Tag Archives: Sedimentary

Geo-pic of the week: Pebble Molds

pebbles-great(photo courtesy of Angela Chandler)

The sedimentary rock in the picture above is a sandstone with pebble molds. If the pebbles were present, this rock would be considered a conglomerate. Conglomerates consist of 2 mm or larger rounded fragments of rock, or clasts, surrounded by finer-grained sediment which geologists call “matrix”. The clasts in the rock above were pebble sized, 2-64 mm, and the matrix is sand sized.

Even though many of the clasts have been removed by erosion, we can tell that they were primarily shale pebbles. The sandy matrix was more resistant to erosion than the softer shale pebbles, so we are left with cavities where the pebbles were (pebble molds) on the rock’s surface. This creates an interesting optical illusion. Did you see the cavities as pebbles or as molds when you first looked at the picture?

This type of conglomerate is deposited by energetic and dynamic water, such as is found in rivers and waves. During higher flow periods, only large clasts are deposited. When flow is lower, finer-grained sediment settles in between the larger clasts.

Geopic of the week: Rosie boulders

Rosie boulders

Pictured above are gigantic sandstone boulders in a gravel pit near Rosie Arkansas.  While these house-sized boulders are not unusual, they’re location is: there is no nearby source for them.  Some geologists speculate that they were transported  to this location by a great tsunami generated during a major meteorite impact.  Perhaps the same meteorite that caused that tsunami created the 110 mile diameter Chicxulub crater on the Yucatan peninsula about 65 million years ago.

For more views of the Rosie boulders look here

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!

 

 

 

Geopic of the Week

 

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Pedestals are a fairly common erosional feature in Arkansas in places where conditions are favorable.  They typically form in massive sandstone units due to an increased rate of erosion along the joint set near a bluff line.  Joints are vertical fractures within almost all rocks that formed in response to the tectonic stresses they have undergone in the distant past.  Joints are most often expressed as sets oriented in rhombohedral patterns.  Water can more easily penetrate the rocks along these joints, eventually opening a gap.  When this happens along joints parallel to a bluff face, the gap essentially cuts off the incipient pedestal from the influence of groundwater, isolating it from most of the processes of chemical weathering.  Once that happens the majority of weathering of the newly formed block of sandstone is done by wind and rain.  Because the corners and edges of a rhombohedron have more surface area, weathering is concentrated there, eventually rounding it off to form the typical pedestal shape.  In many places, a capstone of more resistant sandstone is present which contributes to the top-heavy pedestal or mushroom shape.  Also, the pedestal-forming unit is commonly underlain by shale or silty-shale on which the fully intact pedestal can slowly creep downslope.  Some of them end up quite a distance from the bluff where they started.  If you would like to view several fine examples of this erosional phenomenon, consider a visit to Pedestal Rocks Natural Area in the Ozark National Forest.       

Statemap Field Blog March 24-26, 2014

Taphoni (honeycomb weathering) in massive sandstone.

Taphoni (honeycomb weathering) in massive sandstone.

Hello all!

Sorry about that long hiatus, but I had a couple of extra projects the last couple months that took a lot of extra time.  We’ve been in the field almost every week except for March 3-5 during the 3 inch snow in Van Buren County.  We’ve mostly worked on the Fairfield Bay quad during the last few weeks.  This week was spent tracing a very thick-bedded, massive sandstone unit through the town of Fairfield Bay itself.  It is quite an impressive bluff-former and actually underlies almost the entire Mountain Ranch golf course.

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Danny descending treacherous massive sandstone outcrop

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Danny contemplating how this massive sandstone can all but disappear a few hundred yards north of here

 

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Grotto in massive sandstone

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Most hillsides are composed of a thick sequence of very thin sandstone/siltstone and shale–easily erodible

 

Apparently some structure or perhaps a change in depositional environment made this sandstone climb up 200 feet to the east.  There it forms the cap of the ridge on which the small town of Fairfield Bay sits.  Moving east again, It underlies the Indian Hills Country Club where weathering (and earth-moving equipment) has produced the famous Indian Rock House on the golf course there.  Underlying that massive across the entire area is a very thick sequence of very thin-bedded sandstone/siltstone/shale.  A lot of the roads built in this unit have formed deep gullies making some of them impassable.  Still, there is better access in this area than most that we map, so we’re thankful for that.  Only about two weeks left of the field season.  We’ll probably be jumping around a lot to work out problem areas on both quads during that time.

See you on the outcrop!

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Danny actually seeing through the groundcover to the rock beneath the Mountain Ranch golf course

 

 

 

 

 

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: Tilted rock layers at Cossatot Falls, Cossatot River State Park – Natural Area

Tilted Rock at Cossatot Falls

Tilted rock layers at Cossatot Falls, Cossatot River State Park – Natural Area

The Cossatot River is located in the Ouachita Mountains physiographic province in south-western Arkansas.  Notice the tilted rock layers.  Geologists informally use the term dipping to describe these rock layers.  The majority of rocks in the Ouachita Mountains are dipping and sometimes almost vertical.  Why?  The Ouachita Mountains Region contains sedimentary rocks that were originally deposited as flat-lying layers.  Later on, these rocks were uplifted and compressed northward due to a major mountain building process called the Ouachita Orogeny.  This caused the rock layers to be tilted.  The rocks exposed at Cossatot Falls are sandstones in the Mississippian Stanley Formation.