Magnetostratigraphy of the Upper Triassic Chinle Group of New Mexico: Implications for Regional and Global Correlations Among Upper Triassic Sequences

Home , Alaunian, Blue Mesa Member, Petrified Forest Member, Redonda Formation, Rock Point Formation, Trujillo Formation

Magnetostratigraphy of the Upper Triassic Chinle Group of New Mexico: Implications for regional and global correlations among Upper Triassic sequences

Kate E. Zeigler1,* and John W. Geissman2,* 1Department of Earth and Planetary Sciences, MSC 03-2040 Northrop Hall, University of New Mexico, Albuquerque, New Mexico 87131, USA 2Department of Earth and Planetary Sciences, MSC 03-2040 Northrop Hall, University of New Mexico, Albuquerque, New Mexico 87131, USA, and Department of Geosciences, ROC 21, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080-3021, USA

ABSTRACT polarity chronologies from upper Chinle graphic correlations (e.g., Reeve, 1975; Reeve and strata in New Mexico and Utah suggest that Helsley, 1972; Bazard and Butler, 1989, 1991; A magnetic polarity zonation for the strata considered to be part of the Rock Point Molina-Garza et al., 1991, 1993, 1996, 1998a, Upper Triassic Chinle Group in the Chama Formation in north-central New Mexico are 1998b, 2003; Steiner and Lucas, 2000). Conse- Basin, north-central New Mexico (United not time equivalent to type Rock Point strata quently, the polarity record of the mudstones and States), supplemented by polarity data from in Utah or to the Redonda Formation of east- claystones, which are the principal rock types in eastern and west-central New Mexico (Mesa ern New Mexico. the Chinle Group, is largely unknown. Redonda and Zuni Mountains, respectively), In our study of Triassic strata in the Chama provides the most complete and continuous INTRODUCTION Basin of north-central New Mexico, we sam- magnetic polarity chronology for the Late pled all components of the Chinle Group, with Triassic of the American Southwest yet avail- The Upper Triassic Chinle Group, prominent a focus on mudstones and claystones at Coyote able. Most of the Chinle Group sequence in the Mesozoic stratigraphy of the American Amphitheater, which exposes a nearly complete is composed of hematitic mudrocks that Southwest, is continental in origin and reflects and continuous section of these rocks, as well as typically carry a well-defined, well-grouped a complex environment of fluvial, lacustrine, selected subintervals at other nearby localities magnetization (residing in both pigmen- and aggradational fan deposition (Blakey, 1989; (Fig. 1). Each stratigraphic datum was sampled tary and detrital hematite), with laboratory Dubiel, 1987, 1989a, 1989b, 1994; Weissmann with sufficient density (N = five or more inde- unblocking temperatures as high as 680 °C. et al., 2007) with a drainage basin that encom- pendent samples) to allow for robust evaluation Demagnetization experiments isolate mag- passed most of western North America. Based on of the paleomagnetism of these rocks. netizations of south- or north-seeking dec- vertebrate biostratigraphy, palynostratigraphy, Two additional sections were sampled to lination and shallow inclination, which are and limited geochronology, the Chinle Group test magnetostratigraphic correlations between interpreted as early acquired, Late Trias- has typically been inferred to span most of the modern physiographic basins, and to evaluate sic magnetizations. Our proposed polarity Late Triassic (Stewart et al., 1972; Litwin, 1986; previous lithostratigraphic and biostratigraphic correlations, coupled with biostratigraphic Lucas and Hunt, 1992; Hunt and Lucas, 1993a, correlations. The Six Mile Canyon section in the observations and recent U-Pb age determi- 1993b; Steiner and Lucas, 2000; Lucas et al., Zuni Mountains (central New Mexico) includes nations on detrital zircon–bearing strata in 2003, 2005; Riggs et al., 2003). The global-scale the upper Bluewater Creek Formation and the Chinle Group in western New Mexico, changes in tectonics and evolution over the time the lower Blue Mesa Member of the Petrified West Texas, and Arizona, indicate that period represented by strata of the Chinle Group Forest Formation, from which Irmis and Mundil deposition of Chinle strata likely spanned a emphasize the critical need for accurate corre- (2008) reported a detrital zircon date of 219.2 ± much shorter time span than previously con- lation of marine and nonmarine Upper Triassic 0.7 Ma as a maximum depositional age. The sidered. If this interpretation is correct, the strata on regional and global scales (Olsen et al., Mesa Redonda section (eastern New Mexico) Chinle Group can be correlated with only 2009; Lucas, 2010a). One means of correlating includes the Redonda Formation (Fig. 1), parts part of the Newark Supergroup or the Upper diverse strata, of both continental and marine of which were originally sampled by Reeve and Triassic Tethyan sections. On a local scale, affinity, is through magnetostratigraphy. Helsley (1972), Reeve (1975), and Bazard and lower Chinle strata in the Chama Basin are Early efforts to construct a stratigraphically Butler (1989, 1991). Here we present a new significantly older than the Bluewater Creek continuous magnetic polarity chronology for the magnetic polarity chronology for the Upper Tri- Formation in western New Mexico, and the Chinle Group and to provide key paleomagnetic assic Chinle Group in New Mexico and use this base of the Poleo Formation represents a dis- poles for the Late Triassic primarily involved new information to test both local and regional conformity of >13 m.y. duration. Magnetic sampling sandstones and siltstones from inde- lithostratigraphic correlations and to provide pendent sections of Chinle strata from different tentative correlations of Chinle strata with the *Emails: Zeigler: [email protected], Geissman: subbasins and outcrop belts, and these data were Newark Supergroup in eastern North America [email protected]. stitched together based on lithologic and biostrati- and Tethyan Triassic strata in Europe.

Geosphere; June 2011; v. 7; no. 3; p. 802–829; doi:10.1130/GES00628.1; 15 figures; 4 tables; 1 supplemental text file.

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108° Colorado 106° 104° 37° Chama W. New Mexico N. New Mexico E. New Mexico Basin Okl. after Stewart after Zeigler after Lucas et al., 1972 et al., 2008 et al., 2001 Gallup Moenave- 6 Santa Fe equivalent? I-40 Tucumcari Albuq. 7 35° Redonda Zuni Mtns. ?? ?? xa s

ona Bull Te upper Petrified Petrified Ariz I-25 Forest Canyon 33° Forest Sonsela I-10 Las Cruces Interval lower Petrified Poleo Trujillo Texas Mexico Forest A 200 km Bluewater Salitral Tecovas Creek 107° 106°30′ Shinarump Shinarump Santa Rosa C E. AZ, SE UT N. New Mexico E. New Mexico Chama Basin after Stewart after Zeigler after Lucas et al., 1972 et al., 2008 et al., 2001 36°30′ Moenave- Moenave equivalent? 4 Ghost Ranch Rock Point Redonda Youngsville 2 Owl Rock ?? ?? 1 3 Gallina Abiquiu upper Petrified Bull Petrified Canyon 10 km Forest Forest B D

Figure 1. Chinle Group strata of the American southwest. (A) Distribution of Triassic outcrop area (approximates outline of southeastern Chinle Basin). (B) Expanded view of Chama Basin Chinle Group outcrops. General sampling localities are numbered: 1—Coyote Amphitheater, 2—Youngsville, 3—Abiquiu Dam, 4—Ghost Ranch, 5—El Cobre Canyon, 6—Six Mile Canyon, Zuni Mountains, 7—Mesa Redonda. (C) Stratigraphic nomenclature in New Mexico for Upper Triassic strata (after Stewart et al., 1972; Lucas et al., 2001; Zeigler et al., 2008) (D) Stratigraphic nomenclature for the upper Chinle Group in the Four Corners area (AZ—Arizona; UT—Utah).

GEOLOGIC SETTING AND been assigned to five formations (in ascending a revised nomenclature of Upper Triassic strata STRATIGRAPHY order): Shinarump (Agua Zarca and Zuni Moun- in New Mexico was proposed. The Shinarump tains Formations), Salitral, Poleo, and Petrified Formation is a color-mottled, coarse-grained Chama Basin, North-Central New Mexico Forest Formations and strata interpreted as part quartz sandstone and conglomerate with clasts of the Rock Point Formation (Lucas and Hunt, composed primarily of chert and quartzite, In general, the Chinle Group consists pre- 1992; Hunt and Lucas, 1993a, 1993b; Lucas interbedded with green claystones. The Salitral dominantly of red and purple mudstones, with et al., 2003, 2005; Zeigler et al., 2008; Fig. 1 Formation, which conformably overlies the lesser orange siltstones and buff sandstones and 2). In Zeigler (2008) and Zeigler et al. Shinarump Formation, is a brick-red mudstone and conglomerates. Upper Triassic strata in the (2008), descriptions of the Upper Triassic stra- that is occasionally color mottled (Figs. 2B, 2C). Chama Basin of north-central New Mexico have tigraphy in the Chama Basin were provided and Vertebrate fossil material from the Salitral For-

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Mesa Redonda Coyote Amphitheater r. Ghost Ranch r. Ju Entrada r. Ju M. Ss. Entrada Ju

M. Entrada Ss.

M. Ss. 60 MR 31 5 m 58 MR 30

MR 29 MR 28 56 MR 27 MR 26 Abiquiu Dam 54 6 MR 25

oint Fm. MR 24 erred Rock Pet. For. P 52 5 MR 23 AD-1 Inf MR 22 Fm. AD-2 50 4 AD-3 48 3 not AD-4 Chama Basin (CB) sampled AD-5 46

AD-6 ointFm. AD-7 44 not MR 18 AD-8 sampled rmation MR 17 AD-9 42 AD-10 calcrete Fo MR 16 AD-11 40 9 MR 15 n AD-12 conglomerate erred RockP AD-13 38 8 MR 14 AD-14 sandstone Inf MR 13 iassic Chinle Group 36 Redonda rmatio AD-15 7 MR 12 Tr

AD-16 okout siltstone 34 6 int (GRRP)

AD-17 Lo MR 11 iassic Chinle Group Po AD-18 ’s te 32 5 Tr MR 10 oleo Fo AD-19 mudstone Coelo. quarry P MR 9

AD-20 Upper Coyo iassic Chinle Group CB 1, 2 4 AD-21 30

Tr MR 8 AD-22 28 3 AD-23 Upper MR 7 2

AD-24 Ghost Ranch Rock

rest Fm. 26 MR 6 AD-25 5 m Fo

Upper MR 5 AD-26 24 1 2.5 m AD-27 MR 4 AD-28 22

iassic Chinle Group Pet. For. MR 3 AD-29 Youngsville trified Fm. MR 2 AD-30 Tr 20 Bull Can. AD-31 Pe Bedding Orientation: Fm. MR 1 AD-32 Poleo Fm. 18 Subhoriztonal D Bedding Orientation: AD-33 16 Upper F Subhoriztonal Shin. AD-34 Coy. Amph. Bedding Orientation: Six Mile Canyon 14 Fm. 183/15 from Shinarump to 12 CL 47 - changes to 308/20

A Bedding Orientation: 10

Subhoriztonal Fm. 7 8 r. SMC-1 Fo

Blue Mesa Member SMC-2 1 m 5 m 6 t. SMC-3

6 Pe 4 2 SMC-4 5 8 SMC-5 SMC-6 6 k SMC-7 Salitral Fm. 4 4 SMC-8 2 oungsville Mb r. Poleo SMC-9 Y te Cree

6 rmation 3 Fm. SMC-10 iassic Chinle Group

4 Fo Coyo 2 Tr 2 1 6 El Cer. Bed 5 Upper Salitral Knob 4 ’s Fm. 3 te 5 m Shin. Fm. 2 1 Coyo Bluewater Creek

E.C. s e

6 n’ 4 Bedding Orientation: ve Bedding Orientation:

B Shin. Fm. 2 Ridg Subhoriztonal C Ra E Subhoriztonal

Figure 2. Stratigraphic sections representative of localities sampled. (A) Shinarump (Shin) and Poleo Formations, Abiquiu Dam (Pet. For. Fm— Petrified Forest Formation). (B) Salitral Formation, Youngsville. (C) Chinle Group, Coyote Amphitheater (Coy. Amph; M.Jur.—Middle Jurassic; Mbr—member; Ss—sandstone). (D) Inferred Rock Point Formation, Ghost Ranch. (E) Bluewater Creek Formation and Blue Mesa Member, Petrified Forest Formation, Six Mile Canyon, Zuni Mountains. (F) Redonda Formation, Mesa Redonda, eastern New Mexico (Bull Can.—Bull Canyon).

mation is interpreted as representative of the is the thickest and most widespread unit of the ered Late Triassic in age are those that have Adamanian land vertebrate faunachron (LVF) Chinle Group, and was thought to represent pri- been inferred to be part of the Rock Point For- (Lucas et al., 2003, 2005; Zeigler et al., 2005). marily overbank deposition (Repenning et al., mation, and consist of nonbentonitic, reddish- The Poleo Formation consists of sandstone, 1969; Dubiel, 1987, 1989a, 1989b), although brown massive siltstone and fine sandstone. with intrabasinal calcrete and mud-clast con- the thick mudrock sequence of the Petrified Stewart et al. (1972) originally coined the term glomerate being more prevalent in the lower Forest Formation has also been interpreted to “Siltstone Member” in describing this sequence third of the formation (Figs. 2A, 2C). Clasts in reflect aggradational fan deposition (Weiss- of strata immediately overlying Petrified Forest the conglomerate beds are mudstone or siltstone mann et al., 2007; Fig. 2C). Vertebrate fossils of strata and below the disconformity with the rip-ups, calcrete nodules, occasional carbonized phytosaurs, aetosaurs, and other tetrapods in the Middle Jurassic Entrada Sandstone in northern plant debris, and rare extrabasinal clasts (chert, Petrified Forest Formation are associated with New Mexico. These rocks were lithologically quartzite). Both sandstones and conglomerates the Revueltian LVF (Lucas, 1993, 1998, 2010b). correlated with the type Rock Point Formation contain authigenic hematite cement in varying The stratigraphically highest sedimentary in Arizona and the name transferred into the concentrations. The Petrified Forest Formation rocks in northern New Mexico that are consid- Chama Basin (Lucas and Hunt, 1992; Hunt and

