The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 5 2018 © 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 GIW@utahgeology.org. 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 GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover View of upper part of Morrison Formation at Hub Can- yon near Lamy in Santa Fe County, New Mexico. Pale green sandstone ledge in the foreground is Salt Wash Member overlain by greenish slopes of mudrock-domi- nated Brushy Basin Member capped by white-colored Jackpile Member at top of Morrison Formation. The Cretaceous Dakota Sandstone rests disconformably on the Jackpile Member and is seen as a brown sandstone at the top of the outcrop. Photograph by Spencer G. Lucas. i 2018 President Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 2018 President-Elect Peter Nielsen peternielsen@utah.gov 801.537.3359 2018 Program Chair Emily McDermott ekeller@utah.gov 801.537.3389 2018 Treasurer Zach Anderson zanderson@utah.gov 801.538.4779 2018 Secretary Christopher Kravits ckravitsgeo@gmail.com 2018 Past President Bill Loughlin bill@loughlinwater.com 435.649.4005 UGA Board UGA Committees Education/Scholarship Loren Morton lmorton@utah.gov 801.536.4262 Environmental Affairs Craig Eaton eaton@ihi-env.com 801.633.9396 Geologic Road Sign Terry Massoth twmassoth@hotmail.com 801.541.6258 Historian Paul Anderson paul@pbageo.com 801.364.6613 Membership Rick Ford rford@weber.edu 801.626.6942 Public Education Paul Jewell pwjewell@mines.utah.edu 801.581.6636 Matt Affolter gfl247@yahoo.com Publications Roger Bon rogerbon@xmission.com 801.942.0533 Publicity Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Social/Recreation Roger Bon rogerbon@xmission.com 801.942.0533 AAPG House of Delegates 2017–2020 Term Tom Chidsey tomchidsey@utah.gov 801.537.3364 State Mapping Advisory Committe UGA Representative Jason Blake blake-j@comcast.net 435.658.3423 UGA Newsletter Newsletter Editor Bill Lund uga.newsletter@gmail.com 435.590.1338 UGA Website www.utahgeology.org Webmasters Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Lance Weaver lanceweaver@utah.gov 801.403.1636 Become a member of the UGA to help support the work of the Association and 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. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would 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. Volume 5 2018 This is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. Earthquake Safety Committe Chair Grant Willis gwillis@utah.gov 801.537.3355 Douglas A. Sprinkel Utah Geological Survey 801.391.1977 GIW@utahgeology.org Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Society of Vertebrate Paleontology Editors Kelli C. Trujillo — University of Wyoming John Foster — Museum of Moab Cary Woodruff — University of Toronto Octavio Mateus — Universidade Nova de Lisboa Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 5 2018 117 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. 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. lucas@state.nm.us 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 GIW@utahgeology.org. INTRODUCTION Jurassic strata of the Morrison Formation exposed in north-central New Mexico provide an important link between two extensive Morrison outcrop belts. To the west, Morrison strata of the southern Colorado Pla- teau have been studied intensively, particularly because of the uranium resources they contain (e.g., Kirk and Condon, 1986; Turner-Peterson and others, 1986; An- derson and Lucas, 1997). To the east, on the southern High Plains, the Morrison strata have not received such intensive study, but are clearly genetically related to Morrison strata on the southern Colorado Plateau (e.g., Lucas and others, 1985; Lucas and Woodward, 2001). 