Cenomanian–Turonian and d C, and d O, sea level and salinity variations at Pueblo, Colorado

25 Pages·2004·1.9 MB·English

Published in Palaeogeography, Palaeoclimatology, Palaeoecology 211, issues 1-2, 19-43, 2004 1
which should be used for any reference to this work
Cenomanian–Turonian and d13C, and d18O, sea level and
salinity variations at Pueblo, Colorado
Gerta Kellera,*, Zsolt Bernerb, Thierry Adattec, Doris Stuebenb
aDepartmentofGeosciences,PrincetonUniversity,PrincetonNJ08544,USA
bInstitutfu¨rMineralogieundGeochemie,Universita¨tKarlsruhe,76128Karlsruhe,Germany
cInstitutdeGe´ologie,IIrueEmileArgand,2007,Neuchatel,Switzerland
Abstract
Stable isotopes of the surface dwelling planktic foraminifera Hedbergella planispira, its abundance variations, and
mineralogicalanalysisoftheCenomanian–TuronianatPueblo,CO,revealcyclicvariationsinsurfacesalinityduetochangesin
precipitation,freshwaterinflux,marineincursionsandlong-termsea-levelfluctuations.Hedbergellaplanispiraisaproxyfor
salinityvariations,asindicatedby2–4xmorenegatived18Ovaluesinintervalsofpeakabundancesascomparedtointervals
withreducedpopulations.Negatived18Ovaluesreflectperiodsofbrackishsurfacewaterscausedbyfreshwaterinfluxduring
wethumidperiods,accompaniedbyincreasedclastictransport.Morepositived18Ovaluesreflectmorenormalmarinesalinities
asaresultofaridperiodsand/ormarineincursionsandcorrelatewithintervalsofincreasedbiogeniccarbonatedeposition.The
magnitudeofsalinityvariationsduringthelowsea-leveloftheHartlandShaleistwicethatduringthesea-leveltransgressionof
theBridgeCreekLimestone.Therapidpositived13CshiftthatmarkstheonsetofOceanicAnoxicEvent2(OAE2)atPueblo
occurredoveraperiodofabout100ky(93.90–94.00Ma),andcoincidedwiththemajorsealeveltransgressionthatculminated
in the deposition of the basal Bridge Creek Limestone. A positive d13C shift also occurred in the Rotalipora cushmani zone
priortoOAE2andcoincidedwithasealevelriseandenhancedpreservationofterrestrialorganicmatter.Thelikelycausefor
OAE2isdepletionof12Cinthewatercolumnasaresultofhighprimaryproductivity,whereasanearlierR.cushmanizone
event was primarily caused byincreased input of terrigenous organic matter. Both d13C events are associated with enhanced
organic matterpreservation andanoxic ordysoxic bottom waters.
Keywords:Cenomanian–Turonian;Stableisotopes;Salinity;Sealevel;Pueblo,CO
1. Introduction ofsubsidenceandatectono–eustanichighstand(Cal-
dell and Kauffman, 1993). As a result, a shallow
During thelate Cenomanian toearly Turonian, the seaway, the Greenhorn Sea, extended from the west-
Western Interior basin of North America experienced ern Tethys north to the polar ocean, with the deep
a major sea-level transgression due to a combination central axis passing from New Mexico north through
the Rock Canyon area of Colorado (Fig. 1). Rivers
drainedthehighlandstothewestoftheseaway,andto
a lesser extent the eastern lowlands, transporting
* Correspondingauthor.Fax:[phone].
E-mailaddress:[email protected](G.Keller). abundant siliciclastic sediments. Sediment deposition
2
varied between shales and marls during the low sea westernTethys.Withthesemarineincursions,salinity
levelsofthemiddletolateCenomanian,andbetween sensitive planktic and benthic foraminifera and inver-
marlsandlimestonesduringthehighsealevelsofthe tebrates invaded the hyposaline interior sea, particu-
latest Cenomanina to early Turonian. The limestone/ larly during the major transgression and global
marlcoupletsarecommonlyinterpretedaschangesin perturbation of the carbon system, Oceanic Anoxic
clastic dilution and watermass stratification due to Event 2 (OAE 2), that marks the lower part of the
Milankovitchforcingofprecipitationandrunoff(e.g., Bridge Creek Limestone Member (Eicher, 1969a,b;
Barronetal.,1985;ElderandKirkland,1985;Prattet Eicher and Worstell, 1970; Elder, 1985; Leckie et al.,
al.,1993; Sagemanetal.,1998),orchanges inclastic 1998).
