www.elsevier.com/locate/palaeo

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: tswhite@essc.psu.edu (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,

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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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