New Mexico Geological Society Annual Spring Meeting — Abstracts

The Kelly Limestone (Mississippian) of central New Mexico is located in several 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 Sandia Formation (pennsylvanian). The lower Caloso Member consists of sandstones, shales, lime mudstones and wackestones. The upper Ladron Member disconformably overlies the Caloso and is composed of crinoidal grainstones and packstones.

The basal limestones of the Caloso Member have produced conodonts assigned to the species Patrognathus variabilis, Polygnathus inornatus, and Pseudopolygnathus fuslformis indicating a Kinderhookian age. Previously barren strata at the top of the Caloso have yielded undiagnostic conodont faunas; however, one specimen, tentatively identified as Gnathodus typicus M1, would suggest a Kinderhookian -earliest Osagean time. The Ladron Member has produced conodonts throughout its thickness. The lowest units contain the conodonts Gnathodus texanus and Polygnathus communis communis marking the lower texanus Zone (upper Osagean). 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 endothyrid foraminifers, Latiendothyra, Medioendothyra, and Tuberendothyra. The Ladron Member was assigned to the late Osagean based on brachiopod and blastoid assemblages, and the first occurrences of the foraminifers Prisella, Pseudotaxis, and Tetrataxis.

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