Stratigraphy and depositional environments of middle and upper Jurassic rocks, southeastern San Juan Basin, New Mexico
Orin J. Anderson and Spencer G. Lucas
Middle Jurassic rocks of the southeastern San Juan Basin pertain to the San Rafael Group (in ascending order): Entrada Sandstone, Todilto Formation, consisting of the basal Luciano Mesa Member and the Tongue Arroyo Member (previously gypsum member), Summerville Formation, and Bluff Sandstone. The latter is not everywhere mappable. These units record deposition under an arid climate, on a broad coastal plain and/or in a paralic salina associated with the southern margin of the Curtis-Sundance seaway during Callovian–Oxfordian(?) time. Disconformably overlying Upper Jurassic rocks are assigned to the Morrison Formation, consisting of the basal, sandstone-dominated Salt Wash Member, the claystone-dominated, slope-forming Brushy Basin Member, and the sandstone-dominated Jackpile Member. Basal Morrison sediments record a significant climatic change with the onset of Late Jurassic time. More humid climates are strongly suggested by the relatively thick fluvial sequences containing rip-up clasts, local woody trash impressions, numerous scour-and-fill sequences and coarsegrained to pebbly sandstones. In addition, land-vertebrate fossils are much more abundant in the Morrison Formation than in underlying Jurassic rocks, suggesting a greater carrying capacity and biomass production. The climatic change is in turn related to the continued northward drift of Laurasia, which, by Late Jurassic time, brought this area of North America into the zone of prevailing westerly winds.
- Anderson, Orin J.; Lucas, Spencer G., 1996, Stratigraphy and depositional environments of middle and upper Jurassic rocks, southeastern San Juan Basin, New Mexico, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 205-210.