59th Annual Report on Research 2014

Structural basins are defined by extensional tectonics. Rugged mountain ranges stand in stark relief adjacent to muted structural basins filled with sediment. In simplest terms, this topography is the result of the mountain ranges being uplifted along normal faults. This uplift drives erosion within the upland drainage basins and, consequently, sheds sediment into the subsiding basins that are forming adjacent to the uplifting mountain ranges. In southeastern Arizona's Basin and Range province extensional tectonics waned at approximately 3-5 Myr, and the region's structural basins began transitioning from internal to external drainage, forming the modern Gila River fluvial network. In the Atacama Desert of northern Chile, some basins of the Central Depression remain internally drained while others have integrated to the Pacific Ocean. In northern Chile, rates of landscape evolution are some of the slowest on Earth due to the region’s hyperarid climate. While the magnitude of upland erosion driven by extensional tectonics is largely recorded in the stratigraphy of the structural basins, the landscape's response to post-tectonic forcings was unknown. This project sought to quantify the rates and processes of sedimentation in these two dramatically different basins to assess how the sedimentation processes reflect upland erosion.

We used the full suite of terrestrial cosmogenic nuclides (TCN) – surface exposure dating, conventional burial dating, isochron burial dating, quantifying millennial-scale upland erosion rates, quantifying paleo-erosion rates using ²¹Ne/¹⁰Be and ²⁶Al/¹⁰Be, and assessing sediment recycling and complex exposure histories – to quantify the rates of landscape evolution in southeastern Arizona and northern Chile during the Late Cenozoic. In Arizona, we also used modern remnants of the pre-incision landscape and digital terrain analyses to reconstruct the landscape, allowing the quantification of incision and erosion rates that supplement the erosion rates. Our new chronology for key basin high stand remnants provided a record of incision rates from the Pliocene through the Quaternary, and we assessed how significantly regional incision is driving erosion rates. Paired nuclide analyses in the Atacama Desert revealed complex exposure histories resulting from several rounds of transport and burial by fluvial systems. These results support a growing understanding that geomorphic processes in the Atacama Desert are more active than previously thought.

Southeastern Arizona’s Basin and Range physiographic province is an ideal setting to investigate questions of post-tectonic landscape evolution. It is defined by two stages of extensional tectonics: mid-Tertiary low-angle extension and metamorphic core complex exhumation and high-angle normal faulting associated with the Basin and Range Disturbance (8-12 Myr). The modern landscape is most significantly a result of the latter episode of extension. However, regional tectonics were inactive for approximately 3-5 Myr, and previously internally drained structural basins integrated into the modern Gila River system. Fortuitously, in many basins of the region, remnants of each structural basin’s pre-incision high stand record the geometry of the paleo-landscape and preserve the stratigraphy of each basin’s final stages of post-tectonic filling. Previous studies identified the importance of these deposits, and were able to define rough age constraints for some of the surfaces based on magnetostratigraphy of underlying fill and soil development on the surfaces themselves. However, this landscape had not been revisited and reinvestigated with some of the more recently available tools for process geomorphology - namely our methods of higher resolution topographic data, digital terrain analyses, and our suite of TCN applications.

The Atacama Desert is one of the driest places on Earth, and for this reason it is an ideal place to investigate how rates of landscape evolution may slow under hyperarid conditions. Previous work employing TCN abundances in rock and sediment in the Atacama Desert have identified some of the slowest erosion rates on Earth, leading to the preservation of very old landforms. However, recent work revealed that hillslope and fluvial processes in the Atacama Desert may be more active than previously assumed. We use paired ¹⁰Be and ²¹Ne abundances to determine whether sediment is undergoing simple exposure and erosion or more complex exposure histories. When quantifying exposure ages or erosion rates with a stable nuclide such as ²¹Ne, accounting for inherited TCN abundances developed across multiple rounds of exposure is crucial. Active erosion-deposition cycles in the Atacama Desert are ongoing under the modern hyperarid climate, and these processes should be accounted for when sampling for TCN studies in this region of northern Chile.

1) Quantify Erosion and Sedimentation Rates for Late Stage Sedimentary Basin Fill.

We used TCN burial dates to establish an absolute temporal framework for continuous exposures of Pliocene and Pleistocene sedimentary basin fill. An absolute chronology for these deposits allows the calculation of sedimentation rates between dated strata. TCN concentrations in basin fill sediment provide not only a burial date, but also a paleo-erosion rate at the time of deposition. We compared paleo-erosion rates to inferred sedimentation rates as an internal check for our methods. We then considered these rates in the context of Pliocene-Pleistocene climate shifts as recorded in paleoclimate proxies in the Upper San Pedro Basin.

2) Determine the Regional Pattern of Basin Integration Using Absolute Ages for Basin High Stands. We determined the timing of each basin’s integration with the Gila River by quantifying absolute ages for basin high stand remnants. These surfaces are the highest elevation, undeformed alluvial deposits in each basin, and they record the maximum height of basin fill prior to basin integration and subsequent incision below each high stand. It is likely that basins integrated via sequential spillover, but there is no resolution to which basins integrated first, and how integration proceeded.

3) Quantifying Quaternary Incision and Paleo-Erosion Rates.

We used TCN depth profiles to determine the ages of Quaternary terraces inset into incised sedimentary basin fill. At least one to two terrace levels are present above modern channels throughout the Sonoita, Lower San Pedro, Santa Cruz, and Safford Basins. We assessed whether periods of alternating floodplain stability and channel incision followed were a function of the region’s climate oscillating between wetter and drier periods.