Tag Archives: fault

Geo-pic of the week: Slickensides

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The grooved surface pictured above is a slickenside.  Slickensides indicate the relative direction of movement between fault blocks (hanging wall moved up, down, laterally, etc..).  

Slickensides form when fault blocks move against each other.  The natural irregularities on each scratches grooves into the other.  The grooves are parallel to movement;  for instance in this example, movement was either to the right or the left.  To tell whether it was right or left, you can rub your hand along the slickensides.  They feel smooth in the direction the fault moved and rough in the opposite direction – it’s like petting a dog from tail to head.  Slickensides are a valuable tool because determining fault movement can be a challenge when there are no easily-recognized beds that can be correlated across the fault to show the sense of offset.

The shale above was photographed in Big Rock Quarry, North Little Rock, AR.  It’s a part of the Jackfork Formation (Pennsylvanian).

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Statemap 2015-16 Update

 

Hello all!

Well, another year, another map!  The Brownsville quad is now published (see map below), and a link to it will be posted on our website soon.  This year marks the 22nd anniversary of Statemap, aka the National Cooperative Geologic Mapping Program, in Arkansas.  Statemap is partially funded by a USGS grant, and was established to encourage the states to map their surface geology at the 1:24,000 scale.  To date, our mapping teams have completed thirty-three quadrangles in the West Gulf Coastal Plain and, with the recent publication of the Brownsville quad, forty quads in the Ozark Plateaus.

Geologic map Brownsville, AR

The geology of the area around Greers Ferry Lake has never been mapped in great detail until now.  Previous work had been to produce the 1:500,000-scale Geologic Map of Arkansas.  Because we mapped the Brownsville quad at the 1:24,000 scale, we were able to make some observations new to science.  A fault was discovered that had never been mapped previously.  We named it the Shiloh Fault for the old town, now inundated by the lake, that lies along its trace.  Meanders of the Little Red River channel approached this fault but didn’t cross it, probably due to encountering more resistant rock on the north side of the fault.  The Witts Springs Formation had not been mapped south of the Choctaw Creek Fault before, but we were able to draw in its upper contact with the Bloyd Formation along the Devil’s Fork and several other drainages.

Overturned cross beds in massive sandstone of the undifferentiated Bloyd Formation

As on other quads around Greers Ferry Lake, we continued to find terrace deposits left behind as the Little Red River carved the valley down to its present elevation.  Some of these are stranded as much as 260 feet above the current channel bottom (now located on the bottom of the lake).

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For many years now, our mapping program has focused on completing the Mountain View 1:100,000-scale quad.  This area encompasses thirty-two 1:24,000-scale quads and stretches from Richland Creek to Sylamore Creek on the north side and from the Illinois Bayou to Greers Ferry Lake on the south side.  Now that this area is finished, our Statemap Advisory Committee has decided we should jump over to northwest Arkansas to complete work on the Fly Gap Mountain quad, just west of the Mountain View quad (see map below).

STATEMAP index for blog

So for next year, the Statemap team is going to start work on the Durham quad in the northwest corner of the Fly Gap Mountain quad near Fayetteville.  We’ll have to spend a few weeks getting our feet on the ground, so to speak, because we won’t have the benefit of already mapped quads adjacent.  Fortunately, we will be very close to the type-sections for most of the formations we’ll be mapping, so hopefully, we can study the classic outcrops and trace them into our new field area without too much difficulty.

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A type-section is an area, or even just an outcrop, where a particular formation was first described.  They are named after a local geographic feature.  Formations first described in northwest Arkansas include: the Fayetteville Shale, the Pitkin Limestone, and the Hale Formation which has the Cane Hill and Prairie Grove as members.  Members are smaller, discernable units within a formation.  The type-section for the Bloyd Formation, including the Brentwood, Woolsey, Dye, and Kessler Members, and the Trace Creek, which is the basal member of the Atoka Formation (named for its type locality in Oklahoma), is on Bloyd Mountain near West Fork.

I would like to take this opportunity to thank my field partners that accompanied me this past year.

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I started the year with Ty Johnson, who has since moved into a permanent staff position at the Survey, so congratulations to him!  He was with me for just a year, but we covered a lot of ground together.  He’s now mapping the geology of the Lake Ft. Smith area with an emphasis on landslide mitigation.

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The writer and also principle investigator of the Statemap grant, Angela Chandler, went out a few weeks in the late fall before we could fill the vacancy Ty left behind.  No matter how much I learn, she always manages to teach me something new.

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We hired Garry Hatzell, a recent U of A grad, who started fieldwork in January.  He brings an enthusiastic knowledge of paleontology to the mix, and I look forward to his continued insight into the biostratigraphy of our field areas.

Without the help of these fine folks, we couldn’t have gathered the data or produced the map.  Also, I would have been stuck in the office—a torture for the unrepentant field geologist.

Wish us luck on the Durham quad!  And if you’re in northwest Arkansas during the next twelve months and happen to drive by a Jeep Cherokee with the AGS seal on it, be sure to stop and introduce yourself.

