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
Straight Cliffs Formation Member were subaerially exposed and eroded before Drip Tank deposition. After the period of erosion river systems swept over the Kaiparowits Plateau and deposited the braided fluvial sheet deposits sands of the Drip Tank Member. The most diverse and abundant fossils found in the Straight Cliffs Formation are invertebrate fauna. The fauna observed include oysters, ammonites, inoceramids, bivalves, ostracods and foraminifera. Oysters are one of the most common invertebrate fossils found in the Straight Cliffs Formation and are often preserved in large shell hash beds in marginal marine parts of the section. Only the Tibbet Canyon and John Henry members