118 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 Here, I review the Morrison Formation stratigraphy along a transect from the southern Colorado Plateau to the southern High Plains in north-central New Mexico (figure 1). My goal is to correlate the Morrison Forma- tion strata between San Ysidro on the eastern Colorado Plateau edge, and at Romeroville on the western edge of the southern High Plains. This correlation has import- ant implications for the extent and nature of Morrison unconformities, lithofacies and depositional systems. Lucas and Anderson (1998) reviewed Jurassic stra- tigraphy in New Mexico; the stratigraphic nomencla- ture they advocated is on the current geologic map of New Mexico (NMBGMR, 2003) and is employed here. Lucas and others (1999) and Lucas and Woodward (2001) presented recent reviews of Jurassic stratigraphy along parts of the transect from the Colorado Plateau to the High Plains. The oldest Cretaceous strata above the Jurassic section are referred to the Dakota Sand- stone/Formation in north-central New Mexico, though alternative names have been proposed elsewhere that may provide more appropriate nomenclature for this lithostratigraphic unit (e.g., Mateer, 1987; Carpenter, 2014). I can add new data here in the form of Morrison Formation stratigraphy at Placitas in Sandoval County (figure 2) and on the San Cristobal Ranch near Lamy in Santa Fe County (figures 3 to 6) that fill important gaps in earlier coverage of the regional Morrison Formation lithostratigraphy (figures 1 and 7). STRATIGRAPHIC SECTIONS Introduction The correlation presented here is of six surface stratigraphic sections and one well log across an about 141 km transect of northern New Mexico (figures 1 and 7). Four of these Morrison sections and the well log have already been described in detail (see Lucas and others, 1999 and references cited therein and below), so they are only briefly reviewed here. Two of the sur- face sections of the Morrison Formation (Placitas, San Cristobal Ranch) have not been described previously, so they are new data presented here. San Ysidro Section Anderson and Lucas (1996; also see Woodward, 1987) reviewed Jurassic stratigraphy along the south- eastern edge of the Colorado Plateau in the vicinity of San Ysidro. The Jurassic section there is approxi- Figure 1. Map of part of north-central New Mexico, showing locations of the Jurassic sections discussed in this article. All sections are in shown in figure 7; the detailed section at Placitas is in figure 2; and the detailed sections on the San Cristobal Ranch are in figures 4 and 5. 119 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 Figure 2. Stratigraphic sections of Jurassic strata measured at Placitas, Sandoval County, New Mexico. A, B, C and D are four overlapping sections measured on different fault blocks, and the column on the right is the composite Jurassic section. Locations of sections (GPS coordinates are UTM meters, zone 13, datum NAD 27). (A) Base of section is located at 367734E, 3906520N, top at 367750E, 3906646N; (B) Base of section is located at 366918E, 3906929N, top at 367034E, 3907157N; (C) Base of section is located at 369914E, 3907759N, top at 369933E, 3907813N; and (D) Section is located at 366307E, 3907103N. Member-level stratigraphic nomenclature of the Entrada Sandstone is based on Lucas and Heckert (2003). 120 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 mately 300 m thick and is assigned to the (ascending) Entrada, Todilto, Summerville, and Morrison forma- tions (figure 7). The Morrison Formation consists of the Salt Wash, Brushy Basin, and Jackpile members. The Salt Wash Member is up to 36 m thick and con- sists mostly of pale yellow, trough cross-bedded, fine to coarse-grained sandstone with chert-pebble conglom- eratic zones (mainly in the basal part) interbedded with thinner units of siltstone and mudstone. The overlying Brushy Basin Member is as much as 80 m thick and mostly variegated, smectitic mudstone, and contains a few beds of lenticular, fine- and medium-grained sand- stone. The Jackpile Member (Owen and others, 1984) is as much as 60 m thick and is mostly pale yellow to white, kaolinitic sandstone with some green mudstone interbeds. The Encinal Canyon Member of the Dakota Formation rests disconformably on the Jackpile Mem- ber at San Ysidro (Lucas and others, 1998). Shell Oil Santa Fe No. 1 From the Jurassic outcrops on the eastern edge of the Colorado Plateau near San Ysidro to Jurassic outcrops on the eastern margins of the Rio Grande rift--at Placitas, in the Hagan basin and at Galisteo Dam—a distance of more than 50 km is covered by younger strata (figure 1). To aid in correlation across this gap, a subsurface section about midway between the Colorado Plateau edge and the Sandia uplift along Figure 3. Overview photographs of Morrison Formation sections at Hub Canyon (A) and Hub Mesa (B). 