Monthly Archives: October 2013

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

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GeoPic of the Week: Skeletal Limestone — Happy Halloween!

Crinoidal Imo Face

Skeletal limestone – Happy Halloween!

Limestone varies in color from shades of gray to white and red.  This limestone gets its red color from iron oxide in the rock and contrasts nicely with the white crinoids.  Crinoids are a group of marine invertebrates sometimes referred to as “sea lilies”.  The animal was attached to the ocean floor by a main stem or stalk with long arms flowing outward from a central head on top of the stem.  These animals were abundant in the Mississippian Period in Arkansas.  It is unusual to find a complete crinoid fossil; however, pieces of the stalk (commonly called stem or column) are abundantly preserved in the rocks in northern Arkansas.  This limestone is fossiliferous or since it contains the skeletons of animals can be referred to as a skeletal limestone.  This adjective is especially fitting for the rock considering how it has weathered to resemble a skeleton face and the fact that it is Halloween week.  Happy Halloween!

Statemap Field Blog, Sept. 23-25, 2013

Hello all!

Another good week in the field!  Temperatures are getting low in the morning, but still warming up in the afternoon.  We started out by finishing up the rest of Tick Creek at the north edge of the map.  Got the lower Cane Hill sandstone up Files Hollow and again on the east side of Tick.2013-09-23 001  2013-09-23 014Still have good channel-bedded Imo below in the creek bed with a shale unit between the two.

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Tuesday and Wednesday we finished up the lower end of Indian Creek.  There were several massive-bedded sandstone units that had cut down into lower units of thin- to medium-bedded sandstone interbedded with shale.

This is probably all in the Witts Springs Formation, but we won’t know for sure until we see more of the rock in the area.  One of the indications that it is Witts Springs sandstone is the characteristic curved reentrant at creek level.  This is caused by exfoliation or spalling of curved sheets of rock due to more rapid dissolution of the calcareous cement near the creek.  The Witts Springs sandstone is typically more calcareous than overlying formations.    2013-09-24 0452013-09-24 060

You never know when you might see a classic example of a sedimentary structure while hiking around.  This week we saw some good examples of load casts in the creek float.  A sand deposit formed an irregular bulge as it pressed down (loaded) into the mud below.  Later lithification preserved the cast of that shape on the bottom of the sandstone bed.

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As we approached the southwest/northeast topographic lineation along the Middle Fork at the mouth of Indian Creek, we began to see more and more signs of a major structure.  Aside from a 5 to 7 degree southeast dip, there was an increasing abundance of deformation bands in the massive sandstone.   Also, we began to see a lot of non-vertical joints and small faults.  Took photos of a couple of fault planes with the slickensides still evident.2013-09-25 013  Slickensides are the parallel grooves or scratches left behind on the fault plane caused by the abrasion of one rock surface against another.  They are typically smooth in the direction of movement and rough in the opposite direction, so can indicate which way the fault moved.  Unfortunately there is no way to estimate throw, or the amount of offset on the fault, without knowing which formations are on either side.  That’s why we’ll have to be extra thorough in that area.  There’s probably a big fault somewhere along that lineation, but we’ve yet to find it so far.

By the way, Indian Creek must have gotten its name from the number of moccasins lying around.

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We’re taking a little hiatus next week so will be at least two weeks ’til the next installment.

See you on the outcrop!

Snake count: at least 4

Tick attacks: still severe

GeoPic of the Week: Crinoid Stem In Limestone

Crinoid stem in limestone

Crinoid stem in limestone

Crinoids are a group of marine invertebrates that were abundant in the Mississippian Period (359-323 million years ago). It is unusual to find a complete crinoid fossil; however, pieces of the stalk (commonly called stem or column) are abundantly preserved in the rocks in northern Arkansas.  The center portion of the stem in this picture has dissolved away and ooliths were deposited in it.  Ooliths are small round bodies, usually 0.5-1 mm in size, and are commonly formed of calcium carbonate (calcite).  They are formed as successive concentric layers precipitate around a nucleus such as a shell fragment, algal pellet, or a quartz-sand grain.  The layers form in wave-agitated water such as a shoal environment along a shoreline.  The rock type is called an oolitic limestone.  Fossiliferous and oolitic limestone is present in the Pitkin Limestone in northern Arkansas.

Statemap Field Blog, Sept. 16-18, 2013

Hello all!

Another great week in the field!  Temperatures are coming down some which makes things much more bearable.  Monday we came down from Fox and crossed the Middle Fork at Lydalisk.  Went as far south on the old railroad grade as we could then worked our way back, getting points along the way.  Still finding the Imo near the bottom of the valley.   The river was so low that we could actually cross it on foot to get a drainage on the east side.  The first good outcrops we found there were the lower Cane Hill.   There were good examples of taphoni in the rocks there.  Commonly known as “honeycomb weathering”, taphoni is caused by weathering processes in many rock types and under many conditions.  In the Cane Hill it typically affects the thin-bedded sandstones in dry areas beneath overhangs and is probably due to differential dissolutioning of the cement by groundwater.  This process also causes the concave structures as noted in previous blogs.

