Application of Foreland Basin Detrital-Zircon Geochronology to the

[The Journal of Geology, 2010, volume 118, p. 000–000] � 2010 by The University of Chicago.All rights reserved. 0022-1376/2010/11801-00XX$15.00. DOI: 10.1086/648400

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JG vol. 118, no. 1 2010 Tuesday Oct 27 2009 01:03 PM/80097/MILLERD CHECKED

Application of Foreland Basin Detrital-Zircon Geochronology to theReconstruction of the Southern and Central Appalachian Orogen

Hyunmee Park, David L. Barbeau Jr., Alan Rickenbaker, Denise Bachmann-Krug,and George Gehrels1

Department of Earth and Ocean Sciences, University of South Carolina, Columbia,South Carolina 29208, U.S.A.

(e-mail: [email protected])

A B S T R A C T

We report the U-Pb age distribution of detrital zircons collected from central and southern Appalachian forelandbasin strata, which record changes of sediment provenance in response to the different phases of the Appalachianorogeny. Taconic clastic wedges have predominantly ∼1080–1180- and ∼1300–1500-Ma zircons, whereas Acadianclastic wedges contain abundant Paleozoic zircons and minor populations of 550–700- and 1900–2200-Ma zirconsconsistent with a Gondwanan affinity. Alleghanian clastic wedges contain large populations of ∼980–1080-Ma, ∼2700-Ma, and older Archean zircons and fewer Paleozoic zircons than occur in the Acadian clastic wedges. The abundanceof Paleozoic detrital zircons in Acadian clastic wedges indicates that the Acadian hinterland consisted of recycledmaterial and possible exposure of Taconic-aged plutons, which provided significant detritus to the Acadian forelandbasin. The appearance of Pan-African/Brasiliano- and Eburnean/Trans-Amazonian-aged zircons in Acadian clasticwedges suggests a Devonian accretion of the Carolina terrane. In contrast, the relative decrease in abundance ofPaleozoic detrital zircons coupled with an increase of Archean and Grenville zircons in Alleghanian clastic wedgesindicates the development of an orogenic hinterland consisting of deformed passive margin strata and Grenvillebasement. The younging-upward age progression in Grenville province sources revealed in Taconic through Allegh-anian successions suggest a reverse unroofing sequence that indicates at least two cycles of Grenville zircon recycling.

Online enhancements: appendix, data file.

Introduction

Sediments derived from orogenic hinterlands andadjacent quiescent cratons accumulate in forelandbasins that develop in response to tectonic loadingcaused by subduction, continental collision, and/or terrane accretion (Jordan 1995; DeCelles and Gi-les 1996). In the case of orogenic systems with suf-ficiently diverse sediment sources, spatial and tem-poral variations in foreland basin sedimentprovenance data can provide insight into the ki-nematics of deformation, landscape evolution, andsediment dispersal (Cawood and Nemchin 2001;McLennan et al. 2001). In recent years, U-Pb geo-chronology of individual detrital zircons has be-come one of the most useful approaches for iden-tifying sediment sources in basins (Gaudette et al.

Manuscript received May 15, 2009; accepted August 4, 2009.1 Arizona LaserChron Center, Department of Geosciences,

University of Arizona, Tucson, Arizona 85721.

1981; Gehrels et al. 1995; Fedo et al. 2003). Moderntechniques of U-Pb geochronology using laser ab-lation–multicollector–inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) now allowrapid inexpensive determination of ages (Black etal. 2004; Gehrels et al. 2006). In this article, we usesuch data to address persistent questions relatingto the tectonic development of the southern andcentral segments of the Paleozoic Appalachianorogen.

The Appalachian hinterland is partially com-posed of a complex mosaic of terranes that wereamalgamated to the Laurentian margin during mul-tiple phases of collision and related magmatismthroughout Paleozoic time (fig. 1; Horton et al.1989; Sinha et al. 1989; Hatcher 2005). Existinganalyses of Appalachian detrital-zircon composi-tions indicate that sediments derived from hinter-

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Figure 1. Simplified map of the Appalachian foreland basin and hinterland (modified from Millici and Witt 1988;Hatcher et al. 2004).

land accreted terranes are relatively minor in com-parison to those originally derived from Grenvilleand related rocks that occur pervasively in the east-ern Laurentian subsurface (Eriksson et al. 2004;Thomas et al. 2004; Becker et al. 2005). Despite thesmall sizes of these non-Grenville populations,

such sediment should provide important data forthe evaluation of Appalachian tectonic models,several of which remain poorly constrained or con-troversial. Here we report single-grain detrital-zir-con U-Pb and Pb-Pb crystallization ages from 15samples of Upper Ordovician to Mississippian

Journal of Geology T E C T O N I C D E V E L O P M E N T O F T H E A P P A L A C H I A N O R O G E N CHECKED 3


sandstones collected along the central and southernAppalachians in Tennessee, West Virginia, Virginia,and Pennsylvania. The age distribution of detritalzircons from Pennsylvanian sandstones in the cen-tral and southern Appalachians are well established(e.g., Becker et al. 2005) and can be used to evaluatethe provenance evolution of the Appalachian fore-land basin together with our Ordovician to Missis-sippian samples. With these data, we examine thehistory of terrane accretion and kinematic evolu-tion of the central and southern Appalachians.

Geological Background

The Appalachians are a 3,300-km-long orogen ex-tending from Newfoundland to Alabama thatformed through at least three Paleozoic orogenicevents on the eastern margin of Laurentia (Wil-liams 1978; Bradley 2008). Today the Appalachiansconsist of crystalline basement exhumed from theunderlying Grenville province in addition to vari-ably deformed and metamorphosed rift, passivemargin, and foreland basin sedimentary rocks.These rocks record the development of the Lauren-tian passive margin caused by breakup of the su-percontinent Rodinia and tectonic evolution as-sociated with opening and closing of Atlantic-realmocean basins (fig. 2). The breakup of Rodinia is re-corded in two pulses of magmatic activity, includ-ing a failed rifting event at ∼700–760 Ma and theopening of the Iapetus Ocean at ∼550–620 Ma (figs.2, 3; Aleinikoff et al. 1995; Walsh and Aleinikoff1999; Cawood et al. 2001). The first magmatic ac-tivity is preserved in the Mt. Rogers and RobertsonRiver formations of the Blue Ridge and is charac-terized by bimodal igneous activity in an intracon-tinental rift system (fig. 3; Aleinikoff et al. 1995).Evidence of the younger 550–620-Ma rifting eventis widespread in the northern Appalachians in-cluding the Pound Ridge Granite and the CatoctinFormation of the central and southern Appalachi-ans (fig. 3; Aleinikoff et al. 1995; Rankin et al.1997). Following the breakup of Rodinia, easternLaurentia accumulated 3–5 km of passive marginsedimentary rocks represented by the Erwin,Hampton, and Unicoi formations of the ChilhoweeGroup, as well as the Shady Dolomite and theRome and New Market formations in the centraland southern Appalachians (fig. 2; Diecchio 1986;Fichter 1986; Read 1989).

This passive margin sedimentation was inter-rupted by the Taconic orogeny in the Middle Or-dovician, presumably caused by progressive colli-sion of an arc and continental fragments with theeastern Laurentian margin, resulting in the closure

of the Iapetus Ocean (Drake et al. 1989). This col-lision produced the Taconic foreland basin that iswell preserved in northern New York and involveduplift and carbonate deposition upon the forebulgeand the accumulation of black shales and turbiditesin the foredeep (Bradley 1989, 2008). In the hinter-land, the orogeny involved significant magmaticactivity, penetrative deformation, and granulite-facies and kyanite-grade metamorphism at ∼465Ma (Hatcher 1987; Drake et al. 1989; Bradley 2008).The accreted terranes responsible for this defor-mation and sediment accommodation presumablyincluded 450–470-Ma magmatic arcs preserved inthe Milton, Tugaloo, Potomac, and Chopawamsicterranes and ∼530-Ma rocks of the Smith River Al-lochthon (fig. 1; Horton et al. 1989; Coler et al.2000; Hibbard et al. 2003). In the study area, thesynorogenic clastic wedges associated with this Ta-conic deformation are represented by the Martins-burg Formation, the Oswego Sandstone, and theJuniata Formation (fig. 2).