121 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 the eastern boundary of the Rio Grande rift is the geophysical log of the Shell Oil Santa Fe #1 well in section 18, T. l3 N., R. 3 E. (figure 7). Interpretation of this log by Black and Hiss (1974) and by Lucas and others (1999) indicates that the well penetrated the Entrada, Todilto, Summerville, and Morrison Forma- tions in the subsurface. Black and Hiss (1974) assigned the 145 m (642 ft) of Jurassic strata drilled by the well from 2210 to 2076 m (7252–6810 ft) to the Morrison Formation and at- Figure 4. Stratigraphic section of Jurassic strata exposed at Hub Mesa on the San Cristobal Ranch, Santa Fe County, New Mexico. Location of section (GPS coordinates are UTM meters, zone 13, datum NAD 83): Base of section is located at 420858E, 3908188N, top at 420426E, 3908577N. 122 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 tempted no further subdivisions. However, Lucas and others (1999) concluded that the 79-m-thick (260-ft- thick) mudstone-dominated interval from 2106 to 2185 m (6910–7170 ft) is the Brushy Basin Member. If the underlying 33 m (110 ft) of strata belong to the Salt Wash Member (a thickness comparable to the outcrops at San Ysidro), then the base of the Salt Wash corre- sponds to the base of a prominent sandstone at 2220 m (7280 ft) (figure 7). All of the sandstone strata from 2106 to 2075 m (6910 to 6810 ft), about 31 m (about 102 ft), are assigned to the Jackpile Member of the Morrison For- mation, though it is possible that a few meters of sand- stone at the base of the Cretaceous Dakota Sandstone is included in this sandstone interval (figure 7). Placitas Placitas is located at the northern end of the Sandia Mountains in Sandoval County (figure 1). Here, Jurassic strata are exposed on fault blocks at the northern end of the Sandia uplift. Most of the published information on the Jurassic section at Placitas is on geologic maps (Kelley and Northrop, 1975; Menne, 1999; Connell and others, 1995). In 2000, a complete Jurassic section was measured as four overlapping sections on different fault blocks (figure 2). Anderson and Lucas (2000), in an ab- Figure 5. Stratigraphic section of the Morrison Formation at Hub Canyon on the San Cristobal Ranch, Santa Fe County, New Mexico. Location of section (GPS coordinates are UTM meters, zone 13, datum NAD 83). Base of section is located at 419981E, 3910395N, top at 419527E, 3911025N. 123 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 stract, briefly reported on this section, which consists of the (ascending) Entrada, Todilto, Summerville, and Morrison Formations. At Placitas, the basal member of the Morrison For- mation, the Salt Wash Member, is as much as 71 m thick and consists of fine- to coarse-grained, feldspathic sandstone, with pebbly (conglomeratic) zones as much as several meters thick. In addition to the pebbly beds, other distinguishing features are trough cross-bedding, presence of clay clasts (rip-ups) and nonmarine biotur- bation. The overlying Brushy Basin Member is as much as 49 m thick and consists of greenish, mostly smectitic claystone with thin interbeds of dark-weathering, fine- grained sandstone and nodular limestone. The upper- most member of the Morrison Formation, the Jackpile Member, is as much as 21 m thick locally and consists of trough and planar cross-bedded kaolinitic sandstone. The Cretaceous Oak Canyon Member of the Dakota Sandstone (Cretaceous) unconformably overlies the Jackpile Member at Placitas. Hagan Basin Lucas and others (1995a) described in detail the Figure 6. Photographs of selected outcrops of the Morrison Formation in the Hub Canyon section. (A) Cross-bedded sand- stone in the Salt Wash Member (figure 5, unit 13). (B) Dinosaur limb-bone fragment in conglomeratic sandstone of the Salt Wash Member (figure 5, unit 23). (C) Silica-pebble conglomerate of the Jackpile Member (figure 5, unit 51). (D) Contact of Jackpile Member with overlying basal sandstone of the Oak Canyon Member of the Dakota Sandstone (figure 5, units 63 and 64). 