Download Thrust-top basin formation along a suture

Thrust-top basin formation along a suture zone, Cantwell basin, Alaska Range:
Implications for development of the Denali fault system
Kenneth D. Ridgway*
Jeffrey M. Trop
Arthur R. Sweet
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Department of Earth and Atmospheric Sciences,
Purdue University, West Lafayette, Indiana 47907-1397
Geological Survey of Canada, Calgary, Alberta T2L 2A7, Canada
ABSTRACT
The Cantwell Formation consists of a lower
sedimentary sequence as much as 4000 m
thick and an upper volcanic sequence with a
maximum thickness of 3750 m that was deposited in the Cantwell basin, south-central
Alaska. Previous to this study, the Cantwell
basin was interpreted as a Paleogene, nonmarine (mainly fluvial), pull-apart basin that
formed in response to dextral, strike-slip displacement on the Denali fault system. This
study proposes that the Cantwell basin formed
as part of the Mesozoic accretionary phase of
deformation, prior to the development of the
Cenozoic postaccretionary Denali fault system.
Our reinterpretation is based on several new
lines of data.
(1) Age. New data based on palynologic
analyses of 135 fine-grained samples indicate
that the lower Cantwell Formation was deposited during the late Campanian and early
Maastrichtian. On the basis of previous regional tectonic studies and this new age constraint, the formation of the Cantwell basin
was coeval with regional Late Cretaceous
shortening associated with accretionary tectonics in southern Alaska.
(2) Depositional systems. Our analysis of the
Cantwell Formation demonstrates that sedimentation occurred mainly in stream-dominated alluvial fan, axial braided stream, and
lacustrine settings. These depositional systems
were strongly influenced by a southward dipping, asymmetric basin floor. The presence of
abundant terrestrially derived organic material, together with palynological assemblages
that include marine dinoflagellates and the associated presence of oncolites, may be suggestive of a time of marginal marine influence
during the deposition of the upper part of the
lower Cantwell Formation. The late Campan*E-mail: [email protected]
ian to early Maastrichtian timing of this possible marine influence is within the range of the
Bearpaw transgressive event of the Cordilleran foreland basin and allows for regional
stratigraphic correlation of the Cantwell basin
with other sedimentary basins in northwestern
North America.
(3) Structural controls on basin formation.
Mapping of intraformational angular unconformities and progressively tilted strata along
the southern margin of the Cantwell basin
provides direct evidence that thrust fault deformation and lower Cantwell Formation sedimentation were synchronous. Distinctive
Cantwell Formation conglomerate clasts derived from the uplifted hanging walls of
nearby thrust sheets adjacent to the southern
basin margin also support a syndepositional
thrusting interpretation. Provenance data and
the concentration of proximal alluvial fan deposits along the northwestern basin margin
adjacent to the Hines Creek fault indicate that
it, too, was active during deposition of the
Cantwell Formation.
On the basis of the new data, the Cantwell
basin is interpreted to have formed as a
thrust-top basin (i.e., piggyback basin) along
the Late Cretaceous suture zone between the
accreting Wrangellia composite terrane and
the North American continental margin. In
contrast to previous studies, this reinterpretation of the formation of the Cantwell basin implies that the lower Cantwell Formation is not
a synorogenic deposit directly associated with
strike-slip displacement along the Denali fault
system. Therefore, the Cantwell basin cannot
be used to constrain the timing for the early
development of the Denali fault system.
INTRODUCTION
The Cantwell Formation is presently exposed
in a 45-km-wide and 135-km-long, east-west–
trending outcrop belt referred to here as the
GSA Bulletin; May 1997; v. 109; no. 5; p. 505–523; 16 figures.
505
Cantwell basin (Fig. 1A). The basin is located approximately between the Hines Creek fault to the
north and the McKinley fault to the south within
the central Alaska Range (Fig. 1B). The McKinley fault is part of the 2000-km-long Denali fault
system that extends from British Columbia to
western Alaska (Fig. 1A). The Cantwell Formation consists of two distinct lithologic units, an upper volcanic unit and a lower sedimentary unit
(Wolfe and Wahrhaftig, 1970). The sedimentary
unit of the Cantwell Formation, referred to here as
the lower Cantwell Formation, consists of a thick
sequence of conglomerate, sandstone, siltstone,
mudstone, coal, and minor volcanic rocks (Wolfe
and Wahrhaftig, 1970; Hickman, 1974; Sherwood, 1979). Although thickness varies throughout the basin, the maximum preserved thickness
of the lower Cantwell Formation is 4000 m (Hickman et al., 1990). The upper volcanic unit, informally referred to as the Teklanika formation, consists of intercalated andesite, rhyolite, and basalt
flows, subordinate pyroclastic and intrusive rocks,
and minor sedimentary rocks (Gilbert et al.,
1976). The maximum preserved thickness of the
volcanic sequence is 3750 m (Gilbert et al., 1976).
Although usually conformable, the contact between the upper volcanic unit and the underlying
sedimentary unit is an angular unconformity at
some locations (Gilbert et al., 1976; Csejtey et al.,
1986), especially along the southern basin margin
(Ridgway et al., 1995b). The Cantwell Formation
unconformably overlies a complex assemblage of
Precambrian(?) to Cretaceous rocks that may represent several tectonostratigraphic terranes (Csejtey et al., 1986; Jones et al., 1983, 1987).
Field mapping (Eldridge, 1900; Brooks and
Prindle, 1911; Capps, 1940; Reed, 1961; Wahrhaftig, 1970a, 1970b, 1970c, 1970d; Decker,
1975; Gilbert and Redman, 1975; Hickman and
Craddock, 1976; Sherwood and Craddock, 1979;
Csejtey et al., 1986, 1992), stratigraphic and paleobotanical analyses (Eldridge, 1900; Wolfe and
Wahrhaftig, 1970; McLean and Stanley, 1992),
and structural studies (Hickman, 1974; Sher-
RIDGWAY ET AL.
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I
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lat River
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Denali
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Upper Cantwell Formation
(volcanic rocks)
Index Map
ALASKA
Lower Cantwell Formation
(sedimentary rocks)
Cantwell Basin
lt
ali
Fau
Sys
tem
Den
Gulf of Alaska
0
All blank areas represent non-Cantwell Fm. rocks
Abbreviations represent measured section locations
within the lower Cantwell Formation
500 KM
10
20 km
N
Major fault – oblique motion, dotted where inferred
Minor fault – normal dip-slip motion
Thrust fault – reverse dip-slip motion
Depositional contact
River
section with upper contact exposed
section with basal contact exposed
section with basal and upper contact
Figure 1. (A) Map of Alaska showing the Denali fault system (bold black line) and the location of the Cantwell basin (black area). (B) Geologic map of the Cantwell basin in south-central Alaska. The gray areas represent exposures of the lower Cantwell Formation, a predominantly sedimentary unit. The hashed areas represent exposures of the upper Cantwell Formation, a predominantly volcanic unit. Abbreviations mark locations of measured stratigraphic sections in the lower Cantwell Formation discussed in text. Geology from Reed (1961), Csejtey
et al. (1992), Hickman and Craddock (1976), Sherwood and Craddock (1979), and this study.
wood, 1973, 1979; Gilbert, 1975, 1976; Hickman
et al., 1990) have provided a general framework
for the Cantwell Formation. Hickman et al.
(1990) interpreted the Cantwell basin as having
formed during the Paleogene as a nonmarine,
strike-slip pull-apart basin. Structural analyses interpret the basin as forming due to right-lateral
strike-slip motion on the McKinley fault (Sherwood et al., 1979; Hickman et al., 1990). On the
basis of fossil plant leaves, the age of the lower
Cantwell Formation has been interpreted as
Paleocene (Wolfe and Wahrhaftig, 1970). Previous studies based on the presence of terrestrial
plant fossils, coal beds, and the apparent lack of
marine fossils have interpreted the lower Cantwell Formation as an entirely nonmarine unit deposited in mainly fluvial depositional environments (Hickman, 1974; Decker, 1975; Sherwood,
1979; Hickman et al., 1990).
506
Several new lines of data from this study indicate that the Cantwell basin formed in response to Late Cretaceous thrusting associated
with suturing of the Wrangellia composite terrane to southern Alaska (Plafker et al., 1989)
and is not directly related to Cenozoic strikeslip displacement on the Denali fault system.
Palynological analyses indicate that the lower
Cantwell Formation was deposited during the
late Campanian and early Maastrichtian of the
Late Cretaceous. Reconstruction of depositional
systems demonstrates that sedimentation was
predominantly by stream-dominated (“wet”) alluvial fan, axial braided stream, and lacustrine
environments within an asymmetric basin. The
presence of marine dinoflagellates in association with oncolitic limestones is taken to suggest a marine incursion within the upper part of
the lower Cantwell Formation. Mapping of in-
Geological Society of America Bulletin, May 1997
traformational unconformities and progressively tilted strata along with compositional
data indicate that sedimentation was synchronous with thrust fault deformation in the Cantwell basin. In light of the new data, the Cantwell
basin is interpreted as having formed on a series
of active thrust faults as a thrust-top basin (i.e.,
piggyback basin). We will discuss the evidence
for each of these new interpretations in the following sections.
AGE
The age of the lower Cantwell Formation,
based on interpretations of fossil plant leaf biostratigraphy, has been controversial and unclear.
The formation was originally interpreted as Eocene (Moffitt, 1915), but later assigned to the
Cretaceous (Chaney, 1937). Imlay and Reeside
THRUST-TOP BASIN FORMATION ALONG A SUTURE ZONE
(1954) placed the Cantwell Formation in the Albian of the Early Cretaceous. Most recently, the
lower Cantwell Formation was interpreted as Paleocene, based on reexamination of Chaney’s original fossil collection and analysis of additional
plant fossil localities (Wolfe and Wahrhaftig,
1970; J. A. Wolfe in Hickman, 1974).
Palynologic analysis of 135 samples indicates
that the lower Cantwell Formation was deposited
during the late Campanian and early Maastrichtian
(latest Cretaceous) (Figs. 2 and 3). The recovered
palynomorphs include Aquilapollenites aptus Srivastava, A. ceriocorpus Srivastava, A. contiguus
Tschudy, A. drumhellerensis Srivastava, A. funkhouseri Srivastava, (not figured), A. paplionis Srivastava, A. parallelus Tschudy, A. quadrilobus
Rouse, A. reticulatus (Mchedlishvili) Tschudy and
Leopold, A. scabridus Tschudy, A. senonicus
Tschudy (not figured), A. trialatus Rouse, A. turbidus Tschudy and Leopold (not figured), A. vinosus Srivastava (not figured), Azonia sufflata Wiggins, Callistopollenites sp. (not figured, two poor
specimens), Cranwellia rumseyensis Srivastava,
Expressipollis sp., Fibulapollis mirificus (Chlonova) Chlonova, Kurtzipites andersonii Srivastava,
?Pulcheripollenites sp., Scabratriporites legibilis
Samoilovich, Senipites drumhellerensis Srivastava, Siberiapollis sp., and Trifossapollenites sp.
(not figured).