dilution and productivity (Eicher and Diner, 1989, Palaeoenvironmental conditions and the OAE 2 of
1991; Ricken, 1991, 1994). the Western Interior Seaway have been analyzed in a
Warm temperature to subtropical climates pre- number of stable isotope studies based on organic
vailed in the Cenomanian–Turonian Western Interior matter, bulk rock carbonate, inoceramid and ammo-
with humid to subhumid conditions (Pratt, 1984; niteshells(e.g.,Arthuretal.,1985;Pratt,1984,1985;
Upchurch and Wolfe, 1993), and monsoonal circula- Pagani and Arthur, 1998). All of these studies noted
tion led to fluctuations in the hydrologic cycle (Glan- the anomalously light oxygen isotopic compositions
cey et al., 1993; Park and Oglesby, 1994). Surface and concluded that this depletion is likely due to
salinity was variable with hyposaline waters at times freshwater influx to the seaway.
of shallow seas and high freshwater influx, and more Inthis study,thepalaeoenvironment,sea-level and
normal marine salinity at times of higher sea levels salinity variations during the middle Cenomanian
andtheincursionofsubtropicalwatermassesfromthe through early Turonian of the Greenhorn Formation
Fig.1.LocationofthePueblosectionintheRockCanyonAnticlineareaofLakePuebloStatePark,Colorado.Thesectionisexposedalongthe
roadonthelakeside(seelabels).
3
Fig.2.OutcropofthePueblosectionexposedontheroadalongLakePuebloshowingexposureoftheHartlandShaleandBridgeCreekLimestone.
Fig.3.Scanningelectronmicrographs(SEM)ofHedbergellaplanispira,whichwasanalyzedforstableisotopesatthePueblosection.1and2:
scalebar=100Am.3and4:surfacestructuredetails.Notethatalthoughthereissomerecrystallizationofthetestcalcite,theoriginalporeand
teststructurearepreservedandtheforaminiferalshellsareclearofcalciteinfilling.
4
Table1
Mineralogical(calcite,quartzandphyllosilicates)andstableisotopemeasurementsofHedbergellaplanispira(63–100Am,sizefraction)and
Cibicidoidesspp.atthePueblostratotypesection
Pueblo Sample Height(m) d13C d18C Calcite(%) Phyllosilicates(%) Quartz(%)
Markerbeds Hedbergella
PC1 0.00 0.25 (cid:1)9.98 28.16 18.00 25.26
PC2 1.67 36.98 4.67
PC3 72.19 14.86 10.95
PC4 0.30 (cid:1)0.16 (cid:1)8.48 55.69 14.33 20.65
PC5 76.52 4.78 18.14
PC6 0.00 71.95 0.74
PC7 0.85 0.04 (cid:1)8.59 26.85 20.54 13.60
PC7 0.85 (cid:1)0.02 (cid:1)8.68
PC8 1.10 (cid:1)0.07 (cid:1)9.22 56.32 13.27 22.28
PC9 31.84 24.07 18.70
PC10 1.45 (cid:1)0.57 (cid:1)10.21 26.07 19.44 23.89
PC11 1.65 (cid:1)0.23 (cid:1)9.53 34.00 15.40 15.76
PC12 39.19 10.83 12.78
PC13 1.95 (cid:1)0.22 (cid:1)11.14 21.94 20.92 27.23
PC14 0.00 58.97 4.61
PC16 2.15 0.07 (cid:1)9.00 56.07 12.71 30.38
PC17 93.38 0.00 6.20
PC18 40.94 16.49 14.46
PC19 2.45 0.53 (cid:1)8.27 16.25 21.62 9.63
PC20 36.09 12.16 12.11
PC21 27.09 17.49 7.78
