July 17, 2020
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
Sedimentary layers deformed by extensional movement in rift basins can make very interesting surface outcrop patterns…if they actually end up exposed on the land surface. Several Mesozoic rift basins have been exhumed along the Atlantic margin of North America, creating interesting patterns in sedimentary rock layers and igneous intrusions that originated during the breakup of Pangaea. These Mesozoic structures do not dominate the landscape in the same way as the compressional structures of the Paleozoic Valley and Ridge fold thrust belt to the west, but they can be appreciated thanks to good geologic mapping and digital topography. Of the several Mesozoic basins along the Atlantic margin, I think the Newark Basin offers the best map-scale visuals.
The brown, tan, and red zone running through the middle of the image above is the outcrop zone of the Newark Basin. Brown and tan are sedimentary strata; red is igneous rock (mostly diabase). New York City is the large urban area at the upper right (not covered by the geologic map), and Philadelphia is just left of the “c” in “compressional” at lower right. These USGS-sourced Google Earth overlays do not provide quite enough detail in the basin fill to sharply highlight the structures that are present, but fold noses are clearly visible in the light tan layers just below center. The sweeping patterns of the deformed sedimentary layers are more visible in the 3DEP hillshade overlay available on The National Map Viewer (view below is centered on 74.849W 40.500N).
While extensional deformation does not produce the same type of folds, faults, and outcrop patterns as compressional deformation, the Newark Basin obviously hosts plenty of dipping sedimentary layers that now create surface patterns. I tried to replicate this with a number of sand models, which ended up producing some interesting patterns despite the shortcomings of using granular media to make simple model rift systems. The real Atlantic margin basins have also experienced some compression and inversion after their initial development, which was not included in the models. The effects of these events are indeed visible in the real rocks at various scales, but the overall, large-scale pattern seen below is the result of extension.
The strike-slip offset at the bottom of the model image above resembles the distinct feature just below the center of the geologic map image above it. Basin fill layers folded during deformation due to rotation and drag associated with faulting, producing tightly curving outcrop patterns. A slightly zoomed-in comparison is shown below.
This zone of oblique faulting that forms these outcrop patterns relates to the shape of the moving baseplate in the model; the GIF below shows the model’s formation and erosion to expose the pattern.
A cross section slice through the middle of the model at the end of erosion looks like this:
The colorful tilted layers in the middle are the basin sediment that was added during extension; the “basement” is the pre-extension rock (various Appalachian rock types in the real world). The white material at the base of the model is glass microbeads, which causes the faults in the model to flatten with depth, rotating and tilting the overlying layers. The image above has reasonable resemblance to general Newark Basin cross sections, sourced here.
The post-extension regional uplift and erosion that the model tries to replicate is described in the conceptual images along the bottom of the image below, sourced here.
Overall, I was surprised at the variety of patterns the first sand model shown above produced. The tilted blocks that formed towards the far side of the model were especially interesting, particularly when they began to be exposed next to the younger and less deformed layers in the near side of the model. Below, the tilted blocks are just beginning to be exhumed from beneath the overlying, less folded layers. They “repeat” the same layer at the surface 3 or 4 times due to the rotation of the blocks along closely-spaced normal faults.
Block rotation along normal faults in the real Newark Basin also creates outcrop repetition, visible in this map sourced here.
The model shown below, from a different experiment, shows how slight tilting and normal fault offset can produce outcrop repetition on a low-relief surface. The effect is also evident in the first model section shown a few images up.
Some of the most interesting real-world outcrop patterns occur along the edges of the basin, where faulting is oblique and cuts the basin fill into thin slivers. The resulting narrow folds are visible in The National Map hillshade imagery; the one shown below is just southeast of Reading, Pennsylvania, centered around 40.3051N 75.8060W.
The basin-scale map image below shows where the fold is located in the basin.
Features like these narrow folds formed in the models, which are a bit too small to capture much detail. Even so, tight fold noses are visible along faults near the model basin margins. The one indicated below is actually an anticline, which would be flanked by synclines immediately adjacent to faults.
As with any sand model, the combination of layers of different mechanical properties strongly controls how the layers deform. The model shown below produces a much simpler structural style with an interesting relay ramp linking two fault strands.
The outcrop pattern is simpler, and the layers show less dip. The relay ramp is clearly visible in the outcrop pattern, and structural styles on either side of it are slightly different. The images below show two sides of a slice through the model, which did not gel properly due to the humid summer weather and looks a bit grungy…
The two structural styles are actually quite similar, but the broadly anticlinal block at the center of the bottom image is faulted in the top image. Note that these layers in this model dip much less than the other models, which creates broader (and less interesting!) outcrop patterns.
Interesting discussions of Newark Basin history can be found here and here; particularly interesting are discussions of when the block rotation occurred in the sequence of the basin’s development. Both Rutgers and Columbia have lots of well-illustrated online materials discussing the Newark Basin; Google will take you there through a variety of search terms.
This post was originally published on The Geo Models blog.