Timing and nature of alluvial fan and strath terrace formation in the Eastern Precordillera of Argentina

Abstract

Sixty-eight 10 Be terrestrial cosmogenic nuclide (TCN) surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper-arid San Juan region of the Argentine Precordillera. This region is tectonically active, and numerous fault scarps traverse Quaternary landforms. The three study sites, Marquesado strath complex, Loma Negra alluvial fan and Carpintería strath complex reveal a history of alluvial fan and strath terrace development over the past w225 ka. The Marquesado complex Q3 m surface dates to w17 AE 3 ka, whereas the Loma Negra Q1 ln , Q2 ln , Q3 ln , Q4 ln , and Q5 ln surfaces date to w24 AE 3 ka, w48 AE 2 ka, w65 AE 13 ka, w105 AE 21 ka, and w181 AE 29 ka, respectively. The Carpintería complex comprises eight surfaces that have been dated and include the Q1 c (w23 AE 3 ka), Q2 c (w5 AE 5 ka), Q3a c (w25 AE 12 ka), Q3b c (w29 AE 15 ka), Q4 c (w61 AE 12 ka), Q5 c (w98 AE 18 ka), Q6 c (w93 AE 18 ka), and Q7 c (w212 AE 37 ka). 10 Be TCN depth profile data for the Loma Negra alluvial fan complex and Carpintería strath terrace complex, as well as OSL ages on some Carpintería deposits, aid in refining surface ages for comparison with local and global climate proxies, and additionally offer insights into inheritance and erosion rate values for TCNs (w10 Â 10 4 10 Be atoms/g of SiO 2 and w5 m Ma À1 , respectively). Comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the Carpintería and Loma Negra areas with only younger alluvial fan surfaces preserved both to the north and south. These data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages, which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region.

Late Quaternary Activity of the La Rinconada Fault Zone, San Juan, Argentina

“…We justify our assumption of zero erosion with our observation of desert varnish and large rounded clasts. Similar to previous studies in the area (e.g., Hedrick et al, ; Siame et al, ), while assuming a 0‐mm/ka erosion, we also show the potential effect of assuming different erosion scenarios on the surface clast ages we calculate. For depth profiles, erosion rates are already calculated along with inheritance.…”

“…In computing ages of surfaces from surface clasts, we assumed zero erosion due to the well‐preserved surface as indicated by presence of desert varnish. Previous work done in nearby areas in the Eastern Precordillera and Western Sierras Pampeanas (Hedrick et al, ; Schmidt et al, ; Siame et al, ) also assume zero erosion. In arid environments such as this, it is more likely for computed ages of surfaces to be older due to inheritance rather than younger due to erosion.…”

Reconstructing fluvial landscape evolution using terrestrial cosmogenic nuclide dating: achievements, limitations and applications

“…The second approach, consisting in sampling the alluvial fan deposits along a depth profile (e.g. Hedrick et al 2013), will be tackled in the next text section (3.5). A combination of both approaches is recommended (e.g.…”

“…For instance, 10 Be and 36 Cl concentrations can be measured in quartz-bearing and calcite-bearing material, respectively, and thereby allow an internal control by comparing their concentrations from the same landform (Frankel et al 2007, Le Dortz et al 2011. Likewise, where the grain size distribution of fan sediments allows performing luminescence dating, it is highly recommendable to crosscheck age information from both methods (Nissen et al 2009, Le Dortz et al 2011, Cortés et al 2012, Hedrick et al 2013). An alternative crosscheck might be undertaken with U-series dating of pedogenic carbonate coatings from subsurface clasts (Blisniuk et al 2012).…”

“…Besides its usefulness in palaeoseismology, surface exposure dating of fan sediments yielded new insightful information about climatic forcing on fan formation (e.g. Hedrick et al 2013, Owen et al 2014).…”