PC22 2.60 0.69 (cid:1)8.04 39.41 12.14 14.59
PC23 2.20 46.95 1.30
PC24 3.00 0.47 (cid:1)8.39 69.45 12.15 14.85
PC25 81.04 9.33 9.46
PC26 3.55 0.12 (cid:1)9.58 32.39 17.10 19.55
PC27 3.95 0.36 (cid:1)9.33 29.50 16.39 18.51
PC28 41.26 15.38 20.44
PC29 4.35 0.24 (cid:1)9.71 14.68 44.28 3.10
PC30 4.50 (cid:1)0.10 (cid:1)10.65 43.55 15.30 20.67
PC31 5.40 0.11 (cid:1)9.80 17.51 13.56 15.47
PC32 5.70 (cid:1)0.24 (cid:1)12.06 49.30 7.71 8.24
PC33 6.25 0.45 (cid:1)8.59 28.40 16.12 17.10
PC34 6.85 (cid:1)0.06 (cid:1)11.55 32.86 26.07 15.74
PC35 7.15 0.22 (cid:1)10.92 31.27 13.89 7.94
PC36 7.40 0.73 (cid:1)7.26 47.88 13.82 16.70
PC37 7.95 0.33 (cid:1)10.00 29.58 15.54 16.26
PC38 8.25 0.22 (cid:1)10.64 45.05 12.18 15.58
PC39 8.55 1.35 (cid:1)10.58 77.13 0.00 5.09
PC40 8.75 0.70 (cid:1)7.94 25.53 5.84 8.92
PC41 8.85 0.75 (cid:1)7.99 64.10 17.70 16.07
PC42 8.95 0.21 (cid:1)11.00
PC43 9.30 0.81 (cid:1)9.34 17.88 18.85 55.57
PC44 9.75 0.13 (cid:1)10.68 7.09 8.61 83.65
PC45 9.85 1.02 (cid:1)8.29 8.71 8.60 19.22
PC46 10.20 (cid:1)0.29 (cid:1)11.53 14.52 11.95 19.08
PC47 10.50 0.80 (cid:1)9.21 25.54 7.69 8.37
PC48 10.75 0.64 (cid:1)9.72 38.59 21.28 15.23
PC49 11.00 0.92 (cid:1)9.82 72.08 9.52 17.05
PC50 11.25 1.44 (cid:1)7.53 34.05 17.40 14.57
PC51 11.75 2.07 (cid:1)6.50 62.71 21.36 14.75
5
Table1(continued)
Pueblo Sample Height(m) d13C d18C Calcite(%) Phyllosilicates(%) Quartz(%)
Markerbeds Hedbergella
PC52 11.80 2.19 (cid:1)8.36 40.85 10.42 12.44
PC53 11.85 1.64 (cid:1)8.93 41.41 15.51 14.48
PC54 11.90 1.62 (cid:1)9.07 79.25 10.14 8.05
63 PC55 12.10 2.01 (cid:1)8.55 87.63 4.47 6.48
63 PC56 12.25 2.52 (cid:1)7.60 87.32 4.12 7.93
PC57 12.30 1.60 (cid:1)8.86 76.75 11.98 10.50
PC58 12.60 2.20 (cid:1)8.35 88.83 0.00 10.51
PC59 12.80 2.07 (cid:1)8.25 47.54 16.02 11.46
PC60 12.95 1.60 (cid:1)8.96 56.91 18.93 11.62
PC61 13.20 1.91 (cid:1)7.08
67 PC62 70.23 4.96 8.44
PC63 13.35 1.70 (cid:1)7.45 23.36 6.83 3.75
PC64 13.35 1.83 (cid:1)7.31 9.44 26.98 5.22
PC65 13.55 2.11 (cid:1)8.57 34.12 21.35 4.76
69 PC66 0.49 35.11 1.44
PC67 13.75 2.11 (cid:1)8.75 49.44 19.62 11.18
PC68 86.11 7.04 6.54
PC69 14.00 1.63 (cid:1)8.56 37.21 18.60 6.58
PC70 14.25 2.13 (cid:1)6.71 45.31 16.73 20.76
PC(cid:1)71 79.83 4.57 2.84
PC72 14.45 2.35 (cid:1)8.52 14.36 42.60 2.14
PC73 84.86 7.77 3.69
PC74 14.45 2.41 (cid:1)8.22 15.28 6.59 76.28
PC74 14.75 1.86 (cid:1)8.67 15.28 6.59 76.28
PC75 73.23 10.67 15.57
PC76 14.75 1.78 (cid:1)8.30 56.05 7.92 15.82
PC77 15.30 1.67 (cid:1)9.81 20.66 2.93 7.05
PC77A 60.43 3.65 5.37
PC78 15.55 1.96 (cid:1)7.47 33.54 18.99 16.24
PC79 16.00 1.82 (cid:1)8.65
PC80 16.45 1.81 (cid:1)8.64 66.86 11.55 2.99
PC80 16.65 1.66 (cid:1)7.97 66.86 11.55 2.99
PC81 16.80 1.61 (cid:1)8.77 71.98 8.93 7.30
PC81 17.35 0.94 (cid:1)7.71 71.98 8.93 7.30