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Lucas, 1991, 1993a, b; Lucas, 1993; Lucas et al., fied Forest Formation were considered by Hunt lower Tuvalian (early Carnian). At the Placerias 2003, 2005). At Coyote Amphitheater, much of and Lucas (1993b) to be lithostratigraphically quarry in central Arizona, the lower Blue Mesa the uppermost Chinle Group strata are covered equivalent to the Salitral Formation in northern Member contains Gregoriusella n. sp. and with colluvium from the overlying Entrada New Mexico. Laxitextella dorsorecta and is considered late Sandstone, making it difficult to observe the Tuvalian in age. At the Coelophysis quarry uppermost unit(s) of the Chinle Group in most Mesa Redonda, Eastern New Mexico in north-central New Mexico, a new species places. Stewart et al. (1972), Lucas and Hunt of Anyuanestheria (H. Kozur, 2010, personal (1992), and Lucas et al. (2003, 2005) did not commun.) is reported that Kozur and Weems Mesa Redonda in eastern New Mexico record the presence of this sequence of non (2010) designated as Sevatian (late Norian), in exposes a 120-m-thick section of reddish- bentonitic siltstone and fine sandstone at Coyote contradiction to the assumption by Cleveland purple to reddish-orange mudstones and silt- Amphitheater, but did note the presence of litho- et al. (2008) that these strata are Rhaetian in age. stones, with occasional sandstone beds (Fig. logically similar strata at Ghost Ranch. Dubiel Very few palynostratigraphic data are avail- 2F). Repetitively bedded fine sandstone, silt- (1989a), however, reported the presence of the able for Chinle Group strata. In Litwin (1986) stone, and mudstone form ribbed cliffs and Siltstone Member at Coyote Amphitheater. In and Litwin et al. (1991), palynomorphic data steep mudrock slopes (Lucas et al., 2001). The this study, we sampled the best exposed sec- were used to assign a Norian age to upper Chinle Redonda Formation is disconformably overlain tion of these rocks above the Petrified Forest strata (Petrified Forest and inferred Rock Point by the Middle Jurassic Entrada Formation and strata (sites CL 48-60) at Coyote Amphitheater. Formations) based on a few samples obtained overlies the Bull Canyon Formation. Based on In addition, outcrops of lithologically similar from only two localities. its stratigraphic position and lithologic simi- strata, which have been considered part of the Geochronologic data for the Chinle Group are larity, the Redonda Formation is considered Rock Point Formation at Ghost Ranch (Lucas sparse. Available high precision U-Pb ID‑TIMS equivalent to the Rock Point Formation (Hester and Hunt, 1992; Hunt and Lucas, 1991, 1993a, single zircon isotopic age determinations for the and Lucas, 2001) or to the Owl Rock Formation b; Lucas, 1993; Sullivan et al., 1996; Lucas Chinle Group yield a date of 219.2 ± 0.7 Ma (Lucas et al., 1987). et al., 2003, 2005) were sampled. In this study, (Irmis and Mundil, 2008) for a tuffaceous sand- we refer to the sequence of nonbentonitic siltstone near the base of the Blue Mesa member stone and fine sandstone that is disconformably Biostratigraphic and Geochronologic Data in western New Mexico, a date of 213 ± 1.7 Ma overlain by the Entrada Sandstone at Coyote for the Black Forest Bed of the Painted Desert Amphitheater as the “inferred Rock Point” strata. Biostratigraphic data for the Chinle Group Member (Riggs et al., 1997), and a date of are largely derived from both vertebrate assem- 219.4 Ma for the base of the Blue Mesa Member Zuni Mountains, West-Central New Mexico blages and conchostracans. A complete discus- at Petrified Forest National Park (Ramezani sion of the rapidly improving biostratigraphic et al., 2009). Initially there was some question Upper Triassic strata in the Zuni Mountains data sets applied to Chinle Group strata is as to the stratigraphic position of the sampling are assigned to the Shinarump (including Zuni beyond the scope of this paper. Here we list those interval yielding the 219.4 date at Petrified For- Mountains Formation; Zeigler et al., 2008), specific data points we use and refer the reader est National Park (Kozur and Weems, 2010). In Bluewater Creek, Sonsela, and Petrified For- to the appropriate literature where necessary. fact, recent revisions to the stratigraphy of the est formations. The belt of Triassic outcrops Lucas (1997, 1998) developed a series of LVFs Chinle Formation at Petrified Forest National along the northern flank of the Zuni Moun- based on assemblages of tetrapod vertebrate Park (Martz and Parker, 2010) suggest that the tains is separated from Triassic exposures in fossils (phytosaurs, aetosaur, and amphibians) 219.4 Ma date reported in 2009 (Ramezani the Chama Basin by Jurassic and Cretaceous and divided Late Triassic time for the continental et al., 2009) is from a sample from the Sonsela strata downdropped in Laramide monoclines, southwest into three divisions (oldest to young- interval. More recently reported high precision the Mount Taylor volcanic field and the Sierra est), Adamanian, Revueltian, and Apachean. absolute age determinations for the Sonsela Nacimiento, thus precluding direct correlation Lucas (1998, 2010b) argued that these time divi- interval at Petrified Forest National Park indi- of lithostratigraphic units. At Six Mile Canyon sions are nearly isochronous with the Late Tri- cate that this interval was deposited between ca. in the Zuni Mountains, the Bluewater Creek assic marine stages (Carnian, Norian, and Rhae- 219 and 213 Ma and that the base of the Blue Formation consists of interbedded mudstones tian). Here we use the LVF assemblage names, Mesa Member, as defined stratigraphically in and siltstones with scattered calcrete horizons but do not utilize Lucas’s correlation of these northern Arizona, is ca. 225 Ma (Ramezani et al., interpreted as floodplain deposits (Heckert and continental age divisions to the marine stages 2010). Dickinson and Gehrels (2008), using Lucas, 2003; Fig. 2E). The contact between (discussed further below). the lower precision U-Pb LA-ICPMS method, the Bluewater Creek Formation and the over- Conchostracan data have been obtained from report a maximum depositional age of ca. lying Petrified Forest Formation is marked by strata in west-central and north-central New 215 Ma for the Sonsela Sandstone in northern a 1.5-m-thick, white, tuffaceous sandstone that Mexico and central Arizona. Kozur and Weems Arizona and the Poleo Formation in the Chama is the lowest unit of the Blue Mesa Member (2010) summarized all conchostracan assem- Basin and a range of inferred depositional ages of the Petrified Forest Formation (Heckert and blages from the western United States; we focus between 219 Ma and 226 Ma for the Trujillo Lucas, 2003). Irmis and Mundil (2008) reported on three of their localities, Fort Wingate, the Formation of eastern New Mexico. a detrital zircon date of 219.2 ± 0.7 Ma from this Placerias quarry, and the Coelophysis quarry. In unit. Blue Mesa Member strata above the tuffa- west-central New Mexico, the conchostracans Regional Disconformities ceous sandstone consist of mudstones with sev- Anyuanestheria wingatella, Laxitextella seegisi, eral discontinuous horizons of calcrete nodules. and Howellisaura princetonensis are reported Pipiringos and O’Sullivan (1978) described On the basis of lithologic similarities and simi- from the Fort Wingate area (western Zuni a series of inferred regional unconformities in lar stratigraphic position, the Bluewater Creek Mountains) from the lower Bluewater Creek Triassic and Jurassic strata on the Colorado Pla- Formation and Blue Mesa Member of the Petri- Formation, and the unit is designated as upper- teau. In the Triassic part of the sequence, they

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identified the Tr-1 and Tr-2 unconformities as ~3 m sampling interval may be equivalent to sampled) to investigate the character of the at the base and within Lower–Middle Trias- considerably <0.25 m.y. duration between sites. natural remanent magnetization (NRM) in these sic strata (Moenkopi Formation and equivalent The Salitral Formation was also sampled materials. For sufficiently indurated samples, units), respectively. The Tr-3 unconformity at a 3 m interval (7 sites total) at exposures duplicate specimens were treated with chemical is identified as at the base of the Upper Trias- in the county landfill outside of Youngsville demagnetization, as described in Henry (1979). sic section (Chinle Group and equivalents; (36.185°N, 106.583°W; Fig. 3B). An additional All measurements of the NRM were made using Pipiringos and O’Sullivan, 1978). Lucas (1991, section of subhorizontal strata of the Shinarump a 2G Enterprises Model 760R superconducting 1993) defined two more unconformities, the and Poleo Formations was sampled at Abiquiu rock magnetometer, equipped with DC SQUIDS Tr-4 and Tr-5, within the Upper Triassic section Dam (36.236°N, 106.425°W; Fig. 3C), with (superconducting quantum interference device), that were also considered of regional extent. a site spacing of ~2 m (32 total sites). In the with a magnetic moment noise level of ~1.0– The Tr-4 is defined as occurring at the base upper part of the Poleo Formation, the sampling 3.0 × 10–12 Am2. At least one specimen per sam- of the Sonsela interval in Arizona, the Poleo interval was dictated by bed thickness, so that ple was subjected to thermal demagnetization, Sandstone in northern New Mexico and/or each site was a discrete bed, whereas parts of utilizing either a Shaw Magnetic Measurements the Trujillo Sandstone in eastern New Mexico the lower third of the Poleo Formation and the Thermal Demagnetizer (MMTD) or an ASC 48 (Lucas, 1991, 1993). May (1988), Herrick Shinarump Formation are thick bedded, so that thermal demagnetizer. (1999), Woody (2006), Martz (2008), and Martz some intervals included multiple sites. A sec- Progressive demagnetization data were ana- and Parker (2010) argued that the Tr-4 uncon ond, composite section of strata inferred to be lyzed using the principal component analysis formity is not a regional feature. Here we fol- part of the Rock Point Formation was sampled approach of Kirschvink (1980) and individual low previous work and do not consider the at two sublocalities near Ghost Ranch. There linear segments, usually defined by four to six Tr-4 unconformity to be of regional extent. were 16 sites established at the Coelophysis data points, were accepted if maximum angu- The Tr-5 unconformity is identified as occurring quarry (36.338°N, 106.464°W; Fig. 3D) and lar deviation (MAD) values were <10°. Where at the top of the Owl Rock Formation in Arizona near U.S. Highway 84 (36.307°N, 106.448°W), data were anchored to the origin, results with and at the base of the inferred Rock Point For- south of Ghost Ranch. MAD values <15° were accepted. Bulk mag- mation in New Mexico. As we suggest herein, At Six Mile Canyon in the western Zuni Moun- netic susceptibility was measured for duplicate a possible revision to the age of the youngest tains of west-central New Mexico (35.441° N, specimens from most sites using a Kappabridge inferred Triassic strata in northern New Mexico 108.483° W; Fig. 3F), an ~15-m-thick section KLY4S instrument. Estimated site mean direc- may alter the definition of this unconformity. of Bluewater Creek Formation and Blue Mesa tions were obtained following the methods of Member strata was sampled at ~1.5 m intervals Fisher (1953) and were termed excellent if the

SAMPLING AND METHODS just above and below the white tuffaceous sand- 95% confidence parameter,α 95, is ≤ 10°, good

stone (basal Blue Mesa Member) for a total if α95 is 10°–20°, salvageable if α95 is 20°–30° The entire Chinle Group section was sam- of 10 sites. At Mesa Redonda in eastern New yet the magnetizations clearly had a Late Tri-

pled at Coyote Amphitheater (36.219°N, Mexico (34.997° N, 103.703° W), samples assic affinity, and unacceptable ifα 95 was >30° 106.631°W; Figs. 3A, 3E), from the base of were obtained every ~3 m for a total of 31 sites (Fisher, 1953). the Shinarump Formation where it overlies the that encompassed most of the Redonda Forma- Acquisition of isothermal remanent magne- Lower Permian Cutler Group, to the upper- tion over a thickness of ~75 m. Approximately tization (IRM) to saturation (SIRM) or near- most exposures of strata interpreted as Rock 9 m of section in the middle of the Redonda saturation and backfield DC demagnetization Point Formation (Dubiel, 1989a). A sampling Formation could not be sampled due to thick of SIRM experiments was conducted with (site) interval of ~3 m was used for a total of 82 colluvial cover. a home-built pulse magnetizer that provided a sites (~230 m of strata), except for Poleo strata, Samples from well-indurated sandstone and field up to 2.97 T, capable of nearly saturating which were sampled at the individual bed level siltstone beds (Poleo, Shinarump and/or lower most assemblages of hematite grains. Thermal (site sampling interval varied from 2 to 5 m; Salitral, and upper Rock Point Formations) demagnetization of IRM acquired in DC fields Fig. 2C). As a simplistic, initial approach, if we were obtained by drilling with a water-cooled of 2.97 T, 0.3 T, and 0.03 T along three orthog- assume both a constant sedimentation rate for diamond drill bit, and six to eight independently onal axes (Lowrie, 1990) was also conducted the Petrified Forest Formation and the lack of oriented core samples were taken. Samples on representative specimens. Several mudrock appreciable hiatuses, a 3 m sampling interval were typically prepared into multiple specimens specimens of differing shades of red, purple, is equivalent to a maximum of 0.25 m.y. time (2.2-cm-high right cylinders) for demagnetiza- and orange were dissolved in reagent-grade duration between sites [if the Carnian-Norian tion. Most sampling, however, was of mudstone hydrochloric acid and the residue was mixed boundary is slightly younger than 230.91 Ma or poorly consolidated siltstone (upper Salitral, with high purity alumina cement and subjected (Furin et al., 2006) and the Norian-Rhaetian Petrified Forest, lower inferred Rock Point For- to IRM acquisition experiments to attempt to boundary is ca. 203.6 Ma (Gradstein et al., mations). For these sites, fresh, coherent mate- assess relative abundances of detrital and pig- 2005)]. Our estimate assumes that the entire rial was uncovered by digging at least 0.5 m mentary hematite. 234-m-thick Chinle section at Coyote Amphi- into the exposure. A block sampling method theater was deposited over ~30 m.y. duration, to similar to that of Johnson et al. (1975) was used PALEOMAGNETIC RESULTS near the end of the Triassic, which may be an to obtain typically six to eight oriented blocks Shinarump Formation unrealistically long period of time, as discussed at a site (single datum). Blocks yielded 1–7 herein. The recent high-precision U-Pb age individual specimens, prepared by dry sawing Samples from the Shinarump Formation determination on Bluewater Creek Formation with a nonmagnetic diamond blade, from ~1.0– typically yield uninterpretable demagnetization strata in the Zuni Mountains raises the possibil- 3.0 cm3 in volume. behavior and have average NRM intensities of ity that no lower Chinle Group strata in the Zuni Progressive thermal demagnetization was ~1.2 mA/m for Abiquiu Dam and ~2.8 mA/m Mountains are Carnian in age. We infer that an applied to most specimens (drilled or block for Coyote Amphitheater material. Although

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D E F

Figure 3. Photographs of Upper Triassic sections sampled for this endeavor. (A) Petrified Forest and inferred Rock Point Formations at Coyote Amphitheater. (B) Salitral Formation at Youngsville. (C) Upper Shinarump Formation at Coyote Amphitheather. (D) Poleo Formation at Abiquiu Dam. (E) Upper Bluewater Creek Formation at Six Mile Canyon, Zuni Mountains. (F) Inferred Rock Point Formation, overlain by Middle Jurassic Entrada Sandstone, at Ghost Ranch.

