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

LITHOFACIES AND CONODONTS OF CARBONIFEROUS STRATA IN THE IVOTUK HILLS, BROOKS RANGE 37
half of the unit, but 30 to 50 percent of the upper strata.
Shale makes up 10 percent of the unit and occurs as part-
ings between beds and rare layers as much as 10 cm thick.
Several beds of silicified bioclastic packstone, 5 to 8
cm thick, punctuate the section; these beds contain abun-
dant disarticulated brachiopod valves and lesser crinoid
ossicles, spicules, and phosphatic clasts. Some shells are
rimmed with borings filled with brown or black mud (fig.
70).
Tundra covers the highest beds of the Lisburne Group
along Otuk Creek. The contact between the Lisburne and
the overlying Siksikpuk Formation is not exposed along
the line of our measured section, but is preserved in an
isolated outcrop 100 m to the southeast (fig. 1, sample lo-
cality 85ASi14-18; see also "Age and Biofacies" below).
The stratigraphic relationship between this outcrop and our
measured section is uncertain, but it most likely represents
strata at least in part equivalent to the uppermost beds of
the measured section. The total thickness of the Lisburne
Group in the Ivotuk Hills is probably no more than 250 m.
AGE AND BIOFACIES
KAYAK SHALE
A sample from the 2-m-thick dolomitic packstone bed
at the base of the section in the Kayak Shale yielded con-
odonts of late Early Mississippian (early Osagean) age
(fig. 80, P-T). The fauna represents a polygnathid-
bispathodid biofacies, which generally indicates a rela-
tively shallow-water, normal-marine platform or shelf
depositional environment. The two predominant species,
however, are Polygnathus communis and Bispathodus sp.
indet. (a single-row morphotype that is either B. stabilis
(Branson and Mehl) or B. utahensis Sandberg and
Gutschick). These are pelagic forms (Sandberg and
Gutschick, 1984) that are widespread in most normal-ma-
rine Early Mississippian environments, but are most abun-
dant in the more basinal and (or) anoxic to dysaerobic
environments (Sandberg and Gutschick, 1984; Dumoulin
and others, this volume).
Figure 4. Sedimentary features of Kayak Shale. A, Dolostone
lens in black shale, 6.1 m above base of section. B, Photo-
micrograph of dolomitic rnudstone lens, 8.4 m above base of
section. C, Photomicrograph of layer rich in calcareous sponge
spicules, 22 m above base of section. D, Dolomitic bioclastic
packstone layer containing abundant crinoid columnals (c) and
solitary corals (r), 21 m above base of section. E, Photomicro-
graph of bioclastic packstone lens shown in 40; note clast of
spiculitic mudstone (s), and crinoid ossicles (c) replaced by
silica. F, Photomicrograph of bioclastic packstone lens 0.5 m
above base of section; note phosphatic fish debris (p), and
crinoid ossicles (c) replaced by dolomite.
The collection from the Kayak Shale also contains a
few representatives of species characteristic of shallow-
water (but not the shallowest water) biofacies such as
Bispathodus aculeatus anteposicornis (fig. 8T) and
Synclydognathus geminus (Hinde) (fig. 8D), as well as rep-
resentatives of shallow- to slightly deeper water platform
or shelf biofacies such as Hindeodus crassidentatus (fig.
8S), Pseudopolygnathus sp. indet., and Kladognathus sp.
indet. Of note is the absence of deeper water (outer shelf
or slope) early Osagean genera such as gnathodids and
scaliognathids. Taken together, the species association and
preservation characteristics suggest postmortem transport
within or from a moderate to shallow depth platform or
shelf environment.
LISBURNE GROUP
Two samples from the lower unit of the Lisburne
Group in the Ivotuk Hills section produced conodonts of
early Late Mississippian (late Merarnecian) age, as did a
sample from the massive middle unit. Samples taken 54
and 66 m above the base of the section are from dolostone
with relict crinoidal packlgrainstone texture. Conodonts
are relatively sparse, virtually all are incomplete (the ratio
of generically determinate to indeterminate elements
ranges from 1:3 to 1:2) and bispathodids (fig. 814) and
kladognathids (fig. 8M) predominate. The collections con-
tain a few diagnostically shallow-water forms such as
Synclydognathus geminus (fig. 8E, F) and cavusgnathids
(fig. 8N, 0). Overall species association and preservation
characteristics suggest postmortem transport within or from
a relatively shallow-water platform or shelf environment.
The sample taken from the middle massive unit at
115.4 m above the base of the section is from a dolomitic
bryozoan-crinoidal wackestone. Conodonts are few, some-
what more complete than in samples from the lower unit,
and consist chiefly of representatives of the pelagic species
Bispathodus utahensis, with lesser numbers of Kladognathus
sp. indet. and a few fragments of cavusgathids (fig. 8G, H,
and L). The species association and preservation charac-
teristics suggest a normal-marine depositional environment
with somewhat less postmortem transport than in the two
samples from the lower unit.
Thin-bedded dolostone in the upper unit of the
Lisburne Group, 190 m above the base of the section, was
barren of conodonts. Another sample from the upper unit
taken about 267 m above the base of the section produced
only a few indeterminate conodont fragments. However, a
sample collected by J.P. Siok (Terrasat, Anchorage,
Alaska) from an isolated outcrop about 100 m southeast of
our measured section provides a date for the uppermost
part of the Lisburne Group in the Ivotuk Hills. This out-
crop exposes a few tens of meters of the uppermost strata
of the Lisburne Group, as well as strata of the overlying
Siksikpuk Formation. A sample of thin-bedded, fine-