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For permission to copy, contact [email protected] 2004 Geological Society of America 525

GSA Bulletin; May/June 2004; v. 116; no. 5/6; p. 525–538; doi: 10.1130/B25295.1; 11 figures.

Late Paleozoic tectonism in Nevada: Timing, kinematics,and tectonic significance

James H. Trexler, Jr.†

Patricia H. CashmanDepartment of Geological Sciences, University of Nevada, Reno, Nevada 89557-0180, USA

Walter S. SnyderVladimir I. DavydovDepartment of Geology and Geophysics, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA

ABSTRACT

Three late Paleozoic, angular unconfor-mities, each tightly constrained in age bybiostratigraphy, are exposed in Carlin Can-yon, Nevada. These record deformation aswell as erosion. Folding associated withthese deformation events is roughly coaxial;all three sets of fold axes trend northeast.Each unconformity represents tectonic dis-ruption of the middle part of the westernNorth American margin between the timesof the initiation of the Antler orogeny (LateDevonian–Early Mississippian) and thePermian–Triassic Sonoma orogeny. Thispaper focuses on one of these unconformi-ties in the Middle Pennsylvanian—the C6unconformity—and the deformation andage constraints associated with it.

Our data from Carlin Canyon yield de-tailed glimpses of how the Antler forelandevolved tectonically in Mississippian andPennsylvanian time. Middle Pennsylvanian(Desmoinesian) northwest-southeast con-traction resulted in thin-skinned foldingand faulting, uplift, and erosion. These datarequire reinterpretation of the tectonic set-ting at the time of the Ancestral RockyMountains orogeny and suggest that plateconvergence on the west side of the conti-nent played a significant role in late Paleo-zoic tectonics of the North Americancontinent.

Keywords: tectonics, stratigraphy, struc-ture, Great Basin, Nevada, Pennsylvanian,Ancestral Rocky Mountains.

†E-mail: [email protected].

INTRODUCTION

Although the tectonic evolution of south-western North America (present-day UnitedStates) is generally thought to include onlytwo Paleozoic orogenies, the latest Devonian–Early Mississippian Antler orogeny and theLate Permian–earliest Triassic Sonoma orog-eny, many workers have shown that there isconsiderable evidence for deformation at othertimes in the late Paleozoic. Mechanisms forthis deformation fall into two groups: (1) evo-lution or continuation of the Antler orogeny,or (2) new orogenic events resulting from achanged tectonic setting.

Both the Antler and Sonoma orogenies areinterpreted to have resulted in the tectonic em-placement of deep-marine strata and volcanicrocks eastward onto the continental margin ofwestern North America. However, identifyingspecific structures (and particularly the origi-nal basal thrust fault or suture) unequivocallyassociated with these events has been difficult.In contrast, the Middle Pennsylvanian andearliest Permian deformations we describehere include folding and thrust faulting thatare well constrained in style and timing.This widespread, late Paleozoic contractionaldeformation appears to require poorly under-stood (and in some places unrecognized) tec-tonic activity, thus adding a phase of defor-mation whose timing is well constrained—butwhose nature is poorly known—to the evolu-tion of western North America.

Unusually good exposure and age controlin Carlin Canyon, north-central Nevada (Fig.1), allow us to document the geometry, kine-matics, and timing for three episodes of latePaleozoic tectonism in central Nevada: a mid-dle Mississippian event (not discussed in de-

tail in this paper; see Trexler et al., 2003), aMiddle Pennsylvanian deformation event, andanother in earliest Permian time. Although ev-idence for late Paleozoic deformation of Ant-ler foreland sedimentary deposits has longbeen recognized here (e.g., Dott, 1955; Ketner,1977), it has generally been overlooked indiscussions of the tectonic evolution of west-ern North America. Deformation events thatcannot be attributed to either a narrowly de-fined, Late Devonian Antler orogeny or thePermian–Triassic Sonoma orogeny have alsobeen documented elsewhere in Nevada (e.g.,Erickson and Marsh, 1974; Silberling et al.,1997; Ketner, 1998; Theodore, 1999, personalcommun.). The geologic relationships at Car-lin Canyon are significant for two reasons:they record the geometry of the contractionaldeformation, and they narrowly constrain thatdeformation in time. In Carlin Canyon, it isalso possible to distinguish between the lessintense Mesozoic overprint and the late Paleo-zoic deformations and to determine the ge-ometry of the Mesozoic deformation.

In this paper, we present evidence for Mid-dle Pennsylvanian and Permian deformationin Nevada, and we then discuss the signifi-cance of these events for our understanding ofthe tectonic evolution of North America. Wefirst summarize (1) the generally accepted Pa-leozoic history of the Antler foreland, and (2)our approach to reexamining part of this his-tory by using detailed biostratigraphy to rec-ognize and correlate widespread unconformi-ties in the stratigraphic record. Next, wepresent the structural and stratigraphic resultsof our detailed studies at Carlin Canyon. Wethen briefly summarize evidence for late Pa-leozoic deformation elsewhere in Nevada. Weconclude with a short discussion of the im-

526 Geological Society of America Bulletin, May/June 2004

TREXLER et al.

Figure 1. Location map for northeast and north-central Nevada. Squares—sections de-picted in Figure 10. Star—Carlin Canyon (geologic map in Fig. 5).

portance of late Paleozoic deformation andsome possible implications of this deforma-tion for the tectonic evolution of NorthAmerica.

BACKGROUND

Paleozoic History of the Antler Foreland

The western edge of Paleozoic North Amer-ica is thought to have been a passive marginuntil the Late Devonian–Early MississippianAntler orogeny (e.g., Roberts et al., 1958;Burchfiel and Davis, 1972, 1975). The Antlerorogeny is marked by the formation of a fore-land basin in Nevada and by a deep strike-slipbasin in Idaho (Link et al., 1996). Most work-ers think that it also entailed the eastward ob-duction of the internally deformed RobertsMountains allochthon onto the continental

margin (e.g., Burchfiel and Davis, 1975; Dick-inson, 1977; Speed and Sleep, 1982).

