Applying carbon-isotopestratigraphy using well cuttingsfor high-resolutionchemostratigraphic correlationof the subsurfaceJ. Garrecht Metzger, David A. Fike, and L. B. Smith

ABSTRACT

Basin-scale correlations in the subsurface generally rely on lithos-tratigraphic information synthesized from wireline logs, and, insome cases, well cuttings, and cores. However, lithostratigraphicboundaries are often diachronous, and, as such, the correlationsbased upon them may not provide reliable timelines. In this paper,we use δ13Ccarb data from well cuttings and a core to generatechronostratigraphic logs of Late Ordovician strata spanning theBlack River Group, Trenton Group, and Utica Shale across thesubsurface of New York State. Although particular δ13Ccarb val-ues may be impacted by (primary) variability in local dissolvedinorganic carbon reservoirs and/or (secondary) diagenetic altera-tion, it is possible to identify spatially and stratigraphically coher-ent patterns in δ13Ccarb, which can be used to effectively correlatetime-equivalent strata on a basin-wide (or even global) scale,including across lithologies (e.g., between limestone and calcare-ous shale). The present study emphasizes the use of well cuttings,as these are commonly collected during drilling and can providethe maximum lateral resolution for subsurface correlation.Parallel geochemical (percent carbonate and total organic carbon)and isotopic (δ18Ocarb and δ13Corg) data are used to understand theorigin of stratigraphic and spatial variability in the δ13Ccarb signaland to identify diagenetic alteration. Stratigraphically coherentδ13Ccarb trends across New York were used to identify six isotopi-cally distinct packages of time-equivalent strata within these for-mations. Pairing chemostratigraphic and lithostratigraphic dataimproves our ability to document the diachronous nature of

Copyright ©2014. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received December 20, 2012; provisional acceptance March 26, 2013; revised manuscriptreceived August 01, 2013; final acceptance April 01, 2014.DOI: 10.1306/04011412231

AUTHORS

J. Garrecht Metzger ∼ Department ofEarth and Planetary Sciences, WashingtonUniversity, 1 Brookings Dr., St. Louis,Missouri, 63130; gmetzger@levee.wustl.edu

J. Garrecht Metzger is a 5th year Ph.D.student at Washington University inSt. Louis, Missouri. He holds a B.S. in earthand environmental sciences from FurmanUniversity and an A.M. from WashingtonUniversity in St. Louis. He has studiedinorganic carbon cycling in small streams,gypsum formation in Mammoth Cave(Kentucky), and carbon isotope variability inlate Ordovician rocks. Currently, he isinterested in the Late Ordovicianbiogeochemical carbon cycle and geologicalcontrols on the formation of the Utica Shale.

David A. Fike ∼ Department of Earth andPlanetary Sciences, Washington University,1 Brookings Dr., St. Louis, Missouri, 63130;dfike@levee.wustl.edu

David A. Fike is an associate professor in theDepartment of Earth and Planetary Sciencesat Washington University in St. Louis.He obtained B.S. degrees fromUniversity of Illinois at Urbana-Champaign inengineering physics, astronomy, andgeology, respectively. He obtained aMaster’s of Philosophy in polar studies fromChurchill College, Cambridge University in2002 and a Ph.D. in geochemistry in 2007from the Massachusetts Institute ofTechnology. Subsequently, he was as anO.K. Earl postdoctoral fellow at theCalifornia Institute of Technology in2007–2008, before joining the faculty atWashington University in 2009. His currentresearch interests focus on isotopebiogeochemistry, particularly understandingthe biological, sedimentological, anddiagenetic processes that generate and alterisotopic signatures in modern marinesedimentary phases and ultimately how theyget preserved in the rock record.

L. B. Smith ∼ Smith Stratigraphic LLC, 397State St., Albany, NewYork 122120;smithstrat@gmail.com

Langhorne Smith is a geological consultantspecializing in characterization of both

AAPG Bulletin, v. 98, no. 8 (August 2014), pp. 1551–1576 1551

lithologic contacts, including the base of the Utica Shale, which isprogressively younger moving west through New York.

INTRODUCTION

Several chronostratigraphic tools can be combined with lithostratig-raphy for the temporal correlation of strata. Radiometric dating ofdetrital minerals in ash beds (e.g., U/Pb or Ar/Ar) provides a meansfor absolute time determination in sedimentary strata (Bowring et al.,1993; Goldman et al., 1994; Berkley and Baird, 2002; Maloof et al.,2005; Ramezani et al., 2007). These approaches, however, are oftendifficult to apply in the subsurface because ash beds are infrequentlydeposited, limited subsurface volcanic materials may be availablefor sampling, and the resulting age resolution may be too coarsefor fine-scale correlations. When ash beds are not present, radiomet-ric ages of organic-rich strata can be obtained using rhenium–

osmium (Re–Os) isotopes; however, this approach is unlikely toprovide sufficient resolution (<1 m.y.) for fine-scale correlations(e.g., Selby and Creaser, 2005; Selby et al., 2009), and the potentialimpact of diagenesis on the resulting Re–Os ages remains poorlyunderstood. Biostratigraphy is a powerful tool for chronostrati-graphic correlation of strata on a local, regional, and global scale(Goldman et al., 1994; Brett, 1995; Patzkowsky, 1995; Kirchnerand Brett, 2008). Although biostratigraphy can often be produc-tively applied in the subsurface (Gartner et al., 1983; Gradstein et al.,1988; Gradstein et al., 1992; Armentrout, 1996; Witrock et al.,2003), challenges can arise from the limited sample availabilityfrom cores and destruction of larger biostratigraphically relevantfossils during generation of cuttings fragments. These challengesare most pronounced in the Paleozoic before the rise of abundantplanktonic microfossils (e.g., foraminifera, coccolithophores) thatare so useful for Mesozoic and Cenozoic biostratigraphy (seeWitrock et al., 2003).

Isotope chemostratigraphy has the potential to generate high-resolution chronostratigraphic correlations. The stable isotopecomposition of marine carbonates δ13Ccarb = 13C∕12C sample∕13C∕12C standard − 1 × 1 000 in units of per mil (‰) relative tothe Vienna-PeeDee Belemnite [V-PDB] standard) records thecoeval carbon isotopic composition of ambient dissolved inor-ganic carbon (DIC) in the ocean. Changes in δ13CDIC are thoughtto primarily derive from changes in the global burial flux oforganic carbon, which is enriched in 12C, the lighter stable isotopeof carbon, relative to the local DIC pool, and can occur at time-scales greater than 103 years (Mitchell et al., 1996; Hayes et al.,1999; Kump and Arthur, 1999; Saltzman, 2003). Increased burialof organic matter with low δ13C values, results in a concomitant

conventional and unconventional carbonateand organic-rich source rock reservoirs. Heearned a B.S. in geology at TempleUniversity and a Ph.D. from Virginia Tech.He spent two years working for Chevron,two years doing postdoctoral work at theUniversity of Miami, then thirteen years asthe state oil and gas geologist, and, finally,the acting state geologist of New York State.

ACKNOWLEDGEMENTS

We thank Alexa Stolorow and James Leonefor assistance with Petra and guidance in thefield, Brian Slater and Kathleen Bonk fortheir help with core sampling, Dwight McCayand Claire Beaudoin for their help in the lab.J. G. M. thanks Robert Criss and RichardNyahay for thought provoking discussionsand Martin Pratt for computationalassistance. Digitized logs were processedusing IHS Petra (v. 3.6.2). This manuscriptwas greatly improved from reviews byN. Harris, G. Baird, R. Koepnick, andF. McDonald. Funding was provided by theAgouron Institute and a grant from theAmerican Chemical Society PetroleumResearch Fund (#51357–DN12) awarded toD. A. F. and support from the Hanse-wissenschaftskolleg to D. A. F. and J. G. M. inthe form of a Hanse Fellowship and a TwinFellowship, respectively.The AAPG Editor thanks the followingreviewers for their work on this paper:Gordon C. Baird, Nicholas B. Harris, andRichard B. Koepnick.

