Expansion of the Late Quaternary morphochronology of Atacama’s coastal alluvial fans (northern Chile) by Schmidt hammer exposure-age dating

Last major fluvial modification along the hyperarid coast of the Atacama Desert is relatively young. It has been found that the coastal alluvial fans (CAFs) were formed during the Late Pleistocene and Holocene. No remnants older than the last interglacial period could be constrained as yet. However, robust geochronological frameworks by numerical dating using radiocarbon dating, trapped charge dating techniques, and in situ terrestrial cosmogenic nuclides are restricted to few sites. This is related to both the geomorphic and stratigraphic complexity of the multi-stage CAFs as well as the high costs of those numerical dating methods. Consequently, it has remained unclear so far to what extent fan aggradation and progradation is controlled by large-scale allogenic versus individual autogenic forcing.

As a first study, an application of the cost-effective Schmidt hammer exposure-age dating (SHD) technique was explored for constraining the age of terminal aggradation of the CAF generations along the south-central coast of the Atacama Desert (24°15’S–25°15°S) using an ¹⁰Be exposure-dated telescopic alluvial fan featuring four control surfaces (after Walk et al., 2023) for age calibration. Apart from the calibration site, SHD was applied on, in total, 19 depositional lobes from 11 CAFs featuring at least one phase of progradation following main channel incision. Morphostratigraphies are primarily based on in-field mapping. Rebound (R) values were systematically assessed using an electronic N-type Schmidt hammer for each abandoned fan generation (Q1–Q3) by randomly sampling 50 surface boulders of comparable lithology. For calibration with recent deposits (Q4), multiple impacts were exerted on a careful selection of few boulders. Linear age calibration and error propagation follows the two-point solution by Matthews and Winkler (2022), adapted to a segmented approach for four control surfaces and complemented by Deming regression.

Calibration results in a negative and significant linear relationship between ¹⁰Be exposure ages and R values, presenting a robust regional calibration model for SHD of fan boulders exposed at least since the last interglacial period. SHD of the 19 fan surface generations yield ages of terminal aggradation ranging between the mid MIS 4 (late MIS 3) and early to mid MIS 5. The age range exceeds the usual dating range reported for SHD applied in (sub)humid regions by up to one order of magnitude, which can be explained by the comparatively low weathering rates at the arid-hyperarid transition. The relative age uncertainties amount to 3–20% (10–24%) and allow to deduce a spatial heterogeneity in the Late Quaternary fan morphodynamics. While the CAFs south of 24°53’S show a systematic response probably related to palaeoclimatic changes of the SE Pacific, those to the north are decoupled – indicating a potential control by individual autogenic forcing.

References
Matthews, J.A., Winkler, S. (2022): Schmidt-hammer exposure-age dating: a review of principles and practice. Earth-Science Reviews 230, 104038. DOI:10.1016/j.earscirev.2022.104038

Walk, J., Schulte, P., Bartz, M., Binnie, A., Kehl, M., Mörchen, R., … Lehmkuhl, F. (2023): Pedogenesis at the coastal arid-hyperarid transition deduced from a Late Quaternary chronosequence at Paposo, Atacama Desert. Catena 228, 107171. DOI: 10.1016/j.catena.2023.107171