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The next two days we started at the upper end of Indian Creek and worked our way down.  Lots 2013-09-17-019_thumb.jpgof Fayetteville shale production going on in this area, which means good roads for us!  Seemed to be in Witts Springs sandstone the entire stretch, so not a lot of contacts.  Definitely had a lot of dipping rock and signs of  a large fault further south including deformation bands and non-vertical joints.  Located a couple small faults with what we think is minimal offset along the creek and adjacent hollows.

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Cottonmouth in dry creek bed.

Did have a few encounters with our reptile friends.  We always leave them be so maybe word will spread among their kind that we mean no harm.  It’s worked so far!

See you on the outcrop!

Snake count: 2

Tick attacks: severe

Statemap Field Blog, Sept. 8-13, 2013

Hello all!

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Silence of the cicadas

Signs that summer is almost over despite the continued warm temperatures include the silencing of cicada calls one at a time.  A welcome turn of events!

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More “crazy-bedding” in the upper Imo

This week was a long one because we worked in two different field areas.  On Monday and Tuesday we were back on Tick Creek looking at the last two major eastern drainages.  Found good Cane Hill/Imo contacts in each and a fairly consistent
irregular-, channel-bedded sandstone unit with abundant soft-sediment deformation near the top of the Imo.  It’s so irregular that I was calling it “crazy-bedded” before long. 

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“Crazy-bedding” in the upper Imo

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Yet more “crazy-bedding” in the upper Imo

There was also a section in the middle of the Cane Hill that was so perfectly cut by its east/west joint that it formed a smooth wall on the north side of the creek.  There was a parallel joint face on the south side, but it was not nearly as well exposed.

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Danny views joint face “wall” in Cane Hill

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Buttresses on Bear Creek

On Tuesday evening we joined Angela Chandler and Lea Nondorf, also of the Survey, on Bear Creek in Marshall to work on several other projects the rest of the week.  One ongoing project is to try to resolve edge-matching issues whenever we get a chance.  This involves gathering new data in boundary areas between quads that were mapped by different people or who used a different stratigraphy.  This week we were also looking for localities in the Imo interval for an upcoming field trip that Angela, Erin Smart and I are leading this spring for the GSA (Geological Society of America) conference in Fayetteville.  While looking at various road cuts, we also took new points on an area in the corner of four quads.  

Angela in front of lower Imo sandstone at the type section

Angela and Danny at the foot of the basal Witts Springs sandstone near Bryan Mountain

Wednesday and part of Thursday we looked at several possible field trip stops in the Imo, none of which seemed particularly suitable for one reason or another.  Mostly this is because the Imo doesn’t tend display good outcrops in this area due to its shaley composition and its typically being covered by the flaggy sandstone of the Cane Hill above.  We did visit the type section which is in Sulphur Springs Hollow to see if that could be used as a stop, but deemed the area too rugged unless a very small, sturdy group of geologists sign up.   A type section is an area where a formation or rock unit is first described and studied in detail.  In this case the type section was proposed then summarily  abandoned, but there are those working hard to see it reinstated eventually.

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Coalified wood prints in Imo sandstone at the type section

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Crinoids in red, fossiliferous Imo limestone at the type section

Another project that we are working on for the National Park Service is a compilation of all the quad maps along the Buffalo National River. Our agency and the US Geological Survey have each done about half of the quads in that area.  Most of the quads mapped by our agency did not include the higher terrace levels above the river–some as high as 200 feet!  This week we were able to get points on a few more of these terraces on the Snowball quad. 

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Smart Bluff above Arnold Bend on the Buffalo River

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Calcite fracture-fillings used as building stones in St. Joe

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Quartz crystal encrusted Boone chert used as building stones in St. Joe

We looked in Arnold Bend Thursday afternoon, and differentiated several terrace levels there, then on the way back to town, we stopped at a roadcut on Hwy. 65 that Angela knew about where there are quartz crystals growing in fractured Boone limestone and chert.  Danny had stopped at St. Joe on a previous field trip and so directed us there to see the quartz and calcite crystals encrusted on some of the old building stones in town.  These stones were no doubt found nearby along the several mineralized fault zones in the area.

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Early morning fog on Bear Creek

The next morning there was fog on Bear Creek and we headed up to Jamison Bluff to look for Plattin in the riverbed where it had been mapped previously.  This is part of the 6 mile section of the Buffalo between Woolum and Margaret White Bluff that dries up during the summer.  All we could find was St. Joe, so that part of the map remains as is. 

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Jamison Bluff along the Buffalo River

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River crossing at Woolum on the Buffalo River

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Looking down on Skull Bluff on the Buffalo River

 We crossed the Buffalo at Woolum and walked along the top of Skull Bluff to the Nars Cemetery most of which was covered with a terrace deposit (along with almost impenetrable black locust, cedar and briars).  

And of course, we couldn’t leave the area without a quick trip to “the Nars” itself for Danny and Lea to see for the first time.

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“The Nars”. Buffalo River left, Richland valley right.

“The Nars” is an almost sheer rock wall in the Boone formed as an erosional remnant between the valleys of the Buffalo River and Richland Creek.  Quite impressive as usual.  After that we had to get back to Little Rock and “the real world”.  See you next week.     

Tick attacks: severe