1552 C-Isotope Stratigraphy Using Well Cuttings

increase in δ13C of the marine DIC reservoir fromwhich it is derived. Because δ13CDIC is well mixedin the surface ocean on the thousand-year time scale,coeval carbonates from around the globe preservesimilar δ13Ccarb signatures, which enable high-resolution correlation of strata across vast distances.However, the preservation of a global δ13Ccarb signa-ture can be overprinted either by (primary) environ-mental variability in the local δ13CDIC composition(Gruber et al., 1999) or by (secondary) diageneticalteration (Patterson and Walter, 1994); chemostrati-graphic data need to be screened to assess theseimpacts and reconstruct representative δ13Ccarb

records. The resulting time-varying δ13Ccarb patternspreserved in marine carbonates have been used exten-sively to correlate both outcrop and subsurface sec-tions from around the globe (Knoll et al., 1986;Burns and Matter, 1993; Hayes et al., 1999; Pancostet al., 1999; Veizer et al., 1999; Saltzman et al.,2000; Herrle et al., 2004; Tsikos et al., 2004; Fikeet al., 2006; Hesselbo et al., 2007; Young et al.,2008; Maloof et al., 2010; Jones et al., 2011;Sabatino et al., 2013). Yet, the majority of existingchemostratigraphic work has focused on generatinga time-series record of particular paleoenvironmentalconditions (e.g., ocean redox) or global (basin-to-basin) stratigraphic correlation; such studies do notoften have a sampling density sufficient to generatehigh-resolution chronostratigraphic tie points withinindividual basins. Furthermore, with limited sam-pling locations within a basin, it is difficult to assesslocal variability in δ13Ccarb (e.g., arising from spatialgradients in ocean chemistry or subsequent diageneticalteration) that may be superimposed on the primarytemporal signal of interest (Patterson and Walter,1994; Immenhauser et al., 2003; Swart and Eberli,2005; Swart, 2008). Diagenetic alteration typicallyresults from precipitation of carbonate cements inpore waters, wherein bacterial processes have alteredthe local δ13CDIC composition (e.g., through respira-tion: Coleman and Raiswell, 1981; or methanogene-sis: Hein et al., 2006). Alternatively, later-stagerecrystallization of carbonates can produce alterationunder the influence of meteoric or basinal fluids inwhich ambient δ13CDIC has been similarly impactedby oxidation of organic matter (Allan and Matthews,1982; Joachimski, 1994; Swart and Kennedy 2012).

Here, by combining chemo- and lithostrati-graphic data from multiple subsurface boreholes, wedemonstrate the potential of δ13Ccarb-basedapproaches for high-resolution intrabasinal correla-tions, assessment of diagenetic alteration, and theidentification of diachronous lithostratigraphic con-tacts. The resulting δ13Ccarb chemostratigraphy canbe used to reconstruct how sedimentation and faciesvaried across a basin as a function of space and time.

The purpose of this study is fivefold: (1) tocollect δ13Ccarb data from a series of core and cuttingssamples from multiple subsurface wells; (2) to assessrespective impacts of cuttings-sample resolution (ver-tical) and well density (lateral) on the ability toresolve δ13Ccarb signals in a single well and across aregion; (3) to evaluate diagenetic alteration ofδ13Ccarb in cuttings samples; (4) to produce chemo-stratigraphic logs suitable for correlation through theLate Ordovician subsurface strata of New YorkState (Figure 1); and (5) to identify stratigraphic inter-vals with distinctive δ13Ccarb character that can beused for basin-wide correlation. By focusing onubiquitous cuttings samples, we hope to encouragethe broad application of these techniques for the cor-relation of source rock and reservoir strata in basinsof varying age from around the world.

GEOLOGIC CONTEXT

The study interval ranges from the BeekmantownGroup, which straddles the Cambrian–Ordovicianboundary, through the Upper Ordovician Black RiverGroup, Trenton Group, and Utica Shale (Figure 2).The lowermost unit of the study is the BeekmantownGroup, composed of the Upper Cambrian GalwayFormation, Little Falls Dolomite, and the morelimestone-rich Lower Ordovician Tribes HillFormation (Smith, 2006). The Beekmantown Group isat least partially equivalent with the Knox Group of thesouthern United States (e.g., Tennessee; Patchen et al.,2006). Beekmantown strata are truncated by the Knoxunconformity, a major unconformity that spans most ofthe Lower Ordovician and all of Middle Ordoviciantime in the New York area (Patchen et al., 2006).

Overlying the Knox unconformity, the UpperOrdovician Black River Group consists of clean to

METZGER ET AL. 1553

argillaceous burrowed limestones, variably dolomi-tized, that were deposited on a regionally extensiveperitidal to subtidal ramp (Keith 1989; Patchen et al.,2006). The Black River Group lithology is uniformover large distances extending as far west asMissouri (Thompson, 1991). The Black River Groupis overlain unconformably by the Trenton Group(Mitchell et al., 2004), although in places TrentonGroup strata are found directly above the Knoxunconformity.

The Trenton Group is primarily composed oflimestone and argillaceous limestone and was depos-ited in a subtidal environment in central and westernNew York with deeper basinal sections deposited inthe east (Brett and Baird, 2002). Lithologies are var-ied, ranging from muddy, light gray and brown,mostly pure carbonates to dark, organic-rich calcare-ous shales. Rocks are most often muddy, but do con-tain shelly lag beds, arenites, and other coarsergrained materials (e.g., Brett and Baird, 2002). Inthe western and central parts of New York, theTrenton Group is capped by the Thruway discon-formity (Baird and Brett, 2002; Brett and Baird,2002), which cuts down into progressively olderstrata to the south and east. Where present, theThruway disconformity is overlain by the UticaShale; where the disconformity is absent insoutheastern New York, the Trenton grades upwardinto the Utica Shale.

In eastern New York, the dark gray to black faciesof Mohawkian (Middle Ordovician North Americanstage) and Cincinnatian (Upper Ordovician NorthAmerican stage) strata are referred to here as theUtica Shale, which comprises the Flat Creek andIndian Castle shale members (after Brett and Baird,2002). The Utica Shale is composed of variably calca-reous shales with infrequent micritic beds, with colorsranging from gray to black. Deposition of the TrentonGroup and Utica Shale is concurrent with the last stageof the Taconic orogeny (van Staal and Barr, 2012). Atthis time, organic-rich strata were deposited in a zonebetween the shallow marine carbonates of theTrenton to the west and deep-water turbidite faciesfound in eastern New York (Smith, 2010). TheLorraine Shale overlies the Utica Shale and hasupward-increasing sand and silt content.

Despite much study, it remains uncertain how tobest correlate the Trenton Group of western NewYork with the Utica Shale of the eastern part of thestate (Figure 2), and multiple different correlationshave been proposed (see Brett and Baird, 2002 foran in-depth discussion of correlation history).Outcrop-based correlations using lithostratigraphic,biostratigraphic, and K-bentonite data suggestthat the Trenton Group–Utica Shale contact is time-transgressive with a younger Utica Shale base in thewest (Brett and Baird, 2002). Specifically, the calca-reous and organic-rich Flat Creek, Dolgeville, andlower Indian Castle units are thought to be time-equivalent to the Trenton Group, whereas the upperIndian Castle member (Utica Shale) is younger thanthe Trenton Group in New York State. Additionalwork, particularly in the subsurface, is needed to fur-ther clarify geographic and temporal relationships ofthe Trenton Group–Utica Shale interval.

METHODS

Geophysical wireline and geological sample logs wereobtained from the Empire State Oil and GasInformation system (http://esogis.nysm.nysed.gov/). Alist of all wells and cores used in this study can befound in Table 1. Hereafter, wells are referred to by afive-digit numerical identifier; the full API 10-digitidentifier and well location can be found in Table 1.

Figure 1. Outcrop map of Late Ordovician sedimentary rocks(gray) and study locations (black diamonds). Map modified fromDicken et al. (2008). For study location information, see Table 1.

1554 C-Isotope Stratigraphy Using Well Cuttings

Lithologies for stratigraphic columns were obtainedfrom geologic sampling logs for each well and weredescribed by a geologist soon after the well wascompleted. Gamma-ray values for core C1 wereobtained using a CoreLab Instruments SpectralGamma Logger Model SGL-300 at the OhioDepartment of Natural Resources Division ofGeological Survey. Calibration was run each daywith American Petroleum Institute (API) standards 0and 200 (API units).

Cuttings samples were collected from the NewYork State Geologic Survey and cataloged at

Washington University. A representative subsampleof each cuttings sample was collected for analyses.Cuttings were collected into stratigraphic intervals of∼0.5 to 9 m (1.6 to 30 ft) at the borehole and thereforerepresent a lithologic average over that interval. Tohelp assess the impact of lithologic mixing in theanalysis of cuttings samples, several individual car-bonate chips were taken from selected intervals inwell W7. When picking individual chips, those withmacroscopic pyrite, recrystallized material, or macro-scopic spar were excluded. Individual chips werepowdered for chip-specific analysis, alongside

Cam

bria

n

Upp

erU

pper

Low

Mid

Ord

ovic

ian

Knox Unconformity

Thruway Disconformity

West Lake Erie

EastCatskills

New York Stratigraphy

Utic

a Sh

ale

Fault

Bee

kman

tow

n

Little Falls

Galway

GICE

BR-2BR-3

TR-1

TR-2

UT-1?

δ13 Ccarb

δ

13

CIn

terv

al

-3 0 3

BR-1

Moh

awki

anC

inc.

Whi

t.Sk

ullr

ocko

ckia

n

Syst

em

Seri

es

N.A

. Sta

ge

Tribes Hill

Limestone Key

Dolomitic limestone

Dolomite

Calcareous shale

Shale

Siltstone

Sugar HillLimestone

(‰)W1 W2 W3 W5 W6 W7C1W4G

loba

l Sta

geK

atia

nSa

nd.