124 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 Jurassic section exposed in the Hagan basin (figure 1). The entire Jurassic section here is as much as 440 m thick and consists of the Entrada, Todilto, Summerville, and Morrison Formations. In the Hagan basin, the Salt Wash Member of the Morrison Formation disconform- ably overlies the Summerville and is as much as 75 m of mostly grayish-yellow sheets of fluvial sandstone and conglomerate with some interbeds of olive and brown mudstone. The Salt Wash Member grades upward into and intertongues with the Brushy Basin Member of the Morrison Formation, as much as 128 m of mostly var- iegated olive, gray, and brown bentonitic mudstone/silt- stone. The Jackpile Member of the Morrison Formation disconformably overlies the Brushy Basin Member and Figure 7. Lithostratigraphic correlation of Jurassic sections from San Ysidro on the southeastern edge of the Colorado Pla- teau to Romeroville, on the western edge of the southern High Plains, a transect of about 141 km. For location of sections see figure 1. 125 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 is as much as 62 m of mostly light gray and pale orange, trough-cross bedded kaolinitic sandstone. Cretaceous strata of the Dakota Sandstone (Encinal or Oak Canyon members) disconformably overlie the Jackpile Member in the Hagan basin (Lucas and others, 1998). Galisteo Dam The Jurassic section at Galisteo Dam (figure 1) is assigned to the (ascending) Entrada, Todilto, Sum- merville, and Morrison Formations (Lucas and others, 1999). At Galisteo Dam, the Salt Wash Member is 113 m thick and mostly consists of laterally extensive beds of yellow, gray, and brown arkosic sandstone and con- glomerate. Green bentonitic mudstone lenses and beds appear in the upper half of the member and support the idea that the Salt Wash Member grades upward into the Brushy Basin Member. At Galisteo Dam, the first laterally extensive, thick (9 to 10 m) bed of variegated green and brown smec- titic mudstone is the base of the Brushy Basin Member, which is nearly 82 m thick. This type of mudstone is the dominant lithology of the Brushy Basin Member local- ly, which forms a gentle slope between the cliff- or cues- ta-forming Salt Wash and Jackpile Members. Brushy Basin sandstone beds form thin ledges in that slope and are mostly dark brown, massive, lithic subarkoses. These sandstone beds are less extensive laterally than are sand- stone intervals in the Salt Wash Member. The Jackpile Member is 77 m thick at Galisteo Dam. It is mostly yellowish-gray and very pale orange, trough cross-bedded, kaolinitic, subarkosic sandstone with some interbeds of greenish-colored sandy mudstone. The base of the Dakota Sandstone (Oak Canyon Mem- ber; no Encinal Canyon Member is present at Galisteo Dam: Lucas and others, 1998) is a prominent discon- formity at which clayey, carbonaceous litharenite of the Dakota Sandstone rests directly on kaolinitic subarkose of the Jackpile Member. San Cristobal Ranch South of Lamy, the San Cristobal Ranch is a large holding of patented land that encompasses most of the old San Cristobal Land Grant (figure 1). This land was largely inaccessible to geologists until the current owner of the ranch purchased it in 1985. Thus, the only infor- mation on the Jurassic stratigraphy of the San Cristobal Ranch previously published was geological mapping by Read and others (1944). I measured two sections of Ju- rassic strata on the San Cristobal Ranch, at Hub Mesa and at Hub Canyon (figures 3 to 6). The outcrops at Hub Mesa expose the entire local Jurassic section below the Morrison Formation (Entrada, Todilto, and Summer- ville Formations; Lucas and Cather, 2004) overlain by a nearly complete section of the Salt Wash Member of the Morrison Formation (figure 4). At Hub Canyon, a com- plete section of the Morrison Formation is exposed, be- tween the underlying Jurassic Summerville Formation and below the overlying Cretaceous Dakota Sandstone (figure 5). At Hub Mesa, the Salt Wash Member of the Morri- son Formation is about 90 m thick, but this is an incom- plete section of the unit (figure 4). However, the top of the Salt Wash Member should be a laterally persistent unit overlain by easily eroded Brushy Basin Member mudstone. Thus, the fact that the top of the Hub Can- yon section is a mesa defended by Salt Wash sandstone beds (figure 3B) suggests that it may be very close to the top of the Salt Wash Member. The Salt Wash Member at Hub Mesa (figure 5) is about two-thirds sandstone (68% of the section) and almost one third mudstone (31%). Conglomerate (1%) is a minor constituent of the measured section. Most sandstone beds are subarkosic and have trough cross- beds, but laminar beds are also present. Colors are mostly light gray, grayish yellow and yellowish brown. Mudstone beds that are mostly reddish and less often greenish in color form