Can you guess my age?
A simple scene, a code to crack
This vertical aerial photograph of fractures in flat-lying sandstone of the Jurassic Entrada Formation on the Colorado Plateau in Arches National Park compels a question that is commonly asked when multiple fracture sets are present: what is their relative age?
This scene is about 2.5 kms across its horizontal dimension. The fracture system exposed comprises three joint sets, as defined by their orientations. I’ll refer to them by their apparent azimuth (relative to a fictitious North-is-up photo orientation). The faux azimuths of the three sets are 080, 095, and 165 degrees.
The challenge of this relative-age game is to determine:
1. Which set is the oldest (first fractures to form)?
2. Which set is intermediate in age?
3. Which set is youngest (most-recently formed)?
My answer is below. Before you seek the answer, jot down your interpretation of the relative age sequence as well as the logical basis for that interpretation. Writing down your answer forces you to commit to your thinking. The answer to this relative age puzzle is not a best guess; there is only one right answer.
Before we get to the relative age of these fracture sets there are three characteristics evident here that may lead some to re-think their relative age interpretation.
The first of these additional characteristics is fracture length. Fractures in Set 080 are long and through-going, whereas those is Set 165 are comparatively short.
The second additional characteristic to consider is planarity (or, in this 2D view, linearity) of individual fractures. Sets 080 and 095 are both composed of very straight fractures, whereas fractures in Set 165 are less so.
The final characteristic is fracture spacing, which is defined as the perpendicular distance between fractures. Fractures in Set 165 show the smallest average spacing, hence the highest fracture density. Fracture Set 095 is not uniformly developed across the scene. I think it shows the greatest average spacing of the three sets.
Let’s return to the question of the relative age. Numerous people have misinterpreted the relative age of fractures in this scene due to presumptions about offsetting faults. There are no faults in this image. Nothing has been visibly displaced. The fractures are all joints, which are opening-mode fractures.
Fractures of Set 165, the short fractures, terminate against both of the other fracture sets. They are short because their propagation is arrested when they intersect a pre-existing joint from one of the other sets. In order for them to “know” to terminate at these exact locations, the other two sets MUST have been present already. Therefore, Set 165 is the youngest set (most-recently developed).
Similarly, the fractures of Set 095 terminate against those of Set 080, thus Set 080 must be older, and thus it is the oldest set. In summary, the sequential timing of fracture set development is: Set 080 is the oldest, Set 095 came next, and Set 165 formed last.
Concerning the linearity of the fracture sets, the orientation of propagating fractures is affected by the regional stress field and by heterogeneities in the rock. Fractures of Set 080 are likely to have developed when the stratum was buried to a significant depth, and fracture propagation was controlled by a far-field tectonic stress. Heterogeneities in the rock had little influence on the propagating fractures, resulting in long, straight fractures.
In contrast, Set 165 fractures probably propagated under the influence of a lower differential stress field, hence local heterogeneities were able to steer fractures to a degree, resulting in crooked traces. The long-straight to short-crooked progression of fracture set characteristics is consistent with a fracture system that developed during uplift and erosional unloading of the strata.
Finally, consider the fracture spacing in different sets. In many stratigraphic sequences the spacing of fractures correlates with mechanical layer thickness of the strata. We see this in several of the 50 Photos postings, such as #1, #11, and #30. That simple spacing/thickness relationship may apply for the first-formed Set 080 fractures. But the Set 165 fractures propagated in a different, 3D mechanical environment, influenced by both mechanical stratigraphic layering and the presence of Set 080 fractures. This may have instigated closer spacing of the Set 165 fractures.
Fracture systems in a subsurface reservoir are commonly less complex than those in comparable strata in outcrop. I think this is because fracture systems start simple and gain complexity as the host rock progresses from the subsurface environment to the outcrop.
This photo was taken by Russ Dyer as part of his PhD research at Stanford University in the early 1980s. I appreciate his willingness to share it.
That’s it! The final photo in this series 50 Photos for Fracture Aficionados. I’ve enjoyed compiling & narrating this series. Many thanks to Knowledgette for hosting this series of photos.