New Mexico Geological Society Annual Spring Meeting — Abstracts

Isopach patterns, pinch-outs, and regional lithologic data for rocks of Jurassic and Cretaceous age are either permissive of, or supportive of, large-scale right-slip of Laramide age along the eastern Colorado Plateau boundary in north-central New Mexico. Because of widespread erosion of the southern basin-margins of the Jurassic Todilto and Entrada Formations in east-central New Mexico, no definitive arguments comparing pinch-outs or isopach trends can be made with those of better preserved equivalents on the Colorado Plateau. Perhaps significantly, however, the Picuris-Pecos fault and its southward bifurcations (Tijeras-Canoncito and Glorieta Mesa-Estancia Basin faults), all known or suspected Laramide right-oblique structures, divide thick (30-45 m) gypsiferous Todilto exposures on the west from thin ( <7 m) non-gypsiferous exposures to the east, suggesting either Jurassic syndepositional tectonism or subsequent strike-slip juxtaposition. Recent documentation of exposures of Jurassic Morrison Formation southeast of Socorro and in the northern Sierra Blanca Basin necessitate a minimum 80 km dextral step of possible tectonic origin in the southern pinch-out of this unit across what is now the southern Albuquerque and Socorro Basins of the Rio Grande rift. East-west comparisons of other isopach data for this unit are inconclusive.

Dextral deflection of pinch-outs of three Upper Cretaceous regressive sandstones (Tres Hermanos, Point Lookout and Hosta-Dalton) occur across the eastern Colorado Plateau boundary, but control points are too widely spaced to establish the origin of these deflections. In the western Albuquerque Basin-Sierra Nacimiento area, the pinch-out of the Gallup Sandstone exhibits a closely constrained dextral offset of at least 22 km across a Laramide precursor to the Sand Hill fault and its northern bifurcations (including Nacimiento and Jemez(?) faults). None of these Mesozoic data, unfortunately, provide useful constraints that encompass the entire breadth of the Laramide oblique-slip zone, which may have accommodated 60-120 km of right-slip based on other lines of service.