Silurian to Early Devonian time in the Appala-chians was a period of orogenic quiescence betweenthe Taconic and Acadian orogenies (Johnson et al.1985; Ettensohn 1991). During this time, Upper Si-lurian to Lower Devonian strata accumulated inthe Appalachian foreland basin and are character-ized by eustatically controlled sequences includingthe Tuscarora Sandstone, the Rose Hill Formation,and Keefer Sandstone of the Clinton Group; theMcKenzie Formation; the Helderberg Group; andthe Oriskany Sandstone in West Virginia and Vir-ginia (fig. 2; Johnson et al. 1985; Brett et al. 1990).

The Devonian to Early Mississippian Acadianorogeny is generally regarded as the result of thecollision of the Avalonian microcontinent to themargin of eastern Laurentia in the northern Ap-palachians, and the accretion of the Carolina ter-rane in the southern and central Appalachians (Os-berg et al. 1989; Wortman et al. 2000). Thesecollisions are also recorded by ∼384–423-Ma plu-tonism and the cratonward migration of northernAppalachian deformation front (Bradley et al. 2000).In comparison to widespread evidence of the Aca-dian orogeny in the northern Appalachians, theAcadian orogeny is poorly manifested in the south-ern and central Appalachian hinterland outside of374–382-Ma granitoid plutonism in the easternBlue Ridge and late Acadian metamorphism in theCat Square terrane (Horton et al. 1989; Osberg etal. 1989; Hatcher 2005). Acadian synorogenic de-posits are known broadly as the Catskill clasticwedge and are present from New England to Geor-gia (Faill 1985; Osberg et al. 1989). In West Virginia,these rocks are represented by the Devonian–Lower

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Figure 2. Stratigraphic column depicting the geologic history of the southern and central Appalachians (Johnson etal. 1985; Diecchio 1986; Fichter 1986; Hatcher 1987, 2005; Johnson 1987; Ettensohn 1994; Aleinikoff et al. 1995;Eriksson et al. 2004). On the right, sampled stratigraphic units are shown with circled dots.

Mississippian Chattanooga–Brallier, Chemung, Fo-reknobs, Hampshire, and Pocono formations (fig.2). Following the Acadian orogeny, interorogenicdeposition preceding the Alleghanian orogeny is re-corded by the Mississippian Greenbrier Limestonein West Virginia (Wynn et al. 2006).

The culminating orogenic event of the Appala-chians is the Late Mississippian–Pennsylvanian Al-leghanian orogeny that involved an oblique, tran-spressive, and rotational collision between part ofGondwana and previously accreted peri-Gond-wanan assemblages, causing the formation of the

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Figure 3. A, Laurentian basement provinces and magmatism ages in central North America including cratonic and Grenville provinces (modified from Hoffman1989; Van Schmus et al. 1993), and the late Precambrian–early Cambrian rift margin (Thomas 1991; Williams et al. 1995; Rankin et al. 1997; Walsh andAleinikoff 1999; Cawood et al. 2001; Thomas et al. 2004; Bradley 2008). B, Laurentian and Gondwanan tectonic events (Becker et al. 2005): peri-Gondwananages are from Mueller et al. (1994), Ingle-Jenkins et al. (1998), Coler and Samson (2000), Wortman et al. (2000), and Samson et al. (2001). Laurentian ages arefrom Hoffman (1989), Van Schmus et al. (1993), and Aleinikoff et al. (1995).



Pangean supercontinent (Hatcher et al. 1989). In thecentral and southern Appalachians, the Alleghan-ian orogeny involved: (1) accretion of the ArcheanSuwannee terrane to the southeast margin of Lau-rentia (fig. 1; Horton et al. 1989; Heatherington etal. 1999), (2) lateral translation of previously ac-creted terranes along dextral strike-slip faults(Hatcher and Bream 2002; Dennis 2007), (3) 300–325-Ma subduction-related magmatism and green-schist to amphibolite facies regional metamor-phism of hinterland terranes (Hatcher et al. 1989;Hatcher 2005), and (4) development of a forelandfold-thrust belt that propagated into sedimentaryrocks from the allochthonous pre-Alleghanianmetamorphic rocks in the southern and central Ap-palachians (Hatcher et al. 1989). The orogeny ex-posed Grenville basement and earlier orogenic fore-land basin strata producing the Blue Ridge-Piedmont thrust sheets (Hatcher et al. 1989, 2004).The lower Alleghanian clastic wedges examinedherein record an up-section transition from marinelimestone (Greenbrier Formation) to marine orbrackish-water clastic deposition (Mauch ChunkGroup and younger strata) in the central and south-ern Appalachians (fig. 2).

Geochronologic Provinces

The ages of detrital zircons in Appalachian forelandbasin strata are generally divisible into those ofLaurentian and Gondwanan affinities, althoughthose associated with the Grenville orogen are alsoknown on several Laurentian and Gondwanan frag-ments (Becker et al. 2005). Evaluation of the originof detrital zircons in the basin is not always easyto interpret because some Laurentian crustal agesand/or geological events overlap with those ofGondwanan affinity. For example, the opening ofthe Iapetus Ocean adjacent to Laurentia occurredduring the Pan-African/Brasiliano orogenies inGondwana (fig. 3). Moreover, sediment recyclingknown in the Appalachian basin (McLennan et al.2001) is likely to have mixed Laurentian and Gond-wanan components, complicating the interpreta-tion of sediment provenance. These challenges inidentifying the Gondwanan versus Laurentiancomponents can be overcome by assembling detri-tal zircon ages representative of a certain continentand examining presence and/or absence of uniquecomponents (Samson et al. 2001; Fedo et al. 2003;Bream et al. 2004). Below we review the pre-Ap-palachian orogenic events that occurred in Lauren-tian and Gondwanan provinces that could pertain

to the zircon ages recovered from the Appalachianforeland basin.

Laurentian Craton Interior Provinces. Potentialsource provinces of Laurentian affinity for the Ap-palachian foreland basin include the Superior, Wy-oming, Trans-Hudson, Penokean, Yavapai, Ma-zatzal, and Granite-Rhyolite provinces and orogens(fig. 3A). The Archean Superior province comprisesthe Laurentian shield and largely constitutes amal-gamated island arcs formed at ∼2700–2800 Ma,with small regions of pre-3500-Ma crust (Hoffman1989). The Wyoming province consists mainly of∼2500–2700-Ma granites and gneisses, and includes∼1600–1800-Ma accreted terranes and 13500-Macontinental crust composed of metamorphosedshelf-type sedimentary rocks (Wooden and Mueller1988). The Trans-Hudson province is a collisionalzone between the Superior province and the Wy-oming province and consists mainly of ∼1800–1900-Ma metasedimentary rocks (Hoffman 1989).The Penokean province is a Paleoproterozoic ac-cretionary orogen along the southern margin of theSuperior province and is composed of ∼1800–1900-Ma magmatic terranes (Van Schmus et al. 1993;Sims 1996). In southwestern Laurentia, the Yavapaiprovince is defined by deformed volcanic arc ter-ranes accreted to North America by ∼1700-Ma andincludes a basement of juvenile felsic igneous rocksformed at ∼1700–1760 Ma (Van Schmus et al. 1993;Holm et al. 2007). The Mazatzal province is an as-semblage of ∼1700–1800-Ma accretionary prismblocks accreted to the southern Yavapai provinceduring the ∼1600–1660-Ma Mazatzal orogeny (Ben-nett and DePaolo 1987; Karlstrom and Bowring1988; Amato et al. 2008). The Yavapai and Mazatzalprovinces are adjoined in the southeast to the Gran-ite and Rhyolite province that consists of ∼1360–1500-Ma crust formed as the result of high-silicamagmatism (Nyman et al. 1994). On the basis ofages, position and/or internal arrangement of theprovinces, we include the Yavapai and Mazatzalprovinces within the “Central Plains orogens” andrefer to the Trans-Hudson and Penokean provincesas the “midcontinent orogens” (fig. 3B).

