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 Sevier fold-thrust belt was a mountain range forming to the west of the Kaiparowits while the Cordilleran volcanic arc was further west in California. Although the Straight Cliffs Fm was deposited in an ancient basin it is preserved in a modern physiographic plateau. The Kaiparowits Plateau covers 3,600 km2 and preserves strata located roughly 120 km east of the leading edge of the thrust front at the time of deposition. First analyzed for its coal content, the Straight Cliffs Formation was assessed by Gregory and Moore (1931) and later by Peterson (1969a, 1969b) and Vaninetti (1979). The formation has four