Nichols and Sweet (1993) proposed a series of
palynologically based datums applicable to the
zonation of Late Cretaceous strata throughout
midcontinental North America. The SantonianCampanian boundary is approximated by the
Aquilapollenites datum marked by the first conspicuous presence of A. spp. Given the number of
species of Aquilapollenites in the lower Cantwell
Formation, its age is certainly younger than Santonian. The Campanian-Maastrichtian boundary
is approximated by the Wodehouseia-Kurtzipites
datum marked by the first occurrences of either or
both of the name-bearing genera. Of these two
genera, only Kurtzipites occurs in the upper part
of the lower Cantwell Formation (measured section PC1 in Fig. 1B). The absence of these genera
from all but the PC1 section constrains the age of
much of the lower Cantwell Formation to the
Campanian. As Kurtzipites occurs, albeit rarely, in
section PC1, it is considered to be of Maastrichtian age. The beginning of the late Maastrichtian is
identified by the Wodehousiea spinata–Mancicorpus vancampoae datum (Nichols and Sweet,
1993). No species characteristic of this datum, or
of the late Maastrichtian, was recorded from the
lower Cantwell Formation. This, and the fact that
most species of Aquilapollenites do not range into
the Paleocene (above the K-T boundary extinction event), precludes the sampled portion of the
lower Cantwell Formation from being younger
than early Maastrichtian in age. Analyzed sam-
ples from three of our measured sections (PC1,
DM1, and DC1 on Fig. 1B) were collected within
several meters of the base of the upper Cantwell
Formation volcanic rocks, thereby ensuring that
we have a complete record of the uppermost
lower Cantwell Formation. Further constraints on
the age of the lower Cantwell Formation are
gained by the presence of Cranwellia, whose earliest record is taken to identify the beginning of
the late Campanian Cranwellia Suite of Norris et
al. (1975). Additionally, the presence of Azonia
sufflata (occurs in DM1, JC1, and possibly FM1
sections on Fig. 1B), a species found otherwise in
the Campanian of Alaska (Wiggins, 1976), implies that at least some of the lower Cantwell Formation is no younger than Campanian. The age of
the lower Cantwell Formation is, therefore, determined to span the late Campanian and early
Maastrichtian based on the recovered species. The
Paleocene age previously interpreted from macrofossils (Wolfe and Wahrhaftig, 1970) is based on a
relatively restricted plant assemblage. The age implications of this plant assemblage need to be reexamined in light of our results.
The new palynological data are in general
agreement with previously conflicting Late Cretaceous K-Ar age determinations (71.9 ± 2.7 Ma,
biotite; 78.7 ± 3.6 Ma, biotite) obtained from two
granitic stocks that intrude the lower Cantwell Formation. These were discounted in favor of the
Paleocene fossil ages (Fig. 4) (Hickman, 1974;
Hickman and Craddock, 1976; Sherwood and
Craddock, 1979) and with isotopic age determinations (50.9 ± 2.2 Ma, whole rock; 56.6 ± 2.4 Ma,
whole rock; 58.7 ± 3.5 Ma, plagioclase; 60.4 ±
3.1 Ma, whole rock; 61.0 ± 2.8 Ma, whole rock;
and 64.6 ± 3.4 Ma, hornblende) from the overlying
volcanic rocks of the upper Cantwell Formation
(Fig. 4) (Bultman, 1972; Sherwood, 1973; Hickman, 1974; Gilbert et al., 1976). All the cited K-Ar
age determinations were recalculated using the
constants of Steiger and Jäger (1977) (from Csejtey et al., 1992).
DEPOSITIONAL SYSTEMS
The Cantwell basin was filled by three principal types of depositional systems: (1) southwardflowing, stream-dominated alluvial-fan systems;
(2) eastward-flowing, axial sandy braided stream
systems; and (3) lacustrine systems concentrated
in the south-central part of the basin (Fig. 5).
Measured stratigraphic sections, lithofacies analysis, maximum particle-size data, and paleocurrent analysis indicate that each of the depositional systems was influenced by a southwardtilted basin floor (Fig. 5). We have also documented a marginal marine influence in the upper
part of the lower Cantwell Formation. Facies associations for each of the depositional systems
Geological Society of America Bulletin, May 1997
documented in the Cantwell basin are described
below.
Stream-Dominated Alluvial Fan Systems
Three common facies associations have been
documented for alluvial-fan deposits in the Cantwell basin. A description of each facies association will be presented and then a general interpretation for the three facies associations will be
discussed.
Facies Association 1. The Cantwell Formation
along the northwestern margin of the Cantwell
basin is characterized by clast-supported, boulder
and cobble conglomerate (Fig. 6A). Subrounded
to rounded clasts, polymodal grain-size distribution (very coarse sand to boulder) and a(t)b(i) imbrication (Harms et al., 1982) are common features in facies association 1. Average maximum
particle size in facies association 1 is 11 cm
(Fig. 7A), but clasts as much as 34 cm in diameter
are present. The measured section at Mount Sheldon (MS1) (Figs. 5 and 8) is representative of facies association 1. This section is characterized by
amalgamated, 10–30-m-thick conglomeratic units
that form 50–60-m-thick packages that are laterally continuous for hundreds of meters. Internally,
lenticular channels can be defined within the conglomerate packages. Separating the thick conglomerate packages are laterally continuous,
5–10-m-thick units of mudstone and coal (Fig. 8).
Mudstone lithofacies are thinly bedded, contain
abundant plant fragments (with in situ tree trunks
as much as 2.4 m in height and exquisitely preserved tree leaves), and locally contain burrows.
The best developed coal seams in the Cantwell
basin occur in facies association 1. The thickest
recorded coal seam is 1.75 m.
Facies association 1 is best developed proximal to the Hines Creek fault, which forms the
northern margin of the Cantwell basin (Figs. 1B
and 5). Paleocurrent indictors along the northern basin margin document south-to-southeast
paleoflow (Fig. 7B). This facies association also
occurs locally along the southern basin margin
adjacent to bordering thrust faults (Fig. 5A).
Facies Association 2. Facies association 2 is
characterized by upward fining sequences that
have a lower, 4–6-m-thick, pebble-to-cobble conglomerate unit which grades upward into massive sandstone that is, in turn, overlain by mudstone (Fig. 6B). The upward fining sequences
have sharp erosional lower contacts and are contained within lenticular units that are 200–500 m
wide. Conglomerate lithofacies are clast supported, well sorted, contain rounded clasts, and
have well-developed a(t)b(i) imbrication. Facies
association 2 is well exposed at the Polychrome
Mountain measured section (PC1) (Figs. 5
and 8). At the PC1 measured section, the average
507
THRUST-TOP BASIN FORMATION ALONG A SUTURE ZONE
Figure 2. Photomicrographs of selected spore and pollen taxa that are representative of the assemblages recovered from the lower Cantwell
Formation, Cantwell basin. All figures ×850. Additional detailed information for each specimen shown can be found in the permanent records
at the Geological Survey of Canada under the listed GSC type number. (A) Distaltriangulisporites sp., GSC 112740; (B) Hazaria sp., GSC 112741;
(C) Equisetosporites sp., GSC 112742; (D) Ginkgo-type pollen, GSC 112743; (E) Pesavis parva Kalgutkar and Sweet, GSC 112744; (F) Leptolepidites sp., GSC 112745; (G) Cyathidites minor Couper, GSC 112746; (H) Aquilapollenites reticulatus (Mchedlishvili) Tschudy and Leopold, GSC
112747; (I) A. parallelus Tschudy, GSC 112748; (J, K) A. sp. cf. A. ceriocorpus Srivastava; (J) GSC 112749; (K) GSC 112750; (L) A. trialatus
Rouse, GSC 112751; (M) A. drumhellerensis Srivastava, GSC 112752; (N) A. sp., GSC 112753; (O, P) A. quadrilobus Rouse; (O) GSC 112754;
(P) GSC 112755; (Q) A. paplionis Srivastava, GSC 112756; (R) A. aptus Srivastava, GSC 112757; (S) A. contiguus Tschudy, GSC 112758.
508
Geological Society of America Bulletin, May 1997
RIDGWAY ET AL.
Figure 3. Photomicrographs of selected angiosperm pollen and dinoflagellate taxa that are representative of the assemblages recovered from
the lower Cantwell Formation, Cantwell basin. All figures ×800. Additional detailed information for each specimen can be found in the permanent records at the Geological Survey of Canada under the listed GSC type number. (A) Aquilapollenites scabridus Tschudy, GSC 112759; (B) A.
turbidus Tschudy and Leopold, GSC 112760; (C, D) Fibulapollis mirificus (Chlonova) Chlonova; (C) GSC 112761; (D) GSC 112762; (E) Kurtzipites andersonii Srivastava, GSC 112763; (F) Platanus-type pollen, GSC 112764; (G) Cranwellia rumseyensis Srivastava, GSC 112765; (H) ?Pulcheripollenites sp. (although many specimens were observed, the form of the apertures is indeterminate; possibly an Anacolosidites sp.), GSC
112766; (I) Siberiapollis sp., GSC 112767; (J, Q) Tricolpites sp., (J) GSC 112768; (Q) GSC 112775; (K) Senipites drumhellerensis Srivastava, GSC
112769; (L) Cranwellia? sp., GSC 112770; (M, N) Azonia sufflata Wiggins; (M) GSC 112771; (N) GSC 112772; (O) Expressipollis sp., GSC 112773;
(P) Pleurospermaepollenites sp., GSC 112774; (R) Scabratriporites legibilis Samoilovich, GSC 112776; (S) Liliacidites sp., GSC 112777; (T–Z) Dinoflagellates; (T) ?Alterbidinium sp., GSC 112778; (U) Ovoidinium scabrosum (Cookson and Hughes) Davey, GSC 112779; (V) ?Gardodinium trabeculosum (Gocht) Alberti, GSC 112780; (W) cf. Horologinella sp., GSC 112781; (X) Kallosphaeridium minus (Cookson and Hughes) Helby, GSC
112782; (Y) Laciniadinium arcticum (Manum and Cookson) Lentin and Williams, GSC 112783; (Z) Spinidinium sp., GSC 112784.
Geological Society of America Bulletin, May 1997
509
RIDGWAY ET AL.
Period
T
e
r
t
i
a
r
y
Cretaceous
Epoch/Age
Past
Studies
This
Study
Alaska Range
Stratigraphy
Eocene
Upper
Cantwell
Formation
Paleocene
L
a
t
e
(volcanic rocks)
M
Maastrichtian
Campanian
C E
D J
P
Lower
Cantwell
Formation
(sedimentary rocks)
Figure 4. Time chart showing geochronologic data from various studies of the Cantwell Formation. Dark circles are K-Ar age determinations of two intrusives that crosscut the lower
Cantwell Formation (Hickman, 1974; Hickman and Craddock, 1976; Sherwood and Craddock,
1979). Open circles are K-Ar age determinations from the upper Cantwell Formation volcanic
rocks (Bultman, 1972; Sherwood, 1973; Hickman, 1974; Gilbert et al., 1976). The dark square
with brackets represents age determination of the lower Cantwell Formation based on fossil leaf
biostratigraphy (Wolfe and Wahrhaftig, 1970). Brackets with letters represent palynologic ages
from this study based on 135 mudstone samples from five measured stratigraphic sections. D—
Double Mountain (DM1) measured section, J—Jenny Creek (JC1) measured section, C—Dean
Creek (DC1) measured section, E—East Toklat (ET1) measured section, P—Polychrome Mountain (PC1) measured section. See Figure 1B for locations of measured sections.
maximum particle size is 6 cm (Fig. 7A). Facies
association 2 in the Cantwell basin is located basinward of facies association 1 and contains southto-southeast paleocurrent indicators (Fig. 7B).
