Evolution and degradation of flat‐top mesas in the hyper‐arid Negev, Israel revealed from ¹⁰Be cosmogenic nuclides

Abstract

Mesas are ubiquitous landforms in arid and semiarid regions and are often characterized by horizontal stratified erodible rocks capped by more resistant strata. The accepted conceptual model for mesa evolution and degradation considers reduction in the width of the mesa flat-top plateau due to cliff retreat but ignores possible denudation of the mesa flat-top and the rates and mechanism of erosion. In this study we examine mesas in the northeastern hyperarid Negev Desert where they appear in various sizes and morphologies and represent different stages of mesa evolution. The variety of mesas within a single climatic zone allows examination of the process of mesa evolution through time. Two of the four sites examined are characterized by a relatively wide (200-230 m) flat-top and a thick caprock whereas the other two are characterized by a much narrower remnant flat-top (several meters) and thinner caprock. We use the concentration of the cosmogenic nuclide 10 Be for: (a) determining the chronology of the various geomorphic features associated with the mesa; and (b) understanding geomorphic processes forming the mesa. The 10 Be data, combined with field observations, suggest a correlation between the width of flat-top mesa and the denudation and cliff retreat rates. Our results demonstrate that: (a) cliff retreat rates decrease with decreasing width of the flat-top mesa; (b) vertical denudation rates increase with decreasing width of the flat-top mesa below a critical value (~60 m, for the Negev Desert); (c) the reduction in the width of the flat-top mesa is driven mainly by cliff retreat accompanied by extremely slow vertical denudation rate which can persist for a very long time (>10 6 Ma); and (d) when the width of the mesa decreases below a certain threshold, its rate of denudation increases dramatically and mesa degradation is completed in a short time.

Tracking the <sup>26</sup>Al/<sup>10</sup>Be source-area signal in sediment-routing systems of arid central Australia

“…2). Given that the degree of mesa dissection increases in the down-system direction (west-to-east), according to Boroda et al (2014), we can predict that 10 Be supply to the stream network decreases downstream-and this is essentially what 15 we find. Extremely high to rather low 10 Be content of mesa bedrock overlaps with data from hillslope soil mantles (Fig.…”

“…Based on their work in the Negev, Boroda et al (2014) propose that the erosion rate of caprock-mesas scales with their size and extent. Parallel slope retreat, with negligible vertical erosion, predominates on wide tableland plateaus and with ongoing mesa reduction the rate of vertical and horizontal erosion increases to a maximum at the tor-stage.…”

Tracking the <sup>10</sup>Be–<sup>26</sup>Al source-area signal in sediment-routing systems of arid central Australia

“…Rates are < 5-10 m Myr −1 for bedrock outcrops Heimsath et al, 2010;Struck et al, 2018) and 5-20 m Myr −1 at the catchment scale (Bierman et al, 1998;Heimsath et al, 2010). The slow evolution of the central Australian landscape is a function of low relief due to restricted tectonic uplift (Sandiford, 2002;Jansen et al, 2013) combined with intensified aridity since the Miocene (Bowler, 1976;McGowran et al, 2004;Martin, 2006;Fujioka and Chappell, 2010). Ongoing intra-plate tectonic deformation is driven by far-field compressive stresses (Sandiford et al, 2004;Hillis et al, 2008;Waclawik et al, 2008; together with dynamic processes beneath the lithosphere, which have caused long-wavelength deformation on the order of hundreds of metres in vertical amplitude .…”

“…Silcrete duricrust forms a cap rock that is exceptionally resistant to weathering (Struck et al, 2018) and hence the mesa surfaces tend to accumulate very high 10 Be abundances. Based on their work in the Negev, Boroda et al (2014) propose that the erosion rate of cap rock and mesas scales with their size and extent. Parallel slope retreat, with negligible vertical erosion, predominates on wide tableland plateaus and with ongoing mesa reduction the rate of vertical and horizontal erosion increases to a maximum at the tor , (e), and (h) show 10 Be abundances in stream sediment relative to the distance along-stream from most downstream samples -note that we have reversed the x axes in all panels to illustrate our data from source to sink, left to right.…”

Evolution of sandstone peak‐forest landscapes – insights from quantifying erosional processes with cosmogenic nuclides

“…Assuming densities of 2.7 g cm −3 for the eroded rock mass, sub‐catchment specific unit erosion rates are calculated to be from ~71 to 260 t km −2 yr −1 (Table ) and fall well into the reported range for regional sediment yields of 86 (>2007) to 469 t km −2 yr −1 (1954–1986) (Zheng, ). Overall, our results thus confirm the spatially (and also temporally) varying nature of denudation rates at the summit, escarpment and footslope during tableland formation and degradation (Boroda et al ., ), and suggest that vertical wall retreat is the dominant process of sediment production in the catchments characterized by peak‐forest. Therefore, our findings highlight the importance of wall retreat processes, in addition to fluvial processes, for the longer‐term erosional history of the sandstone peak‐forest.…”