Chalybeate (pronounced ka-lib-e-it), or mineral springs, is a term used when water is highly saturated with iron that precipitates out of solution forming an iron-rich stain along the surface of an outcrop. These iron-rich waters are in solution until reaching the surface, where the iron becomes oxidized, forming an iron coating along the water’s path. The different colors are a result of the different oxidation states of iron. At one time, these mineral springs were believed to have healing powers with many health benefits. Unfortunately, these springs simply leave you with a lingering iron taste with no health benefits, but do provide an aesthetically pleasing photo opportunity.
Well, this is the last week of field work for the 2013-14 season. Of course, there’s always more one would like to have a look at, but we have to stop sometime. On Monday, we started down by the M&NA railroad bridge at Shirley. The big fault that makes the SW/NE lineation goes through here somewhere, but it’s difficult to say where exactly. There are lots of non-vertical joints and deformation bands in the area, which are all good fault signs, but nothing very definitive. The area north of the bridge is about as thick as it could possibly be with greenbriers –only passable with much effort and many scratches. We saw very thick-bedded sandstone there which we took for Witts Springs that day, but when we came back on Wednesday, we decided it may be north of the fault, and therefore would be Imo. We have Imo across the valley, so it’s not out of the question to have it here, but it may be just a relatively thin slice. There are many cut and fill channel beds there, some of them with very nice soft-sediment deformation at the margins.
On Tuesday we finished up some loose ends in the northwest corner of the Shirley quad. After we climbed way down in a hollow that had an old tornado track going through it, Danny realized he had lost his camera somewhere. We hiked back up to the Jeep to see if it was there (it wasn’t), then retraced our steps from earlier that morning. Still nothing. He remembered the last time he had used it was in that horrible briar patch the day before, so after we climbed out again, we headed back there. Sure enough, in the thickest part of the patch, where he had been practically crawling to get through, a briar had reached in his carrying case and pulled it out. It was still dangling there about a foot off the ground right on the river bank. At least we got it back!
On Tuesday afternoon, we went down a drainage on the west side of Middle Fork looking for more signs of a fault we have traced from the Old Lexington quad. We definitely found a lot of deformation bands in the Witts Springs massives down there and figure there might be as much as 80 feet of throw on the fault.
Wednesday was our last day in the field this year, and we spent most of our time on the Middle Fork just north of Shirley where we had left off on Monday. Did look like the fault goes through there because we found very-thick bedded massives on the north side (Imo) and shale interbedded with very thin-bedded sandstone on the south side (Cane Hill). Our last couple of hours we spent getting points in several road cuts in and around Shirley. We took a final photo in front of the town sign.
This will be Danny’s last year out in the field with me, so I’d like to take this opportunity to thank him for putting up with me and the sometimes horrendous field conditions we’ve faced together the last five years. Looks like I’ll have to break in a new field partner next year, so should be interesting. Now comes the time of year when we have to sit in the office and draw the maps, create the layouts, and finish the database, all to be turned in to the USGS by June 30. It seems like a long time, but we’re always editing down to the last minute. By the time we make it back out in the field, it will be mid-July, so the ticks and snakes will be out in full force, it will be nice and hot, and all the vegetation will be full grown. At least that gives us something to look forward to. Until then, I’ll see you in the office. After that, I’ll see you on the outcrop!
Soft sediment deformation in silty shale of the Imo Formation near the Middle Fork of the Little Red River near Shirley, Arkansas. Features like that pictured here result from sediment being exposed to pressure prior to being completely lithified. Many factors can contribute to soft-sediment deformation including uneven settling, slumping, and escape during compaction of water trapped in the sediment.
“Turtle Rocks” are unique, mounded polygonal structures that resemble turtle shells. These features are found along the Arkansas River Valley in the Hartshorne Sandstone, a brown to light gray, massive, medium-grained sandstone deposited during the Pennsylvanian Period by ancient river systems. The processes that generate “turtle rocks” are not clearly understood. One explanation suggests that these features were created by a process known as spheroidal weathering, a form of chemical weathering that occurs when water percolates through the rock and between individual sand grains. These grains loosen and separate from the rock, especially along corners and edges where the most surface area is exposed, which widens the rock’s natural fractures creating a rounded, turtle-like shape. Additionally, iron is leached from the rock and precipitated at the surface creating a weathering rind known as case hardening. These two processes along with the polygonal joint pattern contribute to this weathering phenomenon.
The Arkansas Geological Survey hosted a field trip to Petit Jean State Park for 26 Fountain Lake High School seniors and science club students. The high school seniors are currently in a college geology course taught by Mrs. Jennifer Cox, a former geologist with the AGS. As far as we could tell, these seniors were ready to show off their geologic knowledge. Two students, whom I understand are brothers, were excited enough to buy Muscadine Grape Juice from the Visitor’s Center prior to the start of our trip. Nothing says geology like a good swig to start your day.
Our first stop was to Seven Hollows Trail. Along the trail, we first looked at liesegang banding and a natural shelter within the Hartshorne Sandstone. Liesegang banding (aka box-work) is created when water percolates through the sandstone and comes in contact with the iron minerals present causing the iron to go into solution. As the rock is exposed to air, oxygen is added to the solution, oxidizing the iron and causing it to precipitate out of solution along exposed joints and/or bedding planes in the rock formation. The iron sometimes precipitates out as box-shaped and triangular patterns. The natural shelter within the sandstone was created as a result of weathering. Again, water percolates through the sandstone and between individual sand grains, causing the grains to loosen and separate from the rock. After millions of years of weathering, large voids are created within the rock. This large void appears to be a prime location for the first of many class photos.
Senior class photo in a natural shelter within the Hartshorne Sandstone. The natural archway is lined with great liesegang banding features.