PC81A 58.49 4.40 16.44
PC82 75.84 8.39 4.77
PC83 17.75 1.18 (cid:1)9.87 64.38 11.69 5.76
PC83A 60.30 7.29 5.69
PC84 18.00 1.29 (cid:1)9.47 84.47 7.15 1.18
PC85 86.56 8.70 4.32
PC86 82.36 10.61 5.86
PC86A 62.78 8.74 10.16
PC87 75.97 9.54 10.85
PC87A 63.37 3.97 6.05
Pueblo Sample Height(m) d13C d18C Calcite(%) Phyllosilicates(%) Quartz(%)
Markerbeds Cibicidoides
PC88 11.85 1.91 (cid:1)6.38 71.05 13.99 12.86
PC89 12.10 2.34 (cid:1)5.92 81.14 12.47 6.04
PC90 12.25 2.43 (cid:1)4.96
PC-91 56.45 7.70 5.92
(continuedonnextpage)
6
Table1(continued)
Pueblo Sample Height(m) d13C d18C Calcite(%) Phyllosilicates(%) Quartz(%)
Markerbeds Cibicidoides
PC-92 55.57 4.93 6.02
PC-93 33.50 12.37 4.43
PC94 12.30 1.86 (cid:1)6.71 62.51 10.74 7.25
PC95 90.30 7.20 2.18
PC96 12.80 1.79 (cid:1)6.54 41.71 16.72 5.68
PC97 81.00 10.35 7.08
PC98 48.30 8.77 3.50
PC99 76.18 4.19 5.26
PC100 42.04 4.39 2.94
Notethatbenthicforaminiferaarerareorabsent,exceptintheoxicphaseafterthed13Cexcursion.Height(m)inthesectionismeasuredfrom
thebaseoftheoutcropandhavebeenindexedtothekeymarkerbedsfororientation.
atPueblo,CO,areanalyzedbasedonmineralogyand marls, bentonites, calcarenites), a total of 68 samples
stable isotope compositions of the surface dwelling yielded sufficient specimens for isotope analysis (Ta-
planktic foraminifera Hedbergella planispira and the ble 1). Benthic foraminifera are nearly absent in the
benthictaxonCibicidoidesspp.ThePueblosectionis HartlandShaleandarerareintheLowerBridgeCreek
the Global Stratotype and Point of the Cenomanian/ Limestone, except for five samples that contained
Turonian (C/T) boundary (Kennedy and Cobban, sufficient Cibicidoides for analysis.
1991; Kennedy et al., 2000). The samples were analyzed at the stable isotope
laboratory of the Department of Mineralogy and
Geochemistry at the University of Karlsruhe, Ger-
2. Methods many, using an Optima (Micromass, UK) ratio mass
spectrometerequippedwithanonlinecarbonateprep-
A total of 100 samples were collected at 10-cm aration line (Multi Carb) with separate vials for each
intervals in the changing lithologies of the Bridge sample. The results were calibrated to the PDB scale
CreekLimestone,andat20-to25-cmintervals inthe with standard errors of 0.1x ford18O and 0.05x for
more monotonous shale of the underlying Hartland d13C (Table 1). Replicate sample analyses for d13C
ShaleMember(Fig.2).Allsampleswereanalyzedfor were within 0.06–0.13x and for d18O, they ranged
bulk-rock mineralogy and microfossil content. Pres- from 0.09% to 0.37x. The higher values may reflect
ervation of planktic foraminifera is relatively good, varying degrees of diagenetic alteration and/or sea-
though tests are recrystallized to varying degrees sonality effects.
