Pictured here is a normal fault discovered this week on the Parma quadrangle by our field mapping crew. It’s developed in the Witt Springs formation and offsets thick-bedded sandstone against thin-bedded sandstone. There is about a one foot wide zone just north (left) of the fault plane where the bedding appears to parallel the fault. This could be due to the sandstone of the footwall being dragged into the fault or the near-vertical “beds” may actually be joints that developed due to stress near the fault.
Tight recumbent folds in a fresh exposure of Bigfork Chert, Ouachita Mountains Arkansas. Thin chert beds divided by siliceous shale beds are the basic lithology of this Ordovician age (540 to 490 Ma) formation. Complicated structures like those pictured below are characteristic of the rocks that form the Ouachita Mountains. The complex folds and faults resulted from plate tectonic forces that compressed the rock and caused many of the structures to rotate and overturn.
This photo, recently taken on the Prim quad, shows deformation of the bedding, probably due to dewatering of the sand prior to lithification. This soft-sediment deformation typically occurs in areas where deposition rates are high such as a delta or near shore marine environment. As sediment is deposited, water is trapped in the space between grains. As sedimentation continues, pressure on the trapped water, due to compaction, causes rupture of the sediment and escape of the water to the surface. Evidence for this are “tent” or “pillar” structures left behind in the rock as seen in the photographs above. The black line on the right photo shows the probable path of the water from one bed to the next through what is now a sandstone outcrop of the Pennsylvanian Witts Springs Formation.
Hello all! Sorry I haven’t blogged in awhile. I’ve been so busy trying to complete the maps for this year by the June 30th deadline. But, I am proud to announce that the Geologic Maps for Shirley and Fairfield Bay are now published and available on our website! Click below to view and download the maps.
The process to create these maps takes an entire year. I kept you updated each field week from July to April last year, so I thought you might be interested to know how we take the raw data we collected in the field and use it to make a map. First of all, it’s a collaborative effort. It takes a lot of people who specialize in various disciplines working together to make this product. Basically, drawing the map starts with the notes we took in the field. At each point, we tried to identify the rock formation exposed there. Sometimes this was difficult, especially in the southern portion of the Boston Mountains Plateau where we worked this year. These rocks are all so similar–mostly sandstone and shale. Nevertheless, if you cover as much ground as we did, you begin to discern similarities in the rock types and bedding structures, and can make formation calls based on those similarities. Many of the points are taken on what we considered to be contacts between different formations. These points are used to hand draw contact lines on a blank topographic base map. These lines are continued into areas where the contacts may not be exposed, because we assume lateral continuity of these units. Many times there are topographic breaks along these contacts which can help us draw the lines in areas of poor exposure or in areas we just didn’t get to. Structural lines are drawn along the trace of faults or other structures at the surface in areas where we saw the hallmarks of faulting such as deformation bands and non-vertical joints. Also, the many strike and dip measurements we took were plotted on the map and helped determine orientation of faults and other structures, such as the axis of a monocline. Once all the lines were neatly drawn on the topo, it was scanned into the computer and georeferenced to the grid of all quads in the state. Next, each line was painstakingly digitized in ArcView by one of our cartographers, in this case Brian Kehner. The digitized map was then added to a layout that Danny created in Adobe Illustrator.
The layout includes descriptions of each formation developed from our field notes and are specific to each quad. A correlation of map units, a generalized stratigraphic column, an inset map of the locations of the field points, a symbol chart, and a rose diagram of the frequency of each joint direction are also added to the layout. A cross-section based on formation thickness is hand drawn, digitized, and placed along the bottom of the layout. Formations are symbolized by color and an abbreviation. Sometimes photos are added to balance the layout. Also plotted are any quarries or pits we found or were in the economic commodities database we keep at the Survey.
After we have a reasonably good map, it’s printed and set out for staff review. They really let us have it, but this editing process always greatly improves the maps. After two or three revisions, we finalize it and send it to the USGS by June 30 to fulfill the requirements of the STATEMAP grant. Whew! What a relief! This year, as in years past, I have designed a commemorative STATEMAP t-shirt. I’m taking orders until July 25th if anyone is interested. They are available for the cost of the shirt you choose plus the printing. Please email me at email@example.com for details. Now we get ready to head back out again to our new field area. This year we’ll be mapping the Parma, Prim, and Greers Ferry quads. I’m breaking in a new field partner this time out. Andrew Haner says he’s looking forward to seeing some of the Arkansas wilderness. I just hope the snake count is low this year. From what I’ve see so far, the ticks seem to be at an all time high. I’ll try to keep you posted, but will be out of the office four days a week this year. That will leave little time for blogging. So until my next post, I’ll see you on the outcrop! Richard Hutto
This large quartz vug, or cavity, (approximately 3 ft wide x 3 ft high) was extracted from one of the Coleman Quartz Mines and brought to the Arkansas Geological Survey Learning Center in the 1980s. This vug was found in the Crystal Mountain Sandstone (Early Ordovician, 485-470 mya), a massive, coarse-grained, well rounded, light gray sandstone from Montgomery and western Garland Counties, Ouachita Mountains. These quartz crystals formed secondarily from silica-rich fluids that resided within this large cavity.
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!