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Palaeogeography, Palaeoclimatology, P
Organic carbon production and preservation in response to
sea-level changes in the Turonian Carlile Formation,
U.S. Western Interior Basin
Timothy White a,*, Michael A. Arthur b
a Earth and Environmental Systems Institute 2217 EES Building, The Pennsylvania State University University Park, PA 16802, United Statesb Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, United States
Accepted 12 September 2005
Abstract
A primary sea-level control over the distribution of total organic and carbonate carbon and organic matter type can be inferred in
the early to middle Turonian Carlile Formation (Fm.), Western Interior Basin, United States. The conceptual model relies on
chemo- and lithostratigraphic correlations of lower to mid-Turonian strata in the central KWIS, supported by ammonite
biostratigraphy, and is based primarily on lithologic, gamma-ray spectrometric, and geochemical facies analysis of the USGS
Portland No. 1 Core from central Colorado, the Amoco Rebecca Bounds No. 1 Core from western Kansas, and the Hawarden Core
from northwestern Iowa.
Sedimentation in the central marine axial basin of the Cretaceous Western Interior Seaway (KWIS) during the Turonian mostly
reflects deposition by pelagic settling and from nepheloid layers with winnowing by bottom currents. Relatively high % total
organic carbon (TOC), % carbonate (CaCO3) and Rock-Eval pyrolysis hydrogen index (HI) values correspond to transgressive or
highstand episodes within the overall regressive sequence, whereas low values of these parameters characterize regressive intervals.
The lower Fairport Shale Member of the Carlile Fm and coeval strata in Iowa were deposited during a second-order sea-level
highstand, the waning stages of the Greenhorn cyclothem. An overall shallowing- and coarsening-upward sequence characterizes
the overlying majority of the Carlile Fm. This trend is punctuated by a short-term transgressive episode with associated retrograde
facies and a disconformity.
Earlier studies document relatively high productivity during the Turonian. Nutrient input to the seaway, required to sustain
water-column productivity, is difficult to account for solely by riverine inputs; thus, a model of transgressive flooding of
preconditioned, oxygen-deficient, nutrient-rich water from the global ocean into the KWIS is invoked. This advection of nutrients
and low-oxygen water also helped to create broadly distributed dysoxic to anoxic conditions in the seaway, which would otherwise
have been difficult to maintain in a relatively well-mixed, shallow sea. As the seaway regressed, river-supplied sea-surface
nepheloid layers provided sufficient nutrient inputs and occasionally established temporary stratification of the water column, and
thus contributed to maintaining an environment poised to produce and preserve organic matter.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Carlile Formation; Cretaceous Western Interior Seaway; Rock Eval hydrogen index; Sequence stratigraphy; Sea-level highstands
0031-0182/$ - s
doi:10.1016/j.pa
* Correspondi
E-mail addr
alaeoecology 235 (2006) 223–244
ee front matter D 2005 Elsevier B.V. All rights reserved.
laeo.2005.09.031
ng author. Tel.: +1 814 865 2213.
ess: [email protected] (T. White).
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244224
1. Introduction
The deposits of the KWIS rank among the best
studied foreland basin epicontinental sea systems be-
cause the well-exposed strata contain abundant fossil
fuel and a nearly complete climatic, eustatic and tec-
tonic record for the middle to Late Cretaceous of North
America. Although transgressive nearshore sandstone,
and limestone and shale of the KWIS, and the organic
matter contained therein, have been extensively studied,
less effort has been expended on the study of sediments
in the basin deposited offshore during regression; the
seemingly monotonous character of these strata has
deflected attention and led to a paucity of data and
interpretation. This lack of data for amounts and types
of organic matter buried during overall regressive epi-
sodes of the KWIS provided much of the motivation for
our study. We focus on the upper half of the Greenhorn
Fig. 1. A schematic cross section of the Cretaceous Western Interior Forel
lithofacies distribution map for the early Turonian highstand compiled from
Elder and Kirkland (1994), Gardner and Cross (1994), Ludvigson et al. (1994
cyclothem (early to middle Turonian, Late Cretaceous),
primarily composed of fine-grained sediments deposit-
ed during a major regressive phase in the history of the
KWIS. While this depositional setting is substantially
different than the well-studied pelagic deposits of
Greenhorn maximum transgression, the central basin
of the KWIS (Fig. 1) remained poised to produce,
bury and preserve large quantities of organic matter.
These deposits now exist as potential petroleum source
rocks. We elucidate patterns of lithostratigraphy, sedi-
ment fabric, Th/U, %TOC, organic matter type,
%CaCO3 and organic matter y13C in these overall
progradational sediments, and outline a basinal sedi-
mentation model to account for the trends using data
from three drill holes.
The U.S. Geological Survey (USGS) Portland No. 1
Core (P#1) was drilled in Fremont County, central
Colorado (CO), by the USGS with Department of
and Basin (modified from Kauffman and Pratt, 1985) compared to a
Hattin (1965), Witzke et al. (1983), Merewether and Cobban (1986),
). Cores: USGSP#1=Portland, ARB=Bounds, and HAW=Hawarden.
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244 225
Energy funding and is stored at the USGS Core Re-
search Center in Denver, CO; recovery was effectively
100% (Dean and Arthur, 1998). The Amoco Rebecca
Bounds No. 1 Core (ARB#1) was drilled in Greeley
County, western Kansas (KS), by the Amoco Produc-
tion Company. The Cretaceous portion of this core
(N90% recovery) is also stored in Denver (Dean and
Arthur, 1998). The Hawarden Core (HAW) is housed at
the Iowa (IA) Geological Survey Bureau’s Core Labo-
ratory, Iowa City, IA. The entire Carlile Fm. was not
penetrated in the HAW core; the early Turonian strata
are unconformably overlain by Quaternary deposits.
Overall core recovery through the extant portions of
the Carlile Formation (Fm.), and Greenhorn Fm., was
effectively 100% (Witzke and Ludvigson, 1994). Strata
penetrated in the three cores was deposited in marine
settings within the KWIS; the HAW core sediments
were deposited closer to the eastern paleoshoreline,
whereas the USGSP#1 core sediments were deposited
on the flank of a forebulge that acted as a barrier
between the western foredeep and eastern axial basin.
All three cores record overall seaway shoaling during
the Turonian.
1.1. Controls on Sedimentation in the KWIS
An assessment of the development of accommoda-
tion space must be made to fully understand the factors
controlling organic matter sedimentation and burial in
the KWIS during the Turonian. A major consideration
is to what extent basinal sea-level changes were in-
duced by regional tectonism vs. global (eustatic) pro-
cesses. During the middle Cretaceous, thrust loads were
emplaced in Nevada, Utah (UT), Wyoming (WY) and
Idaho by convergence at the western margin of North
America, thus creating the Sevier Mountains fold and
thrust belt. This crustal loading has been deemed re-
sponsible for isostatic subsidence and the development
of the Western Interior foreland basin (Jordan, 1981).
Active thrusting was mostly continuous in southwest-
ern and central UT during the Cenomanian to Conia-
cian (DeCelles, 1994; Goldstrand, 1994; DeCelles et
al., 1995); to the north extending into WY, active
thrusting had ceased by Cenomanian–Turonian time
(Villien and Kligfield, 1986). Similarly, although spatial
variations in subsidence rates determined by flexural
backstripping analysis are observable throughout the
foredeep, no temporal variation in subsidence rate
was calculated for the middle Turonian (Pang and
Nummedal, 1995). Thus, tectonism was important in
providing primary accommodation space for Carlile
Fm. deposition, but probably played no role in the
development of higher order stacking patterns within
the formation.
Merewether and Cobban (1986) mapped a lacuna
(Fig. 1) that White et al. (2002) surmised was formed
on a rising forebulge. The arguments in support of the
influence of a forebulge on patterns of sedimentation in
the Turonian are given in White et al. (2002) and are
only briefly summarized here. At times, the paleobathy-
metric forebulge high appears to have acted as a barrier
to sediment transport and water circulation between the
western paleoshoreline and foredeep in central and
southern UT and the central axial basin in CO, KS
and IA during deposition of the Carlile Fm. Although
extensive stratigraphic horizons containing TOC con-
tents up to 2% have been identified in Turonian fore-
deep strata, the organic matter type is characteristically
terrestrially derived Type III (Leithold and Dean, 1998;
White, 1999). For this reason we have focused on
Turonian strata of the axial basin where TOC values
range from 4% to 8%, with HI values mostly N600
indicative of marine organic carbon burial. In choosing
this focus, we have also substantially eliminated the
need to further consider the potential effects of subsi-
dence variations because the geochemical facies stack-
ing patterns we describe formed in a tectonically
quiescent distal offshore realm.
Ample conjecture based on intriguing evidence
exists indicating that climate played an important role
in the preservation of organic matter in the KWIS. For
example, many researchers have studied limestone/
marlstone couplets in the Bridge Creek Limestone
Member of the Greenhorn Fm. (e.g., Gilbert, 1895;
Barron et al., 1985; Fischer et al., 1985; Elder et al.,
1994, and many others), which some researchers have
suggested record orbitally forced changes in humid and
arid climate cycles and subsequent runoff to the seaway
(Arthur et al., 1984; Pratt, 1984). More recently, Sage-
man et al. (1998) suggested that constructive and de-
structive interference in the sedimentary expression of
orbital precession and obliquity best explained the bed-
ding pattern of the Bridge Creek Limestone. They
hypothesized that this interference reflected orbitally
forced variations in climate that controlled (1) carbon-
ate productivity through changes in lower latitude evap-
oration and nutrient upwelling and (2) siliciclastic
dilution through variations in higher latitude precipita-
tion. One common characteristic of many of the early
interpretations of the Bridge Creek couplets is that the
episodes of intensified runoff caused long-term stratifi-
cation of the seaway and, consequently, a better pre-
servational setting for organic matter settling to the
seafloor. We suggest that while fluctuations in runoff
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244226
probably played a role in sediment transport in the
KWIS, long episodes of water-column stratification
are not required to produce patterns of organic matter
preservation in the Turonian regressive facies. This
conclusion is consistent with similar recent interpreta-
tions made for the Bridge Creek Limestone Member of
the Greenhorn Fm. (Meyers et al., in press; Arthur and
Sageman, 2005). It is also consistent with results of
numerical modeling of circulation in the KWIS that
take into account large freshwater fluxes but were
unable to produce any long-term stratification (Slinger-
land et al., 1996; Kump and Slingerland, 1999).
Correspondence between the Haq et al. (1988) glob-
al eustatic curve and a KWIS relative sea-level curve
has been established (Kauffman and Caldwell, 1993).
The correspondence has subsequently been verified for
the Cenomanian–Turonian interval, although the mag-
nitude of sea-level change in the KWIS may have been
greater than that indicated in the Haq et al. (1988)
curve. The sea-level curves were constructed primarily
using observations of relative stacking between obvi-
ously marine and nearshore facies. However, in the
regressive facies described here, only subtle variations
in grain size may exist such that the imprints of relative
sea-level change can be elusive. We apply a holistic
geochemical approach to sedimentary facies analysis to
ascertain the effects of relative sea-level variation on
the study interval.
Fig. 2. Geochemical profiles of the Carlile Formation from the USGS Portla
Turonian transgressive interlude in the upper part of the Fairport Chalky Sh
2. Methods
A determination of total organic (TOC) and carbonate
carbon (%CaCO3) by carbon coulometry (Engleman et
al., 1985), and hydrogen (HI) and oxygen index (OI) by
Rock-Eval pyrolysis (Espitalie et al., 1977; Peters, 1986)
was made on samples obtained at 30-cm intervals from
all three cores (Figs. 2–4). In marine facies, low (b1%)
TOC values often characterize intervals containing pri-
marily terrestrial organic matter, whereas higher TOC
contents are most often attributed to marine organic
matter. The type of organic matter, as indicated by
Rock-Eval pyrolysis, also can provide a record of organ-
ic matter and host sediment provenance (Robert, 1985;
Peters, 1986), which can be used to unravel stratigraphic
stacking patterns and to understand the development of
accommodation space in a basin (White, 1999).
Widespread carbonate accumulations are often asso-
ciated with global sea-level highstands, so relative sea-
level rise and fall can greatly affect carbonate sedimen-
tation. Thick carbonate sequences are also deposited as
transgressive systems tracts, and relative sea level
change may affect carbonate sedimentation by subaerial
exposure and diagenesis (Tucker and Wright, 1990).
Relative increases in %CaCO3 values attributed to the
deepening of Cenomanian epeiric seas on the Russian
Craton (Ilyin, 1994) have been considered as a general
proxy for relative sea-level rise.
nd No. 1 core, near Florence, CO. Gray zone marks the lower middle
ale Member of the formation.
Fig. 4. Geochemical profiles of a portion of the Carlile Formation from the Hawarden D-7 core, near Hawarden, IA. Gray zone marks the lower
middle Turonian transgressive interlude in the upper part of the Fairport Chalky Shale Member of the formation. Note that no bentonites were
described in the core–the location of bentonites in the figure is based on lithostratigraphic correlation from nearby outcrops.
Fig. 3. Geochemical profiles of the Carlile Formation from the Amoco Rebecca Bounds core, in western Kansas. Gray zone marks the lower middle
Turonian transgressive interlude in the upper part of the Fairport Chalky Shale Member of the formation.
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244 227
Fig. 5. Relationship between % terrestrial organic matter (as deter-
mined by organic petrography) and hydrogen index, from the USGS
Portland No. 1 core, CO.
Fig. 6. Plots of % total organic carbon and hydrogen index vs. sediment fabri
for the USGS Portland No. 1 core. Note that the dashed lines on Th/U vs. %
and Weaver (1958).
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244228
Visual kerogen analysis was performed using stan-
dard organic petrographic techniques outlined in Taylor
et al. (1998); the results are presented in Fig. 5. Th and
U content was obtained by gamma-ray spectrometry at
30-cm intervals on uniformly sized core chips from the
ARB#1 and USGSP#1 cores (Figs. 6 and 7) using a
lead shield to block background radiation. Th/U values
N7 are considered as indicative of oxic conditions,
whereas Th/U b2 are interpreted as anoxic (Adams
and Weaver, 1958). Our profiling of the USGSP#1
and ARB#1 cores includes an assessment of sediment
fabric, or extent of bioturbation, in the cores, which
supports the Th/U inferences.
Carbon isotopic values for bulk organic carbon were
obtained by EA-IRMS in the Stable Isotope Biogeo-
chemistry Laboratory at Penn State. Samples were trea-
ted with buffered acetic acid to remove carbonate
minerals, freeze-dried and combusted in an elemental
c (from visual descriptions) and Th/U (from gamma-ray spectrometry),
TOC and HI plots mark oxic and anoxic regions delineated by Adams
Fig. 7. Plots of % total organic carbon and hydrogen index vs. sediment fabric (from visual descriptions) and Th/U (from gamma-ray spectrometry),
for the Amoco Rebecca Bounds core. Note that the dashed lines on Th/U vs. %TOC and HI plot mark oxic and anoxic regions delineated by Adams
and Weaver (1958).
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244 229
analyzer using standard techniques. Gases were intro-
duced into a Finnigan Delta XP for isotope ratio mea-
surement. Results were standardized using calibrated in
house standards and are reported with respect to VPDB
in standard y notation.
3. Discusion
The results of organic petrographic analysis of sam-
ples from the Carlile Fm. in the studied cores indicate
that the terrestrial organic component consists of vitri-
nite (woody material) and subordinate amounts of iner-
tinite (bcharcoalQ) and pollen and spores. Marine facies
contain amorphous organic matter, dinoflagellates, and
occasional chitinous inner linings of foraminifera
(White, 1999). A reasonably robust relationship exists
between increasing percent terrestrial organic macerals
and decreasing HI values through the Carlile Fm. as
expected; the trend is pronounced in the Blue Hill Shale
Member (Fig. 5). The overall trend combined with the
petrographic observations mentioned above suggests
that HI values record shifts in organic matter type
and/or preservation. Modified van Krevelen diagrams
indicate that the majority of the material analyzed in the
study contains Type II, marine-derived algal material
(MOM), with fewer horizons containing dominantly
Type III organic matter, i.e., terrestrially derived orga-
nic matter (TOM; White, 1999).
