Monthly Archives: August 2013

GeoPic of The Week: Painted Bluff at Buffalo Point along the Buffalo National River

Painted Rock at Buffalo Point along the Buffalo National River.

Painted Bluff at Buffalo Point along the Buffalo National River

Painted Bluff gets its name from water seeping over the top portion of the bluff.  This darkens the rock giving it a painted look.  The rock formation that is painted is the St. Peter Sandstone.  The rock formation below the painted portion of the bluff is the Everton Formation.  Thin bedded limestone and dolostone layers make up the lower portion of the bluff.  The rock formations are both Ordovician (485-444 million years ago) in age, however there is an unconformity between the two formations.  An unconformity is a rock surface that represents a gap in the geologic record either due to a period of erosion or non-deposition.  Notice the wavy line halfway down the bluff.  This wavy line separates the sandstone from the limestone and is the unconformity surface.  The top of the limestone was at one time the rock exposed at the earth’s surface in this area.  The limestone was eroded, and then the sandstone was deposited upon it.

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:


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

Big Rock Interchange, Little Rock, Arkansas

clip_image002The intersection of I-630 and I-430 is undergoing a major overhaul in west Little Rock, Arkansas. Excavation, during 2011 to the southeastern corner of this intersection, exposed a spine of Pennsylvanian Jackfork Formation sandstone and surrounding shale. Due to hardness of the sandstone at this location, it was decided to leave this rock outcrop and the name Big Rock Interchange was adopted. Anyone traveling through this interchange will be able to see this outcrop as it is very prominent.

The Jackfork was deposited about 318 million years ago in a deep water marine environment by turbidites, landslides, and other mass wasting events some of which happened very rapidly while others very slowly. In total, the formation is about 6500 feet thick, was deposited in about 2.5 million years and is part of the Ouachita Mountain Physiographic Province. Deep water, in this instance, means 5 to 8 thousand feet deep. Fossils of any kind are hard to find in this type of environment. The sandstones at this site are harder than normal and have been upturned to the point they are almost vertical. The process of hydrothermal silica-rich waters passing through the rock in the distant past cemented the sand grains together well enough to now call the rock a quartzite. The difference between sandstone and quartzite is evident when broken. Quartzite breaks across the sand grains while sandstone breaks around the sands grains. A hand lens, magnifying glass or microscope is really needed to see this phenomenon. One may ask why the spine of sandstone is nearly vertical. This is due to tectonic plates colliding and deforming the rocks. The collision between tectonic plates forming the super continent Pangaea caused folding and faulting as the rock in an ancient sea were pushed northward onto the North American tectonic plate. Fault gouge collected in 2011 suggest that a minor ancient fault occurs somewhere on this outcrop which is now hidden.

BigRock2011_ShowingQuartzVeinAt least two generations of quartz veining is evident at this site. Some veins are coarse-grained while others are finer-grained and they cut across each other. This is called cross cutting relationships and demonstrates that the quartz veining occurred at two different times. Cookeite, an accessory mineral, was deposited along with the vein quartz. Cookeite, lithium-rich mica, is known to occur in the Little Rock/North Little Rock area. We found occurrences of Cookeite while investigating this exposure in 2011 in some of the quartz veins. When fresh, the Cookeite has a blue-green color and is less than 1/16inch in diameter.

Additional places to view the Jackfork Formation in the local area are Pinnacle Mountain State Park, along Hwy 10 south of Lake Maumelle (Maumelle Chaotic Zone), Big Rock Quarry and the west side of the McCain Mall parking lot. These locations all have wonderful outcrops which display different depositional styles and settings and are very easy to access. One not so accessible outcrop of the Jackfork that many people see every day occurs along I-430 south of the Arkansas River. Along this section of roadway, Jackfork sandstone and shale beds are nearly vertical and make up the south limb of the Big Rock syncline.

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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

STATEMAP Field Blog August 5-7, 2013

Hello campers!

Wild grape clusterWell, this week we continued to work down the Middle Fork.  We started out on Sally Flat again, but this time came down the west side.  We were told there had been a tornado through the area a few years ago, but this week we actually encountered large areas that have become inpenetrable due to downed trees and subsequent overgrowth of the understory.

Entrance to fracture cavernFracture cavern in basal Witts Springs sandstoneOn Monday, had a good trail to a fracture cavern in the basal Witts Springs sandstone, and a fine overlook of the Middle Fork valley.  Ended the day with a 500 foot drop to the Cane Hill/Imo contact in a small drainage and back up.

View north from the end of Sally FlatOverlooking the Middle Fork valley

Lute Mountain Road appears impassableThe next day we tried to come down the north end of Lute Mountain Road to get to Lydalisk, but that proved impracticable because it’s even more gullied out than last time I was down it 5 years ago.  That meant we had to go around through Arlberg which took a much longer.  Meadow Creek had come up during the night, and was swelling the Middle Fork considerably below its confluence.  It began to rain as we searched for the next big hollow south, and even though it stopped fairly quickly, the leaves continued to be wet and drippy the rest of the day.  Went up a long hollow west of the river and got good contacts at the base of the Cane Hill and the Witts Springs.  The Cane Hill was exposed almost continuously in the creek bed and is nearly all sandy in this area.