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Until then, I’ll see you on the outcrop!

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Richard Hutto

Geopic of the week: well exposed fault

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The picture above shows a normal fault exposed in an old railroad grade just north of Greers Ferry lake in the Ozark Mountains of central Arkansas.  Two different lithologies are juxtaposed against one another: siltstone on the left and sandstone on the right.   Exposures such as this one are relatively rare in the Ozarks where most of the bedrock is covered by loose sediment and vegetation.

This fault is extensive and roughly parallels the northern portion of Greers Ferry lake.  The fault gives the northern portion of the lake its linear shape (see below).

Greers Ferry lake

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!

 

 

 

STATEMAP Field Blog—August 19-21, 2013

Hello campers!

Well, much drier this week, but of course the heat is back!

We’re still working on the Shirley quad this week.  Started out Monday by dropping down a steep side drainage off Nubbin Ridge nearly to the Middle Fork, then right back up another one.  Got good points at the base of the Witts Springs and Cane Hill.  The Imo is at the bottom of the valley but the very large boulders eroding at the top cover most of it.  Relief was around 400 feet.

Tuesday and Wednesday we were back in the headwaters of Lost Creek.  There are many branches to this creek, and there seems to be a fairly large structure crossing them approximately east/west.  Could be a fault, or maybe just a fold of some kind—will take much more data collection to be sure.  We did observe a repeating sequence from higher elevations to lower—now we just have to figure out which formation they are in!  In the Ozarks, the Pennsylvanian-aged rocks are a series of sandstone and shale units deposited in a shallow marine environment.  That means they are all very similar to each other and the differences are subtle and vary from place to place.  To differentiate one formation from another, we depend a lot on context, which means we must take meticulous notes on each outcrop in the hopes we can see a pattern in the stratigraphy that matches something known.  We’re starting in the northwest corner of Shirley because we’ve already mapped the surrounding quads in that area, so have a pretty good idea of the geology there.  Things are getting more complicated as we reach the structure because the strata are displaced somewhat from one side to the other.  We will continue to add points next week in order to discover what the rocks are doing there.

ripple beds

ripple beds

Since I’ve mentioned some of the formations in the area and how difficult they are to differentiate, let me discuss one that has stayed fairly consistent across the Boston Mountains Plateau.  The Cane Hill is a Member of the Hale Formation, and its type section is in northwest Arkansas.  It is typically very thin- to thin-bedded, ripple-bedded, very fine-grained sandstone with interbedded shale and thicker shale units.  There are abundant trace fossils in the Cane Hill including asterosoma which is a somewhat star-shaped feeding burrow.

Trace Fossils:

Asterosoma:

concave solutioning in thin-bedded Cane HillBluffs of this sandstone often affected by a particular weathering phenomenon where the cement holding the sand together actually solutions out which lets the individual grains fall away eventually forming a concave structure.  I have included a good set of photos of the Cane Hill and a few of its characteristic structures.  It is getting to be over 300 feet thick as we continue to map toward the southeast.

See you next week!

Snake count: 2

Conostichus count: 1

Tick attacks: moderate to heavy

STATEMAP Field Blog August 12-14, 2013

Hello campers!

Rather wet in the Shirley quadrangle this week.  Rain over the weekend brought the Middle Fork up, and we crossed it on the old train bridge at Shirley on Monday.  About three miles upstream, there are a couple washouts and parts of the old railroad bed have slumped into the river.  We managed to get around all that, but then ran into a gate at Bear Branch.  That was the drainage we needed to get anyway, so we hiked up and got a Witts Springs/Cane Hill contact in the ditch of an old road.  At that point the rain caught up with the thunder we’d been hearing for a while and we got soaked through, but not for the last time this week.      

The next day we did some work in the headwaters of Lost Creek.  Again the day looked threatening.  We started down the drainage and almost immediately found the trace of a small fault (probably).

We traced it though a couple more side drainages, and got soaked again around noon.  It continued to rain on and off the rest of the afternoon.  We did see some good examples of Asterosoma, ripple bedding, and sandstone concretions (“Prim boulders”).  Also saw what I think is a native spider lily for the first time.  Ended the day by climbing 300 feet up one side drainage, followed a bench over to another, and climbed back down to the main drainage, then 500 feet back up to the Jeep (Goldy).  All was wet and slippery.


The next day the sun came out and the air was much cooler and drier.  What a relief!  We started by going to an area marked “Chimney Rocks” on the map.  It is near the the top of a mountain where a massive sandstone unit has weathered along its joints to form several large blocks and pedestals which are slowly creeping downslope away from the bluff from which they originate.  The weathering of the pedestals slows down tremendously once they detach from the face because they simultaneously become disconnected from groundwater. The area, though small, is very scenic, and I took many photos. 

After that we drove to the bottom of Good Spur Hollow and walked upstream and down until we ran out of time and had to get back to Little Rock.  Couldn’t ask for better field weather on Wednesday.  Hope it holds out!

See you next week!

Snake count: 1

Tick attacks: low to moderate