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

LITHOFACIES AND CONODONTS OF
CARBONIFEROUS STRATA IN THE IVOTUK HILLS,
WESTERN BROOKS RANGE, ALASKA
ABSTRACT
Carboniferous strata in the Ivotuk Hills, in the western
Brooks Range fold and thrust belt, consist of about 45 m
of dark-gray shale, mudstone, dolostone, and spiculitic
chert (upper part of the Kayak Shale, Endicott Group) and
at least 225 m of light- to dark-gray dolostone, chert, and
minor shale (Lisburne Group). The Kayak Shale was de-
posited chiefly below wave base; subordinate beds of dolo-
mitic bioclastic packstone probably formed as storm
deposits. The Kayak contains conodonts of Early Missis-
sippian (early Osagean) age. The Lisburne Group in the
Ivotuk Hills can be divided into three subunits on the basis
of bedding style and lithology. The lower and upper units
are thin-bedded to laminated, consist chiefly of fine-
grained dolostone and spiculite and subordinate dolo-
mitized bioclastic packstone. The middle unit is massive
and resistant and is made up mostly of crinoidal packstone
and lesser bryozoan wackestone. The lower and middle
units of the Lisburne Group yield conodonts of early Late
Mississippian (late Meramecian) age; a sample from the
upper unit produced latest Late Mississippian (late
Chesterian) conodonts.
The Kayak Shale in the Ivotuk Hills is in general
similar to the Kayak elsewhere in the Brooks Range, but
also has some lithologic and faunal similarities to the Kuna
Formation of the Lisburne Group. The Kayak was prob-
ably deposited in a middle to outer platform or shelf set-
ting in somewhat deeper water than that in which most of
the Kayak accumulated, but under more oxygenated condi-
tions than those typical of Kuna environments.
Sedimentologic data and conodont biofacies indicate
that the Lisburne Group in the Ivotuk Hills accumulated
primarily in normal marine, middle to outer platform or
shelf settings. The section correlates well in age and li-
thology with the Lisbume at Lisbume Ridge, about 40 km
to the west. Repeated thrust panels of Carboniferous strata
encountered in the Lisburne Test Well No. 1, drilled 1.5
km northeast of our study area in the Ivotuk Hills, corre-
late biostratigraphically and lithologically with rocks ex-
posed at the surface, but appear to have formed, at least in
By Julie A. Dumoulin and Anita G. Harris
part, in somewhat shallower water and more restricted
depositional environments.
INTRODUCTION
Carboniferous carbonate rocks (Lisburne Group and
carbonate layers in the underlying Kayak Shale) are widely
exposed across northern Alaska in the Brooks Range fold
and thrust belt. Lithofacies, biostratigraphy, and deposi-
tional environments of these units have been delineated at
a reconnaissance scale in the eastern Brooks Range
(Armstrong, 1974; Armstrong and Mamet, 1977), but are
less well known to the west. Both shallow and deeper
water Carboniferous facies occur in thrust sheets in the
western Brooks Range; the structural, stratigraphic, and
paleogeographic relationships of these facies remain con-
troversial (Mayfield and others, 1988; Dumoulin and oth-
ers, this volume).
This paper describes the lithofacies and conodont bio-
stratigraphy of Carboniferous strata in the Ivotuk Hills,
western Killik River B-5 quadrangle (fig. 1). Although
these strata constitute the type section of the Ivotuk Hills
structural sequence of Martin (1970) and Mayfield and
others (1988), no detailed sedimentologic or paleontologic
studies of these rocks have previously been published.
PREVIOUS WORK
Carboniferous and younger rocks of the Ivotuk Hills
area were first described by Martin (1970), who designated
these strata, along with similar sections at Lisburne Ridge
and Mount Bupto in the adjacent Howard Pass quadrangle
(fig. I), as the Ivotuk Hills structural sequence. The Car-
boniferous part of this sequence consists of about 300 m of
dolomite and chert (Lisburne Group) overlying an indeter-
minate thickness of dark-gray shale with subordinate iron-
stone nodules and bioclastic limestone lenses (Kayak
Shale) (Martin, 1970). In the tectonic reconstruction of
Mayfield and others (1988), the Ivotuk Hills structural
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