slopes between sandstone bench- es. These mudstone beds are not the swelling, smec- tite-rich clays found higher in the section (see below). Conglomerate beds are lenses in sandstone intervals of intraformational (mudstone, calcrete) rip-up clasts. At Hub Canyon (figure 5), the Salt Wash Member is about 46 m thick. It is almost equally sandstone (46% of the section) and mudstone (49% of the section) with minor conglomerate (5%). Most of the sandstone beds have trough cross-beds, and some are bioturbated. Mudstone is mostly reddish colored with some green- ish-colored bands. Conglomerate beds are pebbly lens- es of intraformational (mudstone) pebbles, except for 126 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 bed 15, which has extraformational siliceous pebbles. Mudstone in the Salt Wash Member does not swell on outcrop when wet. The Brushy Basin Member at Hub Canyon is about 108 m thick. It is mostly mudstone (89% of the mea- sured section) with a minor amount of sandstone (11%). Mudstone colors are those characteristic of the Brushy Basin Member regionally—pale green, grayish red, reddish, and salmon. The stratigraphically lowest clay that swells on outcrop with the addition of water is bed 39 (figure 5), and this apparently is what has been called the “clay line” in the Morrison Formation section, at which the mineralogy changes from dominantly illit- ic to smectitic clay. However, I note that Trujillo (2006) pointed out that such outcrop observations can be mis- leading because smectitic clays with a high silt content may not swell when wet on outcrop. At Hub Canyon, the Jackpile Member is about 35 m thick. It is mostly sandstone (63% of the measured section), about one-quarter mudstone (27%), and about 10% conglomerate. Sandstone is subarkosic and kaolinitic, and yellowish gray and white, as is characteristic of the Jackpile Member farther west. Romeroville The Jurassic section exposed at Romeroville Gap in San Miguel County (figure 1) was most recently de- scribed by Lucas and others (1995b, 1999). Here, the Jurassic section is approximately 85 m thick and is as- signed to the Entrada, Todilto, Summerville, and Mor- rison Formations (figure 7). Lucas and others (1999) placed the base of the Mor- rison Formation at Romeroville at a change from silt- stone of the Summerville Formation to coarser, pebbly, trough cross-bedded sandstone that characterizes a 5-m-thick interval assigned to the Salt Wash Member (figure 7). Coarse grain size, the presence of clay clasts, and yellowish-brown color characterize the Salt Wash sandstone interval at Romeroville. Bentonitic mud- stone beds that are variegated brownish, greenish, and grayish dominate the overlying Brushy Basin Mem- ber, which is 80 m thick. The Lower Cretaceous (up- per Albian) Mesa Rica Sandstone of the Dakota Group rests disconformably on the Brushy Basin Member at Romeroville (Lucas and others, 1998). LITHOSTRATIGRAPHIC CORRELATION Correlation of Jurassic strata across the northern Rio Grande rift (figure 7) is a straightforward one based on lithostratigraphy. Thickness variation of the units in part reflects unconformities or facies changes and is discussed below. No paleontological control of the cor- relation is available, there are no radioisotopic ages from the sections, and magnetostratigraphy is only available for the Romeroville section (Steiner and others, 1994). Thus, the correlation presented here is purely litho- stratigraphic, matching the member-level lithosomes across the transect (figures 7 and 8). SOME IMPLICATIONS The correlation presented here of Morrison strata across northern New Mexico improves our understand- ing of the nature and extent of regional Jurassic uncon- formities, and of deposition in the Jurassic paleobasin. It documents the continuity of the Morrison depositional systems from the Colorado Plateau eastward onto the southern High Plains. The base of the Salt Wash Member of the Morrison Formation is here considered to be the regional J-5 un- conformity (Anderson and Lucas, 1995, 1998; Lucas and Anderson, 1998). The varied thickness changes in the Salt Wash Member evident across northern New Mexico (figures 7 and 8) likely reflect paleotopogra- phy that developed during the hiatus preceding Mor- rison deposition. Indeed, the thickness trends of the Salt Wash Member across northern New Mexico may identify a large paleovalley in the Hagan