This post isn’t about somebody overturning (I don’t think anyone did), but the reversal of expected geology in mixed sand/mud environments. I start by referring to the wheeling experience at Hawk Pride, where we fought mud that was produced from weathering (erosion) of shale and sandstones. There, we found boulders of Hartselle Fm. sandstone in a hillside with lots of shale/mud, where the ledges (an offroad term for a hard climb over a solid layer of rock) were the more resistant sandstone. BUT, those were 300 my old sediments where the quartz grains produced a silica cement, which made the “sandy” layers really hard.
As I mentioned in a previous post, these rocks are only 50 my old, and the sandstone is not cemented, which is why the layers (in total) are called the “Queen City Sand”. Here is a (poor) image that shows what I mean…
The vehicle in the image was dragged (by winch) over a 2 foot ledge of hard rock, which can be inferred from their exit (sorry about the poor trail photos, but I forget to take them when I am worried about my own escape…a work in progress). The ledge was composed of shales like I showed in the previous post. Here is another photo of a hand sample from this area. The rock doesn’t have as many organic components but it is as resistant.
This post is difficult to write because I didn’t take enough photos of the trail geology, but it is important to learn one lesson…time heals all sedimentary wounds…the young Eocene rocks (~50 mya) haven’t had time to heal all wounds. I mean that the organic material was still observable in a hand sample, and the sands didn’t supply silica (aka SiO2) to cement them as with older rocks. The result is that we were attempting to drive over hard shale (aka mud) layers while the sand formed nice traction surfaces to approach these ledges.
I will try and tie the environmental, geological, and wheeling aspects of this trip together in the next post, the epilogue…
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.
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.
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…
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)…
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!
As noted in the prologue post, we moved back in time as we travel northeast, just as in driving to NE Alabama. The difference between these two drives is the time-interval covered by our 8-hour drive; we time-traveled 330 my (million years) to go to AL whereas the trip to TX only covered about 60 my.
The ancestral Appalachian Mtns. were located all along the eastern seaboard as they still are today. All of those rocks and the titanic collision with Africa pushed the earth’s crust downward as well as upward, as vast quantities of sediment were eroded from the rapidly rising mountains. Thus, the Mississippian Era (~330 mya) rocks deposited in NE AL were buried deep (and produced oil and coal because of it), but the crust has rebounded over the last several HUNDRED million years and these old, previously deeply buried sediments from a delta are now exposed at Hawk Pride ORV park.
When I drove to Gilmer, instead of seeing a regular, albeit incomplete (due to erosion) stack of sediments, I drove over Pleistocene (<2 mya) terraces produced by changes in sea level on a relatively stable coastline. There were also missing rocks as in Mississippi, especially the gap between the Pleistocene terraces (~2 mya) and the Miocene rocks we encountered near Natchitoches (~20 mya), and the even longer diastem (missing rocks) between the Miocene and Eocene sediments we will see in Gilmer (~40 mya). As we get into older sediments, they become more uniform (i.e. the smooth transition from Eocene to Paleocene near the TX-LA border on I-20.
This post is intended to remind me of the similarity of the rocks and history on the east and west sides of the MS embayment, which has little to do with the ancestral Appalachians but it does overprint its impacts on the older ones…We must always think 4-dimensionally with geology…LOL
I am about to leave LA so I will briefly review my trip. I left Alexandria and crossed the Qal (Quaternary alluvium, less than 10 KYA), as seen this photo; Alexandria is where the coin is. The dash line is my path. As I drove north, the pink Qal was flat with farms but where I crossed exposures of the Pleistocene terraces (aka Pt), linear ridges were present as seen in previous post. As I crossed into the Miocene sedimentary rocks (light purple), the terrain became hilly until Shreveport (westward turn near top of path). However there was much more variability than the map suggests. Here is photo of hilly Miocene I am now at Love’s in the Paleocene Wilcox Group, but there is a deep-crustal uplift (Sabine Arch) and erosion has revealed the younger Eocene Claiborne Group sediments at Gilmer.
i drove along the Prairie terrace to Baton Rouge and then spent more than an hour crossing the basin before turning north on I-49 at Lafayette. Although I was in the Quaternary alluvium the land rose because the Qal tends to fill river valleys. Here is one of the many rises I crossed northward; they are oriented E-W.
As I neared Alexandria there were more smaller ridges. I crossed over the alluvium of the Ouachita R just before arriving. Here are some of these smaller ridges. I am probably on the Intermediate (older) terrace now.
