First quantitative evidences of ghost-rock karstification controlling regional karst geometry

2020 ◽  
Author(s):  
Philippe Vernant ◽  
Oswald Malcles ◽  
Jean-François Ritz ◽  
David Fink ◽  
Gaël Cazes ◽  
...  
Keyword(s):  
Classical Model ◽  
Large Drop ◽  
Geochronological Data ◽  
Base Level ◽  
New Finding ◽  
Level Elevation ◽  
Flow Through ◽  
Steep Slopes ◽  
Surface Geomorphology ◽  
Level Lowering

<p>Although more and more processes are discussed and discovered on the genesis and evolution of cave systems, the tiered karsts are often explained by a control of the base level evolution. In this classical model, the horizontal galleries are explained by a stability of the base level elevation. To the contrary, the shafts and network segments with steep slopes are related to incision periods with a base level lowering.</p><p>We use Terrestrial Cosmogenic Nuclide Geochronology to estimate burial ages of alluvium trapped in several caves of the Larzac plateau in Southern France. All the samples are collected in horizontal cave levels, sometimes located between steeper segments. Some caves are opened in river gorge walls, while others are located below the Larzac plateau not farther than 5km away from the river gorges.</p><p>The burial ages for the caves opening in the gorges are consistent with the incision rates given for the area and could be interpreted using the classical model. However, the cave within the plateau show a horizontal level with alluvium deposited 200m above the caves in the gorge with the same burial ages (~1 Myr). Since then, new shafts have been opened without alluvium and are hydrologically connected to the river by deeper[jfr1]  hypogenic galleries. The cave morphologies and the geochronological data suggest that the classical model fails to explain the horizontal levels in cave below the plateau. We postulate that the geometry of the caves in these limestone and dolomite plateaus are related to a previous period of ghost-rock and alteration roots formations. Without the opening of an efficient connection between this primokarst and the valley, no alluvium can flow through the cave. Therefore, we think that our burial ages constrain the emptying of the ghost-rocks leading to the genesis of the cave where water and possibly alluvium can flow through. Furthermore, these new finding explain why the horizontal levels in the caves are not clearly related to horizontal markers in the surface geomorphology and why large shafts (>100m) exist in the area without evidences of long periods of base level stability followed by large drop of the regional base level.</p><div> <div> <div> </div> </div> </div>

Stream channel convexity induced by continuous base level lowering, the Dead Sea, Israel

Geomorphology ◽  
2007 ◽  
Vol 92 (1-2) ◽  
pp. 60-75 ◽  
Author(s):  
Dan Bowman ◽  
Yonit Shachnovich-Firtel ◽  
Shlomo Devora
Keyword(s):  
Dead Sea ◽  
Stream Channel ◽  
Base Level ◽  
The Dead ◽  
Level Lowering

New evidence for high discharge to the Chukchi shelf since the Last Glacial Maximum

2007 ◽  
Vol 68 (2) ◽  
pp. 271-279 ◽  
Author(s):  
Jenna C. Hill ◽  
Neal W. Driscoll ◽  
Julie Brigham-Grette ◽  
Jeffrey P. Donnelly ◽  
Paul T. Gayes ◽  
...  
Keyword(s):  
Sea Level ◽  
High Discharge ◽  
Incised Valley ◽  
Base Level ◽  
Subbottom Profiling ◽  
New Evidence ◽  
Level Lowering ◽  
Chukchi Shelf ◽  
The Last Glacial Maximum ◽  
The Last Glacial

AbstractUsing CHIRP subbottom profiling across the Chukchi shelf, offshore NW Alaska, we observed a large incised valley that measures tens of kilometers in width. The valley appears to have been repeatedly excavated during sea level lowering; however, the two most recent incisions appear to have been downcut during the last sea level rise, suggesting an increase in the volume of discharge. Modern drainage from the northwestern Alaskan margin is dominated by small, low-discharge rivers that do not appear to be large enough to have carved the offshore drainage. The renewed downcutting and incision during the deglaciation and consequent base level rise implies there must have been an additional source of discharge. Paleoprecipitation during deglaciation is predicted to be at least 10% less than modern precipitation and thus cannot account for the higher discharge to the shelf. Glacial meltwater is the most likely source for the increased discharge.

scholarly journals Multiple knickpoints in an alluvial river generated by a single instantaneous drop in base level: experimental investigation

2014 ◽  
Vol 2 (1) ◽  
pp. 271-278 ◽  
Author(s):  
A. Cantelli ◽  
T. Muto
Keyword(s):  
Experimental Investigation ◽  
Equilibrium State ◽  
Hydraulic Jump ◽  
Dam Removal ◽  
Supercritical Flow ◽  
Alluvial Rivers ◽  
Bedrock Rivers ◽  
Alluvial River ◽  
Base Level ◽  
Level Lowering

Abstract. Knickpoints often form in bedrock rivers in response to base-level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstream-progressing degradation; the knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that under conditions of Froude-supercritical flow over an alluvial bed, an instantaneous drop in base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base-level fall can generate a single knickpoint, or multiple knickpoints. Multiple knickpoints take the form of cyclic steps, that is, trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream. In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of base-level fall can trigger the autogenic generation of multiple knickpoints which are sustained until the alluvial reach recovers a graded state.

scholarly journals A lattice grain model of hillslope evolution

2018 ◽  
Vol 6 (3) ◽  
pp. 563-582 ◽  
Author(s):  
Gregory E. Tucker ◽  
Scott W. McCoy ◽  
Daniel E. J. Hobley
Keyword(s):  
Rock Weathering ◽  
Additional Parameter ◽  
Bare Rock ◽  
Near Surface ◽  
Weathering Rate ◽  
Physical Weathering ◽  
Stochastic Disturbance ◽  
Base Level ◽  
Hillslope Evolution ◽  
Level Lowering

Abstract. This paper describes and explores a new continuous-time stochastic cellular automaton model of hillslope evolution. The Grain Hill model provides a computational framework with which to study slope forms that arise from stochastic disturbance and rock weathering events. The model operates on a hexagonal lattice, with cell states representing fluid, rock, and grain aggregates that are either stationary or in a state of motion in one of the six cardinal lattice directions. Cells representing near-surface soil material undergo stochastic disturbance events, in which initially stationary material is put into motion. Net downslope transport emerges from the greater likelihood for disturbed material to move downhill than to move uphill. Cells representing rock undergo stochastic weathering events in which the rock is converted into regolith. The model can reproduce a range of common slope forms, from fully soil mantled to rocky or partially mantled, and from convex-upward to planar shapes. An optional additional state represents large blocks that cannot be displaced upward by disturbance events. With the addition of this state, the model captures the morphology of hogbacks, scarps, and similar features. In its simplest form, the model has only three process parameters, which represent disturbance frequency, characteristic disturbance depth, and base-level lowering rate, respectively. Incorporating physical weathering of rock adds one additional parameter, representing the characteristic rock weathering rate. These parameters are not arbitrary but rather have a direct link with corresponding parameters in continuum theory. Comparison between observed and modeled slope forms demonstrates that the model can reproduce both the shape and scale of real hillslope profiles. Model experiments highlight the importance of regolith cover fraction in governing both the downslope mass transport rate and the rate of physical weathering. Equilibrium rocky hillslope profiles are possible even when the rate of base-level lowering exceeds the nominal bare-rock weathering rate, because increases in both slope gradient and roughness can allow for rock weathering rates that are greater than the flat-surface maximum. Examples of transient relaxation of steep, rocky slopes predict the formation of a regolith-mantled pediment that migrates headward through time while maintaining a sharp slope break.

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