The relationship between %TOC- and HI-vs.-Th/U
in the USGSP#1 and ARB#1 cores (Figs. 6 and 7)
illustrates that low %TOC and generally low HI
(b200) values characterize the widest range of Th/U
values. However, the highest %TOC and HI values, and
highest mean values, are associated with the lowest Th/
U values, i.e., those intervals interpreted as having been
deposited under anoxic conditions (according to Adams
and Weaver, 1958). Figs. 6 and 7 also display plots of
%TOC- and HI-vs.-sediment fabric for both cores. A
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244230
wide range of %TOC and HI values is associated with
laminated intervals, whereas low %TOC and HI hor-
izons characteristic of oxic conditions at the seafloor are
associated with more intense bioturbation. These plots
suggest that marine organic matter enrichment is asso-
ciated with laminated intervals deposited under dysoxic
to anoxic conditions. Under dysoxic to oxic conditions,
sediments were bioturbated, and more refractory MOM
and/or terrestrial organic matter was preserved.
The y13Corg values averaging �26x primarily indi-
cate a marine source for the organic matter in the Fair-
port Shale Member. As indicated by Arthur et al. (1985)
and Dean et al. (1986), marine organic matter in the
Cretaceous, with some exceptions (e.g., associated with
the Cenomanian/Turonian Event, Arthur et al., 1988),
has more depleted y13C than terrestrial organic matter in
contrast to the Neogene to Recent relationship. This
appears to be the result of higher CO2 availability
(aqueous CO2) in Cretaceous surface waters. Thus, in
the absence of major changes in the y13C of total
dissolved inorganic carbon (monitored by carbonate
carbon), changes in y13Corg probably reflect changes
in the dominant type of organic matter. The higher
y13Corg (greater than �25x) of some intervals may
indicate predominance of terrestrial organic matter, for
example, in the upper Blue Hill Shale and Codell
Members in the USGSP#1 Core. This may be a residual
TOM after oxidation of MOM. In the HAW Core,
y13Corg varies little probably reflecting the dominance
of MOM preservation. Some intervals with more 13C-
enriched organic matter appear to correspond with more
oxidizing depositional conditions. This could indicate
residual refractory TOM or possibly minor isotope
effects of MOM oxidation.
3.1. Lithology and geochemistry of major stratigraphic
units
We used the lithostratigraphy for the Pueblo-Canon
City area presented in Kauffman and Pratt (1985) com-
bined with lithostratigraphy of the Bounds and Portland
cores (Dean and Arthur, 1998), and lithostratigraphy and
biostratigraphy for the Bounds Core (Scott et al., 1998;
Bralower and Bergen, 1998), Portland Core (Bralower
and Bergen, 1998; White, 1999) and Hawarden Core
(Witzke and Ludvigson, 1994) to develop a chronostra-
tigraphy for the cores. The lithostratigraphy, based on
core descriptions (Fig. 8), is similar to previously estab-
lished, generalized regional stratigraphy (Hattin, 1965;
Kauffman, 1977). A detailed discussion of lithologic and
geochemical variations through the USGSP#1 and
ARB#1 cores is presented in White (1999).
In CO and KS, fine-grained early to middle Turonian
facies are composed of offshore, calcareous to non-
calcareous, dark black to bluish-gray, fossiliferous and
non-fossiliferous shale and siltstone with carbonate
concretion horizons and bentonites of the Fairport
Shale (Sh)/Chalky Sh and Blue Hill Sh Members
(Mbr) of the Carlile Fm. The same generalized stratig-
raphy has been recognized in Iowa (Ludvigson et al.,
1994). However, while lithologic logs of the HAW core
have been published (Whitley and Brenner, 1981;
Witzke and Ludvigson, 1994), member level terminol-
ogy has not been applied to the core. For reasons
described below, we have concluded that most of the
Carlile Fm. portion of the HAW core is the Fairport Sh
Mbr of the Carlile Fm.
We applied the biostratigraphic ages of Kauffman et
al. (1993) to compare our results from the cores to the
Haq et al. (1988) curve. Observations of the ammonite
Collignoniceras woolgari, which primarily exists in the
Fairport Sh Mbr and coeval strata, were especially
important for constructing a member-level correlation
between the three cores: White (1999) and Scott et al.
(1998) documented the occurrence of C. woolgari in
the USGSP#1 and ARB cores, respectively, while C.
woolgari was found in much of the study interval in the
HAW core; Witzke and Ludvigson (1994) delineated
the entire Carlile Fm. portion of the HAW core as
falling in the C. woolgari biozone.
The Fairport Sh Mbr of the Carlile Fm. was depo-
sited during the waning stages of maximum sea-level
highstand (global highstand T6) and the subsequent
regression in the KWIS, while a second, less extensive
transgression occurred during late Fairport Sh Mbr time
(see Kauffman and Caldwell, 1993). The Blue Hill Sh
Mbr of the Carlile Fm was deposited during relative
sea-level fall (R6). Along the central eastern margin of
the KWIS, i.e., Minnesota, South Dakota, IA and
Nebraska, the overall upward-coarsening regressive
succession is recognized as the Fairport Chalky Sh,
and Blue Hill Sh, that overlies the open-marine carbon-
ate strata of the Greenhorn Formation (Witzke et al.,
1983).
Twenty-one meters of mostly laminated, calcareous
mudstone of the Fairport Sh Mbr of the Carlile Fm.
overlie the Bridge Creek Mbr in the USGSP#1 core,
whereas 37 m of marlstone compose that member in the
ARB#1 core (Fig. 8). In these cores, bentonites, calcar-
enites, and multiple fecal pellet-, fish debris-, shell
fragment-, and foraminiferal- and inoceramid-rich ho-
rizons were observed in discrete intervals of the upper
and lower portions of the member. In the USGSP#1
core, these upper and lower intervals of the Fairport Sh
Fig. 8. Lithostratigraphic correlation for the axial basin of the Turonian Western Interior Seaway including a tie to the Haq et al. (1988) eustatic curve using the biostratigraphic ages of Kauffman et
al. (1993). Note that (1) the eustatic sea level curve is scaled to the Bounds core, and (2) the bentonite thicknesses are not to scale; real thicknesses range from 5 to 20 cm.
T.White,
M.A.Arth
ur/Palaeogeography,Palaeoclim
atology,Palaeoeco
logy235(2006)223–244
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T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244232
Mbr also display relative y13Corg depletion in those
horizons with the most elevated HI values. For exam-
ple, increased TOC and HI values in these zones cor-
respond to trends toward depletion, whereas y13Corg
enrichment exists where TOC and HI drop; variations
are subtle because of the similarity between Cretaceous
TOM and MOM (Arthur et al., 1985; Dean et al.,
1986]). Carbonate content decreases upsection in the
lower Fairport Sh Mbr in both cores (Figs. 2 and 3).
%TOC mostly varies inversely to %CaCO3 in the
USGSP#1 core, whereas no consistent relationship
was observed in this section of the ARB#1 core. Th/
U values determined for the Fairport Sh Mbr in the
USGSP#1 core are indicative of dysoxic to oxic envir-
onments of formation (Fig. 2). Low values of Th/U in
the upper Fairport Sh Mbr coincide with the highest
%TOC, and correspond primarily to Type II organic
matter on the basis of visual kerogen analysis and HI in
the member. On the basis of sediment fabric, the Fair-
port Sh Mbr in the ARB#1 core (Fig. 3) is characterized
by more intense anoxic episodes than the USGSP#1
core (Fig. 2), a trend in keeping with observations for
the Bridge Creek Limestone (Savrda, 1998).
An abrupt upsection increase in %CaCO3 and
%TOC is observable at 122 m in the USGSP#1 core
(Fig. 2), which continues to 116 m (top of the Fairport
Sh Mbr), and contains the highest average HI values
and relatively depleted y13C values; this zone corre-
sponds to the upper bentonite-containing and bioclastic-
rich zone described previously. This zone appears to
correlate to a similar interval at ~247–257 m in the
ARB#1 core (Fig. 3) on the basis of gross stratigraphic
trends. At least four bentonites appear in this zone in
both the USGSP#1 and ARB#1 cores (Fig. 8).