Antler tectonism began in latest Devonianto earliest Mississippian time, as recorded bythe initiation of a foreland basin with sedi-ments derived from the west (Poole, 1974;Smith and Ketner, 1977; Poole and Sandberg,1977; Ketner and Smith, 1982; Johnson andVisconti, 1992; Carpenter et al., 1993). In theoffshore basin that is now the Roberts Moun-tains allochthon, deposition continued untilLate Devonian (Famennian) to Early Missis-sippian (Kinderhookian) time (Coles and Sny-der, 1985). The rocks of the allochthon arenow strongly deformed, and that deformationis widely thought to be ‘‘Antler’’ in age. How-ever, attempts to find the oldest overlap unitson the allochthon (bracketing the end of de-formation) have so far demonstrated only thatthe internal deformation of the Roberts Moun-

tains allochthon occurred before Middle Penn-sylvanian time (e.g., Poole and Sandberg,1977; Rich, 1977; Dickinson et al., 1983;McFarlane, 1997; McFarlane and Trexler,1997; Trexler and Giles, 2000). Thus, the Ant-ler orogeny is currently defined by syntectonicsedimentary rocks ranging in age from LateDevonian through Early Pennsylvanian and bydeformation that is only known to be pre–Middle Pennsylvanian.

Deformation of the foreland-basin rocks (incontrast to those of the allochthon) as youngas middle Mississippian has been attributed toeither the Antler orogeny or to an unrecog-nized younger event. Lower Mississippianrocks in the Piñon Range are deformed, thenoverlapped by Upper Mississippian strata (forlocations of places discussed in text, see Fig.1) (Silberling et al., 1997; Johnson and Pen-dergast, 1981; Johnson and Visconti, 1992).Johnson and Pendergast (1981) interpretedthis deformation as part of the Antler orogenyin earliest Osagean (middle Early Mississip-pian) time. Alternatively, where the same re-lationship occurs in the Diamond Mountains,it was interpreted to postdate Antler defor-mation, because Antler foreland-basin strataare folded beneath the unconformity (Trexlerand Nitchman, 1990). Newer models for evo-lution of foreland systems in which strata ofthe foreland basin are deformed as contractionpropagates inboard (e.g., DeCelles and Giles,1996) suggest that the Antler orogeny mighthave lasted longer than previously thought—i.e., Late Devonian through middleMississippian.

The Antler orogeny created a long-livedcontractional foreland basin that filled withsyntectonic and posttectonic sedimentary de-posits. These strata record most of what weknow about the orogeny (citations above, andreviewed in Trexler et al., 2003). In centralNevada, siliciclastic rocks of the foreland con-sist of conglomerate, sandstone, and shale thatwere subjected to at least two sedimentary cy-cles within the Antler foreland system (Fig.2). Older basin fill is immature, deep-marine,and turbiditic, especially to the west in theforedeep keel. Later sedimentary fill is moremature and was deposited in fluvial, fan-delta,and shallow-marine environments; the com-mon limestone interbeds in most places pre-serve foraminifera. (Foraminifera [includingfusulinids] are ideal for age control. They arerapidly evolving, cosmopolitan, and do notsurvive recycling.) In some areas, the crustalresponse was quite different, e.g., strike-sliptectonics in Idaho (Link et al., 1996) and aremnant foredeep in southern Nevada (Trexleret al., 1996).

Geological Society of America Bulletin, May/June 2004 527

LATE PALEOZOIC TECTONISM IN NEVADA

Figure 2. Tectonostratigraphic boundaries, C1 through P2, in the upper Paleozoic section of the Great Basin and correlation of regionalstratigraphy discussed in text. The basis for this unconformity-based organization is discussed thoroughly in Trexler et al. (2003).

Intermittent post-Antler (informally definedhere as Middle Pennsylvanian through middlePermian time) deformation has been demon-strated in many places, although much of thisdeformation has been interpreted to be localin extent (e.g., Dott, 1955; Stevens et al.,1997). Some, however, have suggested that itreflects regional phases of tectonism (e.g.,Snyder et al., 1991, 1995; Trexler et al., 1991;Ketner, 1977). In the Carlin area, Dott (1955)and Ketner (1977) mapped angular unconfor-mities in the upper Paleozoic section. In cen-tral Nevada, Snyder et al. (1991), Gallegos etal. (1991), Perry (1994, 1995), Trexler et al.(1995), and Crosbie (1997) documented Mis-sissippian through Permian unconformitiesand syntectonic strata. In southwestern Neva-da, there are tectonic as well as eustatic con-trols on sedimentation in the southern Antlerforeland basin (Trexler et al., 1996; Trexlerand Cashman, 1997; Schiappa et al., 1999). Insoutheastern California, tectonism included aDesmoinesian (Middle Pennsylvanian) phase

of basin formation and two phases of EarlyPermian folding and thrusting (Stevens et al.,1997). In Utah and Idaho, important aspectsof Pennsylvanian and Permian tectonics andsedimentation have been documented by Jor-dan and Douglas (1980), Geslin (1998), andMahoney et al. (1991). In summary, there isconsiderable evidence in the literature forPennsylvanian and Permian deformation inthe region, enough to warrant an examinationof whether one or more regional, rather thanlocal, tectonic events are represented.

Unconformities in the StratigraphicRecord—Evidence for Tectonism in theAntler Foreland

Tectonism is recorded by deformation, bysyntectonic basin formation, and by unconfor-mities that result from uplift and erosion. Tec-tonically induced unconformities related tocollisional orogenies can be recognized be-cause they are angular where the deformation

is greatest (the orogenic belt), and both theangularity and the lacuna typically decreaseaway from the orogenic belt. We recognizeseveral widespread late Paleozoic unconfor-mities in the Great Basin that separate pack-ages of genetically related sedimentary rocks(Fig. 2). We have adopted a numberingscheme for the unconformities similar to thatused for the Mesozoic of the Colorado Plateau(e.g., Pipiringos and O’Sullivan, 1978). Theyare numbered sequentially from oldest toyoungest within each time period (for the latePaleozoic, we use Carboniferous 5 C andPermian 5 P) (Snyder et al., 2000).