D-D

Tre

.St

age

10Lorraine Shale

Hillier LstSteuben Lst

Tre

nton

Black River

Upper Indian Castle

Lower Indian Castle

Dolgeville

Flat Creek

Sche

nect

ady

Figure 2. Chronostratigraphic relationships of Upper Ordovician formations in New York (left), generalized δ13Ccarb intervals andδ13Ccarb chemostratigraphic profiles (right, see discussion in text). Stage abbreviations are Whit = Whiterockian, Cinc = Cincinnatian,Sand = Sandbian, D-D = Darriwilian to Dapingian, Tre = Tremadocian. New δ13Ccarb intervals (BR = Black River Group, TR = TrentonGroup, UT = Utica Shale) are named for lowest formation they are found in. GICE = Guttenberg isotopic carbon excursion (see text).“?” next to UT-1 suggests interval is not suitable for correlation (see Discussion). δ13Ccarb reference curve with intervals taken from datain this work. Values are in per mil (‰) relative to V-PDB. Study locations (W1–W7, C1) are shown in their approximate position alongthe west to east transect.

METZGER ET AL. 1555

aggregated subsamples (∼0.5 g [∼0.02 oz], represent-ing ∼10% total cuttings mass) and bulk-homogenizedpowder. For other samples, cuttings were homog-enized by powdering 0.5–10 g (0.02–0.4 oz) of sam-ples. Core samples were microdrilled to collectpowders for isotopic and geochemical analysis.

Weight % carbonate minerals ( carb) was deter-mined by gravimetric analyses following the dissolu-tion of 0.1 to 0.5 g (0.004 to 0.02 oz) of powder with6M HCl at Washington University. Complete disso-lution of carbonate fraction was obtained by additionof excess acid and agitation on a shaker table for>12 hours. Total organic carbon (TOC) was mea-sured at Washington University by combustion ofhomogenized acid-insoluble residues on a CostechECS 4010 Elemental Analyzer, whereby the emittedCO2 was quantified and calibrated against standardsof known TOC compositions. Additional TOC forwell W7 was measured at the New York StateMuseum on a UIC Coulometrics CM 5130Acidification module.

Stable isotope analyses were conducted atWashington University. Carbon isotopes are reportedas δ13Ccarb = 13C∕12C sample∕ 13C∕12C standard− 1 ×1 000 in units of per mil relative to the V-PDB stan-dard. Oxygen isotopes are reported as δ18Ocarb =

18C∕16C sample∕ 18C∕16C standard − 1 × 1 000 inunits of per mil relative to the V-PDB standard.Carbonates (δ13Ccarb and δ18Ocarb) were analyzed ona Gas Bench II attached to a ThermoFischer Delta VPlus isotope ratio mass spectrometer. A test set ofsamples were roasted at 380°C (716°F) for ∼12 hoursto liberate volatile organic compounds and revealedno systematic offset for δ13Ccarb and a minor(<∼0.3‰) effect on δ18Ocarb in roasted and unroastedsamples. The effect on δ18Ocarb was too small relativeto the magnitude of δ18Ocarb to impact our interpreta-tions. Therefore, the remaining samples were notroasted prior to δ13Ccarb and δ18Ocarb analysis.Additional carbonate analyses were done at theUniversity of Albany using a GV Instruments Optimamass spectrometer for well W1 and portions of W2.Calibration of δ13Ccarb and δ18Ocarb was done by com-parison to International Atomic Energy Associationstandards NBS-19 and NBS-18, National Institute ofStandards and Technology standard LSVEC, and in-house standards. Long-term running reproducibilityTa

ble1.

Welland

core

inform

ation

WellID

Type

API#

Operator

County

WellN

ame

Lat.(°)

Long.(°)

S13Ccarb

ref.

W1

Cuttings

31-073-09540

Consolidated

GasSupplyCorp.

Orleans

Maxon

Roger1

43.1885

−78.0376

Smith,2006;thispaper

W2

Cuttings

31-117-04754

William

J.Du

scherer

Wayne

Smith

Frank1

43.0824

−77.2696

Smith,2006;thispaper

W3

Cuttings

31-101-03924

DominionTransm

issionInc.

Steuben

Olin

142.0631

−77.4303

Thispaper

W4

Cuttings

31-011-23158

HensoilInc.

Cayuga

Carter1

42.7843

−76.5421

Thispaper

W5

Cuttings

31-007-05087

FenixandScisson

Inc.

Broome

Richards

142.3235

−75.9474

Thispaper

W6

Cuttings

31-077-10834

AmocoProductionCompany

Otse

goHo

ose1

42.9291

−74.7438

Thispaper

W7

Cuttings

31-071-01001

Crom

s-WellInc.

Orange

FeeHigh

Barney

141.4246

−74.4539

Thispaper

C1Core

NANa

tionalLeadCompany

Montgom

ery

NA42.9291

−74.7438

Thispaper

API=

AmericanPetro

leum

Institute.

1556 C-Isotope Stratigraphy Using Well Cuttings

FCSDIndian Castle Beek.

-700

-600

-500

-400

-300

-200

-100 0

100

200

300

400

500

600

700

800

900

1000

Relative Depth (ft)30 m

-800

BeekmantownTrentonUtica Black River0

200

-12

-3-3

3

GR

δ18O

W1

δ13C

(AP

I)(‰

)(‰

)0

200

-12

-3-3

3

GR

δ18O

δ13C

(AP

I)(‰

)(‰

)0

15-1

2-3

-33

GR

δ18O

δ13C

(AP

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)(‰

)0

200

-12

-3-3

3

GR

δ18O

δ13C

(AP

I)(‰

)(‰

)0

200

-12

-3-3

3

GR

δ18O

δ13C

(AP

I)(‰

)(‰

)0

200

-12

-3-3

3

GR

δ18O

δ13C

(AP

I)(‰

)(‰

)

C1

W2

W3

W5

W6

Trenton-Utica contact

Knox Unconformity

W2

W5

W6

W3

W1

C1

KIL

OM

ET

ER

S

100

200

Figu

re3.

δ13 C

carb,δ

18Ocarb,and

gamma-raylogdataforselectwells.

Isotope

values

areinpermil(‰

)relativeto

V-PD

B.D=Do

lgeville,FCS=FlatCreekShale.Form

ations

unmarkedbelowFCSincoreC1

areSugarR

iverLimestone

andBlackRiverG

roup

(see

text).Map

show

stransectthrough

thesewellswith

Trenton–Utica

outcropbeltshow

ningray.

Where

gamma-raylogs

aretru

ncated,datawerenotavailable.

METZGER ET AL. 1557

(1σ = 1 standard deviation) for multiple-day replicatesfor δ13Ccarb and δ18Ocarb was 0.09‰ and 0.12‰,respectively.

Cuttings samples for organic carbon analyseswere washed prior to processing to reduce contamina-tion from organic carbon in the drilling fluid. Organiccarbon isotopes δ13Corg were measured from theacid-insoluble organic matter (see TOC and % car-bonate [ carb] methods discussed previously) andwere analyzed by combustion to CO2 on a Costech

ECS 4010 Elemental Analyzer attached to a Delta VPlus mass spectrometer. Values for δ13Corg were cali-brated against international standards from the UnitedStates Geological Survey, USGS 24 (graphite), andInternational Atomic Energy Association standardsIAEA CH-6 (sucrose) and IAEA CH-3 (cellulose).During this study, typical precision (1σ) for δ13Ccarb

and δ18Ocarb replicates of NBS-19 for a single runwas 0.04‰. Typical multi-day reproducibility (1σ)for δ13Corg standards was 0.13‰.

Figure 4. Geologic, geophysical, and geochemical results for well W6. carb and % TOC are given as mass percent. Isotope values arein per mil (‰) relative to V-PDB. Symbol size is >1σ long-term average analytical error. Knox unconformity marked by thick wavy line.Flat Creek, Dolgeville, and Indian Castle are all part of the Utica Shale group (see text). BR = Black River. Lithological color correspondsto general shade of rocks.

1558 C-Isotope Stratigraphy Using Well Cuttings

RESULTS

Stratigraphic Trends

The δ13Ccarb and δ18Ocarb data are presented along-side gamma-ray logs in Figure 3. Clear stratigraphicδ13Ccarb trends are apparent in the sections. A rise inδ13Ccarb is observed from ∼−3 to 1‰ above theKnox unconformity to the top of the Black RiverGroup. A conspicuous interval in which δ13Ccarb

changes from 0 to 3‰ and returns to 0‰ is observedin the four westernmost wells above the top of theBlack River Group. This interval thickens towardthe center of the transect. Above this interval,δ13Ccarb values average around 1‰ in all wells,thickening toward the eastern and western edges ofthe transect. Above the 1‰ interval, δ13Ccarb values

average ∼0‰ in all but the two central wells, W3and W5. The δ13Ccarb values fall below 1‰ wherethe gamma-ray values are highest in the uppermostparts of the wells.