depending on size and morphology of the species. Bulk rock mineral analyses were conducted at the
Large species tend to be completely recrystallized Geological Institute of the University of Neuchaˆtel,
with chambers frequently infilled with blocky calcite. Switzerland, based on XRD (SCINTAG XRD 2000
In contrast, small species, such as Hedbergella pla- diffractometer), following the procedure outlined by
nispira, tend to be only partially recrystallized, with Ku¨bler (1987) and Adatte et al. (1996). Bulk rock
the shell calcite and pore structure preserved and no contents were obtained using standard semiquantita-
infilling of test chambers (Fig. 3). This indicates that tive techniques based on external standardization.
small species at Pueblo are more likely to retain
primaryenvironmentalsignals.Stableisotopeanalysis
was thus performed on monospecific samples of the 3. Lithology
surface dwelling planktic foraminifer H. planispira
using40–75specimenspersampleinthesizefraction The studied Pueblo section forms a road cut at the
63–100 Am. Limestone samples were excluded from northeast end Lake Pueblo (Figs. 1 and 2) where
this analysis. Of the remaining lithologies (shales, about 18.5 m of gray shales, bentonites and tan-
7
Fig.4.Lithology,biostratigraphyandplankticforaminiferalandammonitedatumlevelsofthePueblosection.AgesfordatumlevelsarefromHardenboletal.(1998),exceptfor
Rotaliporadeekei,RotaliporagreenhornensisandGlobigerinelloidesbentonensis,whichareextrapolatedfromage/depthandsedimentaccumulationrates.Plankticforaminiferal
biozonationafterKelleretal.(2001,KellerandPardo(2004.AmmonitezonationfromCobban(1985).
8
colored limestones of the Greenhorn Formation are ofvariousammoniteandplankticforaminiferaldatum
exposed. The Greenhorn Formation is divided into events, which are extrapolated from the palaeomag-
Hartland Shale and Bridge Creek Limestone Mem- netictimescaleandradiometricdatesofHardenbolet
bers. The Hartland Shale Member at this outcrop al.(1998)(Table2).Theseagesareingoodagreement
consistsof11.2mofrhythmicallybeddedthincalcar- with Eastbourne, except for Helvetoglobaotruncana
eniteornodularcalcarenitelayersand30-to100-cm- helvetica which appears earlier at Eastbourne, possi-
thick gray shales. Bentonite layers are common and blybecausethisspeciesisdiachronousand/orbecause
vary from 1–2 to 20 cm thick (Fig. 4). the evolutionary transition from praehelvetica to hel-
The Bridge Creek Limestone Member exposed is vetica is difficult to determine.
about 6.5 m thick. A prominent 40- to 50-cm-thick Three 40Ar/39Ar ages determined from three ben-
bioturbated micritic limestone marks the base of the tonite layers (Obradovich, 1993; Kowallis et al.,
Bridge Creek Limestone and contains an Upper Cen-
omanian ammonite assemblage of the Sciponoceras Table2
gracile zone (Cobban, 1985) and a planktic forami- Agesforplankticforaminiferalandammonitefirst(FAD)andlast
(LAD)appearancedatums,andd13Cshifts(discussedbelow)atthe
niferal assemblage indicative of the uppermost Rota-
PuebloGSSP
lipora cushmani zone (Leckie, 1985; Keller and
Datumevents Age(Ma) Reference
Pardo, 2004). Previous studies have identified this
limestone facies as the main sea-level transgression Helvetoglobotruncana 93.29F0.2 Hardenboletal.
helveticaFAD (l998)
neartheendoftheCenomanian(HancockandKauff-
Watinocerasdevonense 93.49F0.2 Hardenboletal.
man, 1979; Kauffman, 1984; Sageman et al., 1998).
FAD (l998)
Upsection, the lithology consists of rhythmically Neocardiocerasjuddii 93.49F0.2 Hardenboletal.
bedded 10- to 20-cm-thick bioturbated micritic lime- LAD (l998)
stones alternating with 10- to 60-cm-thick organic- Neocardiocerasjuddii 93.59 thisstudyat
LAD Pueblo
rich dark shales (Figs. 2 and 4). Bentonite layers are
Metoicocerasgeslineanum 93.73F0.2 Hardenboletal.
common and are of variable thickness ranging from
LAD (l998)
1–2to 20cm, similarto theBridge CreekLimestone Neocardiocerasjuddii 93.73F0.2 Hardenboletal.
havebeenlabeledasmarkerbedsbypreviousworkers FAD (l998)
and in this study we follow the numbering system of Heterohelixshift 93.78F0.02a thisstudy
OAE2d13Cpeak(2) 93.86F0.05a thisstudy
Cobban and Scott (1972; Fig. 4).
excursionmax.
