New Mexico Geology

Fieldwork in the Little Hatchet Mountains of New Mexico was undertaken to investigate the potential of recovering nannofossils from the early Albian Old Hachita Member of the U-Bar Formation. These samples contain the first record of Early Cretaceous nannofossils from the Bisbee Group, which consists of sediments deposited along a marine embayment that extended east-west across southern New Mexico and Arizona in the United States, northern Chihuahua and northern and central Sonora in Mexico, and possibly into the Gulf of California. Samples from the Old Hachita Member were analyzed, and 20 of the 35 samples yielded nannofossils. The nannofossils are generally poorly preserved and of low diversity, but key biostratigraphic markers such as Braarudosphaera batilliformis, Prediscosphaera columnata, and Crucibiscutum bosunensis indicate the Old Hachita Member is of early Albian age; this is supported by the presence of ammonites of the early part of the Douvilleiceras Zone. Maximum sea level attained in the Old Hachita Member is interpreted in a sample 25 m above the base of the unit based on maximum nannofossil diversity. Another potential sea-level increase is identified between 54 and 56 m above the base of the section. This study highlights the potential of using nannofossils in interpreting the precise ages and paleobathymetry of the Early Cretaceous Bisbee Group seaway in other parts of New Mexico, Arizona, and northern Mexico.