The δ18Ocarb values vary between −2 and −13‰.Highest values are found in the Beekmantown Group.Lowest δ18Ocarb values are found in the Utica Shaleand upper Trenton Group. Variation in δ18Ocarb islowest in the Black River Group and lower to mid-Trenton Group.

Gamma-ray values are intermediate and variable inthe Beekmantown Group. Upsection, gamma-ray read-ings are lowest in the Black River, intermediate in theTrenton Group, and highest in the Utica Shale. Thegamma-ray values increase gradually from the TrentonGroup to the Utica Shale in the easternmost wells.Conversely, in the three westernmost wells, the

Figure 5. Geologic, geophysical,and geochemical results for wellW5. carb and %TOC are given asmass percent. Isotope values are inper mil (‰) relative to V-PDB.Symbol size is >1σ long-term aver-age analytical error. One outlier(δ13Corg = −22.68‰,Δ13C = 22.83�) at depth 7765 ft(2367 m) was removed for clarity.Knox unconformity marked by thickwavy line. See Figure 4 for lithologyand symbol legend.

METZGER ET AL. 1559

gamma-ray values sharply increase at the Trenton-Uticacontact.

Figures 4–6 show lithologic and geochemicaltrends of locations W5, W6, and C1. TheBeekmantown Group is dolomitized and locallysandy with high carb and low TOC. The Black

River Group (where present) has a high carb andvery low TOC and can be variably dolomitized inthe lower portions near the contact with theBeekmantown Group (Smith, 2006). The overlyingTrenton Group carbonates are argillaceous withincreasing clay content upsection. The Utica Shale is

Figure 6. Geologic and geochemical results for core C1. Isotope values are in per mil (‰) relative to V-PDB. Symbol size is > 1σ long-term average analytical error. Isotope legend refers to δ13Ccarb and δ18Ocarb logs, in which colors refer to color-specific sampling of lith-ologies (see text). Knox unconformity marked by thick wavy line. BK = Beekmantown Group, BR = Black River Group, SR = Sugar RiverLimestone of Trenton Group. See Figure 4 for lithology and symbol legend.

1560 C-Isotope Stratigraphy Using Well Cuttings

dominated by dark carbonate strata with dark shaleinterbeds in the eastern locations (W6, C1) and ismore correctly described as marl in much of the sec-tion due to its relatively high carbonate content. TheUtica Shale in the western sections (W1–4) is exclu-sively carbonate-poor black shale.

Percent carbonate slowly declines through theUtica Shale in the easternmost locations W6

(Figure 4) and C1 (Figure 6) and the Trenton Groupin W5 (Figure 5). The carb is higher in the UticaShale of eastern locations W6 and C1. TOC averages∼2% the Utica Shale of eastern locations W6(Figure 4) and C1 (Figure 6), whereas it is more var-ied in W5 (Figure 5). In this well, TOC is ∼0%Black River and lower Trenton strata, ∼0.75% in theupper Trenton, and ∼1.5% in the Utica.

Figure 7. Comparison of isotopic results of bulk material (black symbols), multi-chip subsamples (gray symbols), and single chips(empty symbols) for well W7. Three intervals from the entire well (A) were chosen for their distinct δ13Ccarb patterns; upward declining(B), upward increasing (C), and constant (D). Isotope values are reported in per mil (‰) relative to V-PDB. In subplots B, C, and D, bulkmaterial values are shown only when single chips were analyzed at the same depth. Bulk material values are connected by a solid line.Up to three single chips were analyzed for a given depth. Symbol size is >1σ long-term average analytical error. One outlier(δ13Ccarb = 5.13, δ18Ocarb = −6.69‰) from bulk carbonate at depth 4540 ft (1384 m) was removed for clarity.

METZGER ET AL. 1561

In general, δ13Corg is relatively constant in thehighest carb strata where post-Black River Groupgamma-ray values are lowest (Figures 4, 5). Theδ13Corg averages ∼−27‰ in W5 and declines to∼−30 in the Utica Shale. The δ13Corg averages−28.5‰ in W6 (Figure 4) and declines to ∼−30‰in the Indian Castle Member of the Utica Shale.Δ13C Δ13C = δ13Ccarb − δ13Corg averages 28.5‰in W6 and very closely matches the pattern ofδ13Ccarb because δ13Corg has little variability. In W5,Δ13C increases from ∼23 to 30‰ from the BlackRiver Group to the upper Trenton Group values, thenvaries 29‰ upsection. The large-scale changes inΔ13C are not dominantly controlled by δ13Ccarb or

δ13Corg, but rather a combination of changing valuesin both.

Location C1Location C1, the northeasternmost study section andthe only core, is dominated by dark calcareous shalesand shaley carbonates. The strata above the Knoxunconformity are all part of the Utica Shale exceptfor a very thin (tentative) Black River Group andSugar River Limestone of the Trenton Group in the∼10 m (33 ft) of section immediately above theunconformity (Figure 6). Some lithologies containedlighter and darker beds (e.g., the Flat Creek Memberof the Utica Shale). The lighter beds were dominantly

Figure 8. Comparison of sampling resolution for wells and outcrop during the Guttenberg excursion interval (shaded).(A) Correlations between age-equivalent outcrop in Missouri, (Metzger and Fike, 2013) and New York wells presented here. Samplingresolution is coarser moving to the right, associated with decreasing resolution of the GICE interval. Missouri outcrop y axis (left) isexaggerated relative to the three subsurface sections. (B) Bar graph of sampling resolution for the different locations showing numberof feet per sample. High values correspond to low sampling resolution.

1562 C-Isotope Stratigraphy Using Well Cuttings

fine-grained with occasional coarse sand-sized grainscomposed of fossil debris. Lighter beds sometimesdisplayed a diffuse contact with surrounding beds.Evidence of cm-scale scouring was present in theUtica Shale members.

Additional samples were taken at cm-scale reso-lution in select intervals to investigate possible lithol-ogy-dependent variability in the δ13Ccarb signal.Samples were split into five different categories basedupon a qualitative assessment of their color. In gen-eral, the two darkest shades are stratigraphicallycoherent (i.e., have little scatter) in their isotopictrends. In contrast, the two lightest, most carbonate-rich layers displayed high degrees of scatter in bothδ18Ocarb and δ13Ccarb, in which the δ13Ccarb scatter isskewed toward heavier values relative to the overly-ing and underlying strata.

Stratigraphic SummaryWe focus on the stratigraphic trends observed in thestrata of the Black River Group, Trenton Group, andUtica Shale. Gamma-ray logs show similar patternsacross formation boundaries with the Utica Shalehaving highest gamma-ray emission. Percent carbon-ate is highest in the Beekmantown and Black RiverGroups and decreases up section. Trends in carb

and gamma-ray values are roughly opposite oneanother. The inverse relationship between carb andgamma-ray logs suggests that the gamma-ray signa-tures predominantly track potassium-rich siliciclasticinput (e.g., from detrital clay minerals), rather thanorganic carbon. TOC is highest in Trenton Groupand Utica Shale. The δ18Ocarb is generally highest inthe formations with highest carb (i.e.,Beekmantown Group to Trenton Group). Theδ18Ocarb is not stratigraphically consistent betweensections from the middle Trenton upward and is mostvariable in dolomitized zones (as described in the cut-tings geologic sampling logs) and zones with low

carb. Consistent patterns in δ13Ccarb are apparentacross multiple wells. These include (stratigraphicallyascending) (1) a rise from −3 to 0‰ in the BlackRiver, (2) a stable interval of 0.5‰ in the BlackRiver, (3) a rise from ∼1 to ∼1.5‰ in theBlack River, (4) a peak at 3‰ in the Trenton, (5) astable interval of 1‰ in the Trenton and Utica, (6) astable interval of 0‰ in the Trenton and Utica, and

(7) a drop to −1.5‰ in the Utica. This drop inδ13Ccarb below 0‰ is coincident with a decline in

carb as well as a parallel decrease in δ13Corg.

Impact of Lithologic Mixing on δ13Ccarb inCuttings

A series of tests were conducted on select cuttingssamples from well W7 to evaluate the impact of litho-logic mixing in analysis of cuttings samples. Threestratigraphic intervals were investigated in well W7(Figure 7), each containing a different backgroundδ13Ccarb trend (declining, increasing, or constant)with decreasing depth as defined by data from bulk-homogenized cuttings. For each sample investigated,several carbonate chips (single chips) were individu-ally analyzed for δ13Ccarb and δ18Ocarb, as well as apowder representing ∼0.5 g (subsample). These werecompared with the δ13Ccarb and δ18Ocarb data from thehomogenized bulk sample (bulk). A comparison ofthese results is shown in Figure 7, with the intervalscomprising the distinct δ13Ccarb trends represented inFigure 7B–D. Single chip data are characterized byincreased variability in δ13Ccarb and δ18Ocarb whencompared with bulk samples or the subsamples.Single chip δ13Ccarb scatter increases when the base-line bulk δ13Ccarb signal is increasing or decreasing.Single chip δ13Ccarb scatter is low when the bulkδ13Ccarb signal is not changing. Although single chipsreasonably track the bulk δ13Ccarb signal (albeit withincreased variability), the ∼0.5 g (∼0.02 oz) subsam-ple more closely tracks the δ13Ccarb signal of the bulksignal.