Globigerinelloides 93.86F0.06a thisstudy
bentonensisLAD
4. Biostratigraphy and age control DicarinellahagniFAD 93.86F0.05a thisstudy
Rotaliporacushmani 93.90F0.02a Hardenboletal.
LAD (l998)
The biostratigraphy of the Pueblo section is gen-
OAE2d13Cpeak1 93.91F0.02a thisstudy
erally defined by ammonite and inoceramid zones OAE2d13Cexcursion 94.00F0.02a thisstudy
(e.g., Cobban, 1985; Elder, 1985; Kennedy and Cob- onset
ban,1991;Leckie etal.,1998;Kennedyetal.,2000), Rotaliporagreenhornensis 93.95F0.02a thisstudy
though planktic foraminifera are abundant and pro- LAD
RotaliporadeekeiLAD 93.94 thisstudy
vide the means for global correlations. Based on the
Whiteinellaarcheocretacea 94.50 thisstudy
Eastbourne and Pueblo sections, Keller et al. (2001)
FAD
and Keller and Pardo (2004) identified new biostrati- Praeglobotruncana 94.88 thisstudy
graphic markers and subdivided the Whiteinella praehelveticaFAD
archeocretacea and Rotalipora cushmani zones into Mid-Cenomaniand13C 95.71 thisstudy
shift(MCE)
three subzones for improved age control (Fig. 4). In
Whiteinellaparadubia 95.85 thisstudy
bothsections,plankticforaminiferaldatumeventsand
FAD
the structure of the d13C excursion (discussed below)
aDatumeventsatPueblothathavebeencross-correlatedwith
are comparable. Age estimates for species first and
Eastbourne, England. Error margins reflect uncertainty between
last appearances have been calculated based on ages thesetwosections.
9
1995) are compatible with these dates, but show tion curve can be interpreted as two linear segments
considerable spread. Based on these 40Ar/39Ar data, intersectingatlimestonemarkerbed79.Forthelower
the C/T boundary is estimated at 93.1F0.3 to segment (Bed 63 to Bed 79) of the Bridge Creek
93.3F0.2 Ma, as compared with 93.49F0.2 Ma Limestone, sediment accumulation rates average 1.23
based on the first appearance of Watinoceras devon- cm/ky (1.11 cm/ky excluding bentonite layers),
ense, which is the preferred marker for the C/T whereas for the upper segment they average 0.87
boundary,and93.29F0.2Maforthefirstappearance cm/ky (0.75 cm/ky excluding bentonite layers), as-
of Helvetoglobotruncana helvetica, the planktic fora- sumingthatlimestoneandshalelayersweredeposited
miniferal marker species (Hardenbol et al., 1998; at roughly equal rates. The break at limestone marker
Kennedy et al., 2000). bed 79 may be related to the rising sea level and
reduced terrigenous influx in the upper part of the
Bridge Creek Limestone.
5. Sedimentation rates These sedimentation rates compare well with the
average rates for the entire interval of 0.5–1.0 cm/
Sedimentation rates calculated from an age/depth ky estimated by Elder and Kirkland (1985), 0.9 cm/
plot for the Pueblo section (Fig. 5) indicate very low ky by Scott et al. (1998) and 0.57 cm/ky by
rates, suggesting a condensed record. The sedimenta- Sageman et al. (1998). In comparison, the compa-
Fig.5.Age/depthplotof thePueblosectionbasedonplankticforaminiferal andammonitedatumlevelsfromHardenboletal.(1998)and
extrapolated datum level ages for the Heterohelix shift, Globigerinelloides bentonensis, Rotalipora deekei and Rotalipora greenhornensis.
CalculateddatumlevelagesarecoevalatPuebloandEastbourne.40Ar/39AragesforbentonitelayersarefromObradovich(1993)andKowallis
etal.(1995).
