What happened between the Rabbit Creek and
Post-Rabbit-Creek outwash
Fort Richardson advances and moraine develop-
This is a well-stratified, moderate- to well-sorted
ment is also unknown. Those deposits are deeply
sand and gravel above the Rabbit Creek diamicton
buried in the Fort Richardson area. The nature and
or bedrock. The unit is assumed to be variable in
extent of the Fort Richardson moraines suggest
thickness, up to a few tens of meters thick, but may
that they probably record a period when the gla-
locally pinch-out along valley walls. The outwash
cier terminus was relatively stable. This condition,
is unconformably overlain by Fort Richardson
a stillstand, might have resulted from local chang-
diamicton, but pump tests by Cederstrom
es in glacier dynamics as the terminus retreated to
et al. (1964) demonstrated hydraulic linkage to
the mouth of Knik Arm (e.g., stillstands are com-
deposits higher in the sequence. The linkage is
mon at points of valley constriction [Warren and
probably related to high hydrostatic pressures at
Hulton 1990]). Therefore, it is probable that the
depth that drive flow through fractures, faults,
cantonment was not deglaciated between the old-
and local stratigraphic anisotropies in the overly-
er Rabbit Creek and Fort Richardson stages, and
ing diamicton.
thus subglacial and submarine diamictons were
more or less continuously deposited over this time
Fort Richardson and Dishno Pond sequences
interval.
Well log records described by Cederstrom et al.
The best record of glacial activity is associated
(1964) indicate two distinct glacial diamictons bur-
with the advance of ice that built the Elmendorf
ied in the area, while Trainer and Waller (1965) and
Moraine (Elmendorf advance). However, the
Schmoll and Barnwell (1984) tell of up to seven
record of this cycle is not complete below all of
glacial horizons. More than 20 diamicton and 10
Fort Richardson because the Elmendorf advance
silt strata were identified in the borehole in Tikishla
stopped north of the main cantonment area and,
Park, located about 11 km south of the Elmendorf
thus, a basal diamicton was only deposited upgla-
Moraine (Yehle et al. 1986). However, it is not clear
cier of the Elmendorf Moraine.
that these represent more than local variations in
During the Elmendorf advance, marine silt of
sedimentary processes and deposition, or move-
the Bootlegger Cove Formation was probably
ments in the ice margin.
eroded and recycled into a moraine shoal as the ice
For our generalized model (Fig. 10), we accept
advanced over it. The end of the advance led to the
the two-drift theory (Cederstrom et al. 1964)
Elmendorf Moraine being deposited through
because it agrees nicely (although probably coinci-
combined glacial (thrusting, pushing, meltout,
dentally) with the Fort Richardson and Dishno
lodgement), fluvial (outwash streams), and gravi-
Pond moraines recognized by surficial mapping
tational (debris flows) processes. This produced a
programs (e.g., Schmoll et al. 1996). We realize that
complex internal stratigraphy. Interbedded gla-
this is an oversimplification, but suggest that it is
cioestuarine silts (i.e., Bootlegger Cove Forma-
reasonable for building our conceptual strati-
tion), diamictons, gravels, and sands, without a
graphic model, given the current constraints in
regular lateral or vertical repetitive sequence, is
knowledge of the glacial history and stratigraphy.
the result. A local example is the exposed sea
Deposits of these advances are hydraulically
bluffs at Fire Island near the mouth of Knik Arm
connected, as demonstrated by Cederstrom et al.
(Fig. 3). Any of the fjord bottom sediments in shal-
(1964), and compose the bulk of the confined aqui-
low water near the ice margin were probably eroded
fer in the Anchorage Lowland. Despite rapid
by waves and coarsened as a result (similar to the
facies changes and broad, laterally continuous
Presumpscot Formation in coastal Maine [Bloom
diamicton sheets, deposits of these two glacial
1960]). Close to the Elmendorf Moraine, coarse
phases form an extensive aquifer at depth. The
materials probably compose a significant propor-
sedimentary sequences are detailed below.
tion of the Bootlegger Cove Formation, although
this has not been clearly identified.*
Fort Richardson sequence
The stratigraphy below the cantonment (Fig.
A lower diamicton unconformably overlies the
10) consists of several major sedimentary units
Rabbit Creek outwash. The basal contact is proba-
(listed from oldest to youngest) as follows.
bly erosional, although it should be locally inter-
bedded with gravel. This diamicton is probably
stratified, with occasional sand and gravel hori-
zons. Interbedded gravels and diamictons are
expected where ice-proximal debris flows, gener-
* Personal communication with H.R. Schmoll, USGS, 1996.
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