• Abstract and Figures
• Public Full-text

New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/61 Correlation of the upper part of the Middle Jurassic San Rafael Group in northeast Arizona, northwest New Mexico, and southeast Utah Robert B. O'Sullivan, 2010, pp. 91-100 in: Geology of the Four Corners Country, Fassett, James E.; Zeigler, Kate E.; Lueth, Virgil W., [eds.], New Mexico Geological Society 61st Annual Fall Field Conference Guidebook, 246 p. This is one of many related papers that were included in the 2010 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks 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. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. NewCORRELATION Mexico Geological Society OF Guidebook,THE UPPER 61st Field PART Conference, OF Four THE Corners MIDDLE Country, 2010, JURASSIC p. 91-100. SAN RAFAEL GROUP 91 CORRELATION OF THE UPPER PART OF THE MIDDLE JURASSIC SAN RAFAEL GROUP IN NORTHEAST ARIZONA, NORTHWEST NEW MEXICO, AND SOUTHEAST UTAH ROBERT B. O’SULLIVAN U.S. Geological Survey, Denver Federal Center, MS 939, Box 25046, Denver, CO, 80225 ABSTRACT—In east-central Utah, the Middle Jurassic San Rafael Group consists, in ascending order, of the Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation, with a combined thickness of about 365 m. In northwestern New Mexico, the sequence, only about 80 m thick, consists of the Entrada Sandstone overlain by the Wanakah Formation the basal part of which is the distinctive Todilto Limestone Member. The San Rafael Group includes a mixture of terrestrial, marine, and marginal marine deposits. An unconformity, termed “J-3”, underlies the Curtis Formation and can be traced into northwestern New Mexico where it lies near the middle of the Wanakah Formation. Previously, the Curtis Formation had been correlated with the Todilto Limestone Member. However, the Todilto lies beneath the J-3 unconformity and is separated from it by about 20 m of strata. All of the Summerville Formation is truncated by another unconformity, termed “J-5”, in east-central Utah, and the formation does not extend into New Mexico and adjacent areas. INTRODUCTION diagram (Fig. 3) shows the stratigraphic positions and distribu- tion of the formations and some of their subdivisions. The J-3 The Middle Jurassic San Rafael Group underlies large areas of unconformity is used as the datum to construct the diagram to southeastern Utah and adjacent areas of Arizona, Colorado, and emphasize the relations of units above and below the unconfor- New Mexico (Fig.1). In the San Rafael Swell in Utah, the group mity. Descriptions of all units are only briefly discussed in this consists of 5 formations, in ascending order, the Page Sand- report; further details are given in the reports cited. stone, Carmel Formation, Entrada Sandstone, Curtis Formation and Summerville Formation (Fig. 2). In exposures at Beclabito Sources of Data Dome in northwestern New Mexico, the sequence consists of the Entrada Sandstone overlain by the Wanakah Formation. The San The stratigraphic diagram (Fig. 3) is drawn from several Rafael Group thickens from 80 m at Beclabito Dome (Strobell, sources. The section in the San Rafael Swell (sec. 1) is from 1956; Condon, 1989) to 153 m in the Moab area (O’Sullivan, Wright and others (1979, p. 43-56). I assign their units 37-40 of 1980a) and to about 365 m in the San Rafael Swell (Fig.1) (Pipir- the Entrada Sandstone, because of lithology, to the unit labeled ingos and O’Sullivan, 1978, pl. 1). Throughout most of the area, “beds at Goblin Valley” (Fig. 3). Sections at Dellenbaugh Butte the strata are underlain by the Glen Canyon Group of Early Juras- and at Bartlett Flat (sec. 3) are from O’Sullivan (1980a) and sic age and overlain by the Morrison Formation of Late Jurassic Caputo and Pryor (1991, fig. 3), respectively. The stratigraphic age. sections at Photo Gap (sec. 4) and Bluff (sec. 5) are from Carr- This report changes some previous stratigraphic assump- Crabaugh and