Grenville Province. The Grenville orogen wasproduced by final amalgamation of the supercon-tinent Rodinia resulting from numerous continent-continent collisions during the interval ∼900–1300Ma, and it occurs on numerous modern continents,including the north-central Andean regions ofSouth America, northern Mexico, Central America,southwest Australia, east Antarctica, east-centralIndia, west Africa, Scandinavia, and eastern NorthAmerica (Moores 1991; Keppie and Ortega-Gutier-

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rez 1999). In modern North America, the Grenville-age province comprises the eastern margin of Lau-rentia extending from Newfoundland to Texas andcentral Mexico (fig. 3; Mezger et al. 1993). Basedon magmatic and deformational events near theAdirondack Mountains in the northern Appalachi-ans, the Grenville event has been subdivided intophases associated with arc-related and accretionaryevents during the ∼1220–1350-Ma Elzevirian orog-eny and the subsequent Shawinigan orogeny(ca.1160–1190 Ma) and a continent-continent col-lision during the ∼980–1090-Ma Ottawan orogeny(Rivers 1997; Heumann et al. 2006). Widespreadmagmatism resulting from lithospheric delamina-tion associated with orogenic collapse is recordedat ∼1145–1160 Ma, which overlapped with theearly Shawinigan events (McLelland et al. 2004).Subsurface analysis has revealed that the Grenvillerocks underlie much of the central and southernAppalachian foreland basin and also occur as sev-eral basement massifs in the southern Blue Ridgeof the Appalachian mountain belt (Williams andHatcher 1983; Hatcher et al. 2004). As has beendemonstrated by previous detrital-zircon studies,the Grenville province provided a primary sedi-ment source for the Appalachian foreland basin(Gray and Zeitler 1997; McLennan et al. 2001; Er-iksson et al. 2004; Thomas et al. 2004; Becker etal. 2005).

Rift Successions and Appalachian MagmaticRocks. Several late Proterozoic crustal fragmentsformed along the eastern margin of Laurentia dur-ing the Rodinian rifting event that began ∼800 Maand ended ∼550 Ma (Aleinikoff et al. 1995; Hoffman1999). Rodinia rifted apart ∼750 Ma in western andnortheastern North America but failed to rift in thecentral and southern Appalachians (fig. 3). Thefailed rifting is recorded in the Grandfather Moun-tain, Robertson River, and Mount Rogers forma-tions, which consist of marine, nonmarine, and∼732–758-Ma bimodal volcanic rocks in NorthCarolina, Tennessee, and Virginia (Lukert and Bank1984; Aleinikoff et al. 1995; Hatcher 2005). Suc-cessful rifting of Rodinia in the southern and cen-tral Appalachians occurred at ∼550–600 Ma and isrepresented by siliciclastic sedimentary rocks ofthe Swift Run and Lynchburg Groups, the YonkersGneiss, the Pound Ridge Granite (fig. 2; Wehr andGlover 1985; Rankin et al. 1997) and volcanic andsedimentary rocks of the Catoctin Formation(Badger and Sinha 1988; Aleinikoff et al. 1995; Er-iksson et al. 2004). The rift-related rocks are widelyconsidered to have provided significant amounts ofdetritus to the foreland basin during the Appala-

chian orogenies (Thomas et al. 2004). Synorogenicigneous rocks (∼350–490 Ma) crystallized duringthe Taconic and Acadian orogenies also became animportant source for the Acadian and Alleghanianclastic wedges (McLennan et al. 2001; Thomas etal. 2004; Becker et al. 2005).

Gondwanan Components. Gondwana was a com-posite supercontinent consisting of six major cra-tons whose amalgamation was triggered by closureof Neoproterozoic oceans during the Pan-Africanorogeny in Africa and the Brasiliano orogeny inSouth America (Hoffman 1999). These orogeniesbegan at ∼820 Ma by the joining of the Congo andKalahari cratons, which accreted to greater India at∼680 Ma and West Africa at ∼610 Ma and was com-pleted by ∼550 Ma through the accretion of theAustralia-Antarctica and Amazonia cratons (Han-son et al. 1994; Hoffman 1999). These amalgama-tion events involved the generation of new crustalmaterial, including zircon-bearing felsic and inter-mediate units.

Plate reconstructions indicate that the Rondon-ian-San Ignacio province in Gondwana’s westernAmazonian craton was adjacent to modern Ontarioand New York (e.g., Hoffman 1991) as part of theRodinian supercontinent. These rocks are com-posed of 1300–1550-Ma metamorphic belts, accre-tionary domains, and plutonic bodies (Geraldes etal. 2001; Teixeira et al. 2009) and may have trans-ferred material to Laurentia during Rodinian times.

In addition to these Neoproterozoic and Meso-proterozoic ages, western Gondwana cratons pre-served the ∼1800–2250-Ma crustal-forming eventknown as the Trans-Amazonian orogeny in SouthAmerica and the ∼2050–2150-Ma tectonothermalevent known as the Eburnean orogeny in the WestAfrican craton and the western Congo (Hartmann2002; Schofield et al. 2006; Schofield and Gillespie2007; Santos et al. 2008). The Trans-Amazonianorogen is represented by a series of juvenile mag-matic arcs, anorogenic magmatic belts, and largerifts and disruptive structures associated with thedevelopment of cratonic volcano-sedimentary ba-sins (Hartmann 2002). In contrast, the Eburneanorogen is represented by supracrustal basaltic andgranitic rocks and underwent sinistral transpres-sional deformation (Schofield et al. 2006).

The ages of 1900–2250-Ma Trans-Amazonian/Eburnean and ∼530–680-Ma Pan-African/Brasili-ano events are generally distinct from the Lauren-tian crustal ages and thus distinguish the terranesassociated with Gondwana (e.g., the Suwannee,Carolina, and Avalon terranes) that were accretedto Laurentia during the Appalachian orogenies

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(Heatherington et al. 1999; Wortman et al. 2000;Hibbard et al. 2002).

Sample Preparation and Analysis

Samples analyzed for this study along with briefstratigraphic descriptions and GPS coordinates foreach sample location are listed in appendix tableA1, available in the online edition or from the Jour-nal of Geology office. The separation of detrital zir-cons from sampled sandstones was performed inthe Tectonics and Sedimentation Laboratory at theUniversity of South Carolina, mainly following themethods of Gehrels et al. (2006) but with minormodifications. In order to disaggregate sand-sizedgrains, approximately 5–10 kg of each sandstonesample was crushed using a Bico WD ChipmunkJaw crusher, a Bico UD pulverizer, and an 18-meshsieve. Heavy minerals were separated and concen-trated with a MD Mineral Technologies MK 2 Ge-meni table and hand-operated ABS plastic gold pan.An S. G. Frantz L1 magnetic separator was used toremove remaining magnetic heavy minerals. TheFrantz magnetic separator was operated stepwisevia 0.25-A increments from 0.25 to 1.75 A, withhorizontal and vertical angles set to 15� and 25�,respectively. The samples were separated by den-sity through the heavy liquids sodium polytungs-tate (density 2.89 g/cm3), a lithium heteropoly-tungstate solution (density 2.90 g/cm3), and/ormethylene iodide (density 3.30 g/cm3) in 125-mLPyrex separatory funnels and/or 10-mL centrifugetubes.

