MAGNETOSTRATIGRAPHY OF CENOZOIC SEDIMENTS IN THE BIDAHOCHI FORMATION AND THE LOWER COLORADO RIVER CORRIDOR

Abstract

Magnetostratigraphy utilizes Earth’s ancient magnetic field to help define timing of deposition and potential depositional rates of a formation, especially when coupled with other dating methods. The Bidahochi Formation, located upstream of the Grand Canyon, contains fluvial-lacustrine deposits that record the presence of paleo Hopi Lake that was filled by the early Colorado River. Previous geochronological and paleomagnetic studies place the onset of sedimentation in the Bidahochi Formation around 16 Ma and continued until approximately 5.9 Ma. Downstream of the Grand Canyon, the Bouse Formation within the lower Colorado River corridor (LOCO) records the arrival of the Colorado River to the southern drainage basins. Lost Cabin beds within Cottonwood and Mohave valleys of the LOCO were deposited prior to Colorado River integration. This study investigates sediments east of the Grand Canyon and combines new and previous data south of the Grand Canyon (LOCO) to further constrain the timing of Colorado River integration and related sediments using magnetostratigraphic sampling and analytical methods. 81 total samples were taken from two different sites within the upper member of the Bidahochi Formation to determine the magnetic polarity and correlate them to the Geomagnetic Polarity Time Scale (GPTS). Three sites with 27 samples within the LOCO were chosen to further constrain previous magnetostratigraphic work done on the Lost Cabin beds (pre Bouse) and Bouse Formation. Magnetite and titanomagnetite were identified to be the primary minerals that held magnetic remanence within the samples. All the samples from this study are interpreted to have a primary magnetization that is further interpreted as a detrital remanent magnetization (DRM), held in magnetite and/or titanomagnetite. Upper Bidahochi samples yielded majority normal polarity inclinations except for a handful of samples. The normal polarity samples within the upper Bidahochi are interpreted to be within the normal C3An.2n (6.727 Ma – 6.386 Ma) polarity subchron with the handful of reverse polarity samples interpreted to be an excursion within the C3An.2n subchron. New depositional rates within the upper Bidahochi were calculated from this data and range from 0.313 mm/yr to 0.175 mm/yr. In the LOCO, Lost Cabin bed samples bolstered previous magnetostratigraphy studies, with all reverse polarity inclinations interpreted to be within the reverse C3r Gilbert chron (6.023 Ma – 5.235 Ma). A Bouse channel within the older Lost Cabin beds also yielded similar results from previous study, showing three reverse polarity sites and one ambiguous normal polarity site. Hart Mine Wash Bouse samples were taken above and below the ~4.8 Ma Hart Mine wash ash to try and further constrain a potential reversal at this location, but more sampling is needed.