1 Location of the areas to be visited during this field trip. Part 1 will be in
lower Cook Inlet, with a home base in Homer. Part 2 will be devoted to the outer coast of
south-central Alaska, operating out of Cordova. Also shown is the direction of longshore
sediment transportation based on large-scale coastal geomorphic features and resultant
wave-energy flux distribution for the coastal areas of the Gulf of Alaska. Large-scale
coastal features used in establishing long-term transport directions include spits, inlet
offsets, and crescentic embayments. The resultant wave-energy flux is determined by vec
torial addition of values ·for each compartment shown, which are based on deep-water wave
observations. Note the convergence of wave-energy flux toward the Cordova area [from
Nummedal and Stephen, 1976, p. 44].
Acknowledgments Ray Levey, Tom Gustavson, and Jane Zenger.
Part of the work was carried out while the
The earlier fieldwork in the lower Cook senior author was associated with the Depart
Inlet area was sponsored by the Alaska De ment of Geology at the University of Massa
partmentof Fish and Game (ADF&G), Habitat chusetts. Both authors contributed while
Protection Service, M.P. Wennekens and Lance associated with the Department of Geology at
Trasky, project supervisors. P. Jeffrey the University of South Carolina, and later
Brown assisted in all phases of the field and with RPI International, Inc. All three in
laboratory work of the 1976 study. The ear stitutions are gratefully acknowledged for
lier phases of the work on the outer coast of their support and resources.
south-central Alaska (1969-1971) were sup
ported by the Geography Program of the Office
of Naval Research (Contract No. NOOOI4-67-A PART 1. LOWER COOK INLET
0230-0001, Miles O. Hayes, principal investi
gator), and the later phases there, as well Introduction
as in Cook Inlet, were supported under con
tract with the National Oceanic and Atmo The shoreline of lower Cook Inlet, Alaska,
spheric Administration (Contract No. 03-5 which is located in the forearc region of an
022-82; Miles O. Hayes, principal investi active (or converging) continental plate
gator). A large number of people contributed [Bally and Snelson, 1980], displays a variety
to the field research, including Christopher of depositional systems which reflect its
Ruby, Jon Boothroyd, Joe Hartshorn, Skip tectonic and hydrographic setting. As a re
Rhodes, Gail Ashley, Stewart Farrell, Paul sult of the regional tectonics, the area is
Hague, Bob Henry, Frank Raffaldi, Mark Cable, subject to intensive earthquakes and spectac-
T101: 2
ular volcanism. tides up to 6 The information to be in the
the area in the macrotidal realm field is based on a reconnaissance of
1964]• The shorelines are shel the and of the
tered, but the passage of extra entire lower Cook Inlet coastline, which was
generates storm carried out in the summer of 1976 et
wave conditions. This part will al., 1976a; and Michel, 1982], as well
allow the to the mor- as several environmental sensi-
and framework the ects in the area. Field
shoreline lower Inlet, which studies conducted in the summer of 1976 used
can serve as a basis for a the of et al. [1973].
depositional model for shorelines of a fore- These aerial reconnaissance,
arc embayment. during which the shoreline was photographed
For the first three of the field in detail, and 57 survey stations,
, we will operate out of the small town which included 10 detailed study sites
of Homer, which is located 180 stations) representative of the different
air kilometers south of Anchorage. Part of coastal types in the area. A total of 178
the trip will include an overflight of both sediment samples were collected and analyzed
the western and eastern sides of lower Cook for grain size and clast composition. The
Inlet. The rest of this part of the trip study area, which is located in the southern
will be conducted by means of land vehicles part of Cook Inlet (Figs. 2 and 3), included
in the vicinity of Homer, and a boat trip on (1) the coastline on the eastern side of the
Kachemak Bay. inlet from the Chugach Islands north to the
COOK INLET
o
61
Eill
STUDY AREA
o 30 60
i !