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some specimens collected from the Shinarump behaved demagnetization response and yield 4D, and 5B). The magnetization is of distributed Formation at Coyote Amphitheater yield inter- south-directed and shallow inclination ChRMs laboratory unblocking temperatures between 300 pretable demagnetization behavior (Figs. 4A, (reverse polarity). The magnetization is of dis- and at least 620 °C, and decays linearly to the 5A), more often than not, the inclination of the tributed laboratory unblocking temperatures origin. Maximum unblocking temperatures are characteristic remanent magnetization (ChRM) between 300 and about 615 °C, and decays lin- above 615 °C, but demagnetization response isolated is too steep to be of Late Triassic early to the origin. is often unstable above this temperature. The age, and likely was acquired at a considerably in situ grand mean for sites at Coyote Amphi

younger time (e.g., site RR1, mean of declina- Salitral Formation theater is D = 007.0°, I = 3.4°, α95 = 8.4°, k =

tion, D = 352.8°, inclination, I = 58.1°, α95 = 120.3, N/No = 4/6 sites collected (bedding- 9.1°, k = 102.9, number of samples accepted out Salitral Formation strata at Coyote Amphi corrected grand mean: D = 005.8°, I = 3.8°, 2 of total number of samples sampled, N/No = theater typically yield well-defined demagne- sites normal polarity, 2 sites reverse polarity). 4/6). Of the seven sites established in Shinarump tization behavior showing the isolation of both These four sites alternate polarities, whereas strata at Coyote Amphitheater, only three sites at normal and reverse polarity ChRMs with aver- the remaining two yield uninterpretable results. the top of the Shinarump show moderately well age NRM intensities of ~1.9 mA/m (Figs. 4B, The means for sites at Coyote Amphitheater are

Up, W Up, W 645 655 10 mA/m 210°C N 560 CK 2-2 615 400 (Salitral Fm., 3 m)

450 CK 3-5 NRM 10 mA/m S (Salitral Fm., 9 m) 655 A 250°C 575 645 Up, W NRM Up, W B 615 10 mA/m 560 N 210°C 400 YL 3-2a CK 1-3b (Salitral Fm., 9 m) (Salitral Fm., 2 m) NRM D S NRM 10 mA/m 615 C 645 655 Up, W 250°C 450 370 500 615 630 575 210°C 655 600 675 1.0 Am/m S CCP 2b Up, W (Poleo Fm., 6.5 m) 370 500 630 655 685 600 675 690 NRM S E 1.0 mA/m Up, W F CCP 4e 1.0 mA/m 655 S (Poleo Fm., 11.5 m) 500 630 210°C G 210°C AD 14A Up, W (Poleo Fm., 20 m) 625 670 S NRM I 450°C Up, W

AD 10B-2 NRM (Poleo Fm. - chem., 12 m) 1.0 mA/m S SMC 4-5b 4 h 48 h 500°C (Bluewater Crk. Fm., 1 m below tuff. ss) H 5 mins 5 mA/m

NRM NRM

Figure 4. Orthogonal demagnetization diagrams (Zijderveld, 1967) showing examples of response to progressive thermal and chemical demagnetization for specimens. NRM—natural remanent magnetization. (A–D) Salitral Formation. (E–H) Poleo Formation. (I) Bluewater Creek Formation (tuff. ss—tuffaceous sandstone). Diagrams show the simultaneous projection of horizontal (north-south versus east-west) component of the magnetization (filled symbols) and the vertical (north-south versus up, down) component of the magnetization (open symbols). For thermal demagnetization, peak demagnetizing temperatures are given beside the vertical projections. For chemical demagnetization, the total number of hours (cumulative) of leaching in HCl for each specimen is given beside the vertical projection. Meters are position of site sampled above base of stratigraphic unit.

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N N

Shinarump Formation D = 350.5°, I = 61.0°, N/No = 2/3

Salitral Formation, Youngsville: D = 188.8°, I = 8.0°, α95 = 35.6°, k = 7.9, N/No = 4/7 Upper Shinarump, Salitral Fms., Coyote’s Knob (in situ): D = 187.0°, I = 3.4°, α95 = 8.4°, k = 120.3, N/No = 4/6 Coyote’s Knob (corrected): Coyote Amphitheater B D = 185.8°, I = 3.8° A Abiquiu Dam 90 90 Youngsville Site Means Youngsville Grand Mean Coyotes Knob Site Means Coyote’s Knob Reverse Site Mean Coyote’s Knob Normal Site Mean 90 Coyote’s Knob Grand Mean C Poleo Formation, Coyote Creek: D = 179.7°, I = –5.2°, α95 = 6.0°, k = 85.7, N/No = 8/14 Coyote Creek (corrected): D = 184.4°, I = –4.1° Poleo Formation, Abiquiu Dam (in situ): D = 183.1°, I = 0.3°, α95 = 5.7°, k = 34, N/No = 20/30 Coyote Creek Site Means Coyote Creek Grand Mean Abiquiu Dam Site Means Abiquiu Dam Grand Mean 90 Bluewater Creek Fm. & Blue Mesa Mbr. (PF Fm.), Six Mile Canyon: D = 177.6°, I = –3.0°, α95 = 10.4°, k = 34.5, N/No = 7/10

S D

Figure 5. Equal area projections showing estimated site mean directions for accepted sites (see text). (A) Shinarump. (B) Upper Shinarump and Salitral. (C) Poleo. (D) Bluewater Creek Forma- tion and Blue Mesa Member (PF—Petrified Forest). Closed symbols refer to lower hemisphere projections; open symbols refer to upper projections.

summarized in Table 1 (and the Supplemental 5C), and ChRMs of reverse polarity with NRM using either thermal or chemical demagnetiza- Text File1). Two sites in the Salitral Formation at intensities averaging 2.3 mA/m. The ChRM tion. Of the 30 sites collected, 20 site means

Youngsville landfill yield interpretable data, and is overprinted by a small north-directed, steep have associated α95 values <10° and provide a had average NRM intensities of ~2.0 mA/m. positive inclination component. The in situ grand mean direction of D = 183.1°, I = 0.3°,

Site 3 is of normal polarity (Fig. 4C), and site 2 grand mean for the Poleo Formation is D = α95 = 5.7°, k = 33.9, N/No = 20/30 sites (the beds

is of reverse polarity (Table 1). The five rejected 179.7°, I = –5.2°, α95 = 6.0°, k = 85.67, N/No = are flat-lying, thus not requiring tilt correction). sites have high dispersions (k values range 8/14 sites (bedding corrected grand mean: D = For Poleo sites yielding well-defined demagne- between 1.96 and 51.4) and magnetizations with 184.4°, I = –4.1°). All sites in the Poleo Forma- tization results, specimens subjected to chemi- inclinations too steep to be of Late Triassic age. tion at Coyote Amphitheater yield magnetiza- cal demagnetization show an ~80% decrease tions of reverse polarity, including those sites in NRM intensity, and become nearly white in Poleo Formation with considerably higher dispersion. color, after immersion for a total of 370 hours, The Poleo Formation section at Abiquiu Dam with no statistically significant change in direc- Poleo Formation strata at Coyote Amphi typically yields well-defined, well-grouped tion (Fig. 4H). theater typically yield well-defined, linear ChRMs of exclusively reverse polarity, with demagnetization trajectories (Figs. 4E, 4F, and NRM intensities of ~1.0 mA/m and demagne- Petrified Forest Formation tization behavior similar to that of specimens 1Supplemental Text File. Txt file of Paleomagnetic from Coyote Amphitheater (Figs. 4G, 4H, and Specimens from most stratigraphic levels site mean data for Upper Triassic Chinle Group strata 5C). The magnetization exhibits somewhat (sites) in mudstones exposed over ~132 m in the sampled in New Mexico. If you are viewing the PDF discrete laboratory unblocking temperatures Petrified Forest Formation at Coyote Amphi- of this paper or reading it offline, please visit http:// dx.doi.org/10.1130/GES00628.S1 or the full-text arti between 600 and 670 °C, and decays linearly theater yield well-defined and well-grouped cle on www.gsapubs.org to view the supplemental to the origin. For duplicate specimens from ChRMs with NRM intensities of ~7.1 mA/m text file. the same sample, a similar direction is isolated (Figs. 6A–6C and 7A). The magnetization is

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TABLE 1. GRAND MEAN DIRECTIONS FOR EACH FORMATION OF THE CHINLE GROUP Geographic Geographic Stratigraphic Stratigraphic

Formation Locality declination inclination declination inclination α95 k N/No Inf. Rk. Pt. C.A. 015.61.8 014.6 –16.711.137.26/13 G.R. 017.36.0 017.3 6.07.7 52.8 7/15 Redonda M.R. 183.2–3.5 183.2 –3.5 5.783.410/24 Pet. Forest C.A. ––182.1 4.22.7 75.4 39/45 Poleo C.A. 179.7–5.2 184.4 –4.1 6.085.67 8/14 A.D. 183.10.3 183.1 0.35.7 34.0 20/30 Salitral C.A. 007.03.4 005.8 3.88.4 120.34/6 Low. Chinle S.M.C. 177.6–3.0 177.6 –3.0 10.4 34.5 7/10 Note: Subhorizontal strata have no stratigraphic direction listed. Petriﬁed Forest Formation strata have no geographic direction listed because of a change in bedding orientation near the top of the formation at Coyote Amphitheater. Abbreviations: Inf. Rk. Pt.—inferred Rock Point; Pet— Petriﬁed; Low.—lower; C.A.—Coyote Amphitheater; A.D.—Abiquiu Dam; G.R.—Ghost Ranch; M.R.—Mesa Redonda; S.M.C.—Six Mile Canyon.

α95—cone of conﬁdence; k—measure of data dispersion; N/No—number of sites accepted out of total number of sites sampled. The dash means there was no need for a stratigraphic correction because the strata are subhorizontal.

of distributed laboratory unblocking tempera- 52.8, N/No = 7/15 sites. Sites excluded from positive inclination. Average NRM intensities tures between ~300 and 670 °C, and decays lin- the grand mean direction yield either incoherent are ~5.1 mA/m for the siltstones and fine sand- early to the origin. For these strata, 39 of the 45 magnetizations or relatively scattered magneti- stones and ~1.9 mA/m for mudrock-dominated sites (all in mudstone levels) yield reliable site zations of mixed polarity. units. The overall in situ grand mean direction mean data and these provide a corrected grand for the Redonda Formation is: D = 183.2°, I =

mean of D = 182.1°, I = 4.2°, α95 = 2.7°, k = Six Mile Canyon: Bluewater Creek and –3.5°, α95 = 5.7° and k = 83.4, N/No = 9/26, 75.4, N/No = 39/45 sites. All samples from the Lower Petrified Forest Formations with 3 normal polarity and 6 reverse polarity remaining six sites yielded uninterpretable mag- sites. The normal polarity mean is D = 001.1°, netizations. The normal polarity stratigraphic I = 2.1°, α = 11.3° and k = 120.8, N = 3 sites, Samples from 7 of the 10 sites established in 95 grand mean direction is D = 359.2°, I = –3.4°, and the reverse polarity mean is D = 184.3°, I = the Bluewater Creek Formation and Blue Mesa α = 4.2°, k = 90.7, N = 14 sites, and the reverse –4.2°, α = 8.3° and k = 66.8, N = 6. All other 95 Member in the Zuni Mountains yield well- 95 polarity stratigraphic grand mean direction is sites from the Redonda Formation are character- defined and relatively well grouped ChRMs, D = 183.7°, I = 4.6°, α = 3.4°, k = 72.2, N = 25 ized by incoherent magnetizations. 95 and these are predominantly of reverse polar- sites. Most sites are of reverse polarity, with nor- ity with an average NRM intensity of 4.8 mal polarities observed in restricted intervals in Rock Magnetism mA/m (Figs. 4I and 5D). The magnetization is the lower, middle, and upper parts of the section. of distributed laboratory unblocking tempera- Thermal demagnetization of three compo- tures typically between ~300 and 645 °C, with Inferred Rock Point Strata nent IRM for specimens from Chinle Group maximum unblocking temperatures of 670 °C. strata shows that the high coercivity (3.0 T) The seven accepted sites provide a grand mean Specimens from 6 of 13 sampled sites estab- component is the highest intensity and highest direction of D = 177.6°, I = –3.0°, α = 10.4° lished in strata inferred to be part of the Rock 95 laboratory unblocking temperature component, and k = 34.5, N/No = 7/10. Two of the three sites Point Formation at Coyote Amphitheater yield for most samples, regardless of rock type (Figs. excluded from the grand mean yield magnetiza- well-defined and relatively well grouped ChRMs 8A–8F). Laboratory unblocking temperatures tions with north-directed and moderate to steep and average NRM intensities of ~2.8–6.4 mA/m of the 3.0 T IRM for specimens of sandstone inclinations, probably reflecting a younger age (Figs. 6F, 6G, and 7B). The magnetization is of from the Shinarump Formation and mudrock of magnetization acquisition. Sites in the lower distributed laboratory unblocking temperatures from the Salitral Formation range between 630 Blue Mesa Member, all sampled above the between ~300 and 670 °C, and decays linearly and 655 °C (Figs. 8A, 8B). For samples from zircon-bearing horizon reported by Irmis and to the origin. Five of the accepted site means are the uppermost Shinarump Formation, the 3.0 T Mundil (2008), are all of reverse polarity. Sites of normal polarity and one, at the base of the IRM shows a distributed range of laboratory in the Bluewater Creek Formation below the section, is of reverse polarity (Table 1; see foot- unblocking temperatures (from ~300 to 630 °C; zircon-bearing horizon are also of reverse polar- note 1). The in situ (geographic) grand mean Fig. 8C). In all samples, the low coercivity ity, except for the uppermost site in this unit. direction is D = 015.6°, I = 1.8°, α95 = 11.1°, IRM (<0.03T) has a very small to insignificant k = 37.2, N/No = 6/13 sites (stratigraphic grand contribution. mean direction: D = 014.6°, I = –16.7°). The Mesa Redonda: Redonda Formation For Poleo strata sampled at Abiquiu Dam seven sites not included in the calculation of (Fig. 8D), most specimens show that the 3.0 T the grand mean direction yield completely inco Specimens from the mudrock-dominated IRM component has the highest intensity, with herent magnetizations. lower Redonda Formation, and some siltstone laboratory unblocking temperatures above At Ghost Ranch, 15 sites were established in beds in the upper part, yield moderately well 660 °C. Sandstones have a substantial (as high subhorizontal strata that also have been inferred defined and well-grouped ChRMs of dual, as 30%) intermediate coercivity IRM, which is to be part of the Rock Point Formation. Seven of but dominantly reverse polarity (Figs. 6D, 6E, largely unblocked by 580 °C. Mudrock speci- these sites yield relatively well defined and well- and 7D). The ChRM is of discrete laboratory mens from the Petrified Forest Formation in the grouped magnetizations of dominantly normal unblocking temperatures between 640 and 670 Chama Basin (Fig. 8E) and from the Bluewater polarity (5 normal, 2 reverse; Figs. 6H and 7C). °C, and decays linearly to the origin. This mag- Creek Formation in the Zuni Mountains show The in situ grand mean for these strata at Ghost netization is revealed after removal of a promi- that the high-coercivity IRM always dominates

Ranch is D = 017.3°, I = 6.0°, α95 = 7.7°, k = nent overprint that is north directed and of steep the magnetization and is fully unblocked above

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669 Up,W S 642 CL 16-6A 1.0 mA/m (P.F. Fm., 57.5 m) 606 310°C Up,W 1.0 mA/m

NRM 610 S CL 8-1A A 630 B (P.F. Fm., 33.5 m) 575

300°C

400 570 NRM Up, W

615 645 Up, W 210°C 665 670 0.5 mA/m S 680 C CCMM 5-5a (Poleo Fm., 6 m) 660 640 670 600 N 1.0 mA/m NRM Up, W 400 MR 4-1b (Redonda Fm., 1 m)

670 640 1.0 mA/m 210°C 660 D S NRM E 400 600 210°C MR 2-2a Up, W (Bull Canyon Fm., 615 570 2 m below upper contact) N 665 400 645 CL 60-3b 210°C (inferred R.P. Fm., 36 m)

NRM 1.0 mA/m Up, W

NRM 400 NRM 210 570 G CL48-2b 615 (inferred R.P. Fm., 1 m) 645 Up, W 665 670 S 0.5 mA/m 645 570 210 F 665 400 NRM 670 N 0.5 mA/m

GRRP 5-1b H (inferred R.P. Fm., 15 m) Figure 6. Orthogonal demagnetization diagrams (Zijderveld, 1967) showing examples of response to progressive thermal demag netization for specimens. NRM—natural remanent magnetization. (A–B) Petrified Forest Formation. (C) Upper Poleo Forma- tion. (D, E) Bull Canyon and Redonda Formation. (F–H) Inferred Rock Point Formation. Diagrams show the simultaneous projection of the horizontal (north-south versus east-west) component of the magnetization (filled symbols) and the vertical (north-south versus up, down) component of the magnetization (open symbols). For thermal demagnetization, peak demagnetizing temperatures are given beside the vertical projections. Meters are position of site sampled above base of stratigraphic unit unless otherwise stated.

