New Mexico Geological Society Annual Spring Meeting — Abstracts

The Upper Ordovician Montoya Group (southern New Mexico, west Texas) was deposited on a southward-dipping ramp and contains up to six 3rd-order stratigraphic sequences. These sequences have been correlated by Pope (2004) and Harris et al. (2004) to similar sequences across North America and Estonia. These correlations suggest eustatic sea-level change as the driver; their root cause (tectonic or climatic), however, remains an open question. We are testing the hypothesis that the rock magnetic signal covaries with 3rd-order sea-level change in the Late Ordovician.

We measured the lower two sequences (30-70 m) of the Montoya Group in the Cookes Range, near Deming New Mexico, which are composed of deep-to-shallow ramp carbonates (Pope, 2004). We are comparing sea-level-driven facies changes to rock magnetic values (magnetic susceptibility and anhysteretic remnant magnetization) to evaluate the links between My-year scale changes in sea level and climate. Rock magnetics can be used as a climate proxy for investigating changes in wind intensity/aridity, and/or fluvial sediment influx. Preliminary results show that rock magnetics (2-5 m sampling resolution) in the lower sequence and interpreted sea level covary. Here decreasing magnetic values coincide with transgression (and vice versa) suggesting either wind-intensity/aridity decreased during sea-level rise and/or the influx of fluvially-derived material to the deep ramp decreased. Rock magnetics from the younger sequence show a more complex relationship with sea-level change, which we are presently investigating. To better understand Late Ordovician climate change, we also are extracting conodont apatite from the sequences to examine their oxygen isotopic composition (a climatic proxy for both temperature and ice volume). These isotopic values will further constrain the links between My-scale sea-level and climate change.