New Mexico Geological Society Annual Spring Meeting — Abstracts

The oldest Triassic strata in west-central New Mexico have previously been assigned to various formations. Originally classified as Shinarump Conglomerate or Dockum Group, stewart et al. (U.S.G.S. Prof. paper #691, 1972) designated them as Moenkopi (?) Formation. These strata are best exposed in the S.E. 1/4 S.E. 1/4 N.E. 1/4, sec. 10, T5N, R4W on Mesa Gallina in the Lucero uplift, where they consist of as much as 70 m of conglomerate, sandstone, siltstone and mudstone, in three to four repetitive sedimentary packages. The conglomerates range from clast to matrix supported, with predominantly limestone clasts in a matrix of gray, to grayish-red sandstone which resembles the non-conglomeratic sandstones. These sandstone show abundant trough and planar crossbedding. The siltstones range from medium reddish-brown to dusky red, yellowish, and gray, and from coarsely laminated with current ripples to massive. The mudstones are mostly dark red to dusky red-purple with green mottling. The sedimentary packages tend to thicken towards the top of the sequence, and the sands also become more laterally continuous and show higher energy sedimentary structures in the upper units. These strata overlie limestone of the San Andres Formation, and underlie mottled strata of the Chinle Formation. Fragments of fossil bone collected from the basal conglomerate of the uppermost unit include three partial vertebrae and most of a scapula of a non-parasuchian thecodont reptile, and one partial interclavical armor plate from a capitosauroid amphibian, indicating an Early to Middle Triassic age. Ostracodes from these strata suggest an Early Triassic age. These fossils preclude assignment of these strata to the Shinarump Conglomerate or the Dockum Group, and together with lithologic and paleocurrent data suggest that these strata are correlative with the Holbrook Member of the Moenkopi Formation of northeastern Arizona.