New Mexico Geological Society Annual Spring Meeting — Abstracts

In the vicinity of Placitas, on the northern end of the Sandia uplift in Sandoval County, a much faulted Cretaceous section can be pieced together from outcrops on three fault blocks. However, many of the shale-dominated units in this section have been tectonically thinned, so only their minimum thicknesses can be estimated. The base of the section is the Oak Canyon Member of the Dakota Sandstone disconformably overlying the Upper Jurassic Jackpile Member of the Morrison Formation. The overlying sandstone-dominated Cubero Member of the Dakota completes an ~26 m thick Dakota section. The overlying shale-dominated Graneros-Carlile interval is at least 52 m thick and includes a 2-m-thick Greenhorn Limestone ~ 7 m above its base. The overlying Juana Lopez Member is ~ 2 m of bioclastic calcarenite, overlain by a D-Cross equivalent shale interval at least 27 m thick. The overlying silty El Vado Member is at least 76 m thick and overlain by the merged Dalton-Hosta Sandstone, which is up to 72 m thick. The overlying sandy shale-dominated Satan Member is at least 107 m thick. Map data suggest a total Mancos Formation thickness as at least 345 m. The overlying Point Lookout Sandstone is 28 m thick and is overlain by the Menefee Formation, which is at least 150 m thick and caps the Cretaceous section. The Menefee encompasses two coal-bearing intervals split by a sandstone interval ~ 30 m thick. The Cretaceous section at Placitas is very similar to the Cretaceous sections in the Tijeras syncline on the eastern side of the Sandia uplift and in the Hagan basin north of Placitas. These sections document the continuity of Upper Cretaceous lithosomes across the Rio Grande rift and do not support concepts of 50-150 km of right slip along the rift after Cretaceous deposition.