KM
600
i
I
i
I
I
I
~:-'.l"\,
Q t\
~~
FIGURE 2 Regional map of Cook Inlet showing area studied by the authors in lower Cook In
let. The route we plan to take on the overflight is shown. Several locations along the
beach between Homer and Ninilchik will be visited, and a boat trip in Kachemak Bay will al
low inspection of the fan deltas and macrotidal flats in the bay.
T101: 3
circulation data for Kachemak Bay and V1C1n1
ty obtained by continuous Lagrangian measure
ments for up to 23 days. Bouma et al. [1977]
o-==10-=2-0::::3l0 reported on the nature of the bedforms in the
KM
entrance to the inlet. There have been two
major bibliographies of the Cook Inlet area
[Maher, 1969; Evans et al., 1972], which en
compass all aspects of human activity in the
region.
Several studies have been carried out on
the macrotidal flats of upper Cook Inlet in
EROSIONAL SCARPS
IN BEDROCK recent years by the USGS [Ovenshine and
Kachadoorian, 1976; Bartsch-Winkler et al.,
~ _ SCARPS>100m
~SCARPS<100m 1983; Bartsch-Winkler and Ovenshine, 1984;
c:::J DIP-SLOPES Bartsch-Winkler, 1988]. These studies empha
size the impact of earthquakes on the sedi
FIGURE 3 Location of erosional scarps in ments and tidal channel shifting of the ex
bedrock in study area, which make up 41 per pansive flats of the upper inlet. We will
cent of the total shoreline. Highest scarps see analogous features on the macrotidal
occur along the entrance to the inlet, which flats of Kachemak Bay.
is exposed to large storm waves that enter
from the Gulf of Alaska. Physical Setting
Kasilof River; (2) the southern half of Climate. Lower Cook Inlet is situated in
Kalgin Island; and (3) the coastline on the the buffer zone between maritime and interior
western side of the inlet from Harriet Point weather zones, with the interior zone exert
to Cape Douglas, including Augustine Island. ing a greater influence to the north [Evans
Only the eastern shore of lower Cook Inlet et al., 1972]. The maritime zone is charac
has a year-round populace, with Kenai and terized by small seasonal temperature varia
Homer being the largest towns accessible by tions, high humidity, and high precipitation;
road. Smaller communities south of Kachemak the interior zone is drier and shows a wider
Bay, such as Seldovia, Port Graham, and En seasonal temperature variation. Mean annual
glish Bay, are accessible only by airplane or temperatures are generally 15°-20°F milder
boat. The western region is largely pristine (40°F vs. 15°-25°F) in the maritime zone.
in that humans are only temporary residents, Topographic effects on the local climate
few permanent structures have been erected, are quite pronounced. Lower Cook Inlet is
and man's impact on the environment is mini bordered to the west by the Alaska Range and
mal. to the east by the Kenai Mountains (Fig. 3).
Early studies of lower Cook Inlet were Moist air generated in the Gulf of Alaska is
primarily geologic and mineral resource effectively blocked by the Kenai Mountains,
reconnaissance studies by the U.S. Geological resulting in a relatively low annual pre
Survey (USGS), dating from the late 1800s. cipitation rate of 14-22 inches along the
Physical process studies are relatively re northeastern side of the lower Cook Inlet.
cent, especially for the lower inlet. On the other hand, moist southerly winds are
Stanley and Grey [1963] began studying beach readily funneled into the inlet between the
erosion on Homer spit in time to observe the sheltering mountain ranges, resulting in in
changes after the 1964 earthquake [reported creased precipitation on the northwest shore.
in Waller, 1966]. Knull and Williamson The average annual precipitation rate for
[1969] conducted seasonal hydrographic sur- Iniskin (in Kamishak Bay) is 73.2 inches
veys of the Kachemak Bay estuary. Sharma and [Wagner et al., 1969].
