After the mud dries: Wheeling in nearshore sedimentary rocks

Barnwell Mountain consists of one geologic formation, the Queen City Sand. As suggested by the linked report, and others related to water wells in the NE TX area, this formation comprises unlithified (not cemented) sandy layers and muddy layers that are somewhat more resistant to erosion. This formation has been described (reference unknown but government agency source) as a primarily sandy unit that was deposited in a constructive coastal environment (i.e. more sediment than could be transported away) that was part of a lobate delta system, including delta plain, delta front, and prodelta facies (i.e. sedimentary depositional environments). This is a concise description of the Mississippi River delta today, with numerous inactive deltas, lots of sediment (especially mud), and a wave-dominated sandy barrier island system where the river finally must deal with the ocean head-on…

I took a nice photo of an outcrop within the geology compass Ap but it cannot apparently be downloaded…here is a photo (actually more useful) of the blocks of nodular, fossiliferous, dark grey mudstones that formed ledges (<2 feet high) and erosional blocks within the park.

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It is a quirk of younger rocks that the sandy layers are difficult to cement because of the great pressure and temperature necessary to recrystallize quartz into a silica cement (microcrystalline), whereas mud can be lithified more easily because of all of the organic contaminants found in fine-grained sediments in coastal environments. The result of this is that there is a lot of sand found all over the mountain, but no obvious source because the mountain is made of sand (maybe some weak cement but not silica). Here is a photo of a sample I procured from the campsite, which is representative of other layers in the park.

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Note the nodular surface that almost looks like walnuts. This is another example of how bedding layers, like the surface of this sample, are produced by chemical and not physical processes (enough about that!). When I broke this open, it almost looks like some mud from the slough behind my house…

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This photo shows the walnut/nodular surface and inside one of these nodules. We see concentric layers based on slight chemical differences in the original mud, and some darker, partly recrystallized, dark matter that is probably a piece of wood (very resistant to recrystallization,,,I think?) or other crap…geologists must use very expensive methods like electron microscopy to identify these kinds of substances. The gaps in the pore space are obvious…these would be filled with water, gas, or (Texans hope) OIL. The next photo shows how much these Eocene (~55 mya) sedimentary rocks resemble modern sediments (a Hoorah for Uniformitarianism)…

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This sample is difficult to describe…it contains a lot of “stuff” that is apparently biological in origin, a lot like the muck I dig out of the slough behind my house. Some of the material even has a lighter color, suggesting it is quite different mineralogy or even original material, but I have no idea about this without more thorough study of the sample. I will close this post with the thought that mud is mud is shale is mud, ad-nauseam!


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  1. Geology and wheeling upside down | Rocks and (no) Roads - May 9, 2015

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