At Carlin Canyon, three of these late Paleo-zoic unconformities are particularly easy torecognize because they are angular. With theaid of detailed biostratigraphy, these can betraced laterally into their correlative discon-formities. The middle Mississippian (C2) un-conformity is an angular unconformitythroughout much of the Piñon Range and Di-amond Mountains (Figs. 1, 2, and 3) (Trexler


TREXLER et al.

Figure 3. North wall of Carlin Canyon at the west end. Both the C6 and C2 angularunconformities are visible, at right and center, respectively. Here, the C6 unconformityhas removed all intervening boundaries.

et al., 2003) and a disconformity in the Spot-ted Range of southern Nevada (Trexler et al.,1996). The Late Pennsylvanian (C6) uncon-formity is a dramatic angular unconformity atCarlin Canyon and throughout the Piñon andAdobe Ranges. The earliest Permian (P1) un-conformity is a low-angle unconformity atCarlin Canyon, detectable here only throughdetailed biostratigraphy that shows strataltrimming (down to the east) of the underlyinglower Strathearn Formation, which is Mis-sourian. The detailed work necessary to re-solve the P1 relationship elsewhere is in prog-ress (W.S. Snyder and J.H. Trexler, Jr.,unpublished mapping).

RESULTS

Geologic Relationships at CarlinCanyon—Overview

Carlin Canyon, along the Humboldt Riverand Interstate 80 in western Elko County, Ne-vada, exposes the Mississippian throughPermian section in the heart of the Antler fore-land basin (Figs. 1, 4, and 5). Strata dip steep-ly throughout the canyon and display both me-soscopic and macroscopic folds and faults.Most workers have assumed that all of the de-formation there is Mesozoic or Cenozoic inage (e.g., Ketner and Smith, 1974; Smith andKetner, 1977; Thorman et al., 1991; Schwarzet al., 1994). This assumption was based onthe observation that rocks as young as Triassicare deformed nearby in the northern AdobeRange, rather than on field relationships in ornear Carlin Canyon.

The most conspicuous geologic feature inthe canyon is the angular contact between

Mississippian–Middle Pennsylvanian coarseclastic and carbonate strata (Tonka, Moleen,and Tomera Formations) and overlying UpperPennsylvanian–Lower Permian limestone(Strathearn Formation) (Fig. 3; see Fig. 4 forstratigraphy). Dott (1955) recognized this re-lationship as an angular unconformity. Ketner(1977) and Smith and Ketner (1977) attributedit to a Pennsylvanian ‘‘Humboldt orogeny’’and presumed uplift to the west. The conceptof the Humboldt orogeny was challenged bySnyder et al. (1991, 1995) and has been aban-doned by Ketner, who now favors exclusivelyMesozoic deformation in the region (Ketner,1998). Jansma and Speed (1990) reinterpretedthis contact as a Mesozoic omissional fault, aproposition since challenged by Schwarz et al.(1994).

Our new mapping at Carlin Canyon, usingdetailed biostratigraphic age control, revealsthree angular unconformities in the upper Pa-leozoic section (Fig. 2). The stratigraphicallylowest (C2 in Figs. 2 and 4) occurs withincoarse-grained conglomerates mapped as theTonka Formation by Dott (1955) and thereforerequires a redefinition of the Tonka. We callthe rocks below the unconformity MelandcoFormation, and those above, Tonka Formation(Trexler et al., 2003). This angular unconfor-mity is Mississippian in age and cuts downsection to the west. Although it does not cropout elsewhere in the Carlin area, it is foundthroughout much of central Nevada (Trexleret al., 2003). The next younger unconformitypreserved in Carlin Canyon, C6, separates theMoleen and Tomera Formations (local equiv-alents of the lower and upper Ely Limestone)from the overlying Strathearn Formation. Thisunconformity cuts down section toward the

west, removing the Tomera and Moleen en-tirely in the western part of the map area (Fig.5) and placing Strathearn directly on the upperpart of the Tonka. This angular relationship—Strathearn over Tonka—was recognized byDott (1955) as evidence for Pennsylvanian de-formation in the Carlin area. A third angularunconformity (P1) occurs within the Stra-thearn and therefore requires a redefinition ofthe Strathearn. This surface cuts down sectiontoward the east; in the eastern part of the maparea, it has removed all of the lower Strathearnand has placed the upper part of the Strathearndirectly on Moleen-Tomera. A fourth uncon-formity (P2), not obviously angular here, plac-es Buckskin Mountain Formation over Stra-thearn and cuts down section toward the eastin the Carlin Canyon area. This is a majorunconformity at a regional scale; it occursthroughout northern Nevada (Snyder et al.,1991; Sweet et al., 2001; Sweet and Snyder,2002).

The C6 and P1 unconformities truncate me-soscopic and macroscopic structures that re-cord the geometry and kinematics of two de-formation events, one occurring in the MiddlePennsylvanian and the other in earliest Perm-ian. The Middle Pennsylvanian (C6) defor-mation includes thrust faults and macroscopicoverturned folds in addition to mesoscopicstructures. It records more contraction (at leastlocally) than the other deformation events ex-posed in Carlin Canyon. The earliest Permian(P1) deformation is expressed as open, uprightmacroscopic folds. Both fold sets have gentlynortheast-plunging axes, but these folds canbe distinguished at Carlin Canyon on the basisof details of fold geometry and kinematics(see section on Kinematics of Pennsylvanianand Permian Deformations).

