Morrison Formation, San Juan Basin
General Setting
The San Juan Basin formed during the Late Cretaceous-early Tertiary Laramide orogeny as an asymmetric syncline that steepens the northwest limb (Kernodle, 1996). In most areas, the basin is well defined by bounding faults and other structural features (for example, the Defiance and Nutria Monoclines). However, in some areas (for example, the Gallup and Acoma Sags and the Four Corners Platform), the boundary from the San Juan to adjacent basins is much less distinct (Kernodle, 1996).
Anderson and Lucas (1995), called into question the previously accepted stratigraphic definition of the Morrison. They instead defined the Morrison Formation as a two-member unit with a base of Morrison coincident with the "J-5" sequence boundary, thus excluding the Recapture Member from the Morrison and including it in the older San Rafael Group. Anderson and Lucas (1995) also proposed that the so-called "Westwater Canyon Member" is actually equivalent to the Salt Wash Member. They proposed that the base of the Morrison occurs at a regionally traceable scour surface. In the northwest San Juan Basin, this change is significant because the base of the Morrison, as now defined, is the base of what had been called the "Westwater Canyon Sandstone Member" but which Anderson and Lucas consider to be the Salt Wash Member. As early as 1980, Green (1980) identified and mapped this unconformity and suggested that it was a significant surface because, among other things, no uranium ore deposits existed below the unconformity. However, the U.S. Geological Survey study by Kernodle (1996), from which we have taken many data, specifically includes the Recapture and Westwater Canyon Members as part of the Morrison.
Regardless of the sequence stratigraphic interpretation of the Morrison Formation, the literature (for example, Kernodle, 1996) suggests that there is a confined aquifer in the San Juan Basin roughly equivalent to the lower portion of the Morrison. This aquifer is overlain by a less-permeable unit composed of green, olive, and maroon mudstone interbedded with tuff beds (Aubrey, 1992) known as the Brushy Basin Member. Therefore, with regard to the current study, the main consequence of this stratigraphic controversy is not whether there is a potential CO2 sequestration target but whether it includes or excludes certain members. The user should be aware that the maps of Dam and others (1990) and Kernodle (1996) used in this study include the Recapture Member in the Morrison Formation.
References
Anderson, O. J., and Lucas, G., 1995, Base of the Morrison Formation, Jurassic, of northwestern New Mexico and adjacent areas, New Mexico: Geology, v. 17, no. 3, p. 44–53.
Aubrey, W. M., 1992, New interpretations of the stratigraphy and sedimentology of uppermost Jurassic to lowermost Upper Cretaceous strata in the San Juan Basin of northwestern New Mexico: U.S. Geological Survey Bulletin B 1808-J, p. J1–J17.
Dam, W. L., Kernodle, J. M., Thorn, C. R., Levings, G. W., and Craigg, S. D., 1990, Hydrogeology of the Morrison Formation in the San Juan structural basin, New Mexico, Arizona, and Utah: U.S. Geological Survey, Hydrologic Investigations Atlas, HA-0720-J, 2 sheets.
Fassett, J. E., 1991, Oil and gas resources of the San Juan basin, New Mexico and Colorado, in Gluskoter, H. J., Rice, D. D., and Taylor, R. B., eds., The decade of North American geology, v. P-2, Economic Geology, U.S.: Geological Society of America, p. 357–372.
Freethey, G. W., 1987a, Upper Colorado River Basin regional aquifer-systems analysis—Mesozoic rocks in Colorado, Utah and Wyoming, Arizona, and New Mexico, in McLean, J. S., and Johnson, A. I., eds., Regional aquifer systems of the United States—aquifers of the western mountain area: 23rd Annual American Water Resources Association (AWRA) Conference and Symposium, Salt Lake City: AWRA Monograph Series No. 14, p. 57–70.
___________ 1987b, Lithologic and hydrologic properties of Mesozoic rocks in the Upper Colorado River Basin, Mexico, in McLean, J. S., and Johnson, A. I., eds., Regional aquifer systems of the United States—aquifers of the western mountain area: 23rd Annual American Water Resources Association (AWRA) Conference and Symposium, Salt Lake City: AWRA Monograph Series No. 14, p. 81–100.
Galloway, W. E., 1980, Deposition and early hydrologic evolution of the Westwater Canyon and other fluvial fan systems, in Rautman, C. A., compiler, Geology and mineral technology of the Grants uranium region: New Mexico Bureau of Mines and Mineral Resources Memoir 38, p. 59–69.
Green, M. W, 1980, Disconformities in Grants mineral belt and their relationship to uranium deposits, in Rautman, C. A., compiler, Geology and mineral technology of the Grants uranium region: New Mexico Bureau of Mines and Mineral Resources Memoir 38, p. 70–74.
Kernodle, J. M., 1996, Hydrogeology and steady-state simulation of ground-water flow in the San Juan Basin, New Mexico, Colorado, Arizona, and Utah: U.S. Geological Survey, Water-Resources Investigations Report 95-4187, 117 p.
Kirk, A. R., and Condon, S. M., 1986, Structural control of sedimentation patterns and the distribution of uranium deposits in the Westwater Canyon Member of the Morrison Formation, northwestern New Mexico—a subsurface study, in Turner-Peterson, C. E., Santos, E. S., and Fishman, N. S., eds., A basin analysis case study: the Morrison Formation Grants uranium region New Mexico: American Association of Petroleum Geologists Studies in Geology No. 22, p. 105–143.
National Imagery and Mapping Agency, 2000, Digital terrain elevation data (DTED Level 0)
Turner, C. E., and Fishman, N. S., 1991, Jurassic Lake T'oo'dichi': a large alkaline, saline lake, Morrison Formation, eastern Colorado Plateau: Geological Society of America Bulletin, v. 103, no. 4, p. 538–558.
Turner-Peterson, C. E., 1987, Sedimentology of the Westwater Canyon and Brushy Basin Members, Upper Jurassic Morrison Formation, Colorado Plateau, and relationship to uranium mineralization: University of Colorado, Ph.D. dissertation, 169 p.
Prepared by Ramón H. Treviño.