The prepared samples were analyzed by LA-MC-ICP-MS in the Arizona LaserChron Center at theUniversity of Arizona. One hundred zircons wererandomly selected from each sample for ablationwith a DUV193 ArF Exciplex laser using a 35-mmspot for 14 samples and a 25-mm spot for the Fore-knobs Formation sample because of its finer-grained zircons. Laser spot selection targeted areasthat were sufficiently clean and large to accept thelaser spot, which favors zircon cores. Because ourpostacquisition processing excludes grains withlarge spatial variations in age data, which wouldresult from the mixing of rim and core composi-tions, we did not perform cathodoluminescenceimaging of grains before ablation, in line with otherrecent laser-ablation detrital-zircon geochronologystudies that prioritize the rapid generation of largedata sets at the expense of detailed grain charac-terization. All isotopic measurements were madein static mode, using Faraday detectors for 238U,232Th, 208Pb, 207Pb and 206Pb and an ion-countingChanneltron for 204Pb. Data were collected through

reference to a large zircon standard (SL: 564 � 4Ma, 2j), which was analyzed after every fifth un-known. Interpreted ages older than 800 Ma, whichcompose a natural break for the Appalachian crys-tallization ages, were calculated from 206Pb/207Pb,whereas ages younger than 800 Ma were based on206Pb/ 238U. Common Pb correction was achievedby the measured 204Pb in conjunction with an initialPb composition from Stacey and Kramers (1975)with uncertainties of 1.0 for 206Pb/ 204Pb and 0.3 for207Pb/ 204Pb. For each analysis, the 2j error was usedto determine whether the scatter in a given groupwas consistent with the internal errors, but we pre-sent data in our tables and figures at the 1j level.We employed only grains that were !30% discor-dant or !5% reverse discordant for provenanceanalyses. Further details are provided in an Exceldata file with U-Pb data, available in the onlineedition or from the Journal of Geology office.Graphical representation of our data using concor-dia and probability plots used standard routines of-fered by IsoPlot (Ludwig 2003).

Results

The results from 15 individual detrital zircon sam-ples are plotted on concordia diagrams (fig. 4) andage-probability plots (fig. 5). Maximum deposi-tional ages determined from our detrital-zirconanalysis are shown in table 1 along with indepen-dently constrained depositional ages for each sam-ple.In general, the major age populations of detritalzircons from the Appalachian foreland basin strataare ∼900–1300 Ma, correlative with the Grenvilleprovince. The age range of ∼1300–1500 Ma is cor-relative with the Granite-Rhyolite province, thelater stages of Grenville magmatism and meta-morphism, and, potentially, terranes derived fromthe Trans-Amazonian craton (fig. 2B). Althoughthese zircon ages do not distinguish these sources,we consider that the Granite-Rhyolite province isprimary source for the central and southern Ap-palachian foreland basin based on the size of prov-ince and distance to the depositional site (see “Dis-cussion”). Considerable ∼440–480-Ma detritalzircons, which are coeval with Taconic magmatismand metamorphism (Gray and Zeitler 1997; Mc-Lennan et al. 2001), appear in Acadian through Al-leghanian samples (fig. 5). There were minor agepopulations at ∼1600–1900 Ma and ∼2700 Ma, agessimilar to those of the midcontinent and CentralPlains orogens and the Superior province in theLaurentian interior provinces, respectively, as wellas to the ages of terranes that originated in theTrans-Amazonian and/or Ebumean cratons. An

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Figure 4. Concordia plots of 206Pb/238Pb versus 207U/235Pb* and corresponding ages (in 200-m.yr. increments) forzircons analyzed in this study. Data-point error ellipses are at the 68.3% confidence level.

abrupt change in zircon age distribution recognizedby an abundance of Paleozoic zircons is observedat the transition between the Foreknobs and Or-iskany formations (fig. 5). In the Acadian and Al-leghanian clastic wedges, detrital zircon age pop-ulations vary but tend to have a significant youngcomponent that is largely absent from other strata.In the Silurian sandstones, zircon populations arelargely restricted to those of Grenville age. In ad-dition to single sample probability curves, we plotsamples with similar tectonic significance togetherin order to characterize the age distribution of de-trital zircons in different tectonic phases of the Ap-palachian orogeny (fig. 6), which we describe below.

Taconic Clastic Wedges. Samples collected from

Upper Ordovician Taconic foreland basin units( , ) contain prominent populationsN p 2 n p 185consistent with ages of the Grenville and Granite-Rhyolite provinces. Approximately 52% of zirconshave Grenville ages, and 29% fall within the agerange of the Granite-Rhyolite province (fig. 6).

The Grenville-age population consists of two dis-tinct peaks that occur at ∼1088 and 1176 Ma andthat are also observed in our Silurian, Acadian, andAlleghanian samples. The basement of the north-ern Blue Ridge consists of different age groups ofProterozoic granitic gneisses, including ∼1140–1190-Ma and ∼1020–1090-Ma groups, which havebeen respectively correlated with the Shawiniganand Ottawan phases of the Grenville orogeny de-

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Figure 5. Detrital-zircon probability plots from the upper Ordovician to Mississippian sandstones of the Appalachianbasin. PZ p Paleozoic crust, PA p Pan-African/Brasiliano crust, GRN p Grenville province, GR p granite-rhyoliteprovince, MC p midcontinent orogen, CP: Central-Plains orogen, TA p trans-Amazonian crust, SUP p Superiorprovince.

fined in the Adirondacks and Canada (Aleinikoff etal. 2000). The peak ages at 1088 Ma and 1176 Main our Taconic samples coincide with the Ottawanand the Shawinigan orogenies, respectively. TheTaconic samples also contain minor peaks at 1652and 1776 Ma, which correspond with CentralPlains orogen ages, and at 2985 and 3320 Ma, whichcorrespond to the Archean Laurentian provinces,including the Superior province.

Post-Taconic Strata Deposited during Orogenic Qui-escence. The Tuscarora, Rose Hill, Keefer and Or-iskany formations were deposited during the Si-

lurian to early Devonian orogenic quiescence(Fichter 1986; Ettensohn 1994; Castle 2001). Ap-proximately 87% of the analyzed zircons fromthese samples ( , ) cluster at ages con-N p 4 n p 385sistent with the Grenville and Granite-Rhyoliteprovinces (fig. 6). Among the Grenville-age zircons,the Shawinigan-phase (peak at 1173 Ma) zircons aremore abundant than Ottawan-phase (peak at 1063Ma) zircons (fig. 6). The remaining 13% of the zir-cons are older, with U-Pb ages of ∼1600–1900 and∼2700 Ma that are associated with the midconti-nent orogen and Superior province, respectively.

Table 1. Depositional Ages

Period, stratigraphic unit

Maximumdepositionalage (youn-gest age;Ma) from

U-Pbanalysis

Depositional ages

ReferencesEpoch/stage Ma (approximate)

Mississippian:

Bluestone Formation 658 (471) Late Chesterian 316–319Jones 1996; Davydov et al. 2004; Maynard

et al. 2006

Princeton Formation 454 (406) Late Chesterian 320Jones 1996; Davydov et al. 2004; Maynard

et al. 2006

Hinton Formation 392 (392) Middle Chesterian 320–324Jones 1996; Davydov et al. 2004; Maynard

et al. 2006Mauch Chunk Formation

(Undivided) 393 (393) Middle Chesterian 320–339 Cardwell et al. 1968

Grainger Formation 456 (376) Kinderhookian 349–359Cardwell et al. 1968; Matchen and Kam-

mer 1994Price Formation 405 (375) Kinderhookian 349–359 Matchen and Kammer 1994

Devonian:

Hampshire Formation 407 (407) Famennian 359–374Berg et al. 1983; Millici and Witt 1988;

Castle 1998Chemung Formation 390 (390) Frasnian, earliest Famennian 375–385 Millici and Witt 1988; Hughes 2001Foreknobs Formation 385 (385) Frasnian, earliest Famennian 375–385 Hughes 2001Oriskany Formation 963 (963) Pragian 407–411 Tucker et al. 1998; Ver Straeten 2004

Silurian:Keefer Formation 1002 (967) Late Llandovery 428 Cardwell et al. 1968Rose Hill Formation 972 (921) Middle Llandovery 430 Cardwell et al. 1968Tuscarora Sandstone 939 (553) Early Llandovery 435 Cardwell et al. 1968; Diecchio 1986

Ordovician:Oswego Formation 461 (461) Gamachian–late Richmondian 443 Knowles 1966; Robert et al. 1992Fincastle Member, Martinsburg

Formation 1011 (1011)Latest Maysvillian through ear-

liest Richmondian 450 Walker 1978; Wise et al. 2007

Note. Maximum depositional ages are from U-Pb analysis.



