Tectonic development of late Pleistocene (Rancholabrean) animal-trapping fissures in the Middle Jurassic Todilto Formation, north-central New Mexico

Open fissures in the Middle Jurassic Todilto Formation near San Ysidro, Sandoval County, New Mexico, trapped large late Pleistocene mammals whose articulated bones are preserved in the fissure fill. The fissures probably originated as the result of two tectonic events—compression that led to closed conjugate fractures during the Laramide orogeny followed by extension associated with development of the Rio Grande rift. During the later event the conjugate shears were opened and additional extensional, open fractures were formed.