Only 19 m of the Carlile Sh were recovered in the
HAW core, consisting of thinly laminated, calcareous
shale and mudstone, with silty laminations and intervals
containing pelecypods, ammonites, fish fragments and
carbonized woody plant debris. Nearby outcrops of the
Carlile Sh, in the Big Sioux River Valley, northwest IA,
are dominated by silty shale that becomes less calcar-
eous upward in the sequence; inoceramid and plank-
tonic microfossils exist within discrete beds (Witzke
and Ludvigson, 1987). These lithologic attributes are
grossly similar to those observed for the Carlile Fm. to
the west. The geochemical profiles for the HAW core
(Fig. 4), which display an upward %CaCO3 decrease
overlain by a zone of somewhat elevated values, appear
to indicate that the Carlile Fm. in the HAW core is
correlative with the Fairport Sh Mbr in the ARB#1 and
USGSP#1 cores. Bentonites in Big Sioux River Valley
outcrops (Ludvigson et al., 1994), correlative to the top
of the HAW core by lithologic profiling, indicate that
the upper zone of elevated carbonate content in the
HAW core is correlative to the high %TOC/%CaCO3/
HI zone, which also contains bentonites, in the ARB#1
and USGSP#1 cores. The correlation is corroborated by
the observations of C. woolgari through much of the
core (Witzke and Ludvigson, 1994). A prominent fea-
ture of the geochemical profiles for the HAW core is a
zone of depressed HI values, suggestive of terrestrial
organic matter preservation, and depressed %CaCO3
values from 107 to 120 m that surrounds a narrower
interval of relatively enriched y13Corg values. The base
of the Blue Hill Sh Mbr in the HAW core (Fig. 4) is
assigned to the level of the abrupt drop in %CaCO3
above the high %CaCO3 zone at 76 m in the core.
In the ARB#1 core, the Fairport Sh Mbr is grada-
tionally overlain by 9 m of laminated mudstone that
coarsens upward to moderately laminated siltstone and
fine-grained sandstone of the Blue Hill Sh Mbr of the
Carlile Fm. (Fig. 8). Here, the Blue Hill Sh Mbr also
contains calcareous concretions. The steady decline in
%TOC and %CaCO3 observed in the upper Fairport Sh
Mbr continues across the Fairport Sh Mbr/Blue Hill Sh
Mbr contact and through the Blue Hill Sh Mbr in the
core (Fig. 3). HI values are interpreted as reflecting
preserved marine organic matter mixed with terrestrial
organic matter. Th/U ratios in the Blue Hill Sh Mbr in
the ARB#1 core range from anoxic in the lower half to
oxic in the upper half.
Twelve meters of the Blue Hill Sh Mbr overlie the
Fairport Sh Mbr in the USGSP#1 core; phosphate
nodules mark the contact (Fig. 2) and are discussed
later. Here, the Blue Hill Sh Mbr consists primarily of
silty mudstone in a coarsening-upward package. The
basal half is moderately laminated; the upper half is
mostly bioturbated. The base of the Blue Hill Sh Mbr in
Fig. 2 is the abrupt drop in %CaCO3 above the high
TOC/CaCO3/HI zone at the top of the Fairport Sh Mbr
in the USGSP#1 core. Values for %TOC and HI de-
cline, and y13Corg values become enriched, to the top of
the member.
4. Depositional processes and facies
4.1. Relative sea level
A primary control over the distribution of total
organic and carbonate carbon and organic matter type
for each core can be inferred by comparing the sea-level
curve to Figs. 2–4. This approach is supported by
previous research comparing sequence stratigraphy to
variations in sea level that hypothesized a correspon-
Fig. 9. Plot of bulk accumulation rate compared to %CaCO3 for the
three study cores. The relationships indicate that higher carbonate
productivity occurred with lower clastic dilution during deposition of
the Fairport Shale Member, whereas greater clastic input during Blue
Hill Shale Member deposition was accompanied by a reduction in
carbonate productivity. Fairport Shale Member depositional processes
on the eastern shelf (HAW core) more closely resembled those of the
Blue Hill Shale Member in the open-marine basin.
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244 233
dence between stacking patterns in the Cretaceous
Western Interior Basin and the Haq et al. (1988)
curve (Sageman, 1996). Relatively high %TOC,
%CaCO3 and HI values (and relatively depleted
y13Corg values in the USGSP#1 core) correspond to
transgressive or highstand episodes on the sea level
curve as correlated, whereas low values of these para-
meters characterize regressive lowstand intervals. In
general, the highest carbonate and organic carbon con-
tents, and most depleted y13Corg values, exist within the
Upper Bridge Creek Mbr (absolute highstand of the
Greenhorn cyclothem; Meyers et al., in press; Arthur
and Sageman, 2005) to lowermost Fairport Sh Mbr
transition, and during a subsidiary transgressive episode
in Upper Fairport Sh Mbr time. The lowest carbonate
and organic carbon contents, relatively enriched y13Corg
values, and low HI values indicative of terrestrially
derived organic matter, exist within progradational
and lowstand deposits of the Blue Hill Sh.
In the HAW core from the eastern margin of the
KWIS, generally lower values for %TOC, %CaCO3
and HI, and a narrower range in y13Corg were observed
relative to those characterizing the USGSP#1 and
ARB#1 cores (compare Figs. 2–4). Nonetheless, some
similarity in the stratigraphic distribution of %CaCO3 is
apparent in the three cores, and, combined with litho-
logic and biostratigraphic observations, suggests that a
control on carbonate content of eastern margin strata of
the KWIS may also be demonstrated. In the HAW core,
the highest carbonate and organic carbon contents exist
within the Greenhorn Fm., whereas %CaCO3 values
substantially greater than the Carlile Fm. mean for the
core are observable in a zone near the top of the HAW
core. We surmise that this zone was deposited during
the subsidiary transgressive episode observed in the
Upper Fairport Sh Mbr in the USGSP#1 and ARB#1
cores.
A plot of %CaCO3 vs. bulk accumulation rate
(BAR) for member subunits of the cores is shown in
Fig. 9. Bulk accumulation rate was calculated by mul-
tiplying linear sedimentation rates by a mean dry bulk
density of 2.65 gm/cm3. Linear sedimentation rate for
the Bounds and Portland cores was determined using
lithostratigraphic member-level subdivisions (Dean and
Arthur, 1998) and the biostratigraphic stage zonations
of Kauffman et al. (1993), whereas the lithostratigraphy
of Witzke and Ludvigson (1994) was applied for the
Hawarden Core.
Although porosity and therefore dry bulk density
varies, our calculations are not point by point, but
over stratigraphic intervals. Therefore, we contend
that the application of mean values for the intervals is
appropriate though the values may be in error by 10%.
This approach is admittedly crude and fraught with
uncertainty, but is based on the best available data.
In the USGSP#1 and ARB#1 cores, the Blue Hill Sh
Mbr consists of low values for both %CaCO3 and BAR,
whereas the Fairport Sh Mbr has low BAR values but
relatively high %CaCO3. The high carbonate contents
and low BAR are suggestive of somewhat higher car-
bonate productivity with little clastic dilution for the
Fairport Sh Mbr; clastic input increased in Blue Hill Sh
time reflected in the large %CaCO3 decrease, but BAR
remained about the same as that established in the
underlying Fairport Sh Mbr. This observation suggests
that clastic dilution corresponded to a carbonate pro-
ductivity decrease from the Fairport Sh Mbr to the Blue
Hill Sh Mbr. In the HAW core, generally low values for
both BAR and %CaCO3 suggest that the Fairport Sh
Mbr on the eastern margin is more similar to the Blue
Hill Sh Mbr in the axial basin.
%TOC vs. linear sedimentation rate is presented in
Fig. 10. At first glance, data for the Blue Hill Sh and
Fairport Sh Mbrs in all three cores plot as a cluster of
points characterized by low %TOC values and low
sedimentation rates. A closer inspection of these rela-
tionships reveals enhanced organic matter production
occurred in the more distal offshore setting of the Fair-
port Sh Mbr, whereas in the Blue Hill Sh, sedimentation
and dilution controlled organic matter preservation.
A general trend of covariance in mass accumulation
rates (MAR) for carbonate and total organic carbon can
be observed (Fig. 11). Stratigraphically, an increase in
Fig. 10. Plot of linear sedimentation rates compared to %TOC for the
three study cores. The Fairport Shale Member records higher organic
productivity, whereas during Blue Hill Shale time, organic matter
preservation was facilitated by sedimentation and dilution.
ig. 11. Plot of mass accumulation rates (MAR) according to relative
tratigraphic position. Transgressive intervals (defined by elevated
AR values) correspond to episodes of higher primary production
ith less clastic dilution. The record in the HAW core indicates that
arbonate productivity increased during transgression without accom-
anying increases in organic matter production.
