A magnetic polarity zonation for the Upper Triassic Chinle Group in the Chama Basin, north-central New Mexico (United States), supplemented by polarity data from eastern and west-central New Mexico (Mesa Redonda and Zuni Mountains, respectively), provides the most complete and continuous magnetic polarity chronology for the Late Triassic of the American Southwest yet available. Most of the Chinle Group sequence is composed of hematitic mudrocks that typically carry a well-defined, well-grouped magnetization (residing in both pigmentary and detrital hematite), with laboratory unblocking temperatures as high as 680 °C. Demagnetization experiments isolate magnetizations of south- or north-seeking declination and shallow inclination, which are interpreted as early acquired, Late Triassic magnetizations. Our proposed polarity correlations, coupled with biostratigraphic observations and recent U-Pb age determinations on detrital zircon–bearing strata in the Chinle Group in western New Mexico, West Texas, and Arizona, indicate that deposition of Chinle strata likely spanned a much shorter time span than previously considered. If this interpretation is correct, the Chinle Group can be correlated with only part of the Newark Supergroup or the Upper Triassic Tethyan sections. On a local scale, lower Chinle strata in the Chama Basin are significantly older than the Bluewater Creek Formation in western New Mexico, and the base of the Poleo Formation represents a disconformity of >13 m.y. duration. Magnetic polarity chronologies from upper Chinle strata in New Mexico and Utah suggest that strata considered to be part of the Rock Point Formation in north-central New Mexico are not time equivalent to type Rock Point strata in Utah or to the Redonda Formation of eastern New Mexico.
The Upper Triassic Chinle Group, prominent in the Mesozoic stratigraphy of the American Southwest, is continental in origin and reflects a complex environment of fluvial, lacustrine, and aggradational fan deposition (Blakey, 1989; Dubiel, 1987, 1989a, 1989b, 1994; Weissmann et al., 2007) with a drainage basin that encompassed most of western North America. Based on vertebrate biostratigraphy, palynostratigraphy, and limited geochronology, the Chinle Group has typically been inferred to span most of the Late Triassic (Stewart et al., 1972; Litwin, 1986; Lucas and Hunt, 1992; Hunt and Lucas, 1993a, 1993b; Steiner and Lucas, 2000; Lucas et al., 2003, 2005; Riggs et al., 2003). The global-scale changes in tectonics and evolution over the time period represented by strata of the Chinle Group emphasize the critical need for accurate correlation of marine and nonmarine Upper Triassic strata on regional and global scales (Olsen et al., 2009; Lucas, 2010a). One means of correlating diverse strata, of both continental and marine affinity, is through magnetostratigraphy.
Early efforts to construct a stratigraphically continuous magnetic polarity chronology for the Chinle Group and to provide key paleomagnetic poles for the Late Triassic primarily involved sampling sandstones and siltstones from independent sections of Chinle strata from different subbasins and outcrop belts, and these data were stitched together based on lithologic and biostratigraphic correlations (e.g., Reeve, 1975; Reeve and Helsley, 1972; Bazard and Butler, 1989, 1991; Molina-Garza et al., 1991, 1993, 1996, 1998a, 1998b, 2003; Steiner and Lucas, 2000). Consequently, the polarity record of the mudstones and claystones, which are the principal rock types in the Chinle Group, is largely unknown.
In our study of Triassic strata in the Chama Basin of north-central New Mexico, we sampled all components of the Chinle Group, with a focus on mudstones and claystones at Coyote Amphitheater, which exposes a nearly complete and continuous section of these rocks, as well as selected subintervals at other nearby localities (Fig. 1). Each stratigraphic datum was sampled with sufficient density (N = five or more independent samples) to allow for robust evaluation of the paleomagnetism of these rocks.
Two additional sections were sampled to test magnetostratigraphic correlations between modern physiographic basins, and to evaluate previous lithostratigraphic and biostratigraphic correlations. The Six Mile Canyon section in the Zuni Mountains (central New Mexico) includes the upper Bluewater Creek Formation and the lower Blue Mesa Member of the Petrified Forest Formation, from which Irmis and Mundil (2008) reported a detrital zircon date of 219.2 ± 0.7 Ma as a maximum depositional age. The Mesa Redonda section (eastern New Mexico) includes the Redonda Formation (Fig. 1), parts of which were originally sampled by Reeve and Helsley (1972), Reeve (1975), and Bazard and Butler (1989, 1991). Here we present a new magnetic polarity chronology for the Upper Triassic Chinle Group in New Mexico and use this new information to test both local and regional lithostratigraphic correlations and to provide tentative correlations of Chinle strata with the Newark Supergroup in eastern North America and Tethyan Triassic strata in Europe.
GEOLOGIC SETTING AND STRATIGRAPHY
Chama Basin, North-Central New Mexico
In general, the Chinle Group consists predominantly of red and purple mudstones, with lesser orange siltstones and buff sandstones and conglomerates. Upper Triassic strata in the Chama Basin of north-central New Mexico have been assigned to five formations (in ascending order): Shinarump (Agua Zarca and Zuni Mountains Formations), Salitral, Poleo, and Petrified Forest Formations and strata interpreted as part of the Rock Point Formation (Lucas and Hunt, 1992; Hunt and Lucas, 1993a, 1993b; Lucas et al., 2003, 2005; Zeigler et al., 2008; Fig. 1 and 2). In Zeigler (2008) and Zeigler et al. (2008), descriptions of the Upper Triassic stratigraphy in the Chama Basin were provided and a revised nomenclature of Upper Triassic strata in New Mexico was proposed. The Shinarump Formation is a color-mottled, coarse-grained quartz sandstone and conglomerate with clasts composed primarily of chert and quartzite, interbedded with green claystones. The Salitral Formation, which conformably overlies the Shinarump Formation, is a brick-red mudstone that is occasionally color mottled (Figs. 2B, 2C). Vertebrate fossil material from the Salitral Formation is interpreted as representative of the Adamanian land vertebrate faunachron (LVF) (Lucas et al., 2003, 2005; Zeigler et al., 2005).
The Poleo Formation consists of sandstone, with intrabasinal calcrete and mud-clast conglomerate being more prevalent in the lower third of the formation (Figs. 2A, 2C). Clasts in the conglomerate beds are mudstone or siltstone rip-ups, calcrete nodules, occasional carbonized plant debris, and rare extrabasinal clasts (chert, quartzite). Both sandstones and conglomerates contain authigenic hematite cement in varying concentrations. The Petrified Forest Formation is the thickest and most widespread unit of the Chinle Group, and was thought to represent primarily overbank deposition (Repenning et al., 1969; Dubiel, 1987, 1989a, 1989b), although the thick mudrock sequence of the Petrified Forest Formation has also been interpreted to reflect aggradational fan deposition (Weissmann et al., 2007; Fig. 2C). Vertebrate fossils of phytosaurs, aetosaurs, and other tetrapods in the Petrified Forest Formation are associated with the Revueltian LVF (Lucas, 1993, 1998, 2010b).
The stratigraphically highest sedimentary rocks in northern New Mexico that are considered Late Triassic in age are those that have been inferred to be part of the Rock Point Formation, and consist of nonbentonitic, reddish-brown massive siltstone and fine sandstone. Stewart et al. (1972) originally coined the term “Siltstone Member” in describing this sequence of strata immediately overlying Petrified Forest strata and below the disconformity with the Middle Jurassic Entrada Sandstone in northern New Mexico. These rocks were lithologically correlated with the type Rock Point Formation in Arizona and the name transferred into the Chama Basin (Lucas and Hunt, 1992; Hunt and Lucas, 1991, 1993a, b; Lucas, 1993; Lucas et al., 2003, 2005). At Coyote Amphitheater, much of the uppermost Chinle Group strata are covered with colluvium from the overlying Entrada Sandstone, making it difficult to observe the uppermost unit(s) of the Chinle Group in most places. Stewart et al. (1972), Lucas and Hunt (1992), and Lucas et al. (2003, 2005) did not record the presence of this sequence of nonbentonitic siltstone and fine sandstone at Coyote Amphitheater, but did note the presence of lithologically similar strata at Ghost Ranch. Dubiel (1989a), however, reported the presence of the Siltstone Member at Coyote Amphitheater. In this study, we sampled the best exposed section of these rocks above the Petrified Forest strata (sites CL 48-60) at Coyote Amphitheater. In addition, outcrops of lithologically similar strata, which have been considered part of the Rock Point Formation at Ghost Ranch (Lucas and Hunt, 1992; Hunt and Lucas, 1991, 1993a, b; Lucas, 1993; Sullivan et al., 1996; Lucas et al., 2003, 2005) were sampled. In this study, we refer to the sequence of nonbentonitic siltstone and fine sandstone that is disconformably overlain by the Entrada Sandstone at Coyote Amphitheater as the “inferred Rock Point” strata.
Zuni Mountains, West-Central New Mexico
Upper Triassic strata in the Zuni Mountains are assigned to the Shinarump (including Zuni Mountains Formation; Zeigler et al., 2008), Bluewater Creek, Sonsela, and Petrified Forest formations. The belt of Triassic outcrops along the northern flank of the Zuni Mountains is separated from Triassic exposures in the Chama Basin by Jurassic and Cretaceous strata downdropped in Laramide monoclines, the Mount Taylor volcanic field and the Sierra Nacimiento, thus precluding direct correlation of lithostratigraphic units. At Six Mile Canyon in the Zuni Mountains, the Bluewater Creek Formation consists of interbedded mudstones and siltstones with scattered calcrete horizons interpreted as floodplain deposits (Heckert and Lucas, 2003; Fig. 2E). The contact between the Bluewater Creek Formation and the overlying Petrified Forest Formation is marked by a 1.5-m-thick, white, tuffaceous sandstone that is the lowest unit of the Blue Mesa Member of the Petrified Forest Formation (Heckert and Lucas, 2003). Irmis and Mundil (2008) reported a detrital zircon date of 219.2 ± 0.7 Ma from this unit. Blue Mesa Member strata above the tuffaceous sandstone consist of mudstones with several discontinuous horizons of calcrete nodules. On the basis of lithologic similarities and similar stratigraphic position, the Bluewater Creek Formation and Blue Mesa Member of the Petrified Forest Formation were considered by Hunt and Lucas (1993b) to be lithostratigraphically equivalent to the Salitral Formation in northern New Mexico.
Mesa Redonda, Eastern New Mexico
Mesa Redonda in eastern New Mexico exposes a 120-m-thick section of reddish-purple to reddish-orange mudstones and siltstones, with occasional sandstone beds (Fig. 2F). Repetitively bedded fine sandstone, siltstone, and mudstone form ribbed cliffs and steep mudrock slopes (Lucas et al., 2001). The Redonda Formation is disconformably overlain by the Middle Jurassic Entrada Formation and overlies the Bull Canyon Formation. Based on its stratigraphic position and lithologic similarity, the Redonda Formation is considered equivalent to the Rock Point Formation (Hester and Lucas, 2001) or to the Owl Rock Formation (Lucas et al., 1987).
