Inverted channel variations identified on a distal portion of a bajada in the central Atacama Desert, Chile

Abstract

In deserts, the interplay between occasional fluvial events and persistent aeolian erosion can form composite modern and relict surfaces, especially on the distal portion of alluvial fans. There, relief inversion of alluvial deposits by differential erosion can form longitudinal ridges. We identified two distinct ridge types formed by relief inversion on converging alluvial fans in the hyperarid Chilean Atacama Desert. Although they are co-located and similar in scale, the ridge types have different ages and formation histories that apparently correspond to minor paleoclimate variations. Gravel-armored ridges are remnants of deflated alluvial deposits with a bimodal sediment distribution (gravel and sand) dated to a minor pluvial phase at the end of the Late Pleistocene (~12 kyr). In contrast, younger (~9 kyr) sulfate-capped ridges formed during a minor arid phase with evaporite deposition in a pre-existing channel that armored the underlying deposits. Collectively, inverted channels at Salar de Llamara resulted from multiple episodes of surface overland flow and standing water spanning several thousand years. Based on ridge relief and age, the minimum long-term deflation rate is 0.1–0.2 m/kyr, driven primarily by wind erosion. This case study is an example of the equifinality concept whereby different processes lead to similar landforms. The complex history of the two ridge types can only be generally constrained in remotely sensed data. In situ observations are required to discern the specifics of the aqueous history, including the flow type, magnitude, sequence, and paleoenvironment. These findings have relevance for interpreting similar landforms on Mars.

Fluvial Depositional Systems of the African Humid Period: An Analog for an Early, Wet Mars in the Eastern Sahara

“…Moreover, when comparing the rivers with nearly similar discharge from different fluvial depositions environments, our drainage areas are close to those formed under flashier climates (Figure 8). A similar paleoenvironmental interpretation has been observed from fluvial ridge systems in the Kumtagh Desert, China (Wang et al, 2015), Ad Dwasir, Saudi Arabia (Matter et al, 2016), and the Atacama Desert (e.g., Morgan et al, 2014;Williams et al, 2021), where coarse materials from local bedrock sources are prevalent and were deposited over short durations, that is, thousands to tens of thousands years. Hayden, Lamb, and McElroy (2021) calculated the median of intermittency of the gravel-bed river at 0.092, which means that the formative fluvial events could occur over a few days to several weeks every year, which could be applied to the ancient gravel-bed rivers investigated here.…”

“…Hayden et al (2019) suggested that ridges recording long-lived fluvial activity that persisted over millions of years may favor habitability on early Mars. However, Williams et al (2021) reported fluvial ridges from the Atacama Desert, Chile, preserving potential biosignatures, including plants and subsurface organisms over short durations, that is, thousands of years. This study also indicates that fluvial landforms deposited by short-duration fluvial activity (10-100s years over thousands of years) also supported habitable environments for biosignature, including bacterial structures and organisms in the eastern Sahara (e.g., Cremaschi et al, 2010;Nicoll & Sallam, 2017).…”

“…The investigation of these landforms on Mars is now only possible through remotely sensed data from orbiters (although this might soon change, at least in Jezero crater, when the Perseverance rover arrives at the deltaic landform in Figure 1d; Mangold et al., 2021), hindering our ability to measure their exact geometries, understand their development, how much time they record, and the mechanisms of their preservation (e.g., sediment cementation) during the shift from wet to arid environments. Although fluvial ridges on Earth have been widely investigated (e.g., Pain & Ollier, 1995; Williams et al., 2007, 2009, 2021, Clarke et al., 2020; Hayden et al., 2019; Hayden, Lamb, & Carney, 2021; Hayden, Lamb, & McElroy, 2021; Zaki et al., 2018; Zaki, Pain, et al., 2021, further studies of similar landforms in dry climates on Earth help provide uniformitarian analogs of the physical principles of landform evolution processes, including streamflow mechanisms, duration of fluvial activity, patterns of fluvial deposition, processes of relief inversion, and the nature of the transition from wet to arid climates. Constraining such information can refine our understanding of martian landforms with similar planforms and sedimentary structures by providing new insights into the paleoenvironmental conditions associated with their development.…”

Impact of CaSO4-rich soil on Miocene surface preservation and Quaternary sinuous to meandering channel forms in the hyperarid Atacama Desert

“…Capped duricrust are known to have a protective effect, resistant enough to preserve ancient landscapes 15 , 35 . The observation of sulphate-capped ridges from the Atacama Desert, resembling inverted fluvial stream channel indicate that sulphate cementation is able to increase the erosion resistance against fluvial and aeolian processes 57 . Studies of gypcrete in Egypt 36 supports the protective nature of gypsum crusts against erosion and weathering, consolidating surface layers by cementation.…”

“…the evolution of duricrusts, might have relevance to other arid to hyperarid climates and for the geomorphology of Mars, where similar channel forms are evident 45 – 48 , 50 – 53 . The Atacama Desert serves as a natural laboratory for potential surface processes on Mars due to the broadly similar climatic conditions and geochemical/mineralogical compositions 32 , 57 , 59 . Sulphates (albeit MgSO 4 60 ) are abundant on Martian surface 60 – 66 and atmospheric deposition of sulphates from volcanic origin 67 , 68 may give rise to surface encrustations that are similar to those found in the Atacama 9 – 11 , 27 .…”