Impact of Sampling Resolution

A comparison of δ13Ccarb profiles sampled at differentresolutions through the same interval is shown inFigure 8. The variable vertical sampling resolutionbetween locations results in differing stratigraphic mor-phologies of the same δ13Ccarb signal, where a lowersampling resolution results in a smoother isotopic rec-ord. The interval of elevated δ13Ccarb (gray) is ∼30 ft(∼9 m) at the New London outcrop in Missouri (seeMetzger and Fike, 2013 for discussion of locality),whereas the same interval is >300 ft (>91 m) in NewYork well W4. The sampling resolution in well W4 is

METZGER ET AL. 1563

1564 C-Isotope Stratigraphy Using Well Cuttings

30 ft (9 m); a similar sampling resolution in NewLondon would reduce the entire interval of elevatedδ13Ccarb values to a single data point.

DISCUSSION

The δ13Ccarb can be used to chronologically correlatestrata because the δ13CDIC value synchronouslychanges across any well-mixed portion of the ocean.Stratigraphically meaningful correlations requirepreservation of the original δ13Ccarb signal, and vari-ous post-depositional processes may alter δ13Ccarb

values. Sampling methods (e.g., vertical samplingresolution and sample selection) are additional con-trols on the resolution and accuracy of δ13Ccarb logs.Here, we identify seven intervals with distinctiveδ13Ccarb signatures that can be used for correlation.We then examine evidence of alteration of the origi-nal δ13Ccarb signal and the impact of sampling resolu-tion on the morphology of δ13Ccarb logs.

Seven discrete δ13Ccarb intervals, bounded bycorrelation points in the carbon isotope logs, havebeen identified within the stratigraphic interval exam-ined (Figure 9): three in the Black River Group (BR-1,BR-2, and BR-3) and four in the Trenton Group andUtica Shale (GICE, TR-1, TR-2, and UT-1). Theseare defined as follows:

• BR-1: an interval of increasing δ13Ccarb from−3 to 0‰ in the Black River Group.

• BR-2: a stable interval of ∼+0.5‰ in δ13Ccarbin the Black River Group above BR-1.

• BR-3: an interval starting at ∼1‰ in δ13Ccarband increasing to ∼1.5% in the Black RiverGroup above BR-2.

• GICE: a positive excursion in δ13Ccarb, increas-ing from ∼0.5 to 3‰ before decreasing to

∼0‰. Known as the Guttenberg isotopic car-bon excursion (GICE), this positive δ13Ccarbexcursion has been identified in much of centraland eastern Laurentia (e.g., Ludvigson et al.,2004; Young et al., 2005; Bergström et al.,2010; Metzger and Fike, 2013) as well asSweden (Bergström et al., 2004), China(Young et al., 2005; Bergström et al., 2009),and Malaysia (Bergström et al., 2010). In thispaper, it begins near the base of the TrentonGroup.

• TR-1: a stable interval of +1‰ in δ13Ccarb inthe Trenton Group (western-central New York)and Utica Shale (eastern New York) that post-dates the Guttenberg excursion.

• TR-2: a stable interval of ∼0‰ in δ13Ccarbabove TR-1.

• UT-1: an interval of decreasing δ13Ccarb from∼0 to −1.5‰.

Figure 2 shows a δ13Ccarb reference curve (di-vided into seven δ13Ccarb intervals) for the NewYork region and its relationship to formation bounda-ries across the state.

Evaluating Diagenetic Alteration

Diagenetic (postdepositional) processes can alteroriginal δ13Ccarb values in marine carbonates (Allanand Matthews, 1982; Patterson and Walker, 1994;Melim et al., 2004; Metzger and Fike, 2013), givingrise to apparent spatial heterogeneities in δ13Ccarb val-ues and limiting the ability to use δ13Ccarb for correla-tions. Several recognized mechanisms exist by whichdiagenesis can alter δ13Ccarb. The most common isoxidation of organic matter δ13Corg ∼ −30‰ ,

Figure 9. Proposed correlations of the New York subsurface based on δ13Ccarb. Red line (map) shows sampling transect for both(A) and (B). GR = gamma ray (in API units). (Note that API range is 0–200 API units for all locations except for W3 for which the rangewas 0–15 API units due to a different gain setting during analysis. The 0–15 API unit range in this well is roughly equivalent to the 0–200units in other wells.) δ13C = δ13Ccarb given in per mil (‰) relative to V-PDB, δ18O = δ18Ocarb given in ‰ relative to V-PDB, BK =Beekmantown Group, BR = Black River Group, SR = Sugar River Limestone. (A) Isotopic intervals (colored zones) show shift in the sedi-ment depocenter from western and central New York from δ13Ccarb to eastern New York following the GICE. Isotopic intervals aredefined and discussed in text (see Discussion). Generalized δ13Ccarb curve for New York region and chronostratigraphic relationshipsare found in Figure 1. The basal contact of the Utica Shale is marked by a thick dashed red line. Wells are hung on base of δ13Ccarb inter-val TR-1. (B) Sampling transect showing time-equivalent nature of lower Utica (dark gray) and upper Trenton (light gray) facies. Wellshung on top of δ13Ccarb interval TR-2.

METZGER ET AL. 1565

which results in a decrease in δ13CDIC of pore fluidsand subsequently a decrease in δ13Ccarb of any stratathat re-equilibrate with such pore fluids. This canhappen either as meteoric diagenesis, in which soilcarbon is the source (e.g., Knauth and Kennedy,2009; Swart and Kennedy, 2012), or during burialdiagenesis (e.g., Derry, 2010), in which oxidationresults from thermal breakdown of migrating hydro-carbons or in situ organic matter.

Covariation in δ13Ccarb, δ18Ocarb, and %carb

Geochemical crossplots can provide context for identi-fying possible postdepositional alteration. An exami-nation of covariation between geochemical

parameters and δ13Ccarb can constrain possible diage-netic pathways. Often, covariation in δ13Ccarb andδ18Ocarb is examined, because changes in δ18Ocarb arelargely or entirely independent of changes in δ13Ccarb

prior to alteration, and therefore covariation betweenthese two parameters is not expected. However, inmeteoric fluids both δ13C and δ18O are low (e.g.,Bathurst, 1975; Allan and Matthews, 1982;Lohmann, 1988), and covarying δ13Ccarb and δ18Ocarb

can indicate isotopic alteration by meteoric diagenesis(we note that although covariation can indicate diage-netic alteration, the absence of covariation cannot betaken as evidence of primary δ13Ccarb values beingretained). For example, covariation in δ13Ccarb andδ18Ocarb is observed in formations that have

D E GF

Black River

-12 -10 -8 -6 -4-4

-2

0

2

4

-12 -9 -6 -3

-4

-2

0

2

4

-4

-2

0

2

4 CBA

δ18Ocarb (‰)

δ13C

carb

(‰)

δ13C

carb

(‰)

δ18Ocarb (‰)δ18Ocarb (‰)

δ13C

carb

(‰)

δ18Ocarb (‰)

W3

= 0.55r2

= N.A.r2

= 0.36*r2

= 0.03r2

W2

= 0.65*r2

= N.A.r2= 0.22*r2

W1

= 0.33 r2

= N.A.r2

= 0.13 r2

= 0.00r2

Indian Castle Shale

Dolgeville

Flat Creek Shale

Sugar River

Eastern Legend

-12 -10 -8 -6 -4

= 0.41r2 = N.A.r2

Utica

Trenton

Black River

Beekmantown

Cambrian (undifferentiated)

Western Legend

Lorraine

-4

-2

0

2

-3 -1 1 3-30

-29

-28

-27

0 25 50 75 100-4

-2

0

2

0 25 50 75 100-12

-10

-8

-6

I J

L

W6

= 0.47r2

= 0.24r2

= 0.53*r2

δ18Ocarb (‰)

δ13C

carb

(‰)

δ13C

carb

(‰)

% carb

δ13C

org

(‰)

δ13Ccarb (‰)

δ18O

carb

(‰)

K

% carb -12 -10 -8 -6 -4

= N.A.r2

= 0.67r2

= 0.95r2

= 0.35r2

= 0.40*r2

= N.A.r2

= 0.36r2

= 0.52r2

= 0.19r2

= 0.63*r2

= N.A.r2

= 0.17r2

= 0.56r2

= 0.40r2

= 0.44*r2

= N.A.r2

= 0.02r2

0 25 50 75 100-4

-2

0

2

4

-4 -2 0 2 4-30

-28

-26

-24

0 25 50 75 100-13

-10

-7

-4

δ18Ocarb (‰)

