Tag Archives: deformation band

Statemap Field Blog, April 7-9, 2014

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Hello all,

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!

Deformation bands in massive sandstone near Middle Fork north of Shirley2014-04-07 017

2014-04-09 026 2014-04-09 011On 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.

2014-04-08 0192014-04-08 027Wednesday 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.

2014-04-08 048 (2)2014-04-09 047This 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!

 

 

 

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STATEMAP Field Blog Oct. 21-23, 2013

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Hello all! 

Another good week in the field!  Continued fairly dry fall weather means we’re still in the big drainages this week.  Finished up in Weaver Creek on Monday.  Still have the channel-bedded mostly shaly unit in the valley and the thin- to very thick-bedded sandstone unit on the southeast side of the valley.  Got to see the thickness of the alluvial cover in the high wall of a shale pit in the middle of the valley.  The owner states that the pit is 60 feet deep and is shaly to the bottom.  That’s a lot of shale!   The sandstone above it is at least that thick and probably more like 100 feet.  Still seeing channel beds within the sandstone unit as well.  A ribbon snake crossed our path heading toward the water. 

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The next day, we tried to cross the Middle Fork as we have done in past weeks, but there is already too much water in it.  There are some deep hollows on the east side that we needed to get to, but after fording the river proved impracticable, we resigned to going north on highway 9 and dropping off the top edge of the valley.  At least we started from the lowest saddle.  It was a fairly easy descent with a few washed out 4-wheeler trails helping out.  We knew right away we were in Witts Springs when we reached the bottom, but were surprised by how low the contact was when we reached the Cane Hill.  When we reached the river valley, we decided to follow a slough upstream to the next drainage on the east side.  This looked like the best route on the map, but was actually a thick canebrake with very few places to cross the slough.  We had a hard time getting in the upper end of this drainage a few weeks ago, and the lower end proved no different.  We were rewarded by finding a lot of deformation bands along non-vertical joints in the Witts Springs, and a drop of about a hundred feet in the contact with the Cane Hill between the drainages, which are only separated by about a mile.  At best may be a monocline in there.  On the way out we nearly ran over what I hope is the last moccasin of the season sunning on a very steep hillside above the drainage.  We gave him as wide a berth as we could and continued back to the river bottom.  This time we tried to avoid the slough and stayed closer to the river where the switch cane was thinner.  We backtracked up the hollow we came down earlier, then climbed back up the side to the highway.  So to summarize, river unfordable so drove around, hiked down 420 feet, bushwhacked a mile through canebrake, hiked up 320 feet, saw deformation bands, back down 320 feet, avoided snake, hiked a mile through woods, climbed back up 420 feet.  What a day!  

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On Wednesday we started on the west side of the river downstream of where we were the day before, and walked up one of two remaining large drainages before the river turns northeast at Shirley to follow the lineament.  We were rather surprised to find Imo near the mouth of the hollow.  This would make it the southernmost exposure of Imo mapped to date.  Saw good exposures of Cane Hill, which is mostly sandy through here, on the way to a classic example of the basal Witts Springs sandstone complete with exfoliation weathering. 

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Once we got out of there, it was back to Little Rock and back to reality. 

Oh, and I almost forgot.  I found this graph on the USGS website and thought you might find it interesting.  You can see that flood event on the last day of May this year that we are still seeing evidence of.  Apparently it was right up there with some of the biggest recorded events on the Middle Fork in that area.   

Stages on the Middle Fork at Shirley

Looks like we’ll be in the office next week, so next installment in two.  Until then, I’ll see you on the outcrop!

Tick attacks: very light

Snake count: 2

Statemap Field Blog–Oct 7-9, 2013

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Hello all!

Well, it was good to be back in the field again after a week of “vacation” (painting my daughter’s bedroom).  The weather is really getting a lot nicer now, and the ticks have almost given up.  We’re still working on the larger tributaries to the Middle Fork, so that they will be done by the time the dry season ends.  The Middle Fork is still low enough for us to cross it easily which is good because road access on the north side of the river turned out to be very limited.

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We finished up the last two big drainages that flow south into the Middle Fork east of Shirley.  Mostly a big stack of massive-bedded, calcareous sandstone, but little sign of the shalier Cane Hill which is still prominent along the river to the west.  This might suggest that the Cane Hill is either faulted out or that the tilt of the rocks to the southeast brings the Witts Springs to a lower elevation near the large southwest-northeast trending fault just to the south along the river.  Whichever it is, we’ll need more data to determine.  We did see a good example of travertine precipitating on some of the thin-bedded calcareous sandstone.  Slightly acidic groundwater is solutioning the calcium carbonate (calcite) with which this particular sandstone is cemented.  It is then precipitated where the water seeps out of the rock, in this case along a joint.  This lets us know that the adjacent rocks are calcareous, which in turn may help us determine what formation it’s in.2013-10-08 009 

We also saw some good deformation bands, which as I’ve said in a previous post is one of the signs that a fault is nearby.  These were in float, and though I looked high and low, I couldn’t find the source.  Certainly evidence of a lot of stress in these rocks!

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2013-10-09 004The other drainage we looked at is north of Shirley and is extremely steep and badly overgrown.  The sides were so boxed in we couldn’t come down the main channel, and had to try a side branch.  We did make it down that way, but with great difficulty.  Once we got to the bottom, we could only get within about 40 feet of the Witts Springs/Cane Hill contact in the main channel, because the lower end was boxed in as well.  We usually don’t give up until we get our point, but this one was deemed impenetrable.  The break is visible in the contours, so shouldn’t be too difficult to draw in anyway.  On the way down the side branch, I spied a round rock or “Prim boulder”.  These are definitely coming out of the Witts Springs.

2013-10-09 009Also saw more deformation bands, this time way up at the top of a 40 foot massive of basal Witts Springs sandstone.  That band you see in the close-up is about 6 inches wide!  The last photo is the Cane Hill in the box canyon below the contact.  The sheer bluff of massive Witts Springs sandstone was visible on both sides above.  I was rewarded by finding a large patch of muscadines on the way out.  They continue to get sweeter each week!  Wish I could have stayed to fill a bucket, but had to make do with a pocketful.  Made a nice snack during the hike back to the Jeep.  This is definitely a banner year for the muscadine crop!

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Hope y’all have a good week, and see you on the outcrop!

Tick attacks: mild