The Dakota Sandstone
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New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/25 The Dakota Sandstone (Cretaceous) of the southern part of the Chama Basin, New Mexico--A preliminary report on its stratigraphy, paleontology, and sedimentology Keith Grant and Donald E. Owen, 1974, pp. 239-249 in: Ghost Ranch, Siemers, C. T.; Woodward, L. A.; Callender, J. F.; [eds.], New Mexico Geological Society 25th Annual Fall Field Conference Guidebook, 404 p. This is one of many related papers that were included in the 1974 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. 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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. New Mexico Geol. Soc. Guidebook, 25th Field Conf., Ghost Ranch (Central-Northern N.M.), 1974 239 THE DAKOTA SANDSTONE (CRETACEOUS) OF THE SOUTHERN PART OF THE CHAMA BASIN, NEW MEXICO- A PRELIMINARY REPORT ON ITS STRATIGRAPHY, PALEONTOLOGY, AND SEDIMENTOLOGY by KEITH GRANT and DONALD E. OWEN Department of Geology Bowling Green State University, Ohio 43403 INTRODUCTION with a few lenticular sandstones (Smith and others, 1961, p. 13 & 15). The Dakota-Morrison contact is a regional discon- The purpose of this report is to summarize present informa- formity with local channeling and local angularity as, for tion on the Dakota Sandstone of the southern part of the example, at the Highway 84 section (Fig. 2). The contact is Chama basin in southern Rio Arriba County, northern New commonly unexposed due to slumping of resistant lower beds Mexico. The Dakota forms an arcuate, generally south-facing of Dakota sandstone downslope over weak Morrison mud- rim rock which caps the various mesas east of the Chama stones. Canyon extending from Mesa de los Viejos on the west to In the study area the Dakota Sandstone can be subdivided Mesa del Yeso on the east and continuing northeastward as into lower sandstone, middle carbonaceous shale, and upper part of Magote Ridge (Fig. 1). Detailed thickness measure- sandstone units that stand out prominently on the steep cliffs ments, lithological data, trace-fossil descriptions, and primary of the area (Fig. 3). These general subunits have been recog- sedimentary structure measurements were emphasized at eight nized by others in our study area (Smith and others, 1961), measured-section locations (Fig. 1) during field work in traced throughout the Chama basin (Bingler, 1968, p. 24-25), August, 1973. The orientation, type, size, shape, and bound- and extend north into most of adjacent Archuleta County, aries of crossbedding, by far the most abundant inorganic sed- Colorado. imentary structure, were measured in detail. Later laboratory Although the three subunits of the Dakota are lithologically measurements were made of grain-size distribution by sieving, distinct, subunit contacts are difficult to trace precisely. Sand- shale mineralogy by X-ray diffraction, and sandstone petrog- stone beds and lenses, similar to those of the upper sandstone raphy from samples collected. A computer analysis of the unit, occur within the middle carbonaceous shale unit, and crossbedding data is in progress in May, 1974. This data is carbonaceous shale beds commonly subdivide the upper sand- being integrated with earlier observations reported by Owen stone unit (Fig. 4). The subdivision used in Figure 4 is based (1963, 1966, 1969, and 1973) to form a M.S. thesis by Grant mainly on physiographic expression. The lower sandstone unit, (1974, in preparation). The authors' names are listed alphabet- which contains a few thin lenses of light green to locally red- ically in this paper to indicate an equal share of the work. dish noncarbonaceous mudstone, is most easily traced laterally The authors appreciate research grants from the Society of because many of the mudstone lenses can usually be seen to Sigma Xi and Bowling Green State University which made the wedge-out within a single exposure. Good exposures of the latest study possible. We also thank John R. Kostura of Bowl- lower sandstone are laterally discontinuous due to downslope ing Green for designing the computer analysis used and Kath- movement of higher Dakota material over them. leen Grant for assistance with field work. Lower Sandstone Unit STRATIGRAPHY Much, if not all, of the lower sandstone unit has been dis- cussed by several authors (Smith and others, 1961, p. 15; General McPeek, 1965, p. 25-26; Landis and Danke, 1967, p. 2-3) as The Dakota Sandstone of the San Juan-Chama basin region the possible thin, southeasternmost equivalent of the Burro is a complex nonmarine and marine elastic rock stratigraphic Canyon Formation, a probable Lower Cretaceous conglom- unit deposited probably during Albian and Cenomanian (latest eratic sandstone and green-red mudstone unit mainly present Early Cretaceous and earliest Late Cretaceous) time in re- in southeastern Utah and adjacent parts of Colorado. Young sponse to a regional transgression of the epeiric Cretaceous sea (1960, p. 164 and 1973, p. 13) preferred to call these beds the (Landes and others, 1973, p. 1-2, and Owen, 1973, p. 38-39). Cedar Mountain Formation of the Dakota Group. However in Fluvial rocks in the northwestern part of the San Juan Basin this paper the possible Burro Canyon or Cedar Mountain beds and marine rocks in the southeastern part form most of the are included in the lower sandstone unit and are not recog- Dakota while the Chama basin Dakota contains intermediate nized separately because these beds have not been definitely amounts of marine and nonmarine sediments, as is also the traced back to the Burro Canyon type locality in southwestern case in the southwestern part of the San Juan Basin. Colorado or Cedar Mountain type locality in southeastern In the southern part of the Chama basin the Dakota forms Utah and it would be impossible to map the formation in the basal strata of the Cretaceous System unconformably over- question as a separate unit at a scale of 1:25,000 as is required lying the probable Upper Jurassic Morrison Formation—a suc- by the American Commission on Stratigraphic Nomenclature cession of approximately 600 feet of varicolored mudstones (1970, p. 6). This practice of not separating possible Burro Canyon strata follows that of Smith and others (1961) who braided streams in the Chama basin area (Owen, 1973, p. 39- mapped most of the study area at 1:24,000 and Landis and 42). Dane (1967), who mapped the Tierra Amarilla Quadrangle to At most exposures, the uppermost part of the lower sand- the north. stone unit is finer grained and more carbonaceous than sand- As so defined, the lower sandstone unit has an average stones below, and consists of channel-type sandstone beds in- thickness of 135 feet and consists of light colored, highly terbedded with overbank carbonaceous shale. A Dakota-Burro cross-bedded, coarse-grained quartz sandstone, some of Canyon contact could be placed at the base of these carbona- which is pebbly, and locally conglomerate. Lenses and pebble ceous strata, although with great difficulty, especially in map- to boulder-size clasts (Fig. 5) of green, locally reddish, ping anything less than excellent exposures. This contact mudstone occur within the sandstone. This sandstone is a probably records a gradient and climatic change from braided- fluvial deposit apparently laid down by a widespread series of 242 GRANT and OWEN ronments between fluvial and shallow-marine conditions. Owen (1973, p. 45) interpreted it largely as a coastal shale, probably deposited in a variety of floodplain, paludal, and low-salinity paralic locations along a low coastal area. What may be delta-foreset beds are prominent in this unit in the Mesa del Yeso area. Upper Sandstone Unit The upper sandstone unit is quite distinct from the lower sandstone unit, being a tan to brown, generally fine-grained sandstone, containing only a few crossbedded strata and dis- playing abundant burrow-cast trace fossils which are especially evident on exposed bedding planes. The upper contact with the Mancos Shale is rarely well exposed. A prominent, thick bed of sandstone, with a locally ripple-marked upper surface, caps the rim of mesas of the area and forms a good physio- graphic break, but the top of a thinner sandstone bed with a distinctive yellowish weathering color lies about 30 to 60 feet above the rimrock (Fig.