Basin-Galisteo Dam-San Cristobal Ranch area that was filled during early Salt Wash Member deposition (figure 8). Jurassic strata in northern New Mexico are overlain by Cretaceous strata at what Pipiringos and O’Sullivan (1978) termed the K unconformity. These are strata of the Dakota Group (Formation, Sandstone) of late Albi- an or middle Cenomanian age (Lucas and others, 1998). This unconformity (which represents as much as a 50 million-year-long hiatus) has substantial stratigraphic relief on top of the Morrison Formation, and the disap- pearance of the Jackpile Member between the San Cris- 127 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 tobal Ranch and Romeroville may in large part be due to erosion at this unconformity (figure 8). The base of the Jackpile Member also has substan- tial stratigraphic relief on top of the Brushy Basin Mem- ber (figure 8). This suggests that the regional (confined to northern New Mexico) low sinuosity river system that deposited the Jackpile Member (e.g., Schlee and Moench, 1961; Moench and Schlee, 1967; Flesch, 1974; Owen and others, 1984) was either (1) incised into the extensive muddy floodplain deposits of the Brushy Basin Member, or (2) there is a hiatus between the Brushy Basin and Jackpile Members during which an incised topography developed through erosion, to be later filled during Jackpile deposition. Unfortunately, there is insufficient age control on the Jackpile Member with which to evaluate these alternatives. Reconstructions of Morrison depositional systems in the southern Western Interior have largely been con- fined to the Colorado Plateau (e.g., Peterson, 1994). These studies do not include the Morrison strata on the southern High Plains, and therefore do not encompass the southern or the eastern portion of the Morrison pa- leobasin. Thus, it is clear that Salt Wash Member depo- sition was generally by easterly-flowing rivers (Craig and others, 1955; Peterson, 1994; Anderson and Lucas, 1997), but this river system continued well east of the Colorado Plateau, and did not simply end in mudflats and dunes on the Colorado Plateau as depicted by Pe- terson (1994, figure 20). Salt Wash rivers flowed gen- erally from west to east across the Morrison depo- sitional basin, and clastic input to and subsidence of the basin was highly asymmetrical, with the greatest accumulations of Salt Wash Member in the western part of the basin (e.g., Craig and oth- ers, 1955; Anderson and Lucas, 1997). Continuity of the Brushy Basin Member off the Col- orado Plateau onto the High Plains also has particular significance for interpretation of depositional systems. Turner and Fishman (1991) argued that Brushy Basin Member deposition took place in a large, semi-arid lake centered in the Four Corners area with a maxi- mum northwest-southeast dimension of 500 km. How- ever, Brushy Basin Member outcrops off the Colorado Plateau were not included in this lake system, which was termed “Lake T’oo’dichi’” by Turner and Fishman (1991). Dunagan and Turner (2004) later revised this interpretation to recognize an extensive wetland (palus- trine depositional system) instead of a lake centered in the Four Corners area. However, the continuity of the Brushy Basin Mem- ber, and its characteristic zeolite-rich clay facies, onto the High Plains suggests that these localized deposi- tional models need to be re-evaluated. Instead, Brushy Basin Member deposition on a vast muddy floodplain, with some localized lacustrine and palustrine depocen- ters, better encompasses the distribution and facies of the Brushy Basin Member (e.g., Anderson and Lucas, 1997; Galli, 2003, 2014; Tanner and others, 2014). Mor- rison depositional systems encompassed a significant area east of the Colorado Plateau and need to be inter- preted across their entire outcrop area. Figure 8. Restored cross section of Morrison Formation from San Ysidro on the southeastern edge of the Colorado Plateau to Romeroville, on the western edge of the southern High Plains, a transect of about 141 km. For location of sections see figure 1. Datum is base of Brushy Basin Member. J-5 and K unconformities are indicated on the right side of the dia- gram. 128 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 ACKNOWLEDGMENTS Orin Anderson was instrumental in the fieldwork to obtain the stratigraphic data presented here. John Es- tep, Jess Hunley and Clarence Pigman assisted