Facies Association 3. The Cantwell Formation in the north-central part of the Cantwell
basin is characterized by interbedded mudstone
and sandstone. Individual mudstones and sandstones occur mainly in <2-m-thick beds that can
be traced for hundreds of meters (Fig. 6C). The
measured section at Double Mountain (DM1)
(Fig. 8) is typical of facies association 3. The
sandstones are massive to trough cross-stratified, and fine upward. Fluid escape structures
are common in the sandstones. In the 353.5 m of
lower Cantwell Formation exposed at DM1,
only one conglomeratic unit was present. Average maximum particle size in this single unit is
3 cm. Facies association 3 in the Cantwell basin
is located basinward of facies association 2
(Fig. 5). The thicker mudstone units at section
DM1 are part of facies association 5 and will be
discussed in that section.
Interpretation of Facies Associations 1, 2,
and 3. Sedimentary structures in the conglomerates and sandstones of facies associations 1, 2, and
510
3 are characteristic of stream-flow deposits. Evidence for stream-flow processes in the conglomerates and sandstones includes the framework
support, a(t)b(i) imbrication, crude upward-fining
trends, lenticular geometries of individual units,
trough cross-stratification, and unimodal paleocurrent indicators. The organized and imbricated
conglomerates of facies associations 1 and 2 are
typical of modern fluvial conglomerates that form
when gravels are transported as bedload and deposited under waning flow by accretion of progressively smaller clasts in channels and on longitudinal bars of low-sinuosity stream systems
(Collinson, 1986). Facies association 3 is interpreted as sheet flood deposits. Sheet flood deposits are formed by surges of sediment-laden water
that spread out from the ends of stream channels
on alluvial fans, resulting in laterally continuous
deposits of sand (Bull, 1972). Evidence for sheet
flood deposition includes the following: (1) the
dominance of laterally continuous, normally
graded sandstone, which suggests nonchannelized flows; and (2) individual bed thickness of
less than 2 m, which implies that flows were
rarely very deep.
Several basinward facies transitions can be inGeological Society of America Bulletin, May 1997
ferred from the fluvial facies documented for facies associations 1, 2, and 3. Massive, boulder and
cobble conglomerates (average maximum particle size = 11 cm) of facies association 1, located
along the northern basin margin (i.e., Mount Sheldon (MS1) measured section adjacent to the
Hines Creek fault) (Figs. 5 and 7), grade basinward into channelized cobble conglomerates (average maximum particle size = 6 cm) of facies association 2 (i.e., Polychrome Mountain, PC1,
measured section). In turn, channelized cobble
conglomerates grade basinward into massive and
trough cross-stratified sheet sandstones of facies
association 3 (i.e., Double Mountain, DM1, measured section). These basinward trends therefore
include (1) a decrease in average maximum particle size (Fig. 7A), (2) a decrease in conglomerate
(Fig. 5B), and (3) an increase in the organization
of conglomerates. Paleocurrent and maximum
particle size data also indicate southward sediment transport in all three facies associations
(Fig. 7, A and B). Our interpretation of basinward
facies transitions is supported by palynological
age correlation which shows that all three facies
associations (1–3) were deposited at roughly the
same time, from late Campanian to early Maastrichtian (see Trop, 1996, Appendices 1 and 2, for
details of age constraints).
We interpret the dominance of stream-flow facies, the abrupt south-to-southeastward change in
grain size (Fig. 7A), and the basinward lithofacies trends as evidence for stream-dominated
alluvial-fan deposition. Stream-dominated alluvial fans are fans whose surface processes are
dominated by perennial stream flow (Collinson,
1986); they also have been referred to as wet fans
(Schumm, 1977), and humid fans (Fraser and
Suttner, 1986). Similar, ancient stream-dominated alluvial fan deposits have been documented
in other northwestern Cordilleran sedimentary
basins (Ridgway and DeCelles, 1993b). Coal distribution in the Cantwell basin, as well as in other
studied coal-bearing basins, is related to depositional processes on stream-dominated alluvial
fan deposits. The thickest coal seams in the Cantwell alluvial fan deposits occur in the proximal
lithofacies. Coal development in the intervening
middle and distal fan areas was probably suppressed by the high frequency of unconfined flow
events and lateral channel mobility.
Axial Sandy Braided Stream Systems
Facies Association 4. The Cantwell Formation exposed along the east-west axis of the basin
is characterized by a dominance of planar and
trough cross-stratified sandstone and subordinate
clast-supported, well-organized conglomerate
(Fig. 9A). Sandstone and conglomerate units are
laterally continuous. Internally, the sandstone
sheets are marked by lateral discontinuity, with
THRUST-TOP BASIN FORMATION ALONG A SUTURE ZONE
A
B
Figure 5. (A) Block diagram illustrating the types and locations of depositional systems that
filled the Cantwell basin. Abbreviations mark the approximate location of measured stratigraphic sections used to develop this depositional model. The Hines Creek fault (HCF) formed
the northern basin margin; the southern margin was defined by a series of thrust faults. Fluvial
systems changed southward from large, confined channels on the upper and middle alluvial fan
surfaces (MS1, JC1, PC1) (facies associations 1 [FA1] and 2 [FA2]) to unconfined channels on the
outer fan (DM1) (facies association 3 [FA3]). Alluvial fan systems flowed into a large, eastwardflowing axial fluvial system (RV1, FM1) (facies association 4 [FA4]) that drained the Cantwell
basin. Lacustrine systems were best developed along the deeper southern basin margin (PA1). See
text for additional information. (B) Stratigraphic cross section across the Cantwell basin. Base of
the upper Cantwell Formation volcanic rocks was used as a datum for constructing the cross section. Measured section columns show true thickness and generalized lithostratigraphy. Open circles represent conglomerate facies; stippled patterns represent sandstone facies; and dashed lines
represent mudstone facies. FA1—facies association 1, FA2—facies association 2, FA3—facies association 3, FA4—facies association 4, FA5—facies association 5. Note the southward-dipping
basin floor and the asymmetry of the basin. See Figure 1B and 5A for measured section locations.
numerous reactivation surfaces, and local scours
(Fig. 9B). Individual foresets in the stratified
sandstones have a maximum thickness of 75 cm.
Facies association 4 is well exposed in the
Dean Creek (DC1) measured section (Fig. 8). The
624-m-thick section of lower Cantwell Formation
Geological Society of America Bulletin, May 1997
exposed at DC1 consists of amalgamated beds of
massive and planar cross-stratified sandstone, and
clast-supported conglomerate (Figs. 8 and 9B).
Individual bed thicknesses range from 0.5 to
1.5 m. In the upper part of DC1, 40–60-m-thick,
coarsening-upward packages have been documented (Fig. 9A). The base of the package is defined by a 2–8-m-thick laminated mudstone and
coal unit, which grades upward into massive and
planar cross-stratified sandstone, which is, in turn,
overlain by a 10–20-m-thick unit of conglomerate
marked by a channelized base (Fig. 9A).
Facies association 4 is also well exposed in
the Refuge Valley (RV1) and Fang Mountain
(FM1) measured sections (Fig. 1B). Paleocurrent data from this facies association indicate
mainly eastward paleoflow in the western and
central part of the basin (RV1, FM1) and northward paleoflow in the eastern part of the basin
(DC1) (Figs. 1B and 7B). Maximum particle
size data from conglomerates in these deposits
usually range from 4 to 6 cm (Fig. 7A).
Interpretation of Facies Association 4. Facies
association 4 is interpreted as being formed by
stream-flow deposits. Evidence for stream-flow
processes includes the presence of planar- and
trough cross-stratified sandstones; clast-supported,
imbricated, well-organized conglomerates; and
unidirectional paleocurrents recorded at each measured section. The abundance of planar and trough
cross-stratified sandstones, with individual foresets less than 75 cm in thickness, indicates that
deposition was primarily by subaqueous twodimensional and three-dimensional ripples
(Harms et al., 1982) in shallow channels. The lack
of definable large channels in facies association 4
implies that deposition was laterally extensive and
not restricted to a single main channel. We interpret facies association 4 as the product of deposition in a large axial, sandy braided stream system
(Fig. 5). We further interpret that the axial braided
stream system was partly fed by the streamdominated alluvial fan systems to the north (facies
associations 1, 2, and 3) (Fig. 5A). The axial
stream system was the major trunk stream system
that drained the entire Cantwell basin and, on the
basis of paleocurrent analysis (Fig. 7B), exited the
basin in the northeastern corner near the present
Dean Creek area (DC1; Fig. 1B). The upwardcoarsening packages documented at measured
section DC1 may represent the progradation of
coarser facies across the basin as a result of displacement on nearby basin-margin faults.
Lacustrine Systems
Facies Association 5. The Cantwell Formation within the south-central part of the basin is
often characterized by 10–40-m-thick units of
trough cross-stratified and massive sandstone in-
511
RIDGWAY ET AL.
Figure 6. (A) Boulder
and cobble conglomerate
deposits of facies association B
1 at Mount Sheldon (MS1
in Figs. 1B and 5) located
along the northern basin
margin. (B) Facies association 2 in the Cantwell basin.
These deposits are characterized by an upward fining
sequence with a channelized base. A typical sequence consists of a lower
4–6-m-thick, well imbricated pebble conglomerate
(Gcmi) that grades upward
into massive sandstone
(Sm), that is overlain by
mudstone (Fsm). Person
(arrow) for scale. (C) Laterally continuous, interbedded sandstone and mudstone typical of facies
association 3. Arrows mark
the base of a single sandstone bed. Dark beds are mudstone and lighter colored beds are sandstone. Individual mudstone and sandstone units are usually less than 2 m thick. An igneous
intrusion (Int) is located in the center of the outcrop.
terbedded with 10–50-m-thick units of mudstone (Fig. 9, C and D). The trough crossstratified and massive sandstones are lithologically similar to facies association 4 sandstones.
The defining feature of facies association 5 is the
thick mudstone deposits. Mudstone lithofacies
in facies association 5 have varvelike laminations (Fig. 9E), and often contain ripple stratification with clay drapes, dewatering structures,
flaser bedding, graded beds, convoluted bedding
(Fig. 9F), and abundant large plant fragments.
Facies association 5 is well exposed at the
Panorama Mountain measured section (PA1)
(Figs. 5 and 8). At this locality, mudstones with
minor coals are interbedded with trough crossstratified sandstones. Both the mudstones and
sandstones are laterally continuous at the scale
of the outcrops.
Interpretation of Facies Association 5. The
abundance of fine-grained deposits in facies association 5 implies low-energy depositional
processes with a large component of suspension
fallout. The varvelike laminations in the mudstones indicate that sediment transport involved
a component of gravitational settling of finer
grain sizes. We interpret facies association 5 as
512
A
C
suspension fallout (represented by the mudstones) and stream-flow (represented by the
trough cross-stratified sandstones) deposits related to lacustrine and sandy braided stream environments, respectively. The presence of clay
drapes on ripple laminations and flaser bedding
indicates that flow velocity varied from a few
tens of centimeters per second to stagnant. Convoluted bedding and dewatering structures in the
mudstones are also indicators of episodic rapid
deposition on top of existing water-saturated deposits. These types of processes are common in
lacustrine depositional systems (Fouch and
Dean, 1982). Periodically, axial sandy braided
stream systems (part of facies association 4) migrated into lacustrine environments and deposited the trough cross-stratified sandstone.