The presence of these late Paleozoic uncon-formities requires revision of the local strati-graphic nomenclature, in particular, the Tonkaand Strathearn Formations. Original mappingin the area (Dott, 1955) designated all Missis-sippian conglomerate sections as Tonka For-mation, defined at Tonka siding in CarlinCanyon. The unconformity within the Missis-sippian section was not recognized, and thelower section has no known fossil occurrencesthat would allow biostratigraphic recognitionof the older conglomerates. We use the nameMelandco Formation for these Lower andmiddle Mississippian rocks, consistent withunit names nearby to the east (Trexler et al.,2003). The unconformity within the Missis-sippian conglomerate section is expressed asan angular discordance accompanied by a fa-cies change at the west end of Carlin Canyon,west of the highway and railroad tunnels



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TREXLER et al.

Figure 5. Two cross sections; for locations see Figure 4. Sense of motion on strike-slip faults is shown by T (5 toward) and A (5 away)from the viewer.

(Figs. 3 and 5). Melandco strata are polymictconglomerate and sandstone that are matrix-rich to matrix-supported and that generallylack sedimentary fabric. The overlying Tonkastrata are well bedded; they consist of clean,chert–quartzite–lithic conglomerate and sand-stone with interbedded calcareous litharenite.

Our detailed biostratigraphy also documentsa hiatus within the Strathearn Formation. InCarlin Canyon, the lower Strathearn Forma-tion was folded and erosionally trimmed priorto deposition of the upper Strathearn. TheStrathearn, defined southeast of Carlin Canyonat Grindstone Mountain (Dott, 1955), is lith-ologically uniform from bottom to top, mak-

ing recognition and mapping of the twomembers very difficult where they are con-formable. We have not yet renamed theseunits and have referred to them informally asupper and lower members of the StrathearnFormation.

Evidence for Erosion at the MappedUnconformity—It Is Not a Fault

Our field work has convinced us that theangular contact at the base of the Strathearnin Carlin Canyon is the regionally importantC6 unconformity. Locally, the basal part ofthe Strathearn Formation is characterized by a

rusty-colored, iron oxide–rich regolith zoneabout 1 m thick. This zone contains clasts de-rived from the subjacent strata. Where the reg-olith zone overlies the Tonka, the number andsize of Tonka clasts increases downward with-in the zone, until the rock becomes a recog-nizable paleosol C-horizon developed on in-tact Tonka Formation. Vertical, openpaleofractures in the Tonka are filled with reg-olith material. The same regolith zone can bemapped laterally along the base of the Stra-thearn to where the Strathearn overlies otherunits such as the Moleen and Tomera For-mations. The regolith zone is overlain para-conformably by quartz-arenaceous limestone



Figure 6. Panoramic photograph looking at the north wall of east-central Carlin Canyon(‘‘the amphitheater’’). ‘‘Duplex Canyon’’ is the southwest-trending gulch on the left (west)end of the amphitheater. Mesoscopic folds are asymmetric to overturned and have am-plitudes of meters to tens of meters and northeast-plunging axes (see Fig. 9A for a ste-reogram of this folding). Vergence is toward the northwest for some structures and south-east for others (see Fig. 5). For example, the lowest of the three closely spaced thrust faultsnear the center of Figure 6 cuts upsection to the northwest and has a northwest-vergenthanging-wall anticline. The middle thrust cuts upsection to the northwest in the footwallhere and cuts upsection to the northwest in the hanging wall in ‘‘Duplex Canyon’’ (seethe hanging-wall ramp in the upper thrust in Fig. 8). The upper thrust has a southeast-vergent hanging-wall anticline and cuts upsection to the southeast. Stratigraphic throw onthese thrust faults is tens to hundreds of meters; faulting duplicates the Lower Pennsyl-vanian section at least hundreds of meters. Horizontal displacement is unknown, but maybe large.

of the lower and/or upper Strathearn Forma-tion. No recognizable clasts of Strathearnlimestone occur in this regolith zone, makingit unlikely that the zone is, or contains, a fault-slip breccia. We have not found evidence oforganic activity in the regolith zone, althoughit has extensive iron oxide development sug-gesting an aridisol.

Our field observations do not support afault interpretation (Jansma and Speed, 1990)for the sub-Strathearn contact. Although thereappears to have been local slip on the contactduring subsequent folding, throughgoing,well-developed slip surfaces are absent, andthere is no evidence for measurable offsetalong the base of the Strathearn. We were un-able to find any kinematic features consistentwith fault-zone deformation along or near thecontact. Instead, the base of the Strathearn isa mappable regolith zone. We conclude thatany slip along this contact is insignificant.

Age Control on Pennsylvanian andPermian Deformations

The strata that most tightly bracket the C6unconformity in Carlin Canyon are the sub-jacent Middle Pennsylvanian Tomera Forma-tion (sensu Dott, 1955), and the superjacent,Upper Pennsylvanian–Lower Permian Stra-thearn Formation (Fig. 4). Below the uncon-formity, the youngest sub-C6 strata in CarlinCanyon are Atokan (Fig. 4), and angular trim-ming of the section suggests that younger stra-ta may be preserved nearby. On a regionalscale, we have shown that sub-C6 strata areas young as medial to possibly late Desmoi-nesian (Bissel, 1964; our unpublished work).The oldest Strathearn rocks overlying C6 inCarlin Canyon are lower to middle Missourian(Fig. 4). Therefore, the C6 hiatus in CarlinCanyon (and the tectonic event it records) isbracketed to be 5 m.y. or less in duration, de-pending on the time scale used for the extrap-olation of numerical ages from biostratigraph-ic ages.

All Mississippian–Pennsylvanian strata inCarlin Canyon were folded again in the ear-liest Permian, during an event bracketed bythe lower Strathearn Formation, below, andthe upper Strathearn Formation, above. Theresulting angular unconformity is the P1boundary. Fusulinids from the upper Stra-thearn are upper Asselian to lower Sakmarianin age (Fig. 4). Therefore, in Carlin Canyon,the P1 hiatus is 5 to 7 m.y. in duration.

