Arjun Heimsath, PhD, Arizona State University
An initial suite of samples for cosmogenic surface exposure and burial dating collected during previous field seasons in southeastern Arizona have been processed and analyzed for 10Be and 21Ne abundances. Graduate student Matthew Jungers (MJ) helped with the processing of the samples from quartz rich surface clasts and buried cobbles beryllium oxide targets ready for mass spectrometer analysis. Beryllium ratios were measured at PRIME Lab (Purdue University) and 21Ne activities were measured at the Berkeley Geochronology Center (BGC) in Berkeley, CA. Initial results have guided an addititional 3-4 weeks of field work in southeastern Arizona this past year with a focus on collecting burial dating samples to constrain the Plio-Pleistocene evolution of Safford Basin crucial to understanding the drainage integration history of the modern Gila River. We also collected river sediment samples from streams draining the Pinaleño Mountains adjacent to the Safford Basin in order to quantify both millennial scale erosion rates for this landscape (using 10Be) and the background levels of non-cosmogenic 21Ne in the sediment to improve our 21Ne analyses.
MJ is also using DEM analyses to reconstruct the paleo-landscape of Aravaipa Creek Basin, a tributary to the San Pedro River. These reconstructions allow the quantification of incision and erosion rates driven by the base level fall following Aravaipa Creek’s integration with the San Pedro.
1) 10Be/21Ne ratios measured in samples from the highest elevation undeformed basin fill surfaces in four key structural basins of southeastern Arizona are lower than expected, thus these samples are not suitable for simple surface exposure interpretations. This finding is especially important in directing our current and future sampling of these deposits. We now focus on burial dating samples to avoid the complications introduced by surface erosion and exhumation of buried clasts.
2) 21Ne concentrations from a depth profile sampled in a perched mid-elevation piedmont surface within the Lower San Pedro Basin suggest a surface exposure age of 500 – 600 ka. This date provides an important minimum age bound for when the San Pedro River began incising into Miocene-Pliocene basin fill, and it provides a benchmark for the pace of incision through the Pleistocene.
3) DEM analyses of Aravaipa Creek Basin suggest integration with the Lower San Pedro via basin overflow, and the subsequent incision rates could have been as low as 60 m/Ma and as high as 130 m/Ma depending on how the timing of integration is inferred. We currently have no absolute dates to constrain this timing for Aravaipa Creek. Our reconstructions also show that at least 13 km3 of sediment have been eroded from the basin forced by drainage integration alone.
Training and development:
Graduate student MJ has been trained by AH to make quantitative observations and process based interpretations from remotely sensed topography data and in the field. MJ was also trained by AH to process terrestrial cosmogenic radionuclide samples for 10Be and 26Al analyses. David Shuster and Greg Balco trained MJ in the analysis of samples for the stable cosmogenic nuclide 21Ne.
We have begun working closely with the Arizona Geological Survey (AZGS) so that our work complements their mapping efforts in southeastern Arizona, and their years of expertise in the region help inform our future sampling efforts. The collaboration included several days of fieldwork with Phil Pearthree, Ann Youberg, and Joe Cook (AZGS) in the Safford Basin during May 2012.
We also conducted a weekend fieldtrip with Ronald Amundson from University of California, Berkeley, to initiate collaboration on a paleoclimate reconstruction of southeastern Arizona using stable isotopes in paleosol carbonates preserved in the Sonoita Creek Basin of our field area. Three graduate students (in addition to MJ) attended this fieldtrip to broaden Dr. Amundson’s contact with ASU’s School of Earth and Space Exploration.