The concentration of lacustrine mudstones in
the southern part of the basin indicates that the
deepest part of the Cantwell basin was often adjacent to the thrust-faulted southern margin of
the basin (Fig. 5, A and B). This interpretation is
consistent with data from our measured stratigraphic sections of the lower Cantwell Formation, which record a southward increase in
thickness (Fig. 5B). The increase in strati-
Geological Society of America Bulletin, May 1997
graphic thickness of the lower Cantwell Formation provides documentation of an asymmetric,
southward-dipping basin profile.
Marginal Marine Systems
Previous studies have interpreted the depositional systems of the lower Cantwell Formation as
entirely nonmarine. This study has recognized dinoflagellates and acritarch-bearing mudstones
(Fig. 3, T–Z), interbedded with oncolitic(?) limestones (Fig. 10) in the upper part of the lower
Cantwell Formation. If the dinoflagellates and
acritarchs are indigenous to the sampled horizons,
they would be indicative of a brackish water to
marine depositional environment. In the Dean
Creek section (DC1; Figs. 1B and 8), the samples
from 234 and 331 m above the base of the section
yielded an algal cyst assemblage dominated by cf.
Horologinella (16 specimens seen) accompanied
by rare occurrences of the dinoflagellates Ovoidinium, Alterbidinium, and Laciniadinium, and
other unidentified cysts. In the Polychrome
Mountain section (PC1; Figs. 1B and 8), rare dinoflagellates occurred in samples from 48, 58.5,
and 93 m that included one to three specimen
,, ,,,
,,,,,,,,,,
,,,,,
,
,,
,,
,
,,
,,
,
,
,
,,
RIDGWAY ET AL.
I
I
A
A)
I
150°W
148°W
63°45'N
Creek
Fault
s
Hine
Fault
)
Kinley
Denali
(Mc
- Average Maximum Particle Size (MPS):
3-7 cm.
8-10 cm.
11-12 cm.
I
I
B
B)
I
Figure 7. (A) Maximum particle-size distribution of the lower
Cantwell Formation. Maximum
particle-size data points (black
circles) are calculated from the
average length of the ten largest
conglomerate clasts in each individual bed of the measured section. (B) Map showing sediment
transport directions of the lower
Cantwell Formation. Each arrow
represents the average paleocurrent direction based on measurements of imbricated conglomerate clasts and trough cross-strata
(using Method 1 of DeCelles et
al., 1983). See text for additional
discussion.
63°45'N
Creek
Fault
s
Hine
Fault
)
Kinley
Denali
records of Cliestospheridium, Hystrichoshaera,
Ovoidinium, Odontochitina, and Spiniferites (in
order of decreasing number of records). The Double Mountain section (DM1; Figs. 1B and 8)
yielded two different styles of algal cyst assemblages. The sample from 32 m yielded from one
to three specimens of Scenedesmus, Pediastrum,
Horologinella, Ovoidinium, and Cliestosphaeridium. Modern Scenedesmus lives in fresh water,
Pediastrum in fresh to brackish, and the other taxa
in brackish-to-marine waters. The six samples in
DM1 section examined from between 241 and
353 m all contained dinoflagellates including
from one to six specimens of Ovoidinium, Laciniadinium, Horologinella, Oligosphaeidium, Spinidinium, and Gardodinium.
The relative concentration of Horologinella
and sometimes Ovoidinium in a restricted number of samples appears to support a conclusion
that these algal cysts were indigenous to the sampled horizons and indicative of a marine influence. Ovoidinium and Gardodinium, however,
are considered to have Early Cretaceous to Cenomanian and Early Cretaceous ranges, respectively (Lentin and Williams, 1985). These age
(Mc
ranges raise the question as to whether some or
all the recovered dinoflagellates are indigenous to
the sampled horizons or are reworked from Early
Cretaceous marine rocks exposed on the margins
of the Cantwell basin. Limestones, interbedded
with dinoflagellate-bearing mudstones, having a
maximum thickness of 50 cm, appear to contain
oncolitic algal structures (Fig. 10). Recrystallization of the limestones often obliterated much of
the original texture and made precise identification of any fossils difficult. Although oncolitic
limestones have been documented from nonmarine deposits (Jerzykiewicz and Sweet, 1988),
they are frequently associated with marine environments (Wilson, 1975; Scholle et al., 1983).
On the basis of the close association between oncolitic limestones and mudstones containing marine dinoflagellates, we conclude that a marine
influence in the upper part of the lower Cantwell
Formation is a possibility. The possible marine
deposits, based on the low diversity assemblage
of dinoflagellates and the abundance of terrestrially derived, dispersed organic material, would
have been the product of a restricted, marginal
marine depositional environment.
Geological Society of America Bulletin, May 1997
0
10
20 km
N
STRUCTURAL CONTROLS ON BASIN
DEVELOPMENT
Southern Basin Margin
Previous studies of the Cantwell basin have
considered the McKinley segment of the Denali
fault system as defining the southern basin margin (Fig. 1B) (Hickman et al., 1990). In past
studies, the Cantwell basin has been interpreted
as a strike-slip pull-apart basin that formed between the Hines Creek fault and the McKinley
fault (Fig. 1B). Strike-slip displacement along
the McKinley fault has been interpreted as the
primary control on basin subsidence (Hickman
et al., 1990).
Several new lines of evidence from this study
indicate that a system of thrust faults (originally
mapped by Hickman and Craddock, 1976; Sherwood and Craddock, 1979; Csejtey et al., 1986;
Csejtey et al., 1992) defined the southern margin
of the Cantwell basin (Fig. 1B). Our data
(Fig. 11) indicate that these southward-dipping
thrust faults, located north of the McKinley fault
(Fig. 1B), were active during Cantwell Formation
513
RIDGWAY ET AL.
Figure 8. Measured stratigraphic sections of the Cantwell basin showing changes in lithofacies from the northern basin margin (MS1) to
the southern basin margin (PA1). See Figures 1B and 5 for section locations. Open circles represent clast-supported conglomerate. Stippled
pattern represents massive and trough cross-stratified sandstone. Horizontal dashed lines represent mudstone. Double hashed symbols represent igneous intrusion. S, silt; M, sand (medium grain size); P, pebble; B, boulder. Due to space limitations, only a 300 m representative part
of each measured section is shown. MS1—Mount Sheldon; PC1—Polychrome Mountain; DM1—Double Mountain; DC1—Dean Creek;
PA1—Panorama Mountain.
deposition. Intraformational angular unconformities and progressively tilted strata within the
lower Cantwell Formation provide compelling
evidence that the adjacent thrust faults were syndepositional. The intraformational unconformities and tilted strata are well exposed in the area
near measured section PA1 at Panorama Mountain (Figs. 1B and 12). Here the stratigraphically
lowest Cantwell Formation beds are dipping
85°NW (Unit A in Fig. 12A). Unit A is separated
from the overlying deposits of Cantwell Formation by an angular unconformity with 24° of discordance (Fig. 12A). This second package of rotated Cantwell Formation deposits has an average
dip of 58°NW (Unit B in Fig. 12A). An angular
unconformity with 19° of discordance separates
Unit B from the highest stratigraphic package
(Unit C in Fig. 12A). The uppermost package has
514
an average dip of 36°NW. The contact with the
upper Cantwell Formation volcanic rocks is located at the top of this package. The entire lower
Cantwell Formation in this specific area has undergone 49° of northward (i.e., basinward) rotation and contains at least two major intraformational unconformities.
The amount of rotation within the lower Cantwell Formation, proximal to the southern basin
margin, varies along strike. For example, directly east of Revine Creek, our mapping indicates 35° of progressive northward rotation
(Fig. 11). In contrast, west of Revine Creek,
mapping indicates 50°of progressive northward
rotation within the lower Cantwell Formation
(Fig. 11). Figure 12B shows a typical example of
an angular unconformity and Figure 12C shows
the progressive tilting of strata documented
Geological Society of America Bulletin, May 1997
along the southern margin of the Cantwell basin.
The presence of angular unconformities along
the southern basin margin has been recognized
in previous studies (Newell, 1975, Moose Creek
area; Sherwood, 1979, Wood River area). On the
basis of our mapping and structural data from
previous studies, the package of progressively
tilted strata and unconformities can be traced for
most of the 135 km length of the southern margin of the Cantwell basin.
We interpret the progressive tilting within the
lower Cantwell Formation strata and the associated intraformational angular unconformities as
forming when Cantwell deposits, located in the
footwall of bordering thrust faults, were progressively tilted basinward during fault displacement
and incorporated into a growth footwall syncline. Footwall growth synclines, as defined
RIDGWAY ET AL.
A
B
C
D
E
F
Figure 9. (A) Facies association 4 of the Cantwell basin. Fsm—mudstone; St—trough cross-stratified sandstone; Gct—trough cross-stratified conglomerate. Arrows outline erosional surface between sandstone and conglomerate. Person (lower right) for scale. (B) Amalgamated
channels of cross-stratified sandstone typical of facies association 4. Arrows outline one channelized surface. Bedding dips to right. Person
(lower right center) for scale. (C) Steeply dipping, interbedded mudstone (F) and trough cross-stratified sandstone (St) of facies association 5
common in the southern part of the Cantwell basin. Dark recessive units are mudstone (F) and prominent lighter units are sandstone (St).
Bedding dips 85° to left. (D) Close-up photograph of thinly bedded mudstone (Fsm) and trough cross-stratified sandstone (St) typical of facies
association 5. Bedding dips steeply to left. Person (arrow) for scale. (E) Laminated mudstone lithofacies of facies association 5 common along
the southern margin of the Cantwell basin. Scale is 10 cm long. (F) Convoluted bedding (arrows) associated with soft sediment deformation
that often characterizes mudstones of facies association 5. Scale is 10 cm long.
Geological Society of America Bulletin, May 1997
515
RIDGWAY ET AL.
Figure 10. Oncolitic(?) limestone (L) interbedded with marine dinoflagellate-bearing mudstone
(M) in the lower Cantwell Formation. Arrow points to a recrystallized oncolite. Stratigraphically,
this bed is 210 m below the contact with the upper Cantwell Formation volcanic rocks in measured
stratigraphic section DM1 (Fig. 1B) (Trop, 1996). Location of measured section DM1 is Healy (C5)
quadrangle, Alaska, SW 1/4 sec. 15, T. 15 S., R. 10 W.
here, are footwall folds in which sedimentation
took place during structural growth of the syncline. During each major episode of thrust fault
deformation, Cantwell Formation sediments in
the proximal footwall area of the fault were tilted
and eroded, and subsequent lithofacies were deposited on an angular unconformity (Fig. 13).
Packages of progressively tilted strata, each
bounded by angular unconformities, are common elements of thrust-derived deposits within
growth footwall synclines (Riba, 1976; Anadon
et al., 1986; DeCelles et al., 1991, 1993; Burbank and Vergés, 1994; Vergés et al., 1996; Hoy
and Ridgway, 1997). The progressively tilted
strata associated with growth folds have been referred to as growth strata (see Suppe et al., 1992;
Schneider et al., 1996).
Compositional data from the lower Cantwell
Formation along the southern basin margin also
provides evidence for syndepositional thrusting
(Ridgway et al., 1994; Trop and Ridgway, 1995).
Limestone conglomerate clasts, for example,
common in measured section RV1 (Fig. 14), lo-
Figure 11. Sketch map of part of the south-central margin of the Cantwell basin showing bedding orientation and location of intraformational unconformities in the lower Cantwell Formation (Kcs). Note the northward decrease in dip away from the thrust faults that formed the
southern boundary of the Cantwell basin. Area shown on map is located east of the Nenana River near measured section PA1 on Figure 1B.