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244234
%CaCO3 and %TOC MARs from the lower Fairport to
the upper Fairport Sh Mbr, followed by a decrease in
both parameters into the lower Blue Hill Sh Mbr, is
present. These trends, using the conclusions from Figs.
9 and 10, demonstrate that higher %CaCO3 and %TOC
MAR values from transgressive intervals in the
USGSP#1 and ARB#1 cores are indicative of higher
primary production by carbonate and organic-carbon-
producing organisms, with less clastic dilution at the
seafloor. This conclusion has also been made with
respect to the Bridge Creek Limestone (Meyers et al.,
in press). In our study, the observed MAR increases
might be explained by enhanced nutrient flux by incur-
sion of a Tethyan oxygen minimum zone (discussed
below), and condensation from up-dip coastal sediment
trapping. In the HAW core, primary production by
carbonate-secreting organisms increased during trans-
gression, while less organic carbon was preserved in
seafloor sediments; these observations can be explained
by coastal sediment trapping but in this case with no
increase in nutrient flux because circulation to the
shallow eastern shelf was beyond the effect of the
advected oxygen minimum zone. The conditions in
which this setting developed are described below.
4.2. Oceanic anoxic events and advection of
extrabasinal nutrient-rich water
Secondary effects of eustasy during the late Ceno-
manian and early Turonian included the expansion of
oceanic oxygen deficiency associated with so-called
oceanic anoxic events (OAEs). During these episodes,
a wide variety of oceanic sediments, including U.S.
Western Interior Basin strata, sequestered large quanti-
ties of marine-produced organic carbon (Schlanger et
al., 1987). Arthur et al. (1987) emphasized that eustasy
may have been bthe driving force for and common link
in the originQ of high %TOC strata of the Cenomanian–
Turonian OAE. They suggested that major transgres-
sion bmay also have raised the upper part of a midwater
oxygen-minimum zone onto the shelf...thereby aiding
in the development of more widespread oxygen
deficiency,Q as well as having instigated greater rates
of oceanic overturn that upwelled nutrient-rich water
and intensified mid-water oxygen minimum zones.
Low sediment Th/U values and preservation of fine
lamination (lack of bioturbation; Figs. 6 and 7) indicate
the existence of delicately poised, dysoxic conditions in
the KWIS low enough for burial of large quantities of
marine-derived algal matter during the Turonian. A
substantial proportion of the organic matter flux from
the KWIS surface waters probably arrived at the sea-
floor since remineralization was inhibited during the
short transit through the relatively shallow water col-
umn. Elevated surface water productivity may have
caused increased fluxes of organic matter to sediments.
The nutrient input to the seaway from rivers alone was
likely insufficient to support high productivity and an-
oxia especially during transgressive episodes when
nutrients would tend to have been sequestered in near-
F
s
M
w
c
p
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244 235
shore settings. Thus, we favor an additional nutrient
source: preconditioned, oxygen-deficient, relatively nu-
trient-rich water may have been advected into the KWIS
(Fig. 12) during transgression, a process previously
invoked to describe the development of organic C-rich
horizons in the Bridge Creek Limestone Mbr (Arthur
and Sageman, 2005). This mechanism provides addi-
tional nutrients and explains the development of widely
distributed oxygen-poor conditions, which are difficult
to explain by long-term stratification in a relatively well-
mixed, shallow sea (Slingerland et al., 1996).
As this poorly oxygenated, nutrient-rich Tethyan
water may have been advected into the KWIS as the
result of an estuarine circulation pattern (Slingerland et
al., 1996), water mass flow from the south (Tethys)
would have been directed to the north toward the
eastern margin of the KWIS. Our data sets support
these modeling results. The highest values for
%CaCO3, %TOC and HI exist in the ARB#1 core in
those transgressive intervals when the advective pro-
cess is proposed to have occurred; the ARB#1 core is
positioned directly within the region of Tethyan inflow
predicted by Slingerland et al. (1996). These same
principles were applied to explain the distribution of
organic matter and ichnocoenoses in the Bridge Creek
Limestone Mbr of the Greenhorn Fm. (Savrda, 1998;
Arthur and Sageman, 2005). TOC and HI increase (and
y13Corg becomes enriched) in coeval strata of the
USGSP#1 core, though not as pronounced as in the
ARB#1 core. This productivity increase can be
explained by caballing of easterly sourced waters, and
surface mixing associated with a large cyclonic gyre
(Slingerland et al., 1996). The lower TOC and HI
values observed in the HAW core, and narrower
range in y13Corg values, are a function of deposition
on the shallower eastern inner shelf inboard of the
influence of Tethyan inflow to the seaway and the
eastern shelf depositional regime dominated by conti-
nentally derived sediment and organic matter.
4.3. Coastal and bottom currents and nepheloid layers
More abundant and thicker calcarenites in the retro-
grade facies of the Fairport Sh Mbr in the ARB#1 core
occur at the same general stratigraphic levels as calcar-
enites and calcisiltstones in the USGSP#1 core (see Fig.
8; White, 1999). A close stratigraphic association of the
calcarenites, or skeletal limestones, within intervals
high in %TOC, HI and %CaCO3 intervals suggests a
shared mechanism of formation.
One explanation for this association is that the avail-
ability of carbonate detritus on the seafloor was greater at
these times. The calcarenites most often contain forami-
niferal tests, inoceramid prisms, and fecal pellets, fea-
tures that are compatible with the high %CaCO3, %TOC
and HI nature of the transgressive intervals, as is the
observation of more abundant inoceramid remains with-
in the strata surrounding the calcarenites. A second
explanation is that these intervals are condensed hori-
zons, i.e., lower rates of sedimentation during transgres-
sion, allowed for longer periods of seafloor winnowing.
Sageman (1996) interpreted similar Greenhorn Fm.
skeletal limestones as formed through bwinnowing by
storm events during relative sea-level fall and conden-
sation due to starvation during subsequent riseQ. Thismodel of sedimentation is difficult to resolve with the
observation that water depths in the seaway during
Cenomanian/Turonian maximum transgression are con-
sidered to have been at least 300 m (Eicher, 1969;
Asquith, 1970; Sageman and Arthur, 1994), a water
depth at which even storm wave energy is generally not
transmitted to the seafloor. Furthermore, the Sageman
(1996) model invokes sea-level falls ranging from N50
to 150 m to explain shallower water depths at which
storm wave-induced winnowing formed skelelal lime-
stones. Although the model does not offer a mechanism
for the 100-m sea level change, the regressive episodes
are shown to correspond to regressions on the Haq et al.
(1988) sea-level curve (Sageman, 1996). This observa-
tion is highly suggestive of a global (eustatic) mecha-
nism for the sea level falls, even though it is difficult to
invoke such large ice volume effects on sea level in a
virtually non-glaciated Turonian world (Barron, 1983,
1994; and many others), and the time scale for mid
ocean ridge spreading-induced sea-level variations is
much greater than those studied by Sageman (1996).
Thus, we conclude that the magnitude of sea-level fall
required by the Sageman (1996) hypothesis is unlikely
and thus, we find little direct support for the lowstand
tempestite model. We favor an alternative hypothesis
for the formation of Carlile calcarenites by winnowing
from bottom currents for the following reasons.
1) Isopach maps of the lower Carlile Fm. in the Denver
Basin (Weimer, 1978, and Weimer and Sonnenberg,
1983) indicate that much less sediment accumulated
in the early Turonian Front Range region of CO (up to
30 m) than at the Frontier depocenter (~90 m), or so
called bVernal DeltaQ (Hale, 1961), to the north inWY
(see Fig. 1 for locations). In addition, a mid-Cenoma-
nian through early Turonian hiatus in central CO and
southern WYoccurs south of the depocenter and west
of the Front Range (Merewether and Cobban, 1986).