Kocurek (1998, fig. 3), but are slightly modified tions. For example, Anderson and Lucas (1992, p. 80) stated for this report. The “bed at Butler Wash” of the Wanakah For- “…Wanakah Formation is another name for Summerville….” mation (Fig. 3) is extended into Dry Valley to a point just north However, the 2 units discussed here are different and unrelated of Photo Gap, where Carr-Crabaugh and Kocurek (1998, fig. 3) formations. The Summerville Formation is truncated in Utah 200 recognized the Moab Tongue of the Entrada Sandstone. Section km north of Four Corners and does not extend into New Mexico 6 at Beclabito Dome is from Condon (1989) and Strobell (1956), where it is now recognized,for example ,by Kirtland and others and sections 7 (Mexican Water), 8 (Garnet Ridge), and 9 (Red (1995, fig. 15). The basal unit of the Wanakah Formation, the Point) are from O’Sullivan (1978). Todilto Limestone Member, thought to be equivalent to the Curtis Formation, is here shown, instead, to be older and unrelated. J-3 Unconformity STRATIGRAPHY An unconformity at the base of the Curtis Formation, in the San Rafael Swell, described by Gilluly (1929, p. 105-108), In this report only the uppermost parts of the San Rafael Group was later identified as the J-3 unconformity by Pipiringos and are discussed, including the upper part of the Entrada Sandstone O’Sullivan (1978, p. A 23). The eastern limits of the J-3 uncon- and the Curtis and Summerville Formations as well as the related formity as drawn by Pipiringos and O’Sullivan (1978) and, as Wanakah Formation. Two unconformities are associated with now interpreted, are shown in Figure 4. The revision is based on these rocks: (1) the J-5 unconformity, which separates the Mor- later studies by (1) Doelling (2001, pl. 2), who extended the J-3 rison Formation from underlying rocks; and (2) the J-3 uncon- surface from Dellenbaugh Butte on the Green River through the formity, which lies within the San Rafael Group. A stratigraphic Moab area and into western Colorado; and (2) Carr-Crabaugh 92 O’SULLIVAN FIGURE 1. Index map of southeastern Utah and adjacent areas. Shows the distribution of rock units directly under the J-3 unconformity. Stippled area underlain by Entrada Sandstone- the beds at Goblin Valley and the beds at Baby Rocks west and east of the Colorado River, respectively. Short-dash pattern underlain by the lower member of the Wanakah Formation. Unpatterned areas underlain by “slick rock facies” of Entrada Sandstone. Locally, in southwest part of map, the “slick rock facies” of the Entrada Sandstone underlies the J-5 unconformity where the J-3 surface has been truncated (see Peterson, 1988, figs. B5, B7, B13). Triangles labeled f5A-f5E are locations of views of the J-3 shown in Figure 5. The irregular line labeled EF shows the eastern and southern limits of the “earthy facies” of the Entrada Sandstone. CORRELATION OF THE UPPER PART OF THE MIDDLE JURASSIC SAN RAFAEL GROUP 93 near Moab, 38 m thick at Bluff, and according to Strobell (1956) is only 28 m thick at Beclabito Dome. As noted earlier, only the upper part of the formation is discussed in this report. A silty unit at the top of the Entrada Sandstone is here referred to as the “beds at Goblin Valley” west of the Colorado River, and as “the beds at Baby Rocks” in northern Arizona (Fig. 3). Both units directly underlie the J-3 unconformity, are similar lithologically, show similar weathering features, and both display contorted bedding at many localities (Fig. 5). They undoubtedly once formed a continuous north-trending unit (Fig. 1) but the connection has been eroded away by the Colorado River. The beds at Goblin Valley at the top of the Entrada Sandstone were discussed by McKnight (1940, p. 92), who described them in outcrops extending about 10 km east of the Green River as “… a brownish-red muddy sandstone that weathers into rounded “rock baby” surfaces. The bedding is poor but appears to be largely horizontal ..

Load more

  12

Copy Link