Burrell [1970] divided bottom sediments of Ice break-up and freeze-up data for lower
the entire inlet in three sedimentary facies Cook Inlet are sketchy, but apparently, the
based on grain size, which coarsens toward upper reaches of the lower portion of the in
the mouth. Tidal hydraulics have been iden let (near Kenai and Kasilof) freeze up in
tified [Carlson, 1970], modeled [Mungall and early December and are free of ice by early
Mathews, 1970], and related to sediment to mid-April [U.S. Coast and Geodetic Survey,
transfer [Wright et al., 1973]. Gatto [1975] 1974]. Mobile sea ice tends to accumulate on
combined ERTS, other satellite, and aircraft the western and southwestern portions of the
remote sensing with ground truth from field inlet [Gatto, 1975], due to the strong
work and the literature to compile baseline northerly winds that blow during the fall,
data on the oceanography of the inlet. winter, and spring.
Wennekens et al. [Section I, 1975] reported
T101: 4
Wind and Wave Regime. Synoptic wind data Cook Inlet, we mapped all geomorphic indica
for the study area are available only for tors of wave-generated longshore transport,
Homer and Kenai. No data are available for such as recurved spits, natural groins, and
the western side of the inlet [National Cli crenulate bays. The longshore transport di
matic Center, pers. comrn.]. These data are rections thus derived are shown on Figure 4.
sufficient to illustrate the strong topo These trends show that, in general, longshore
graphic control placed upon the wind regime transport is directed toward the heads of all
by the Alaska Range and Kenai Mountains (Fig. the major embayments, such as Kamishak Bay.
3) • During the fall and winter, northerly The exception is the northeastern coastline,
and northeasterly winds prevail, although where the longshore transport is consistently
there is a strong southwesterly component. northward. These directions determined from
During the spring and summer seasons, the geomorphic indicators are consistent with the
southwesterly winds are dominant. The moun wave conditions as described above.
tains tend to funnel the winds along the
trend of the inlet while blocking winds from Bathymetry and Tides. Lower Cook Inlet
other directions. has extremely irregular bathymetry, averaging
It is apparent that the mountains, espe between 40 and 8 m in depth, with troughs up
cially the Kenai Mountains, are effective in to 200 m deep occurring near inlet con
neutralizing most of the effect of the cy strictions. The inlet generally shallows in
clonic activity 'associated with the Gulf of a northward direction [Sharma and Burrell,
Alaska. Historically, lower Cook Inlet has 1970]. Perhaps the most notable irregular
in fact been a point of refuge for oceangoing ities in bottom topography are two deep chan
vessels during periods of intense storm ac nels situated on either side of Kalgin Is
tivity in the Gulf [Wennekens et al., 1975]. land.
Large accumulations of logs and other debris Embayments on the western side of the in
were observed in parts of Kamishak Bay south let are generally very shallow, averaging
west of Augustine Island (Fig. 3), which less than 20 m in depth. Extensive mud flats
tends to suggest that some of these areas may are associated with the heads and flanks of
receive the westerly component of cyclonic these bays.
induced winds. In addition to generally shallowing to the
To gain insights into the local climate of north, the inlet constricts from a width of
lower Cook Inlet, maximum obtainable deep approximately 80 kilometers (km) at the mouth
water wave conditions were computed from the to approximately 35 km at the head. This
synoptic wind data for Kenai and Homer, using narrowing causes amplification of the tidal
a method adopted from Bretschneider [1971, in wave to the north, resulting in a tidal range
Coastal Engineering Research Center, 1973, at the head which is more than twice as great
pp. 3:33-3:38]. These data indicate that as at the mouth [Gatto, 1975].
lower Cook Inlet has a moderate wave climate Coriolis forces, which are very pronounced
and that maximum waves occur in the Homer in the upper latitudes, cause a piling up of
area during the later winter and at Kenai in water on the eastern side of Cook Inlet.
the early spring. The predicted values do Hence, mean diurnal tidal ranges are smallest
suggest, however, that wave climate is fairly in the southwestern portion of the inlet
constant year-round. Predicted maximum mouth (approximately 4 m) and increase
heights vary from a minimum of less than 2 gradually toward Anchorage (approximately 9
feet (ft) [0.6 meter (m)] [less than 3-second m). The tidal range in the area we will vis
(sec) period] for winds blowing from the west it on this trip ranges between 4 and 6 m
at Kenai during the winter to a maximum of [Wagner et al., 1969; Carlson, 1970].