These attributes may be evidence for riverine-induced
Fig. 12. Schematic conceptual model of the primary factors effecting sedimentation in the KWIS (Turonian) of CO, KS and IA. Bottom current winnowing of a paleobathymetric structural high led
to the development of a lacuna in north-central Colorado and Wyoming, as well as the formation of calcarenites at the P and B locales when bottom currents reached these distal locations; fine
materials were advected from the region by bottom current flow to deeper water. (a) During episodes of sea-level highstand and low river discharge, the main source of nutrients to the seaway was an
advected, nutrient-rich Tethyan watermass, whereas (b) during high river discharge, nutrients were also derived from riverine input; surface nepheloid plumes may also have contributed to water-
column stratification at this time. During lowstand (c and d), the advected water mass withdrew from the KWIS; sedimentation was affected primarily by fluctuations in river input; water-column
stratification occurred during high river discharge settings. Note: not to scale; H=Hawarden D-7 core, B=Amoco Rebecca Bounds 1 core, and P=USGS Portland No. 1 core.
T.White,
M.A.Arth
ur/Palaeogeography,Palaeoclim
atology,Palaeoeco
logy235(2006)223–244
236
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244 237
dilute suspensions and current flow in the seaway. For
example, Slingerland et al. (1996) used seaway cir-
culation models to, among other things, interpret the
lacuna as having formed by bottom-current erosion in
a shallow region of the KWIS, while White et al.
(2002) suggested that bottom-current erosion swept
a forebulge (USGSP#1 region) forming the lacuna,
and sediment was bypassed to the east where sedi-
mentation continued offshore in the ARB#1 region.
The bypassed sediment may account for, in part, the
isopach trends discussed next.
2) Weimer and Sonnenberg’s (1983) isopach map for
the lower Carlile Fm. shows the thickest accumula-
tion in a southeast-trending belt from the Frontier
depocenter in WY to eastern CO. Their cross sections
show southeast-dipping prograding clinoforms
(interpreted from wireline logs) where the Fairport
and Blue Hill Sh Mbrs are thickest near the Frontier
source. The ARB#1 locality is southeast of their
isopach map, but within the trend of thicker lower
Carlile Fm. accumulation. A simple explanation for
the lower Carlile Fm. belt of greater accumulation is
distal sediment dispersal from riverine-induced
nepheloid layers introduced to the seaway from the
Frontier depocenter and swept to the south by long-
shore drift associated with proposed coast-parallel
currents in the early Turonian (Slingerland et al.,
1996), in accord with the southeasterly clinoform
dips reported by Weimer and Sonnenberg (1983).
In this manner the observations are similar to pro-
cesses operating on the East Texas continental shelf
driven by outflow from the Colorado and Brazos
Rivers. In the water column of the East Texas conti-
nental shelf, high-energy coastal processes and high
sediment input from rivers maintain nepheloid layers,
while resuspended fine material is advected from the
bottom layer at the outer shelf (Sahl et al., 1987).
3) Calcareous nannoplankton in the Fairport Sh Mbr
were restricted to the southwest and central portions
of the seaway (Watkins et al., 1993). Watkins et al.
(1993) interpreted this restricted distribution as in-
dicative of somewhat less saline surface waters as-
sociated with outflow from rivers that produced the
Frontier depocenter to the north and west. This
observation further suggests that outflow from the
bVernal DeltaQ (Frontier depocenter in WY) was a
significant force in shaping sediment and microfossil
distributions in the seaway at great distances from
the delta.
In the modern Atlantic Ocean, the capacity of bottom
currents to resuspend and redeposit seafloor sediment
varies with current velocity (Biscaye and Eittreim,
1977). In the KWIS, flooding and deepening of the
seaway during the T6 eustatic highstand may have led
to intensified regional rainfall (Barron and Washington,
1982). Under these conditions, increased rainfall would
have increased riverine discharge to the seaway, ampli-
fied coast-parallel current flow, and increased the energy
and winnowing capacity of bottom currents. Nepheloid
plumes may have caused higher water column turbidity
and degraded living conditions for carbonate-producing
organisms. At the same time, current winnowing of the
seafloor was most effective and fine material (including
organic matter) was resuspended and advected from the
original site of deposition. On the forebulge, these pro-
cesses led to the formation of the aforementioned lacu-
na, whereas further offshore, calcarenites resulted from
these conditions. The stratigraphic distribution of the
calcarenites indicates that these processes would have
been most influential during sea-level highstands, in line
with Barron and Washington’s (1982) interpretation of
amplified precipitation during highstand.
Occasional phosphate nodule horizons exist in the
intervening strata between calcarenites in the Fairport
Sh Mbr in the ARB#1 core and consist of higher
%TOC than adjacent beds. Low Th/U values for the
laminated to moderately laminated strata are suggestive
of poorly oxygenated bottom-water conditions. This
interpretation is supported by the observations of Mac-
farlane et al. (1989) and Doveton (1991), who inter-
preted Th/U results for the Fairport Chalk in central KS
as indicative of formation in an anoxic to dysoxic
setting. The phosphate nodules are associated with
pyrite, inoceramids, planktonic foraminifera, and fecal
pellets. The abundance of planktonic foraminifera and
fecal pellets indicate that surface water productivity was
active at this time.
The T6 peak highstand and subsidiary transgression
during upper Fairport time each encompass hundreds of
thousands of years. Even during these longer term
highstand events, higher order relative sea-level low-
stands occurred in which the amplified hydrologic cycle
of the overall highstand would have been reduced. At
these times, and during the majority of Fairport time
when sea level was falling and the breadth of the
seaway was shrinking, waning river discharge and bot-
tom current velocity diminished the capacity of bottom
currents to erode and transport sediment. The phosphate
nodules likely formed by increased delivery of organic
matter to the seafloor, or perhaps by the liberation of P
from Fe–Mn oxyhydroxides during dysoxia/anoxia
(Sanudo-Wilhelmy et al., 2004). Organic matter settled
to a dysoxic to anoxic seafloor less hospitable to ben-
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244238
thic fauna and more amenable to organic matter pres-
ervation. Occasional winnowing of the seafloor by a
weak bottom current was sufficient to leave phosphate
pebble lags. It was under these conditions that much of
Carlile sedimentation occurred.
While the advection of extrabasinal water to the
KWIS provides an attractive mechanism for explaining
elevated productivity and dysoxic to anoxic conditions,
it is important to recall that overall, the Fairport Sh Mbr
of the Carlile Fm. was deposited during regression.
Therefore, the effects of an advected water mass were
waning as the sea withdrew from the basin, and other
processes may have overlapped with the lagging
advected water to create a seafloor environment poised
to preserve organic matter. One mechanism is the in-
creasing relative importance of river inputs as the sea-
way shallowed. This conclusion is similar to those of
Arthur and Sageman (2005), who attributed enhanced
stratification and higher organic matter productivity to
high fluvial input in Western Interior Basin shales prior
to deposition of the Greenhorn and Carlile Fms.
Rabalais et al. (1991) reported that the Mississippi
River creates seasonally stratified nearshore waters that
flow offshore along the Louisiana and Texas coastline,
and Sen Gupta et al. (1996) found seasonal bottom-
water oxygen depletion of the Louisiana continental
shelf, driven by water-column stratification and phyto-
plankton productivity, which they attributed to nutrient
loading from Mississippi and Atchafalaya River dis-
charges. Even during episodes of low river discharge,
remnant surface plumes, like those observed at Barba-
dos from periods of increased Amazon River discharge
(Kidd and Sander, 1979), may have been present. Given
recovery periods of several years for benthic commu-
nities subject to infrequent oxygen depletion (Boesch
and Rabalais, 1991), an occasionally stratified water
column with dysoxic to anoxic bottom waters could
conceptually produce a sedimentary record of oxygen
depletion, even though oxic conditions may exist in the
intervening times between oxygen depletion events. A
model for occasional (seasonal?), but not necessarily
annual, water-column stratification during deposition of
the Fairport Sh Mbr can be envisioned and is more
palatable than earlier models invoking permanent strat-
ification of the seaway (Arthur et al., 1984; Pratt, 1984),
particularly considering the well-mixed nature of the
relatively shallow water column of the KWIS.
5. Depositional history
A gradual regression and resulting shallower basin
occurred during Fairport Sh Mbr deposition. The lower
part of the member in the Front Range of CO has been
interpreted as being deposited under normal marine
conditions in warm, well-circulated, oxic bottom
water (Glenister and Kauffman, 1985). As eustatic sea
level fell and reduced the breadth and depth of the
KWIS during lower Fairport Sh Mbr deposition, the
effects of an advected oxygen-poor and nutrient-rich
bottom-water mass, introduced from the global ocean
during the Cenomanian–Turonian boundary OAE, were
also reduced; as sea level fell below a critical sill depth
these waters were no longer advected into the KWIS
and the effects of riverine discharge-induced nepheloid
layer stratification and dysoxia discussed above became
a more important factor in controlling organic produc-
tivity and preservation in the seaway.