Biostratigraphic and Geochronologic Data
Biostratigraphic data for the Chinle Group are largely derived from both vertebrate assemblages and conchostracans. A complete discussion of the rapidly improving biostratigraphic data sets applied to Chinle Group strata is beyond the scope of this paper. Here we list those specific data points we use and refer the reader to the appropriate literature where necessary. Lucas (1997, 1998) developed a series of LVFs based on assemblages of tetrapod vertebrate fossils (phytosaurs, aetosaur, and amphibians) and divided Late Triassic time for the continental southwest into three divisions (oldest to youngest), Adamanian, Revueltian, and Apachean. Lucas (1998, 2010b) argued that these time divisions are nearly isochronous with the Late Triassic marine stages (Carnian, Norian, and Rhaetian). Here we use the LVF assemblage names, but do not utilize Lucas's correlation of these continental age divisions to the marine stages (discussed further below).
Conchostracan data have been obtained from strata in west-central and north-central New Mexico and central Arizona. Kozur and Weems (2010) summarized all conchostracan assemblages from the western United States; we focus on three of their localities, Fort Wingate, the Placerias quarry, and the Coelophysis quarry. In west-central New Mexico, the conchostracans Anyuanestheria wingatella, Laxitextella seegisi, and Howellisaura princetonensis are reported from the Fort Wingate area (western Zuni Mountains) from the lower Bluewater Creek Formation, and the unit is designated as upper-lower Tuvalian (early Carnian). At the Placerias quarry in central Arizona, the lower Blue Mesa Member contains Gregoriusella n. sp. and Laxitextella dorsorecta and is considered late Tuvalian in age. At the Coelophysis quarry in north-central New Mexico, a new species of Anyuanestheria (H. Kozur, 2010, personal commun.) is reported that Kozur and Weems (2010) designated as Sevatian (late Norian), in contradiction to the assumption by Cleveland et al. (2008) that these strata are Rhaetian in age.
Very few palynostratigraphic data are available for Chinle Group strata. In Litwin (1986) and Litwin et al. (1991), palynomorphic data were used to assign a Norian age to upper Chinle strata (Petrified Forest and inferred Rock Point Formations) based on a few samples obtained from only two localities.
Geochronologic data for the Chinle Group are sparse. Available high precision U-Pb ID‑TIMS single zircon isotopic age determinations for the Chinle Group yield a date of 219.2 ± 0.7 Ma (Irmis and Mundil, 2008) for a tuffaceous sandstone near the base of the Blue Mesa member in western New Mexico, a date of 213 ± 1.7 Ma for the Black Forest Bed of the Painted Desert Member (Riggs et al., 1997), and a date of 219.4 Ma for the base of the Blue Mesa Member at Petrified Forest National Park (Ramezani et al., 2009). Initially there was some question as to the stratigraphic position of the sampling interval yielding the 219.4 date at Petrified Forest National Park (Kozur and Weems, 2010). In fact, recent revisions to the stratigraphy of the Chinle Formation at Petrified Forest National Park (Martz and Parker, 2010) suggest that the 219.4 Ma date reported in 2009 (Ramezani et al., 2009) is from a sample from the Sonsela interval. More recently reported high precision absolute age determinations for the Sonsela interval at Petrified Forest National Park indicate that this interval was deposited between ca. 219 and 213 Ma and that the base of the Blue Mesa Member, as defined stratigraphically in northern Arizona, is ca. 225 Ma (Ramezani et al., 2010). Dickinson and Gehrels (2008), using the lower precision U-Pb LA-ICPMS method, report a maximum depositional age of ca. 215 Ma for the Sonsela Sandstone in northern Arizona and the Poleo Formation in the Chama Basin and a range of inferred depositional ages between 219 Ma and 226 Ma for the Trujillo Formation of eastern New Mexico.
Pipiringos and O'Sullivan (1978) described a series of inferred regional unconformities in Triassic and Jurassic strata on the Colorado Plateau. In the Triassic part of the sequence, they identified the Tr-1 and Tr-2 unconformities as at the base and within Lower–Middle Triassic strata (Moenkopi Formation and equivalent units), respectively. The Tr-3 unconformity is identified as at the base of the Upper Triassic section (Chinle Group and equivalents; Pipiringos and O'Sullivan, 1978). Lucas (1991, 1993) defined two more unconformities, the Tr-4 and Tr-5, within the Upper Triassic section that were also considered of regional extent. The Tr-4 is defined as occurring at the base of the Sonsela interval in Arizona, the Poleo Sandstone in northern New Mexico and/or the Trujillo Sandstone in eastern New Mexico (Lucas, 1991, 1993). May (1988), Herrick (1999), Woody (2006), Martz (2008), and Martz and Parker (2010) argued that the Tr-4 unconformity is not a regional feature. Here we follow previous work and do not consider the Tr-4 unconformity to be of regional extent. The Tr-5 unconformity is identified as occurring at the top of the Owl Rock Formation in Arizona and at the base of the inferred Rock Point Formation in New Mexico. As we suggest herein, a possible revision to the age of the youngest inferred Triassic strata in northern New Mexico may alter the definition of this unconformity.
SAMPLING AND METHODS
The entire Chinle Group section was sampled at Coyote Amphitheater (36.219°N, 106.631°W; Figs. 3A, 3E), from the base of the Shinarump Formation where it overlies the Lower Permian Cutler Group, to the uppermost exposures of strata interpreted as Rock Point Formation (Dubiel, 1989a). A sampling (site) interval of ∼3 m was used for a total of 82 sites (∼230 m of strata), except for Poleo strata, which were sampled at the individual bed level (site sampling interval varied from 2 to 5 m; Fig. 2C). As a simplistic, initial approach, if we assume both a constant sedimentation rate for the Petrified Forest Formation and the lack of appreciable hiatuses, a 3 m sampling interval is equivalent to a maximum of 0.25 m.y. time duration between sites [if the Carnian-Norian boundary is slightly younger than 230.91 Ma (Furin et al., 2006) and the Norian-Rhaetian boundary is ca. 203.6 Ma (Gradstein et al., 2005)]. Our estimate assumes that the entire 234-m-thick Chinle section at Coyote Amphitheater was deposited over ∼30 m.y. duration, to near the end of the Triassic, which may be an unrealistically long period of time, as discussed herein. The recent high-precision U-Pb age determination on Bluewater Creek Formation strata in the Zuni Mountains raises the possibility that no lower Chinle Group strata in the Zuni Mountains are Carnian in age. We infer that an ∼3 m sampling interval may be equivalent to considerably <0.25 m.y. duration between sites.
The Salitral Formation was also sampled at a 3 m interval (7 sites total) at exposures in the county landfill outside of Youngsville (36.185°N, 106.583°W; Fig. 3B). An additional section of subhorizontal strata of the Shinarump and Poleo Formations was sampled at Abiquiu Dam (36.236°N, 106.425°W; Fig. 3C), with a site spacing of ∼2 m (32 total sites). In the upper part of the Poleo Formation, the sampling interval was dictated by bed thickness, so that each site was a discrete bed, whereas parts of the lower third of the Poleo Formation and the Shinarump Formation are thick bedded, so that some intervals included multiple sites. A second, composite section of strata inferred to be part of the Rock Point Formation was sampled at two sublocalities near Ghost Ranch. There were 16 sites established at the Coelophysis quarry (36.338°N, 106.464°W; Fig. 3D) and near U.S. Highway 84 (36.307°N, 106.448°W), south of Ghost Ranch.
At Six Mile Canyon in the western Zuni Mountains of west-central New Mexico (35.441° N, 108.483° W; Fig. 3F), an ∼15-m-thick section of Bluewater Creek Formation and Blue Mesa Member strata was sampled at ∼1.5 m intervals just above and below the white tuffaceous sandstone (basal Blue Mesa Member) for a total of 10 sites. At Mesa Redonda in eastern New Mexico (34.997° N, 103.703° W), samples were obtained every ∼3 m for a total of 31 sites that encompassed most of the Redonda Formation over a thickness of ∼75 m. Approximately 9 m of section in the middle of the Redonda Formation could not be sampled due to thick colluvial cover.
Samples from well-indurated sandstone and siltstone beds (Poleo, Shinarump and/or lower Salitral, and upper Rock Point Formations) were obtained by drilling with a water-cooled diamond drill bit, and six to eight independently oriented core samples were taken. Samples were typically prepared into multiple specimens (2.2-cm-high right cylinders) for demagnetization. Most sampling, however, was of mudstone or poorly consolidated siltstone (upper Salitral, Petrified Forest, lower inferred Rock Point Formations). For these sites, fresh, coherent material was uncovered by digging at least 0.5 m into the exposure. A block sampling method similar to that of Johnson et al. (1975) was used to obtain typically six to eight oriented blocks at a site (single datum). Blocks yielded 1–7 individual specimens, prepared by dry sawing with a nonmagnetic diamond blade, from ∼1.0–3.0 cm3 in volume.
Progressive thermal demagnetization was applied to most specimens (drilled or block sampled) to investigate the character of the natural remanent magnetization (NRM) in these materials. For sufficiently indurated samples, duplicate specimens were treated with chemical demagnetization, as described in Henry (1979). All measurements of the NRM were made using a 2G Enterprises Model 760R superconducting rock magnetometer, equipped with DC SQUIDS (superconducting quantum interference device), with a magnetic moment noise level of ∼1.0–3.0 × 10−12 Am2. At least one specimen per sample was subjected to thermal demagnetization, utilizing either a Shaw Magnetic Measurements Thermal Demagnetizer (MMTD) or an ASC 48 thermal demagnetizer.
Progressive demagnetization data were analyzed using the principal component analysis approach of Kirschvink (1980) and individual linear segments, usually defined by four to six data points, were accepted if maximum angular deviation (MAD) values were <10°. Where data were anchored to the origin, results with MAD values <15° were accepted. Bulk magnetic susceptibility was measured for duplicate specimens from most sites using a Kappabridge KLY4S instrument. Estimated site mean directions were obtained following the methods of Fisher (1953) and were termed excellent if the 95% confidence parameter, α95, is ≤ 10°, good if α95 is 10°–20°, salvageable if α95 is 20°–30° yet the magnetizations clearly had a Late Triassic affinity, and unacceptable if α95 was >30° (Fisher, 1953).
