Geologic Studies in Alaska by the U.S. Geological Survey, 1992

56 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY. 1992
Additional sequence boundaries may be present within
the Chandler Formation at Slope Mountain, but given the
relatively poor exposure of the lithologies between sand-
stones and the fair exposure of the sandstones, they are not
outlined for this study. More definition of the environ-
ments represented by the Chandler Formation would help
to define these sequence boundaries, if present.
GILEAD CREEK SANDSTONE UNIT
The informally named Gilead Creek sandstone unit
(Reifenstuhl, 1989; Pessel and others, 1990) is exposed in
the foothills about 60 km northeast of Slope Mountain
(figs. 1, 3). This sandstone interval, 600-900 m thick, is
anomalous in that it lies in close proximity to the Kingak
Shale. Although exposures are poor, present mapping
does not allow for hundreds to thousands of meters of
mudstones of the Torok or Okpikruak Formations that
typically occur in this position. Because this is a structur-
ally complex area, it is possible that these mudstones,
once present, are now faulted out. Such faulting, how-
ever, would require either a normal fault (in a dominantly
compressional regime) or a thrust fault that places
younger over older strata. We have no evidence to sup-
port either of these possibilities. We suggest that the
Gilead Creek sandstone unit is the stratigraphic equivalent
of part of the Torok Formation, or that it is somewhat
younger and has scoured away an unknown amount of the
Torok. Such relations could be explained by a lowstand
fan deposit.
During relative sea-level low stands, the depositional
sequence model predicts the development of lowstand fans
in a downdip position at the same time that canyons and
valleys are cut in an updip position. We concur with the
interpretation of Molenaar and others (1984) of the Gilead
Creek sandstone.unit as composed mainly of turbidites. Such
an interpretation is consistent with a lowstand fan deposit.
The age of the Gilead Creek sandstone unit is con-
strained by a marine pelecypod identified by W.P. Elder
(personal communication, 1992) as Inocerarnus
dunveganensis of Cenomanian age from the upper part of
the Gilead Creek sandstone unit (R. Reifenstuhl, personal
communication, 1991) and marine pelecypods Buchia of
Valanginian age from the underlying Kingak Shale (Pessel
and others, 1990). With such age constraints, the Gilead
Creek sandstone unit could represent a Cenomanian
lowstand fan that scoured away Albian and Aptian mudstones,
or it more likely represents an amalgamation of lowstand fan
deposits spanning several depositional sequences.
DISCUSSION
Five depositional sequence boundaries have been out-
lined from exposures of the Fortress Mountain Formation,
Torok Formation, and the Chandler Formation of the
Nanushuk Group in the area from Atigun syncline and
Slope Mountain, and from examination of other sandstones
including the Gilead Creek sandstone unit. These are not
the only sequence boundaries possible in this stratigraphic
interval; only these five are postulated from limited field
observations. Other sequence boundaries may be present
within the Fortress Mountain Formation and the Nanushuk
Group, and their interpretation will have to be based on
detailed field examination of these exposures.
One of the questions that arose during our field work
was the stratigraphic relationship between the Fortress
Mountain Formation and the Nanushuk Group in the
Atigun syncline-Slope Mountain area. In this area there
is a strong similarity of lithologic composition between
these stratigraphic units, and one could argue that the For-
tress Mountain at Atigun Gorge is simply the more proxi-
mal facies of the Nanushuk Group at Slope Mountain.
Although the Fortress Mountain Formation-Torok Forma-
tion-Nanushuk Group transition is illustrated across the
North Slope in cross sections (Molenaar, 1988), actual
field evidence showing this succession is limited, as far as
we could ascertain, to the area of Igloo Mountain near the
Kokolik River, which was described by Chapman and
Sable (1960). In the Igloo Mountain area (fig. lA), the
Fortress Mountain Formation appears to be in depositional
contact with the Torok Formation, which is overlain by
the Kukpowruk Formation (part of the Nanushuk Group).
As the Kokolik River area is approximately 550 km west
of the Atigun syncline area, the stratigraphic relationships
between these three stratigraphic units are not clear. We
speculate that the two sequences outlined in the Fortress
Mountain Formation at Atigun syncline could be equiva-
lent to two sequences of the Nanushuk Group at Slope
Mountain, given the lack of good stratigraphic and
biostratigraphic control. Although many seismic lines
are available in the NPRA (Molenaar, 1985, 1988),
none are properly positioned to resolve these strati-
graphic relationships.
We attempted to point out the possible sequence
boundaries that were present in this part of the strati-
graphic section based on limited field work. We did not
attempt to relate these boundaries to a causal mechanism,
such as tectonics or eustasy. The Albian to Cenomanian
section in the Western Interior Seaway of the United States
contains many sequence boundaries ascribed by some to
eustatic mechanisms (Haq and others, 1988), or to purely
tectonic effects in the active Colville basin (Noonan, 1987)
or in similar-aged rocks in Arctic Canada (Embry, 1989).
Others have developed hybrid tectonic-eustatic models for
the formation of depositional sequences in the Brookian
sequence of Alaska (Rudolph and others, 1990). Whatever
the cause, relative base-level changes had a significant ef-
fect on the depositional sequences as they developed in the
Lower Cretaceous section of the Colville basin.