δ13C

carb

(‰)

δ13C

carb

(‰)

% carb

δ13C

org

(‰)

δ13Ccarb (‰)

δ18O

carb

(‰)

W5

= 0.46*r2

= 0.01r2

= 0.09r2

= 0.28r2

= 0.34r2 = 0.01r2

= 0.01r2

= 0.18r2

= 0.00r2= 0.43r2

= 0.16r2

= 0.00r2

= 0.01r2= 0.19r2

= 0.07r2

= 0.06r2

= 0.00r2

% carb -14 -12 -10 -8 -6 -4

-6

-4

-2

0

2

4= 0.10r2

H

δ13C

carb

(‰)

= 0.02 r2

= 0.14 r2

= 0.50 r2

= 0.11 r2

C1

Beekmantown-10 -8 -6-4

-2

0

2

0 25 50 75 100

δ13C

carb

(‰)

-4

-2

0

2

% carb

= 0.30 r2

= N.A. r2

= 0.00 r2

= 0.24 r2

M

0 25 50 75 100-10

-8

-6

% carb

δ18O

carb

(‰)

N

= N.A. r2

= N.A.r2

= 0.30 r2

= N.A. r2

= 0.00 r2

= 0.24 r2

Figure 10. Geochemical cross-plots of well (A) W1, (B) W2, (C) W3, (D–G) W5, (H–K) W6, and (L–N) core C1. Western sections havegray background. Eastern sections have white background. * indicates outliers omitted from r2 calculation. These outliers are shown byarrows. For all isotope values, symbols are larger than 1σ long-term average analytical error. No r2 is given for data sets with n < 3.

1566 C-Isotope Stratigraphy Using Well Cuttings

undergone visible karstification (e.g., Lohmann,1988). When considering diagenetic alteration ofδ13Ccarb, two important points must be considered:(1) most diagenetic mechanisms decrease δ13Ccarb;and (2) δ18Ocarb is easier to reset than δ13Ccarb duringdiagenesis because of the relative abundance of O rel-ative to C in any diagenetic fluid (e.g., Banner andHanson, 1990). This offset in oxygen and carbonconcentrations in diagenetic fluids allows quantitativedescription of the volume of diagenetic fluid requiredto change the rock’s δ13Ccarb and δ18Ocarb values.This is normally described in terms of fluid to rockratios (fluid:rock). Most relevant here is the molar ratioof the cumulative flux of pore fluids migrating througha sample to the volume of carbonate material withinthe sample. Under a low fluid:rock ratio, the δ18Ocarb

is vulnerable to change, whereas the δ13Ccarb is lesssusceptible due to the lower carbon content of the flu-ids (Banner and Hanson, 1990). For a given flux ofdiagenetic fluids, siliciclastic-dominated strata withlow percent carbonate will have their δ13Ccarb andδ18Ocarb signatures more easily altered compared tothe same volume of carbonate-rich strata. However,the low porosity/permeability of fine-grained silici-clastics (e.g., shales) results in decreased fluid flux,thereby increasing preservation potential of a primaryisotopic signature in lower carb strata. Bulk rockδ13Ccarb signatures can also be altered by the additionof pore-filling secondary cement, which adds secon-dary material to void spaces, rather than sequentialequilibration of the bulk rock itself with the diageneticfluid. These factors must be considered when inter-preting δ13Ccarb records in fine-grained lithologieswith low carbonate contents.

Most of the formations in this study do not showstrong covariation in δ13Ccarb and δ18Ocarb

(Figure 10), and, therefore, we find no formation-scaleevidence of meteoric diagenesis. However, in TrentonGroup strata, δ13Ccarb and δ18Ocarb show moderate tostrong correlation in all wells (r2 = 0.33 to 0.65).Understanding the origin of the covariation in theTrenton Group requires closer examination, and wewill look at well W3 as an example. There are threeδ13Ccarb intervals present in the Trenton Group: theGICE, TR-1, and TR-2. These intervals are all charac-terized by minimal stratigraphic variability (i.e., scat-ter) and relatively high values of δ13Ccarb, both

inconsistent with the variably decreasing δ13Ccarb

expected from alteration by diagenetic fluids withlow δ13C values. Examining δ18Ocarb values in thesethree intervals, we see a general trend towarddecreased δ18O values. This trend, however, is notconsistent from well to well. Thus, whereas the upper-most GICE, TR-1, and TR-2 show consistent δ13Ccarb

signatures between sections, they are characterized byvariable δ18Ocarb profiles. The low andspatially variable δ18Ocarb values and spatially repro-ducible δ13Ccarb profiles support diagenetic alterationto δ18Ocarb, but not δ13Ccarb (Figure 9). This is consis-tent with low water:rock ratio (because δ18Ocarb ismore easily reset), suggesting that the low scatterδ13Ccarb did not result from homogenization duringdiagenesis. The simultaneous decrease in bothδ13Ccarb and δ18Ocarb in the uppermost GICE is merelycoincidental, because the GICE shows the samedecline in other wells (wells W2 and W1) with nochange in δ18Ocarb (Figure 9). It is the combination ofvariably altered δ18Ocarb values against a changing pri-mary δ13Ccarb trend, rather than diagenetic alterationof δ13Ccarb itself, that produces a moderate degree ofcovariance in each of the three isotopic intervals inTrenton Group strata (e.g., Figure 10, well W5:r2 = 0.55). Further evidence of a low water:rock ratiocan be seen in TR-1 of wells W1, W2, and W3(Figure 9), in which δ13Ccarb is consistent whereasδ18Ocarb shows great disparity between wells.

Another interval of possible isotopic alteration isUT-1. The δ13Ccarb interval UT-1 shows an upwarddecrease in δ13Ccarb from 0 to −1.5‰, a paralleldecrease in carb, and constant, but low, δ18Ocarb

(Figures 4, 5). Because δ18Ocarb can be reset underlow water:rock ratios (in this case “rock” is equiva-lent to carbonate content), then UT-1 can plausiblybe the result of (1) meteoric diagenesis under a fixedflux of fluid in which strata with lower carb havelower δ13Ccarb, or (2) differential fluid:rock ratiosacross the strata with a fixed carb where δ13Ccarb islower in strata where fluid flux was higher. Ifδ13Ccarb interval UT-1 is diagenetic in origin, thenthis unit may not represent coeval strata, but insteadcapture a regionally extensive diagenetic event or epi-sode, information which may be useful for other pur-poses (e.g., reservoir development) beyondchronostratigraphic correlation.

METZGER ET AL. 1567

δ13Corg and Δ13C

Another isotopic parameter, δ13Corg, can be used toidentify potential alteration of δ13Ccarb. This is becauseorganic matter is derived from the local dissolved inor-ganic carbon (DIC) reservoir and is isotopically offsetfrom DIC as a result of biological carbon fixation(e.g., Hayes et al., 1999). The offset is relativelyconstant when the conditions controlling the source(e.g., growth rate, CO2 aq; Popp et al., 1997) and pres-ervation (e.g., thermal maturity, Des Marais, 1997) oforganic matter are stable, so any variation in δ13CDIC

(i.e., what is eventually recorded in δ13Ccarb) shouldappear as a parallel variation in δ13Corg. This, in turn,leads to a constant Δ13C = δ13Ccarb − δ13Corg; e.g.,Kump and Arthur, 1999). Variations in δ13C can pro-vide constraints on possible processes that changeδ13Ccarb or δ13Corg signatures. For example, in themiddle of the GICE interval of well W5 (Figure 6),Δ13C increases for ∼90 ft (∼27 m) up section beforereturning to a relatively stable value of 29‰. Thisresults from a larger decrease in δ13Corg than δ13Ccarb.Meteoric diagenesis should not affect δ13Corg, and thestable and high δ18Ocarb values in this interval providefurther evidence that meteoric diagenesis is unlikelythe source of the change in Δ13C. Rather, a short-termchange in organic carbon source or composition (witha different isotopic fractionation from DIC and there-fore a different δ13Corg value) could explain the pat-tern, and this explanation has been proposed for strataof similar age in Iowa (Pancost et al., 1999) and else-where (Pancost et al., 2013).

Stratigraphic ReproducibilityConsistent δ13Ccarb patterns across multiple locationsin a region are deemed stratigraphically reproducible.The lack of regional stratigraphic reproducibility canreveal local (diagenetic or primary) overprints on theδ13Ccarb signals. A primary oceanic δ13Ccarb signalwill be consistent over a large area, whereas the iso-topic composition of diagenetic fluids (or a localdeviation in δ13CDIC) are inherently variable overlarge distances. To have stratigraphically reproduc-ible diagenetic δ13Ccarb patterns means all locationsmust have been modified in such a way that the com-bination of (1) the degree of diagenesis and (2) theisotopic signature of the diagenetic fluid(s) resulted

in the same alteration of the δ13Ccarb signal. The largelocal range in O and C isotopic signatures of diage-netic fluids and the differing lithological propertiesof the rocks in different areas will tend to producescatter in δ13Ccarb and δ18Ocarb of altered rocks. Theinverse, low scatter (particularly in δ13Ccarb) is con-sistent with a primarily oceanic signal, which shouldproduce a smoothly changing isotopic record throughtime. Intervals BR-2, BR-3, GICE, TR-1, and TR-2,show very consistent, smoothly changing δ13Ccarb

patterns between sections, strong evidence for a pri-mary signal. Some δ13Ccarb intervals show minor dis-crepancies between wells (TR-1 of W2, BR-1 of W1),but the overall trends are clear enough to distinguishbetween δ13Ccarb intervals.