Acquisition of isothermal remanent magnetization (IRM) to saturation (SIRM) or near-saturation and backfield DC demagnetization of SIRM experiments was conducted with a home-built pulse magnetizer that provided a field up to 2.97 T, capable of nearly saturating most assemblages of hematite grains. Thermal demagnetization of IRM acquired in DC fields of 2.97 T, 0.3 T, and 0.03 T along three orthogonal axes (Lowrie, 1990) was also conducted on representative specimens. Several mudrock specimens of differing shades of red, purple, and orange were dissolved in reagent-grade hydrochloric acid and the residue was mixed with high purity alumina cement and subjected to IRM acquisition experiments to attempt to assess relative abundances of detrital and pigmentary hematite.
Samples from the Shinarump Formation typically yield uninterpretable demagnetization behavior and have average NRM intensities of ∼1.2 mA/m for Abiquiu Dam and ∼2.8 mA/m for Coyote Amphitheater material. Although some specimens collected from the Shinarump Formation at Coyote Amphitheater yield interpretable demagnetization behavior (Figs. 4A, 5A), more often than not, the inclination of the characteristic remanent magnetization (ChRM) isolated is too steep to be of Late Triassic age, and likely was acquired at a considerably younger time (e.g., site RR1, mean of declination, D = 352.8°, inclination, I = 58.1°, α95 = 9.1°, k = 102.9, number of samples accepted out of total number of samples sampled, N/No = 4/6). Of the seven sites established in Shinarump strata at Coyote Amphitheater, only three sites at the top of the Shinarump show moderately well behaved demagnetization response and yield south-directed and shallow inclination ChRMs (reverse polarity). The magnetization is of distributed laboratory unblocking temperatures between 300 and about 615 °C, and decays linearly to the origin.
Salitral Formation strata at Coyote Amphitheater typically yield well-defined demagnetization behavior showing the isolation of both normal and reverse polarity ChRMs with average NRM intensities of ∼1.9 mA/m (Figs. 4B, 4D, and 5B). The magnetization is of distributed laboratory unblocking temperatures between 300 and at least 620 °C, and decays linearly to the origin. Maximum unblocking temperatures are above 615 °C, but demagnetization response is often unstable above this temperature. The in situ grand mean for sites at Coyote Amphitheater is D = 007.0°, I = 3.4°, α95 = 8.4°, k = 120.3, N/No = 4/6 sites collected (bedding-corrected grand mean: D = 005.8°, I = 3.8°, 2 sites normal polarity, 2 sites reverse polarity). These four sites alternate polarities, whereas the remaining two yield uninterpretable results. The means for sites at Coyote Amphitheater are summarized in Table 1 (and the Supplemental Text File1). Two sites in the Salitral Formation at Youngsville landfill yield interpretable data, and had average NRM intensities of ∼2.0 mA/m. Site 3 is of normal polarity (Fig. 4C), and site 2 is of reverse polarity (Table 1). The five rejected sites have high dispersions (k values range between 1.96 and 51.4) and magnetizations with inclinations too steep to be of Late Triassic age.
Poleo Formation strata at Coyote Amphitheater typically yield well-defined, linear demagnetization trajectories (Figs. 4E, 4F, and 5C), and ChRMs of reverse polarity with NRM intensities averaging 2.3 mA/m. The ChRM is overprinted by a small north-directed, steep positive inclination component. The in situ grand mean for the Poleo Formation is D = 179.7°, I = –5.2°, α95 = 6.0°, k = 85.67, N/No = 8/14 sites (bedding corrected grand mean: D = 184.4°, I = –4.1°). All sites in the Poleo Formation at Coyote Amphitheater yield magnetizations of reverse polarity, including those sites with considerably higher dispersion.
The Poleo Formation section at Abiquiu Dam typically yields well-defined, well-grouped ChRMs of exclusively reverse polarity, with NRM intensities of ∼1.0 mA/m and demagnetization behavior similar to that of specimens from Coyote Amphitheater (Figs. 4G, 4H, and 5C). The magnetization exhibits somewhat discrete laboratory unblocking temperatures between 600 and 670 °C, and decays linearly to the origin. For duplicate specimens from the same sample, a similar direction is isolated using either thermal or chemical demagnetization. Of the 30 sites collected, 20 site means have associated α95 values <10° and provide a grand mean direction of D = 183.1°, I = 0.3°, α95 = 5.7°, k = 33.9, N/No = 20/30 sites (the beds are flat-lying, thus not requiring tilt correction). For Poleo sites yielding well-defined demagnetization results, specimens subjected to chemical demagnetization show an ∼80% decrease in NRM intensity, and become nearly white in color, after immersion for a total of 370 hours, with no statistically significant change in direction (Fig. 4H).
Petrified Forest Formation
Specimens from most stratigraphic levels (sites) in mudstones exposed over ∼132 m in the Petrified Forest Formation at Coyote Amphitheater yield well-defined and well-grouped ChRMs with NRM intensities of ∼7.1 mA/m (Figs. 6A–6C and 7A). The magnetization is of distributed laboratory unblocking temperatures between ∼300 and 670 °C, and decays linearly to the origin. For these strata, 39 of the 45 sites (all in mudstone levels) yield reliable site mean data and these provide a corrected grand mean of D = 182.1°, I = 4.2°, α95 = 2.7°, k = 75.4, N/No = 39/45 sites. All samples from the remaining six sites yielded uninterpretable magnetizations. The normal polarity stratigraphic grand mean direction is D = 359.2°, I = –3.4°, α95 = 4.2°, k = 90.7, N = 14 sites, and the reverse polarity stratigraphic grand mean direction is D = 183.7°, I = 4.6°, α95 = 3.4°, k = 72.2, N = 25 sites. Most sites are of reverse polarity, with normal polarities observed in restricted intervals in the lower, middle, and upper parts of the section.
Inferred Rock Point Strata
Specimens from 6 of 13 sampled sites established in strata inferred to be part of the Rock Point Formation at Coyote Amphitheater yield well-defined and relatively well grouped ChRMs and average NRM intensities of ∼2.8–6.4 mA/m (Figs. 6F, 6G, and 7B). The magnetization is of distributed laboratory unblocking temperatures between ∼300 and 670 °C, and decays linearly to the origin. Five of the accepted site means are of normal polarity and one, at the base of the section, is of reverse polarity (Table 1; see footnote 1). The in situ (geographic) grand mean direction is D = 015.6°, I = 1.8°, α95 = 11.1°, k = 37.2, N/No = 6/13 sites (stratigraphic grand mean direction: D = 014.6°, I = –16.7°). The seven sites not included in the calculation of the grand mean direction yield completely incoherent magnetizations.
At Ghost Ranch, 15 sites were established in subhorizontal strata that also have been inferred to be part of the Rock Point Formation. Seven of these sites yield relatively well defined and well-grouped magnetizations of dominantly normal polarity (5 normal, 2 reverse; Figs. 6H and 7C). The in situ grand mean for these strata at Ghost Ranch is D = 017.3°, I = 6.0°, α95 = 7.7°, k = 52.8, N/No = 7/15 sites. Sites excluded from the grand mean direction yield either incoherent magnetizations or relatively scattered magnetizations of mixed polarity.
Six Mile Canyon: Bluewater Creek and Lower Petrified Forest Formations
Samples from 7 of the 10 sites established in the Bluewater Creek Formation and Blue Mesa Member in the Zuni Mountains yield well-defined and relatively well grouped ChRMs, and these are predominantly of reverse polarity with an average NRM intensity of 4.8 mA/m (Figs. 4I and 5D). The magnetization is of distributed laboratory unblocking temperatures typically between ∼300 and 645 °C, with maximum unblocking temperatures of 670 °C. The seven accepted sites provide a grand mean direction of D = 177.6°, I = –3.0°, α95 = 10.4° and k = 34.5, N/No = 7/10. Two of the three sites excluded from the grand mean yield magnetizations with north-directed and moderate to steep inclinations, probably reflecting a younger age of magnetization acquisition. Sites in the lower Blue Mesa Member, all sampled above the zircon-bearing horizon reported by Irmis and Mundil (2008), are all of reverse polarity. Sites in the Bluewater Creek Formation below the zircon-bearing horizon are also of reverse polarity, except for the uppermost site in this unit.
Mesa Redonda: Redonda Formation
Specimens from the mudrock-dominated lower Redonda Formation, and some siltstone beds in the upper part, yield moderately well defined and well-grouped ChRMs of dual, but dominantly reverse polarity (Figs. 6D, 6E, and 7D). The ChRM is of discrete laboratory unblocking temperatures between 640 and 670 °C, and decays linearly to the origin. This magnetization is revealed after removal of a prominent overprint that is north directed and of steep positive inclination. Average NRM intensities are ∼5.1 mA/m for the siltstones and fine sandstones and ∼1.9 mA/m for mudrock-dominated units. The overall in situ grand mean direction for the Redonda Formation is: D = 183.2°, I = –3.5°, α95 = 5.7° and k = 83.4, N/No = 9/26, with 3 normal polarity and 6 reverse polarity sites. The normal polarity mean is D = 001.1°, I = 2.1°, α95 = 11.3° and k = 120.8, N = 3 sites, and the reverse polarity mean is D = 184.3°, I = –4.2°, α95 = 8.3° and k = 66.8, N = 6. All other sites from the Redonda Formation are characterized by incoherent magnetizations.
Thermal demagnetization of three component IRM for specimens from Chinle Group strata shows that the high coercivity (3.0 T) component is the highest intensity and highest laboratory unblocking temperature component, for most samples, regardless of rock type (Figs. 8A–8F). Laboratory unblocking temperatures of the 3.0 T IRM for specimens of sandstone from the Shinarump Formation and mudrock from the Salitral Formation range between 630 and 655 °C (Figs. 8A, 8B). For samples from the uppermost Shinarump Formation, the 3.0 T IRM shows a distributed range of laboratory unblocking temperatures (from ∼300 to 630 °C; Fig. 8C). In all samples, the low coercivity IRM (<0.03T) has a very small to insignificant contribution.
For Poleo strata sampled at Abiquiu Dam (Fig. 8D), most specimens show that the 3.0 T IRM component has the highest intensity, with laboratory unblocking temperatures above 660 °C. Sandstones have a substantial (as high as 30%) intermediate coercivity IRM, which is largely unblocked by 580 °C. Mudrock specimens from the Petrified Forest Formation in the Chama Basin (Fig. 8E) and from the Bluewater Creek Formation in the Zuni Mountains show that the high-coercivity IRM always dominates the magnetization and is fully unblocked above 650 °C. The intensity of both lower coercivity IRMs in these rocks is typically at least one order of magnitude less than the 3.0 T IRM. Mudstone and sandstone samples from inferred Rock Point strata show that the high-coercivity IRM dominates and is characterized by a distributed range of laboratory unblocking temperatures from 300 to 630 °C. Mudstone and sandstone of the Redonda Formation show similar responses, with a distributed range of laboratory unblocking temperatures for the 3.0 T IRM (Fig. 8F).
