Monthly Archives: March 2014

Statemap Field Blog—Dec. 2-4, 2013

2013-12-02 031

Hello all!

This week we finished up a few odds and ends on the Shirley quad.  We needed to get to a few suspected outcrops along the north side of the Middle Fork just east of Shirley.  As we were looking for a way to access them, we stumbled upon the Sid Burgess Historic M&NA Trail which starts in downtown Shirley and ends up about a mile distant at the historic Cottrell-Wilson Cemetery.  As luck would have it, this trail happened to access the very areas we needed to see.  If you’re ever in Shirley, it’s definitely worth checking out!

2013-12-02 0112013-12-02 0152013-12-02 018

We saw mostly thin-bedded sandstone and shale units of the same variety as on the south side of the Middle Fork and Weaver Creek upstream.  There are a few low dips toward the lineation, but nothing indicating a major structure.  I’m thinking this may all be the unit above the Witts Springs (Bloyd Formation) brought down to the southeast by a monocline.  The trouble is, we don’t really know what the Bloyd/Witts Springs contact looks like in this area yet.  That’s something we still need to work out.

Tuesday was wet again, but we set out to finish up the southernmost branch of Lost Creek anyway.  Seems to be mostly Witts Springs in there with some Cane Hill at the bottom of the valley.  We saw some great examples of soft-sediment deformation in some of the silty units on the way down.  Soft sediment deformation occurs during sedimentation when the rapid loading of usually more dense, overlying sediments causes the less dense, buried deposits beneath them to become partially liquefied, which forms various types of disruptions in the original bedding.  This can take the form of simple reorientation of the bedding as we have here, to more complex convolute bedding and flame structures.  I took a photo later in the week of a good flame structure in the Bloyd Formation.  Notice where the shale has been squeezed up between the thick, contorted beds of sandstone.

2013-12-03 0072013-12-04 019

Several massive calcareous sandstone units in the Witts Springs again illustrated the dramatic difference between outcrops weathered with and without the influence of groundwater.  Notice how rotten the outcrop of massive sandstone in the photo below left appears.  Also note the green color.  There is a layer of moss and lichen growing over almost the entire rock surface, made possible by its relative saturation by groundwater.  These organisms help accelerate the weathering of the rock, and there are places where you can actually see clumps of moss peeling off the surface along with a layer of sand.  This type of chemical weathering is known as chelation and results in the effective removal of the residual iron cement still holding the rock together after the calcite cement has been dissolved by groundwater.  The photo below right shows how “dry weathering” of a boulder of the same material can result in well-defined liesegang bands.  Highly concentrated iron has cemented these bands within the massive sandstone, and without the influence of groundwater, they are preferentially resistant to weathering, leaving them in bold relief.

2013-12-03 015

2013-12-03 034

On Friday, we looked at some of the last steep areas we haven’t vistited north of the Middle Fork east of Shirley.  Definitely still have Witts Springs right down to the river there, but there is also a thin- to very thick-bedded unit above it that is probably in the Bloyd.  We saw a fairly recent landslide above the river composed of material from that upper unit.  There was also a good cut and fill channel bed exposed in that unit as well.

2013-12-04 0052013-12-04 0142013-12-04 0152013-12-04 020

It was warm enough for the critters to be out again this week.  Just when I thought it was safe to put my foot down anywhere I pleased, I nearly stepped on a moccasin.  That’s him slinking back in his hole.  We also saw a western slimy salamander (plethodon albagula?) under some storm debris, which was subsequently replaced.

2013-12-04 0032013-12-03 046

Hopefully the warm weather holds out, but the forecast says the bottom may drop out on Friday.  We’ll see!

Until next week, see you on the outcrop!

 

Statemap Field Blog Nov. 25-27, 2013

2013-11-26 047

Hello all!

A cold rain on Monday was freezing on the trees, so we explored some of the many undeveloped road networks in Fairfield Bay, especially along Dave Creek and down to the lake on the east side of the map.

2013-11-25 009

2013-11-25 014

Not quite sure what we’re in here, but there is a calcareous sandstone massive not too far above lake level which could indicate that we’re still in the Witts Springs even though this is south of the lineation along the Middle Fork.  We’re getting a lot of strong southerly dips along the north edge of the lake which indicate there is a fault along that lineation, unfortunately the lake covers it.  Too bad this detailed geologic mapping was not done prior to 1963!

Tuesday we finished up the upper end of Big Branch.  The ice was still on the branches when we started, but soon began to melt which made it seem like it was raining again until about noon.

2013-11-26 021

At first, we thought we were finding additional Witts Springs/Cane Hill contacts, which was surprising since we were so far above where we had them downstream last week.  But we definitely had a thin-bedded sandstone that was shaly near the top beneath a classic basal Witts Springs sandstone.

2013-11-26 0742013-11-26 025

Or did we?  As it turned out, the thin-bedded sandstone was only about 40-60 feet thick and was above at least two other massive sandstone units.  Another Cane Hill look-alike!  That’s why you always have to look at the entire section, or you may miss something!

2013-11-26 0282013-11-26 052

What we took for the basal sandstone massive may actually have been the uppermost sandstone massive in the Witts Springs.  As we hiked on downstream, we did eventually find the actual Witts Springs/Cane Hill contact that lined up much better with the points we already had.

2013-11-26 065

Wednesday, it was so cold the moisture being wicked up certain grasses was making “frost flowers”.

2013-11-27 051

We walked up the lower end of Little Creek along the western edge of the map.  We had already seen the upper portion when we mapped Old Lexington, and it seems to be all Witts Springs in there.  We saw some good examples of “zebra rock” and “Prairie Grove weathering” in some of the massive sandstone units (see previous blog).

2013-11-27 0012013-11-27 023

Some of the calcareous sandstone is also fossiliferous, and I was lucky enough to find a good rugose coral weathering out in one fossiliferous zone.

2013-11-27 040 (2)

Well, looks like winter is here to stay!  At least I don’t have to watch for snakes anymore!  Until next week, see you on the outcrop!