Sediments in the HAW core deposited during Fairport
time was likely deposited by the influx of prodelta mud in
quiet water under nearshore marine conditions (Whitley
and Brenner, 1981). Whereas relatively lower values for
%TOC and HI were observed in the HAW core compared
to the more westerly study cores, the values for these
parameters in the core indicate that marine productivity
occurred along with substantial inputs of terrestrially
derived or oxidized marine organic matter. This observa-
tion explains the much lower values for %CaCO3 in the
core relative to the ARB#1 and USGSP#1 cores; fewer
carbonate-secreting organisms may have survived in the
turbid and/or lower salinity water. Laminated to moder-
ately laminated strata in the HAW core are suggestive of
poorly oxygenated bottom-water conditions at this time.
In the ARB#1 core, lower %CaCO3 values in the
lower Fairport Sh Mbr relative to the Bridge Creek
Mbr are probably due to dilution by an increase in
terrigenous flux associated with eustatic sea level fall.
The overall decrease in %CaCO3 upsection is compati-
ble with observations of declining trends in planktic and
benthic foraminiferal numbers and diversity through the
Fairport Sh Mbr (Eicher and Diner, 1985). They inter-
preted the trends to be indicative of an oxygen-depleted
seafloor uninhabitable by benthic forms, with an over-
lying surface layer unable to sustain abundant planktic
forms. A modern example of this phenomenon may be
manifest as subdued populations of carbonate-secreting
organisms in turbid, high-nutrient water (Hallock, 1987).
In addition, however, %CaCO3-vs.-BAR relation-
ships (Fig. 9) demonstrate that clastic dilution also
had a role in declining carbonate content upsection
through the Fairport Sh Mbr. During this time, sedi-
mentation in the USGSP#1 region was dominated by
calcareous mud, whereas further offshore in the ARB#1
region, marlstones were deposited. Sedimentary fabrics
of the Fairport Sh Mbr in both cores (Figs. 2 and 3)
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244 239
exhibit an upsection change from poorly laminated to
laminated, though the trend is best displayed in the
ARB#1 core, an observation supported by Eicher and
Diner’s (1985) microfossil-based interpretation of oxy-
gen depletion upsection through the member.
Organic matter preservation at the seafloor was high
during Fairport time. Water-column productivity may
have been sufficient, at times, to induce and maintain
bottom-water dysoxia. We expected an upsection TOM
increase in the progradational setting as strata recorded
shoreline (TOM source) migration closer to the depo-
sitional site. However, HI and OI values are indicative
of mostly MOM with less preservation of TOM, a
continuation of organic matter sedimentation/preserva-
tion style in the underlying upper Bridge Creek Mbr.
Perhaps productivity was so high in Fairport Sh Mbr
time that the terrestrial signal was swamped by marine
fluxes. These factors led to an increase in production
and preservation of MOM.
Marlstone deposition in the ARB#1 core region was
interrupted by calcareous mud, whereas in the
USGSP#1 core calcareous mudstone is overlain by
poorly laminated, non-calcareous mud- and claystone.
Eicher and Diner’s (1985) identification of arenaceous
benthic foraminifera, which they interpreted as indica-
tive of reduced salinity within strata coeval to the
lithologic change from marlstone to calcareous mud
in the ARB#1 core, supports a progradational interpre-
tation for the calcareous mud. We suggest that the
prominent zone of depressed HI values and relatively
enriched y13Corg values in the HAW core is the eastern
shelf correlative to this progradational interval.
Marlstone deposition in the ARB#1 core, and lami-
nated calcareous mudstone in the USGSP#1 core,
returned during a transgressive pulse near the top of the
unit, when the eustatic and KWIS sea-level curves (see
Kauffman and Caldwell, 1993) show a sea-level rise
affected the region during upper Fairport Sh Mbr time.
Low Th/U values in this interval of the Fairport Sh Mbr
in the USGSP#1 core suggest a dysoxic to anoxic setting
and coincide with the highest %TOC (Type II organic
matter) in the member and laminated calcareous mud-
stone, bentonites, and a horizon of phosphate nodules.
This zone correlates to the zone of high %TOC,
%CaCO3 and MOM observed from ~247 to 257 m in
the ARB#1 core, which is also moderately to well lam-
inated, and contains bentonites. A renewal of sediment
trapping along the coast may have provided a more
hospitable environment for calcareous-secreting plank-
tonic organisms in the offshore realm, as increased water
depth and lower sedimentation were manifested as great-
er organic matter and carbonate preservation. The return
of dysoxic to anoxic conditions may also signal an
incursion of an oxygen-minimum zone from the global
ocean into the KWIS with nutrient bconcentrationQ alongthe seaway’s eastern margin through estuarine circula-
tion. Contemporaneous lower values for TOCMAR and
elevated values for CaCO3 MAR in the HAW core
indicate a renewal of carbonate production along the
eastern inner shelf. As sea-level rose and sediment was
again temporarily trapped along the coastlines, the flux
of nutrients decreased. At the same time, the advection of
Tethyan bottom waters was again insufficient to flood
this region; thus, a reduction in the production and burial
of organic carbon resulted; carbonate-secreting organ-
isms, while not thriving, were able to fill the niche in the
slightly less turbid water column.We suggest that marine
onlap of Carlile strata onto Precambrian highlands in
eastern South Dakota and western Minnesota, which in
places overstep the Greenhorn Fm. (Shurr, 1981), was
established during this transgressive interlude in the
overall Carlile regressive phase of the Greenhorn sea-
level cycle.
Above the subsidiary transgressive (high TOC/
CaCO3/HI) zone in the ARB#1 and HAW core, the
geochemical data (TOC, HI, CaCO3) show declining
trends that we interpret as representing progradation
associated with a return to eustatic fall. In the ARB#1
region, marlstone again gave way to calcareous mud in
the upper part of the Fairport Sh Mbr, whereas in the
HAW region more silt laminations were deposited. At
least four bentonites exist in the zone of declining
%TOC/%CaCO3/HI values in the ARB#1 core. How-
ever, they are not observed in the USGSP#1 core. This
missing interval in the USGSP#1 core is considered to
be a manifestation of the disconformity surmised by
Glenister and Kauffman (1985) at the Fairport-Blue
Hill Sh Mbr contact in the Front Range of CO, which
is probably the lacuna mapped by Merewether and
Cobban (1986). This hiatus is probably the result of
bottom-current erosion on the migrating forebulge
(White et al., 2002). Phosphate nodules at the top of
the Fairport Sh Mbr in the USGSP#1 core most likely
are a lag on the disconformity.
The Blue Hill Sh Mbr in the ARB#1 core consists of
a coarsening-upward sequence recording continued in-
flux of terrigenous detritus associated with regression.
In the USGSP#1 region, moderately laminated silty
mudstone deposited in a dysoxic setting lies above
the upper Fairport/Blue Hill Mbrs disconformity,
whereas a conformable, gradually coarsening-upward
package of laminated mudstone to burrowed siltstone to
sandy siltstone were observed in the ARB#1 core. The
Blue Hill Sh Mbr in the shallower, proximal setting
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244240
records very little carbonate preservation. Abundant
pyrite and fish bones were observed through the Blue
Hill Sh Mbr in the USGSP#1 core; siderite nodules
exist near the top in the USGSP#1 core.
%CaCO3 decreased to zero through the Blue Hill Sh
Mbr in the ARB#1 core as carbonate productivity
declined. In Fig. 9, the Blue Hill Sh Mbr is character-
ized by low values for both %CaCO3 and bulk accu-
mulation rate, whereas the Fairport Sh Mbr has low
bulk accumulation rates, but relatively high %CaCO3.
We previously concluded that clastic dilution occurred
simultaneously with a decrease in carbonate productiv-
ity from Fairport Sh Mbr to the Blue Hill Sh Mbr time.