IRM acquisition response for mudrock specimens from the Salitral, Petrified Forest, and inferred Rock Point Formations shows a concave-up response, with specimens gaining no more than 20% of the maximum IRM at inductions of 0.5 T (Fig. 8G). Between ∼0.8 and 1.2 T, acquisition curves rise steeply, and reach near saturation at ∼3.0 T. In backfield demagnetization, the mudstone specimens exhibit a marked inflection, suggesting two phases with distinct coercivities, and an ultimate demagnetization in fields (coercivity of remanence, Hcr) between 0.25 and 1.2 T. Sandstone specimens from the upper Shinarump Formation and Poleo Formation show a rapid (concave downward) acquisition of isothermal magnetization over a range of low fields, acquiring 70%–90% of the total IRM below 0.5 T (Fig. 8H). Samples approach saturation at ∼3.0 T and backfield DC demagnetization in these specimens reveals a coercivity of remanence <0.4T.
Overall, IRM acquisition and IRM three-component thermal demagnetization results are consistent with hematite as the dominant magnetic phase, although most sandstones show a significant contribution from a lower coercivity phase. Based on laboratory unblocking temperature information, the lower coercivity phase is interpreted to be a mixture of magnetite and maghemite, which could yield laboratory unblocking temperatures above 600 °C. Mudstones from the Redonda, inferred Rock Point, and upper Shinarump Formations have demagnetization characteristics that suggest that very fine grained pigmentary hematite is a significant contributor to the remanence. This contribution is less important in sandstones of the Poleo Formation and in the mudstones of the Petrified Forest Formation. Petrographic observations of selected Chinle Group rocks confirm these inferences. Thin sections of mudrock samples from Coyote Amphitheater show an abundance of fine-grained hematite, both as detrital specularite grains and as thin pigmentary coatings on quartz grains (Fig. 9).
Bulk magnetic susceptibilities of sandstone-dominated strata (Shinarump, Poleo, and upper Redonda Formations) are lower than finer-grained hematitic mudstones and siltstones (Salitral, Petrified Forest, lower Redonda, and inferred Rock Point Formations) (Table 2). The Shinarump Formation has the lowest bulk susceptibility values of all Chinle Group strata sampled, and the highest values are from the upper Poleo Formation and the mudstones of the Petrified Forest Formation. The Poleo Formation records an abrupt change in bulk susceptibility from relatively low (30–120 × 10−6) to higher values (70–225 × 10−6) ∼21 m above its base.
For each formation, magnetic susceptibility values show little correlation with the overall quality of the remanence signature, as defined by demagnetization behavior and dispersion of the ChRM at a site level. For example, specimens from three sites with the lowest bulk susceptibility and three with the highest in the Petrified Forest Formation all exhibit nearly univectorial demagnetization of a well-grouped ChRM of reverse polarity. Specimens of high bulk magnetic susceptibility tend to be very pale red and show some sedimentary structures (laminations or intraformational grains), though there are other horizons with similar features that do not yield high bulk susceptibility values. Medium-grained sandstone has lower bulk susceptibility values than fine sandstone or mudstone samples. Mudstones, on average, have much higher bulk susceptibility values than either medium- or fine-grained sandstones.
Magnetic Polarity Zonation
From oldest to youngest strata, we summarize the polarity record of Chinle Group strata obtained in this study. The Shinarump Formation at Abiquiu Dam typically yields either incoherent magnetizations or a north-directed, steeply inclined magnetization unblocked by 150–210 °C that probably reflects complete remagnetization at a younger time. In the Chama Basin, Shinarump strata have been interpreted as a paleovalley fill sequence (Dubiel, 1987, 1989a) dominated by coarse to very coarse grained sandstone with intense color mottling and silcrete lenses. The intense color mottling and associated diagenetic phases suggest postdepositional fluid alteration of Shinarump strata, consistent with the pervasive remagnetization documented in this study. Only the uppermost, medium- to fine-grained sandstone interval of the Shinarump at Coyote Amphitheater records a magnetization consistent with a Late Triassic age, and these samples are of reverse polarity (Fig. 10C).
Salitral Formation strata at Coyote Amphitheater typically yield well-defined and well-grouped magnetizations of mixed polarity (Figs. 10B, 10C). Sites at Youngsville yield lower-quality data, although still of mixed polarity. Salitral Formation strata yielding incoherent magnetizations may reflect intense diagenetic and/or pedogenic modification, as they are commonly color mottled, bioturbated, and lack preservation of original sedimentary structures.
Poleo Formation strata at both Coyote Amphitheater and Abiquiu Dam yield well-defined ChRMs that are almost exclusively of reverse polarity (Figs. 10A, 10C), with the exception of sites developed in conglomeratic horizons. These sites yield uninterpretable results.
The Petrified Forest Formation at Coyote Amphitheater is characterized by several polarity magnetozones, with individual samples and discrete horizons usually yielding internally uniform, well-defined ChRMs (Fig. 10C). Levels providing incoherent magnetizations tend to be intensely color mottled and bioturbated. Despite the heavy reliance on such fine-grained materials for polarity information for this part of the Chinle Group, the paleomagnetism of these strata is of very high quality. Of the 45 sites established in mudrock horizons within the Petrified Forest Formation, 39 of these provide acceptable site means with α95 values <15°.
Strata inferred to be Rock Point Formation at both Coyote Amphitheater and Ghost Ranch are predominantly of normal polarity (Figs. 10C, 10D). At Coyote Amphitheater, a single site of reverse polarity is in strata immediately above the inferred Rock Point–Petrified Forest Formation contact. Several sites yield completely incoherent magnetizations, which may reflect pedogenic modification of some stratigraphic levels. The thick paleosol dominated sequence sampled near Ghost Ranch yields several sites that are dominated by completely incoherent magnetizations, although two intervals of reverse polarity were identified, one of which is in strata directly above the Coelophysis quarry.
The Six Mile Canyon section, north-central Zuni Mountains, yields a predominantly reverse polarity magnetozone (Fig. 10E). The site immediately below the tuffaceous sandstone that provided the detrital zircon date reported by Irmis and Mundil (2008) shows intense pedogenic modification with strong color mottling and distorted bedding, and yields poorly defined shallow inclination magnetizations that are both north and south seeking.
The Mesa Redonda section in eastern New Mexico consists almost entirely of the Redonda Formation and yields moderately well defined, well-grouped ChRMs in the lower two-thirds of the section (Fig. 10F). Only a few sites in the upper third provide interpretable polarity data. Overall, our results are consistent with those previously reported by Reeve (1975) and Reeve and Helsley (1972), in that the Redonda Formation is characterized by predominantly reverse polarity with two stratigraphically narrow, well-defined normal polarity magnetozones.
Paleomagnetism and Antiquity of Magnetization
Most Chinle Group strata examined at localities in northern, western, and eastern New Mexico yield internally consistent paleomagnetic behavior, with ChRMs interpreted to be of Late Triassic age and of primary or near-primary origin. The inclination of the ChRM is shallower than expected magnetizations derived from paleomagnetic poles of late Early Jurassic or younger age for North America (Van der Voo, 1993; Besse and Courtillot, 2002; Molina-Garza et al., 2003; Kent and Irving, 2010), and this supports a Triassic age of ChRM acquisition. Approximately 60% of the site mean results are considered to be of excellent or good quality, and therefore relatively high fidelity with low within-site dispersion (k >> 20), and are statistically acceptable recorders of a Late Triassic field. Data from the Salitral, Petrified Forest, and Redonda Formations are sufficiently robust in terms of total number of dual polarity ChRMs to pass the reversal test of McFadden and McElhinny (1990). Petrified Forest Formation strata yield a positive, class A, reversal test with 25 reverse polarity sites and 14 normal polarity sites, with an angle between observed means of 4.6°. Salitral Formation strata at Coyote Amphitheater are characterized by three sites of reverse polarity and three of normal polarity and pass a reversal test with an angle of 11.9° (class C), and Redonda Formation strata pass with an angle of 3.8° between observed means, with six sites of reverse and three sites of normal polarity (class A). Reversal tests could not be carried out on Bluewater Creek, Poleo, or inferred Rock Point strata, as the accepted site mean ChRMs for these formations are dominated by a single polarity.
The combination of IRM acquisition data, response to both thermal and chemical demagnetization, and three-component IRM thermal analyses indicates that much of the ChRM in these rocks is carried by pigmentary hematite, with lesser contributions by fine-grained detrital hematite and magnetite. Rocks where the high-coercivity IRM is dominant, and shows a wide range of elevated laboratory unblocking temperatures in three component thermal analysis, typically provide the highest quality and most interpretable demagnetization results, as well as the best-defined site mean directions. Although Chinle Group strata overall yield well-defined and well-grouped magnetizations, many specific intervals (e.g., Shinarump Formation mudstones) exhibit considerable pedogenic modification and yield no coherent remanence. We rule out the possibility that the entire Upper Triassic section was subjected to a younger basin-wide chemical remagnetization after deposition because of the presence of dual polarity magnetozones, some of which can be correlated across the Chama Basin, as discussed in the following.
Paleosols are common components of Chinle Group strata (Parrish et al., 1982; Dubiel, 1987, 1989a, 1989b; Dubiel et al., 1991; Parrish, 1993; Mack et al., 1993; Therrien and Fastovsky, 2000; Tanner, 2003, and references therein; Cleveland et al., 2008). The observed correlation between pedogenically modified strata and poor to uninterpretable demagnetization response contrasts with results from paleosol horizons in some types of sedimentary deposits where paleosol magnetizations are well defined and often of higher quality than surrounding strata (e.g., loess and shallow basinal deposits) (Wang et al., 2005; Clyde et al., 2007). Liu et al. (2006) demonstrated that hematite was less likely to be modified during pedogenic processes than other early formed phases such as goethite, thus prompting the question of what process results in incoherent remanent magnetizations in pedogenically modified Chinle strata, even if characterized by abundant hematite. In the case of Chinle Group strata, we infer that deposition of detrital hematite grains led to an early acquisition of a magnetic remanence that was enhanced to some degree by later stage pigmentary hematite cement. Any disturbance of these sediments after deposition by bioturbation and/or pedoturbation could have disrupted the alignment of detrital hematite grains, thus distorting the ChRM vector.