Note the change in strike and dip across the intraformational unconformities. See text for additional discussion.
516
Geological Society of America Bulletin, May 1997
THRUST-TOP BASIN FORMATION ALONG A SUTURE ZONE
A
B
C
Figure 12. Photographs of intraformational unconformities and rotated beds in the lower Cantwell Formation along the southern basin margin. Wavy black lines indicate intraformational unconformities and tadpole symbols represent dip of bedding. (A) View to the east from Nenana
River on Figure 11. Stratigraphic up and north are to the left. The stratigraphically lowest beds (Unit A) are dipping 85°. Unit A is separated from
the overlying deposits by an angular unconformity. The second package of rotated beds (Unit B) has an average dip of 58°. An angular unconformity separates Unit B from the highest stratigraphic package (Unit C), which has an average dip of 36°. The entire lower Cantwell Formation
in this area has undergone 49° of northward (i.e., basinward) rotation and contains at least two major intraformational unconformities. (B) Intraformational unconformity in the Cantwell Formation. View is of outcrops located east of Revine Creek on Figure 11. Stratigraphic up and
north are to the right. Beds south (left) of the unconformity are dipping 59°, whereas beds north of the unconformity are dipping 46°. (C) Progressive rotation of strata in the lower Cantwell Formation. Stratigraphic up and north are to the right. Bedding systematically decreases from
75° on the left (south) to 65° on the right (north). View is of outcrops located west of Revine Creek on Figure 11. Person (arrow) for scale.
cated adjacent to the southern basin margin thrust
fault system, are similar to nearby limestones located in the hanging walls of adjacent thrust
faults. Conodonts extracted from these limestone
conglomerate clasts in the Cantwell Formation
yield an Ordovician to Early Devonian age (Csejtey et al., 1986), indicating that they were likely
derived from the Ordovician–Devonian limestones found in the hanging wall of a nearby
thrust fault (Fig. 14). A detailed petrographic
study of the sandstones of the lower Cantwell Formation along the southern basin margin, coupled
with paleocurrent data, indicates that rocks exposed by the thrust faults contributed detritus to
the Cantwell basin (Trop et al., 1995; Trop and
Ridgway, 1997). We interpret the provenance
documented for the Cantwell Formation along the
southern basin margin, the angular unconformities, the growth strata, and the northward-directed
paleocurrent indicators (Fig. 7B) as direct evidence that thrust fault deformation and Cantwell
Formation sedimentation were synchronous.
Northern Basin Margin
The Hines Creek fault forms the northern structural boundary of the Cantwell basin (Fig. 1B).
The type of structural feature represented by the
Hines Creek fault in the study area has been controversial, partly due to a lack of unequivocal surface exposure data (Brease, 1995). The 275-kmlong Hines Creek fault was originally interpreted
as an older (post–95 Ma) segment of the strikeslip Denali fault system that represented a major
terrane boundary (Grantz, 1966; Wahrhaftig et al.,
1975). On the basis of additional geologic mapping and geophysical data, Csejtey et al. (1982)
interpreted the Hines Creek fault as a mid-Cretaceous fault that was not a major terrane boundary
fault. More recent studies (Nokleberg et al., 1989,
1992; Nokleberg et al., 1994), east of the Cantwell
basin in the Mount Hayes quadrangle, interpret
the Hines Creek fault as part of a series of Cenozoic thrust faults that are contained within the
Yukon-Tanana terrane.
Geological Society of America Bulletin, May 1997
Several important findings from our study suggest that the Hines Creek fault was active during
the Late Cretaceous from the Campanian to Maastrichtian, and that it played an important role in the
formation of the Cantwell basin. First, the proximal alluvial fan deposits concentrated along the
Hines Creek fault in the lower Cantwell Formation
are consistent with deformation and uplift during
Cantwell Formation deposition (Figs. 5 and 6A).
Second, the Cantwell Formation conglomerates
along the Hines Creek fault contain as much as
40% reworked Cantwell Formation clasts. An
abundance of reworked Cantwell Formation conglomerate clasts is well documented at the Polychrome Mountain measured section (Fig. 14). We
interpret the clast composition data, coupled with
paleocurrent data (Fig. 7B), as evidence that periodically during lower Cantwell Formation deposition, the Cantwell Formation proximal to the
Hines Creek fault was uplifted, eroded, and incorporated into younger Cantwell Formation deposits
along the northwestern basin margin. In the central
517
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,,,,
,,,,
,,,,
,,,,
,,,,
,,,,
,,,
,,,
,,,,
,,,
,,,
,,,,
,,,
,,,
,,,
,,,,
,,,,
,,,
,,,
,,,,
,,,,
,,,
,,,,
RIDGWAY ET AL.
gested by previous studies (Grantz, 1966; Wahrhaftig et al., 1975; Gilbert and Bundtzen, 1983).
STAGE 1
Depositional surface
IMPLICATIONS OF THE STUDY
Regional Tectonic Setting
Hanging Wall
1
Unconformity
Footwall
Figure 13. Schematic diagram
of the development of angular unconformities and progressively
tilted strata within a growth footwall syncline. During each stage of
thrust fault displacement, footwall
deposits adjacent to the fault are
tilted and eroded, and subsequent
sediments are deposited across an
angular unconformity. Additional
stages of fault displacement result
in packages of progressively tilted
strata, each bounded by angular
unconformities.
STAGE 2
Depositional surface
Hanging Wall
2
1
Footwall
STAGE 3
Depositional surface
Hanging Wall
3
2
1
Footwall
and eastern parts of the Cantwell basin, the Hines
Creek fault is covered by the Cantwell Formation
(Wahrhaftig et al., 1975; Csejtey et al., 1992) and
is interpreted as a blind fault during Cantwell Formation deposition. Despite being covered by the
Cantwell Formation in the central and eastern
parts of the Cantwell basin, stratigraphic evidence
suggests that the Hines Creek fault actively influenced Cantwell Formation sedimentation along
this part of the northern basin margin. For example, Sherwood (1979) has interpreted the Cantwell
Formation, north of the Hines Creek fault in the
eastern part of the basin, as a condensed section
518
based on minor angular unconformities and abrupt
northward thinning of the lower Cantwell Formation. In summary, we interpret the Hines Creek
fault as a reverse or thrust fault (in agreement with
geologic mapping of Csejtey et al., 1992, and Nokleberg et al., 1992) that was active during lower
Cantwell Formation deposition. Periodic Late
Cretaceous displacement on the Hines Creek fault
resulted in southward transport of sediments into
the Cantwell basin (Fig. 7B). Our interpretation of
Late Cretaceous thrust displacement on the Hines
Creek fault does not preclude the possibility of an
earlier history of strike-slip displacement as sug-
Geological Society of America Bulletin, May 1997
We interpret the thrusting associated with the
formation and development of the Cantwell
basin to be a result of regional shortening related to the Mesozoic accretionary development
of southern Alaska. In south-central Alaska,
mid-Cretaceous to Late Cretaceous shortening
was in response to accretion of the Wrangellia
composite terrane (terminology of Plafker et al.,
1989) to the continental margin of North America (i.e., the Yukon-Tanana terrane) (Fig. 15)
(Csejtey et al., 1982; Gilbert and Bundtzen,
1983). The Wrangellia composite terrane consists of the Alexander, Peninsular, and Wrangellia terranes and represents the largest allochthonous crustal fragment of southern Alaska. The
precise time of final Cretaceous accretion of the
Wrangellia composite terrane is not perfectly
constrained. Geologic data point to a late Early
to early Late Cretaceous (Albian–Cenomanian)
docking of the Wrangellia composite terrane in
its present position (Nokleberg et al., 1994),
whereas plate reconstructions indicate that
northward displacement and docking of the
Wrangellia composite terrane could not have
occurred prior to the Campanian (Engebretson
et al., 1985). Mid-Cretaceous to Late Cretaceous docking of the Wrangellia composite terrane resulted in underthrusting, folding, and
metamorphism of an extensive Mesozoic flysch
basin (Kahiltna assemblage in Fig. 15) located
between the impinging Wrangellia composite
terrane and the North American continental
margin (Yukon-Tanana terrane on Figure 15)
(Csejtey et al., 1982; Nokleberg et al., 1994).
Continued mid-Cretaceous to Late Cretaceous
convergence resulted in the obduction of the
Wrangellia composite terrane onto the continental margin of North America (Csejtey et al.,
1982) and the development of an extensive suture zone. The present surface expressions of
this Late Cretaceous suture zone are the tectonic
melange deposits located along the southern
Alaska Range (Csejtey et al., 1992), ≈5–10 km
south of the Cantwell basin. The tectonic
melange deposits consist of intensely deformed
and sheared blocks of Paleozoic limestone,
ophiolitic rocks, and Mesozoic flysch-related
argillite and chert (units Kms and Kmn of Csejtey et al., 1992). Development of the suture zone
associated with obduction was characterized by
extensive thrusting and folding of the Wrangellia composite terrane, the intervening Mesozoic
flysch basin, and the ancient North American
THRUST-TOP BASIN FORMATION ALONG A SUTURE ZONE
Polychrome Mtn. (PC1)
40
n = 1279
%
20
0
,
,,
,
,,
,
, ,
,,,
,,
,,,
,,
Q C A Cw L S
es
Hin
I M
Fault
Creek
F
F
Fault
ley)
Denali
(McKin
Refuge Valley (RV1)
n = 668
60
Ordovician-Devonian
Limestone
%
40
20
0
Q C A Cw L S
N
I M
Figure 14. Conglomerate clast-type distribution in the RV1 measured section located along the southern margin of the Cantwell basin and
the PC1 measured section located along the northern margin of the Cantwell basin. Clast types: Q—quartz, C—chert, A—black argillite,
Cw—lower Cantwell Formation, L—limestone, S—other sedimentary rocks, I—igneous, M—metamorphic, n—number of clasts identified.
The limestone clasts common in section RV1 conglomerates are identical to Ordovician–Devonian limestones found in the hanging wall of
thrust faults located along the southern basin margin (block pattern). Locations of lower Cantwell formation conglomerate clasts, which
yielded Ordovician–Devonian conodont fossils, are labeled F on map (Csejtey et al., 1986). The abundance of reworked Cantwell Formation
conglomerate clasts in the PC1 section indicates that periodically the Cantwell Formation proximal to the Hines Creek fault was uplifted,
eroded, and incorporated into younger Cantwell Formation deposits.
continental margin (Csejtey et al., 1982; Csejtey
et al., 1992).
The importance of this mid-Cretaceous to Late
Cretaceous orogenic event in the tectonic development of south-central Alaska and the formation of the Cantwell basin is documented by
magnetotelluric surveys across the central and
eastern Alaska Range (Stanley et al., 1990). Magnetotelluric data suggest that Mesozoic flysch deposits have been tectonically emplaced northward beneath the southern Yukon-Tanana terrane
for more than 50 km (Fig. 15). One of the northsouth magnetotelluric lines transects the central
Cantwell basin (Fig. 5 of Stanley et al., 1990),
and indicates that at deeper structural levels (below 5 km) the Cantwell basin is underlain by underthrust Mesozoic flysch deposits (Fig. 15).
