New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/11 The Dakota Sandstone and Mancos Shale of the eastern side of San Juan Basin, New Mexico Carle H. Dane, 1960, pp. 63-74 in: Rio Chama Country, Beaumont, E. C.; Read, C. B.; [eds.], New Mexico Geological Society 11th Annual Fall Field Conference Guidebook, 129 p. This is one of many related papers that were included in the 1960 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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The concept of this unconformity was briefly Mexico, the Dakota sandstone of Late Cretaceous age is outlined in a paper by the writer that appeared in April about 200 feet thick. It is overlain by 2,000 feet of 1960, in the Bradley volume of the American Journal of Mancos shale divided into five members, successively up- Science. The present discussion reinterprets the obser- ward, the Graneros shale, Greenhorn limestone, Carlile vations made many years ago in the northeastern part of shale, Niobrara calcareous shale, and an unnamed upper the San Juan Basin in the light of the above mentioned member. Beds of Niobrara age in the northeastern part reconnaissance studies in other parts of the Basin and also of the Basin rest on an unconformity, which is represented adds .some details to the writers previously published by about 400 feet of beds in the southern part of the descriptions of the Dakota sandstone and Mancos shale. As basin including much of the Gallup sandstone. Other a necessary background for the present description of stratigraphic changes from southwest to northeast are stratigraphic relations on the east side of the Basin, figures described, principally those that result from the northeast- 1, 2, and 3 of the previously cited paper in the American ward thinning and termination of tongues of littoral and Journal of Science are reproduced in this guidebook al- offshore sandstones that form part of the Mesaverde group though reference will be made principally to figure 3. of the southern part of the Basin. Dakota Sandstone INTRODUCTION The Dakota sandstone includes very fine to coarse- In the northeastern part of the San Juan Basin, New grained, locally conglomeratic sandstone, crossbedded and Mexico, the lower part of the section of Cretaceous rocks parallel-bedded, and dark-gray to black carbonaceous includes the Dakota sandstone, about 200 feet thick; the shale and siltstone. On the eastern side of the San Juan overlying Mancos shale, about 2,000 feet thick; and three Basin it ranges from about 120 feet to somewhat more than formations of the Mesaverde group that merge northeast- 200 feet in thickness. It varies markedly from .place to ward into an indivisible sequence of sandstone and shale. place in lithologic character, in some places consisting These formations are overlain by younger Cretaceous and chiefly of coarser grained clastic rocks, in others including Tertiary rocks that dip gently westward into the central half or more of fine grained carbonaceous shale and silt- part of the basin. stone. In many places, it appears to rest on an erosional The present paper deals only with the stratigraphy unconformity cut in the underlying Morrison formation, and correlation of the Dakota sandstone and Mancos but no angular discordance has been observed and the shale of the northeastern part of the Basin with equivalent unconformity is nowhere conspicuous. Because both the beds of the southern part. It is based chiefly on mapping top of the Morrison and the base of the Dakota may con- done by the writer and several associates in 1933 and sist of sandstone or conglomeratic sandstone not strikingly 1936 to 1938 from the west side of the southern part of dissimilar in gross lithology, the contract can in some places the Nacimiento Mountains northward to the Colorado— be located only by close inspection. On such inspection, New Mexico boundary, in the vicinity of Chama, New however, the formations are readily discriminated, the Mexico (fig. 1). A preliminary map showing geologic sandstone of the Morrison being white or having a slightly structure (Dane and Bryson, 1938) and a geologic map of greenish cast in distinction to the normal weathered colors part of the area and a short description of the geology have of the Dakota sandstone, very pale orange, tan, or brown. appeared (Dane, 1948). From time to time during the succeeding years recopnaissance stratigraphic observations The Morrison sandstone units are ordinarily coarser grained, commonly conglomeratic, and more strikingly crossbedded of the Cretaceous rocks in other parts of the basin have been made by the writer and associates and reported on and lenticular. A somewhat more tangible criterion is the in part (Dane, Wanek, and Reeside, 1957; Dane, Bach- almost universal occurrence in the sandstone of the Dakota man, and Reeside, 1957; Dane and Bachman, 1957). These of carbonaceous material, either as specks distributed observations, although scattered and incomplete, have re- through the rock, as films or sheets along the bedding sulted in some modification of previous interpretations of surfaces, as recognizable residues of stems or parts of plants, and more rarely as small blocks of mineral charcoal. the regional stratigraphic relations of the lower part of the section of Upper Cretaceous rocks in the San Juan Such carbonaceous plant debris is absent or exceedingly Basin. One important modification is the concept of an rare in the Morrison, in which, however, silified wood is extensive unconformity in the northern part of the Basin not uncommon. Most typically the Dakota of this region consists of a at the base of the rocks of the age of the Inoceramus de- formis zone. This zone is present in the middle to upper top hard massive ledge of fine-grained tan to brown part of the Fort Hays limestone member at the base of the quartzitic sandstone, 50 feet or more thick, and an under- Niobrara formation in the Arkansas Valley region in east- lying sequence of generally softer beds consisting of an alternation of velvety black thin-bedded shale, gray sandy Publication authorized by the Director, U. S. Geological shale, and light-tan bedded sandstone (fig. 7). The Survey. upper ledge of sandstone in places breaks up into an Figure 1. Index map of part of northwestern New Mexico and southeastern Colo- rado showing locality numbers projected into lines of sections A-A (fig. 2) and B-B (fig. 3). Tertiary and Quaternary rocks and surficial deposits (stip- pled), Jurassic and older rocks (lined), and Cretaceous rocks (unpatterned). [From Dane, 1960, American Journal of Science, v. 258A, Bradley Volume, fig. 1] 1 .•.:::.1:7MESAVERDE SOUTH GROUP B Poink:A.00kouti.:sandatone 8 EXPLANATION Vertical scale Lithology greatly generalized -600 FEET Satan tongue :0C.2 Sandstone Carbonaceous shale, coal and other rocks ) -400 Hosta tongue of Point Limestone Marine shale Lookout sandstone Unconformify -200 a Localitongyfnuue shown 0 1 O U >- -0 —p z 10 0 lo 20 30 stray sandstone* of Sears, Horizontal scale Hunt, and Hendricks Dalton sandstone member Mulatto tongue . " ul 2 S44+4S"• Galiego landatene . , maw), foss tongue of Pescado? tongue d Mancos shale 0. Juana Lopez sandstone member (of Rankin) of Carlile shale Greenhorn limestone membe r.. mr-w • - — E E Y.) b o i E Graneros shale member DAKOTA I SANDSTONE DAKOTA 55 South end North end D-Cross Cebolleta Cebolleta Rinconada San Casa Caberon- Senorita Regina Gallina El Vado Monero Chrome), Mountain Mesa Mesa Can on Ygnacio Salazar San Luis River Colorado 0 0 (.3 0 () (1) . FIgure 3. Diagrammatic interpretation of the intertonguing relations of the Dakota sandstone, Mancos shale, and the lower formation of the Meraverde group on the east side of the San Juan Basin from D-Cross Mountain, Socorro County, New Mexico, to Archuleta County, Colorado.