Incoherent demagnetization behavior is typically associated with both conglomerates and fine-grained strata that are color mottled and show evidence of paleosol formation. Mud rip‑up clast conglomerates (common in the Poleo Formation) contain clasts of a range of colors and yield incoherent magnetizations, likely reflecting early acquisition of remanence in each clast and a very heterogeneous remanence. Sandstones, siltstones, and mudrocks with color mottling, bioturbation, and obvious disruption of sedimentary structures, as well as those that are white or pale green yield either moderate quality or incoherent demagnetization behavior. We subjected several specimens exhibiting poor demagnetization behavior to leaching in reagent grade (12.1 M) HCl for as long as 500 h. Each residue was dispersed in high-purity alumina cement and subjected to IRM acquisition experiments. Saturation was reached well below 0.8 T (Fig. 11), indicating the presence of a fine-grained, low-coercivity phase in the residue. The low-coercivity phase is interpreted to be magnetite and of detrital origin. Although magnetite has a higher dissolution rate in concentrated HCl than hematite (Sidhu et al., 1981), its preservation is interpreted to reflect a substantial difference in grain size between magnetite and hematite, as well as hydrated iron oxides.
In general, dark purple, dark red, and brick-red mudstone and claystones all yield excellent response to thermal demagnetization. Brown and buff massive as well as cross-laminated siltstones and buff and red, fine- to medium-grained sandstones also typically yield a ChRM of sufficient quality for an unambiguous polarity determination and estimate of a bed-level mean direction. In studies of red beds, numerous workers (e.g., Herrero-Bervera and Helsley, 1983; Shive et al., 1984; Bazard and Butler, 1991) concluded that the finest grained hematitic detrital rocks tend to be better recorders of the magnetic field and the NRMs are less complex than those of substantially coarser grained equivalents with both authigenic and detrital hematite. Bazard and Butler (1991) hypothesized that the simpler NRMs may reflect lower permeability and thus less prolonged diagenetic effects.
The relative abundance of detrital hematite grains versus other magnetic phases (including authigenic pigment hematite) has been a subject of considerable interest (Collinson, 1965, 1974; Tauxe et al., 1980) and clearly influences the demagnetization behavior of red beds. To assess the relative abundance of detrital grains in selected parts of the Chinle Group, specimens from the Petrified Forest, Redonda, and inferred Rock Point Formations were crushed to sub-0.5 cm3 fragments and leached in reagent grade (12.1 M) HCl until completely disaggregated. The residue was mixed in ultrahigh-purity alumina cement and subjected to IRM acquisition. After ∼24–48 h of leaching, the residue from red and purple mudstone is only slightly paler than the original specimen, and the response of the residue to IRM acquisition is essentially identical to that of the bulk specimen, with saturation by 3.0 T. The experiment shows that both fine-grained, authigenic hematite, as revealed in petrographic examination (Fig. 9), and detrital hematite are important magnetic components in these rocks and that magnetite is only present in small concentrations. For lighter orange mudstones and siltstones, the leached residue is pale green and shows IRM saturation below 0.8 T (Fig. 11). The orange color is due primarily to fine-grained hydrated iron oxides and some pigment hematite, which are both leached in the experiments, leaving a fine-grained low-coercivity phase, which we interpret to be magnetite. As with experiments described herein, the preservation of magnetite is assumed to reflect a substantially coarser grain size for this low-coercivity phase.
Paleolatitudes and Inclination Shallowing
Stratigraphically corrected grand mean directions (Table 1) from all formations of the Chinle Group place northern New Mexico at near-equatorial latitudes (ranging from 0.2°N to 8.5°N) during the Late Triassic (Table 3), consistent with paleogeographic reconstructions that suggest deposition of Upper Triassic strata in the Four Corners between lat 5°S and 15°N (e.g., Ziegler et al., 1983; Blakey and Gubitosa, 1983; Golonka, 2007). Paleolatitudes calculated for strata of the inferred Rock Point Formation are farther north than those calculated for Salitral through Redonda Formation strata, although the difference is not statistically distinct. In the initial absence of any consideration of the possibility of inclination shallowing, there is no statistically significant temporal progression in estimated mean inclinations and thus paleolatitudes among these results.
Considerable research has convincingly demonstrated that inclination shallowing affects the remanence of most if not all sedimentary rocks (e.g., Anson and Kodama, 1987; Deamer and Kodama, 1990; Tan et al., 2002, 2007; Kent and Tauxe, 2005), resulting in erroneous (shallower) paleolatitude estimates, thus affecting paleogeographic reconstructions and definitions of apparent polar wander paths. Inclination shallowing can affect sedimentary sequences where the remanence is carried by detrital hematite (Tan and Kodama, 2002; Tan et al., 2003; Kent and Tauxe, 2005); however, it is not certain to what extent any remanence recorded by authigenic, grain-coating hematite is susceptible to the same processes that result in inclination shallowing. In addition, the net effect of inclination shallowing is minimized in rocks deposited at very low latitudes.
Chemical demagnetization has long been utilized to isolate a remanence residing in very fine grained, authigenic hematite (e.g., Collinson, 1965; Park, 1970). Hematitic sandstone samples of the Poleo Formation show no statistically significant change in direction after cumulative chemical demagnetization of 370 h and a decrease of at least 80% in NRM intensity (e.g., Fig. 4H). The Poleo samples are slightly lighter in color after treatment, but are not completely bleached after more than 300 h of leaching. Although pigment hematite is leached in this process, resulting in a decrease in NRM intensity, the lack of change in direction during demagnetization suggests that detrital (specular) hematite also is an important contributor to the ChRM in these sandstones and that detrital and pigmentary hematite carry essentially the same overall direction.
For all samples from the Petrified Forest and Poleo Formations that provide line segments with MAD values <10°, we used the inclination shallowing approach of Kent and Tauxe (2005) to correct for potential flattening of inclination (Table 4). For the data from both formations, in spite of a substantial flattening factor, the observed inclination is not statistically different from the corrected inclination. For the other units of the Chinle Group, we did not have a sufficient number of independent specimen directions to apply the Kent and Tauxe (2005) approach. Kent and Tauxe (2005) also provided data supporting a rapid northward drift of North America during the Late Triassic, a result that is not apparent in the available data from the Chinle Group. Further discussion of this discrepancy is beyond the scope of this contribution.
Magnetostratigraphy: Local Correlations
Within the Chama Basin, parts of the Chinle Group can be correlated based on magnetostratigraphic data obtained in this study (Figs. 10 and 12). The Salitral Formation at Coyote Amphitheater contains three normal polarity magnetozones (ca1n, ca2n, and ca3n) and three of reverse polarity (ca1r, ca2r, and ca3r). At the Youngsville locality, the Salitral Formation contains two intervals of normal polarity (yo1n, yo2n), one interval of reverse polarity (yo1r), and one interval of uninterpretable magnetization within the lower normal polarity interval (yo1n). Both sections display normal polarity at the base of the section and normal polarity in the middle part. The uppermost part of the section was not sampled at Youngsville, but a reasonable correlation can be made between these localities.
Poleo Formation rocks at both Coyote Amphitheater (N = 6 sites, ca4r) and Abiquiu Dam (N = 30 sites) are exclusively of reverse polarity, despite local differences in thickness (Figs. 10 and 12). The thicker Poleo section at Abiquiu Dam defines a single reverse polarity magnetozone, suggesting that the entire section was deposited over a relatively short time interval, given that field reversals during the Late Triassic were relatively frequent (e.g., Olsen et al., 1996, 2002; Hounslow and Muttoni, 2010). The average duration of reverse polarity chrons in the Newark Supergroup record is ∼650 k.y. for the Norian part of the section and 180 k.y. for the Carnian part of the section (calculated from Olsen et al., 1996; assuming that the approximate age of the Carnian-Norian boundary is ca. 228 Ma; Hounslow and Muttoni, 2010). Hounslow and Muttoni (2010) estimated an average magnetochron duration of 380 k.y. for the entire Triassic.
Only one complete section of the Petrified Forest Formation was sampled in this study, thus precluding any internal correlation within the Chama Basin. The Petrified Forest Formation magnetic polarity zonation is characterized by a short reverse interval at the base (ca5r) and top (ca8r), and two relatively long reverse polarity magnetozones (ca6r, ca7r), with shorter normal polarity zones near the base (ca5n), in the middle (ca6n), and near the top of the section (ca7n; Fig. 12). The lower 15 m of the Petrified Forest Formation was sampled at 0.5–1.0 m intervals at a second locality within Coyote Amphitheater. Although most specimens from the second locality yielded uninterpretable magnetizations, those specimens that did yield relatively linear trajectories matched the overall polarity zonation seen at the primary sampling locality.
Strata that have been inferred to be part of the Rock Point Formation were sampled at both Coyote Amphitheater and Ghost Ranch and are almost exclusively of normal polarity. The many well-developed paleosol horizons in these sections (Cleveland et al., 2008) yield incoherent magnetizations at both localities. Two short reverse polarity magnetozones were identified at the Ghost Ranch locality (gr1r, gr2r); one was directly above the Coelophysis quarry (Fig. 12). The Coyote Amphitheater and Ghost Ranch localities are ∼19 km apart and, although the general prevalence of normal polarity magnetizations characterizes both localities, polarity zonation cannot be directly correlated. We attribute this to pedogenic processes that have locally destroyed a primary magnetization.
The Coyote Amphitheater section is the first where all formations in the Chinle Group have been sampled in sequence, albeit with a sampling interval that requires future densification, at least over part of the section. Because Chinle strata were deposited in a large-scale fluvial system, the presence of multiple disconformities, many of which are not obvious, will lessen the completeness of any magnetostratigraphic record (e.g., Hall and Butler, 1983). For example, the contact between the Salitral and Poleo Formations has been described as disconformable (Tr-4 disconformity; Lucas, 1991, 1993; Lucas et al., 2003, 2005), and some workers have argued for a disconformity between the Petrified Forest and inferred Rock Point Formations (Tr‑5, locally of Lucas, 1991, 1993). We note that magnetozone boundaries in the Coyote Amphitheater sequence rarely coincide with substantial changes in lithology, at least suggesting that not all magnetozone boundaries are disconformities. At the resolution of current sampling, the Salitral-Poleo, the Petrified Forest–Rock Point, and the Poleo–Petrified Forest Formation contacts in the Chama Basin are all within reverse polarity magnetozones (Fig. 12).
Our sampling interval at Coyote Amphitheater is clearly too coarse to provide a complete polarity zonation across all formation and lithologic boundaries, as it corresponds to a time duration of possibly several hundred thousand years between sites, assuming constant sedimentation rates within mudstone-dominated intervals. The Newark Supergroup record includes numerous chrons of duration considerably <∼0.36 m.y. (Olsen et al., 1996, 2002). For example, the Norian part of the Newark polarity record contains 40 magnetozones, compared to our present estimate of ∼12 magnetozones in Norian age strata of the Chinle Group (assuming that Shinarump and Salitral strata in the Chama Basin are Carnian in age). It is clear that several short duration polarity intervals have not been captured in this study.
