Paleomagnetism and mangetostratigraphy of the Upper Triassic Petrified Forest and Poleo formations, north-central New Mexico, and the Bluewater Creek and Lower Petrified Forest Formations, central New Mexico

Abstract

—As demonstrated by vertebrate biostratigraphy and palynostratigraphy, the Upper Triassic Chinle Group spans most of the Late Triassic. The Chinle Group was deposited by a fluvial system and consists predominantly of red to purple mudstones with some red to orange siltstones and red to buff sandstones. In the Chama basin, north-central New Mexico, both lower and upper Chinle Group strata are well-exposed and are being used to develop a more complete magnetic reversal chronology for the Late Triassic for the American Southwest. Our sampling methods concentrate on hematitic mudrocks, using an intricate block sampling technique. These materials typically carry a well-defined, well-grouped magnetization dominated by pigmentary hematite that is unblocked below about 660oC. For example, a single horizon (level 39) in the Painted Desert Member of the Petrified Forest Formation yields an estimated site mean of D=172.9o, I=7.0o, α95=5.5o and k=102.5 (N = 8 independent samples). Indurated, coarser-grained deposits are sampled by more conventional methods. Sandstones and siltstones of the Poleo Formation contain both detrital and authigenic hematite and some detrital magnetite as the magnetization carriers and typically yield a well-defined magnetization at the site (single bed) level (e.g., site 11, eight independent samples, yields an estimated site mean of D=187.4o, I=0.8o, α95=3.4o and k=268.1). A very preliminary polarity reversal stratigraphy has been developed for the Poleo and Petrified Forest formations in the Chama basin. The Petrified Forest Formation includes multiple polarity intervals. All accepted sites in the Poleo Formation are of reverse polarity. The upper part of the Bluewater Creek Formation and lower Blue Mesa Member (Petrified Forest Formation) in the Zuni Mountains, central New Mexico, appear to be entirely of reverse polarity. FIGURE 1. Distribution of Triassic outcrop area (approximates outline of Chinle basin) (from Molina-Garza et al., 1993), with expanded view of Chama basin Chinle Group outcrops. 1 = Abiquiu Dam, 2 = Coyote Amphitheater, star = Zuni Mountains. 116 ZEIGLER, GEISSMAN, AND LUCAS as well as tie the magnetostratigraphy of these rocks to a tuffaceous, zircon-bearing sandstone within the section for which we are obtaining a high-precision U-Pb zircon age. Here we present preliminary magnetostratigraphic data from the mudstone-dominated Petrified Forest Formation and the Poleo Sandstone, as well as the Bluewater Creek Formation and Blue Mesa Member (Petrified Forest Formation), of the Chinle Group. GEOLOGIC SETTING AND STRATIGRAPHY The Chinle Group was deposited in a back-arc basin in western Pangea by a large and widespread fluvial system (Stewart et al., 1972; Blakey and Gubitosa, 1983; Dubiel, 1987) and consists predominantly of red mudstones, with lesser orange siltstones and buff sandstones. Upper Triassic strata in the Chama basin of north-central New Mexico have been assigned to six formations (in ascending order): Zuni Mountains (=mottled strata), Shinarump (=Agua Zarca), Salitral, Poleo, Petrified Forest and Rock Point formations (Lucas and Hunt, 1992; Hunt and Lucas, 1993a,b; Lucas et al., 2003) (Fig. 2). We briefly describe those formations sampled for this study. The Poleo Formation consists primarily of sandstones but intrabasinal calcrete and mud-clast conglomerates are more prevalent in the lower third of the unit (Fig. 2). Clasts in the conglomerate beds are mudstone or siltstone rip-ups, calcrete nodules, and occasionally carbonized plant debris. Both the sandstones and conglomerates contain authigenic hematite cement in variable abundance. The sandstones’ dominant bedform is trough crossbedding, and they are micaceous litharenites and subarkoses, indicating a lack of compositional maturity (Lucas et al., 2003 and references therein). Most Poleo rocks are dusky yellow to grayish yellow. The formation lies disconformably above the Salitral Formation and conformably below the Petrified Forest Formation. It is thickest at Abiquiu Dam, where over 53 m of section is exposed in the side walls of the canyon below the dam. The Petrified Forest Formation is the thickest and most widespread part of the Chinle Group (Repenning et al., 1969). It is predominantly a reddish brown bentonitic mudstone, although it includes thin beds of arkosic sandstones (Dubiel, 1987, 1989) and rip-up clast conglomerates (Zeigler, 2002, 2003; Tanner et al., 2003) (Fig. 2). The mudstones are mottled at distinct stratigraphic levels, which represent paleosol formation (Tanner, 2003). Overall, the Petrified Forest Formation has been interpreted as floodplain deposits (Lucas et al., 2003 and references therein). Vertebrate fossils found in the Petrified Forest Formation indicate a Revueltian (early to mid-Norian) age (Lucas, 1993, 