10
rable interval at Eastbourne averages 2.4 cm/ky. The 6.1. Carbon isotopes
low sediment accumulation rate is probably due to
short periods of non-deposition between marl/lime- 6.1.1. OAE 2 d13C excursion
stone couplets and cyclic climate variations with The major features of the planktic foraminifera H.
humid wet periods accompanied by high sediment planispirad13COAE2excursionatPuebloare: (1)a
and freshwater influx depositing shales, and more rapid increase of 2x to reach the first peak, (2) a
arid periods accompanied by reduced sedimentary 0.6xdecreaseformingatrough,(3)a1xincreaseto
influx and increased production of biogenic carbon- formasecondpeak,thoughfollowedbylowvaluesin
ate (foraminiferal tests, coccoliths, invertebrates) the bentonite bed 69, and (4) prolonged high, but
depositing calcarenites and limestones. These dilu- variable, d13C values into early Turonian (Fig. 6).
tion/productivity cycles have been interpreted as The same features are better seen in the more
obliquity and precession cycles, respectively (Sage- expanded coeval interval at Eastbourne, England
man et al., 1998). (Paul et al., 1999; Keller et al., 2001) and suggest
that this pattern reflects a series of widespread or
global oceanographic events (Fig. 7). A decrease, or
6. Stable isotopes trough, occurs between peak 1 and peak 2 at East-
bourne and these intervals correspond to the G.
Oxygenandcarbon isotope analysesofthePueblo bentonensis and D. hagni subzones, respectively. At
section are based on the surface dwelling planktic Pueblo, the trough interval is very condensed though
foraminifer Hedbergella planispira (Fig. 6). Hedber- marked by a 0.6x decrease in d13C (Figs. 6 and 7).
gella planispira populations range between (cid:1)0.5x Thed13Cpeak2isasharpincreasefollowedbya1x
and 1.0x d13C for the Hartland Shale, and between drop in the bentonite marker bed 69 of Cobban and
1.5xand2.5xy13CfortheBridgeCreekLimestone Scott (1972), and return to high values in the marl
above the positive d13C excursion (Fig. 6). Similar above. It is possible that the bentonite value is a
values are observed in whole-rock d13C values of the diagenetic signal and peak 2 spans through the marl
same section (Pratt, 1984), and the overall trend is and limestone bed 73 above it, as suggested by
similar to the organic d13C curve (Pratt, 1985; Pratt biostratigraphy (see shaded interval in Fig. 7). Alter-
and Threlkeld, 1994). In general, the observed d13C natively, the second peak may be condensed and/or
trends are very similar to published curves based on partially missing at Pueblo. Above this interval, d13C
bulk-rock or fine-fraction carbonates across the late valuesremainhighwithatemporarydecreasenearthe
Cenomanian to early Turonian that mark the organic first appearance of H. helvetica about 0.75 m above
carbon-rich marine sediments known as Oceanic the C/T boundary at Pueblo (Fig. 7).
Anoxic Event 2 (OAE 2, Scholte and Arthur, 1980; The onset of the rapid d13C excursion at Pueblo
Jarvis et al., 1988, 2001; Jenkyns et al., 1994; begins about 65 cm below the base of the Bridge
Accarie et al., 1996; Nederbragt and Fiorentino, CreekLimestone,andreachesamaximuminthemarl
1999;Pauletal.,1999;Kelleretal.,2001).However, layer above limestone marker bed 63 (Fig. 6). Based
at Pueblo, the absolute values of both bulk rock and on sediment accumulation rates the d13C excursion
planktic foraminifera are about 2.5–3x lighter. beganat94.00Maandreachedthefirstpeakabout90
Pagani and Arthur (1998) observed a similar differ- ky later at 93.91 Ma. The equivalent interval at
ence in unaltered aragonite and calcite shells of Eastbourne is also estimated between 94.00 and
primarily early Turonian macroinvertibrates (ammon- 93.90 Ma (Keller et al., 2001).
ites, inoceramids and oysters) from other localities in
the Western Interior, and attributed this to early 6.1.2. Benthic oxic event
diagenetic cementation and organic carbon degrada- The second important feature of this dataset is the
tion in the sediments. Because of the vital effects appearance of diverse benthic assemblages, including
between macroinvertibrates and planktic foraminif- Cibicidoides species, indicating normal oxic bottom
era, differences in age and region, no direct compar- waters beginning with d13C peak (1) and continuing
ison can be made. just above d13C peak (2), or about 100 ky (benthic