Still higher in the section at Carlin Canyon,the P2 boundary is defined at the base of theBuckskin Mountain Formation, where it over-lies the upper Strathearn Formation. The old-


TREXLER et al.

Figure 7. View toward the south, showing deformation in the Moleen and Tomera (5 ElyLimestone) Formations at the east end of Carlin Canyon. Vertical bedding occurs in theforeground and across the Humboldt River on the south canyon wall in the distance.Stratigraphic ‘‘tops’’ are toward the northwest, indicating that this large fold is northwest-vergent. This macroscopic fold is coaxial with the thrust-related folds (see Figs. 6 and9A); all are consistent with a single northwest-southeast shortening event.

est Buckskin Mountain strata are early Artin-skian in age. The P2 unconformity trims thesection down to rocks as old as Sakmarian atCarlin Canyon, but regionally (e.g., SecretCanyon, in the Fish Creek Range) the ero-sional surface cuts as far down as Chesterianrocks (Schwarz, 1987; Snyder et al., 1991).Where the P2 hiatus is the most tightly con-strained, it is possibly as short as 5 m.y.

Kinematics of Pennsylvanian and PermianDeformations

Deformation, expressed as angular discor-dance, folding, or imbricate thrusting, can bedocumented below the unconformities in theupper Paleozoic section at Carlin Canyon(Fig. 6). Wherever fold axis orientations canbe determined, they trend northeast, making itdifficult to distinguish between the foldingevents on the basis of their general orientation.However, there are consistent differences inboth fold style and precise axis orientationthat make it possible to distinguish these de-formations, even where good age control isabsent. In the following paragraphs we docu-ment the geometry and kinematics of each de-formation event, named—for simplicity—after the unconformity that represents it (Fig.2). The events are discussed in chronologicorder, from oldest to youngest.

C2 EventThe pre–late Late Mississippian (Chesteri-

an) C2 unconformity is marked by Melandco(lower Tonka) below and (upper) Tonkaabove. Deformation below the C2 unconfor-mity in Carlin Canyon is expressed only as anangular truncation (Fig. 3). After the steep tiltof the overlying Tonka is removed, the bed-ding in the Melandco dips east, and the un-conformity cuts down section to the west. Thecontact between the Melandco and Tonka isnot exposed over a wide enough area in CarlinCanyon to determine the geometry of the pre-Chesterian deformation. However, this defor-mation event is regional in extent (Trexler etal., 2003) and has also been mapped in thenorthern Piñon Range ;30 km to the southwhere there is middle Mississippian foldingand thrusting (Silberling et al., 1997; R.M.Tosdal’s mapping in Trexler et al., 2003) andin the Diamond Mountains 120 km to thesouth where an angular truncation exists be-low the C2 unconformity (Trexler and Nitch-man, 1990).

C3, C4, and C5 EventsAt Carlin Canyon, the Late Mississippian

and Early Pennsylvanian C3, C4, and C5

boundaries are conformable contacts, are con-tained within other unconformities, or are oth-erwise not preserved. Each is expressed else-where, however, and is a stratigraphicboundary that is of potential tectonic signifi-cance. These unconformities, not preserved atCarlin Canyon, are not described in detailhere.

C6 EventThe post–early Middle Pennsylvanian (mid-

dle Desmoinesian), pre–late Late Pennsylva-nian (middle Missourian) C6 event has jux-taposed the Tonka, Moleen, and TomeraFormations below and Strathearn Formationabove. The deformation involves overturned

folding and imbricate thrusting within theTonka, Moleen, and Tomera Formations atCarlin Canyon. Faulting and folding are de-veloped at both the macroscopic (Fig. 7) andmesoscopic (Figs. 6 and 8) scales and aredominantly northwest vergent. Mesoscopicfolds visible on the north side of I-80 (Fig. 6)have northeast-trending axes and verge bothnorthwest and southeast; some are fault-propagation folds preserved in the hangingwalls of mesoscopic thrusts. Imbricate thrust-ing in ‘‘Duplex Canyon’’ is the best evi-dence that the deformation was dominantlynorthwest-directed. Here, individual thrustsstep up-section to the northwest, and folds as-sociated with hanging-wall ramps, which oc-



Figure 8. ‘‘Duplex Canyon’’ in the north wall of Carlin Canyon; see Figure 6 for location.View is toward the east. Although the thrust faults now dip gently toward the northwest,the truncation of hanging-wall bedding into the thrusts represents hanging-wall rampsand documents northwest-directed thrusting.

cur in several of the imbricate sheets, all vergenorthwest.

Macroscopic folding also records northwestvergence. A macroscopic anticline-synclinepair that is coaxial with the mesoscopic fold-ing is well exposed along I-80 east of the tun-nels. It crops out adjacent to the hoodoos ofTertiary Humboldt Formation on the northside of the highway and continues along strikesouth of the highway (Fig. 7). The subverticallimb of this fold has stratigraphic ‘‘tops’’ tothe northwest, indicating northwest vergence.

The overlying Strathearn Formation is notpreserved immediately above many of thesestructures, so the later deformation cannot beremoved with confidence. In their present ori-entation, however, fold axes characteristic ofthis deformation are subhorizontal and trend0408–0658 (usually 0608–0658) (Fig. 9A).