Correlation Models and Development of a Composite Magnetic Polarity Chronology
Magnetostratigraphic data from Chinle Group strata in north-central New Mexico may, in a general sense, be compared with composite polarity zonations recorded in Upper Triassic strata elsewhere, including eastern North America (Witte and Kent, 1989; Kent and Olsen, 1999; Olsen et al., 1996, 2002) and the Tethys realm (Krystyn et al., 2002; Channell et al., 2003; Muttoni et al., 2004; Gallet et al., 2007), as well as the composite geomagnetic polarity time scale (GPTS) of Hounslow and Muttoni (2010) (Fig. 13). Although compromised by the absence of a near-continuous magnetostratigraphic record, the polarity sequence of Chinle strata is nonetheless important as a continental record in that it is supported by a growing number of high-precision isotopic age determinations, whereas others are defined solely by biostratigraphy or cyclostratigraphy. Here we discuss two possible correlation approaches. One combines the lithostratigraphic correlations of Triassic strata in New Mexico (Lucas and Hunt, 1992; Lucas, 1993; Hunt and Lucas, 1993a, 1993b; Lucas et al., 2001, 2003, 2005; Heckert and Lucas, 2003) with geochronologic data, including the Irmis and Mundil (2008) detrital zircon date for the basal Blue Mesa Member. The second age model compares Chinle Group magnetic polarity chronologies from the region to the new Triassic GPTS (Hounslow and Muttoni, 2010) and includes the compilation of a tentative composite Chinle Group magnetic polarity chronology for the region.
Correlation Model 1: Lithostratigraphic Correlations on a Regional Scale
The first age model we examine is based on lithostratigraphic and vertebrate biostratigraphic correlations among localities as developed by Hunt and Lucas (1993a, 1993b), Lucas and Hunt (1992), and Lucas et al. (2005). A composite magnetic polarity stratigraphy (Fig. 12) is based on data from the Chinle Group, including inferred Rock Point Formation strata, from northern, western, and eastern New Mexico, and eastern Arizona (Reeve and Helsley, 1972; Molina-Garza et al., 1991, 1993, 1996, 1998b; Steiner and Lucas, 2000) as well as results presented here.
In this model, lower Chinle Group strata in the Zuni Mountains are considered lithostratigraphically and biostratigraphically equivalent to the lower Chinle Group in the Chama Basin. Vertebrate fossils from the Bluewater Creek–Blue Mesa interval in Arizona and the Salitral Formation in northern New Mexico are Adamanian, and the age determination of ca. 219 Ma from the base of the Blue Mesa Member is projected into the upper Salitral Formation. If the age of the Carnian-Norian boundary is ca. 228 Ma (Furin et al., 2006; Hounslow and Muttoni, 2010), the implication is that most if not all of the Chinle Group sampled in this study is of Norian age.
The oldest parts of the Chinle Group that provide a reasonably robust magnetic polarity zonation are the uppermost Shinarump, Salitral, and Bluewater Creek Formations. The Bluewater Creek Formation and Blue Mesa Member (Petrified Forest Formation) of the Zuni Mountains have been correlated by lithostratigraphic means to the Salitral Formation in northern New Mexico (Fig. 12). In both regions this stratigraphic interval is characterized by predominantly reverse polarity and several relatively short normal polarity magnetozones. Only a small part of the Bluewater Creek Formation and Blue Mesa Member section was sampled at Six Mile Canyon in the Zuni Mountains, but additional data are available for localities near Fort Wingate (Molina-Garza et al., 1998a). Correlation between the Salitral and Bluewater Creek–Blue Mesa intervals is tenuous at present. The magnetostratigraphic record allows correlation of zones ca1n and ca2n with normal polarity intervals in the Bluewater Creek Formation (Fig. 12). Magnetostratigraphic data for the lower Chinle Group in the Sangre de Cristo Mountains and Tucumcari Basin (Molina-Garza et al., 1996) suggest an acceptable correlation of the Garita Creek Formation with the Blue Mesa Member (Fig. 12).
We recognize that a Norian to Rhaetian age assignment for all Chinle Group strata contrasts with previous age interpretations based on the vertebrate fossil and palynostratigraphy record for Chinle Group strata in New Mexico. The Carnian-Norian boundary has typically been estimated to be at the base of the Poleo Formation, based primarily on sparse fossil pollen data (Litwin, 1986; Litwin et al., 1991), as well as vertebrate biostratigraphy (Zeigler et al., 2005). However, this boundary is poorly defined, as flora and fauna identified both above and below the Poleo Formation have been used to approximate it (e.g., Litwin, 1986; Lucas and Hunt, 1992; Hunt and Lucas, 1993a, 1993b; Lucas et al., 2005). No palynostratigraphic or biostratigraphic information is available from the Poleo Formation. The Dickinson and Gehrels (2008) maximum detrital zircon depositional age of ca. 215 Ma for the Poleo Formation clearly supports a Norian age for this part of the Chinle Group. Strata lithologically similar to either the Sonsela interval or the Poleo Formation have not been recognized in the Zuni Mountains area. However, the older ca. 219 Ma age estimate for the tuffaceous sandstone in the Blue Mesa Member, Zuni Mountains, suggests that most sites collected in this area are below the Sonsela interval–Poleo Formation. The 219 Ma age estimate for basal Blue Mesa strata mandates a major disconformity below the Blue Mesa Member in order for any underlying Chinle strata to be Carnian in age.
The Steiner and Lucas (2000) magnetostratigraphic record for the Chinle Group at Petrified Forest National Park included the Bluewater Creek Member and lower Sonsela Sandstone, as well as the Petrified Forest Member up to the base of the Black Forest Bed. Notably, there is a gap in their sampling of ∼75 m (W. Parker, 2009, 2010, personal communs.), and this negates use of this sequence as a complete section of the Chinle Group for correlation with the Newark Supergroup. The composite section presented by Steiner and Lucas (2000) contains eight magnetozones with no obvious polarity bias (Fig. 12). The Sonsela Sandstone, considered to be lithostratigraphically equivalent to the Poleo Formation, is reported by Steiner and Lucas (2000) to be within an interval of normal polarity (Fig. 12). Nonetheless, other sections that have been lithostratigraphically correlated with the Sonsela, such as the Trujillo Formation of eastern New Mexico (Molina-Garza et al., 1993) and the Sonsela in southeast Utah (Molina-Garza et al., 1993), are of reverse polarity. All Poleo Formation strata at both Abiquiu Dam and Coyote Amphitheater that yield interpretable results are of reverse polarity.
An alternative interpretation of the disparity in polarity between the Sonsela Sandstone in eastern Arizona and the Poleo Formation in north-central New Mexico is that the interval of deposition was so brief that, from a correlation perspective, Sonsela and Poleo strata are still effectively equivalent but diachronous. The Poleo Formation disconformably overlies Salitral strata at Abiquiu Dam, and the Sonsela Sandstone in the Petrified Forest National Park (PFNP) area may have been deposited during the interval represented by this hiatus or during a similar thickness normal polarity interval within the lower Petrified Forest Formation at Coyote Amphitheater (ca4n), which is the interpretation we consider (Fig. 12).
In the PFNP area, the lower third of the Petrified Forest Member is dominated by reverse polarity and the upper two thirds are predominantly of normal polarity (Steiner and Lucas, 2000) (Fig. 12). In contrast, at Coyote Amphitheater the lower half of the Petrified Forest Formation is dominated by reverse polarity (a single normal polarity magnetozone is identified) and the upper half contains a normal-reverse-normal sequence with all three magnetozones being of similar thickness.
If we assume that a wide sampling gap exists between the Sonsela and Petrified Forest sections at Petrified Forest National Park, a viable correlation with the Coyote Amphitheater magnetic polarity chronology associates the relatively long normal polarity interval high in the Painted Desert Member at PFNP with one of the long normal magnetozones at Coyote Amphitheater (ca6n or ca7n). Our preferred correlation is with ca6n (Fig. 12), based on the presence of additional strata above the Black Forest Bed that have not been sampled.
In eastern New Mexico, Redonda Formation strata yield high-quality data and are dominated by reverse polarity, as reported by Reeve and Helsley (1972), Reeve (1975), and Bazard and Butler (1991), and demonstrated by the new Mesa Redonda information reported here. Our Mesa Redonda section shows two reverse polarity magnetozones, one beginning at the contact with the underlying Bull Canyon Formation (mr1r) and the other low in the mudstone-dominated interval (mr2r; Fig. 12). A normal polarity interval was also identified by Molina-Garza et al. (1996) in the lower Redonda Formation near the contact with the Bull Canyon Formation at Sebastian Canyon. A normal polarity zone in the uppermost Redonda Formation is preserved in the Sebastian Canyon section, but not at Mesa Redonda. We correlate the lower normal magnetozone mr1n with magnetozone ca7n at Coyote Amphitheater (Fig. 12) and correlate magnetozone mr2n with the lowest normal interval reported by Reeve and Helsley (1972).
The Redonda Formation has been lithostratigraphically correlated to the inferred Rock Point Formation of north-central New Mexico (Lucas, 1997), and this correlation was utilized by Cleveland et al. (2008), who also assumed both formations to be exclusively Rhaetian in age, in their study of the stable isotope record of pedogenic carbonate in both sections. However, strata termed Rock Point Formation are predominantly of normal polarity (Figs. 10 and 12). The contrast in polarity between the Mesa Redonda and Ghost Ranch sections may be due to temporal differences in accumulation and preservation (i.e., siltstones near the top of the Redonda Formation at Mesa Redonda captured polarity intervals not recorded in the Rock Point Formation). An alternative explanation involves a major disconformity at the base of the inferred Rock Point Formation in the Chama Basin, such that the dominantly reverse polarity interval in the Redonda Formation is the time lost at the Petrified Forest–inferred Rock Point disconformity in the Chama Basin (Fig. 12). Also, we correlate the dominantly normal polarity interval in inferred Rock Point strata at Ghost Ranch to the uppermost normal magnetozone in the Luciana Mesa section (Reeve and Helsley, 1972) and the uppermost normal interval in the Redonda Formation at Sebastian Canyon (Molina-Garza et al., 1996).
The Rock Point Formation in Utah is characterized by dominantly reverse polarity, and the overlying Wingate Formation and the laterally equivalent Moenave Formation are dominated by normal polarity (Fig. 11; Molina-Garza et al., 2003; Donohoo-Hurley et al., 2010). Because strata inferred to be Rock Point Formation in north-central New Mexico are predominantly of normal polarity, we argue that these strata cannot be directly correlated with the type Rock Point Formation exposed in Arizona and Utah. Rather, we speculate that the strata inferred to be Rock Point Formation in northern New Mexico are younger and temporally equivalent to Wingate and/or Moenave strata in Utah and northern Arizona, which are considered to span latest Triassic (Rhaetian) to earliest Jurassic (Hettangian) time (Litwin, 1986; Kirkland and Milner, 2006; Lucas et al., 2006a, 2006b; Donohoo-Hurley et al., 2010). The Redonda Formation of eastern New Mexico, however, is primarily of reverse polarity and we still consider it to be time equivalent to the type Rock Point Formation in Utah.