Liesegang bands, or carpet rocks based on their square pattern, adjacent to natural shelter.
Our last stop was to Natural Bridge and the turtle rocks above natural bridge. We had some excited young geologists who immediately began to climb on top of the natural bridge.
Natural Bridge (left). Again notice the great liesegang bands in the archway. Young adventurous, soon-to-be geologists climbing above natural bridge to the turtle rocks above (right).
Another senior photo op at natural bridge.
Turtle rocks above natural bridge are some of the best features in Petit Jean State Park. “Turtle Rocks” are unique, mounded polygonal structures that resemble turtle shells. These features are found along the Arkansas River Valley in the Hartshorne Sandstone deposited during the Pennsylvanian Period by ancient river systems. The processes that generate “turtle rocks” are not clearly understood. One explanation suggests that these features were created by a process known as spheroidal weathering, a form of chemical weathering that occurs when water percolates through the rock and between individual sand grains. These grains loosen and separate from the rock, especially along corners and edges where the most surface area is exposed, which widens the rock’s natural fractures and creates a rounded, turtle-like shape. Additionally, iron is leached from the rock and precipitated at the surface creating a weathering rind known as case hardening. These two processes along with the polygonal joint pattern contribute to this weathering phenomenon.
Exploring these great turtle rocks. Everyone was thinking that these features were definitely worth the hike.
After exploring these sedimentary features, we headed back up the trail toward the bus, ready for lunch. I’m not sure how anyone had any energy left after the hike, but it seems most of the students finished their lunch pretty quickly so they could play around on the playground.
After lunch we headed to Rock House Cave, a large rock shelter within the Hartshorne Sandstone. Honeycomb weathering and cross bedding features are easily visible around Rock House Cave. Honeycomb weathering is created very similarly to how the natural shelters are formed (e.g. Rock House Cave, natural shelter along Seven Hollows Trail), in that water percolates through the sandstone, loosening and separating the sand grains from the rock creating a void. Cross beds are diagonal lines that represent movement of large ripples within the sandstone deposited by an ancient river system that existed here 300 million years ago. These cross beds indicate the direction the river once flowed.
Notice the nice cross beds in the middle section of the large boulder above Ms. East’s head.
We ended the day with a final photo session in both Rock House Cave and on the turtle rocks located on the trail.
There are those Sig Figs (FLHS Science Club). They are reminiscing about the day’s awesome geology field trip.
Another great week in the field. Signs of spring are everywhere, and unfortunately the field season is drawing to a close. We skipped around all over the Fairfield Bay quad this week, still trying to trace the very thick-, massive-bedded sandstone that we’re calling the base of the Bloyd for now. Just off the eastern edge of the Fairfield Bay quad is a locally famous outcrop of that sandstone that was supposedly visited by Hernando Desoto himself in 1542. Whether or not that’s true, it is a very impressive bluff shelter known as the Indian Rock House. A lot of eroded material was removed from the floor of the shelter when the adjacent Indian Hills Golf Club was built, leaving behind the fine sandstone amphitheater we see today. One could see how this formation could later become a natural bridge if erosion continues along the joint set parallel to the bluff face. If that interior arch were to fall out, then the remaining one would form a bridge. This is how most of the sandstone natural bridges in Arkansas are formed. Lots of graffiti has been scratched into the friable rock over the years, including some that may have been carved by native people.
On Tuesday, we finished up our field work on the lake. We still had a couple islands we needed to visit, and the entire south side of the lake is so steep that access by land would be difficult. We were excited to find more old river terraces on the islands, including one that would have been deposited on a cut-off meander in the area of Harpers Cove. The deposit is about 80 feet above and over a half mile north of the current river channel (before the lake was there, that is). The high end of the range for the downcutting rate for the Colorado River in the western Grand Canyon is 16 centimeters/1000 years, and I think we can all agree that downcutting there probably exceeds that in Arkansas. Using that rate, an estimated 152,000 years would have passed since that terrace was deposited. That gravel has been there a long time! Of course, cutting off the meander would have stranded that deposit at that time, but don’t forget that this stream is developed in bedrock, so meander cut-off would be a fairly infrequent event. To get a better estimate of these events, methods such as luminescence dating are being developed to age date the sand in these stranded river terraces. With this new technology, perhaps someday we will know when these terraces were deposited.
On the south side of lake below Stevens Point is a good example of a modern landslide, and a bit of a cautionary tale. Sometimes clearing trees for roads and houses can have catastrophic results. The photo tells the story. The major part of this landslide occurred March 28, 2005 just after a road was cleared from the house down to the lake. Most of the material at the edge of the lake on the north side of Hunter Mountain is there as a result of old landslides, therefore any development in this area can cause it to become unstable, as evidenced here. That’s why part of our project includes mapping areas where landslides have occurred.
Speaking of Hunter Mountain, we ran across one of the now ubiquitous gas well pads up there, and I thought you might be interested to know the function of each piece of typical well head production equipment. At each wellhead is a set of valves that regulate the flow of gas. These are often controlled remotely, thus the solar panels which power the system. The big tanks near them contain hydrogen sulfide which is introduced into the gas right away to give it a strong odor. This odor is, of course, quite useful to determine if there are any gas leaks since natural gas is odorless. From the wellhead, the gas flows to the separators which remove any fluids contained in the gas. This fluid could include heavy hydrocarbons, but is mostly produced water. These fluids are stored in large tanks which are built inside a berm. The berm is designed to hold 1 1/2 times the capacity of one of the storage tanks in case of a spill. The level in the tanks is also monitored remotely and emptied on a regular basis. From here, the gas is piped to a compression station where it undergoes further treatment. Then it is sent through a transmission line and on to your house. It’s not pretty, but for now, we have to have it.
Well, next week will be the last of our field season.
Until then, see you on the outcrop!