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

LITHOFACIES AND CONODONTS OF CARBONIFEROUS STRATA IN THE IVOTUK HILLS, BROOKS RANGE
grained carbonate rock taken 11 m below the stratigraphic
top of the Lisburne Group at this outcrop yields represen-
tatives of Gnathodus bilineatus (Roundy), G. defectus
Dunn, and G. girryi simplex Dunn (fig. 8A-C). The col-
lection restricts the age of the uppermost part of the
Lisburne in this area to the latest Late Mississippian (latest
Chesterian).
DEPOSITIONAL ENVIRONMENT
KAYAK SHALE
The Kayak Shale in the Ivotuk Hills accumulated in
relatively quiet water, below fair-weather wave base.
Preservation of laminae and rarity of bioturbation in these
strata suggest that conditions inimical to bottom fauna,
such as high turbidity and (or) low oxygen content, pre-
vailed during deposition. Rocks rich in sponge spicules
form both in shallow-water, restricted platform and deeper
water, outer shelf or basin environments (Murchey and
others, 1988; Dumoulin and Harris, 1992). Spiculites in
the Kayak probably formed in deeper water, because they
contain rare radiolarians and lack features characteristic of
shallow-water, restricted settings (for example, peloids, al-
gal stromatolites, and fenestral fabric).
Concretions and lenses of finely crystalline dolostone
in the Kayak Shale resemble dolomitic lenses in the Kuna
Formation, a deeper water facies of the Lisburne Group
widely distributed in the western Killik River and eastern
Howard Pass quadrangles (Mull and others, 1982). Dolo-
mite in the Kuna appears to have formed during early di-
agenesis as a result of marine microbial activity in
relatively organic-rich sediments (Dumoulin and others,
this volume); dolomite concretions in the Kayak were
probably similarly produced.
Bioclastic packstone beds in the Kayak Shale were not
generated in place, but were redeposited, probably by
storm waves and (or) turbidity currents. Bouma sequences
were not observed in these beds, but typical tempestite fea-
tures (Fliigel, 1982) such as graded bedding, shale rip-up
*
Figure 5. Sedimentary features, lower unit of Lisburne Group.
A, Thinly interbedded light-weathering dolostone and dark-
weathering spiculitic chert, 79 m above base of section. B,
Photomicrograph of spiculitic mudstone, 45.7 m above base of
section; light-colored layer is concentration of siliceous sponge
spicules. C, Photomicrograph of spiculitic chert with minor
rhombs of dolomite, 45.9 rn above base of section. D, Graded
bed of crinoidal packstone, 65 m above base of section. E,
Photomicrograph of packstone shown in 5D; small dolomite
crystals (d) grow inside calcite crinoid ossicles (c) in a matrix of
spiculitic rnudstone. F, Photomicrograph of bioclastic packstone,
54.5 rn above base of section; crinoidal debris here has been
partly pyritized and replaced by silica.
clasts, and a mix of fossils derived from various shallow-
water biofacies characterize these strata. Conodont ele-
ments in these layers are mostly incomplete, indicating
transport during high-energy conditions. Storm waves
may have produced atypically well-oxygenated bottom
conditions, because bioturbation in the Kayak appears con-
fined to tempestite layers. Packstone beds may have been
dolomitized by the same processes which produced the
finely crystalline dolomite concretions, but could also have
formed during later (reflux related?) dolomitization events.
Lithologic and faunal evidence thus suggest that the
Kayak Shale in the Ivotuk Hills formed in moderately deep
water on a middle to outer platform or shelf setting. The
depositional environment was characterized by persistent
influx of pelitic detritus and periodic accumulation of
tempestites rich in calcareous bioclasts. These bioclasts
were probably derived from the leading edge of a
prograding carbonate platform.
LISBURNE GROUP
The Lisburne Group in the Ivotuk Hills represents
progradation of a carbonate bank across a shelf or platform
setting. Fine-grained dolostone and spiculitic mudstone
beds in the lower unit of the Lisburne are similar to those
in the underlying Kayak Shale, but the lower unit contains
less pelitic detritus and more bioclastic carbonate material
than the Kayak. Both units were probably deposited in
similar water depths, but the lower unit accumulated closer
to the source of the carbonate debris. "Background" sedi-
ment in the lower unit consists of finely laminated
noncarbonate mudstone, chert, and diagenetic dolomite;
sponge spicules and rare radiolarians are the only bioclasts
in these beds. Subordinate layers of skeletal packstone,
like those in the underlying Kayak Shale, have features in-
dicative of tempestite origin, such as graded bedding, shale
rip-up clasts, and bioclasts derived from several biofacies.
Some conodonts (cavusgnathids) and skeletal grains (fora-
minifers) indicate input from shallow- to very shallow-wa-
ter depositional environments.
The massive middle unit appears to have formed in a
shallower water setting and (or) during maximum
progradation of the carbonate platform. Skeletal (chiefly
crinoidal) packstone and lesser bryozoan wackestone
dominate this unit and probably formed in sand shoals and
crinoid-bryozoan gardens of the middle to inner shelf.
Sedimentologic or paleontologic evidence of very shallow-
water, restricted environments, such as mud cracks,
peloids, fenestral fabric, stromatolites, calcispheres, or con-
odonts representative of the mestognathid biofacies, have
not been found in this unit. If any part of the middle unit
was deposited in shallow subtidal to supratidal environ-
ments, evidence of these settings has been obliterated by
later dolomitization and (or) silicification.
39