Seismic refraction data from the trans-Alaska
crustal transect, located 100 km to the east of the
Cantwell basin, also indicate that the southern
Yukon-Tanana terrane is underlain by underthrust flysch deposits (Beaudoin et al., 1992). We
interpret the Late Cretaceous Cantwell basin as
having formed in the upper structural levels,
above the deeper zone of underthrusting, along
the suture zone between the Wrangellia composite terrane and the North American continental
margin (Fig. 15). The presence of Cenomanian
fossils in strongly deformed and regionally metamorphosed Mesozoic flysch deposits (Csejtey et
al., 1992) suggests that the unmetamorphosed
Campanian and Maastrichtian Cantwell Formation deposits record later stages of the major Late
Cretaceous orogenic event.
Geological Society of America Bulletin, May 1997
Basin Formation
New data from this study, coupled with regional tectonic models (Csejtey et al., 1982;
Plafker et al., 1989; Nokleberg et al., 1994), indicate that the Cantwell basin formed on a series of
active thrust faults that were associated with a
Late Cretaceous suture zone (Fig. 15). This new
thrust-top basin interpretation is based on (1) the
regional intraformational unconformities and
growth strata documented in the Cantwell Formation associated with thrust faults along the
southern basin margin; (2) the coarse, alluvial fan
deposits concentrated along the northwestern
basin margin adjacent to the Hines Creek fault;
(3) provenance and paleocurrent data that indicate both the southern basin margin thrust faults
519
,,,,
,,,,
,,,,
,,,,
,,,,
RIDGWAY ET AL.
Dv
Yukon - Tanana
Terrane
Tr
Cantwell
Basin
Tr
Kahiltna Assemblage
Mesozoic flysch rocks
Regional Antiform
N
?
Wrangellia
Terrane
Talkeetna
Thrust
Dv
Hines
Creek
Fault
,,,
,,,
,,,
,,,
,,,
Trace of modern
Denali fault
?
Zone of underthrusting of
Mesozoic flysch rocks
?
Not To Scale
Figure 15. Schematic paleotectonic block diagram of south-central Alaska during the Late Cretaceous based on the regional tectonic models of Csejtey et al. (1982), Plafker et al. (1989), and Nokleberg et al. (1994), and this study. The surface data used to construct this diagram is
taken from the geologic map of the Healy quadrangle by Csejtey et al. (1992). The length of the block diagram is ≈180 km. During the Late
Cretaceous, accretion of the Wrangellia composite terrane (WCT) resulted in regional shortening throughout south-central Alaska. The southeast-dipping Talkeetna thrust fault marks the northern extent of the WCT (Csejtey et al., 1982). Docking of the WCT resulted in underthrusting and folding of the Mesozoic flysch basin (Kahiltna assemblage) located between the impinging WCT and the North American continental margin. The Yukon-Tanana terrane represents the Late Cretaceous North American continental margin. Magnetotelluric and seismic
data indicate that Mesozoic flysch deposits of the Kahiltna assemblage have been tectonically emplaced northward beneath the southern
Yukon-Tanana terrane for more than 50 km (Stanley et al., 1990; Beaudoin et al., 1992). The Late Cretaceous Cantwell basin formed in the
upper structural levels, above the deeper zone of underthrusting, along the suture zone between the WCT and the North American continental margin. See text for additional discussion.
and the Hines Creek fault were active during
Cantwell Formation deposition; and (4) the Late
Cretaceous age documented for the lower Cantwell Formation. The Hines Creek fault defined
the northern leading edge of the thrust-top basin,
whereas the syndepositional thrust faults, located
along the southern margin of the basin, formed
the trailing edge of the basin (Fig. 15). This type
of basin configuration would explain the present
distribution of Cantwell Formation outcrops to
the area between the southern thrust faults and
the Hines Creek fault (Fig. 1B). The packages of
progressively tilted strata bounded by unconformities, the abrupt changes in lithofacies, the complex provenance and paleodrainage, and the evidence for deformation along both margins of the
basin, documented in our analysis of the Cantwell basin, are common features in many ancient
piggyback basins (i.e., Ori and Friend, 1984;
Beer et al., 1990; Lawton and Trexler, 1991;
Coogan, 1992; Talling et al., 1995).
520
Our analysis suggests that there were two major controls on the formation and stratigraphic
evolution of the Cantwell basin. First, episodic
uplift and translation of the Cantwell basin along
the Hines Creek fault resulted in cannibalization
of existing Cantwell deposits and produced a
condensed section (≈700 m) of Cantwell Formation along the eastern basin margin. The other
major control on basin development was subsidence along the southern basin margin induced
by hinterland thrust loading proximal to the suture zone. The accumulation of over 4000 m of
Cantwell deposits adjacent to the thrust-bounded
southern basin margin and the asymmetric basin
geometry (Fig. 5B) are interpreted to be the result of subsidence due to thrust loading. The
presence of (1) intraformational unconformities
and sequentially tilted strata and (2) diagnostic
Cantwell Formation conglomerate clasts derived
from the uplifted hanging walls of nearby thrust
sheets provides strong evidence for syndeposi-
Geological Society of America Bulletin, May 1997
tional thrusting during the development of the
southern basin margin.
Strike-Slip Displacement Along the Denali
Fault
The Denali fault system is considered one of
the most important tectonic features in the northwestern Cordillera (Lanphere, 1978; Hickman et
al., 1978; Eisbacher, 1985; Plafker and Berg,
1994). On the basis of previous studies that have
interpreted the Cantwell basin as a Paleogene
strike-slip basin, the lower Cantwell Formation
was interpreted as representing the oldest synorogenic deposit directly associated with strikeslip displacement along the entire Denali fault
system (Ridgway et al., 1992b). The age of the
Cantwell Formation, therefore, was thought to
provide one of the few geochronologic constraints on the timing of the early development
of the Denali fault system, especially along the
THRUST-TOP BASIN FORMATION ALONG A SUTURE ZONE
Figure 16. Regional correlation map of the
Cantwell basin with other sedimentary basins in northwestern North America. Late
Campanian–early Maastrichtian marine or
marginal marine strata have been documented in all of these basins. The marine influence recognized in the lower Cantwell
Formation probably represents the same major, late Campanian transgressive event documented in the Alberta foreland basin (i.e.,
the Bearpaw transgression). The solid lines
show the extent of the late Campanian transgressive event in northwestern North America (Nichols and Sweet, 1993). The dashed
line shows our interpretation of the paleogeographic relationship between the late Campanian seaway and the Cantwell basin. See
text for additional discussion.
McKinley segment. Results of this study indicate that the Denali fault system is younger than
the tectonics involved in the formation of the
Cantwell basin. This interpretation of the Late
Cretaceous Cantwell basin is in agreement with
the geologic mapping of Csejtey et al. (1992)
which documents offset of Late Cretaceous
thrust faults by the Cenozoic Denali fault system. Within the general framework of previous
regional tectonic models (Csejtey et al., 1982;
Nokleberg et al., 1994), we interpret the Cantwell basin as part of the Cretaceous accretionary
orogenic event, whereas the Cenozoic, postaccretionary strike-slip Denali fault system developed within the older Cretaceous suture zone.
In light of this reinterpretation of the lower
Cantwell Formation, the oldest documented synorogenic strike-slip deposits along the eastern Denali fault system are the late Eocene–Oligocene
Amphitheatre Formation located in the Saint Elias
Mountains, Yukon Territory (Ridgway and DeCelles, 1993a; Ridgway et al., 1995a). Along the
western part of the Denali fault system, several
strike-slip basins have been described that contain
mostly Oligocene and Miocene deposits, and
smaller amounts of possibly Paleocene deposits
(Gilbert, 1981; Dickey, 1984). Additional detailed
geochronologic studies are needed on all the
strike-slip basins along the Denali fault system, but
much of the available data from studied synorogenic sedimentary deposits suggest that the earliest major displacement on the Denali fault system
occurred during the late Eocene and Oligocene
(Ridgway et al., 1992b).
Regional Stratigraphic Correlations
The marine influence recognized in the upper
lower Cantwell Formation may represent the same
major Campanian transgressive event (i.e., the
Bearpaw transgression) found in several sedimentary basins in western Canada. From a regional
stratigraphic perspective, the lower Cantwell Formation is correlative to the upper Tango Creek and
Brothers Peak Formations of the Sustut Group,
Bowser basin, British Columbia (A. R. Sweet,
1995, unpub. data) (Fig. 16); the Belly River/
Brazeau, Bearpaw, and St. Mary River/Horseshoe
Canyon Formations,Alberta foreland basin (Jerzykiewicz and Sweet, 1988) (Fig. 16); the middle
part of the Bonnet Plume Formation of the Yukon
Territory; and the Little Bear and East Fork Formations of the Brackett basin, Northwest Territories (Nichols and Sweet, 1993) (Fig. 16). These
stratigraphies contain marine or marginal marine
strata of late Campanian–early Maastrichtian age.
Geological Society of America Bulletin, May 1997
Documentation of marine influence during the late
Campanian in the Cantwell basin permits the first
regional correlation of the eustatic Bearpaw transgressive event (Bearpaw cyclothem; Table 3 in
Hancock and Kauffman, 1979) into south-central
Alaska. On the basis of previous studies of the extent of the Bearpaw transgression (Hancock and
Kauffman, 1979; Nichols and Sweet, 1993), the
main body of the Bearpaw sea was located to the
north of the Cantwell basin (Fig. 16). The sea
probably entered the Cantwell basin from the fluvial drainage outlet in the northeast corner of the
basin (Fig. 7B). Recognition of the Bearpaw transgression event in south-central Alaska should
allow for more accurate paleogeographic reconstructions of North America during the Late
Cretaceous.
CONCLUSIONS
(1) Previous geochronologic studies have
placed the Cantwell Formation in the Paleocene.
New data based on palynologic analyses indicate
that the lower Cantwell Formation was deposited
during the late Campanian to early Maastrichtian.
On the basis of published regional tectonic studies and this new age constraint, the formation of
the Cantwell basin was coeval with Late Creta521
RIDGWAY ET AL.
ceous thrusting associated with accretionary tectonics in southern Alaska.
(2) Reconstruction of lower Cantwell Formation depositional systems demonstrates that sedimentation was strongly influenced by an asymmetric basin floor. The basin asymmetry resulted
in a concentration of fine-grained lacustrine environments adjacent to the deeper southern basin
margin. In the central part of the basin, axial
braided stream systems and lacustrine environments interfingered with stream-dominated alluvial fan systems that flowed south to southeast
away from the northern basin margin.
(3) Previous studies have interpreted the lower
Cantwell Formation as an entirely nonmarine
unit. The presence of abundant terrestrially derived organic material, marine dinoflagellates
and oncolitic(?) limestones is indicative of a marginal marine depositional environment in the upper part of the lower Cantwell Formation. The
marine influence recognized in the Cantwell Formation may represent the same major Campanian transgressive event as the Bearpaw of the
Cordilleran foreland basin.
(4) New data indicate that the Cantwell basin
formed as a thrust-top basin along a Late Cretaceous suture zone between the Wrangellia composite terrane and the North American continental
margin in south-central Alaska. This interpretation is based on (1) the Late Cretaceous age documented for the lower Cantwell Formation; (2) the
regional intraformational unconformities and
growth strata recognized in the Cantwell Formation associated with thrust faults along the southern basin margin; (3) the proximal alluvial fan deposits concentrated along the northwestern basin
margin adjacent to the Hines Creek fault; and
(4) compositional and paleocurrent data indicating that the southern basin margin thrust faults
and the Hines Creek fault (i.e., the northern basin
margin) were both active during Cantwell Formation deposition. The Hines Creek fault defined the
northern leading edge of the thrust-top basin,
whereas the syndepositional thrust faults, located
along the southern margin of the basin, formed
the trailing edge of the basin.