in the field. Will Singleton granted access to the San Cristobal Ranch, and Wesley Layman facilitated fieldwork there. John Foster, Ken Galli, and Kelli Trujillo provided help- ful reviews of the manuscript. REFERENCES Anderson, O.J., and Lucas, S.G., 1995, Base of the Morrison For- mation, Upper Jurassic, of northwestern New Mexico and ad- jacent areas: New Mexico Geology, v. 17, p. 44–53. Anderson, O.J., and Lucas, S.G., 1996, Stratigraphy and deposition- al environments of Middle and Upper Jurassic rocks, south- eastern San Juan Basin, New Mexico: New Mexico Geological Society Guidebook 47, p. 205–210. Anderson, O.J., and Lucas, S.G., 1997, The Upper Jurassic Morri- son Formation in the Four Corners region: New Mexico Geo- logical Society Guidebook 48, p. 139–155. Anderson, O.J., and Lucas, S.G., 1998, Redefinition of Morrison Formation (Upper Jurassic) and related San Rafael Group stra- ta, southwestern U.S.: Modern Geology, v. 22, p. 39–69. Anderson, O.J., and Lucas, S.G., 2000, Jurassic stratigraphy at Plac- itas, New Mexico and its regional significance [abs.]: Geolog- ical Society of America Abstracts with Programs, v. 32, no. 7, p. A-458. Black, B.A., and Hiss, W.L., 1974, Structure and stratigraphy in the vicinity of the Shell Oil Co. Santa Fe Pacific No. 1 test well, southern Sandoval County, New Mexico: New Mexico Geo- logical Society Guidebook 25, p. 365–370. Carpenter, K., 2014, Where the sea meets the land—the unresolved Dakota problem in Utah: Utah Geological Association Publi- cation 43, p. 357–372. Connell, S.D., Cather, S.M., Ilg, B., Karlstrom, K.E., Menne, B., Pi- cha, M., Andronicus, C., Read, A.S., Bauer, P.W., and Johnson, P.S., 1995, Geology of the Bernalillo and Placitas quadrangles, Sandoval County, New Mexico: New Mexico Bureau of Mines and Mineral Resources Digital Open-File OF-GM-2 and OF- GM-16, 1 plate, scale 1:24,000. Craig, L.C., Holmes, C.N., Cadigan, R.A., Freeman, V.L., Mullens, T.E., and Weir, G.W., 1955, Stratigraphy of the Morrison For- mation and related formations, Colorado Plateau region, a preliminary report: U.S. Geological Survey Bulletin 1009-E, p. 125–168. Dunagan, S.P., and Turner, C. E., 2004, Regional paleohydrologic and paleoclimatic settings of wetland/lacustrine depositional systems in the Morrison Formation (Upper Jurassic), Western Interior, USA: Sedimentary Geology, v. 167, p. 269–296. Flesch, G.A., 1974, Stratigraphy and sedimentology of the Morri- son Formation (Jurassic), Ojito Spring quadrangle, Sandoval County, New Mexico—a preliminary discussion: New Mexico Geological Society Guidebook 25, p.185–195. Galli, K.G., 2003, Sedimentology and petrology of the Brushy Basin Member, Morrison Formation (Late Jurassic), western Colorado: Amherst, University of Massachusetts, Ph.D. dis- sertation, 402 p. Galli, K.G., 2014, Fluvial architecture analysis of the Brushy Basin Member, Morrison Formation, western Colorado: Volumina Jurassica, v. 12, p. 69–106. Kelley, V.C., and Northrop, S.A., 1975, Geology of the Sandia Mountains and vicinity: New Mexico Bureau of Mines and Mineral Resources Memoir 29, 136 p. Kirk, A.R. and Condon, S.M., 1986, Structurally controlled sed- iment distribution patterns in the Jurassic Morrison forma- tion of northwestern New Mexico and their relationship to uranium deposits, in Peterson, J.A., editor, Paleotectonics and sedimentation in the Rocky Mountain region, United States: American Association of Petroleum Geologists Memoir 41, p. 597–622. Lucas, S.G., and Anderson, O.J., 1998, Jurassic stratigraphy and correlation in New Mexico: New Mexico Geology, v. 20, p. 97–104. Lucas, S.G., and Cather, S.M., 2004, Stratigraphy of the Jurassic San Rafael Group along the Picuris-Pecos fault system, Santa Fe County, New Mexico: New Mexico Geology, v. 26, p. 71. Lucas, S.G., and Heckert, A.B., 2003, Jurassic stratigraphy in west-central New Mexico: New Mexico Geological Society Guidebook 54, p. 289–301. Lucas, S.G., and Woodward, L.A., 2001, Jurassic strata in east-cen- tral New Mexico and their regional significance: New Mexico Geological Society Guidebook 52, p. 203–212 Lucas, S.G., Anderson, O.J., and Estep, J.W., 1998, Stratigraphy and correlation of middle Cretaceous rocks (Albian-Cenomanian) from the Colorado Plateau to the southern High Plains, north-central New Mexico: New Mexico Museum of Natu- ral History and Science Bulletin 14, p. 57–66. Lucas, S.G., Anderson, O.J., and