New Mexico Geological Society Annual Spring Meeting — Abstracts

The Kelly Limestone (Mississippian) of central New Mexico is located in widespread outcrops along the western margins of the Rio Grande rift. It nonconformably overlies Precambrian igneous and metamorphic rocks and is unconformably overlain by the Pennsylvanian Sandia Formation. The lower Caloso MenIDer consists of sandstones, shales, and various limestones. The Ladron Member disconformably overlies the Caloso and is composed mostly of crinoidal grainstones.

Basal limestones of the Caloso Member have produced conodonts assigned to Patrognathus variabilis, Polygnathus inornatus, and Bispathodus aculeatus plumulus indicating upper sulcata Zone (early Kinderhookian). The top of the caloso and lower Ladron Member have yielded diagnostic conodont faunas from upper typicus to lower anchoralis-Iatus Zones (early Osagean): Gnathodus cuneiformis, Gnathodus delicatus, Gnathodus typicus M1 and M2, Polygnathus communis communis and Pseudopolygnathus oxypageous M1 . and M3. The Ladron Member produced conodonts throughout its thickness except for the silver pipe facies. The Ladron conodont fauna above the silver pipe consists of elements diagnostic of upper texanus Zone (upper Osagean): Cloghergnathus texanus, "Spathognathodus" deflexus, and Taphrognathus varians. The top of the Ladron is assigned a latest Osagean-Meramecian age because it contains specimens belonging to the genus Cavusgnathus.

The Caloso Member was determined to be Kinderhookian by early workers based on brachiopod faunas. It was later reassigned a middle Osagean age based on the endothyrids Latiendothyra, Medioendothyra, and Tuberendothyra. The Ladron Member was assigned to the late Osagean based on brachiopod and blastoid assemblages, and the first occurrence of the foraminifers Prisella, Pseudotaxis, and Tetrataxis.

Results of this study have shown that the Caloso Member is early Kinderhookian at its base and early Osagean at the top. The disconformity between the two members corresponds to the upper anchoralis-latus to lower texanus Zones (middle Osagean).