GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 5 2018 THE UPPER JURASSIC MORRISON FORMATION IN NORTH-CENTRAL NEW MEXICO—LINKING COLORADO PLATEAU STRATIGRAPHY TO THE STRATIGRAPHY OF THE HIGH PLAINS Spencer G. Lucas Theme Issue An Ecosystem We Thought We Knew— The Emerging Complexities of the Morrison Formation SOCIETY OF VERTEBRATE PALEONTOLOGY Annual Meeting, October 26 – 29, 2016 Grand America Hotel Salt Lake City, Utah, USA © 2018 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to [email protected]. GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 5 2018 Editors UGA Board Douglas A. Sprinkel Thomas C. 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The Newsletter Editor Bill Lund [email protected] 435.590.1338 Cretaceous Dakota Sandstone rests disconformably on the Jackpile Member and is seen as a brown sandstone at Become a member of the UGA to help support the work of the Association and the top of the outcrop. Photograph by Spencer G. Lucas. receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. This is an open-access article in which the Utah The UGA board is elected annually by a voting process through UGA members. Geological Association permits unrestricted use, However, the UGA is a volunteer-driven organization, and we welcome your distribution, and reproduction of text and figures that voluntary service. If you would like to participate please contact the current are not noted as copyrighted, provided the original president or committee member corresponding with the area in which you would author and source are credited. like to volunteer. Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. Affiliated with the American Association of Petroleum Geologists. i GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 5 2018 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Spencer G. Lucas New Mexico Museum of Natural History, 1801 Mountain Road N. W., Albuquerque, NM 87104, USA; spencer. [email protected] ABSTRACT Most study of the Upper Jurassic Morrison Formation has focused on its spectacular and extensive outcrops on the southern Colorado Plateau. Nevertheless, outcrops of the Morrison Formation extend far off the Colorado Plateau, onto the southern High Plains as far east as western Oklahoma. Outcrops of the Morrison Formation east of and along the eastern flank of the Rio Grande rift in north-central New Mexico (Sandoval, Bernalillo, and San- ta Fe Counties) are geographically intermediate between the Morrison Formation outcrops on the southeastern Colorado Plateau in northwestern New Mexico and on the southern High Plains of eastern New Mexico. Previous lithostratigraphic correlations between the Colorado Plateau and High Plains Morrison Formation outcrops using the north-central New Mexico sections encompassed a geographic gap in outcrop data of about 100 km. New data on previously unstudied Morrison Formation outcrops at Placitas in Sandoval County and south of Lamy in Santa Fe County reduce that gap and significantly add to stratigraphic coverage. At Placitas, the Morrison Formation is about 141 m thick, in the Lamy area it is about 232 m thick, and, at both locations, it consists of the (ascending) sandstone-dominated Salt Wash Member, mudstone-dominated Brushy Basin Member, and sandstone-dominat- ed Jackpile Member. Correlation of Morrison strata across northern New Mexico documents the continuity of the Morrison depositional systems from the Colorado Plateau eastward onto the southern High Plains. Along this transect, there is significant stratigraphic relief on the base of the Salt Wash Member (J-5 unconformity), the base of the Jackpile Member, and the base of the Cretaceous strata that overlie the Morrison Formation (K unconfor- mity). Salt Wash Member deposition was generally by easterly-flowing rivers, and this river system continued well east of the Colorado Plateau. The continuity of the Brushy Basin Member, and its characteristic zeolite-rich clay facies, onto the High Plains suggests that localized depositional models (e.g., “Lake T’oo’dichi’) need to be re-eval- uated. Instead, envisioning Brushy Basin Member deposition on a vast muddy floodplain, with some localized lacustrine and palustrine depocenters, better interprets its distribution and facies. INTRODUCTION of the uranium resources they contain (e.g., Kirk and Condon, 1986; Turner-Peterson and others, 1986; An- Jurassic strata of the Morrison Formation exposed derson and Lucas, 1997). To the east, on the southern in north-central New Mexico provide an important link High Plains, the Morrison strata have not received such between two extensive Morrison outcrop belts. To the intensive study, but are clearly genetically related to west, Morrison strata of the southern Colorado Pla- Morrison strata on the southern Colorado Plateau (e.g., teau have been studied intensively, particularly because Lucas and others, 1985; Lucas and Woodward, 2001). Citation for this article. Lucas, S.G., 2018, The Upper Jurassic Morrison Formation in north-central New Mexico–linking Colorado Plateau stratigraphy to the stratigraphy of the High Plains: Geology of the Intermountain West, v. 5, p. 117–129. © 2018 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to [email protected]. 117 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Here, I review the Morrison Formation stratigraphy (figure 2) and on the San Cristobal Ranch near Lamy in along a transect from the southern Colorado Plateau to Santa Fe County (figures 3 to 6) that fill important gaps the southern High Plains in north-central New Mexico in earlier coverage of the regional Morrison Formation (figure 1). My goal is to correlate the Morrison Forma- lithostratigraphy (figures 1 and 7). tion strata between San Ysidro on the eastern Colorado Plateau edge, and at Romeroville on the western edge of STRATIGRAPHIC SECTIONS the southern High Plains. This correlation has import- ant implications for the extent and nature of Morrison Introduction unconformities, lithofacies and depositional systems. The correlation presented here is of six surface Lucas and Anderson (1998) reviewed Jurassic stra- stratigraphic sections and one well log across an about tigraphy in New Mexico; the stratigraphic nomencla- 141 km transect of northern New Mexico (figures 1 and ture they advocated is on the current geologic map of 7). Four of these Morrison sections and the well log have already been described