A Remotely Sensed and Paleomagnetic Perspective on the Bonelli Tuff of NW AZ and SE CA

Abstract:

The southern Black and Cerbat Mountains of NW AZ and the Sacramento Mountains of SE CA preserve ignimbrites associated with multiple episodes of volcanic activity that span at least a million years. Unraveling the stratrigraphy of these deposits, as well as their eruptive centers, is critical for constraining the volcanic history of this ignimbrite, the 18.8 Ma Peach Spring Tuff, is the recently identified 17.7 Ma Tuff of Bonelli House (TB) (Ferguson & Cook 2015) and may also occur in the southern Black and Sacramento Mountains. To help determine the extent and possible source of the TB, we have performed a combined remote sensing and paleomagnetic study of this unit, including possible correlatives.

Paleomagnetic work involved Remanence and anisotropic magnetic susceptibility methods. Drill samples were collected and processed at Scripps Institute of Oceanography & Pomona College. An AC current was run to obtain the Paleomag current, as opposed to the traditional of heating up the cores at specific intervals. Sacramento Mountains samples produced an average direction of 200.9 / -26.4, which contrasts the Peach Spring Tuff paleodirection of 036.4/33 (Wells & Hillhouse, 1989). An AMS direction was determined using a MFK1 Kappabridge instrument and consistently showed similar flow direction to that of the PST. In compiling our data on a map, we took into account the Whipple Detachment Fault, ~40 km westward (Lister & Davis, 1989).

We were able to identify a spectral signature and remnant paleomagnetic direction for the TB and identify potential additional outcrops in the southern Black mountains. AMS showed us that the ignimbrites originated from a source in the Silver Creek Caldera, which may indicate the PST at TB were produced from a similar source. The remnant paleomagnetic direction allows us to closely correlate these tuff units as occurring within a similar timeframe. The contrasting paleodirection of the TB and the PST allows us to confidently say that the Peach Spring and Bonelli Tuffs occurred at different times when the Earth’s magnetic field directions were different.