Cane Hill in the jungleAre we ever going to get to the top? More Cane Hill Watch your step!

The author on the Middle Fork-I've mapped the geology of this river all the way to Tilly.

The author on the Middle Fork-I’ve mapped the geology of this river all the way to Tilly.

On the way out we found a new fossil locality in the Imo shale.  Nice calcareous siltstone concretions encrusted with pyrite and fossils with a few solitary crinoids, gastropods, bivalves, and rugosa in the shale.  Looked up a small hollow for a cave marked on the topo, but were disappointed.  Wednesday, we decided to walk the Lute Mountain Road to get some points on the north end of the mountain and down along the west side of the river.

It was a good idea because everywhere else along there is much steeper.  Again we found good Witts Springs/Cane Hill/Imo contacts.  The walk out was quite a climb, and after exchanging our drenched clothes for dry ones, we drove back to Little Rock.

Lepidodendron in Imo sandstone

Snake count: still 0

Tick attacks: low to moderate

Conostichus count: 1

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STATEMAP Field Blog for week of July 29, 2013:

Should be a bumper crop of muscadines this year.Well, I’m new to blogging, but thought it might be interesting to post a running update of our field activity this year. This year’s project is to map the surface geology of the Shirley and Fairfield Bay quads in north central Arkansas. For those of you unfamiliar with the STATEMAP program, it is a cooperative grant program administered by the USGS to encourage the states to map their surface geology at the 1:24,000 scale.  This is important because it is a basic inventory of materials at the earth’s surface in a given area.  The topographic maps drawn at this scale are the typical 7.5-minute quadrangles which we use as a base map for the geologic data we generate from our field work. Each quad covers approximately 60 square miles, and we currently map two quads per year. To do this we spend 3 days a week in (or near) our field area from mid-July to mid-April each year. Basically, we try to find a town close by where there is lodging, restaurants and stores for basic supplies. This year we are working out of Greers Ferry.

By the way, let me introduce myself. I am Richard Hutto, and I have been mapping with the STATEMAP program since the 2005-6 field season. My field partner this year and for the past 4 years has been Danny Rains. This is our first week out this season, and it is hot and steamy as usual this early on. There has been plenty of rain this summer, and the rocks are covered by lush, dense growth.

This makes it hard to spot outcrops unless you’re right next to them. Danny and I could only make it out Tuesday and Wednesday this week, so only made it up two drainages.  The reason we like to have a look at the drainages in an area is because that is one of the few places we are likely to see bedrock exposed at the surface.  Another would be bluffs, but they are usually more difficult to get to and get around on.  We need to see bedrock in order to determine what rock units (formations) underlie the erosional cover and hopefully discern contacts between them.  We take points on these contacts, and when there are enough points, we can draw an inferred contact line on the base map and describe the formations between them.  All this information goes on the geologic map of the quads we publish each year   at the end of the season (June 30).

On Tuesday, we found an old Jeep trail down to the Middle Fork of the Little Red River almost 700 feet below Sally Flats which is in the northwest corner of the Shirley quad.  We parked on the abandoned M&NA railroad grade and hiked up Arnold Hollow.

Middle Fork of the Little Red River.Jeep on old M&NA railroad grade.

Found a good Cane Hill base and climbed on up to the Witts Springs; about 460 feet.

View north (toward Meadow Creek) from atop the basal Witts Springs sandstone.  Moon Bluffs are visible.View south from atop the basal Witts Springs sandstone.

Massive-, irregular-, channel-bedded sandstone in the Imo.

We followed a contour back to the Jeep trail and returned to the bottom.

The next day we came down the same road and walked up a small drainage south of Arnold Hollow. This one actually revealed a massive-, irregular-, channel-bedded Imo sandstone which has a calcareous, fossiliferous section above it.

On up was a thin-, ripple-bedded lower Cane Hill sandstone outcrop with a dry waterfall.

Same Cane Hill outcrop as in the previous 2 pics from above.Danny by the thin-, ripple-bedded sandstone of the lower Cane Hill.Danny by lower Cane Hill sandstone and dense jungle above.

Atypical massive-bedded sandstone in Cane Hill.On up there was some atypical massive sandstone units in the Cane Hill.

But higher still were some beautiful bluffs of basal Witts Springs sandstone.

Classic massive-bedded sandstone of the basal Witts Springs.Pedestal of basal Witts Springs sandstone.Danny beside the basal Witts Springs sandstone.

After getting a couple points there, we had to bail off down the steep hillside and back to the Jeep for our return trip to Little Rock.

Snake count: 0

Tick attacks: light

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