I will cross back over the Qal and expect to see Miocene rocks soon
The next series of posts discuss the geologic time travel across the Mississippi Embayment into Texas. My destination is Barnwell Mtn Recreational Area, which is run by the state, unlike Hawk Pride, a private park. Before I can get to Texas, however, I must cross the down-warped Cenozoic (<65 mya) rocks of the MS embayment. I am going to try and take pictures of the contacts seen in the below image as I drive up, because I have installed the WordPress Ap on my phone (good luck with that!). Thus, I won't say much here…
These rocks are time-synchronous with the rocks I briefly described as I drove to Hawk Pride last month, but they are different because the sediments that formed them were further from a source of erosion (i.e. Ancestral Southern Appalachian Mountains). This trip, as I drive northwest, the rocks get progressively older because of the down-warping beneath the MS river (note that erosion removed any rocks that were exposed to the wind and rain…etc).
This photo shows the geologic map of NE TX, with the park indicated by a black circle. The oldest rocks I will encounter are Eocene Claiborne Group sedimentary rocks, which were deposited in an environment just like the mouth of the MS River today!
The dominant rock type in the park (Gilmer, TX, Ushur County) is a sandstone, but anyone who has been wheeling there will tell you about the sticky! mud. The name of the primary stratigraphic unit (aka formation) is the Queen City Sandstone (a group comprises several formations), but I bet there is some mud in there if the sediments were deposited in a delta front environment!
This topographic map, with the very rough outline of the park, shows the topographic relief to be about 200 feet, but the park encompasses the entire mountain. I hope to see if I can identify changes in the sedimentary facies within this small area.
Looking forward to a great time-travel trip…see you then!
I want to add one last comment, which addresses my original objective in this wheeling/geology trip; why is Mountainside, which is a low-gradient road cut down the north side of Thompson Hollow, more difficult to traverse when it rains than Big Hill, which goes right up the mountain?
My limited field work suggests that the answer lies in the original distribution (horizontally as well as vertically) of clay sediments in the Mississippian. The transition from shallow ocean (i.e. fossil-rich thin beds of limestone and dark shales) to coastal sediments (i.e. the Hartselle Fm. red sandstones and shales with some limestone) during this period produced rapid changes horizontally in facies, which are environmentally dependent groups of sediments (later rocks). I think that coincidentally, there was a barrier island system on the south side of Thompson Draw with thinner layers of mud between thin sand layers. Thus, Big Hill and Mountainside must traverse more shale (now mud) than 1/2 mile to the south. When Mountainside was cut with a dozer, it produced a trail/road that has a greater extent of slippery mud when it rains. This same section (from ~500 -700 feet) on Big Hill is crossed in a series of swales and sandy ridges that are easier to traverse in sections.
There are slippery sections of John’s Way (the escape route out of Thompson Draw) but overall it is sandier.
This post attempts to relate the previous comments to the four-wheeling experience in northern AL. First, I am going to show some photos of trucks meeting these rocks up close. This photo says a lot about HPORV park…my truck is parked on a flat, muddy, area with 3-6 foot ledges to the right side.
This was taken in the Hartselle Fm., which contains sandstones and shales (i.e. sand and mud). The shale was originally mud and when the shale, which is the rock that results from ~20 km depth of burial, is exposed to rain and weathering, the result is mud! This same spot is slimy when it rains (which it did the same night after this photo!). But it is not a mud hole because it is underlain by sandstone rock, which prevents deep erosion by truck tires.
I just had to show my trail companions’ response to this wheeling experience…they only get up when thrown to the floor during downhill stretches and when I set the parking break…LOL
This photo, taken the next day, shows how the trail changes after a heavy rain and the mud is liquefied by vehicles and hysteresis.
When the trail crosses a natural drainage (it rains in AL!), the crossing is very difficult, as demonstrated by our trail leader, who had to winch himself…the rest of us used tow straps or made it (50/50).
The geologic map uses the symbol Qal for the recent (~10000 years) sediment that accumulates at the of bottom hills like those surrounding Thompson’s Draw. Here is a photo of everyone breathing a sigh of relief near the lowest point of the park…the lowest point includes the “mud bog”, but it was just too hard to get there after the rain…LOL!
One of the results of weathering of a sandstone is the accumulation of blocks downhill. I showed some photos of these in a previous post. We took a trail on the north side of Big Hill that shows what this is like up close. This first photo shows the entrance to a natural gulley, which really works the suspension.