P1 EventThe post–late Late Pennsylvanian (middle–

upper Virgilian), pre–middle Early Permian(late Asselian) P1 event is expressed as an un-conformity separating lower Strathearn belowand upper Strathearn above. The deformationwithin the lower Strathearn is expressed as up-right, open folding. After the tilt of the over-lying upper Strathearn has been removed, thefold axes plunge gently toward 0308–0358(Fig. 9B). This folding is easily visible in theeastern half of the big meander loop of theHumboldt River at the I-80 tunnels. Here, in-terbedded conglomerate and limestone of thelower Strathearn are folded into a broad syn-form on the west side of the Humboldt River,

and this synform terminates against the homo-clinal upper Strathearn on the east side of theriver (see box on map, Fig. 4). Note that theMesozoic ‘‘Adobe syncline’’ has been mappedthrough this area (Ketner and Ross, 1990), andbedding orientations within the highest pre-served part of the section (Permian) are con-sistent with a large syncline. This syncline isa different, and later, structure than the ero-sionally truncated syncline below the P1 un-conformity. The overlying unconformity cutsdown section to the east, removing the entirelower Strathearn 1.5 km east of the east por-tals of the I-80 tunnels and suggesting thatdeformation-related uplift might have beengreater farther to the east.

High-angle normal faults of late Paleozoicage are locally developed in Carlin Canyonand have meters to hundreds of meters of dis-placement (Fig. 6). Brecciation is typicalalong these faults, and drag folding occurs lo-cally. A normal sense of motion is shown bothby the offset of marker units across the faultsand by steeply plunging oblique-slip striaealong the fault surfaces. The high-angle faultscut thrust-related structures, but they are olderthan the sub–upper Strathearn (P1) unconfor-mity. At least one of these high-angle faultshas been reactivated by post-Pennsylvaniandeformation. Although this fault can bemapped across the unconformity into the baseof the upper Strathearn Formation (Fig. 5),displacement of the Strathearn is an order ofmagnitude smaller than displacement of theunderlying Moleen Formation (lower ElyLimestone).

P2 EventThe post–early Early Permian (late Asselian–

early Sakmarian), pre–late Early Permian (ear-ly Artinskian) P2 unconformity separates up-per Strathearn Formation below from Buck-skin Mountain Formation above. The de-formation features in the upper Strathearn arenot as well developed as those of the preced-ing two deformations in the Carlin Canyonarea. The unconformity below the BuckskinMountain Formation cuts down section to-ward the east in Carlin Canyon. We have notyet collected enough structural data along thiscontact to determine the geometry of the de-formation below the unconformity. Althoughnot dramatic in the Carlin Canyon area, theP2 is a major unconformity at a regional scale;it occurs throughout northern Nevada (Snyderet al., 1991; Sweet et al., 2001; Sweet andSnyder, 2002).

Post–Late Early Permian (Late Artinskian)Events

In the Carlin Canyon area, the Permian sec-tion is deformed. The immediately overlyingTertiary Humboldt Formation remains unde-formed. Therefore, the timing of the open, up-right folding exhibited in the younger Permiansection is not well constrained—it postdatesall of the Permian units present in this area(upper Strathearn, Buckskin Mountain, Bea-con Flat, and Carlin Canyon Formations) andpredates the Tertiary Humboldt Formation. Intheir present orientation, the folds are openand upright, and fold axes plunge gently (258)toward the north-northeast (0208) (Fig. 9C).These folds are commonly meters to tens ofmeters in amplitude. A set of open, uprightfolds in the Moleen and Tomera Formationsis also attributed to the post–Early Permiandeformation event on the basis of their foldstyle and orientation; the axes are subhorizon-tal or plunging gently toward 0258 (Fig. 9C).

Open, upright folding is demonstrably post-Triassic elsewhere in the region, but this agecontrol is not available in the Carlin Canyonarea. In the northern Adobe Range, the Adobesyncline involves rocks as young as Triassic(Ketner and Smith, 1974; Smith and Ketner,1977; Thorman et al., 1991). Broad folding inthe Carlin Canyon area has been correlatedwith the Adobe syncline, but the youngestrocks involved are Early Permian in age. Fold-ing cannot be directly connected between thenorthern Adobe Range and Carlin Canyon;therefore, we can only constrain the latestfolding seen at Carlin Canyon to be late EarlyPermian or younger in age.


TREXLER et al.

Figure 9. Stereograms of the three different fold sets in the Carlin Canyon map area. Allare lower-hemisphere, equal-area plots; poles to bedding are shown as circles, squares, oropen triangles. Different symbols represent individual mesoscopic or macroscopic folds.Fold axes for individual folds are shown as filled triangles. (A) Pennsylvanian (C6) defor-mation, characterized by asymmetric to overturned folding and northwest-directed thrustfaulting. Stereogram shows poles to bedding in macroscopic, mesoscopic, and hanging-wall folds; also plotted are individual fold axes. Fold axes most commonly trend east-northeast (0508–0658). Note: The subsequent deformation has not been rotated out formost of these folds, because the overlying Strathearn Formation is not preserved. How-ever, the three folds where subsequent tilt has been removed do not differ systematicallyfrom the other folds on this stereogram, indicating that subsequent rotation is not signif-icant here. (Fold axis orientations, presented as plunge/trend: 11/037, 08/039, 04/056, 04/063, 18/059, 02/246, 12/228.) (B) Lower Permian (P1) deformation, characterized by open,upright folds (see also box, Fig. 5). The tilt of the overlying upper Strathearn has beenremoved, and the P1 fold axes plunge gently toward 0308–0328. (Fold axis orientations,presented as plunge/trend: 16/032, 13/030.) (C) Post-Permian deformation, characterizedby open, upright folds. The tilt of the overlying upper Strathearn has been removed, andthe P1 fold axes plunge gently toward 0208–0268. (Fold axis orientations, presented asplunge/trend: 21/026, 25/020, 05/205.)