Correlation Model 1: Correlations to Newark Supergroup and European Strata
In a crude sense, Chinle Group magnetic polarity records from New Mexico can be compared with the astronomically tuned polarity time scale for the Upper Triassic Newark Supergroup of the east coast of North America (Witte and Kent, 1989; Olsen and Kent, 1999; Olsen et al., 1996, 2002, and references therein) (Fig. 13). Initial correlations between Chinle and Newark strata were based primarily on biostratigraphy (Lucas and Huber, 1993) with the assumption that the lower Chinle Group (Shinarump, Salitral, and Bluewater Creek Formations) was Carnian in age. The lower part of the Chinle Group is of mixed polarity, with a reverse polarity interval straddling the inferred Carnian-Norian boundary, assuming that the Poleo Formation was deposited across the Carnian-Norian boundary. This pattern is similar to that of the Carnian section of the Newark Supergroup (chrons E1–E7n), except, of course, that fewer magnetozones are observed in the Chinle section. Alternatively, if we accept a considerably older age for the Carnian-Norian boundary, as proposed by Furin et al. (2006) and adapted by Hounslow and Muttoni (2010), then the ca. 219 Ma age estimate for lower Blue Mesa strata results in a preferred correlation of the Salitral–Bluewater Creek–Garita Creek–Blue Mesa sequence with the Newark polarity interval from E8 to E13 (Fig. 13). In support of this correlation, an age of ca. 219 Ma for magnetozone E13 has been extrapolated from cyclostratigraphy in the Newark basin (Olsen et al., 2002).
The recent U-Pb age date from the upper Bluewater Creek Formation of ca. 219.2 ± 0.7 Ma (Irmis and Mundil, 2008) implies that few or no Carnian strata are preserved in western New Mexico if a Carnian-Norian boundary age estimate of ca. 228 Ma is valid. Assuming lithostratigraphic correlations between the Bluewater Creek Formation and Blue Mesa Member and the Salitral Formation are correct, then all Chinle strata in western and northern New Mexico, with the possible exception of the inferred Rock Point Formation, are of Norian age, and our correlation of Chinle Group strata with the Newark interval represented by magnetochrons from E8 to E13 is preferred (Fig. 13).
High-quality magnetic polarity compilations have been obtained from several carbonate rock–dominated sequences in the Tethyan region of southern Europe (Fig. 13). These sections include a Tethyan composite section (Krystyn et al., 2002), a revised Pizzo Mondello section in Sicily (Muttoni et al., 2004), and a section from Oyuklu, Turkey (Gallet et al., 2007). The lower Chinle Group polarity sequence does not show a strong correlation with the upper Carnian Tethyan marine sections (Fig. 13). The composite sequence proposed by Channell et al. (2003) shows that the inferred Carnian sequence is of mixed normal and reverse polarity and that the Norian is predominantly of reverse polarity in the lower half to two-thirds of these sections, and of mixed polarity in the upper half to third. If Chinle Group strata are entirely of Norian age, then the correlation between the Chinle Group and the Tethyan sections improves, at least in principle, as more magnetozones are represented in the Chinle Group over a shorter time interval.
However, the stratigraphic completeness of the marine sections used to provide a composite magnetic polarity sequence remains open to question. The marine sequences from which the composite of Channell et al. (2003) was constructed are, on average, <40 m thick and contain thin layers interpreted as turbidite deposits (Krystyn et al., 2002; Channell et al., 2003; Muttoni et al., 2004). Although these sections were sampled at a very high density, the thin sequences, coupled with repeated higher energy deposits, have prompted concerns over the true completeness of these sequences (Lucas et al., 2004). In addition, polarity data for the middle Norian are sparse, complicating efforts to correlate among sections that sample parts of Norian time. The section at Sicily (Muttoni et al., 2004) has a gap near the base of the Norian section, corresponding to a part of the Lacian, and the Tethyan composite (Channell et al., 2003) has a gap in the upper half of the Norian section, which includes most of the Alaunian interval.
Correlation Model 2: Correlation of Magnetic Polarity Chronologies with the Late Triassic GPTS
The new GPTS for the entire Triassic (Hounslow and Muttoni, 2010), coupled with maximum depositional age estimates from detrital zircon assemblages (Dickinson and Gehrels, 2008) and the detrital zircon date of Irmis and Mundil (2008), calls into question previously held assumptions regarding the lithostratigraphic correlation of the Bluewater Creek–Blue Mesa interval to the Salitral Formation as well as correlation of the Trujillo Formation in eastern New Mexico to the Poleo Sandstone. The medial Bluewater Creek–Blue Mesa interval, which encompasses the tuffaceous sandstone dated as ca. 219 Ma (Irmis and Mundil, 2008), is defined by primarily reverse polarity with a short possible normal polarity magnetozone at the base of the sandstone. The GPTS has a relatively long normal polarity interval at ca. 219 Ma, whereas the magnetic polarity chronology from the lower Chinle (Shinarump-Salitral interval) in the Chama Basin shows three short normal polarity intervals and four short reverse polarity intervals below the Poleo Formation (Fig. 12, ca1n–ca3n). Short stratigraphic intervals sampled in the Zuni Mountains and at Fort Wingate show primarily reverse polarity with a possible short normal chron (Fig. 12), which could be tentatively correlated to one of the short normal polarity magnetozones in northern New Mexico.
Alternatively, the Shinarump-Salitral interval in the Chama Basin correlates with the Carnian-Norian boundary interval, representing some part of Hounslow and Muttoni's (2010) chron sequence UT12–UT15. Lower Chinle Group strata in the Zuni Mountains, dated as 219 Ma, may have imperfectly captured the normal polarity interval UT17 in the GPTS. If these correlations are viable, then lower Chinle Group strata in the Chama Basin cannot be time equivalent to lower Chinle strata in the Zuni Mountains and Fort Wingate area. In addition, such correlations allow an estimate of time missing at the disconformity below the Poleo Formation (Tr-4 unconformity of Lucas, 1991, 1993). If the Salitral Formation is older than the Bluewater Creek–Blue Mesa interval and straddles the Carnian-Norian boundary, and if we assume a maximum depositional age for the Poleo Formation of 215 Ma (Dickinson and Gehrels, 2008) and a Carnian-Norian boundary of ca. 228 Ma (after Hounslow and Muttoni, 2010), then at least 13 m.y. are missing at the base of the Poleo Formation.
The Trujillo and Poleo Formations have also been considered to be lithostratigraphically equivalent (Lucas et al., 2001), although maximum depositional ages from detrital zircon assemblages suggest a lack of time equivalence between these two units. The Trujillo Formation has a wide maximum depositional age range of ca. 226–219 Ma (Dickinson and Gehrels, 2008) and appears to be substantially older than the Poleo Formation, with a maximum depositional age of ca. 215. If we correlate the upper Trujillo Formation (ca. 219 Ma) to the tuffaceous sandstone in the Zuni Mountains, the Trujillo interval must be stratigraphically below the Poleo Formation as well as the Blue Mesa Member (Petrified Forest Formation) in northern Arizona. Hounslow and Muttoni (2010) correlated the Trujillo Formation to the upper Stockton and Lockatong Formations of the Newark Supergroup, which are considered early Norian in age (Fig. 13). In light of the new GPTS, the Trujillo Formation is thus probably much older than the Sonsela or Poleo Formations and would be considered equivalent to lower Bluewater Creek strata in Arizona. We place the Trujillo interval above the Salitral Formation in the Chama Basin in our composite geomagnetic polarity chronology (Figs. 10 and 12).
If we compare the composite Chinle polarity chronology to Hounslow and Muttoni's (2010) GPTS for the Late Triassic, the Shinarump–Salitral interval is correlated to the middle Carnian (UT4–UT7 or UT5–UT8), and the Trujillo–Bull Canyon–Blue Mesa interval is correlated to the early Norian (UT16-UT17). The Poleo, Sonsela, and Petrified Forest intervals correspond to almost all of the Norian. The Poleo–Sonsela interval is correlated here to UT18 and the Petrified Forest interval is correlated to upper UT18–UT22. The red siltstone (inferred Rock Point) interval is interpreted to correlate with UT25–UT26.
Composite Chinle Group Magnetostratigraphy
In summary, our composite polarity stratigraphy for Chinle Group strata (Fig. 12) assumes that the Poleo Formation–Sonsela interval likely represents a relatively short period of deposition. It also reflects our tentative assumption that most strata previously referred to as Rock Point Formation in northern New Mexico are younger than Redonda Formation strata and that the Redonda Formation is younger than Petrified Forest strata, which contradicts the direct correlation assumed by Lucas et al. (2005) and Cleveland et al. (2008). The composite magnetic polarity stratigraphy contains 11 well-defined polarity zones (depending on how uninterpretable zones are considered) in the lower Chinle Group (below the Poleo Formation) and 16 in the upper Chinle Group, with a distinct bias, based on stratigraphic thickness of magnetozones, toward reverse polarity.
Lithostratigraphic and Biostratigraphic Implications
The new magnetic polarity data from northern New Mexico illustrate the inherent problems associated with correlation of terrestrial rock units over long distances based solely on lithostratigraphic characteristics. In addition, the tentative correlations we propose imply complex deposition in the Chinle Basin as a whole (Fig. 14). For example, the northern New Mexico area appears to have received sediment only sporadically throughout the Late Triassic, and disconformities, some quite subtle, are pervasive. Fluvial systems are complex depositional environments and lithologic units should not be expected to be isochronous, nor continuous, over long distances.
