Point bars and counter point bars: Why is sediment deposited on the 'wrong' side of a meander bend?

The simplest definition of point bars is that they are sedimentary deposits forming on the inner, convex bank of river bends. [‘Inner’ and ‘convex’ bank means that, looking toward the river, the bank is curving around you.] Amadeus W. Grabau, an early ‘influencer’ in stratigraphy and sedimentary geology, published a beautiful drawing of how meanders grow, point bars evolve, and oxbow lakes form, more than a hundred years ago: Diagram illustrating the development of meanders in a river. From Grabau (1920). ...

March 13, 2021 · Zoltán Sylvester

Does river migration slow down in high-curvature bends?

The answer, at least for seven rivers of the Amazon Basin, seems to be negative, as we try to demonstrate in a paper that was recently published in Geology. My coauthors are Paul Durkin, at the University of Manitoba, and Jake Covault, at the Bureau of Economic Geology, The University of Texas at Austin. In this blog post, I try to provide a bit more background to our paper. Why is this an interesting result? After all, it makes sense that there is more outer bank erosion in sharper bends. Erosion is primarily a function of the shear stress exerted on the bank; and shear stress is high where the high-velocity core of the river gets close to the bank (because in this case flow velocity has to quickly decrease from a maximum to zero, and shear stress is a function of the rate of change in velocity). The high-velocity core gets pushed close to the bank if the centrifugal force is large; and the centrifugal force is directly proportional to curvature, 1/R (where R is the radius of curvature). In short, if we use the simplest, most pedestrian physical reasoning, we would expect that erosion, and therefore bank migration, are high in high-curvature bends. However, a lot of previous work on meandering rivers suggests that this is not the case. ...

February 6, 2019 · Zoltán Sylvester

Stratal patterns in salt-withdrawal minibasins

These are animations that accompany our 2015 article on the stratigraphy of salt-withdrawal basins on the slope (Sylvester, Z., Cantelli, A., and Pirmez, C., 2015, Stratigraphic evolution of intraslope minibasins: Insights from surface-based model: AAPG Bulletin, v. 99, no. 6, p. 1099–1129). We have used a simple model that investigates the interplay between subsidence and sedimentation and helps in the understanding of how stratal termination patterns relate to variations in sediment input and basin subsidence. Conventional sequence stratigraphy focuses on what is happening on the shelf and the shelf edge; and the origin of stratal patterns that characterize the typical ‘slug diagram’ are not trivial to relate to the three main parameters that influence continental-margin stratigraphy: changes in sea level, sediment supply, and subsidence. Our minibasin model is simpler because (1) it only has two parameters: sediment supply and subsidence (the impact of sea level - if any - is included in the sediment supply curve); and (2) sedimentary layers are deposited horizontally during each time step. The basic idea is to investigate the geometries of a system that consists of a depression that deepens through time and sediment is deposited in it with a certain rate. ...

July 18, 2018 · Zoltán Sylvester

Exploring (de)compaction with Python

All clastic sediments are subject to compaction (and reduction of porosity) as the result of increasingly tighter packing of grains under a thickening overburden. Decompaction - the estimation of the decompacted thickness of a rock column - is an important part of subsidence (or geohistory) analysis. The following exercise is loosely based on the excellent basin analysis textbook by Allen & Allen (2013), especially their Appendix 56. Import stuff import numpy as np import matplotlib.pyplot as plt import functools from scipy.optimize import bisect %matplotlib inline %config InlineBackend.figure_format = 'svg' plt.rcParams['mathtext.fontset'] = 'cm' Posing the problem Given a sediment column of a certain lithology with its top at $y_1$ and its base at $y_2$, we are trying to find the thickness and average porosity of the same sediment column at a different depth (see figure below). We are going to set the new top $y_1’$ and work towards finding the new base $y_2’$. ...

April 12, 2017 · Zoltán Sylvester

Exploring the diffusion equation with Python

Ever since I became interested in science, I started to have a vague idea that calculus, matrix algebra, partial differential equations, and numerical methods are all fundamental to the physical sciences and engineering and they are linked in some way to each other. The emphasis here is on the word vague; I have to admit that I had no clear, detailed understanding of how these links actually work. It seems like my formal education both in math and physics stopped just short of where everything would have nicely come together. Papers that are really important in geomorphology, sedimentology or stratigraphy seemed impossible to read as soon as they started assuming that I knew quite a bit about convective acceleration, numerical schemes, boundary conditions, and Cholesky factorization. Because I didn’t. ...

February 6, 2015 · Zoltán Sylvester

Rivers through time, as seen in Landsat images

Thanks to the Landsat program and Google Earth Engine, it is possible now to explore how the surface of the Earth has been changing through the last thirty years or so. Besides the obvious issues of interest, like changes in vegetation, the spread of cities, and the melting of glaciers, it is also possible to look at how rivers change their courses through time. You have probably already seen the images of the migrating Ucayali River in Peru, for example here. This river is changing its course with an impressive speed; many – probably most – other rivers don’t show much obvious change during the same 30-year period. What determines the meander migration rate of rivers is an interesting question in fluvial geomorphology. ...

March 16, 2014 · Zoltán Sylvester