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N N

Inferred Rock Point Formation, Coyote’s Lookout (in situ):

D = 015.6°, I = 1.8°, α95 = 11.1°, k = 37.2, N/No = 6/13 Petrified Forest Formation, Coyote’s Lookout : Coyote’s Lookout (corrected):

D = 182.1°, I = 4.2°, α95 = 2.7°, k = 75.4, N/No = 39/45 D = 014.6°, I = –16.7° 90 A Coyote’s Lookout Reverse Site Mean B Coyote’s Lookout Normal Site Mean Coyote’s Lookout Grand Mean

Inferred Rock Point Formation, Ghost Ranch: Redonda Formation, Mesa Redonda:

D = 017.3°, I = 6.0°, α95 = 7.7°, k = 52.8, N/No = 7/15 D = 183.2°, I = –3.5°, α95 = 5.7°, k = 83.4, N/No = 9/26 90 C D Redonda Fm. Normal Site Mean Redonda Fm. Reverse Site Mean Redonda Fm. Grand Mean

S S

Ghost Ranch “RP” GM Coyote Creek Poleo GM Coyote “RP” GM Youngsville Salitral GM Redonda Fm GM Coyote Creek Salitral GM Coyote Petrified Forest GM S.M.C. Lower Chinle GM Abiquiu Dam Poleo GM E 90 Figure 7. Equal area projections showing estimated site mean directions for accepted sites (see text). (A) Petrified Forest For- mation. (B, C) Inferred Rock Point “RP”. (D) Redonda. (E) Equal area projections showing site grand mean (GM) directions for accepted sites in Chinle Group strata from northern, western, and eastern New Mexico. Closed symbols refer to lower hemisphere projections; open symbols refer to upper projections. S.M.C.—Six Mile Canyon.

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5.0 RR 1f-2 45.0 CK 1-1b (Shinarump (Salitral 4.0 35.0 sandstone) mudstone) 3.0 25.0 2.0 15.0 1.0 5.0 0 0 100 200 300 400 500 600 700 100 200300 400500 600700 A B 10.0 RR5g-3 8.0 (Shinarump 16.0 sandstone) 3.0 T 6.0 0.03 T 12.0 0.3 T AD 30e-2 4.0 8.0 (Poleo conglomerate)

2.0 4.0

0 0 100 200 300 400 500 600 700 100 200 300 400 500 600 700 C D Magnetization Intensity (A/m) 1.8 10.0 CL35-4 MR 31-a2 1.4 (Petrified Forest 8.0 (Redonda mudstone) red sandstone) 1.0 6.0 4.0 0.6 2.0 0.2 0 0 100 200 300 400 500 600 700 100 200 300 400 500 600700 E F Temperature (°C)

1.0 y 1.0 AD 9g-2 0.7 0.7 YL5-1c (Poleo sandstone) 0.4 (Salitral mudstone) 0.4 0.1 0.1 –0.5 0.4 1.0 1.6 2.2 2.8 3.0 –1.1 –0.5 0.4 1.0 1.6 2.2 2.8 3.0 –0.2 –0.4 Normalized Intensit GH Peak DC Field (Tesla)

Figure 8. Plots showing three component progressive thermal demagnetization response. (A) Shinarump. (B) Salitral. (C) El Cerrito Bed, Shinarump Formation. (D) Poleo. (E) Petrified Forest. (F) Redonda. Specimens of isothermal remanent magnetization (IRM) acquired along three orthogonal axes (following the method of Lowrie, 1990). (G, H) Plots showing IRM acquisition (to saturation or near saturation) and backfield demagnetization curves for specimens from Salitral and Poleo specimens.

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650 °C. The intensity of both lower coercivity demagnetization in these specimens reveals a (Salitral, Petrified Forest, lower Redonda, and IRMs in these rocks is typically at least one order coercivity of remanence <0.4T. inferred Rock Point Formations) (Table 2). of magnitude less than the 3.0 T IRM. Mudstone Overall, IRM acquisition and IRM three- The Shinarump Formation has the lowest bulk and sandstone samples from inferred Rock Point component thermal demagnetization results are susceptibility values of all Chinle Group strata strata show that the high-coercivity IRM domi- consistent with hematite as the dominant mag- sampled, and the highest values are from the nates and is characterized by a distributed range netic phase, although most sandstones show upper Poleo Formation and the mudstones of the of laboratory unblocking temperatures from a significant contribution from a lower coer- Petrified Forest Formation. The Poleo Formation 300 to 630 °C. Mudstone and sandstone of the civity phase. Based on laboratory unblocking records an abrupt change in bulk susceptibility Redonda Formation show similar responses, temperature information, the lower coercivity from relatively low (30–120 × 10–6) to higher with a distributed range of laboratory unblocking phase is interpreted to be a mixture of magnetite values (70–225 × 10–6) ~21 m above its base. temperatures for the 3.0 T IRM (Fig. 8F). and maghemite, which could yield laboratory For each formation, magnetic susceptibility IRM acquisition response for mudrock unblocking temperatures above 600 °C. Mud- values show little correlation with the overall specimens from the Salitral, Petrified Forest, stones from the Redonda, inferred Rock Point, quality of the remanence signature, as defined by and inferred Rock Point Formations shows a and upper Shinarump Formations have demag- demagnetization behavior and dispersion of the concave-up response, with specimens gaining netization characteristics that suggest that very ChRM at a site level. For example, specimens no more than 20% of the maximum IRM at fine grained pigmentary hematite is a significant from three sites with the lowest bulk suscepti- inductions of 0.5 T (Fig. 8G). Between ~0.8 and contributor to the remanence. This contribu- bility and three with the highest in the Petrified 1.2 T, acquisition curves rise steeply, and reach tion is less important in sandstones of the Poleo Forest Formation all exhibit nearly univectorial near saturation at ~3.0 T. In backfield demag- Formation and in the mudstones of the Petrified demagnetization of a well-grouped ChRM of netization, the mudstone specimens exhibit a Forest Formation. Petrographic observations reverse polarity. Specimens of high bulk mag- marked inflection, suggesting two phases with of selected Chinle Group rocks confirm these netic susceptibility tend to be very pale red and distinct coercivities, and an ultimate demagne- inferences. Thin sections of mudrock samples show some sedimentary structures (laminations tization in fields (coercivity of remanence, Hcr) from Coyote Amphitheater show an abundance or intraformational grains), though there are between 0.25 and 1.2 T. Sandstone specimens of fine-grained hematite, both as detrital specu- other horizons with similar features that do not from the upper Shinarump Formation and Poleo larite grains and as thin pigmentary coatings on yield high bulk susceptibility values. Medium- Formation show a rapid (concave downward) quartz grains (Fig. 9). grained sandstone has lower bulk susceptibility acquisition of isothermal magnetization over Bulk magnetic susceptibilities of sandstone- values than fine sandstone or mudstone samples. a range of low fields, acquiring 70%–90% of dominated strata (Shinarump, Poleo, and upper Mudstones, on average, have much higher bulk the total IRM below 0.5 T (Fig. 8H). Samples Redonda Formations) are lower than finer- susceptibility values than either medium- or approach saturation at ~3.0 T and backfield DC grained hematitic mudstones and siltstones fine-grained sandstones.

A B C

D E F

Figure 9. Transmission light photomicrographs of selected mudstone to siltstone samples from the Petrified Forest Formation at Coyote Amphitheater showing abundant pigmentary hematite, as well as detrital, specular hematite (opaque grains, D). Scale bars in A–D are 1.0 mm; in E and F, scale bars are 0.2 mm.

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Magnetic Polarity Zonation TABLE 2. BULK SUSCEPTIBILITY VALUES FOR EACH FORMATION SAMPLED NORMALIZED BY VOLUME FormationAverage*Standard deviation RangeN From oldest to youngest strata, we summa- Inf. Rock Point 230.02 94.34 122–3524 rize the polarity record of Chinle Group strata Redonda sandstone 54.72 34.26 23–105 6 obtained in this study. The Shinarump Forma- Redonda mudstone 230.10 29.01 191–2726 Petriﬁed Forest mudstone 277.28 61.01 187–46135 tion at Abiquiu Dam typically yields either Poleo (C.A.) 295.16 79.67 209–3725 inco herent magnetizations or a north-directed, Poleo (A.D.) 103.30 54.59 38–224 28 steeply inclined magnetization unblocked by Salitral 252.92 39.58 225–2812 Shinarump 57.61 25.69 33–103 6 150–210 °C that probably reflects complete Note: Abbreviations: Inf.—inferred; C.A.—Coyote Amphitheater; remagnetization at a younger time. In the Chama A.D.—Abiquiu Dam. Basin, Shinarump strata have been interpreted as *Average values are ×10–6. a paleovalley fill sequence (Dubiel, 1987, 1989a) dominated by coarse to very coarse grained sandstone with intense color mottling and silcrete incoherent magnetizations, which may reflect et al., 2003; Kent and Irving, 2010), and this lenses. The intense color mottling and associ- pedogenic modification of some stratigraphic supports a Triassic age of ChRM acquisition. ated diagenetic phases suggest postdepositional levels. The thick paleosol dominated sequence Approximately 60% of the site mean results are fluid alteration of Shinarump strata, consistent sampled near Ghost Ranch yields several sites considered to be of excellent or good quality, with the pervasive remagnetization documented that are dominated by completely incoherent and therefore relatively high fidelity with low in this study. Only the uppermost, medium- to magnetizations, although two intervals of within-site dispersion (k >> 20), and are statis- fine-grained sandstone interval of the Shinarump reverse polarity were identified, one of which is tically acceptable recorders of a Late Triassic at Coyote Amphitheater records a magnetization in strata directly above the Coelophysis quarry. field. Data from the Salitral, Petrified Forest, consistent with a Late Triassic age, and these The Six Mile Canyon section, north-central and Redonda Formations are sufficiently robust samples are of reverse polarity (Fig. 10C). Zuni Mountains, yields a predominantly reverse in terms of total number of dual polarity ChRMs Salitral Formation strata at Coyote Amphi- polarity magnetozone (Fig. 10E). The site to pass the reversal test of McFadden and theater typically yield well-defined and well- immediately below the tuffaceous sandstone McElhinny (1990). Petrified Forest Formation grouped magnetizations of mixed polarity that provided the detrital zircon date reported by strata yield a positive, class A, reversal test with (Figs. 10B, 10C). Sites at Youngsville yield Irmis and Mundil (2008) shows intense pedo- 25 reverse polarity sites and 14 normal polarity lower-quality data, although still of mixed polar- genic modification with strong color mottling sites, with an angle between observed means of ity. Salitral Formation strata yielding incoherent and distorted bedding, and yields poorly defined 4.6°. Salitral Formation strata at Coyote Amphi- magnetizations may reflect intense diagenetic shallow inclination magnetizations that are both theater are characterized by three sites of reverse and/or pedogenic modification, as they are com- north and south seeking. polarity and three of normal polarity and pass monly color mottled, bioturbated, and lack pres- The Mesa Redonda section in eastern New a reversal test with an angle of 11.9° (class C), ervation of original sedimentary structures. Mexico consists almost entirely of the Redonda and Redonda Formation strata pass with an Poleo Formation strata at both Coyote Amphi- Formation and yields moderately well defined, angle of 3.8° between observed means, with six theater and Abiquiu Dam yield well-defined well-grouped ChRMs in the lower two-thirds of sites of reverse and three sites of normal polar- ChRMs that are almost exclusively of reverse the section (Fig. 10F). Only a few sites in the ity (class A). Reversal tests could not be carried polarity (Figs. 10A, 10C), with the exception upper third provide interpretable polarity data. out on Bluewater Creek, Poleo, or inferred Rock of sites developed in conglomeratic horizons. Overall, our results are consistent with those Point strata, as the accepted site mean ChRMs These sites yield uninterpretable results. previously reported by Reeve (1975) and Reeve for these formations are dominated by a single The Petrified Forest Formation at Coyote and Helsley (1972), in that the Redonda Forma- polarity. Amphitheater is characterized by several polar- tion is characterized by predominantly reverse The combination of IRM acquisition data, ity magnetozones, with individual samples and polarity with two stratigraphically narrow, well- response to both thermal and chemical demag- discrete horizons usually yielding internally defined normal polarity magnetozones. netization, and three-component IRM thermal uniform, well-defined ChRMs (Fig. 10C). Lev- analyses indicates that much of the ChRM in els providing incoherent magnetizations tend DISCUSSION these rocks is carried by pigmentary hematite, to be intensely color mottled and bioturbated. with lesser contributions by fine-grained detrital Despite the heavy reliance on such fine-grained hematite and magnetite. Rocks where the high- materials for polarity information for this part Paleomagnetism and Antiquity coercivity IRM is dominant, and shows a wide of the Chinle Group, the paleomagnetism of of Magnetization range of elevated laboratory unblocking tem- these strata is of very high quality. Of the 45 peratures in three component thermal analysis, sites established in mudrock horizons within the Most Chinle Group strata examined at locali- typically provide the highest quality and most Petrified Forest Formation, 39 of these provide ties in northern, western, and eastern New Mex- interpretable demagnetization results, as well as

acceptable site means with α95 values <15°. ico yield internally consistent paleomagnetic the best-defined site mean directions. Although Strata inferred to be Rock Point Formation at behavior, with ChRMs interpreted to be of Late Chinle Group strata overall yield well-defined both Coyote Amphitheater and Ghost Ranch are Triassic age and of primary or near-primary ori- and well-grouped magnetizations, many specific predominantly of normal polarity (Figs. 10C, gin. The inclination of the ChRM is shallower intervals (e.g., Shinarump Formation mudstones) 10D). At Coyote Amphitheater, a single site of than expected magnetizations derived from exhibit considerable pedogenic modification and reverse polarity is in strata immediately above paleomagnetic poles of late Early Jurassic or yield no coherent remanence. We rule out the the inferred Rock Point–Petrified Forest For- younger age for North America (Van der Voo, possibility that the entire Upper Triassic section mation contact. Several sites yield completely 1993; Besse and Courtillot, 2002; Molina-Garza was subjected to a younger basin-wide chemical

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Coyote Amphitheater Mesa Redonda Ghost Ranch

? 5 m

Abiquiu Dam oint Fm. erred Rock P Inf not rmation sampled Fo oint Fm. not sampled Redonda erred Rock P rmation ? Inf

oleo Fo ? 5 m P rest Fm. Fo ? Bull Can. Fm. –90 090 VGP Latitude

etrified –90 090 F 2.5 m P D VGP Latitude Shin. Fm. –90 0 90 Six Mile Canyon VGP Latitude B Fm . r.

5 m Fo t.

Youngsville Blue Mesa Member 219.2 ± 0.7 Ma Pe

(Irmis & Mundil, 2008) .

