Monthly Archives: November 2013

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. 14 – 16, 2013

Hello all!

It was a rather wet week in the field this week.  Starting to see a little fall color as well, but still mostly green.  We worked in Weaver Creek which runs along that southwest/northeast lineation on the southern part of the Shirley quad.  It appears that the wide valley there and in the Middle Fork to the northeast is developed in a very thick shale sequence that is all dipping southeast, probably southeast of the major fault causing the lineation.  The valley can widen significantly in the shale, but once it reaches the 200+ feet thick, thin- to massive-bedded sandstone unit on the southeast side of the valley, it can’t go much farther.  Essentially the lineation is caused by the stream eroding upsection across the valley through the shale unit, until it reaches the contact with the thick sandstone unit above.  The lineation is therefore along the strike of (perpendicular to) the southeast dipping rock in this area.  One big question is what happens to the fault on the east side of the map where the lineation stops.  May have to wait to find out next year when we map the Parma quad.

Started out on Buffalo Hump, which I think may be a reference to a series of rounded “humps” along the end of Weaver Creek where the sandstone bluff is deeply incised by four drainages.   

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Saw several sets of non-vertical joints crossing each other in one of those drainages.  This usually indicates that there is a structure close-by, but we didn’t see anything unusual.

The next two days were rainy, so we stayed close to the highway so we could bale out in the event of a deluge.  The shaly unit was well exposed in the valley and displayed abundant soft-sediment deformation and load casts.

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It was difficult to ascertain the true dip of the rocks because this unit is a stack of very broad channels, so the beds dip consistently for a long way, then abruptly turn about 90 degrees and continue on like that for a ways, then switch back. The exposed edges of these channels typically dip sharply and are sandier.    Wish we had more time to study this unit to determine paleocurrent direction, facies, and depositional environment, but as usual, we’re just passing through.

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The channel-bedding continues into the sandy unit above as well.  The big question is what formation these units are in.  Now that we’ve worked our way south of the fault, we’re in a totally different section than we were in north of it.  Still haven’t seen anything that looks too familiar.  May have to pull well logs and work our way up from the subsurface to get started.

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Well, looks like better weather next week!  See you on the outcrop!

Tick attacks: 1

GeoPic Of The Week: Ripple Marks In Sandstone

Ripple Marks In Sandstone

Ripple Marks In Sandstone

Ripple marks are sedimentary structures preserved in sandstone and limestone. They may be asymmetrical in shape, with the steep side pointing downstream in the direction of current flow.   In this picture the steep side is toward the viewer and so is the current direction.  Ripples form naturally by the movement of water currents in rivers and streams, on beaches of tidal and long-shore currents, and in deep-ocean basins.  This picture was taken of Ordovician age sandstone in the Everton Formation along Beaver Lake.

 

GeoPic of the Week: Dolomite (pink) and Sphalerite (brown) In Dolostone

Dolomite and Sphalerite in DolostoneDolomite and Sphalerite in Dolostone

Dolomite (pink) and Sphalerite (brown) In Dolostone

Dolomite and sphalerite are two minerals present in limestone and dolostone in the lead and zinc districts of north Arkansas.  Dolomite commonly occurs with the sphalerite, however it is not an ore mineral and is considered worthless.  Sphalerite is the primary ore of zinc.  Zinc was mined in the lower end of the Buffalo National River in the late 1800s and early 1900s.  One of the largest mining communities was located at Rush, Arkansas.  Zinc is used as a coating of iron or steel to protect it from corrosion.  It is also used in batteries, small non-structural castings, and alloys, such as brass.  This mineralization is present in the Everton Formation.  It is thought that migration of warm mineral-rich fluids expelled by the pressure of the mountain building event that caused the Ouachita Mountains is responsible for the mineralization in northern Arkansas.  Note the brecciated texture (angular fragments) of the rock.  Open spaces, called cavities, in the rock caused the overlying rock to collapse, and break into angular pieces.  Mineralized water then flowed around the broken pieces and the dolomite and sphalerite precipitated in the open spaces.