All but one of the δ13Ccarb intervals in this workare characterized by stable, high δ13Ccarb values orduring positive rather than negative trends. The singlenegative interval, UT-1, does not carry a stratigraphi-cally reproducible morphology and is found in stratawith low carbonate abundance and low δ18Ocarb, sug-gesting this interval is the product of alteration.However, the stable Δ13C values in UT-1 suggestalteration is minimal. There are stratigraphic argu-ments arguing against UT-1 being a correct chrono-stratigraphic correlation. The base of the Utica Shaleis younger in the west (Ruedemann, 1925; Kay,1937), because of this we would predict that the iso-topes would not align. At this time, the primarynature of this interval is unlikely.

Absence of covariation in geochemical parame-ters and stratigraphic reproducibility cannot provealteration did not occur, they merely provide no evi-dence for alteration. Conversely, covariation and lack

Figure 11. Isotopic cross-plots for light (concretionary) anddark samples from core C1. r2 refers to linear least-squares fit.* = one outlier (arrow) omitted from regression calculation.

1568 C-Isotope Stratigraphy Using Well Cuttings

of stratigraphic reproducibility can show areas wherealteration most likely did occur (e.g., UT-1), animportant distinction. Minor differences in δ13Ccarb

for a given isotope interval do exist between wells(e.g., small negative shifts in TR-1 of W2) and maybe the result of primary or secondary overprints tothe δ13Ccarb trend, but the larger δ13Ccarb patterns(at the scale of 10s–100s of feet) in each interval aresufficient to distinguish them from the other δ13Ccarb

intervals.

Relationship between Color and Isotopes in Core C1Centimeter-scale sampling of light and dark layers(Figures 6, 11) in core C1 reveals significant color-specific covariation between δ13Ccarb and δ18Ocarb.Stratigraphically, the intervals TR-1 and TR-2 areclearly defined in the darkest layers of core C1,whereas interbedded lighter layers showed high iso-topic scatter as well as δ13Ccarb values higher thanobserved in other wells (Figure 6). Although datafrom the dark layers cluster together in a δ13Ccarb ver-sus δ18Ocarb crossplot (Figure 11), the lighter layersare arranged along a line that intersects the centroidof the samples from the darker layers, a pattern con-sistent with a linear mixture of two isotopically dis-tinct components: a dark primary carbonate phaseand a lighter colored diagenetic carbonate compo-nent. Therefore, δ13Ccarb values obtained from thelighter layers are not representative of global (or evenbasin-wide) δ13Ccarb and should not be used forchemostratigraphic correlation.

One possible source for the higher δ13Ccarb valuesobserved in the lighter component is precipitation influids that were impacted by microbial methanogene-sis. Because methane carries a low δ13C value rela-tive to the surrounding dissolved inorganic carbon(DIC) reservoir (e.g., Conrad, 2005), methane pro-duction leads to an increase in local δ13CDIC if themethane is not re-oxidized and returned to the DICreservoir. Methanogensis has been argued to be thesource of high δ13Ccarb values in septarian concre-tions of the Devonian Marcellus Shale of centralNew York (Siegel et al. 1987). Some of the lighterbeds in core C1 are fossil-poor micrite in thin section,display diffuse contacts with darker beds, and appearconcretionary in outcrop (G. C. Baird, 2013, personalcommunication) suggesting that some of the lighter

bands may be concretionary or impacted by in situcarbonate precipitation during lithification and/or dia-genesis. Widespread, abundant concretions have beenobserved in the Kope Formation of Kentucky andOhio (Brett et al., 2008), which is partially contempo-raneous with the Utica Shale of New York. The highdegree of scatter, lack of regionally reproducibleδ13Ccarb values or vertical trends, and petrographiccharacteristics of some of the lighter-colored layersis consistent with a significant secondary overprintof isotope values and suggests these materials shouldbe avoided when correlating.

The higher δ13Ccarb values in the lighter carbonatelayers may explain the origin of the heavier thanexpected δ13Ccarb values in depths 5350–5370 ft(1631–1637 m) of well W6 (Figure 4) as the lighter col-ored layers correlate lithologically to the higher thanexpected δ13Ccarb values in W6. Although cuttingssamples were screened for obvious alteration(e.g., macroscopic spar or pyrite), they were notscreened by color, so it is possible that these three sam-ples contain the lighter material with higher δ13Ccarb

values as seen in core C1. Alternatively, the higherδ13Ccarb values in W6 could be explained by a localδ13CDIC signal. Grain-specific δ13Ccarb analyses andan increased well density could test this hypothesis.

Evaluation of Lithologic Mixing on δ13Ccarb inCuttings

Results from the single-chip test in well W7 indicatemoderate degrees of isotopic heterogeneity on a scalesmaller than the cuttings sampling resolution (i.e.,within a single cuttings sampling interval).Variability in δ13Ccarb of single chips is higher whenthe baseline δ13Ccarb values of bulk cuttings is chang-ing. This variability is interpreted to result from themixture of stratigraphically varying componentswithin the cuttings sample. The δ13Ccarb values ofmultiple individual chips approximate the subsampleδ13Ccarb values (∼0.5 g [∼0.04 oz]), which in turnapproximates the bulk δ13Ccarb values of the bulkmaterial (except in cases of strongly varying back-ground δ13Ccarb and/or δ18Ocarb signals). The rangeof values in δ13Ccarb suggests that only a small numberof chips (coarse sand and larger) need to be powderedto satisfactorily approximate the total sample values.

METZGER ET AL. 1569

In general, the magnitude of δ18Ocarb variabilitywas higher than that of the stratigraphically equiva-lent δ13Ccarb. This is consistent with the theoreticalprediction that δ18Ocarb is easier to reset during dia-genesis (e.g., Banner and Hanson, 1990). Small-scalevariation in porosity and permeability can result invariable δ18Ocarb over the stratigraphic interval that asingle cuttings sample represents (e.g., 10 ft [3 m]).Those materials with higher permeability will havemore fluids pass through them and therefore are moresusceptible to alteration of the original δ18Ocarb signal(for fixed carbonate content). When picking materialin a given sample, micritic components seem to havethe best potential to record a primary oceanic δ13Ccarb

signal (e.g., Metzger and Fike, 2013).

Evaluation of Sampling Resolution

In chemostratigraphic studies, the geographic resolu-tion is set by the lateral well spacing, whereas thetemporal resolution is set by the stratigraphic (verti-cal) sampling resolution. A high stratigraphic sam-pling density most closely reproduces the evolvingmarine δ13Ccarb record. Cuttings typically averagelithology over depths of ∼1 to 10 m (∼3 to 33 ft),whereas core and outcrop studies only average overthe width of the drill bit used for sampling (a fewmm). For this reason, at a given sampling resolution,chemostratigraphic profiles from cuttings will besmoother than the equivalent profiles obtained fromcores or outcrops. As seen in Figure 8, low samplingresolution in well W4 dampens the magnitude of iso-topic variations and can give the appearance of lateralisotopic variability between localities where there isnone. Low sampling resolution also obscures thestratigraphic position of distinct isotopic tie points(e.g., the low sampling resolution in well W4 is notappropriate for detailed chemostratigraphic correla-tion). Similar effects of averaging are present in othergeochemical records, such as TOC and carb. Thisstratigraphic averaging is important for calibrating iso-topic data against other borehole data such as wirelinelogs (e.g., gamma ray), which more accurately tracktrue depth. When choosing wells for a study, espe-cially across a large range of sedimentation rates, thesampling resolution must be sufficient to identify,resolve, and correlate geochemical features of interest.

SummaryCovariation in isotopic and geochemical parameterscan supply one method for evaluating diageneticalteration. Pairing of δ13Ccarb with δ13Corg can beused to test for meteoric diagenesis during negativeδ13Ccarb excursions; however, this method is limitedby the natural variability in organic carbon sourcesand should be used in tandem with other screeningprocedures. Stratigraphic reproducibility of δ13Ccarb

trends in closely spaced wells can discriminatebetween basin-wide and local δ13Ccarb signals. Thiscombined approach helped in identifying the diage-netic origin of both high δ13Ccarb (e.g., core C1) andlow δ13Ccarb values (e.g., TR-1 in well W2,Figure 9). Low degrees of scatter and stratigraphicreproducibility of δ13Ccarb intervals BR-2, BR-3,GICE, TR-1, and TR-2 strongly suggest these inter-vals represent a primary basin-wide or globalδ13CDIC signal. Analysis within this paper suggeststhat the vast majority of the rocks within the intervalBR-1 retain the original δ13Ccarb signal, but is varia-bly impacted by local diagenetic processes. The pri-mary nature of the UT-1 δ13Ccarb values is unlikely.