1998). In the Zuni Mountains, similar Upper Triassic strata are exposed and have been assigned to the following formations (in ascending order): Zuni Mountains, Shinarump, Bluewater Creek, Sonsela and Petrified Forest. The Bluewater Creek Formation has three distinct lithofacies (Heckert and Lucas, 2003): interbedded mudstones and siltstones with scattered calcrete horizons that are interpreted as overbank/floodplain deposits, ripple laminated to plane bedded sandstones with minor intraformational conglomerates that are interpreted as low sinuosity fluvial deposits, and greenish bentonitic mudstones and black shales that have been interpreted as lacustrine. At Six Mile Canyon in the Zuni Mountains, strata of the Bluewater Creek Formation are predominantly the first of these lithofacies (Fig. 2). The contact between the Bluewater Creek Formation and the overlying Petrified Forest Formation is marked by a white, tuffaceous sandstone that is the lowest unit of the Blue Mesa Member (Heckert and Lucas, 2003). The rest of the Blue Mesa Member strata at Six Mile Canyon consists of mudstones with common, discontinuous horizons of calcrete nodules. The Blue Mesa Member has been interpreted as floodplain deposits with paleosol horizons (Heckert and Lucas, 2003). FIGURE 2. Generalized stratigraphic column of the Chinle Group in northern New Mexico. Note Zuni Mountains Formation is only locally present. 117 PALEOMAGNETISM AND MAGNETOSTRATIGRAPHY OF THE UPPER TRIASSIC SAMPLING AND METHODS We sampled sub-horizontal Poleo strata at Abiquiu Dam (36.2oN, 106.2oW), with a stratigraphic spacing of about 2 m between each site (32 total sampled sites). In the upper part of the Poleo Formation, this sampling interval was dictated by bed thickness, so that each site is a discrete bed, whereas parts of the lower third of the section had large-scale bedding and some beds included several sites. At each site, 6 to 8 samples were typically drilled and oriented in the field. Most samples could be prepared into multiple specimens for demagnetization. Petrified Forest Formation strata were sampled at Coyote Amphitheater (36.1oN, 106.4oW), where strata dip 18o to the west, using an initial sampling interval of about 5 m, for a total of 40 sampling sites. Fresh, in situ mudrock was exposed by digging at least a half meter into the outcrop. We used an intricate block sampling method where individually oriented blocks were carved out in the field and then dry-cut into oriented, ~2 cm3 cubes (e.g., Johnson et al., 1975) (Fig. 3). At each site, typically six to seven blocks were removed, each of which yields from one to seven or more individual specimens. Progressive thermal demagnetization was applied to most specimens to investigate the character of the natural remanent magnetization (NRM) in these materials. Bluewater Creek Formation and Blue Mesa Member strata were sampled at Six Mile Canyon (35.4oN, 108.5oW) at about 1 m intervals above and below the white, tuffaceous sandstone at the base of the Blue Mesa Member for a total of 10 sampling sites. As at Coyote Amphitheater, we used the block sampling method described previously to sample mudrocks. At each site five to eight blocks were removed, with each block yielding from one to eight individual specimens. All measurements of the natural remanent magnetization (NRM) were made using a 2G Enterprises Model 760R superconducting rock magnetometer, equipped with DC SQUIDS, with a magnetic moment noise level of less than 1.0x10-6 A/m. At least one specimen per sample was subjected to progressive thermal demagnetization, using either a Shaw MMTD or an ASC 48 thermal demagnetizer . Where possible for sandstones, duplicate specimens were subjected to chemical demagnetization, essentially following the procedures of Henry (1979). Progressive demagnetization data were analyzed using the principal component analysis approach (Kirschvink, 1980) with sequences of demagnetization data typically unanchored to the origin; individual linear segments were accepted if maximum angular deviation (MAD) values were less than 10o. Where data were anchored to the origin, we only accepted results if MAD values were less than 15o. Acquisition of isothermal remanent magnetization (IRM) to saturation (SIRM) and backfield DC demagnetization of SIRM experiments were conducted using a home-built pulse magnetizer that provided a DC field up to 2.97 T, which is capable of 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 conducted on representative specimens to further evaluate the magnetic mineralogy. PALEOMAGNETIC RESULTS Poleo Formation, Abiquiu Dam Samples from most sites in horizontal strata of the Poleo Formation yield relatively well-defined (Fig. 4a-d) and well-grouped magnetizations (Fig. 4e) that are of reverse polarity (south-seeking and very shallow inclination). We accepted site mean directions from 21 of 30 sites analyzed to date (Appendix), and these yield a grand mean direction of D = 182.7°, I = -0.3°, α95 = 5.3° and k = 36.5 (Fig. 4e). Samples