Evidence for Pennsylvanian and PermianDeformations Elsewhere in Nevada

The relationships we document at CarlinCanyon are also found both west and east ofCarlin Canyon (Figs. 1, 10). Map relationshipsdocument unequivocal Pennsylvanian orPermian tectonism and indicate that late Pa-leozoic deformation is widespread. In somecases, the deformation is in rocks of the Antlerforeland basin—so it either postdates the Ant-ler orogeny or represents propagation of Ant-ler contraction into the foreland basin. In othercases, deformation is in rocks of the Antlerallochthon and would probably be attributedto the narrowly defined Antler orogeny (i.e.,Late Devonian–Early Mississippian) exceptthat rocks of Carboniferous age are involvedin the deformation. In both cases, it is thepresence of Pennsylvanian or Permian sedi-mentary rocks unconformably overlying thedeformation that demonstrates the late Paleo-zoic age. These ‘‘overlap assemblage’’ rocksare absent in most places, so interpretation ofthe age of deformation has often been basedon inference: Antler deformation for rocks ofthe allochthon and Mesozoic deformation forrocks of the Antler foreland basin.

At Beaver Peak west of Carlin Canyon,both members of the Strathearn Formationhave been identified and mapped (Figs. 1, 10)(Berger et al., 2001). There they are both con-glomeratic, and conodont ages there confirmour fusulinid dates for both units. The lowermember of the Strathearn, along with lowerPaleozoic rocks of the Roberts Mountains al-lochthon, is involved in thrusting on a faultthat is unconformably overlapped by the upperStrathearn, tightly bracketing the age ofthrusting as middle to late Asselian (EarlyPermian), equivalent to the deformation atCarlin Canyon below the P1 boundary.

The age of the folding and thrusting can beconstrained only to pre-Permian at Coal MineCanyon, in the northern Adobe Range (Fig. 1).There, Mississippian shale of the Antlerforeland basin is in the footwall of a thrustfault (Ketner and Ross, 1990) that clearlypostdates the Antler foreland basin. Meso-scopic northeast-trending folds in the upperplate and in the thrust itself are erosionallytrimmed and overlain by silicified sandy car-bonates dated only as ‘‘Permian’’ on the basisof macrofossils (Ketner and Ross, 1990). Ourattempts to refine this age by using small fo-raminifera or fusulinids were unsuccessful be-cause of poor fossil preservation in these si-licified, coarse-grained rocks.

Erickson and Marsh (1974) documented Pa-leozoic deformation in the rocks of the Rob-

erts Mountains allochthon and recognized thatthis orogenic event could not be attributed toeither the Antler or Sonoma orogenies as thenconceived. Lower Pennsylvanian to Lower

Permian rocks that overlap pre-Pennsylvanian(Antler age?) deformation in the Edna Moun-tain quadrangle, in Humboldt County, Neva-da, are asymmetrically folded and thrust fault-



Figure 10. Measured sections in upper Paleozoic strata of central and northeast Nevada discussed in the text. Locations of measuredsections are shown in Figure 1. Data for Beaver Peak are from Berger et al. (2001). All other sections are from our unpublished work.Note that unconformities generally span less time in eastern sections than they do in western ones. RMA—Roberts Mountains allochthon.

ed and are themselves unconformably overlainby Upper Permian rocks. Erickson and Marsh(1974) emphasized that ‘‘orogenic movementswhich do not conveniently relate to either theAntler or Sonoma orogeny deserve moreattention and must be accounted for in thegeologic history of the southern Cordillera’’(p. 336).

In northeastern Nevada, the C5 and C6boundaries have been documented indepen-dently by Sweet et al. (2001) and Sweet andSnyder (2002) in the Pequop Mountains (Fig.1) and by McFarlane (1997) and McFarlaneand Trexler (1997, 2000) in the Snake Moun-tains. In the southern Pequop Mountains, theEly Limestone is folded and faulted, erosion-ally truncated, and overlain by the Hogan For-mation (C5). In the Snake Mountains north ofWells, Nevada, an imbricate stack of thrustsheets containing rocks of both the continental-shelf autochthon and the Roberts Mountains

allochthon is erosionally truncated and over-lain by Missourian Strathearn Formation(along the C6 unconformity).

DISCUSSION

The fortuitous combination of exposure,preservation, and datable rocks is the reasonthat several late Paleozoic deformation eventscan be unequivocally documented in CarlinCanyon, although they have not yet beenwidely recognized elsewhere. Paleozoic rocksare exposed discontinuously and poorlythroughout central and western Nevada be-cause of widespread Tertiary volcanic cover inthe ranges and Tertiary and Quaternary sedi-mentary cover in the basins. Furthermore,clear preservation of a complex deformationalrecord is unlikely because of the very natureof the late Paleozoic deformations; the accom-panying uplift and erosion remove or obscure

the evidence of earlier deformations. Finally,many of the upper Paleozoic sedimentaryrocks, and particularly the syntectonic sedi-mentary rocks, are clastic rocks that are un-likely to contain or preserve age-diagnosticfossils. However, all of the necessary condi-tions are met in the Carlin Canyon area, andearlier workers correctly interpreted the upperPaleozoic rocks there to record deformation inPennsylvanian and/or Permian time (e.g.,Dott, 1955; Ketner, 1977).

Even within the Carlin Canyon area, the ev-idence for some of the late Paleozoic defor-mation is cut out laterally by higher uncon-formities. For example, the P1 unconformitycuts down section toward the east, eventuallyplacing the upper Strathearn Formation di-rectly on the Moleen and Tomera Formations,completely removing the lower StrathearnFormation and with it, all evidence of the C6unconformity (Fig. 4). On the basis of the dis-


TREXLER et al.

Figure 11. Tectonic setting for North America during the Pennsylvanian Ancestral RockyMountains orogeny. Many workers have contributed to the ideas present in this conceptualmap (see Discussion in the text).

tribution of unconformities in Carlin Canyon,even with 100% exposure one would not ex-pect to find the C6 unconformity east of themap area or the C3 unconformity west of themap area. The problem is not unique to CarlinCanyon—Sweet et al. (2001) and Sweet andSnyder (2002) described similar relationshipsin the Pequop Range. There, a higher uncon-formity (C6) cuts deeply enough that it locallyremoves all record of an older unconformity(C5) that is preserved elsewhere in the range.This repeated erosional trimming (due to up-lift during later tectonic events) is probablythe reason that the late Paleozoic deformationevents have not been widely recognized, inspite of the accurate interpretations by earlyworkers in the area (e.g., Dott, 1955; Ketner,1977).

