Entrenching tools

So here in Luxembourg, the “Gibraltar of the North” and have been surprised at the entrenched meanders. So in the view of the model at the Musée D’Histoire de la Ville de Luxembourg (below) you can see in the foreground the Bock, the site of the original castle built in the 900s, and its presence on a height encircled by an entrenched meander of the Alzette River, which flows left to right in this view towards the southwest.  Another curve by the tributary Petrusse is in the left background.

Model of Luxembourg early in its development, Musee D'Histoire de la Ville de Luxembourg (Luxembourg City History Museum)

Model of Luxembourg early in its development, Musee D’Histoire de la Ville de Luxembourg (Luxembourg City History Museum)

Geologically, the city sits on a Jurassic sandstone (the Luxembourg sandstone), which lies over Triassic gypsiferous beds that outcrop to the north.  The section overall dips gently towards the south, so as you go to the north along the Alzette, the valley walls retreat and the river occupies a broad valley.

Coming here with the bias from the western U.S. of entrenched meanders seeming to reflect rapid erosion, GG kind of wondered what was up.

[A warning that the suppositions below are the wandering thoughts of a wandering geophysicist and might be a poor representation of reality.]

For instance, a poster child for entrenched meanders is the Goosenecks of the San Juan in SE Utah:

One entrenched meander, Goosenecks of the San Juan State Park, Utah.

One entrenched meander, Goosenecks of the San Juan State Park, Utah.

Now in the western U.S., we often attribute such features to unusually rapid erosion, perhaps driven by uplift or climate change. It is easy to imagine a lazy San Juan river in a floodplain when a waterfall from some downstream knickpoint migrates on through, freezing those meanders in position. Incision rates in the Colorado River drainage are measured in the range of 80-450 m/million years, so the first question might be, what are the incision rates in Europe? Surely they are a lot lower given the absence of significant tectonism away from the Alps, right?

Well, the nearest well-studied (in English) river system with a similar geology to the Alzette would be the Meuse (Maas), to the west. Overall incision rates there of river terraces are on the order of 50 m/million years (buried in text of this paper among other places), though variations of a factor of 2-4 are possible over the past million years. It is a bit less clear how rates are varying within a watershed (i.e., are highlands eroding at about the same rate as rivers are incising–this seems plausible from what GG has seen). So, rather surprisingly, the rates are not really that much slower here than in the desert southwest.

Given all this, it would seem unlikely that the entrenched meanders of the Alzette (and several other rivers in this general region) are the result of sudden drop in base level.  Instead it seems it is nothing more than a measure of the relative rates of incision vs. slope retreat, where incision has to be significantly more than slope retreat in order to get the entrenched meanders. What is happening in Luxembourg City is that, as the Alzette there is within the Luxembourg Sandstone, the slope retreat is small.  Farther north, the slope retreat rate is greater owing to the exposure of the underlying Triassic gypsiferous units, which presumably lead to undercutting of the sandstone.  This is nearly precisely the same mechanism going on in Zion Canyon in Utah, where the massive Navajo Sandstone has no cliff retreat from the Virgin River while that river is within the sandstone, leading to the Narrows of the Virgin, but downstream, where the river reaches the Kayenta and Chinle Formations, the valley opens up as the combination of sapping at the base of the Navajo cliffs and undercutting because of easier erosion of those underlying units.

So the lesson is, what exactly does the presence of entrenched meanders mean? Presumably it is telling of the balance between downward incision and lateral retreat of river banks.  If you see such meanders in rock that is pretty weak, you might suspect very rapid downcutting.  Entrenched meanders in the Desolation Canyon stretch of the Colorado River might be such a case as the upper Cretaceous rocks there are not particularly strong. Entrenched meanders in really strong rocks might mean little.  It would seem that the number you really need to understand such features is the cliff retreat rate of the rocks in the cliff walls. Qualitatively, if you see one stretch of river cutting these rocks with very pronounced entreated meanders and another stretch of river cutting the same material but within a broad, open valley, you would probably be right to guess the first river was cutting down faster than the second.  But if the wall rocks are different, you would need more information.  Least wise, that is how it seems to GG….

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