Estuarine and glacioestuarine sediments

Figure 6. Interbedded alluvial sediments as might be produced by periodic discharge events (sand and gravel) and silt deposition in abandoned channels.

GLACIAL GEOLOGICAL HISTORY

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Transitional Fan Complex

Figure 7. Incised fans in flank of Elmendorf Moraine

Dishno Pond Moraine (?)

and interfingering of sedimentary units along the

moraine margin.

fans along the southern margin of the Elmendorf

eral meters to a few tens of meters thick and con-

Moraine (Fig. 4; Plate 1), with their apices in small

solidated (e.g., Combellick 1990, 1991, 1994).

channels cut into the moraine (Fig. 7). The well-

Glacioestuarine deposits of late Pleistocene age

preserved fan morphology tells us that they were

accumulated in an ancestral Cook Inlet that was

active during the late stages of moraine formation

larger and deeper than at present, although no

(Plate 1). Because the toes of these fans are not

true shorelines have yet been identified.* The

truncated, they were probably created during late

upper limit of marine submergence during this

phases of Mountain View fan formation and may

time may have been as great as 183 to 213 m above

have developed after the catastrophic discharges

modern levels (Yehle and Schmoll 1987a, 1988),

from the Eagle River Valley. Elmendorf ice must

although Karlstrom (1964) suggests water may

have remained close enough to its maximum

have been as high as 305 m above present sea level.

position at the terminal moraine to provide sedi-

Deposits that consist of varying combinations of

ment and water to the aggrading fans and to keep

interbedded diamicton, stony silt, fine sand, silt

meltwater from being diverted into the ancestral

and silty clay, and coarse sandgravel often indi-

drainages of Six Mile Lake or Eagle River to its

cate deposition at elevations above present sea level,

north.

probably with ice terminating in water nearby.

A major stratigraphic unit considered glacio-

Estuarine and glacioestuarine sediments

estuarine in origin is the Bootlegger Cove Forma-

Estuarine deposits are formed in present-day

tion (Bootlegger Cove Clay of Miller and Dobro-

Cook Inlet and its major arms, Knik and Turna-

volny [1959], redesignated as Formation by

gain, or along similar water bodies of the recent

Updike et al. [1982]). It was apparently deposited

past (Fig. 2 and 4). Glacioestuarine deposits accu-

during a much higher sea level, when ice that cre-

mulated in an ancestral Cook Inlet that probably

ated the Elmendorf moraine advanced into the

differed from the present-day inlet in configura-

area (Reger et al. 1995). The unit is composed of

tion because of base level changes and the pres-

silty clay and clayey silt, sometimes interbedded

ence of glaciers.

with silt, fine to medium sand, and thin diamicton

Modern estuarine deposits are peripheral to

beds (Updike et al. 1988). Brackish-marine micro-

Cook Inlet, where macrotidal fluctuations of 79

fossils are present throughout much of the forma-

tion (Schmidt 1963, Smith 1964), which reaches a

m intermittently expose recent silty deposits.

thickness of 35 m (Updike et al. 1988). This unit

Older Holocene estuarine deposits occur exten-

occurs widely throughout the subsurface at eleva-

sively at the upper end of Knik Arm and in Eagle

tions typically below about 30 m asl, and underlies

River Flats (ERF). Estuarine deposits are generally

surficial deposits exposed in the Elmendorf Mor-

composed of well-bedded and sorted silt and fine

sand, and may locally include thin beds of peat,

driftwood, and other organic or windblown

*Personal communication with H.R. Schmoll and L.A. Yehle,

material. Deposits of this unit are commonly sev-

USGS, 1996.

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