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New geochronologic and stratigraphic evidence confirms the paleocene age of the dinosaur-bearing ojo alamo sandstone and animas formation in the San Juan Basin, New Mexico and Colorado

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Dinosaur fossils are present in the Paleocene Ojo Alamo Sandstone and Animas Formation in the San Juan Basin, New Mexico, and Colorado. Evidence for the Paleo-cene age of the Ojo Alamo Sandstone includes palynologic and paleomagnetic data. Palynologic data indicate that the entire Ojo Alamo Sandstone, including the lower dinosaur-bearing part, is Paleocene in age. All of the palynomorph-productive rock samples collected from the Ojo Alamo Sandstone at multiple localities lacked Creta-ceous index palynomorphs (except for rare, reworked specimens) and produced Paleocene index palynomorphs. Paleocene palynomorphs have been identified strati-graphically below dinosaur fossils at two separate localities in the Ojo Alamo Sand-stone in the central and southern parts of the basin. The Animas Formation in the Colorado part of the basin also contains dinosaur fossils, and its Paleocene age has been established based on fossil leaves and palynology. Magnetostratigraphy provides independent evidence for the Paleocene age of the Ojo Alamo Sandstone and its dinosaur-bearing beds. Normal-polarity magnetochron C29n (early Paleocene) has been identified in the Ojo Alamo Sandstone at six localities in the southern part of the San Juan Basin. An assemblage of 34 skeletal elements from a single hadrosaur, found in the Ojo Alamo Sandstone in the southern San Juan Basin, provided conclusive evidence that this assemblage could not have been reworked from underlying Cretaceous strata. In addition, geochemical studies of 15 vertebrate bones from the Paleocene Ojo Alamo Sandstone and 15 bone samples from the underlying Kirtland Formation of Late Creta-ceous (Campanian) age show that each sample suite contained distinctly different abundances of uranium and rare-earth elements, indicating that the bones were miner-alized in place soon after burial, and that none of the Paleocene dinosaur bones ana-lyzed had been reworked.
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... This specimen was discovered by a Navajo Indian in 1983. This man visited the Farmington Bureau of Land Management Fassett and Lucas (2000), Fassett et al. (2002), andFassett (2009a). This paper summarizes and paraphrases from those papers for the benefit of the attendees of the New Mexico Geological Society 2010 Fall Field Conference, because we pass so close to this important locality on this field trip. ...
... These Paleocene palynomorphs unequivocally demonstrated that the Ojo Alamo Sandstone and the large hadrosaur femur found within it were Paleocene in age. (Samples from the San Juan River site were also independently analyzed by two other palynologists for their pollen and spore content, and those analyses confirmed the presence of Paleocene index fossils in those collections (Fassett, 2009a). Figure 3.10 contains stratigraphic columns on which are depicted the relationship of the bone and palynologic collection levels within the Ojo Alamo Sandstone. ...
... This paper was puzzling because its second author had been the coauthor of the Fassett and Lucas (2000) paper in which Lucas, five years earlier, had stated the exact opposite -that this bone could not possibly have been reworked from underlying Cretaceous strata. Fassett's (2009a) detailed monograph on the Paleocene dinosaurs of the San Juan Basin included a discussion of the San Juan River site and presented new palynologic, paleomagnetic, and geochemical data establishing unequivocally that the dinosaur fossils of the San Juan Basin were Paleocene in age. Fassett also responded in this paper to the criticisms by Sullivan, Lucas, and Braman (2005) .11 ...
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Description and impact of a large fossil hadrosaur dinosaur femur found at the San Juan River locality.
... At five of these localities in the southern part of the basin (Figs. 1, 2), the reversal is in the continental Fruitland or Kirtland Formation and at the sixth, in the northeast part of the basin (Fig. 3), it is in the marine Lewis Shale. The five paleomagnetic sections in the southwest part of the basin (Figs. 1, 2) are from Butler et al. (1977), Butler and Lindsay (1985), Fassett and Steiner (1997), and Fassett (2009). The sixth paleomagnetic section at the Chimney Rock locality in the northeast part of the basin is from Fassett and Steiner (1997) and Fassett (2009). ...