Under these conditions, productivity may have de-
creased as nepheloid layers associated with river dis-
charge reached the offshore. Perhaps a threshold in
stressful salinity was exceeded during Blue Hill Sh
Mbr deposition. Th/U values indicate that anoxia dom-
inated the lower half of this unit whereas oxic condi-
tions prevailed in the upper half of the Blue Hill Sh
Mbr. The lower half of the member is laminated,
whereas the upper half is moderately laminated, an
observation supportive of an upward increase in sea-
floor oxygenation. Organic matter input became in-
creasingly terrestrial through the member as %TOC
decreased. The concomitant grain size increase and
carbonate decrease may represent progradational delta
front to prodelta deposition. The paucity of oxygen was
likely a continuation of conditions established during
upper Fairport Sh Mbr sedimentation. As regression
and progradation continued, increasingly bfreshenedQwater was input to the delta-front resulting in a shift
from oxygen-poor to oxygenated conditions.
Carbonate concretions exist at 238 and 236 m in the
Blue Hill Sh Mbr of the ARB#1 core. Ludvigson et al.
(1994) described Blue Hill Sh Mbr carbonate concre-
tions in southeastern South Dakota as containing
bspectacularly dense accumulations of marine mollusks
. . .concentrated in bedforms through current activityQand interpreted the concretions as b implying slow rates
of sediment accumulation.Q Similar concretions were
identified by Glenister and Kauffman (1985) in the
Front Range of CO, which they suggested represent
regionally significant marker beds. The concretions in
the ARB#1 core contain relatively high %TOC, high
HIs indicative of MOM, and low Th/U ratios. We
interpret the concretions as having formed at marine
flooding surfaces during delta-lobe switching or higher
frequency fluctuations in sea level superimposed on the
overall regression.
Pyritic siderite nodules are found near the top of the
Blue Hill Sh Mbr in the USGSP#1 core. Th/U values
from the sideritic horizons are indicative of a dysoxic to
slightly oxic early diagenetic setting. Carpenter et al.
(1988) outlined a diagenetic sequence for similar fos-
siliferous concretions in the younger Fox Hills Fm. of
North Dakota and concluded that the concretions were
deposited under marine conditions with marine pore
fluids gradually replaced by brackish and meteoric
water. Coniglio et al. (2000) interpreted C and O iso-
topes in Blue Hill Sh Mbr pyritic siderite nodules from
the Cretaceous Western Interior reference sections at
Pueblo, CO, as recording a transition from marine to
meteoric pore waters. In the ARB#1 core, a geochem-
ical facies change exists in this transitional interval to
the overlying Codell Sandstone Mbr, whereas in the
USGSP#1 region, a change from deposition of silty
mudstones to muddy siltstones is observed. Shallowing,
in this case indicative of the sea-level fall rate exceed-
ing the subsidence rate, led to the cessation of fine-
grained deposition and progradation of coarser material
into the basin at this time.
6. Conclusion
Our conclusions regarding the upper half of the
Greenhorn cycle concur with earlier interpretations of
a generally progradational sequence with concomitant
coarsening and shallowing upsection. Our work shows
that carbonate contents decrease and inputs of terres-
trial detritus apparently increase in this overall trend.
This study provides some new perspectives on sedi-
mentation and the development of accommodation
space in the central axial basin of the early to mid-
Turonian Western Interior Seaway of the United States
as follows:
1) The majority of the organic matter preserved in the
Fairport Sh Mbr of the Carlile Fm. is Type II marine
organic matter associated with laminated intervals
deposited on a periodically dysoxic to anoxic sea-
floor. Type III terrestrial organic matter preserved in
bioturbated sediments deposited under predominant-
ly dysoxic to oxic conditions in limited horizons in
the Fairport Sh Mbr, but becomes increasingly dom-
inant upsection through the Blue Hill Sh Mbr.
2) A first-order, eustatic control over the distribution of
%TOC, %CaCO3, HI, and y13Corg is inferred through
comparison of these parameters to the KWIS relative
sea-level curve of Kauffman and Caldwell (1993)
and the eustatic sea-level curve of Haq et al. (1988)
and synthesized by Arthur and Sageman (2005). In
general, high %TOC, %CaCO3, and HI values, and
relative depletions in y13Corg, correspond to transgres-
T. White, M.A. Arthur / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 223–244 241
sive or highstand intervals in the upper Bridge Creek
Mbr of the Greenhorn Fm/lower Fairport Sh Mbr of
the Carlile Fm, and in a transgressive interlude in the
upper Fairport Sh Mbr, whereas low values for these
parameters, and relative enrichment in y13Corg, char-
acterize the regressive Blue Hill Sh Mbr of the Carlile
Fm. The subsidiary transgressive interlude in the
upper Fairport Sh Mbr is correlatable from the Port-
land and Bounds cores in CO and KS, respectively, to
the Hawarden core in IA through similar biostratigra-
phy, relative stratigraphic distribution of bentonites,
and CaCO3 content. In the Hawarden core, a zone of
lower TOC and HI values and relatively enriched
y13Corg is suggestive of increased terrestrial organic
matter input and likely correlates to a progradational
zone below the subsidiary transgression, manifested
as calcareous mud in the Bounds core, and mud- and
claystone in the Portland core.
3) The transition between the upper Bridge Creek Mbr
of the Greenhorn Fm. and the lower Fairport Sh Mbr
of the Carlile Fm. records episodes of enhanced
organic matter production and preservation. High
%TOC and HI values are indicative of high produc-
tivity, little remineralization in the shallow water
column, and deposition on a dysoxic seafloor. A
model of preconditioned, relatively oxygen-poor
and phosphate-rich Tethyan water advected into the
KWIS is invoked as a source of nutrients and as a
cause of the dysoxic conditions established in the
generally well-mixed water column of the shallow
seaway. These conditions returned during the trans-
gressive interlude in upper Fairport Sh Mbr time.
4) The paleogeographic distribution of geochemical
facies supports a numerical model for advection
driven by estuarine circulation (Slingerland et al.,
1993). The most pelagic facies exist in the Bounds
core positioned directly where modeled inflow from
the Gulf of Mexico to the south is predicted to have
occurred. In the Portland core, the marine facies
have, somewhat more terrigenous characteristics
than in the Bounds core this difference in character
is attributed to caballing and surface mixing associ-
ated with the estuarine flow. Marine facies encoun-
tered in the Hawarden core on the eastern shelf of
the Western Interior Seaway were mostly inboard of
the effects of the advected water mass.
5) Most of themiddle Fairport ShMbr reflects deposition
by pelagic settling and deposition from nepheloid
layers punctuated by episodes of bottom current win-
nowing, during an overall progradational phase in
seaway history. Coastal and bottom currents driven
by riverine discharge and by estuarine circulation in
the seaway, at times, maintained an offshore bottom
nepheloid layer. During periods of increased coastal
jet flow, produced as the result of greater river input,
nepheloid plumes dominated the water column, and
riverine-driven bottom-current winnowing was most
effective, both processes manifest in the sedimentary
record as bioclastic horizons. At times of waning
coastal jet flow, the erosive capacity of bottom currents
was diminished, remnant surface nepheloid plumes
led to a temporarily (seasonally?) stratified water col-
umn, and organic matter settled to a dysoxic seafloor.
6) A disconformity based on geochemical data in the
upper Fairport Sh Mbr of the USGSP#1 core was
identified and is probably equivalent to Merewether
and Cobban’s (1986) lacuna mapped in sections to
the west. The development of this disconformity is
attributed to erosion and marine bypass by riverine-
derived south-flowing bottom currents on the flank
of a migrating forebulge. Higher clastic dilution and
diminished carbonate productivity characterized de-
position of organic and carbonate carbon on the
overlying Blue Hill Sh Mbr seafloor, as the influence
of rivers grew increasingly dominant.
Acknowledgements
Much of this work was completed under the aus-
pices of the Continental Scientific Drilling Program
with Department of Energy (DOE) funding, DE-
FG02-92ER14251 to Penn State University. We also
acknowledge DOE and the US Geological Survey for
funding of the drilling and coring program that resulted
in acquisition of the Portland core. We thank Walter
Dean for his leadership and collaboration in the coring
and analysis of core samples from other lithologic
units. Support from the Petroleum Research Fund of
the American Chemical Society (grants 32573-AC8
and 39503-AC8) was applied to study of the Hawarden
core. The authors thank Dan Leppold and Leah Young
for help with sample preparation and analysis; Brian
Witzke for his introduction of one of us (TW) to the
Cretaceous of Iowa, and the Hawarden Core; and Phil
Kolb for his graphics expertise. Critical reviews were
obtained from B. Sageman and an anonymous reviewer.
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