THE EFFECTS OF GLOBAL COOLING AFTER THE MID-MIOCENE CLIMATIC OPTIMUM ON THE TESUQUE FORMATION WITHIN THE ESPAÑOLA BASIN IN NORTH-CENTRAL NEW MEXICO

This study attempts to identify the effects of the Mid-Miocene Climatic Optimum (MMCO) and the progressive global cooling thereafter on the local paleoenvironments during deposition of the Tesuque Formation in north-central New Mexico. The Tesuque Formation is well exposed within the northeastern region of the Española Basin. In the study area, the Tesuque Formation has been divided into five formally named members. The lowermost Nambé and the overlying Skull Ridge Members were deposited during the MMCO based on previous ⁴⁰Ar/³⁹Ar age dating of ash beds, whereas the overlying Pojoaque, Cejita, and Cuarteles Members were deposited during the following decline in global temperatures. This research interprets climatic variations across the MMCO through observations of lithostratigraphic, chemostratigraphic, and fossil data. Lithostratigraphic observations, in one example, have indicated a change from warmer conditions in the Skull Ridge Member as shown by the presence of extensive aeolian sandstones to a cooler climate due to their absence in the Pojoaque Member. High resolution chemostratigraphic analyses of the Pojoaque Member using x-ray diffraction have indicated climate fluctuations with the alternating presence and absence of evaporitic minerals. Previous fossil flora discoveries substantiate these interpretations with a palm stump, indicating warmer conditions, in the Skull Ridge and bristlecone pine, indicating cooler subalpine conditions, in the Pojoaque. There is also a noticeable transition of megafauna between the two members. In one instance, camelid fossils are more commonly found within the Skull Ridge, whereas antilocaprid fossils are found more abundantly in the Pojoaque, probably a result of cooling temperatures. Although the changing sedimentation patterns within the study area can be attributed to either tectonic or climatic changes, observed paleontological variations are likely to be the result of shifting environmental conditions. The paleoenvironmental changes as a result of climate change could have ramifications for modern climate change studies.