Nacimiento Formation Nacimiento Formation The Nacimiento Formation is a sedimentary rock formation found in the San Juan Basin of western New Mexico (United States) and named for the Nacimiento Mountains. It is a heterogeneous nonmarine formation composed of shale, siltstone, and sandstone, deposited in floodplain, fluvial and lacustrine settings, and made up of sediment shed from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east. It was deposited mostly between ~64.5 and ~61 million years ago, during the early and middle Paleocene. The climate was humid and warm to hot. This unit interbeds with the
Organ Rock Formation been definitively recovered from this formation . As one moves younger in time through the Organ Rock Formation, temnospondyls becomes less abundant with respect other major groups such as diadectomorphs and early synapsids. This could track the general trend of aridification as indicated by sedimentology of the formation. This trend is globally recognized during the Early Permian, supported by many lines of geologic evidence including significant deposits of calcretes, eolianites, and evaporites. Organ Rock Formation The Organ Rock Formation or Organ Rock Shale is a formation within the late Pennsylvanian to early Permian Cutler Group and is deposited across southeastern