Figure 6. Detrital-zircon probability plots for Taconic, interorogenic, Acadian, and Alleghanian foreland strata fromWest Virginia and Virginia. Arrows represent different orogenic events in the Grenville Province: Shawinigan orogeny,∼1140–1190 Ma; Ottawan orogeny, ∼1020–1090 Ma. Same abbreviations as in figure 5.

The Silurian to Devonian samples do not containzircons younger than 800 Ma, suggesting no con-tributions from Paleozoic Appalachian magmaticrocks or from the Pan-African orogen.

Acadian Clastic Wedges. Samples collected fromDevonian synorogenic clastic wedges of the centralAppalachian foreland basin ( , ) con-N p 4 n p 379tain a more varied distribution of ages, contrastingwith the Silurian formations deposited during pre-Acadian tectonic quiescence (fig. 6). Approximately79% of the zircons in Acadian foreland basin stratawere derived from Laurentian sources, primarilyfrom the Grenville and Granite-Rhyolite provinces(67%), the midcontinent and Central Plains oro-gens (9%), and the Superior and other provinces(3%). Remarkable aspects of detrital-zircon ages inthe Acadian strata include (a) the occurrence of asignificant amount of young (!500-Ma) zircons con-sistent with ages of magmatism associated with theTaconic (11%) and Acadian (3%) orogenies and (b)the occurrence of zircons similar to the 500–700-

Ma Pan-African (4%) and 1900–2250-Ma Trans-Amazonian/Eburnean (2%) events, which are con-sidered to have Gondwanan affinities (Excel datafile). Moreover, zircons derived from Shawiniganand the Granite-Rhyolite provinces are consider-ably less abundant in the Acadian strata comparedto older strata (fig. 6).

Alleghanian Clastic Wedges. Samples collectedfrom Mississippian foreland basin units ( ,N p 3

) contain 30% zircons associated with then p 278Ottawan orogeny (peak at 1064 Ma), and relativelyfew zircons coeval with the Shawinigan (peak at1156 Ma) orogeny and Granite-Rhyolite province(minor peaks at 1393 and 1493 Ma), representing18% and 14% of the total population, respectively(fig. 6; app. A). Approximately 8% of zircons haveages similar to those of the midcontinent and Cen-tral Plains orogens, and 4% of zircons are consis-tent with the Superior (peak at 2775 Ma) and olderArchean (peak at 2920 Ma) provinces. Among theyoung zircon populations (!500 Ma), 5% are con-



sistent with the age of Taconic magmatism (peaksat 425 and 480 Ma), and 2% of zircons have agessimilar to those of the Acadian orogeny. Zircons ofGondwanan affinity in Alleghanian strata include2% with Pan-African ages and 1% with Trans-Am-azonian/Eburnean crust ages. The main differencebetween Alleghanian and Acadian samples is therelative increase in abundance of the Ottawan-agepopulations and relative decrease in abundance ofthe Paleozoic populations with minor increases ofmidcontinent and Superior age populations (fig. 6).

Discussion

Stratigraphic Evolution of Sedimentary Sources.Our data reveal the evolution of sediment prove-nance in the Appalachian foreland basin during theTaconic through Alleghanian phases of the Appa-lachian orogeny. Within the dominant Mesopro-terozoic and early Neoproterozoic age populationsgenerally associated with the Grenville orogeny,the Shawinigan signal became progressivelysmaller while the Ottawan signal increased duringthe time period recorded by our samples. The pro-gressive upward-younging of Grenville-aged detri-tal zircons is coupled with a decrease in abundanceof zircons with ages consistent with the Granite-Rhyolite province. Simultaneously, the populationof Paleozoic zircons decreases stratigraphically up-ward in the Acadian and the Alleghanian samples,and the population of Archean zircons generallyincreases in Alleghanian samples (figs. 5, 6). Do thesystematic waning and waxing of these zircon pop-ulations simply indicate depletion of sources in theShawinigan and Granite-Rhyolite provinces? Or dothey reflect new exhumation of basement as theposition of thrusting migrated through the largerGrenville basement during the various phases ofthe Appalachian orogeny?

Previous Appalachian detrital zircon studies gen-erally agree that Paleozoic zircons are derived fromthe Appalachian orogenic hinterland but suggestthat the immediate source of Grenville-age andolder zircons is not well constrained. Many studiesinterpret Grenville-age and older zircons in the Ap-palachian synorogenic clastic wedges as being de-rived through recycling of intermediate units suchas passive margin successions that were mainly fedfrom Laurentian provinces through northeastwardor southeastward dispersal systems in the centraland southern Appalachians (Gehrels et al. 1995;McLennan et al. 2001; Thomas et al. 2004; Beckeret al. 2005), although some workers emphasize thepossibility of direct derivation of pre-Grenville zir-cons from distal cratonic sources through a north-

ern axial river system (Robinson and Prave 1995;Thomas et al. 2004).

Paleocurrent directions in the Taconic throughAlleghanian foreland strata vary, but southwesternand southeastern sources are dominant in Taconicclastic wedges (e.g., Austin Glen Formation: Powerand Garver 2004), while southeastern or easternsources are predominant in Silurian (e.g., Thoroldand Grimsby sandstones: Lundegard 1979; Tusca-rora Formation: Whisonant 1977) through Missis-sippian strata (e.g., Upper Devonian Brallier For-mation: Lundegard 1979; Lower MississippianPrice Formation: Murphy and Kammer 2001).These general trends of paleocurrent data appear toindicate derivation from orogenic hinterlandsources to the east for the Taconic through Allegh-anian foreland strata rather than distal cratonsources to the north and west.

The relative increase in abundance of Ottawanand Archean zircons in our Alleghanian samplesresemble the distribution of detrital zircons col-lected from passive margin successions (Neopro-terozoic Unicoi and Cambrian Erwin formations)that displays the dominance of Ottawan-age zir-cons and occurrence of Granite-Rhyolite province-age and Archean zircons (Eriksson et al. 2004).Thus, in light of these similarities and paleocurrentconsiderations, we interpret the similar distribu-tion of zircon ages between the passive margin suc-cession and our Alleghanian strata to be the resultof exhumation of Grenville basement and Neopro-terozoic–early Ordovician passive margin strataduring the Alleghanian phase, which are well doc-umented to contain significant sediment popula-tions that presumably derived from the older cra-tonic interior provinces (Robinson and Prave 1995;Thomas et al. 2004).

Within the Grenville-age zircon populations, theShawinigan signal (peaks at 1173 and 1176 Ma) pro-gressively decreases in the Taconic to Alleghanianforeland basin strata and is replaced by an increas-ing abundance of the younger Ottawan signal(peaks at 1055–1068 Ma), with its greatest abun-dance in the Alleghanian samples (figs. 5, 6).McLennan et al. (2001) found a similar stratigraphicprogression as that revealed in our data, such thatthe mean ages of Grenville-age zircon becomesyounger stratigraphically upward from passive mar-gin (Poughquag Quartzite, New York) through Ta-conic foreland basin strata (Austin Glen memberof the Normanskill Formation and the Shawan-gunk Formation, New York). McLennan et al.(2001) interpreted that the evolution of detrital zir-cons represents more distal (northwestern) Gren-ville provinces for the passive margin strata and the



more proximal (southeastern) Grenville sources foryounger Taconic strata.