? Poleo Fm. Creek Fm

? 5 m Bluewater

Salitral ? Normal Polarity Fm. –90 0 90 Salitral Reverse Polarity E VGP Latitude 1 m Fm. Uninterpretable Shin. Fm. A Shin. Fm. –90 090 C VGP Latitude

Figure 10. Summary magnetic polarity chronologies for sections sampled in this study, along with associated virtual geomagnetic pole (VGP) latitudes for estimated site mean directions for accepted sites. VGPs were not calculated for Youngsville specimens due to poor-quality data. Shin.—Shinarump; Pet. For.—Petrified Forest; Can.—Canyon.

remagnetization after deposition because of the deposits) (Wang et al., 2005; Clyde et al., 2007). Incoherent demagnetization behavior is presence of dual polarity magnetozones, some of Liu et al. (2006) demonstrated that hematite typically associated with both conglomerates which can be correlated across the Chama Basin, was less likely to be modified during pedogenic and fine-grained strata that are color mottled as discussed in the following. processes than other early formed phases such and show evidence of paleosol formation. Paleosols are common components of Chinle as goethite, thus prompting the question of what Mud rip‑up clast conglomerates (common in Group strata (Parrish et al., 1982; Dubiel, process results in incoherent remanent magneti- the Poleo Formation) contain clasts of a range 1987, 1989a, 1989b; Dubiel et al., 1991; zations in pedogenically modified Chinle strata, of colors and yield incoherent magnetizations, Parrish, 1993; Mack et al., 1993; Therrien and even if characterized by abundant hematite. In likely reflecting early acquisition of remanence Fastovsky, 2000; Tanner, 2003, and references the case of Chinle Group strata, we infer that in each clast and a very heterogeneous rema- therein; Cleveland et al., 2008). The observed deposition of detrital hematite grains led to an nence. Sandstones, siltstones, and mudrocks correlation between pedogenically modified early acquisition of a magnetic remanence that with color mottling, bioturbation, and obvious strata and poor to uninterpretable demagne- was enhanced to some degree by later stage pig- disruption of sedimentary structures, as well as tization response contrasts with results from mentary hematite cement. Any disturbance of those that are white or pale green yield either paleosol horizons in some types of sedimentary these sediments after deposition by bioturbation moderate quality or incoherent demagnetiza- deposits where paleosol magnetizations are well and/or pedoturbation could have disrupted the tion behavior. We subjected several specimens defined and often of higher quality than sur- alignment of detrital hematite grains, thus dis- exhibiting poor demagnetization behavior to rounding strata (e.g., loess and shallow basinal torting the ChRM vector. leaching in reagent grade (12.1 M) HCl for as

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long as 500 h. Each residue was dispersed in (Fig. 11). The orange color is due primarily to lighter in color after treatment, but are not high-purity alumina cement and subjected to fine-grained hydrated iron oxides and some pig- completely bleached after more than 300 h of IRM acquisition experiments. Saturation was ment hematite, which are both leached in the leaching. Although pigment hematite is leached reached well below 0.8 T (Fig. 11), indicating experiments, leaving a fine-grained low-coer- in this process, resulting in a decrease in NRM the presence of a fine-grained, low-coercivity civity phase, which we interpret to be magne- intensity, the lack of change in direction during phase in the residue. The low-coercivity phase is tite. As with experiments described herein, the demagnetization suggests that detrital (specular) interpreted to be magnetite and of detrital origin. preservation of magnetite is assumed to reflect hematite also is an important contributor to the Although magnetite has a higher dissolution a substantially coarser grain size for this low- ChRM in these sandstones and that detrital and rate in concentrated HCl than hematite (Sidhu coercivity phase. pigmentary hematite carry essentially the same et al., 1981), its preservation is interpreted to overall direction. reflect a substantial difference in grain size Paleolatitudes and Inclination Shallowing For all samples from the Petrified Forest and between magnetite and hematite, as well as Poleo Formations that provide line segments hydrated iron oxides. Stratigraphically corrected grand mean with MAD values <10°, we used the inclination In general, dark purple, dark red, and brick- directions (Table 1) from all formations of the shallowing approach of Kent and Tauxe (2005) red mudstone and claystones all yield excellent Chinle Group place northern New Mexico at to correct for potential flattening of inclination response to thermal demagnetization. Brown near-equatorial latitudes (ranging from 0.2°N (Table 4). For the data from both formations, and buff massive as well as cross-laminated to 8.5°N) during the Late Triassic (Table 3), in spite of a substantial flattening factor, the siltstones and buff and red, fine- to medium- consistent with paleogeographic reconstruc- observed inclination is not statistically different grained sandstones also typically yield a tions that suggest deposition of Upper Trias- from the corrected inclination. For the other units ChRM of sufficient quality for an unam- sic strata in the Four Corners between lat 5°S of the Chinle Group, we did not have a sufficient biguous polarity determination and estimate and 15°N (e.g., Ziegler et al., 1983; Blakey number of independent specimen directions to of a bed-level mean direction. In studies of and Gubitosa, 1983; Golonka, 2007). Paleo- apply the Kent and Tauxe (2005) approach. Kent red beds, numerous workers (e.g., Herrero- latitudes calculated for strata of the inferred and Tauxe (2005) also provided data supporting Bervera and Helsley, 1983; Shive et al., 1984; Rock Point Formation are farther north than a rapid northward drift of North America during Bazard and Butler, 1991) concluded that the those calculated for Salitral through Redonda the Late Triassic, a result that is not apparent in finest grained hematitic detrital rocks tend to Formation strata, although the difference is not the available data from the Chinle Group. Fur- be better recorders of the magnetic field and statistically distinct. In the initial absence of ther discussion of this discrepancy is beyond the the NRMs are less complex than those of sub- any consideration of the possibility of inclina- scope of this contribution. stantially coarser grained equivalents with both tion shallowing, there is no statistically signifi- authigenic and detrital hematite. Bazard and cant temporal progression in estimated mean Magnetostratigraphy: Local Correlations Butler (1991) hypothesized that the simpler inclinations and thus paleolatitudes among NRMs may reflect lower permeability and thus these results. Within the Chama Basin, parts of the Chinle less prolonged diagenetic effects. Considerable research has convincingly dem- Group can be correlated based on magneto- The relative abundance of detrital hematite onstrated that inclination shallowing affects the stratigraphic data obtained in this study (Figs. grains versus other magnetic phases (including remanence of most if not all sedimentary rocks 10 and 12). The Salitral Formation at Coyote authigenic pigment hematite) has been a sub- (e.g., Anson and Kodama, 1987; Deamer and Amphitheater contains three normal polarity ject of considerable interest (Collinson, 1965, Kodama, 1990; Tan et al., 2002, 2007; Kent and magnetozones (ca1n, ca2n, and ca3n) and three 1974; Tauxe et al., 1980) and clearly influences Tauxe, 2005), resulting in erroneous (shallower) of reverse polarity (ca1r, ca2r, and ca3r). At the the demagnetization behavior of red beds. To paleolatitude estimates, thus affecting paleo- Youngsville locality, the Salitral Formation con- assess the relative abundance of detrital grains geographic reconstructions and definitions of tains two intervals of normal polarity (yo1n, in selected parts of the Chinle Group, specimens apparent polar wander paths. Inclination shal- yo2n), one interval of reverse polarity (yo1r), from the Petrified Forest, Redonda, and inferred lowing can affect sedimentary sequences where and one interval of uninterpretable magnetiza- Rock Point Formations were crushed to sub- the remanence is carried by detrital hematite tion within the lower normal polarity interval 0.5 cm3 fragments and leached in reagent grade (Tan and Kodama, 2002; Tan et al., 2003; Kent (yo1n). Both sections display normal polarity at (12.1 M) HCl until completely disaggregated. and Tauxe, 2005); however, it is not certain to the base of the section and normal polarity in the The residue was mixed in ultrahigh-purity alu- what extent any remanence recorded by authi- middle part. The uppermost part of the section mina cement and subjected to IRM acquisition. genic, grain-coating hematite is susceptible was not sampled at Youngsville, but a reasonable After ~24–48 h of leaching, the residue from red to the same processes that result in inclination correlation can be made between these localities. and purple mudstone is only slightly paler than shallowing. In addition, the net effect of inclina- Poleo Formation rocks at both Coyote the original specimen, and the response of the tion shallowing is minimized in rocks deposited Amphitheater (N = 6 sites, ca4r) and Abiquiu residue to IRM acquisition is essentially iden- at very low latitudes. Dam (N = 30 sites) are exclusively of reverse tical to that of the bulk specimen, with satura- Chemical demagnetization has long been polarity, despite local differences in thickness tion by 3.0 T. The experiment shows that both utilized to isolate a remanence residing in very (Figs. 10 and 12). The thicker Poleo section at fine-grained, authigenic hematite, as revealed in fine grained, authigenic hematite (e.g., Collin- Abiquiu Dam defines a single reverse polarity petrographic examination (Fig. 9), and detrital son, 1965; Park, 1970). Hematitic sandstone magnetozone, suggesting that the entire section hematite are important magnetic components in samples of the Poleo Formation show no sta- was deposited over a relatively short time inter- these rocks and that magnetite is only present tistically significant change in direction after val, given that field reversals during the Late Tri- in small concentrations. For lighter orange mud- cumulative chemical demagnetization of 370 h assic were relatively frequent (e.g., Olsen et al., stones and siltstones, the leached residue is pale and a decrease of at least 80% in NRM intensity 1996, 2002; Hounslow and Muttoni, 2010). The green and shows IRM saturation below 0.8 T (e.g., Fig. 4H). The Poleo samples are slightly average duration of reverse polarity chrons in

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–0.1 –0. 3 1. 1 Magnetization Normalized 0. 9 0. 7 0. 5 0. 3 0. 1 .2 N –0.4 N 400

NRM 1. 1 0. 9 0. 7 0. 5 0. 3 0. 1 Normalized Magnetization Normalized –0.2 210°·C –0. 30 N 210°C 56 0 64 5 W , 3.0 rest Fm. Up 3.0 NRM etrified 40 0 66 5 CL 31-2b P Fo 3.0 rest Fm. etrified 50 mA/m CL 20-3c P Fo 61 5 2.7 2.7 rest Fm. etrified

61 5

CL 35-5a P Fo 56 0 64 5 mA/m 10 56 0 W W , , 61 5 Up Up 67 0 2.2 67 0 2.2 2.3 10 mA/m 40 0 1.7 1.8 1.7 210°C ield (T) ield (T) ield (T) NRM 1.2 DC F 1.2 1.3 DC F DC F CL 31-4 Residue 0.7 CL 20-3 Residue 0.8 0.7 0.2 0.3 0.2 CL 35-3 Residue –0.2 0 0 Figure 11. Results of reagent HCl leaching experiments showing isothermal remanent magnetization (IRM) acquisition curves and associ - HCl leaching experiments showing isothermal remanent Results of reagent 11. Figure the same site. specimens from ated orthogonal thermal demagnetization diagrams for

1.0 0.8 0.6 0.4 0.2

Normalized Magnetization Normalized 1.05 0.25 0.85 0.65 0.45 0.05

Normalized Magnetization Normalized –0.15 1.0 0.8 0.6 0.4 0.2 Normalized Magnetization Normalized –0. 2 –0.3 –0.3 –0. 2

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TABLE 3. PALEOLATITUDES AND ASSOCIATED CONFIDENCE LIMITS years between sites, assuming constant sedi- Paleolatitude Upper limit Lower limit mentation rates within mudstone-dominated Formation Locality (°) ( +) ( –) λ λ intervals. The Newark Supergroup record Inf. Rock PointG.R.3.1 6.9–0.9 C.A. 8.514.82.8 includes numerous chrons of duration consider- Redonda M.R. 1.84.6 –1.1 ably <~0.36 m.y. (Olsen et al., 1996, 2002). For Petriﬁed Forest C.A. 2.13.5 0.8 PoleoC.A.2.1 5.1–1.0 example, the Norian part of the Newark polarity A.D. –0.2 3.0–2.7 record contains 40 magnetozones, compared to Salitral C.A. 1.96.2 –2.3 our present estimate of ~12 magnetozones in Lower Chinle S.M.C. 1.56.8 –3.7 Norian age strata of the Chinle Group (assum- Note: Abbreviations: Inf.—inferred; A.D.—Abiquiu Dam; C.A.—Coyote Amphitheater; G.R.— Ghost Ranch; M.R.—Mesa Redonda; S.M.C.—Six Mile Canyon. ing that Shinarump and Salitral strata in the Chama Basin are Carnian in age). It is clear that several short duration polarity intervals have not TABLE 4. COMPARISON OF INCLINATION VALUES BEFORE been captured in this study. ANDAFTER INCLINATION SHALLOWING CORRECTION I I o f Correlation Models and Development of a Formation (°) α95 Flattening factor (°)N Petriﬁed Forest –4.9 2.7 0.63 –7.4 426 Composite Magnetic Polarity Chronology Poleo–0.4 1.7 0.57 –0.7 274

Note: After Kent and Tauxe (2005). Inclination, Io has been calculated with all available specimens with maximum angular deviation <10° for each formatio. n Magnetostratigraphic data from Chinle N—Number. Group strata in north-central New Mexico may, in a general sense, be compared with composite polarity zonations recorded in Upper Triassic the Newark Supergroup record is ~650 k.y. for (Fig. 12). The Coyote Amphitheater and Ghost strata elsewhere, including eastern North Amer- the Norian part of the section and 180 k.y. Ranch localities are ~19 km apart and, although ica (Witte and Kent, 1989; Kent and Olsen, for the Carnian part of the section (calculated the general prevalence of normal polarity mag- 1999; Olsen et al., 1996, 2002) and the Tethys from Olsen et al., 1996; assuming that the netizations characterizes both localities, polar- realm (Krystyn et al., 2002; Channell et al., approximate age of the Carnian-Norian bound- ity zonation cannot be directly correlated. We 2003; Muttoni et al., 2004; Gallet et al., 2007), ary is ca. 228 Ma; Hounslow and Muttoni, attribute this to pedogenic processes that have as well as the composite geomagnetic polarity 2010). Hounslow and Muttoni (2010) estimated locally destroyed a primary magnetization. time scale (GPTS) of Hounslow and Muttoni an average magnetochron duration of 380 k.y. The Coyote Amphitheater section is the first (2010) (Fig. 13). Although compromised by for the entire Triassic. where all formations in the Chinle Group have the absence of a near-continuous magnetostrati- Only one complete section of the Petrified been sampled in sequence, albeit with a sampling graphic record, the polarity sequence of Chinle Forest Formation was sampled in this study, interval that requires future densification, at least strata is nonetheless important as a continental thus precluding any internal correlation within over part of the section. Because Chinle strata record in that it is supported by a growing number the Chama Basin. The Petrified Forest For- were deposited in a large-scale fluvial system, of high-precision isotopic age determinations, mation magnetic polarity zonation is charac- the presence of multiple disconformities, many whereas others are defined solely by biostratig- terized by a short reverse interval at the base of which are not obvious, will lessen the com- raphy or cyclostratigraphy. Here we discuss two (ca5r) and top (ca8r), and two relatively long pleteness of any magnetostratigraphic record possible correlation approaches. One combines reverse polarity magnetozones (ca6r, ca7r), (e.g., Hall and Butler, 1983). For example, the the lithostratigraphic correlations of Triassic with shorter normal polarity zones near the base contact between the Salitral and Poleo Forma- strata in New Mexico (Lucas and Hunt, 1992; (ca5n), in the middle (ca6n), and near the top of tions has been described as disconformable (Tr-4 Lucas, 1993; Hunt and Lucas, 1993a, 1993b; the section (ca7n; Fig. 12). The lower 15 m disconformity; Lucas, 1991, 1993; Lucas et al., Lucas et al., 2001, 2003, 2005; Heckert and of the Petrified Forest Formation was sampled 2003, 2005), and some workers have argued for Lucas, 2003) with geochronologic data, includ- at 0.5–1.0 m intervals at a second locality a disconformity between the Petrified Forest and ing the Irmis and Mundil (2008) detrital zircon within Coyote Amphitheater. Although most inferred Rock Point Formations (Tr‑5, locally of date for the basal Blue Mesa Member. The sec- specimens from the second locality yielded Lucas, 1991, 1993). We note that magnetozone ond age model compares Chinle Group mag- uninterpretable magnetizations, those speci- boundaries in the Coyote Amphitheater sequence netic polarity chronologies from the region to mens that did yield relatively linear trajectories rarely coincide with substantial changes in lithol- the new Triassic GPTS (Hounslow and Muttoni, matched the overall polarity zonation seen at ogy, at least suggesting that not all magnetozone 2010) and includes the compilation of a tenta- the primary sampling locality. boundaries are disconformities. At the resolu- tive composite Chinle Group magnetic polarity Strata that have been inferred to be part of tion of current sampling, the Salitral-Poleo, the chronology for the region. the Rock Point Formation were sampled at both PetrifiedForest–Rock Point, and the Poleo– Coyote Amphitheater and Ghost Ranch and are Petrified Forest Formation contacts in the Correlation Model 1: Lithostratigraphic almost exclusively of normal polarity. The many Chama Basin are all within reverse polarity Correlations on a Regional Scale well-developed paleosol horizons in these sec- magnetozones (Fig. 12). The first age model we examine is based on tions (Cleveland et al., 2008) yield incoherent Our sampling interval at Coyote Amphi lithostratigraphic and vertebrate biostratigraphic magnetizations at both localities. Two short theater is clearly too coarse to provide a com- correlations among localities as developed by reverse polarity magnetozones were identi- plete polarity zonation across all formation and Hunt and Lucas (1993a, 1993b), Lucas and fied at the Ghost Ranch locality (gr1r, gr2r); lithologic boundaries, as it corresponds to a time Hunt (1992), and Lucas et al. (2005). A com- one was directly above the Coelophysis quarry duration of possibly several hundred thousand posite magnetic polarity stratigraphy (Fig. 12)