(5) The Denali fault system is considered to
be one of the key tectonic features in the development of the northwestern Cordillera. On the
basis of previous studies that have interpreted the
Cantwell basin as a Paleogene strike-slip basin,
the Cantwell Formation was interpreted as representing the oldest synorogenic deposit directly
associated with strike-slip displacement along
the Denali fault system. The age of the Cantwell
Formation, therefore, was thought to provide
one of the few geochronologic constraints on the
timing of the early development of the Denali
fault system. Results of this study document that
the Cantwell basin formed as a part of the Cretaceous accretionary-collisional orogenic event,
522
prior to the development of the postaccretionary
Cenozoic Denali fault system.
ACKNOWLEDGMENTS
Funding was provided by the National Science
Foundation (grant EAR-9406078), the Purdue
Research Foundation–Summer Faculty Research
Grant, and the Geological Society of America
(5489-94). We gratefully acknowledge the National Park Service personnel at Denali National
Park and Preserve, in particular Phil Brease and
Joe VanHorn, for their support and cooperation
with our field studies. We thank D. McIntyre for
identification of the dinoflagellates. This study
benefited from discussions with and/or field assistance from R. Stanley, R. Cole, R. Hoy, M.
Keyes, D. Newell, J. Epps, R. Nichols, W. J. Nokleberg, A. Grantz, F. Wilson, A. Till, R. Blodgett,
J. Decker, G. Plafker, and A. Meigs. A study of
this type would not have been possible without
the initial careful mapping of the rugged Alaska
Range by C. Wahrhaftig, J. C. Reed, Jr., R. G.
Hickman, K. W. Sherwood, C. Craddock, W. M.
Brewer, D. Newell, B. Csejtey, Jr., W. G. Gilbert,
J. Decker, W. J. Nokleberg, and others. We thank
Bulletin reviewers John Decker, Wyatt Gilbert,
Richard Stanley, and Lynn Walter.
REFERENCES CITED
Anadon, P., Cabrera, L., Colombo, F., Marzo, M., and Riba, O.,
1986, Syntectonic intraformational unconformities in alluvial fan deposits, eastern Ebro basin margins (NE
Spain), in Allen, P. A., and Homewood, P., eds., Foreland
basins: International Association of Sedimentologists
Special Publication 8, p. 259–271.
Beaudoin, B. C., Fuis, G. S., Mooney, W. D., Nokleberg, W. J.,
and Christensen, N. I., 1992, Thin, low-velocity crust beneath the southern Yukon-Tanana terrane, east-central
Alaska: Results from trans-Alaska crustal transect refraction/wide-angle reflection data: Journal of Geophysical
Research, v. 97, p. 1921–1942.
Beer, J. A., Allmendinger, R. W., Figueroa, D., and Jordon,
T. E., 1990, Seismic stratigraphy of a Neogene piggyback
basin, Argentina: American Association of Petroleum Geologists Bulletin, v. 74, p. 1183–1202.
Brease, P., 1995, The Hines Creek fault in the Denali Park area,
in Brease, P., and Till, A., eds., The Geology and glacial
history of Denali National Park and vicinity: Geological
Society of America Cordilleran Section, Field Trip 9
Guidebook and Roadlog, p. 17–21.
Brooks, A. H., and Prindle, L. M., 1911, The Mount McKinley
region, Alaska: U.S. Geological Survey Professional Paper 70, 234 p., scale 1:625 000.
Bull, W. B., 1972, Recognition of alluvial fan deposits in the
stratigraphic record, in Rigby, J. K., and Hamblin, W. K.,
eds., Recognition of ancient sedimentary environments:
Society of Economic Paleontologists and Mineralogists
Special Publication 16, p. 63–83.
Bultman, T. R., 1972, The Denali fault (Hines Creek strand)
near the Nenana River, Alaska [Master’s thesis]: Madison, University of Wisconsin, 161 p., scale 1:63 360.
Burbank, D. W., and Vergés, J., 1994, Reconstruction of topography and related depositional systems during active
thrusting: Journal of Geophysical Research, v. 99,
p. 20281–20297.
Capps, S. R., 1940, Geology of the Alaska railroad region: U.S.
Geological Survey Bulletin 907, 201 p., scale 1:250 000.
Chaney, R. W., 1937, Age of the Cantwell Formation [abs.]:
Geological Society of America Proceedings 1936,
p. 355–356.
Collinson, J. D., 1986, Alluvial sediments, in Reading, H. G.,
Geological Society of America Bulletin, May 1997
ed., Sedimentary environments and facies: Oxford,
United Kingdom, Blackwell, p. 20–62.
Coogan, J. C., 1992, Structural evolution of piggyback basins
in the Wyoming-Idaho-Utah thrust belt, in Link, P. K.,
Kuntz, M. A., and Platt, L. B., eds., Regional geology of
eastern Idaho and western Wyoming: Geological Society
of America Memoir 179, p. 55–81.
Csejtey, B., Jr., Cox, D. P., Evarts, R. C., Stricker, G. D., and
Foster, H. L., 1982, The Cenozoic Denali fault system and
the Cretaceous accretionary development of southern
Alaska: Journal of Geophysical Research, v. 87,
p. 3741–3754.
Csejtey, B., Jr., and 13 others, 1986, Geology and geochronology of the Healy quadrangle, Alaska: U.S. Geological
Survey Open-File Report 86-396, 92 p.,scale 1:250 000.
Csejtey, B., Jr., Mullen, M. W., Cox, D. P., and Stricker, G. D.,
1992, Geology and geochronology of the Healy quadrangle, south-central Alaska: U.S. Geological Survey Miscellaneous Investigation Series I-1961, 63 p., scale
1:250 000.
DeCelles, P. G., Langford, R. P., and Schwartz, R. K., 1983,
Two new methods of paleocurrent determination from
trough cross-stratification: Journal of Sedimentary Petrology, v. 53, p. 629–642.
DeCelles, P. G., Gray, M. B., Ridgway, K. D., Cole, R. B., Srivastava, P., Pequera, N., and Pivnik, D. A., 1991, Kinematic history of a foreland uplift from Paleocene synorogenic conglomerate, Beartooth Range, Wyoming and
Montana: Geological Society of America Bulletin, v. 103,
p. 1458–1475.
DeCelles, P. G., Pile, H. T., and Coogan, J. C., 1993, Kinematic
history of the Meade thrust based on provenance of the
Belcher Conglomerate at Red Mountain, Idaho, Sevier
thrust belt: Tectonics, v. 12, p. 1436–1450.
Decker, J. E., 1975, Geology of the Mount Galen area, Mount
McKinley National Park, Alaska [Master’s thesis]: Fairbanks, University of Alaska, 77 p.
Dickey, D. G., 1984, Cenozoic non-marine sedimentary rocks
of the Farewell fault zone, McGrath quadrangle, Alaska:
Sedimentary Geology, v. 38, p. 443–463.
Eisbacher, G. H., 1985, Pericollisional strike-slip faults and
synorogenic basins, Canadian Cordillera, in Biddle, K. T.,
and Christie-Blick, N., eds., Strike-slip deformation,
basin formation, and sedimentation: Society of Economic
Paleontologists and Mineralogists Special Publications
no. 37, p. 265–282.
Eldridge, G. H., 1900, A reconnaissance in the Susitna basin
and adjacent territory, Alaska in 1898, in Explorations in
Alaska in 1889: U.S. Geological Survey Annual Report
20, pt. 7, p. 1–29.
Engebretson, D. C., Cox, A., and Gordon, R. G., 1985, Relative
motions between oceanic and continental plates in the Pacific basin: Geological Society of America Special Paper
206, 59 p.
Fouch, T. D., and Dean, W. E., 1982, Lacustrine and associated
clastic depositional environments, in Scholle, P. A., and
Spearing, D., eds., Sandstone depositional environments:
American Association of Petroleum Geologists Memoir 31, p. 87–114.
Fraser, G. S., and Suttner, L., 1986, Alluvial fans and fan
deltas—A guide to exploration for oil and gas: Boston,
International Human Resources Development Corporation, 199 p.
Gilbert, W. G., 1975, Outline of the tectonic history of westcentral Alaska Range: Geological Society of America Abstracts with Programs, v. 7, p. 320.
Gilbert, W. G., 1976, Evidence for early Cenozoic orogeny in
central Alaska Range, in Short notes on Alaskan geology—1976: Alaska Division of Geological and Geophysical Surveys Geological Report 51, p. 5–7.
Gilbert, W. G., 1981, Preliminary geologic map of the Cheeneetnak River area, Alaska: Alaska Division Geological
and Geophysical Survey, Open-File Report AOF-153,
scale 1:63,360, 10 p.
Gilbert, W. G., and Bundtzen, T. K., 1983, Two-stage amalgamation of west-central Alaska: Geological Society of
America Abstracts with Programs, v. 15, p. 428.
Gilbert, W. G., and Redman, E., 1975, Geologic map and structure sections of Healy C-6 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Open-File
Report 80.
Gilbert, W. G., Ferrell, V. M., and Turner, D. L., 1976, The
Teklanika Formation—A new Paleocene volcanic formation in the central Alaska Range: Alaska Division of Geo-
THRUST-TOP BASIN FORMATION ALONG A SUTURE ZONE
logical and Geophysical Surveys Geologic Report 47,
16 p.,scale 1:63 360.
Grantz, A., 1966, Strike-slip faults in Alaska: U.S. Geological
Survey Open-File Report 267, 82 p.
Hancock, J. M., and Kauffman, E. G., 1979, The great transgressions of the Late Cretaceous: Journal of Geological
Society London, v. 136, p. 175–186.
Harms, J. C., Southard, J. B., and Walker, R. G., 1982, Structures and sequences in clastic rocks: Society of Economic
Paleontologists and Mineralogists, Short Course No. 9,
p. 3-1–3-51.
Hickman, R. G., 1974, Structural geology and stratigraphy
along a segment of the Denali fault system, central Alaska
Range, Alaska [Ph.D. dissert.]: Madison, University of
Wisconsin, 276 p., scale 1:63 360.
Hickman, R. G., and Craddock, C., 1976, Geologic map of central Healy quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Open-File Report AOF95, scale 1:63 360.
Hickman, R. G., Craddock, C., and Sherwood, K. W., 1978,
The Denali fault system and the tectonic development of
southern Alaska: Tectonophysics, v. 47, p. 247–273.
Hickman, R. G., Sherwood, K. W., and Craddock, C., 1990,
Structural evolution of the early Tertiary Cantwell basin,
south-central Alaska: Tectonics, v. 9, p. 1433–1449.
Hoy, R. G., and Ridgway, K. D., 1997, Structural and sedimentological development of footwall growth synclines along
an intraforeland uplift, east-central Bighorn Mountains,
Wyoming: Geological Society of America Bulletin (in
press).
Imlay, R. W., and Reeside, J. B., Jr., 1954, Correlations of the
Cretaceous formations of Greenland and Alaska: Geological Society of America Bulletin, v. 65, p. 223–246.
Jerzykiewicz, T., and Sweet, A. R., 1988, Sedimentological and
palynological evidence of regional climatic changes in the
Campanian to Paleocene of the Rocky Mountain
foothills, Canada: Sedimentary Geology, v. 59, p. 29–76.