The upward-younging age progression of detritalzircons in our Taconic through Alleghanian sam-ples may represent reverse unroofing resulting frommultiple phases of sediment recycling. Clastic sed-imentary rocks often record the progressive un-roofing of stratigraphy in the source area, whichtypically yields a normal unroofing sequence con-taining framework grains in inverted stratigraphicorder in comparison to the source area. If these de-rived sedimentary strata are subsequently upliftedin the frontal parts of the orogen by further evo-lution of the mountain belt, they may become thedominant source for the foreland basin at the ex-pense of the more hinterland sources, resulting inreinversion of preexisting unroofing sequences,thereby yielding a reverse unroofing sequence. Al-though the Grenville rocks are not layered, themultiple phases of the Grenville orogeny were firstrecognized on the basis of different deformation andmagmatism patterns below and above the late Me-soproterozoic Flinton Group (Tollo et al. 2004 ). Asa result, it may be possible to interpret progressiveunroofing of the Grenville rocks through an ex-amination of the Grenville-aged zircons in the Ap-palachian foreland basin. Therefore, the younging-upward age progression in our samples mayrepresent a reverse unroofing history, in which casethe Grenville zircons in the Taconic to Alleghanianstrata should have experienced at least two cyclesof sediment recycling. One challenge in detrital-zircon geochronology is that many sediments arethemselves derived from preexisting sedimentaryrocks and the process of multiple recycling is notdetectable because the U-Pb age provides infor-mation pertaining only to the initial source(McLennan et al. 2001; Fedo et al. 2003). Theyounging-upward age progression in our samplessuggests the possibility that vertical evaluation ofdetrital zircon age distributions may provide in-sight into the number of times that sedimentsrecycled.

In addition to the aforementioned evolution ofMesoproterozoic and early Neoproterozoic detrital-zircon age populations, the presence/absence andabundance of Paleozoic zircon populations vary be-tween the Acadian and Alleghanian foreland basinstrata. An abrupt increase in the abundance of Pa-leozoic zircons occurs between the Frasnian (∼375–385 Ma) to earliest Famennian (∼368–375 Ma) Fo-reknobs Formation (Hughes 2001) and the Pragian(∼407–411 Ma) Oriskany formations (Tucker et al.1998; Ver Straeten 2004), which approximately co-incides with the transition from the post-Taconic

orogenic quiescence to the Acadian orogeny (figs.2, 5; table 1; Fichter 1986; Ver Straeten 2008). Weinterpret these results to indicate orogenic exhu-mation and incorporation of preexisting magmaticbelts (i.e., Taconic belt for the Acadian clasticwedges; Taconic and Acadian belts for the Allegh-anian clastic wedges) into the sediment productionand deposition systems of the Appalachian orogenand foreland basin. These Paleozoic zircons aremore abundant in Acadian clastic wedges than inAlleghanian clastic wedges, and they decrease inabundance stratigraphically upward within the Al-leghanian clastic wedges (figs. 5, 6). This progres-sive decrease of Paleozoic zircons suggests that ageographical barrier may have developed betweenPaleozoic magmatic arcs and the Alleghanian fore-land basin as the Alleghanian deformation ex-humed the Grenville basement. This interpretationis also consistent with petrographic data from Aca-dian and Alleghanian sandstones wherein abundantmetamorphic lithic fragments and mica and the rel-ative deficiency of extrabasinal lithic fragments in-dicate exhumation of deep crust (Eriksson et al.2004).

Taconic Accretion. The age distribution of detri-tal zircons in the Ashgillian (∼439–443 Ma) OswegoFormation is significantly different from that of theCaradoc (∼449–450 Ma) Fincastle Member of theMartinsburg Formation, although both units arewidely considered to have been deposited duringthe Taconic orogeny (fig. 5; table 1). The zircon agedistribution in the Oswego Formation is very broad,ranging from 460 to 3300 Ma, whereas zircon agesin the Fincastle Member more narrowly cluster be-tween 1010 and 2750 Ma (fig. 5). The Oswego For-mation sample contains 1900- to 2100-Ma zirconsand 670-Ma zircons. The combination of those zir-con ages has been interpreted as derivation from asource with Gondwanan affinity (Heatherington etal. 1997; Murphy and Hamilton 2000; Steltenpohlet al. 2001). Moreover, Grenville and Granite-Rhy-olite age zircons (peaks at 1109, 1163, and 1345 Ma)in the Oswego Sandstone are common to other Ta-conic foreland samples, but provenance compo-nents with ∼460-Ma zircon ages are absent in otherTaconic foreland samples including our samplefrom the Fincastle Member of the Martinsburg For-mation (Gray and Zeitler 1997; Cawood and Nem-chin 2001; McLennan et al. 2001; Eriksson et al.2004). Central Plains and midcontinent zircons(∼1600–1800 Ma) in the Oswego sample also reveala larger relative abundance than those in manyother Taconic foreland samples (McLennan et al.2001; Eriksson et al. 2004). The differences of de-trital zircon age populations between the Oswego

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Formation and the Fincastle Member suggest thatthey are derived from different sources. This pos-sibility is further supported by complementary pet-rologic and sedimentological studies that indicatethe depositional environment of Oswego Sandstonewas a fluvio-deltaic system sourced from the east-ern hinterland, whereas the Fincastle Member ofthe Martinsburg Formation was deposited in a sub-marine fan system that contributed sediment fromsoutheastern sources (Walker 1978; Fichter andDiecchio 1986).

Many paleogeographic and tectonic reconstruc-tions have positioned peri-Gondwanan terranes andthe west Gondwanan cratons of South Americanear or adjacent to the eastern Laurentian marginafter rifting of Rodinia (Dalziel 1997; Karlstrom etal. 2001; Hatcher et al. 2004). The Dahlonega ter-rane of North Carolina, which is located betweenthe western and eastern Blue Ridge provinces, wasone such peri-Gondwanan terrane, and it recordedthe Neoproterozoic and Paleozoic tectonic historyof opening and subduction of the Iapetus Ocean (fig.1; Bream et al. 2004). Composed of immature sil-iciclastic metasediments intruded by 440–470-Mamafic and magmatic arc rocks, the Dahlonega ter-rane is generally interpreted as a short-lived back-arc basin (Spell and Norrell 1990; Berger et al. 2001).On the basis of detrital zircon data obtained fromthe Otto Formation in the Dahlonega terrane to-gether with structural relationships within the BlueRidge, Bream et al. (2004) suggested that the Dah-lonega terrane was sourced from a Grenville beltin western Gondwana and later accreted to the Lau-rentian margin during the Taconic or Acadian orog-enies. The age distribution of detrital zirconsknown from the Dahlonega terrane resembles ourdata from the Oswego Sandstone in that both con-tain age populations similar to western Gond-wanan components at ∼600–900 Ma and ∼1600–2100 Ma, and so we interpret the Oswego Sand-stone to contain sediments potentially recycledfrom the Dahlonega terrane.

Another sediment source of known Gondwananaffinity in the southern central Appalachians is theCarolina terrane, which accreted to the easternmargin of Laurentia during Appalachian orogenesis(e.g., Horton et al. 1989), although the precise tim-ing of accretion is uncertain. If the Carolina terranewas accreted to the eastern margin of Laurentiaduring the Late Ordovician, it could have been asource for the Oswego Formation. Single-grain zir-con ages collected from the Aaron and UwharrieFormations of the Carolina terrane range from ∼510to 620 Ma (Samson et al. 2001; Eriksson et al. 2004),whereas xenocrystic zircon ages cluster at ∼1000,

2100, and 2500 Ma (Ingle et al. 2003). However, thecorrelation of detrital-zircon ages from the Carolinaterrane with zircons in the Oswego Sandstone iscomparatively weak, such that there are no zirconswith ages between ∼620 and 510 Ma, nor ages of∼2100 Ma. Hence, the Dahlonega terrane currentlyappears to be the best candidate for the source ofthe non-Laurentian zircons contained in the Os-wego Sandstone.