9.9 ft (3 m) (7-sec period) for winds blowing Tides in Cook Inlet are semidiurnal. The
from the southwest in February at Homer. inlet's positioning in the high latitudes re
Undoubtedly, even larger waves can be ex sults in a pronounced tidal diurnal inequali
pected to occur off the exposed portions of ty; that is, the 2 high tides and the 2 low
the western shoreline, particularly in win tides that occur each day are of significant
ter. ly different magnitudes.
It should be kept in mind that the shore The large tidal prism of Cook Inlet gen
lines may be coated with ice during a major erates currents of substantial velocity.
portion of the time that the most vigorous Current velocities of 3.5 meters per second
wave energy conditions exist, which will in (m/sec) are common; during spring tides, ve
hibit effective longshore sediment transport. locities may exceed 7 m/sec in the inlet con
Free-floating ice also can affect the wave strictions [Horrer, 1967]. Clearly, the cir
conditions by damping wave action and reduc culation patterns in the inlet are tide
ing wave height. controlled; it has been estimated that cur
During our reconnaissance study of lower rents generated by wind stress contribute on-
T101: 5
ly 3-5 percent of the total velocity of the continues up the eastern side comes in con
tidal currents [Marine Advisers, 1964]. tact with suspended sediment-rich ebb cur
The dominance of tidal prism over river rents from the upper inlet in the vicinity of
discharge, plus the turbulence associated Ninilchik. The meeting of these two water
with the swift tidal currents, causes lower masses generates a complex series of currents
Cook Inlet to generally be a well-mixed es just north of the study area and around
tuary [Pritchard, 1955], with very poorly de Kalgin Island. The waters from the upper and
veloped density stratification. Stratifica lower inlet also thoroughly intermix [Gatto,
tion may develop in sheltered embayments and 1975]. Many smaller current gyres develop in
along the western side in some areas, but lower Cook Inlet because of current de
this is a seasonal phenomenon associated with flection and the meeting of the two water
the high river discharge during the warmer masses. Wennekens et al. [1975] discovered a
months [Gatto, 1975]. counterclockwise gyre in the waters off the
Coriolis forces have a pronounced effect Bluff Point/Homer area, and Gatto [1975]
on inlet circulation patterns and suspended found a clockwise back eddy offshore near
sediment distribution. On the rising tide, Cape Kasilof. The ebbing currents, which are
the cold, clear ocean waters of the tidal partially dilute and carry suspended sedi
bore are deflected along the eastern side of ments, generally proceed down the western
the bay. Ebbing waters, somewhat diluted by side of lower Cook Inlet, passing on either
fresh water and carrying abundant suspended side of Augustine Island and out around Cape
sediments, are deflected to the west by the Douglas [Burbank, 1974].
same forces.
The resulting counterclockwise circulation Geology. Lower Cook Inlet is a struc
is complicated by irregularities in the inlet turally-controlled basin that has been re
configuration and by the fact that there is a ceiving both marine and nonmarine sedimentary
4.5-hour (hr) lag time between high tide at and volcanic deposits since the late Paleo
the inlet mouth and the inlet head. Flooding zoic. The major feature of the western
currents are partially deflected to the west region is the 320-km-Iong, northeast-trending
by protuberances along the coast, particular Bruin Bay fault system, which generally sepa
ly by the East Foreland. The current that rates volcanic rocks and intrusives of the
"
~~~
~ Kalgin
Island
10
km
FIGURE 4 Wave-generated, longshore sediment transport patterns in lower Cook Inlet. Based
on geomorphic evidence such as recurved spits, cuspate spits, and beach protuberances.
T101: 6