... The five paleomagnetic sections in the southwest part of the basin (Figs. 1, 2) are from Butler et al. (1977), Butler and Lindsay (1985), Fassett and Steiner (1997), and Fassett (2009). The sixth paleomagnetic section at the Chimney Rock locality in the northeast part of the basin is from Fassett and Steiner (1997) and Fassett (2009). In addition, eight 40 Ar/ 39 Ar sanidine ages for volcanic ash beds ( Fig. 1) were reported in Fassett and Steiner (1997), , and Fassett, (2000). ...
... Two additional Kirtland Formation paleomagnetic reversals were reported by Butler and Lindsay (1985) at the northwest and southeast ends of the outcrop map of Figure 1. Those sections are labeled MM (Moncisco Mesa) and EM (Eagle Mesa) on Figures 1 and 2. (Other paleomagnetic sections in the southern San Juan Basin that did not include the C33n-C32r reversal boundary were obtained and discussed in subsequent papers by the Butler-Lindsay team and so are not relevant to this discussion (those sections are discussed in detail in Fassett, 2009). Fassett and Steiner (1997) sampled the HW (Hunter Wash) paleomagnetic section in Hunter Wash ( Fig. 1) about nine km west of the Hunter Wash-Alamo Wash section of Butler et al. (1977). ...
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Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. AbstrAct-Three new 40 Ar/ 39 Ar sanidine dates of tephra deposits, along with existing stratigraphic and paleomagnetic data, are used to constrain the magnetochron C33n-C32r reversal to 73.7 Ma. These 3 ages agree well with previously published dates when the later are corrected to equal 40 Ar/ 39 Ar flux monitor age and 40 K decay constant. This age contrasts significantly with the Geologic Time Scale of Gradstein et al. (2012) that places the boundary at 74.6 Ma; 600 ka older. A wide range of error estimates (0.05-0.4 Ma) for the reversal age depend on the statistical approach chosen and demonstrate the difficulty in unambiguously determining the uncertainty of magnetochron boundaries that are based on ages of ash layers that stratigraphically bracket the reversal. Both discrepancies in reported ages of reversal boundaries and their uncertainties plague global time scales and impact the ability to unambiguously determine rates of important evolutionary processes and to make stratigraphic correlations. Here we recommend that our new age of 73.7±0.4 Ma (a conservative error) be adopted for the C33n/ C32r paleomagnetic reversal and to be the established age for this reversal in future global time scales. 115
... 63 Ma MDA estimate, these fossil occurrences must all be a product of reworking from underlying strata and do not convey the actual depositional age of the "odd conglomerate. " Occurrences of apparently reworked fossils in both marine and continental K-Pg boundary strata have been a subject of much interest and debate, with some workers arguing for a "conventional" interpretation-that such fossils record exhumation and re-burial of remains derived from Cretaceous strata (e.g., Olsson and Liu, 1993;Arz et al., 2004;Sullivan et al., 2005); others argue, however, that such fossils are not reworked and that the impact horizon therefore preceded the later extinction of Cretaceous taxa (e.g., Keller et al., 2004), or that such fossils record the survival of "Cretaceous" taxa into Paleocene time (e.g., Sloan et al., 1986;Fassett, 2009;Fassett et al., 2010). The MDA assessment reported herein indicates that fossils found in the "odd conglomerate" must certainly be reworked, although the time span separating the original burial of the remains from their later exhumation and re-burial as fossils (~3 million years) seems extraordinary. ...