Geologic Implications

Correlations using δ13Ccarb intervals reveal strati-graphic relationships not apparent using gamma-raylogs and geologic sampling logs. For example, theBlack River Group can be confidently subdivided intothree chronstratigraphic units based on δ13Ccarb logs.These chronostratigraphic divisions are very similarto divisions that would be made using gamma-ray logs(except for the uppermost part of the Black RiverGroup in W3, in which isotopic data suggest variableerosion of the upper Black River strata) and suggestthat the environmental change was synchronous acrossthe basin. Conversely, δ13Ccarb interval TR-1 showsthe diachronous nature of environmental changeacross the basin as the shaley facies form in the easternportion of the basin first. Further, δ13Ccarb intervalsBR-1, BR-2, and BR-3 show that differences in thick-ness of the Black River Group between sections resultmainly from differences in the thickness of the upperportion of the Black River (i.e., BR-3). The fact thatBR-3 is proportionally much thicker in the well W3may result from non-deposition or erosion of the

1570 C-Isotope Stratigraphy Using Well Cuttings

uppermost Black River strata in the regions of theflanking wells. In this interpretation, the top of intervalBR-3 in wells W1 and W2 is truncated and would cor-relate to the middle of interval BR-3 in well W3. Thiscorrelation is supported by the fact that the highestδ13Ccarb values in BR-3 are found in the upper part ofthis interval in well W3 and not observed in BR-3from adjacent wells with a thinner BR-3 interval. Anunconformity between the Black River and overlyingTrenton groups is known in outcrop strata north ofthe western wells near Lake Erie (Mitchell et al.,2004) and the unconformity may extend down to ourstudy area, manifested as variable truncation of theuppermost BR-3 interval.

The δ13Ccarb interval above BR-3 is the GICE.This interval appears to be fully preserved in the threewestern wells examined here (W1, W2, and W3) andmostly present in W5. This is evidenced by thegradual thickening and thinning of the GICE intervaland the preservation of both the rise and fall inδ13Ccarb values across the basin. In eastern NewYork, either non-deposition occurred or erosion sub-sequently removed GICE strata. This is shown inFigure 9A in which δ13Ccarb intervals BR-2, BR-3,and the GICE interval pinch out between wells W5and W6. TR-1 sediment accumulation as encounteredin the eastern wells began immediately following theGICE interval, as evidenced by the thick rising limbof interval TR-1 prior to the 1‰ plateau. If more timewere missing, the rise in δ13Ccarb would not be cap-tured and would instead appear as a step change inδ13Ccarb. This places tight chronostratigraphic boun-daries on the events and factors that controlled sedi-mentation across the basin. Assuming thatsedimentation was restricted in eastern New Yorkdue to insufficient accommodation space, then newaccommodation space was created just after the endof the GICE. The increase in accommodation spacelikely arose from extensional faulting in whicheastern New York was rapidly thrust downward fol-lowing the GICE interval. The tectonic activityshifted the basin depocenter from southwestern NewYork (near well W3) before the GICE to easternNew York (near core C1) after the GICE.

Coupled lithostratigraphy and chemostratigraphycan be used to reconstruct basin dynamics. Figure 9shows the relative position of the base of the Utica

Shale contact (lithostratigraphic contact; red wavy linein Figure 9) and top of TR-1 (chemostratigraphic con-tact; top of blue zone in Figure 9). These lines crossmoving from east to west across New York State.This relationship demonstrates the time-transgressivenature of the Utica Shale contact, in which the basalUtica Shale is progressively younger moving westacross New York. This is in general agreement withinterpretations of Brett and Baird (2002), who usedlithostratigraphic, biostratigraphic, and K-bentonitedata to construct their chronostratigraphic correlationsof outcrops in central and eastern New York.

Figure 9B shows two sets of correlations arisingfrom lithostratigraphic (light and dark gray-shadedbackgrounds) and chemostratigraphic methods (col-ored lines). A detailed chronostratigraphic under-standing of the diachronous nature of the lithologicchange is made possible using the δ13Ccarb data andproduces a different depositional history than mayotherwise be constructed for the Trenton–Uticasequence using gamma-ray logs and geologic samplelogs alone. Correlations based upon shifts in gamma-ray logs and lithologic transitions from limestones toshales would suggest more strata missing in easternNew York than the correlations based upon δ13Ccarb

data alone. Although the time-transgressive nature ofthe Trenton–Utica contact has been identified in pre-vious studies (see Baird and Brett, 2002; Brett andBaird, 2002), these were conducted using outcropswith abundant fossils, event beds, and K-bentonites,materials that are rare or unavailable in the subsur-face. This agreement suggests δ13C chemostratigra-phy is a fruitful way to link together subsurfaceregions and their correlative outcrop belts within aunified chronostratigraphic framework. This paperdemonstrates the power of δ13Ccarb stratigraphy forunderstanding chronostratigraphic relationships inthe subsurface, especially across different depositio-nal facies and lithologies (Figure 9B) for which bio-stratigraphy may not be practical or feasible.

CONCLUSIONS

The ubiquity of well cuttings and the ability to rapidlyanalyze large numbers of samples for δ13Ccarb,δ18Ocarb, and δ13Corg have created an opportunity to

METZGER ET AL. 1571

enhance chemostratigraphic studies from a few out-crops and cores to a network of wells to obtain thespatial density needed to assemble a high-resolution,basin-wide correlative framework in the subsurface.The sampling of wells in a high spatial density thatresults in large data sets can be used to resolve small(‰-level) changes in δ13Ccarb that may otherwise beascribed to noise when considering single sectionsalone. Multiple δ13Ccarb intervals can be found withina single formation or lithology demonstrating the poten-tial high-resolution correlative power of δ13Ccarb che-mostratigraphy from large data sets. We believe thispaper demonstrates that δ13Ccarb can provide robustchronostratigraphic correlations across thick strati-graphic packages and over large lateral distances withsmall sample volumes and limited labor investments.The δ13Ccarb chemostratigraphy may be particularlyuseful in basins where environmental segregation ofspecies during deposition limits the utility of biostratig-raphy, such as the Permian Khuff Formation of SaudiArabia (Dasgupta et al., 2001), the Jurassic ArabCycles of Saudi Arabia (Alsharhan and Whittle,1995), and the Silurian of the Michigan basin(Mesolella et al., 1974).

Several points must be kept in mind for acuttings-based chemostratigraphic study:

1. Wells for cuttings-based studies must be chosencarefully based on their maximum verticalsampling resolution and the number of availablewells in a given area needed to create the desiredspatial resolution. Wells with coarse samplingintervals can limit the stratigraphic resolutionspossible, as well as shift the isotopic values andstratigraphic position of local isotopic minimaand maxima.

2. Isotopic analysis of single chips show that only asmall fraction (e.g., <0.5 g [<0.02 oz]) of a stan-dard cuttings sample must be homogenized toobtain representative isotopic values.

3. The resulting data must be screened for diageneticalteration that could otherwise skew resulting che-mostratigraphic correlations. In particular, parallelδ13Ccarb and δ13Corg trends are strong evidencefor a primary δ13C signal in both phases.Similarly, covariation in δ13Ccarb, δ18Ocarb, TOC,and carbonate abundance can be indicators for dia-genetic alteration.

4. Diagenetic alteration of a single location cannot befully assessed without comparison of isotopictrends (here, δ13Ccarb, δ13Corg, and δ18Ocarb) frommultiple sections within a basin, because tradi-tional techniques for assessing diagenetic altera-tion (e.g., covariation in δ13Ccrab, δ18Ocarb) canalso result from primary environmental signals.Testing for reproducible δ13Ccarb patterns fromclosely spaced wells (stratigraphic reproducibility)is a rigorous technique for assessing diageneticalteration at the basin scale.

Chemostratigraphic study of Upper Ordovicianstrata of New York has revealed that

1. Consistent with results using biostratigraphic, lith-ostratigraphic, and event-bed stratigraphic meth-ods from outcrops in central New York State, theTrenton Group–Utica Shale contact is time-transgressive in subsurface regions south and westof the outcrop belt with the basal Utica Shale beingprogressively younger moving west across NewYork. This can be observed in the δ13Ccarbcomposite presented herein for the New Yorkregion of the Middle and Upper OrdovicianMohawkian and Cincinnatian Series, respectively.

2. The locus of sedimentation shifted fromsouthwestern New York (during the deposition ofthe Black River Group to middle Trenton Group)to eastern New York during deposition of theUtica Shale because of local tectonic forces likelyrelated to the Taconic orogeny.

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