Jointing and mechanical layering

Two sets of joints in Permian eolian sandstone, Canyonlands National Park

Two sets of joints in Permian eolian sandstone, Canyonlands National Park. Two joint sets at high angles are quite common in competent beds.

Jointed Cretaceous sandstone layers, Book Cliffs, Utah

Jointed Cretaceous sandstone layers, Book Cliffs, Utah. Note how the distance between joints is larger in the thick layers.

Joints occur pretty much everywhere in rocks exposed at the surface of the Earth. And they can be pretty important, both during construction activities, intrusion of magma and flow of fluids (groundwater, hydrocarbons, CO2, magma). Joints represent conduits of fluids, particularly in the vertical direction, and are made artificially during fracking (hydraulic fracturing) operations.

Jointed silica-rich layers in green shale, Green River Formation, Utah

Jointed silica-rich layers in green shale, Green River Formation, Utah. Note higher joint spacing in thick layer at the top.

Joints form naturally in a variety of settings, but particularly during exhumation due to pressure release and cooling. Thermal contraction is very important, and it is the stiff layers that fracture most easily. Hence we typically see fractured sandstone or limestone layers between non-fractured or less fractured siltstones or shale layers. Layers that develop different joint patterns define what is called mechanical stratigraphy.

Relationship between layer thickness and joint spacing

Relationship between layer thickness and joint spacing, based on field observations.

Typically, stiff layers develop fractures with a characteristic spacing that depends on layer thickness: The thicker the layer, the farther apart the fractures, as shown in some of these pictures.

About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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2 Responses to Jointing and mechanical layering

  1. David Loope says:

    Hello. I am very interested in what you say in your textbook (p. 86-88; 147; Fig. 5.9) about joints that develop during uplift. I’m a sedimentologist working on the Colorado Plateau. Our recent work has been concerned with iron-oxides in the Navajo Sandstone that form concretions and thick rinds associated with close-spaced joints (you have probably seen them; they are especially abundant at Zion and along the White Cliffs of the Grand Staircase). From my website you could download our 2011 paper in Journal of Geology. Our major points in this and several other papers is that siderite formed below CO2 reservoirs, and that the iron oxides in the Navajo are products of siderite oxidation. This oxidation was mediated by iron-oxidizing microbes. Our view is that this oxidation is very young, and is still probably going on in the shallow subsurface. Figure 1D (from the JG article) was taken near Escalante, Utah and shows six joints that cut a 1.5 m-long, tabular mass that was cemented by siderite. The joints are lined by iron-oxide cement, and the outer perimeter of the mass is sheathed in iron-oxide cement. We think the joints post-date the siderite, and with uplift, became conduits for oxidizing water. I’ve been trying to find out more about joints, and the caption to figure 5.9 caught my eye.

    I’ve looked at Engelder’s book Stress Regimes in the Lithosphere, but didn’t really make much progress. Any suggestions for further reading?

  2. Very interesting. In the next edition of my book there will be a new chapter on joints and veins. It may not be advanced enough to cover your need, but perhaps worth a look. Doing things like what you do is very important and highly needed, because really, our understanding of jointing on the Colorado Plateau and in general is very limited.

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