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The iridium zone has been found at 2 separate localities in the Raton Basin coincident with the palynologic Cretaceous/Tertiary boundary. The iridium zone has been searched for in the San Juan Basin at several localities where the palynologic Cretaceous/Tertiary boundary has been bracketed, but it has not been found. Dinosaur bone has been found within the Ojo Alamo Sandstone in the San Juan Basin stratigraphically higher than the palynologic Cretaceous/Tertiary boundary; this bone apparently apparently postdates the event that created the iridium zone. -from Author
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The Ojo Alamo Sandstone of the San Juan Basin of New Mexico is composed of interbedded conglomeratic sandstone, sandstone, siltstone, and mudstone. Conglomerates are found in the western part of the basin; siliceous pebbles diminish in size both southward and eastward across the basin, becoming rare to nonexistent in the eastern part. There is great variation in the internal stratigraphy of the Ojo Alamo: Individual sandstone or mudstone beds thicken, thin, and pinch out laterally. The thickness of the Ojo Alamo varies from 6 m (20 ft) to more than 122 m (400 ft). The formation varies in composition from one to as many as five sandstone beds with interbeds of siltstone or mudstone. Dinosaur bone has been found within the Ojo Alamo at several sites in the western part of the basin. Paleocene pollen has been found within the Ojo Alamo in the eastern part of the basin. To date, no Cretaceous pollen has been found at or above the stratigraphic level of dinosaur bone within the Ojo Alamo Sandstone. Near Barrel Spring, in the southwest part of the basin, both dinosaur bone and Paleocene pollen have been found. One bone, found at the top of the Ojo Alamo, was loose on the surface, and its significance is therefore equivocal. Dinosaur bone, however, has also been found in place in the upper part of the Ojo Alamo about 1.6 km (1 mi) west of Barrel Spring, at about the same stratigraphic level as Paleocene pollen from a site just east of Barrel Spring. Because there is no apparent unconformity between the highest in-place bone level and the Paleocene pollen level in this area, the Ojo Alamo dinosaurs, if not reworked, are Paleocene in age at this site and probably throughout the San Juan Basin.
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Dinosaurs from the Upper Cretaceous Fruitland Formation and Kirtland Shale in the San Juan Basin, New Mexico, pertain to the following taxa: Ornithomimidae, cf. Ornithomimus edmonticus, cf. Struthiomimus alius, Dromaeosauridae, Albertosaurus sp., cf. Tyrannosaurus rex, Alamosaurus sanjuanensis, ?Pachycephalosauridae, Ankylosauria, Ankylosauridae, Nodosauridae, lEuoplocephalus sp., IPanoplosaurus sp., Hadrosauridae, Kritosaurus navajovius, Parasaurolophus tubicen, P. cyrtocristatus, Ceratopsidae, cf. Chasmosaurus sp., Pentaceratops sternbergii, P. fenestratus, and Torosaurus cf. T. utahensis. The dinosaur fauna of the Fruitland Formation is temporally equivalent to the dinosaur faunas of the Judith River (Montana) and Oldman (Alberta) Formations and is of late Campanian (Judithian) age. This correlation is based primarily on the absence in the Fruitland Formation of dinosaurs typical of post- Judithian dinosaur faunas elsewhere in western North America. The dinosaur fauna of the Kirtland Shale below the Naashoibito Member is virtually identical to that of the Fruitland Formation. Based on stratigraphic relationships, the Kirtland Shale must be younger than the Fruitland Formation and may be as young as Edmontonian (latest Campanian-early Maastrichtian) below the Naashoibito Member. The Naashoibito Member of the Kirtland Shale contains cf. Tyrannosaurus rex, Alamosaurus sanjuanensis, and Torosaurus cf. T. utahensis, taxa indicative of a Lancian (middle-late Maastrichtian) age. Therefore, Kritosaurus from the Naashoibito Member represents the youngest known occurrence of this genus. The Lancian age of the Naashoibito Member indicates that the unconformity at the base of the Ojo Alamo Sandstone is not of as great a temporal magnitude as most previous workers had believed. Thus, there is a nearly complete record of the Cretaceous-Tertiary transition in the west-central San Juan Basin, indicated by dinosaur-based correlation of the Fruitland and Kirtland Formations. This correlation is consistent with most other evidence, except magnetostratigraphy, that has been used to determine the age of the Fruitland and Kirtland Formations.
Schowalteria clemensi n.g. n.sp., from the Late Cretaceous Scollard Formation, Red Deer River Valley is the first Mesozoic taeniodont to be discovered. Although more primitive in important features of the postcanine dentition than the conoryctid Onychodectes (of mid-Puercan age, New Mexico, and previously the most primitive tacniodont known), S. clemensi most resembles the advanced, stylinodontid taeniodonts in incisor morphology, canine specializations, facial proportions, and zygomatic arch construction. The dentition of Schowalteria indicates that the purported affinity of palaeoryctid insectivorans with Onychodectes is based only on homoplastic resemblances, leaving taeniodont relationships unresolved.