Jones, D. L., Silberling, N. J., Gilbert, W. G., and Coney, P. J.,
1983, Tectonostratigraphic map and interpretive bedrock
geologic map of the Mount McKinley region, Alaska:
U.S. Geological Survey Open-File Report 83-11, scale
1:250 000.
Jones, D. L., Silberling, N. J., Coney, P. J., and Plafker, G.,
1987, Lithotectonic terrane map of Alaska (west of the
141st meridian): U.S. Geological Survey Miscellaneous
Field Studies Map MF-1874-A, scale 1:2 500 000.
Lanphere, M. A., 1978, Displacement history of the Denali
fault system, Alaska and Canada: Canadian Journal of
Earth Science, v. 15, p. 817–822.
Lawton, T. F., and Trexler, Jr., J. H., 1991, Piggyback basin in
the Sevier orogenic belt, Utah: Implications for development of the thrust wedge: Geology, v. 19, p. 827–830.
Lentin, J. K., and Williams, G. L., 1985, Fossil dinoflagellates:
Index to genera and species, 1985 Ed.: Canadian Technical Report of Hydrography and Ocean Sciences 60, 451 p.
McLean, H., and Stanley, R. G., 1992, Reconnaissance sandstone petrology and provenance of the Cantwell Formation, central Alaska, in Bradley, D. C., and Ford, A. B.,
eds., Geologic Studies in Alaska by the U.S. Geological
Survey, 1990: U.S. Geological Survey Bulletin 1999,
p. 170–179.
Moffitt, F. H., 1915, in Pogue, J. E., ed., The Broad Pass region,
Alaska, with sections on Quaternary deposits, igneous
rocks, and glaciation: U.S. Geological Survey Bulletin
608, 80 p.,scale 1:250 000.
Newell, K. D., 1975, Stratigraphy and structural geology of the
Moose Creek area, central Alaska Range, Alaska [Master’s thesis]: Madison, University of Wisconsin, 177 p.
Nichols, D. J., and Sweet, A. R., 1993, Biostratigraphy of upper Cretaceous non-marine palynofloras in a north-south
transect of the Western Interior basin, in Caldwell,
W. G. E., and Kauffman, E. G., eds., Evolution of the
Western Interior Basin: Geological Association of
Canada, Special Paper 39, p. 539–584.
Nokleberg, W. J., Foster, H. L., and Aleinikoff, J. N., 1989, Geology of the northern Copper River basin, eastern Alaska
Range, and southern Yukon-Tanana basin, southern and
east-central Alaska, in Nokleberg, W. J., and Fisher,
M. A., eds., Alaska geological and geophysical transect:
International Geological Congress, 27th, Guidebook
T104, p. 34–64.
Nokleberg, W. J., Aleinikoff, J. N., Lange, I. M., Silva, S. R.,
Miyaoka, R. T., Schwab, C. E., and Zehner, R. E., 1992,
Preliminary geologic map of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Open-File Report 92-594, scale 1:250 000, 39 p.
Nokleberg, W. J., Plafker, G., and Wilson, F. H., 1994, Geology
of south-central Alaska, in Plafker, G., and Berg, H. C.,
eds., The geology of Alaska: Geological Society of America, Geology of North America, v. G-1, p. 311–366.
Norris, G., Jarzen, D. M., and Awai-Thorne, B. V., 1975, Evolution of the Campanian terrestrial palynoflora in western
Canada, in Caldwell, W. G. E., The Cretaceous System in
the Western Interior of North America: Geological Association of Canada, Special Paper 13, p. 333–364.
Ori, G. G., and Friend, P. F., 1984, Sedimentary basins formed
and carried piggyback on active thrust sheets: Geology,
v. 12, p. 475–478.
Plafker, G., and Berg, H. C., 1994, Overview of the geology
and tectonic evolution of Alaska, in Plafker, G., and Berg,
H. C., eds., The geology of Alaska: Geological Survey of
America, Geology of North America, v. G-1,
p. 989–1021.
Plafker, G., Nokleberg, W. J., and Lull, J. S., 1989, Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach terranes along the trans-Alaska crustal
transect in the northern Chugach Mountains and southern
Copper River basin: Journal of Geophysical Research,
v. 94, p. 4255–4295.
Reed, J. C., Jr., 1961, Geology of the Mount McKinley quadrangle, Alaska: U.S. Geological Survey Bulletin 1108-A,
36 p.,scale 1:250 000.
Riba, D., 1976, Syntectonic unconformities of the Alto Cardener, Spanish Pyrenees: A genetic interpretation: Sedimentary Geology, v. 15, p. 213–233.
Ridgway, K. D., and DeCelles, P. G., 1993a, Petrology of midCenozoic strike-slip basins in an accretionary orogen, St.
Elias Mountains, Yukon Territory, Canada, in Johnsson,
M., and Basu, A., eds., Processes controlling the composition of clastic sediments: Geological Society of America Special Paper 284, p. 67–89.
Ridgway, K. D., and DeCelles, P. G., 1993b, Stream-dominated
alluvial-fan and lacustrine depositional systems in Cenozoic strike-slip basins, Denali fault system, Yukon Territory, Canada: Sedimentology, v. 40, p. 645–666.
Ridgway, K. D., DeCelles, P. G., Cameron, A. R., and Sweet,
A. R., 1992a, Cenozoic syntectonic sedimentation and
strike-slip basin development along the Denali fault system, Yukon Territory: Yukon Geology, v. 3, p. 1–26.
Ridgway, K. D., Skulski, T., and Sweet, A. R., 1992b, Cenozoic
displacement along the Denali fault system, Yukon Territory [abs.]: Eos (Transactions, American Geophysical
Union), v. 73, p. 534.
Ridgway, K. D., Trop, J. M., and Sweet, A. R., 1994, Depositional systems, age, and provenance of the Cantwell Formation, Cantwell basin, Alaska Range: Geological Society
of America Abstracts with Programs, v. 26, no. 7, p. A-492.
Ridgway, K. D., Sweet, A. R., and Cameron, A. R., 1995a, Climatically induced floristic changes across the
Eocene–Oligocene transition in the northern high latitudes, Yukon Territory, Canada: Geological Society of
America Bulletin, v. 107, p. 676–696.
Ridgway, K. D., Trop, J. M., and Sweet, A. R., 1995b, Sedimentary and tectonic history of the Cantwell basin,
Alaska Range: Geological Society of America Abstracts
with Programs, v. 27, no. 5, p. 73.
Schneider, C. L., Hummon, C., Yeats, R. S., and Huftile, G. L.,
1996, Structural evolution of the northern Los Angeles
basin, California, based on growth strata: Tectonics, v. 15,
p. 341–355.
Scholle, P. A., Bebout, D. G., Moore, C. H., 1983, Carbonate
depositional environments: American Association of Petroleum Geologists Memoir 33, 708 p.
Schumm, S. A., 1977, The fluvial system: New York, Wiley and
Sons, 338 p.
Sherwood, K. W., 1973, Geologic structure along the Hines
Creek fault west of the Wood River, north-central Alaska
Range [Master’s thesis]: Madison, University of Wisconsin, 131 p., scale 1:63 360.
Sherwood, K. W., 1979, Stratigraphy, metamorphic geology,
and structural geology of the central Alaska Range,
Alaska [Ph.D. dissert.]: Madison, University of Wiscon-
sin, 692 p., scale 1:63 360.
Sherwood, K. W., and Craddock, C., 1979, General geology of
the central Alaska Range between the Nenana River and
Mount Deborah: Alaska Division of Geological and Geophysical Surveys Open-File Report AOF-116, map scale
1:63 360.
Sherwood, K. W., Hickman, R. G., and Craddock, C., 1979,
Structural evolution of the Paleocene Cantwell basin
along the Denali fault system: Geological Society of
America Abstracts with Programs, v. 11, no. 7,
p. 515–516.
Stanley, W. D., Labson, V. F., Nokleberg, W. J., Csejtey, B., Jr.,
and Fisher, M. A., 1990, The Denali fault system and
Alaska Range of Alaska: Evidence for underplated Mesozoic flysch from magnetotelluric surveys: Geological Society of America Bulletin, v. 102, p. 160–173.
Steiger, R. H., and Jäger, E., 1977, Subcommission on
geochronology: Convention on the use of decay constants
in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359–362.
Suppe, J., Chou, G. T., and Hook, S. C., 1992, Rates of folding
and faulting determined from growth strata, in McClay,
K. R., ed., Thrust Tectonics: New York, Chapman and
Hall, p. 105–121.
Talling, P. J., Lawton, T. F., Burbank, D. W., and Hobbs, R. S.,
1995, Evolution of latest Cretaceous–Eocene nonmarine
deposystems in the Axhandle piggyback basin of central
Utah: Geological Society of America Bulletin, v. 107,
p. 297–315.
Trop, J. M., 1996, Sedimentological, provenance, and structural
analysis of the lower Cantwell Formation, Cantwell
Basin, central Alaska Range: Implications for thrust-top
basin development along a suture zone [Master’s thesis]:
West Lafayette, Indiana, Purdue University, 222 p.
Trop, J. M., and Ridgway, K. D., 1995, Provenance of the
Cantwell Formation, Cantwell basin, Alaska Range: Geological Society of America Abstracts with Programs,
v. 24, no. 5, p. 82.
Trop, J. M., and Ridgway, K. D., 1997, Petrofacies and provenance of a Late Cretaceous thrust-top basin, Cantwell
Basin, central Alaska Range: Journal of Geophysical Research, v. 67 (in press).
Trop, J. M., Ridgway, K. D., and Sweet, A. R., 1995, Geologic
history of the lower Cantwell Formation, Cantwell basin,
south-central Alaska, in Brease, P., and Till, A., eds., The
geology and glacial history of Denali National Park and
vicinity: Geological Society of America Cordilleran Section, Field Trip 9 Guidebook and Roadlog, p. 28–55.
Vergés, J., Burbank, D. W., and Meigs, A., 1996, Unfolding: An
inverse approach to fold kinematics: Geology, v. 24,
p. 175–178.
Wahrhaftig, C., 1970a, Geologic map of the Healy D-2 quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-804, scale 1:63 360.
Wahrhaftig, C., 1970b, Geologic map of the Healy D-3 quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-805, scale 1:63 360.
Wahrhaftig, C., 1970c, Geologic map of the Healy D-4 quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-806, scale 1:63 360.
Wahrhaftig, C., 1970d, Geologic map of the Healy D-5 quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-807, scale 1:63 360.
Wahrhaftig, C., Turner, D. L., Weber, F. R., and Smith, T. E.,
1975, Nature and timing of movement on Hines Creek
strand of Denali fault system, Alaska: Geology, v. 3,
p. 463–466.
Wiggins, V. D., 1976, Fossil oculata pollen from Alaska: Geoscience and Man, v. 15, p. 51–76.
Wilson, J. E., 1975, Carbonate facies in geologic history: New
York, Springer-Verlag, 471 p.
Wolfe, J. A., and Wahrhaftig, C., 1970, The Cantwell Formation
of the central Alaska Range, in Cohee, G. V., Bates, R. G.,
and Wright, W. B., eds., Changes in stratigraphic nomenclature by the U.S. Geological Survey, 1968: U.S. Geological Survey Bulletin 1294-A, p. A41–A46.
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MANUSCRIPT ACCEPTED AUGUST 16, 1996
REVISED MANUSCRIPT RECEIVED JULY 3, 1996
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