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rest Fm. rest Fo trified Pe rred Rock Rock rred fe In Fm . 5 m y Fm . Fm. oleo P Salitral Shin. y te Amphitheate oyo disconformit y y C olarit olarit disconformit ? ? ? m Figure 12. Correlation of magnetic polarity records for Chinle Group sections sampled in New Mexico, Arizona, and Utah and a proposed Arizona, and Utah a proposed sections sampled in New Mexico, Chinle Group for of magnetic polarity records 12. Correlation Figure Can.—Canyon. Shin.—Shinarump; PF—Petrified Forest; the Chinle Group. composite magnetic polarity stratigraphy for erse P

yo1n Normal P yo3n yo2n Uninterpretable Rev

rmation Fo oleo P oungsvill e 5 m Fm. Y Fm. 5 m Fm. Abiquiu Da Salitral Shin. Shin.

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GPTS Turkey

UT27 201.6 (Oyuklu) Eastern North America ? UT26 (Newark S.G.) 20 4 Rhaetian E24 UT25 ? Possible hiatus Rhaetian E23 UT24

E22 UT23

E21 UT22 205

E20 UT21 E19 E18

Norian rt rest/

E17 UT20 Fo

E16 trified inted Dese Pe Pa Gallet et al. E15 (2007) UT19 ca. 213 Ma: Black Forest Bed, PFNP Norian

Tethys Composite 215 UT18 Sicily (Slovakia) E14 (Pizzo Mondello)

E13 UT17 219 E12 E11 UT16 220 Norian UT15 Sonsela Interval UT14 E10 ca. 215 Ma, Poleo Interval E9 UT13 227 Possible hiatus 225 UT12 ca. 219 Ma: Zuni Mountains E8 UT11 229 Blue Mesa Interval UT10

E7 UT9 UT8 230. 9 Possible hiatus UT7 Late? UT6 Salitral Interval Carnian UT5 Carnian Muttoni et al. E6 UT4 230 E5 UT3 Chinle Group 2004 E4 E3 UT2 Composite E2 E1

Channell et al. 235 Olsen et al. UT1 2003 1996 Hounslow & Muttoni 2010

Figure 13. Proposed correlations of marine and nonmarine magnetic polarity records from North America and Europe with composite magnetic polarity chronology for the Chinle Group, as obtained in this study, and with Hounslow and Muttoni’s (2010) Late Triassic geomagnetic polarity time scale (GPTS). PFNP—Petrified Forest National Park.

is based on data from the Chinle Group, includ- Vertebrate fossils from the Bluewater Creek– The oldest parts of the Chinle Group that ing inferred Rock Point Formation strata, from Blue Mesa interval in Arizona and the Salitral provide a reasonably robust magnetic polarity northern, western, and eastern New Mexico, Formation in northern New Mexico are Adama- zonation are the uppermost Shinarump, Salitral, and eastern Arizona (Reeve and Helsley, 1972; nian, and the age determination of ca. 219 Ma and Bluewater Creek Formations. The Blue Molina-Garza et al., 1991, 1993, 1996, 1998b; from the base of the Blue Mesa Member is water Creek Formation and Blue Mesa Member Steiner and Lucas, 2000) as well as results pre- projected into the upper Salitral Formation. (Petrified Forest Formation) of the Zuni Moun- sented here. If the age of the Carnian-Norian boundary is tains have been correlated by lithostratigraphic In this model, lower Chinle Group strata in ca. 228 Ma (Furin et al., 2006; Hounslow and means to the Salitral Formation in northern New the Zuni Mountains are considered lithostrati- Muttoni, 2010), the implication is that most if Mexico (Fig. 12). In both regions this stratigraphically and biostratigraphically equivalent not all of the Chinle Group sampled in this study graphic interval is characterized by predomi- to the lower Chinle Group in the Chama Basin. is of Norian age. nantly reverse polarity and several relatively

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short normal polarity magnetozones. Only a the Newark Supergroup. The composite section Mesa Redonda information reported here. Our small part of the Bluewater Creek Formation presented by Steiner and Lucas (2000) contains Mesa Redonda section shows two reverse polar- and Blue Mesa Member section was sampled eight magnetozones with no obvious polarity ity magnetozones, one beginning at the contact at Six Mile Canyon in the Zuni Mountains, but bias (Fig. 12). The Sonsela Sandstone, consid- with the underlying Bull Canyon Formation additional data are available for localities near ered to be lithostratigraphically equivalent to (mr1r) and the other low in the mudstone-domi Fort Wingate (Molina-Garza et al., 1998a). the Poleo Formation, is reported by Steiner and nated interval (mr2r; Fig. 12). A normal polar- Correlation between the Salitral and Bluewater Lucas (2000) to be within an interval of normal ity interval was also identified by Molina-Garza Creek–Blue Mesa intervals is tenuous at pres- polarity (Fig. 12). Nonetheless, other sections et al. (1996) in the lower Redonda Formation ent. The magnetostratigraphic record allows that have been lithostratigraphically correlated near the contact with the Bull Canyon Forma- correlation of zones ca1n and ca2n with normal with the Sonsela, such as the Trujillo Forma- tion at Sebastian Canyon. A normal polarity polarity intervals in the Bluewater Creek For- tion of eastern New Mexico (Molina-Garza zone in the uppermost Redonda Formation is mation (Fig. 12). Magnetostratigraphic data for et al., 1993) and the Sonsela in southeast Utah preserved in the Sebastian Canyon section, but the lower Chinle Group in the Sangre de Cristo (Molina-Garza et al., 1993), are of reverse polar- not at Mesa Redonda. We correlate the lower Mountains and Tucumcari Basin (Molina-Garza ity. All Poleo Formation strata at both Abiquiu normal magnetozone mr1n with magnetozone et al., 1996) suggest an acceptable correlation of Dam and Coyote Amphitheater that yield inter- ca7n at Coyote Amphitheater (Fig. 12) and cor- the Garita Creek Formation with the Blue Mesa pretable results are of reverse polarity. relate magnetozone mr2n with the lowest normal Member (Fig. 12). An alternative interpretation of the dispar- interval reported by Reeve and Helsley (1972). We recognize that a Norian to Rhaetian age ity in polarity between the Sonsela Sandstone The Redonda Formation has been lithostrati- assignment for all Chinle Group strata contrasts in eastern Arizona and the Poleo Formation in graphically correlated to the inferred Rock with previous age interpretations based on the north-central New Mexico is that the interval Point Formation of north-central New Mexico vertebrate fossil and palynostratigraphy record of deposition was so brief that, from a correla- (Lucas, 1997), and this correlation was utilized for Chinle Group strata in New Mexico. The tion perspective, Sonsela and Poleo strata are by Cleveland et al. (2008), who also assumed Carnian-Norian boundary has typically been still effectively equivalent but diachronous. both formations to be exclusively Rhaetian in estimated to be at the base of the Poleo Forma- The Poleo Formation disconformably overlies age, in their study of the stable isotope record tion, based primarily on sparse fossil pollen data Salitral strata at Abiquiu Dam, and the Sonsela of pedogenic carbonate in both sections. How- (Litwin, 1986; Litwin et al., 1991), as well as Sandstone in the Petrified Forest National Park ever, strata termed Rock Point Formation are vertebrate biostratigraphy (Zeigler et al., 2005). (PFNP) area may have been deposited during predominantly of normal polarity (Figs. 10 and However, this boundary is poorly defined, as the interval represented by this hiatus or dur- 12). The contrast in polarity between the Mesa flora and fauna identified both above and below ing a similar thickness normal polarity interval Redonda and Ghost Ranch sections may be due the Poleo Formation have been used to approxi- within the lower Petrified Forest Formation at to temporal differences in accumulation and mate it (e.g., Litwin, 1986; Lucas and Hunt, Coyote Amphitheater (ca4n), which is the inter- preservation (i.e., siltstones near the top of the 1992; Hunt and Lucas, 1993a, 1993b; Lucas pretation we consider (Fig. 12). Redonda Formation at Mesa Redonda captured et al., 2005). No palynostratigraphic or biostrati- In the PFNP area, the lower third of the Petri polarity intervals not recorded in the Rock Point graphic information is available from the Poleo fied Forest Member is dominated by reverse Formation). An alternative explanation involves Formation. The Dickinson and Gehrels (2008) polarity and the upper two thirds are predomi- a major disconformity at the base of the inferred maximum detrital zircon depositional age of ca. nantly of normal polarity (Steiner and Lucas, Rock Point Formation in the Chama Basin, such 215 Ma for the Poleo Formation clearly supports 2000) (Fig. 12). In contrast, at Coyote Amphi- that the dominantly reverse polarity interval in a Norian age for this part of the Chinle Group. theater the lower half of the Petrified Forest For- the Redonda Formation is the time lost at the Strata lithologically similar to either the Sonsela mation is dominated by reverse polarity (a single Petrified Forest–inferred Rock Point discon interval or the Poleo Formation have not been normal polarity magnetozone is identified) and formity in the Chama Basin (Fig. 12). Also, recognized in the Zuni Mountains area. How- the upper half contains a normal-reverse-normal we correlate the dominantly normal polarity ever, the older ca. 219 Ma age estimate for the sequence with all three magnetozones being of interval in inferred Rock Point strata at Ghost tuffaceous sandstone in the Blue Mesa Member, similar thickness. Ranch to the uppermost normal magnetozone in Zuni Mountains, suggests that most sites col- If we assume that a wide sampling gap exists the Luciana Mesa section (Reeve and Helsley, lected in this area are below the Sonsela inter- between the Sonsela and Petrified Forest sec- 1972) and the uppermost normal interval in val–Poleo Formation. The 219 Ma age estimate tions at Petrified Forest National Park, a viable the Redonda Formation at Sebastian Canyon for basal Blue Mesa strata mandates a major correlation with the Coyote Amphitheater mag- (Molina-Garza et al., 1996). disconformity below the Blue Mesa Member netic polarity chronology associates the rela- The Rock Point Formation in Utah is charac- in order for any underlying Chinle strata to be tively long normal polarity interval high in the terized by dominantly reverse polarity, and the Carnian in age. Painted Desert Member at PFNP with one of the overlying Wingate Formation and the laterally The Steiner and Lucas (2000) magnetostrati- long normal magnetozones at Coyote Amphi- equivalent Moenave Formation are dominated graphic record for the Chinle Group at Petrified theater (ca6n or ca7n). Our preferred correlation by normal polarity (Fig. 11; Molina-Garza et al., Forest National Park included the Bluewater is with ca6n (Fig. 12), based on the presence of 2003; Donohoo-Hurley et al., 2010). Because Creek Member and lower Sonsela Sandstone, as additional strata above the Black Forest Bed that strata inferred to be Rock Point Formation in well as the Petrified Forest Member up to the have not been sampled. north-central New Mexico are predominantly base of the Black Forest Bed. Notably, there is In eastern New Mexico, Redonda Formation of normal polarity, we argue that these strata a gap in their sampling of ~75 m (W. Parker, strata yield high-quality data and are dominated cannot be directly correlated with the type 2009, 2010, personal communs.), and this by reverse polarity, as reported by Reeve and Rock Point Formation exposed in Arizona and negates use of this sequence as a complete sec- Helsley (1972), Reeve (1975), and Bazard and Utah. Rather, we speculate that the strata inferred tion of the Chinle Group for correlation with Butler (1991), and demonstrated by the new to be Rock Point Formation in northern New