Acadian Accretion. In our view, the occurrenceof zircons with unambiguous Gondwanan affinity(i.e., 510–680- and 1900–2100-Ma zircons) alongwith the sudden increase in abundance of Paleozoicdetrital zircons in the Devonian-aged Acadian clas-tic wedges indicate an important terrane accretionevent (figs. 5, 6).

Significant numbers of Paleozoic zircons havebeen identified in the foreland basin strata of thecentral Appalachians including the Catskill “redbeds” of the lower Walton Formation in New York(McLennan et al. 2001) and the Pottsville Forma-tion of Pennsylvania (Gray and Zeitler 1997), butthey have only rarely been reported from the south-ern Appalachian foreland basin (e.g., Cloyd Con-glomerate, Virginia: Eriksson et al. 2004). However,in our Acadian clastic wedge samples, abundantzircons younger than 500 Ma start to appear begin-ning with the Foreknobs Formation. These Paleo-zoic zircons comprise an average of 13% of the totalzircon population (peak at 451 Ma), which werepresumably derived from magmatic rocks associ-ated with Acadian and Taconic orogenesis in theAppalachian hinterland (figs. 6, 7; Excel data file).The abundance (11%) of ∼450-Ma zircons in ourDevonian samples indicates that Taconic mag-matic and metasedimentary rocks were importantsources of the Acadian foreland basin sediment inWest Virginia and Virginia. Synorogenic zircons(3%), which are consistent with Acadian volcanismas represented by ∼390–417-Ma K-bentonites thatoccur pervasively in the central and southern Ap-palachians (Ver Straeten 2004), also indicate thatsome parts of the Appalachian foreland basin suc-cession preserve sediment derived directly fromcontemporaneous orogenic activity.

Trans-Amazonian/Eburnean and Brasiliano/Pan-African zircons first appear in the ∼368–385-Ma Fo-reknobs Formation (figs. 5–7). The Trans-Amazo-nian/Eburnean (∼1900–2200 Ma) zircons are fewbut persistent in all Acadian clastic wedge samples,with three grains in the Foreknobs Formation, twograins in the Chemung Formation, two grains inthe Grainger Formation, and one grain in the PriceFormation (fig. 7; Excel data file). Brasiliano/Pan-African (∼510–680 Ma) zircons are present with



Figure 7. Detrital-zircon age probability plots from Aca-dian clastic wedges.

Figure 8. Detrital-zircon probability plots from Allegha-nian clastic wedges. Same abbreviations as in figure 5.

seven grains in the Chemung Formation, two grainsin the Foreknobs Formation, two grains in theHampshire Formation, and one grain in the PriceFormation (fig. 7; Excel data file).

These Gondwanan zircon peaks in our Devoniansamples are temporally correlative to zircon agesin the Carolina terrane (∼430, 540, and 600 Ma,with small populations of zircons from ∼1900–2200and ∼2700–2900 Ma), where peak metamorphicevents occurred between ∼535 and 538 Ma at thetime of the Brasiliano/Pan-African orogeny (Dennis2007), and later overprinted by amphibolite andgranulite facies metamorphism from ∼350 to 360Ma in the western Carolina terrane (Hatcher 2005).Dennis (2007) interpreted the ∼350–360-Ma meta-morphism as related to dextral shear in the easternLaurentian transform boundary that may have ex-isted during the Devonian-Mississippian transi-tion. Considering the first appearance of Gond-wanan zircons in the Acadian clastic wedgesamples, we interpret accretion of the Carolina ter-rane to have occurred after deposition of the Pra-

gian (∼408–411 Ma) Oriskany Formation and beforedeposition of the Frasnian to the earliest Famen-nian (∼368–385 Ma) Foreknobs Formation. Forsome, the Acadian orogeny is considered to consistof as many as four separate large-scale cycles oftectonism from the beginning of the Early Devo-nian (∼410 Ma) to the early Mississippian (∼340 Ma)in the southern and central Appalachians (Etten-sohn 1985; Bradley and Hanson 1989). The accre-tion of Carolina terrane, which appears to have oc-curred between ∼385 and 408 Ma, may have causedone of the Acadian orogenic phases.

Middle Mississippian Onset of Alleghanian Orog-eny. Detrital-zircon age signatures in our Missis-sippian samples record a sediment provenance shiftwith respect to underlying strata. In comparison toDevonian to early Mississippian samples whereinthe proportions of Shawinigan and Ottawan age zir-cons are similar, the population of Ottawan-age zir-cons increases in upper Mississippian (upper Viseanand Serpukhovian) samples, contemporaneouswith an increase in the relative abundance of mid-continent- and Superior-age zircons (figs. 6, 8).

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The combination of abundant zircons from theOttawan phase of the Grenville orogen with olderLaurentian crustal components in our Mississip-pian samples is similar to the zircon age distribu-tions found in Neoproterozoic and lower Paleozoicpassive margin strata of eastern Laurentia (Gleasonet al. 2002; Thomas et al. 2004). Thus, we interpretthat the passive margin succession was exhumedby the Alleghanian orogeny in the Valley and Ridgefold-thrust belt and parts of the crystalline core ofthe orogen, thereby providing recycled cratonic andother sediment to the foreland basin as has beenpreviously interpreted in Pennsylvanian forelandbasin samples (Thomas et al. 2004; Becker et al.2005). However, the depositional ages of our sam-ples containing this recycled signature range fromthe late Visean to Serpukhovian (Excel data file),suggesting that the Alleghanian orogeny may haveinitiated in the Visean, consistent with interpre-tation of detrital-zircon data from the middle Mis-sissippian Hartselle Sandstone of northern Ala-bama (fig. 8: Barbeau et al. 2006; Russell 2006) andVisean Ar-Ar cooling ages from the western BlueRidge (Hames et al. 2007). Together these resultssuggest a middle Mississippian onset of the Al-leghanian orogeny: 20 m.yr. or more earlier thanthe commonly accepted date.

As indicated in figure 8, there is a greater abun-dance of 1600–1800-Ma zircons in the HartselleSandstone sample of northern Alabama than in theMississippian samples reported herein. In light ofthe Central Plains orogen’s position to the south-west of the Appalachian foreland basin, the higherproportions of Central Plains orogen zircons in theHartselle Sandstone appear to reflect the variabledistances from the sources (figs. 1, 3, 5). Hence, weinterpret that the detrital-zircon signature of theHartselle Sandstone may reflect the sedimentarydispersal processes associated with deposition re-lated to the late Paleozoic Ouachita orogeny ofsouthern North America (Thomas 1995).

Conclusions

1. U-Pb ages of detrital zircons collected from Or-dovician to Mississippian foreland basin strata inthe southern and central Appalachians (West Vir-ginia, Virginia, Tennessee, and Pennsylvania) in-clude the usually prominent population of grainsaged ∼1100–1400-Ma and presumably derived fromthe Grenville province, but they also contain majorcomponents of zircons from ∼430 to 700 Ma, alongwith lesser populations of zircons with ages of∼1900–2200 and 2700–2900 Ma.

2. The age distribution of detrital zircons in our

samples records the relationship between orogenicactivity and provenance of foreland sediments de-posited in response to the Appalachian orogeny.The age distribution of detrital zircons is relativelywide in Taconic, Acadian, and Alleghanian clasticwedges, ranging from the middle Paleozoic to Ar-chean ages, whereas the age distribution is narrowin pre- or interorogenic strata, typically clusteringbetween ∼900 and 1800 Ma. These data indicatethat Acadian and Alleghanian orogenic activity ex-humed and exposed preexisting hinterlands, in-cluding parts of the Taconic orogenic belt, and de-veloped new sedimentary dispersal systems thatprovided various sources to the foreland basin dur-ing orogenic activity. In contrast, the basin was fedfrom less diverse sediment sources during orogenicquiescence in the Silurian.

