Paleomagnetic Constraints on Timing of Reddening in Ferruginous Draping Beds, Sacramento Mountains, New Mexico

Kevin E. Nick, R. Douglas Elmore

Red and orange beds in carbonate sequences are commonly attributed to syngenetic oxidation due to prolonged subaerial exposure. In this study, we demonstrate that stratigraphically continuous ferruginous beds do not necessarily represent early oxidation. In the Upper Pennsylvanian Holder Formation in the Sacramento Mountains, New Mexico, successive carbonate cycles and biohermal mounds are draped by 30-60 cm thick ferruginous-stained beds. The staining has been interpreted as recording syngenetic oxidation and precipitation of iron oxides during times of low sea level. Our work demonstrates that the iron oxides and hydroxides formed due to modern weathering of ferroan dolomite.

Paleomagnetic analysis of the ferruginous beds by stepwise thermal demagnetization gives a mean declination of 10°E and inclination of +61° (k = 330, ^agr₉₅ = 1.9), which corresponds to the modern field direction for the study location. Preliminary investigation of samples from the biohermal mounds indicate a complex magnetization with unresolved, although apparently secondary components of magnetization. Maximum blocking temperatures less than 110°C for the major component and other rock magnetic studies suggest the remanence is carried by goethite. Petrographic analysis indicates that goethite is present associated with calcitized dolomite in the samples. The amount of ferruginous stain and total magnetic intensity of the samples also decrease away from the exposed edge of the carbonate beds. These results suggest that the remanence in the ferruginous beds is a chemical remanent magnetization that formed as a result of modern, not syngenetic, diagenetic processes.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.