Prior to Irmis and Mundil's (2008) detrital zircon age determination, the lower Chinle (Shinarump, Salitral, and Poleo Formations) sequence was assigned to the Adamanian LVF and thus given a Carnian age. The upper Chinle Group (Petrified Forest and inferred Rock Point Formations) were designated as Revueltian on the basis of their faunal assemblages, and assigned a Norian age (Lucas, 1998; Hunt and Lucas, 1993a). Irmis et al. (2010) argued that the LVFs proposed by Lucas (1998) do not appear to be perfectly time correlative to the marine stage divisions of the Late Triassic and thus are of limited use for correlation purposes on more than a regional scale. Irmis et al. (2010) argued that it is nearly impossible to correlate continental stratigraphic sequences with marine sequences based solely on biostratigraphic tie points, as also noted by Donohoo-Hurley et al. (2010) in their attempts to correlate the magnetostratigraphic record of the Moenave Formation with existing continental as well as marine records. One very rare example of vertebrate fossil material found with invertebrate material is a specimen of the aetosaur Aetosaurus found in marine sedimentary rocks (Lucas, 1998, 2010b). The conchostracan record, which is excellent for the Germanic Basin of Europe and the Newark Basin, is very sparse for the Chinle Group (Kozur and Weems, 2010; H. Kozur, 2010, personal commun.). If our tentative correlation of at least part of the inferred Rock Point Formation to Rhaetian-age strata (Wingate–Moenave interval, Oyuklu section) is viable, then the magnetic polarity data are directly at odds with conchostracan biostratigraphy for the inferred Rock Point strata, and furthermore suggest that the direct correlation of inferred Rock Point and Redonda strata by Cleveland et al. (2008) may be problematic. Kozur and Weems (2010) identified latest Norian–earliest Rhaetian conchostracans in the Redonda Formation and latest Norian (latest Sevatian) conchostracans from the inferred Rock Point Formation (Rinehart et al., 2009; Kozur and Weems, 2010; H. Kozur, 2010, personal commun.). Reconciling these disparate data sets will require further magnetostratigraphic and biostratigraphic research. Because attempts to tie LVFs to marine stages are currently of limited utility, we refrain from attempting to correlate the composite Chinle Group magnetic polarity chronology to marine sections using biostratigraphic tiepoints until the relationships between continental and marine faunal assemblages are better determined. Instead, we choose to incorporate the few detrital zircon age determinations available as well as the GPTS proposed by Hounslow and Muttoni (2010).
As noted herein, Litwin (1986) and Litwin et al. (1991) used palynomorphic data to assign a Norian age to upper Chinle strata (Petrified Forest and inferred Rock Point Formations) based on a limited number of samples. Both palynomorphs and conchostracans from strata of the inferred Rock Point Formation at Ghost Ranch may reflect provincialism during the latest Triassic and thus may not accurately reflect global palynostratigraphic or biostratigraphic changes.
If our proposal that the inferred Rock Point strata in the Chama Basin do not directly correlate with Redonda strata in eastern New Mexico is viable, yet Redonda strata can be correlated to the Rock Point Formation of Utah and Arizona (and potentially to the Owl Rock Formation), then this relation suggests the presence of a north-south–trending topographic high in central New Mexico during deposition of Rock Point and Redonda strata. Studies of the Upper Triassic Dockum Group of West Texas have led to the hypothesis that the Dockum Group was deposited in a separate basin from that of the Chinle Group in New Mexico, Arizona, and Utah (McKee et al., 1959; Finch and Wright, 1983; McGowen et al., 1983; Johns and Granata, 1987; Dubiel, 1989b, 1994; Lehman, 1994a, 1994b). Riggs et al. (1996) demonstrated that the two basins were probably connected during early Late Triassic deposition. Detrital zircons unique to the Amarillo-Wichita uplift have been identified both in the Santa Rosa Formation of eastern New Mexico and West Texas, the Poleo Formation of north-central New Mexico, and the Shinarump and Osobb Formations of Arizona and Nevada (Riggs et al., 1996; Dickinson and Gehrels, 2008; Dickinson et al., 2010).
By the latest Late Triassic, however, sediment transport between the Dockum Basin and the rest of the Chinle Basin appears to have been disrupted as indicated by paleocurrent directions (May, 1988; Riggs et al., 1996). The hypothesis of a topographic high in central New Mexico during late Late Triassic time is also supported by field observations in the Dry Cimarron Valley of northeastern New Mexico (Baldwin and Muehlberger, 1959), where the Middle Jurassic Entrada Sandstone directly overlies in angular unconformity the Upper Triassic Dockum Formation (Fig. 15). The Entrada Sandstone varies considerably in thickness in this area, ranging from ∼10 m or more in thickness at Steamboat Butte to nonexistent to the west. We hypothesize that the dramatic variation in thickness of the Entrada Sandstone reflects infilling of topography developed on deformed Upper Triassic strata.
The inferred topographic high may have been related to rifting in the area of the Gulf of Mexico, prior to the onset of deposition of the Redonda Formation or to dynamic flexure parallel to the backarc basin formed inland of the Cordilleran arc (Dickinson and Gehrels, 2008; Dickinson et al., 2010). Alternatively, the proposed topographic high may be related to the prerift uplift in Texas proposed by Dickinson et al. (2010), although it remains unclear if detrital zircon populations in uppermost Chinle strata relate to the Ouachita uplift. The correlations of the magnetic polarity chronologies presented here limit the timing of the development of such a feature because latest Norian, and probably earliest Rhaetian, strata (Redonda, Owl Rock, and Rock Point) may not have been deposited in north-central New Mexico.
The possibility of the preservation of the Triassic-Jurassic boundary in strata of the inferred Rock Point Formation in north-central New Mexico should be further explored. Recent magnetostratigraphic and paleontologic information suggests that the older Dinosaur Canyon Member of the Moenave Formation is largely Rhaetian in age and that the Triassic-Jurassic boundary is within the younger Whitmore Point Member of the Moenave Formation, above at least two short reverse polarity magnetozones, the younger of which is in the lower part of the Whitmore Point Member (Donohoo-Hurley et al., 2010; Lucas et al., 2011). Correlation of Moenave strata to the uppermost Newark Supergroup and the Hartford Basin section (e.g., Kent and Olsen, 2008) may imply that the oldest reverse polarity magnetozone in the Moenave Formation (in the Dinosaur Canyon Member) is equivalent to E23r, a short duration reverse polarity zone just below the Triassic-Jurassic boundary in the Newark Supergroup (Kent and Olsen, 2008).
If E23r is the youngest reverse polarity magnetochron before the Triassic-Jurassic boundary and the uppermost strata (inferred Rock Point Formation) in the Chama Basin are considered at least partially age equivalent to the Dinosaur Canyon Member of the Moenave Formation, as noted by Lucas and Tanner (2007), then we speculate that the Triassic-Jurassic boundary may be in strata a few meters above the world-renowned dinosaur mass death assemblage at Ghost Ranch, the Coelophysis quarry, within strata that were previously inferred to be part of the Rock Point Formation (Lucas et al., 2003, 2005; Fig. 13). If this speculation is proven valid, then strata hosting the Coelophysis quarry were deposited during late Rhaetian time and thus Coelophysis is Rhaetian in age.
The dominantly normal polarity strata historically considered to be part of the Rock Point Formation in northern New Mexico have been lithostratigraphically correlated to the Redonda Formation (in eastern New Mexico; Lucas et al., 2001), which is dominated by reverse polarity (Reeve and Helsley, 1972; Reeve, 1975; Bazard and Butler, 1991). The polarity chronology of the uppermost strata in northern New Mexico is similar to the Rhaetian part of the Oyuklu section (Gallet et al., 2007). Lucas (1997) hypothesized that strata inferred as Rock Point Formation 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 has been used to assign the uppermost Chinle 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 et al., 2010; Lucas et al., 2011).
A possible lithostratigraphic correlation of inferred Rock Point strata in northern New Mexico to the predominantly Rhaetian–earliest Hettangian Moenave Formation in Utah and Arizona would also call into question the biostratigraphic utility of the aetosaur genus Aetosaurus, long considered a Norian index fossil (Lucas, 1998, 2010b), as well as the age assigned to these strata based on palynostratigraphy and conchostracan biostratigraphy (Litwin, 1986; Litwin et al., 1991; Kozur and Weems, 2007; Rinehart et al., 2009; Kozur and Weems, 2010). In addition, as noted herein, such a correlation, as proposed, would limit the approach taken by Cleveland et al. (2008) to directly correlate the stable isotope record of pedogenic carbonate in the Redonda Formation and inferred Rock Point Formation in the Chama Basin.
Chinle Group strata in the Chama Basin of north-central New Mexico, the Zuni Mountains of west-central New Mexico, and Mesa Redonda of eastern New Mexico, allow development of a more complete composite magnetic polarity chronology for the Late Triassic of the American Southwest. Hematitic mudrocks constitute most of the materials sampled in the Chama Basin, Zuni Mountains, and eastern New Mexico, and overall yield high-quality paleomagnetic data.
The magnetostratigraphic information presented here implies that some lithostratigraphic correlations of Upper Triassic strata in the American Southwest may require revision (e.g., Zeigler et al., 2008). The lower Chinle Group in northern New Mexico is probably late Carnian in age, and not equivalent to the Bluewater Creek–Blue Mesa interval in western New Mexico, which is Norian in age, based on Irmis and Mundil's (2008) detrital zircon age determination of ca. 219 Ma for the base of the Blue Mesa Member. The Poleo Formation, considered equivalent to the Sonsela Member in Arizona and Utah, is exclusively of reverse polarity, whereas the Sonsela Member is dominated by normal polarity (Steiner and Lucas, 2000). Comparisons of the magnetic polarity chronologies for these strata as well as the Trujillo Formation of eastern New Mexico with the recently proposed GPTS for the complete Triassic (Hounslow and Muttoni, 2010) demonstrate that these units are also not time equivalent. In fact, correlation of magnetic polarity chronologies for northern New Mexico with the GPTS suggests that the disconformity at the base of the Poleo Formation must represent at least 13 m.y. of missing time. Strata inferred to be Rock Point Formation in northern New Mexico, although previously considered lithostratigraphically equivalent to the Redonda Formation of eastern New Mexico, do not appear to be time correlative to the Redonda Formation, nor to actual Rock Point strata in southern Utah (Molina-Garza et al., 2003). On the basis of their polarity record and the overall absence of firm biostratigraphic tiepoints, the inferred Rock Point strata in the Chama Basin may be comparable in age to much of the Dinosaur Canyon Member of the Moenave Formation, and thus dominantly late Rhaetian in age. Based on this information, it is possible that strata hosting the famous Coelophysis quarry are latest Rhaetian in age and may have been deposited just before the termination of the Triassic.
Reviews by two anonymous reviewers served to greatly improve the organization and focus of the manuscript. Special thanks to Coyote District (F. Sanchez, Carson National Forest), Ghost Ranch, and the Army Corps of Engineers at Abiquiu Dam (D. Dutton and E. Garner) for permission to conduct sampling. We also thank the Branch family of Coyote, New Mexico, for permission to sample and camp on their land. Field assistance was provided by V. Morgan, J. Stiegler, G. Peacock, P. Zeigler, D. Yeck, and D. Chaney. Funding for this research was provided by the Geological Society of America and the New Mexico Geological Society. We thank the Pueblo of Jemez and the Madalena family for permission to examine Chinle Group strata on their tribal lands. We thank Linda Donohoo-Hurley for assistance with inclination shallowing corrections. We also thank S.G. Lucas for initially suggesting this project. Roberto Molina-Garza commented on an earlier version of this manuscript. Dedicated to the memory of V.L. Morgan (1945–2010).