Earliest Jurassic U-Pb ages from carbonate deposits in the Navajo Sandstone, southeastern Utah, USA

Abstract

New uranium-lead (U-Pb) analyses of carbonate deposits in the Navajo Sandstone in southeastern Utah (USA) yielded dates of 200.5 ± 1.5 Ma (earliest Jurassic, Hettangian Age) and 195.0 ± 7.7 Ma (Early Jurassic, Sinemurian Age). These radioisotopic ages—the first reported from the Navajo erg and the oldest ages reported for this formation—are critical for understanding Colorado Plateau stratigraphy because they demonstrate that initial Navajo Sandstone deposition began just after the Triassic and that the base of the unit is strongly time-transgressive by at least 5.5 m.y. INTRODUCTION The Navajo Sandstone in southeastern Utah (United States) represents the largest erg in Earth history (e.g., Blakey et al., 1988), cropping out on much of the Colorado Plateau (Fig. 1). Fossils are generally lacking, poorly preserved, and/ or not age diagnostic. Determining the age of the Navajo Sandstone has, therefore, been challenging. The Navajo Sandstone contains carbonate deposits laid down in lakes and springs (Parrish et al., 2017) from which we report the first radioisotopic ages, acquired by U-Pb dating. The dates reported here set the stage for future work on the development of the Navajo erg and provide a framework for understanding mechanisms of the evolution of the basin in the Jurassic Period. PREVIOUS WORK ON THE AGE OF THE NAVAJO SANDSTONE The Navajo Sandstone is the upper formation of the Glen Canyon Group, which in southern Utah and northern Arizona includes the Wingate Sandstone, Moenave Formation, Kayenta Formation, and Navajo Sandstone (generalized in Fig. DR1A in the GSA Data Repository1). These are lithostratigraphic units, and definition of the boundaries between them is commonly arbitrary, particularly where they interfinger and/ or have gradational relationships. The literature on the Glen Canyon Group is voluminous, and a comprehensive review is outside the scope of this report. Useful general reviews are Blakey (2008) and Dickinson (2018), and additional reviews can be found in Peterson and Pipiringos (1979), Blakey (1989), and Sprinkel et al. (2011), as well as others. The Wingate and Navajo Sandstones are predominately eolian (Blakey, 1989); the Moenave and Kayenta Formations contain interbedded eolian, fluvial, and lacustrine deposits (e.g., Clemmensen et al., 1989; Blakey, 2008; Suarez et al., 2017). The Moenave Formation is divided into two members, the Dinosaur Canyon Member and the overlying Whitmore Point Member. The Moenave Formation is overlain by the Springdale Sandstone, which has been assigned variously as the upper member of the Moenave Formation (Steiner, 2014a) or, more commonly, the lower member of the overlying Kayenta Formation (Lucas and Tanner, 2006; Dickinson, 2018, p. 93). In east-central Utah, the Glen Canyon Group consists solely of the Wingate Sandstone, Kayenta Formation, and Navajo Sandstone. The Kayenta Formation interfingers with the lower part of the Navajo Sandstone across a broad region from southwestern Utah to northeastern Arizona (Blakey, 1989; Hassan et al., 2018). The Glen Canyon Group is underlain by the Upper Triassic Chinle Formation, which includes the Black Ledge sandstone (e.g., Blakey, 2008; Fig. DR1A). Age assignments of the Glen Canyon Group and underlying formations have been in a state of flux (Marsh, 2014), partly because of discrepancies between the geochronological and biostratigraphic records—a global problem in chronostratigraphy—but also because of possible overreliance on the continental biostratigraphic record (Olsen et al., 2010; see also Irmis et al., 2010; Marsh, 2014). Quantitative age estimates have come from detrital zircon (DZ) and magnetostratigraphy (Fig. 2). The Kayenta Formation, Moenave Formation, and Black Ledge sandstone have yielded DZ ages broadly consistent with their depositional ages as determined from biostratigraphy or magnetostratigraphy (Dickinson and Gehrels, 2009a; Table DR1 in the Data Repository). Marsh (2014) dated the Kayenta Formation with DZ in northern Arizona at 183.7 ± 1.7 Ma, ∼12 m.y. younger than the dates obtained by Dickinson and Gehrels (2009a) from their closest sample location of the Kayenta Formation near Moenkopi, Arizona (Fig. 1; Table DR2). DZ ages from the Moenave Formation west of Kanab, Utah, yielded dates ca. 201.3 Ma (Suarez et al., 2017; Fig. 2; Table DR2). The Chinle Formation, which underlies the Glen Canyon Group, is well dated using DZ (Irmis et al., 2011; see also Riggs et al., 2003; Ramezani et al., 2011). The youngest dates from the Chinle Formation are 203–204 Ma (Rhaetian) from the *E-mail: [email protected] 1GSA Data Repository item 2019359, details of methods, sample descriptions, and supplementary tables and figures, is available online at http://www.geosociety. org/datarepository/2019/, or on request from [email protected]. Published online 4 September 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/11/1015/4852055/1015.pdf by guest on 31 October 2019 1016 www.gsapubs.org | Volume 47 | Number 11 | GEOLOGY | Geological Society of America lower part of the Church Rock Member in the San Rafael Swell (Umbarger, 2018). Efforts to define ages based on magnetostratigraphy of the Glen Canyon Group have also yielded age estimates. These results were calibrated against the Newark Basin (northeastern United States) magnetostratigraphy (Molina-Garza et al., 2003; Kent et al., 2017; Suarez et al., 2017), paleopole paths (Steiner and Helsley, 1974; Bazard and Butler, 1991), or the Paris Basin (France) (Steiner, 2014a, 2014b; Steiner and Tanner, 2014; Yang et al., 1996). With the exception of the estimates by Bazard and Butler (1991), these results are broadly consistent with the observed stratigraphic succession (Fig. DR1A; Table DR2). Importantly, Steiner and Tanner (2014) reported that the Kayenta Formation in the Moab, Utah, region was not correlative with the Kayenta Formation near Kanab, Utah; Steiner and Helsley (1974) placed the Kayenta Formation in the Moab area in the Late Triassic. The Moenave Formation in the Kanab area has yielded bio-, chemo-, and magnetostratigraphic, as well as DZ, data (Donohoo-Hurley et al., 2010; Lucas et al., 2011; Suarez et al., 2017). Suarez et al. (2017) built on and corrected the magnetoand biostratigraphy of DonohooHurley et al. (2010) and Lucas et al. (2011) at Potter Canyon, Utah, who placed a brief reversed polarity interval in the Whitmore Point Member in the latest Triassic. Suarez et al.’s (2017) DZ ages led them to conclude that the reversed polarity interval likely correlates to one of such intervals in the Hettangian in the Newark Basin (Kent et al., 2017). METHODS Samples were collected from six carbonate units described by Parrish et al. (2017) in the erg margin of the Navajo Sandstone (Fig. 1; see sample descriptions in Table DR3). They were analyzed by laser ablation for U/Pb (Table DR4); three samples were then chosen for isotope dilution analyses. The carbonate samples were slabbed (1–2 cm long, 1–2 mm thick) and polished for laser ablation–inductively coupled plasma–mass spectrometry (LA-ICPMS). Line scans with LA-ICPMS helped identify areas of elevated U/Pb, which were subsequently targeted for spot analyses. National Institute of Standards and Technology standard NIST612 (trace elements in glass) and the carbonate standard WC-1 (Roberts et al., 2017) were used to correct for drift in U and Pb and for Pb isotope fractionation, and to correct downhole fractionation between U and Pb using Iolite software (Paton et al., 2011). Laser ablation results are provided in Table DR4. The samples subjected to full analysis were 0805123 and 0805132 from the Rocky Tops locality (Fig. DR1B) and 0805032 from the Horsethief Road locality of Parrish et al. (2017). Samples were microsampled with a Dremel tool, spiked with a mixed 236U-205Pb spike, and dissolved in nitric acid. The samples were capped and left on a hotplate for >12 h before being dried for column chemistry. The Pb and U cuts were both run on a NuII MC-ICPMS using a desolvating nebulizer in the Facility for Isotope Research and Student Training (FIRST) lab at Stony Brook University, New York, USA. The lead isotope standard reference material (SRM) 981 was used for Pb analyses, and New Brunswick Laboratory (NBL) CRM-112-a (uranium metal assay and isotopic standard) was used for U analyses. Samples were bracketed by standards, and the bracketing Ma * Kayenta Fm * Springdale Ss * Black Ledge ss * Whitmore Point Mbr, Moenave Fm Navajo Ss (this study) Wingate Ss Dinosaur Canyon Mbr, Moenave Fm