It took some human weight to keep this truck on track…
And then he had to drop over a ledge of Hartsell Fm. sandsone, which proved too high for his truck…
He got some winching from the trail leader and made it until an even bigger ledge appeared (sorry, no photos…I was helping…LOL). I made it down this ravine with no assistance partly because it is always easier to follow and see how the rocks fall (so to speak). My last photo is of the tranquil scene from the top of the Hartselle, where rain and snow haven’t produced the difficult landscape we traverse below.
My final comments relate to the similarity of the environment ~320 mya and those today, notwithstanding the elevation changes. These rock were all flat and all relief is due to erosion. This is very different from the situation 1000 miles to the north, where the slightly younger Pennsylvanian Pottsville Fm. represents the mountain building occurring there at this time (see my previous post). The northern ancestral Appalachian Mtns were close to the massive collision whereas the southern extent (i.e. AL) was a more peaceful environment, with barrier islands, bays, mud flats, and shallow seas. Even after ~300 million years we encounter this geologic memory of those environments, remembering that our ~400 feet drive up and down Big Hill traverses about 20 million years…LOL!
As noted in the previous post, the Pride Mtn formation was dark shales and sandstones at the type locality I visited. The previous post discussed the younger (on top…Upper Mississippian…~330 mya) Harstelle Formation. This post shows some photos of the Pride Mtn Formation. After meeting Big Hill from Mountainside, I stopped and collected a hand sample, which is limestone (HCl fizzes in contact), and took a photo of the exposure. Here is the exposure, after hundreds of drive-overs by four-wheelers.
After driving around a while, I found this great exposure behind the barn near the bottom of Thompson’s Draw. The gray massive rock above is what I showed in the previous photo. The lower unit is more interesting… …
This exposure was accessible from the truck (the dogs were happy) and it showed this remarkable fossile bed, which includes Spiniferid brachiopods, bryozoan calyxes (kinda like spinal disks), and other fossils. I couldn’t get a hand sample of this, however…
This kind of fossil assemblage is used to know were these rocks fit into geological time. The exposure photo above shows how different these layers appear but the following hand sample photos show how similar they are internally..first, the more massive layer from Big Hill:
and then the fossiliferous layer from the barn…
Weathering sometimes causes slight differences in limestone mineralogy (calcite) to accentuate fossils against their containing rock, as seen in the photo above with the fossil bed, even though the rock looks the same when broken apart. These limestones are described in the formal description of the Pride Mtn Formation, but what about the shales (i.e. mud)? I didn’t document the change from redder mud (from the overlying Hartselle FM) as I drove down Mountainside, but the mud (i.e. weathered shale) a Big Hill was darker. I will discuss this in the final post from Hawk Pride
The USGS briefly describes the Hartsell Fm. in this general area as “Thin-bedded, fine-grained sandstone and greenish-gray shale interbedded with coarse limestone. Thickness 0 to 60 feet.” After returning to HPMORV park, I took a drive down the trail called “Mountainside”, which drops from ~850 feet to ~600 feet elevation. I collected a sample and took some photos, which I will show in this post, along with some (quite different) photos of this formation on the other side of Thompson’s Draw. This image shows my traverse downhill to join “Big Hill” trail.
There is a steep, muddy, drop down the top of the hill on Mountainside but then the trail (road, actually when weather is dry..never!) levels off. I climbed back up “Powerline Cutoff” and took this photo, which shows thin bedding in sandy rocks.
This is a pretty easy climb in the 4×4; the steps are like an asphalt highway for going uphill. The hand sample I collected was not particularly interesting.
I think the original top of this layer is to the left in the photo. It has a reddish (buff…LOL) hue and shows some fine cross bedding. The reddish color suggests that the sediments were deposited above sea level ~320 mya, and the fine layering only occurs in fairly quiet water with rhythmic variations in water depth (e.g. tides) and low energy from waves. I drove a little further (location 2 in the above photo) and took this photo, which suggests an intertidal location because of the lack of wave ripples, and the irregular surface.
I will jump ahead to the drive up the southern flank of Thompson’s Draw, where I found these great exposures of massive bedded sandstone with layers of thin-bedded sandstone with cross bedding. This is where the buggies go…there are trails over this ledge!
This doesn’t quite fit the USGS description, but rocks vary rapidly over short distances. The reason in this case is probably the environment in which these rocks were deposited, and the time interval over which the sediments were deposited. This was a shallow-water environment a lot like the Gulf coast, with lagoons, barrier islands, sand bars, and mud flats. My drive down Mountainside also covered several million years of history, but we cannot date these sediments…we only know for sure that the rocks get older downward…LOL!