Joints in juxtaposed, mechanically-distinct layers
The impact of paleoclimate on tectonic joints
Three dipping layers provide a somewhat 3D view of a system of joints.These strata are carbonates of the Pennsylvanian Bird Spring Formation in Arrow Canyon, S. Nevada.Notice the barrel cacti, which are common in this part of the Mojave Desert.They tend to occur along joints, into which their roots can probe for moisture and to anchor themselves in these otherwise impenetrable rocks.
Two sets of joints predominate in each of the three beds, which are similar in several respects.The joints are perpendicular to bedding.Did they develop vertically, before the strata were folded?Or did they form after folding, under the control of the mechanical geometry of the bedded strata?My guess is the latter, but this is generally difficult or impossible to determine without careful study.The joint sets are orthogonal to each other, as is evident on the bedding surfaces and along the eroded edges of the strata.The spacing of joints in each set are controlled by the thickness of their host layer.The thicker the layer, the more widely spaced the joints.
The similarity of the joints in each layer suggests they formed under the influence of similar stress conditions (orientation and magnitude).The independence of joints in each layer suggests that the mechanical boundaries between these depositional layers are effective at hindering fractures from propagating across from layer to layer.The well-developed, thin-bedded character of bedding in this part of the Pennsylvanian is believed to result from shallow-water deposition during greenhouse climatic conditions.Icehouse conditions, present in the underlying Mississippian strata (not shown) results in thicker, more-massive, less-mechanically-distinct strata, resulting in less-regularly developed, more widely-spaced joints.Thus we see an effect of paleoclimate on subsequent natural fracture characteristics.