Pigman, C., 1995a, Jurassic stra- tigraphy in the Hagan basin, north-central New Mexico: New Mexico Geological Society Guidebook 46, p. 247–255. Lucas, S.G., Anderson, O.J., and Steiner, M.B., 1995b, The Jurassic section at Romeroville, San Miguel County, New Mexico: New Mexico Geological Society Guidebook 46, p. 51–53. Lucas, S.G., Estep, J.W., and Anderson, O.J., 1999, Correlation of 129 The Upper Jurassic Morrison Formation in North-Central New Mexico–Linking Colorado Plateau Stratigraphy to the Stratigraphy of the High Plains Lucas, S.G. Geology of the Intermountain West 2018 Volume 5 Jurassic strata from the Colorado Plateau to the High Plains, across the Rio Grande rift, north-central New Mexico: New Mexico Geological Society Guidebook 50, p. 317–326. Lucas, S.G., Kietzke, K.K., and Hunt, A.P., 1985, The Jurassic Sys- tem in east -central New Mexico: New Mexico Geological So- ciety Guidebook 36, p. 213–242. Mateer, N.J., 1987, The Dakota Group of northeastern New Mex- ico and southern Colorado: New Mexico Geological Society Guidebook 38, p. 223–236. Menne, B., 1989, Structure of the Placitas area, northern Sandia uplift, Sandoval County, New Mexico: Albuquerque, Universi- ty of New Mexico, M.S. thesis, 163 p. Moench, R.H., and Schlee, J.S., 1967, Geology and uranium depos- its of the Laguna district, New Mexico: U.S. Geological Survey Professional Paper 519, 117 p. NMBGMR (New Mexico Bureau of Geology and Mineral Resourc- es), 2003, Geologic map of New Mexico: Socorro, New Mex- ico Bureau of Geology and Mineral Resources, 2 plates, scale 1:500,000. Owen, D.E., Watters, L.J., Jr., and Beck, R.G, 1984, The Jackpile Sandstone Member of the Morrison Formation in west-cen- tral New Mexico–a formal definition: New Mexico Geology, v. 6, p. 44–52. Peterson, F., 1994, Sand dunes, sabkhas, streams, and shallow seas—Jurassic paleogeography in the southern part of the Western Interior basin, in Caputo, M.V., Peterson, J.A., and Franczyk, K.J., editors, Mesozoic systems of the Rocky Moun- tain region, USA: Denver, Rocky Mountain Section, SEPM (Society for Sedimentary Geology), p. 233–272. Pipiringos, G.N., and O’Sullivan, R.B., 1978, Principal unconfor- mities in Triassic and Jurassic rocks, Western Interior, U.S.—a preliminary survey: U.S. Geological Survey Professional Paper 1035A, 29 p. Read, C.B., Wilpolt, R.H., Andrews, D.A., Summerson, C.H., and Wood, G.H., 1944, Geologic map and stratigraphic sections of Permian and Pennsylvanian rocks of parts of San Miguel, Santa Fe, Sandoval, Bernalillo, Torrance, and Valencia coun- ties, north central New Mexico: U.S. Geological Survey Oil and Gas Investigations Preliminary Map OM-21, 1 plate, scale 1:190,080. Schlee, J.S., and Moench, R.H., 1961, Properties and genesis of “Jackpile” sandstone, Laguna, New Mexico, in Peterson, J.A., and Osmond, J.C., editors, Geometry of sandstone bodies: American Association of Petroleum Geologists Special Publi- cations 22, p. 134-150. Steiner, M.B., Lucas, S.G., and Shoemaker, E.M., 1994, Correlation and age of the Upper Jurassic Morrison Formation from mag- netostratigraphic analysis, in Caputo, M.V., Peterson, J.A., and Franczyk, K.J., editors, Mesozoic systems of the Rocky Moun- tain region, USA: Rocky Mountain Section, SEPM (Society for Sedimentary Geology, p. 315–330. Tanner, L.H., Galli, K.G., and Lucas, S.G., 2014, Pedogenic and la- custrine features of the Brushy Basin Member of the Upper Ju- rassic Morrison Formation in western Colorado—Reassessing the paleoclimatic interpretations: Volumina Jurassica, v. 12, p. 115–130. Trujillo, K.C., 2006, Clay mineralogy of the Morrison Formation (Upper Jurassic-?Lower Cretaceous), and its use in long dis- tance correlation and paleoenvironmental analyses: New Mexico Museum of Natural History and Science Bulletin 36, p. 17–24. Turner, C.E., and Fishman, N.S., 1991, Jurassic Lake T’oo’dichi’—a large saline lake, Morrison Formation, eastern Colorado Pla- teau: Geological Society of America Bulletin, v. 103, p. 538– 558. Turner-Peterson, C.E., Santos, E.S., and Fishman, N.S., editors, 1986, A basin analysis case study—the Morrison Formation Grants uranium region New Mexico: American Association of Petroleum Geologists Studies in Geology 22, 391 p. Woodward, L.A., 1987, Geology and mineral resources of Sierra Nacimiento and vicinity, New Mexico: New Mexico Bureau of Mines and Mineral Resources Memoir 42, 84 p.