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Mexico are younger and temporally equiva- with the Newark interval represented by magneto well as correlation of the Trujillo Formation in lent to Wingate and/or Moenave strata in Utah chrons from E8 to E13 is preferred (Fig. 13). eastern New Mexico to the Poleo Sandstone. and northern Arizona, which are considered to High-quality magnetic polarity compila- The medial Bluewater Creek–Blue Mesa inter- span latest Triassic (Rhaetian) to earliest Juras- tions have been obtained from several carbon- val, which encompasses the tuffaceous sand- sic (Hettangian) time (Litwin, 1986; Kirkland ate rock–dominated sequences in the Tethyan stone dated as ca. 219 Ma (Irmis and Mundil, and Milner, 2006; Lucas et al., 2006a, 2006b; region of southern Europe (Fig. 13). These 2008), is defined by primarily reverse polarity Donohoo-Hurley et al., 2010). The Redonda sections include a Tethyan composite section with a short possible normal polarity magneto Formation of eastern New Mexico, however, is (Krystyn et al., 2002), a revised Pizzo Mondello zone at the base of the sandstone. The GPTS has primarily of reverse polarity and we still con- section in Sicily (Muttoni et al., 2004), and a sec- a relatively long normal polarity interval at ca. sider it to be time equivalent to the type Rock tion from Oyuklu, Turkey (Gallet et al., 2007). 219 Ma, whereas the magnetic polarity chronol- Point Formation in Utah. The lower Chinle Group polarity sequence does ogy from the lower Chinle (Shinarump-Salitral not show a strong correlation with the upper interval) in the Chama Basin shows three short Correlation Model 1: Correlations to Newark Carnian Tethyan marine sections (Fig. 13). The normal polarity intervals and four short reverse Supergroup and European Strata composite sequence proposed by Channell et al. polarity intervals below the Poleo Formation In a crude sense, Chinle Group magnetic (2003) shows that the inferred Carnian sequence (Fig. 12, ca1n–ca3n). Short stratigraphic inter- polarity records from New Mexico can be is of mixed normal and reverse polarity and that vals sampled in the Zuni Mountains and at Fort compared with the astronomically tuned polar- the Norian is predominantly of reverse polarity Wingate show primarily reverse polarity with ity time scale for the Upper Triassic Newark in the lower half to two-thirds of these sections, a possible short normal chron (Fig. 12), which Supergroup of the east coast of North America and of mixed polarity in the upper half to third. could be tentatively correlated to one of the (Witte and Kent, 1989; Olsen and Kent, 1999; If Chinle Group strata are entirely of Norian short normal polarity magnetozones in northern Olsen et al., 1996, 2002, and references therein) age, then the correlation between the Chinle New Mexico. (Fig. 13). Initial correlations between Chinle Group and the Tethyan sections improves, at Alternatively, the Shinarump-Salitral interval and Newark strata were based primarily on bio- least in principle, as more magnetozones are in the Chama Basin correlates with the Carnian- stratigraphy (Lucas and Huber, 1993) with the represented in the Chinle Group over a shorter Norian boundary interval, representing some assumption that the lower Chinle Group (Shina time interval. part of Hounslow and Muttoni’s (2010) chron rump, Salitral, and Bluewater Creek Forma- However, the stratigraphic completeness sequence UT12–UT15. Lower Chinle Group tions) was Carnian in age. The lower part of the of the marine sections used to provide a com strata in the Zuni Mountains, dated as 219 Ma, Chinle Group is of mixed polarity, with a reverse posite magnetic polarity sequence remains open may have imperfectly captured the normal polar- polarity interval straddling the inferred Carnian- to question. The marine sequences from which ity interval UT17 in the GPTS. If these correla- Norian boundary, assuming that the Poleo For- the composite of Channell et al. (2003) was tions are viable, then lower Chinle Group strata mation was deposited across the Carnian-Norian constructed are, on average, <40 m thick and in the Chama Basin cannot be time equivalent to boundary. This pattern is similar to that of the contain thin layers interpreted as turbidite depos- lower Chinle strata in the Zuni Mountains and Fort Carnian section of the Newark Supergroup its (Krystyn et al., 2002; Channell et al., 2003; Wingate area. In addition, such correlations allow (chrons E1–E7n), except, of course, that fewer Muttoni et al., 2004). Although these sections an estimate of time missing at the disconformity magnetozones are observed in the Chinle sec- were sampled at a very high density, the thin below the Poleo Formation (Tr-4 unconformity tion. Alternatively, if we accept a considerably sequences, coupled with repeated higher energy of Lucas, 1991, 1993). If the Salitral Formation is older age for the Carnian-Norian boundary, as deposits, have prompted concerns over the true older than the Bluewater Creek–Blue Mesa inter- proposed by Furin et al. (2006) and adapted completeness of these sequences (Lucas et al., val and straddles the Carnian-Norian boundary, by Hounslow and Muttoni (2010), then the ca. 2004). In addition, polarity data for the middle and if we assume a maximum depositional age 219 Ma age estimate for lower Blue Mesa strata Norian are sparse, complicating efforts to corre- for the Poleo Formation of 215 Ma (Dickinson results in a preferred correlation of the Salitral– late among sections that sample parts of Norian and Gehrels, 2008) and a Carnian-Norian bound- Bluewater Creek–Garita Creek–Blue Mesa time. The section at Sicily (Muttoni et al., 2004) ary of ca. 228 Ma (after Hounslow and Muttoni, sequence with the Newark polarity interval from has a gap near the base of the Norian section, 2010), then at least 13 m.y. are missing at the base E8 to E13 (Fig. 13). In support of this correla- corresponding to a part of the Lacian, and the of the Poleo Formation. tion, an age of ca. 219 Ma for magnetozone E13 Tethyan composite (Channell et al., 2003) has The Trujillo and Poleo Formations have has been extrapolated from cyclostratigraphy in a gap in the upper half of the Norian section, also been considered to be lithostratigraphi- the Newark basin (Olsen et al., 2002). which includes most of the Alaunian interval. cally equivalent (Lucas et al., 2001), although The recent U-Pb age date from the upper maximum depositional ages from detrital zircon Bluewater Creek Formation of ca. 219.2 ± Correlation Model 2: Correlation assemblages suggest a lack of time equivalence 0.7 Ma (Irmis and Mundil, 2008) implies that of Magnetic Polarity Chronologies between these two units. The Trujillo Formation few or no Carnian strata are preserved in west- with the Late Triassic GPTS has a wide maximum depositional age range of ern New Mexico if a Carnian-Norian boundary The new GPTS for the entire Triassic ca. 226–219 Ma (Dickinson and Gehrels, 2008) age estimate of ca. 228 Ma is valid. Assuming (Hounslow and Muttoni, 2010), coupled with and appears to be substantially older than the lithostratigraphic correlations between the Blue- maximum depositional age estimates from detri- Poleo Formation, with a maximum depositional water Creek Formation and Blue Mesa Mem- tal zircon assemblages (Dickinson and Gehrels, age of ca. 215. If we correlate the upper Trujillo ber and the Salitral Formation are correct, then 2008) and the detrital zircon date of Irmis and Formation (ca. 219 Ma) to the tuffaceous sand- all Chinle strata in western and northern New Mundil (2008), calls into question previously stone in the Zuni Mountains, the Trujillo inter- Mexico, with the possible exception of the held assumptions regarding the lithostrati- val must be stratigraphically below the Poleo inferred Rock Point Formation, are of Norian graphic correlation of the Bluewater Creek– Formation as well as the Blue Mesa Member age, and our correlation of Chinle Group strata Blue Mesa interval to the Salitral Formation as (Petrified Forest Formation) in northern Ari-

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zona. Hounslow and Muttoni (2010) correlated only sporadically throughout the Late Trias- marine faunal assemblages are better deter- the Trujillo Formation to the upper Stockton and sic, and disconformities, some quite subtle, are mined. Instead, we choose to incorporate the Lockatong Formations of the Newark Super- pervasive. Fluvial systems are complex deposi- few detrital zircon age determinations available group, which are considered early Norian in age tional environments and lithologic units should as well as the GPTS proposed by Hounslow and (Fig. 13). In light of the new GPTS, the Trujillo not be expected to be isochronous, nor continu- Muttoni (2010). Formation is thus probably much older than ous, over long distances. As noted herein, Litwin (1986) and Litwin the Sonsela or Poleo Formations and would be Prior to Irmis and Mundil’s (2008) detrital zir- et al. (1991) used palynomorphic data to assign a considered equivalent to lower Bluewater Creek con age determination, the lower Chinle (Shina- Norian age to upper Chinle strata (Petrified Forest strata in Arizona. We place the Trujillo inter- rump, Salitral, and Poleo Formations) sequence and inferred Rock Point Formations) based on a val above the Salitral Formation in the Chama was assigned to the Adamanian LVF and thus limited number of samples. Both palynomorphs Basin in our composite geomagnetic polarity given a Carnian age. The upper Chinle Group and conchostracans from strata of the inferred chronology (Figs. 10 and 12). (Petrified Forest and inferred Rock Point For- Rock Point Formation at Ghost Ranch may reflect If we compare the composite Chinle polarity mations) were designated as Revueltian on the provincialism during the latest Triassic and thus chronology to Hounslow and Muttoni’s (2010) basis of their faunal assemblages, and assigned may not accurately reflect global palynostrati GPTS for the Late Triassic, the Shinarump– a Norian age (Lucas, 1998; Hunt and Lucas, graphic or biostratigraphic changes. Salitral interval is correlated to the middle 1993a). Irmis et al. (2010) argued that the LVFs If our proposal that the inferred Rock Point Carnian (UT4–UT7 or UT5–UT8), and the proposed by Lucas (1998) do not appear to be strata in the Chama Basin do not directly corre- Trujillo–Bull Canyon–Blue Mesa interval is perfectly time correlative to the marine stage late with Redonda strata in eastern New Mexico correlated to the early Norian (UT16-UT17). divisions of the Late Triassic and thus are of lim- is viable, yet Redonda strata can be correlated The Poleo, Sonsela, and Petrified Forest inter- ited use for correlation purposes on more than a to the Rock Point Formation of Utah and Ari- vals correspond to almost all of the Norian. The regional scale. Irmis et al. (2010) argued that it is zona (and potentially to the Owl Rock Forma- Poleo–Sonsela interval is correlated here to nearly impossible to correlate continental strati- tion), then this relation suggests the presence UT18 and the Petrified Forest interval is cor- graphic sequences with marine sequences based of a north-south–trending topographic high in related to upper UT18–UT22. The red siltstone solely on biostratigraphic tie points, as also central New Mexico during deposition of Rock (inferred Rock Point) interval is interpreted to noted by Donohoo-Hurley et al. (2010) in their Point and Redonda strata. Studies of the Upper correlate with UT25–UT26. attempts to correlate the magnetostratigraphic Triassic Dockum Group of West Texas have led record of the Moenave Formation with exist- to the hypothesis that the Dockum Group was Composite Chinle Group ing continental as well as marine records. One deposited in a separate basin from that of the Magnetostratigraphy very rare example of vertebrate fossil material Chinle Group in New Mexico, Arizona, and In summary, our composite polarity stratig- found with invertebrate material is a specimen Utah (McKee et al., 1959; Finch and Wright, raphy for Chinle Group strata (Fig. 12) assumes of the aetosaur Aetosaurus found in marine 1983; McGowen et al., 1983; Johns and Granata, that the Poleo Formation–Sonsela interval likely sedimentary rocks (Lucas, 1998, 2010b). The 1987; Dubiel, 1989b, 1994; Lehman, 1994a, represents a relatively short period of deposi- conchostracan record, which is excellent for 1994b). Riggs et al. (1996) demonstrated that tion. It also reflects our tentative assumption that the Germanic Basin of Europe and the New- the two basins were probably connected during most strata previously referred to as Rock Point ark Basin, is very sparse for the Chinle Group early Late Triassic deposition. Detrital zircons Formation in northern New Mexico are younger (Kozur and Weems, 2010; H. Kozur, 2010, per- unique to the Amarillo-Wichita uplift have been than Redonda Formation strata and that the sonal commun.). If our tentative correlation of identified both in the Santa Rosa Formation of Redonda Formation is younger than Petrified at least part of the inferred Rock Point Forma- eastern New Mexico and West Texas, the Poleo Forest strata, which contradicts the direct cor- tion to Rhaetian-age strata (Wingate–Moenave Formation of north-central New Mexico, and relation assumed by Lucas et al. (2005) and interval, Oyuklu section) is viable, then the the Shinarump and Osobb Formations of Ari- Cleveland et al. (2008). The composite mag- magnetic polarity data are directly at odds with zona and Nevada (Riggs et al., 1996; Dickinson netic polarity stratigraphy contains 11 well- conchostracan biostratigraphy for the inferred and Gehrels, 2008; Dickinson et al., 2010). defined polarity zones (depending on how Rock Point strata, and furthermore suggest that By the latest Late Triassic, however, sedi- uninterpretable zones are considered) in the the direct correlation of inferred Rock Point ment transport between the Dockum Basin lower Chinle Group (below the Poleo Forma- and Redonda strata by Cleveland et al. (2008) and the rest of the Chinle Basin appears to tion) and 16 in the upper Chinle Group, with a may be problematic. Kozur and Weems (2010) have been disrupted as indicated by paleocur- distinct bias, based on stratigraphic thickness of identified latest Norian–earliest Rhaetian con- rent directions (May, 1988; Riggs et al., 1996). magnetozones, toward reverse polarity. chostracans in the Redonda Formation and lat- The hypothesis of a topographic high in central est Norian (latest Sevatian) conchostracans from New Mexico during late Late Triassic time is Lithostratigraphic and the inferred Rock Point Formation (Rinehart also supported by field observations in the Dry Biostratigraphic Implications et al., 2009; Kozur and Weems, 2010; H. Kozur, Cimarron Valley of northeastern New Mexico The new magnetic polarity data from north- 2010, personal commun.). Reconciling these (Baldwin and Muehlberger, 1959), where the ern New Mexico illustrate the inherent prob- disparate data sets will require further mag Middle Jurassic Entrada Sandstone directly lems associated with correlation of terrestrial netostrati graphic and biostratigraphic research. overlies in angular unconformity the Upper rock units over long distances based solely on Because attempts to tie LVFs to marine stages Triassic Dockum Formation (Fig. 15). The lithostratigraphic characteristics. In addition, are currently of limited utility, we refrain from Entrada Sandstone varies considerably in thick- the tentative correlations we propose imply attempting to correlate the composite Chinle ness in this area, ranging from ~10 m or more complex deposition in the Chinle Basin as a Group magnetic polarity chronology to marine in thickness at Steamboat Butte to nonexistent whole (Fig. 14). For example, the northern New sections using biostratigraphic tiepoints until to the west. We hypothesize that the dramatic Mexico area appears to have received sediment the relationships between continental and variation in thickness of the Entrada Sandstone

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GPTS Ghost Ranch (Hounslow & Muttoni, 2010) Correlation Models 5 m

Geochron t No ample d

Bio & Coyote UT27 201.6 Lithostrat & S Lithostrat & 219 Ma Amphitheater GPTS gr3n Mesa Redonda UT26 gr2n 204 Rhaetian Rhaetian ? gr1n UT25 Rhaetian ca8n UT24 t

No UT23 Sampled

ca7n UT22

UT21 mr2n

Norian

ca6n UT20 5 m mr1n Norian

UT19 Norian ca. 213 Ma

ca5r Norian UT18

UT17

ca4n 219

UT16 ca. 215 Ma

UT15 Six Mile Canyon UT14 ca4r UT13 219.2 ± 0.7 Ma 227 (Irmis & Mundill, 2008) ? UT12 ca3n UT11 229 5 m ca2n ? UT10 Carnian Carnian ca1n 5 m UT9 ? UT8 230.9 UT7 UT6 UT5 Carnian UT4 Figure 14. Magnetic polarity chronology correlations of Chinle Group strata in New Mexico, showing disconformities in the section exposed in the Chama Basin. GPTS—geomagnetic polarity time scale. UT3 UT2

