Geopic of the week: Unconformity

 

brownstown and jackfork contact

Pictured above is the contact between two very different rocks: the Brownstown gravel above, and the Jackfork Sandstone below.  This is what geologists call an angular unconformity.  The contact represents a vast period of time.

The lower sandstone was deposited in the Pennsylvanian period, around 300 million years ago, when Arkansas was at the bottom of an abyss.  It was deposited in horizontal layers, but later, when the South American continent collided with the North American continent, it was deformed into mountains and raised above sea level.  For about the next 200 million years it eroded and subsided, until eventually, a shallow sea covered it again.  Rivers washed gravel into that sea, which became the Brownstown gravel.

This picture was taken near De Gray Lake State Park in south central Arkansas.

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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

Geo-pic of the week: Graptolites

 

Graptolites

This is a picture of shale, collected from the Womble Formation, near Lake Ouachita State Park, Arkansas.  The photo shows examples of the, now extinct, Graptolites: fossilized colonies of tiny marine animals.

There were many types of Graptolites.  Some were attached to the sea floor, like corals, while others floated in the water, like plankton.  The feather-shaped fossils pictured here are actually the nests in which the animals lived.  Each tooth-like tube, on the edges of the nests, housed a tiny animal.  Several of these nests would be linked together into a larger colony.

At one time the oceans were full of Graptolites, but by about 300 million years ago they died out for unknown reasons.  Because they were abundant, widespread, and continually evolving, Graptolites are important fossils for dating ancient marine rocks. 

To download a copy of our self-guided tour of Lake Ouachita geology, click here http://www.geology.ar.gov/pdf/Lake%20Ouachita%20Geologic%20Float.pdf

Geopic of the week: Coprolites

Poop_coprolite

Ok, I know what you’re probably thinking, and well….. you’re right!  Coprolites are fossilized feces.  The sample on the right is from out of state, but the fine specimen on the left came from good ole Arkansas, near the town of Saratoga.

Because poop has no hard parts (such as a skeleton or shell) it’s rarely fossilized, but under the right circumstances it can get preserved.  Often, it isn’t certain what animal was responsible for the coprolite, unless it’s discovered in the abdominal area of a fossilized animal.  In cases where it can be identified, it tells us something about the animal, that um…made it.  By closely examining  coprolite, it’s possible to recognize fragments of animals or plants, which give us information about the diet of that species.

Geopic of the week: Ancient delta deposits

Deltaic deposits edited

The above picture shows rocks that were deposited in a delta.  A delta is a place where a river or stream empties into a standing body of water, typically the ocean or a lake.  As it enters the larger body of water, the stream loses velocity and starts dumping the sediment it’s carrying.  Coarse sediment settles out first and finer sediment is carried further from the shore before being deposited. 

As the delta develops, it builds a lobe of sediment out into the ocean.  The stream advances into the sea atop its own lengthening delta, and new sediment is carried ever further seaward.  Eventually, today’s coarse sediment is being deposited on yesterdays fine sediment.  Coarse-grained rock at the top, and fine-grained rock at the bottom, is characteristic of delta deposits.

The picture above shows two lobes of deltaic sediment, each with the characteristic coarsening upward architecture.  The coarsest, top-most bed of each lobe juts out, creating  a waterfall.  This picture is of the Atoka Formation and was taken near Lake Fort Smith, Crawford County, Arkansas.

 

Below is a land satellite image of recent Mississippi River deltas in the Atchafalaya Bay.

AtachfalaDelta

Geo-pic of the week: Fossiliferous mudstone

colorized

Above is a picture of loosely-consolidated, fossiliferous mudstone from the Midway Group: a group of near-shore marine rocks exposed in southwestern Arkansas.  The Midway Group was deposited in a variety of near-shore marine environments that were common in what is now Arkansas around 60 million years ago. The majority of the fossils pictured here are casts of a variety of sea snails (gastropods), but there are also oysters and tiny bryozoans (marine filter feeders).

 

To see more views of fossiliferous mudstone click here