3. Samples from Taconic clastic wedges revealhigh proportions of Shawinigan (∼1160–1190 Ma)and Granite-Rhyolite zircons (∼1300–1500 Ma).Samples in Acadian clastic wedges are character-ized by large populations of Paleozoic zircons andthe occurrence of Pan-African/Brasiliano (∼500–700 Ma) and Eburnean/Trans-Amazonian (∼1900–2250 Ma) zircons. Samples from Alleghanian clas-tic wedges are representative of large amounts ofOttawan zircons (∼980–1080 Ma), an increasedabundance of distal craton zircons, and a decreasedabundance of Paleozoic zircons. The general pat-tern of detrital-zircon age distribution indicatesthat distal sources from Grenville and associatedGranite-Rhyolite provinces are the most importantsources during the Taconic orogeny, but frontal oro-gen and orogenic hinterland sources from Paleozoicmagmatic arcs and associated metasedimentaryrocks comprise more important components in sed-imentary record during the Acadian and Allegh-anian orogenies.

4. There is a stratigraphic evolution of Grenville-age sources recorded in the foreland basin from theTaconic through Alleghanian orogenies. The Sha-winigan signal progressively decreased while theOttawan signal increased through the Taconic toAlleghanian orogenies. We interpret the younging-upward age progression to represent the reverse un-roofing history of the Appalachians, suggesting atleast two cycles of Grenville-age zircon recycling.

5. The different age distribution of detrital zir-cons between the Oswego and Fincastle formationsin Taconic clastic wedges may be attributed tochanges in sources associated with accretion of theDahlonega terrane.

6. The appearance of abundant Paleozoic detritalzircons in Acadian synorogenic clastic wedges in-dicates that: (a) the Taconic hinterland, which con-

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sisted of recycled material and possible exposure ofTaconic-aged plutons, provided a significantamount of erosional detritus to the Appalachianforeland basin, and (b) a significant terrane accre-tion event to the Laurentian margin occurred inDevonian time.

7. The Foreknobs and Chemung formations inAcadian clastic wedges contain zircons with agesof 385–400 Ma, indicating that these detrital zir-cons record contemporaneous magmatic activity.

8. The presence of Pan-African/Brasiliano andEburnean/Trans-Amazonian zircons in the Acadianclastic wedges supports ∼385–408-Ma (Devonian)accretion of the Carolina terrane.

9. The decrease of Paleozoic detrital zircons cou-pled with an increase of Ottawan- and Superior-agezircons in Alleghanian clastic wedges indicates thatthe passive margin succession and Grenville base-ment were exhumed by thrusting associated withthe Alleghanian orogeny, which provided erosionaldetritus to the foreland basin while preventing sed-iment input from Taconic hinterland sourcesthrough the development of a topographic barrier.

10. The detrital-zircon signatures in Mississip-pian clastic wedges from Pennsylvania to Alabamaimply a Visean onset of the Alleghanian orogeny.

A C K N O W L E D G M E N T S

David Gombosi is gratefully acknowledged for ac-companying us on our sampling trip, for assistancewith sample preparation, and for helpful manu-script reviews. We thank Victor Valencia for op-eration and assistance on laser ablation–multi-collector–inductively coupled plasma-massspectrometry at the Arizona LaserChron Center ofthe University of Arizona. This research was sup-ported by the American Associated Petroleum Ge-ologists Grant-In-Aid program, the Geological So-ciety of America Graduate Student Grantprogram, and the Magellan Scholar program of theUniversity of South Carolina. Matt Kohn andDwight Bradley provided valuable reviews thatsignificantly improved the manuscript.

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Wortman, G. L.; Samson, S. D.; and Hibbard, J. P. 2000.Precise U-Pb zircon constraints on the earliest mag-matic history of the Carolina terrane. J. Geol. 108:321–338.

Wynn, T. C., Al-Tawil, A., and Read, J. F. 2006. Outcropanalogs for subsurface Mississippian Greenbrier Lime-stone reservoirs, West Virginia, USA. Geol. Soc. Am.Abstr. Program 38:85.

q26

q27



QUERIES TO THE AUTHOR

1 I inserted a temporary running head. Please re-vise or replace as necessary.

2 Here and throughout, you preface many of theage ranges with “ca.” A hyphen is necessary afterca. when these ranges are used as adjectives, whichI think makes the phrases somewhat cumbersome.To prevent that problem, I replaced “ca.” with atilde where appropriate (e.g., ∼980–1080 Ma)throughout the article. Changes okay?

3 Re the figure 3 legend, Williams et al. 1995does not have a corresponding entry in Refs Cited.Please provide info or mark this citation for dele-tion. Also, correct to change Becker et al. 2004 to2005 as shown?

4 In the sentence that begins “In West Virginia,”the relationships of the terms shown are not clearto me. Is a word missing after Chattanooga (i.e., theChattanooga [something])? Would it be correct tosay “...represented by the Brallier, Chemung, Fo-reknobs, Hampshire, and Pocono formations in theDevonian—Lower Mississippian Chattanooga(_____)”?

5 Correct here to change Fedo et al. 2004 to 2003as shown?

6 Lukert and Bank 1984 does not appear in Ref-erences Cited. Please provide reference info or markthis citation for deletion.

7 In the sentence that begins “Synorogenic ig-neous,” the phrase “contributed an importantsource” seemed awkward to me. Okay to changeas shown?

8 Hoffman 1991 does not appear in ReferencesCited. Please provide reference info or mark thiscitation for deletion.

9 Table A1 cites Englund 1979, but there is nocorresponding entry in Refs Cited. Please providereference information or mark this citation for de-letion. There is an identical problem with Folk1960.

10 In “Laser spot selection,” are you saying thezircon cores are favored by the targeted areas or bythe laser spot? Also, I don’t understand what youmean by “favored.” Do you mean zircon cores aremore likely to be found in targeted areas that aresufficiently clean and large?

11 Re the zircon standard here, please tell mewhat SL stands for.

12 Stacey and Kramers 1975 does not appear inReferences Cited. Please provide reference info ormark this citation for deletion.

13 Re table 1, Robert et al. 1992 does not appearin Refs Cited. Please provide reference info or markthis citation for deletion.

14 The first clause of “Minor age populations”was not clear to me. Okay to change sentence asshown?

15 Re N p 2, what does the capital italic N rep-resent? (I assume n p 185 is the sample size.)

16 Tollo et al. 2004 does not have its own entryin Ref Cited. Instead, it appears as the “parent”source in the reference entries for Bream et al. 2004and Hatcher et al. 2004. Do you want to providecomplete reference info for Tollo et al. so it can belisted in Ref Cited, or should the Tollo et al. 2004citation here be changed to Bream et al. 2004 and/or Hatcher et al. 2004?

17 Bradley and Hanson 1989 does not appear inRefs Cited. Please provide reference info or markthis citation for deletion.

18 In “Together these,” the phrase “before theconventional wisdom” was grammatically incom-plete. Okay to revise as shown?

19 In “The different age,” the phrase “is likelyattributed” didn’t seem quite right. Okay to changeas shown, or should it say “is probably attributableto”?

CHECKED 24

23 Re Cardwell et al., please provide scale andname and location for the map’s publisher.

24 Davis and Ehrlich 1974 is not cited in thetext. Please provide a citation or mark this refer-ence for deletion.

25 Hammer 1998 is not cited in the text. Pleaseprovide a citation or mark this reference fordeletion.

26 Secor et al. 1983 is not cited in the text. Pleaseprovide a citation or mark this reference fordeletion.

27 Thomas et al. 2009 is not cited in the text.Please provide a citation or mark this reference fordeletion.

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