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

DEEP-WATER LITHOFACIES AND CONODONT FAUNAS OF THE LISBURNE GROUP, BROOKS RANGE 13
13) established by Mull and others (1982). In the study
area, the Kuna is at most 70 m thick; it depositionally
overlies the Kayak Shale and generally underlies fine-
grained sedimentary rocks of the Etivluk Group. Locally,
particularly in the eastern part of the study area, the upper
contact of the Kuna is a fault.
As originally defined by Mull and others (1982), the
Kuna Formation consists of black carbonaceous shale,
black chert, fine-grained limestone, and dolostone. In the
study area, some Kuna sections consist chiefly of black,
carbonaceous, noncalcareous shale; such sections are gen-
erally poorly exposed and include only minor amounts of
carbonate rocks and (or) chert (fig. 3A). These shale-rich
outcrops are commonest in the western half of the Howard
Pass quadrangle (fig. 2).
Most sections of the Kuna Formation examined in this
study, however, consist primarily of siliceous mudstone
with subordinate (5-20 percent) shaly beds or partings
(fig. 3B). Mudstone beds are even to irregular and 2-20
cm thick; shale intervals are generally a few centimeters or
less, but may be as much as 10 cm thick. The thicker
beds, called chert by some previous workers (for example,
Mull and others, 1982), are siliceous mudstone, not true
chert; they have an earthy rather than vitreous luster and
do not fracture conchoidally. In thin section, they contain
abundant biosiliceous material, chiefly sponge spicules but
also subordinate radiolarians, in a matrix of organic-rich
mud (fig. 30. Some mudstone layers have been locally
STUDY
gOw AREA
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silicified; silica for this process may have come, at least in
part, from dissolution and remobilization of the bio-
siliceous component of these rocks.
Fine lamination is the only sedimentary structure vis-
ible in most outcrops of the Kuna Formation (fig. 30).
Laminae consist of local concentrations of sand- to silt-
sized clasts, mostly of mudstone, or alternating concentra-
tions of sponge spicules and mud (fig. 3E). Most
laminated intervals probably represent distal turbidites, or
lags left by bottom currents. But cyclic changes in pro-
ductivity and (or) detrital influx into the Kuna basin(s)
could also have played a role in forming some laminae.
Preservation of these laminae, as well as the appar-
ently high organic content of much of the Kuna Formation,
suggest that anoxic or dysaerobic bottom-water conditions
prevailed during its deposition. Some intervals within the
Kuna are burrowed, however, so oxygen levels at the sedi-
ment-water interface were locally or periodically high
enough to support a bottom fauna. Bioturbated intervals
(fig. 3F) are most common in the easternmost exposures of
the Kuna in the study area. Burrows are round to ovoid in
cross section, about 0.5-1 cm in diameter, and generally
richer in siliceous bioclasts than the surrounding mudstone.
Carbonate forms 30 percent or less of most outcrops
of the Kuna Formation that we studied. Carbonate occurs
as concretions, 4-85 cm in diameter (fig. 4A), or more
continuous layers a few millimeters to 30 cm thick (fig.
4B). Primary calcareous material, such as obvious
f ALASKA 1
0 100 200 KILOMETERS
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Figure 1. Distribution of Lisburne Group (pattern) in northern Alaska (generalized from Armstrong and Mamet, 1978) and
location of quadrangles mentioned in text.