Structures we have mapped in CarlinCanyon may (understandably) have beenseen but misinterpreted as to age elsewherein Nevada, especially where age control islimited. In Carlin Canyon, three fold sets(two unequivocally late Paleozoic and thethird probably Mesozoic) all have generallynortheast-plunging axes. It would be hard todistinguish them in the absence of datablePennsylvanian and Permian rocks that occurin overlap relationships.

We suggest that much of the deformationattributed to the Antler orogeny in Nevadamay in fact be Pennsylvanian or Permian inage (see also Trexler et al., 1999; Snyder etal., 2000). This possibility may be particularlytrue within the Antler allochthon, where Or-dovician to Devonian chert, argillite, and vol-canic rocks are folded, but usually there is nosedimentary overlap assemblage to constrainthe time of folding. Our interpretation requiressignificant revision to the conventional under-standing of the Antler orogenic event, extend-ing the activity of this orogen well into thePennsylvanian.

Although the Pennsylvanian (C6) defor-mation in Nevada is synchronous with the An-cestral Rocky Mountains orogeny, the style,intensity, and orientation of deformation sug-gest that, unlike the Ancestral Rocky Moun-tains, it could not be a far-field effect of col-lision with Gondwana along the southeasterncontinental margin (Kluth, 1986). The defor-mation style we describe here is thin-skinnedcontraction, similar to the Rocky Mountainfold-and-thrust belt of late Mesozoic age. Wedo not yet know whether the scale approachesthat of the Sevier thrust belt; that determina-tion will require detailed work to define theages of potentially correlative structuresthroughout the Great Basin. In contrast to thethin-skinned deformation in Carlin Canyon,

Ancestral Rocky Mountains structures gener-ally comprise steep thrust faults involvingbasement and very thick sedimentary sectionsdocumenting rapid and deep subsidence (e.g.,Paradox basin). In addition, Ancestral RockyMountains–related structures are generallyoriented northwest-southeast, orthogonal tothe folds and thrusts in Carlin Canyon.

Several plate boundaries may have influ-enced the tectonic development of westernNorth America in Pennsylvanian time (Fig.11). On the south and east, the Marathon-Ouachita orogenic belt is a complex and well-documented record of the collision of NorthAmerica with Africa and South America (e.g.,Kluth and Coney, 1981; Ross, 1991). To thesouth or southwest, Ye et al. (1996) have sug-gested a Pennsylvanian low-angle, northeast-dipping subduction zone (not shown in Fig.11), although subsequent work has failed tocorroborate this hypothesis. To the southwestand west, Stone and Stevens (1988) and Ste-vens et al. (1998) have proposed a left-lateraltruncation of the continental margin in Penn-sylvanian time. Dickinson and Lawton (2001)also invoke a sinistral transform fault alongthe western edge of North America, and sug-gest that the major displacement along thetransform occurred from Early Permian toMiddle Triassic time. Walker (1988) suggeststhat this transform margin was initiated as ear-ly as Pennsylvanian time. Convergence alongthe western margin is a consistent themeamong many workers, and has recently been

refined by Dickinson (2000), who suggests anortheast-southwest trend.

The recurrent and well-dated deformationevents at Carlin Canyon appear to require anactive orogenic belt to the west of NorthAmerica at the latitude of the Great Basin inCarboniferous and Permian time. Continuedshortening during the late Paleozoic may havereactivated older structures in the Antler fore-land and orogenic belt as well as producingoverprinted folding, thrusting, and erosionsuch as that at Carlin Canyon. Although tra-ditional ‘‘Antler age’’ (Late Devonian) struc-tures may have been reactivated during thisshortening, we prefer the interpretation thatmost, or all, of the structures originated in latePaleozoic time. The contrasting deformationstyle in Nevada relative to coeval deformationin Utah and Colorado may result from themarked difference in crust: old, thinned, pas-sive continental margin and accreted terranesto the west and thick, homogeneous, largelycrystalline continental crust to the east. Also,note that the Oquirrh basin of western Utah,usually thought to be the westernmost expres-sion of the Ancestral Rocky Mountains, is alsoamong the thickest of the Ancestral RockyMountains basins (Erskine, 1997). This, likethe northwest-southeast shortening at CarlinCanyon, is an unexplained anomaly if for-mation of the Oquirrh basin resulted solelyfrom plate convergence along the southernmargin of the continent (Geslin, 1998), but is



consistent with convergence along the westernmargin of North America.

In conclusion, we think that late Paleozoicdeformation in the Great Basin is much morewidespread and important than has been rec-ognized. This contractional deformation oc-curred repeatedly between the times generallyaccepted for the Antler and Sonoma oroge-nies. Continent-continent collision along theMarathon-Ouachita orogenic belt, widely citedas the driving force for the Ancestral RockyMountains orogeny, cannot alone explain con-current deformation in the Great Basin. Weconclude that plate convergence along thewestern margin of the continent must haveplayed a significant role deformation and sed-imentation in western North America duringthe late Paleozoic.

ACKNOWLEDGMENTS

National Science Foundation grants to Trexler,Cashman, and Snyder (EAR-9304972, EAR-9305097, EAR-9903006) funded much of the re-search that led to this paper. Our attention wasdrawn to this area by the spectacular exposure, butthe thorough and prescient work by Bob Dott madeit possible to begin to understand it; his mappingwas our starting point to unravel the stratigraphyand structure. We thank Scott Ritter and DaveSchwarz for their help in the field early in the proj-ect. Mapping and paleontology by graduate studentsat both Boise State University and the University ofNevada, Reno, played a role in sorting out this com-plex locality. The manuscript was much improvedby reviews from Tim Lawton, Paul Link, and JoanFryxell.

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