UT1 235

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tion in New Mexico were late Norian in age, again based primarily on palynostratigraphy (Litwin, 1986; Litwin et al., 1991) and vertebrate biostratigraphy (Small, 1998). Vertebrate fossil material from the Coelophysis quarry M. Jurassic has been used to assign the uppermost Chinle U. Triassic ang. unconformity strata in the Chama Basin to the Apachean LVF (Lucas and Tanner, 2007; Rinehart et al., 2009). If the polarity record for strata inferred to be Rock Point Formation in the Chama Basin can be correlated to the Rhaetian part of the Oyuklu section, we offer the alternative hypothesis that these strata are almost entirely Rhaetian in age. The inferred Rock Point Formation in the Chama Basin may correlate with at least part of the Dinosaur Canyon Member of the Moenave Formation, which is largely Rhaetian in age (Litwin, 1986; Kirkland and Milner, 2006; Lucas et al., 2006a, 2006b; Donohoo-Hurley Figure 15. Photograph of Steamboat Butte in the Dry Cimarron Valley, northeastern New et al., 2010; Lucas et al., 2011). Mexico, showing angular (ang.) unconformity between the Upper Triassic Dockum Forma- A possible lithostratigraphic correlation tion and Middle (M.) Jurassic Entrada Sandstone. View is to the east. of inferred Rock Point strata in northern New Mexico to the predominantly Rhaetian–earliest Hettangian Moenave Formation in Utah and Ari- reflects infilling of topography developed on Kent and Olsen, 2008) may imply that the oldest zona would also call into question the biostrati- deformed Upper Triassic strata. reverse polarity magnetozone in the Moenave graphic utility of the aetosaur genus Aetosaurus, The inferred topographic high may have Formation (in the Dinosaur Canyon Member) long considered a Norian index fossil (Lucas, been related to rifting in the area of the Gulf is equivalent to E23r, a short duration reverse 1998, 2010b), as well as the age assigned to of Mexico, prior to the onset of deposition of polarity zone just below the Triassic-Jurassic these strata based on palynostratigraphy and the Redonda Formation or to dynamic flexure boundary in the Newark Supergroup (Kent and conchostracan biostratigraphy (Litwin, 1986; parallel to the backarc basin formed inland of Olsen, 2008). Litwin et al., 1991; Kozur and Weems, 2007; the Cordilleran arc (Dickinson and Gehrels, If E23r is the youngest reverse polarity mag- Rinehart et al., 2009; Kozur and Weems, 2010). 2008; Dickinson et al., 2010). Alternatively, the netochron before the Triassic-Jurassic boundary In addition, as noted herein, such a correlation, proposed topographic high may be related to and the uppermost strata (inferred Rock Point as proposed, would limit the approach taken by the prerift uplift in Texas proposed by Dickin- Formation) in the Chama Basin are considered Cleveland et al. (2008) to directly correlate the son et al. (2010), although it remains unclear if at least partially age equivalent to the Dinosaur stable isotope record of pedogenic carbonate in detrital zircon populations in uppermost Chinle Canyon Member of the Moenave Formation, the Redonda Formation and inferred Rock Point strata relate to the Ouachita uplift. The corre- as noted by Lucas and Tanner (2007), then we Formation in the Chama Basin. lations of the magnetic polarity chronologies speculate that the Triassic-Jurassic boundary presented here limit the timing of the develop- may be in strata a few meters above the world- CONCLUSIONS ment of such a feature because latest Norian, renowned dinosaur mass death assemblage at and probably earliest Rhaetian, strata (Redonda, Ghost Ranch, the Coelophysis quarry, within Chinle Group strata in the Chama Basin of Owl Rock, and Rock Point) may not have been strata that were previously inferred to be part of north-central New Mexico, the Zuni Moun- deposited in north-central New Mexico. the Rock Point Formation (Lucas et al., 2003, tains of west-central New Mexico, and Mesa The possibility of the preservation of the Tri- 2005; Fig. 13). If this speculation is proven Redonda of eastern New Mexico, allow devel- assic-Jurassic boundary in strata of the inferred valid, then strata hosting the Coelophysis quarry opment of a more complete composite magnetic Rock Point Formation in north-central New were deposited during late Rhaetian time and polarity chronology for the Late Triassic of Mexico should be further explored. Recent thus Coelophysis is Rhaetian in age. the American Southwest. Hematitic mudrocks magnetostratigraphic and paleontologic infor- The dominantly normal polarity strata histori constitute most of the materials sampled in the mation suggests that the older Dinosaur Canyon cally considered to be part of the Rock Point Chama Basin, Zuni Mountains, and eastern Member of the Moenave Formation is largely Formation in northern New Mexico have been New Mexico, and overall yield high-quality Rhaetian in age and that the Triassic-Jurassic lithostratigraphically correlated to the Redonda paleomagnetic data. boundary is within the younger Whitmore Point Formation (in eastern New Mexico; Lucas et al., The magnetostratigraphic information pre- Member of the Moenave Formation, above at 2001), which is dominated by reverse polarity sented here implies that some lithostratigraphic least two short reverse polarity magnetozones, (Reeve and Helsley, 1972; Reeve, 1975; Bazard correlations of Upper Triassic strata in the the younger of which is in the lower part of the and Butler, 1991). The polarity chronology of American Southwest may require revision (e.g., Whitmore Point Member (Donohoo-Hurley the uppermost strata in northern New Mexico is Zeigler et al., 2008). The lower Chinle Group et al., 2010; Lucas et al., 2011). Correlation similar to the Rhaetian part of the Oyuklu section in northern New Mexico is probably late Car- of Moenave strata to the uppermost Newark (Gallet et al., 2007). Lucas (1997) hypothe nian in age, and not equivalent to the Blue Supergroup and the Hartford Basin section (e.g., sized that strata inferred as Rock Point Forma- water Creek–Blue Mesa interval in western

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New Mexico, which is Norian in age, based on Royal Astronomical Society Geophysical Journal, Juan Basin and vicinity, New Mexico: U.S. Geological Irmis and Mundil’s (2008) detrital zircon age v. 88, p. 673–692, doi: 10.1111/j.1365-246X.1987. Survey Bulletin 1801B, 22 p. tb01651.x. Dubiel, R.F., 1989b, Depositional and climatic setting of the determination of ca. 219 Ma for the base of Bazard, D.R., and Butler, R.F., 1989, Paleomagnetism of Upper Triassic Chinle Formation, Colorado Plateau, in the Blue Mesa Member. The Poleo Formation, the Chinle and Kayenta Formations, Arizona and New Lucas, S.G., and Hunt, A.P., eds., Dawn of the age of Mexico; implications for North American Mesozoic the dinosaurs in the American Southwest: Albuquerque, considered equivalent to the Sonsela Member apparent polar wander: Geological Society of America New Mexico Museum of Natural History, p. 171–187. in Arizona and Utah, is exclusively of reverse Abstracts with Programs, v. 21, p. 55. Dubiel, R.F., 1994, Triassic deposystems, paleogeography, polarity, whereas the Sonsela Member is domi- Bazard, D.R., and Butler, R.F., 1991, Paleomagnetism of the and paleoclimate of the western interior, in Caputo, Chinle and Kayenta Formations, New Mexico and Ari- M.V., et al., eds., Mesozoic systems of the Rocky nated by normal polarity (Steiner and Lucas, zona: Journal of Geophysical Research, v. 96, p. 9847– Mountain region, USA: Rocky Mountain Section, 2000). Comparisons of the magnetic polarity 9871, doi: 10.1029/91JB00336. SEPM (Society for Sedimentary Geology), p. 133–168. chronologies for these strata as well as the Tru- Blakey, R.C., and Gubitosa, R., 1983, Late Triassic paleo- Dubiel, R.F., Parrish, J.T., Parrish, J.M., and Good, S.C., geography and depositional history of the Chinle For- 1991, The Pangean megamonsoon—Evidence from jillo Formation of eastern New Mexico with the mation, southeastern Utah and northern Arizona, in the Upper Triassic Chinle Formation, Colorado Pla- recently proposed GPTS for the complete Trias- Reynolds, M.W., and Dolly, E.D., eds., Mesozoic paleo- teau: Palaios, v. 6, p. 347–370, doi: 10.2307/3514963. geography of the west-central United States: Rocky Finch, W.I., and Wright, J.C., 1983, Measured stratigraphic sic (Hounslow and Muttoni, 2010) demonstrate Mountain Paleogeography Symposium 2: Rocky Moun- sections of uranium-bearing Upper Triassic rocks that these units are also not time equivalent. In tain Section, Society of Economic Paleontologists and in eastern New Mexico, West Texas, and the Okla- fact, correlation of magnetic polarity chronolo- Mineralogists, p. 57–76. homa Panhandle with brief discussion of stratigraphic Besse, J. and Courtillot, V., 2002, Apparent and true polar problems: U.S. Geological Survey Open-File Report gies for northern New Mexico with the GPTS wander and the geometry of the geomagnetic field over 83–701, 118 p. suggests that the disconformity at the base of the the last 200 Myr: Journal of Geophysical Research, Fisher, R.A., 1953, Dispersion on a sphere: Royal Society of Poleo Formation must represent at least 13 m.y. v. 107, no. B11, doi: 1029/2000JB000050. London Proceedings, ser. A, v. 217, p. 295–305, doi: Baldwin, B., and Muehlberger, W.R., 1959, Geologic studies 10.1098/rspa.1953.0064. of missing time. Strata inferred to be Rock Point of Union County, New Mexico: New Mexico Bureau Furin, S., Preto, N., Rigo, M., Roghi, G., Gianolla, P., Formation in northern New Mexico, although of Mines and Mineral Resources Bulletin 63, 171 p. Crowley, J.L., and Bowring, S.A., 2006, High-precision Blakey, R.C., 1989, Triassic and Jurassic geology of U-Pb zircon age from the Triassic of Italy: Implications previously considered lithostratigraphically the southern Colorado Plateau, in Jenney, J.P., and for the Triassic time scale and the Carnian origin of cal- equivalent to the Redonda Formation of east- Reynolds, S.J., eds., Geologic evolution of Arizona: careous nannoplankton and dinosaurs: Geology, v. 34, ern New Mexico, do not appear to be time cor- Arizona Geological Society Digest 17, p. 369–396. p. 1009–1012, doi: 10.1130/G22967A.1. Channell, J.E.T., Kozur, H.W., Sievers, T., Mock, R., Aubrecht, Gallet, Y., Krystyn, L., Marcoux, J., and Besse, J., 2007, relative to the Redonda Formation, nor to actual R., and Sykora, M., 2003, Carnian-Norian biomagneto- New constraints on the end-Triassic (Upper Norian– Rock Point strata in southern Utah (Molina- stratigraphy at Silickà Brezovà (Slovakia): Correlation to Rhaetian) magnetostratigraphy: Earth and Planetary Garza et al., 2003). On the basis of their polarity other Tethyan sections and to the Newark Basin: Palaeo Science Letters, v. 255, p. 458–470, doi: 10.1016/ geography, Palaeoclimatology, Palaeoecology, v. 191, j.epsl.2007.01.004. record and the overall absence of firm biostrati- p. 65–109, doi: 10.1016/S0031-0182(02)006545. Golonka, J., 2007, Late Triassic and Early Jurassic paleo- graphic tiepoints, the inferred Rock Point strata Cleveland, D.M., Nordt, L.C., Dworkin, S.I., and Atchley, geography of the world: Palaeogeography, Palaeo- S.C., 2008, Pedogenic carbonate isotopes as evidence climatology, Palaeoecology, v. 244, p. 297–307, doi: in the Chama Basin may be comparable in age for extreme climatic events preceding the Triassic- 10.1016/j.palaeo.2006.06.041. to much of the Dinosaur Canyon Member of the Jurassic boundary: Implications for the biotic crisis?: Gradstein, F., Ogg, J., and Smith, A., 2005, A geologic time Moenave Formation, and thus dominantly late Geological Society of America Bulletin, v. 120, scale 2004: Cambridge, Cambridge University Press, p. 1408–1415, doi: 10.1130/B26332.1. 589 p. Rhaetian in age. Based on this information, it is Clyde, W. C., Hamzi, W., Finarelli, J.A., Wing, S.L., Hall, S.A., and Butler, J.C., 1983, Potential problems in possible that strata hosting the famous Coelo- Schankler, D., and Chew, A., 2007, Basin-wide mag the magnetostratigraphic studies of shallow water physis quarry are latest Rhaetian in age and may netostratigraphic framework for the Bighorn Basin, sequences: Journal of Geology, v. 91, p. 693–705, doi: Wyoming, Geological Society of America Bulletin 10.1086/628820. have been deposited just before the termination v. 119, p. 848-859, doi: 10.1130/B26104.1. Heckert, A.B., and Lucas, S.G., 2003, Triassic stratigraphy of the Triassic. Collinson, D.W., 1965, Depositional remanent magnetiza- in the Zuni Mountains, west-central New Mexico, in tion in sediments: Journal of Geophysical Research, Lucas, S.G., et al., eds., Geology of the Zuni Plateau: ACKNOWLEDGMENTS v. 70, p. 4663–4668, doi: 10.1029/JZ070i018p04663. New Mexico Geological Society Fall Field Conference Collinson, D.W., 1974, The role of pigment and specula- Guidebook, v. 54, p. 245–262. Reviews by two anonymous reviewers served rite in the remanent magnetism of red sandstones: Henry, S., 1979, Chemical demagnetization—Methods, pro- Royal Astronomical Society Geophysical Journal, cedures, and applications through vector analysis: Cana- to greatly improve the organization and focus of v. 38, p. 253–264, doi: 10.1111/j.1365-246X.1974 dian Journal of Earth Sciences, v. 16, p. 1832–1841. the manuscript. Special thanks to Coyote District .tb04119.x. Herrero-Bervera, E., and Helsley, C.E., 1983, Paleomagne- (F. Sanchez, Carson National Forest), Ghost Ranch, Deamer, G.A., and Kodama, K.P., 1990, Compaction- tism of a polarity transition in the Lower (?) Triassic and the Army Corps of Engineers at Abiquiu Dam induced inclination shallowing in synthetic and natural Chugwater Formation, Wyoming: Journal of Geophys- (D. Dutton and E. Garner) for permission to con- clay-rich sediments: Journal of Geophysical Research, ical Research, v. 88, p. 3506–3522. duct sampling. We also thank the Branch family of v. 95, p. 4511–4529, doi: 10.1029/JB095iB04p04511. Herrick, A.S., 1999, Telling time in the Triassic: Biochronol- Coyote, New Mexico, for permission to sample and Dickinson, W.R., and Gehrels, G.E., 2008, U-Pb ages of ogy and stratigraphy of the Chinle Formation in Petri- camp on their land. Field assistance was provided by detrital zircons in relation to paleogeography: Triassic fied Forest National Park, Arizona [M.S. thesis]: Rhode paleodrainage networks and sediment dispersal across Island, University of Rhode Island, 93 p. V. Morgan, J. Stiegler, G. Peacock, P. Zeigler, D. Yeck, southwest Laurentia: Journal of Sedimentary Research, Hester, P.M., and Lucas, S.G., 2001, Lacustrine depositional and D. Chaney. Funding for this research was pro- v. 78, p. 745–764, doi: 10.2110/jsr.2008.088. environments of the Upper Triassic Redonda Forma- vided by the Geological Society of America and the Dickinson, W.R., Gehrels, G.E., and Stern, R.J., 2010, Late tion, east-central New Mexico, in Lucas, S.G., and New Mexico Geological Society. We thank the Pueblo Triassic Texas uplift preceding Jurassic opening of the Ulmer-Scholle, D., eds., Geology of Llano Estacado: of Jemez and the Madalena family for permission to Gulf of Mexico: Evidence from U-Pb ages of detrital New Mexico Geological Society Fall Field Conference examine Chinle Group strata on their tribal lands. We zircons: Geosphere, v. 6, p. 641–662, doi: 10.1130/ Guidebook, v. 52, p. 153–168. thank Linda Donohoo-Hurley for assistance with incli- GES00532.1. Hounslow, M.W., and Muttoni, G., 2010, The geomagnetic nation shallowing corrections. We also thank S.G. Lucas Donohoo-Hurley, L.L., Geissman, J.W., and Lucas, S.G., polarity timescale for the Triassic: linkage to stage 2010, Magnetostratigraphy of the uppermost Triassic boundary definitions, in Lucas, S.G., ed., The Triassic for initially suggesting this project. Roberto Molina- and lowermost Jurassic Moenave Formation, western Timescale: London, Geological Society, Special Publi- Garza commented on an earlier version of this manu United States: Correlation with strata in the United cation 334, p. 61–102. script. Dedicated to the memory of V.L. Morgan Kingdom, Morocco, Turkey, Italy, and eastern United Hunt, A.P., and Lucas, S.G., 1993a, Triassic vertebrate (1945–2010). States: Geological Society of America Bulletin, v. 120, paleontology and biochronology of New Mexico, in p. 2005–2019, doi: 10.1130/B30136.1. 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