Tag Archives: sandstone

Notes from the Field: Japton and Witter Quadrangles

 

Geologic mapping of the Japton and Witter 7.5-minute quadrangles was recently completed by the Arkansas Geological Survey’s STATEMAP field team. In Arkansas, the STATEMAP Program is currently focused on detailed 1:24,000-scale mapping in the Ozark Plateaus Region, located in the northern part of the state.

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Figure 1. Japton and Witter Quadrangles on the 1:500,000-scale Geologic Map of Arkansas (Haley et al., 1993)

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Geologic Map of the Japton Quadrangle, Madison County, Arkansas. Download a digital copy at:

http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/Japton_24k_geologic.pdf

Geological Map of the Witter Quadrangle

Geologic Map of the Witter Quadrangle, Madison County, Arkansas.  Download a digital copy at:

http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/Witter_24k_geologic.pdf

STATEMAP is a cooperative, matching-funds grant program administered by the U. S. Geological Survey. The goal of the program is to classify surface rocks into recognizable units based on a common lithology–basically, an inventory of surface materials. Also, we strive to locate and depict any structural elements that may have deformed the rocks. The rock units are classified into formations and members, and structures are described as synclines, anticlines, monoclines, and faults. During the project, a rich dataset was recorded in the field using a portable data collector/global positioning satellite receiver as well as by traditional methods. This made possible a more detailed depiction of geological and structural features and a more comprehensive description of lithology than previous studies had done. Data collection included:

  • 629 field locations recorded and described in detail
  • 3,385 photographs taken at recorded field locations
  • 72 strike and dip measurements, most depicted on the maps
  • 950 joint orientations, depicted in a rose diagram of strike frequency
  • 1 shale pit
  • 8 springs, previously undocumented
  • 108 rock samples collected and described

The new map is useful to landowners interested in developing their land for personal or commercial purposes, to scientists seeking a better understanding of landscape evolution and geologic history, and to planners responsible for resource development and mitigating environmental impacts.

Angela Chandler, Principal Investigator for the project, wrote the grant for fiscal year 2018 and we received funding adequate to produce two maps.  Two geologists, Richard Hutto and Garrett Hatzell, began their field season last July and after putting in 76 days in the field, concluded that portion of their work in February of this year. The area of investigation lies within the Interior Highlands Physiographic Region in north Arkansas, specifically the Boston Mountains Plateau portion of the Ozark Plateaus Province. Previous work by the AGS in this area had been done in preparation for the 1:500,000-scale Geologic Map of Arkansas by Haley et al. circa 1976 (see Fig. 1). That mapping project delineated five stratigraphic units in this area, but through extensive field reconnaissance, we were able to define ten units on these maps at the 1:24,000 scale. Further division is possible, but several units were considered too thin to depict on the 40-foot contours of the topographic map currently available, or too difficult to delineate by lithology alone.

Several tributaries of the White River are located on these quadrangles including Lollars Creek, Drakes Creek, and War Eagle Creek. The White River is a major water resource in Arkansas and southern Missouri, and as such we need to learn as much as we can about this important watershed. Included in the field work was hiking, wading, or swimming the entire 13-mile stretch of War Eagle Creek located within the Witter quadrangle, the 10 miles of Lollars Creek within the Japton, and many smaller drainages. The reason we concentrate our efforts on stream beds is that there, erosion has typically removed soil and loose rock leaving well-exposed outcrops of bedrock for us to study. Also, being able to see the entire stack of the rock sequence as we move up or downstream helps put each formation in context with the others. Discovering where one formation contacts another is one of the most important things we do while mapping. Because formations are laterally extensive, similar contacts can be connected into a contact line separating one formation from another. Figuring out where to draw these lines on the map is a major goal of the project.

From mid-February through the end of June, we analyzed field data, classified rock specimens, drew formation contacts and structures on the map, then handed it off to our cartography staff to digitize. Final layout and production of the maps was accomplished by the geologists, after which they were subjected to an extensive review and editing process by fellow staff.

The following images were taken during this year’s field season. Hopefully, they will provide a small glimpse into some of what we were privileged to experience in the field this year.  They are arranged in stratigraphic order from youngest to oldest:

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Alluvium in War Eagle Creek (left). Landslide on Highway 23 above Dry Fork (right).

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Ball and pillow structures in the Atoka Formation in Drakes Creek.

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Sequence of photos zooming into herringbone cross-beds in the Greenland Member of the Atoka Formation.

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Large blocks of Kessler Limestone sliding into Lollar’s Creek.

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Sequence of photos zooming into oncolitic limestone of the Kessler Member of the Bloyd Formation. The oncolite pictured far right is nucleated on a tabulate coral.

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Lycopod (tree-like plant) fossil weathering out of the Dye Shale.

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Top of the Parthenon sandstone (Bloyd Formation) in Lollar’s Creek (left). Parthenon resting on the Brentwood Limestone (Bloyd Formation) with travertine precipitating at the drip line (right).

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Siltstone unit in the upper Brentwood Limestone. Cross-bedded (left) and bioturbated (right). 

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Biohermal mounds in the Brentwood Limestone in Jackson Creek.

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Massive bluff of limey sandstone in the Prairie Grove Member of the Hale Formation.

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Sandy limestone in the Prairie Grove. Stream abrasion has revealed cross-bedding (left) and an ammonoid (right).

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Typical thin-, ripple-bedded sandstone of the Cane Hill Member of the Hale Formation (left). A basal conglomerate in the Cane Hill contains fossiliferous and oolitic limestone pebbles and fossil fragments (right).  This unit probably rests on the Mississippian-Pennsylvanian unconformity.

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The Pitkin Limestone in War Eagle Creek.

This year we will be mapping the Weathers quadrangle which is just east of the Witter, and the Delaney quadrangle which is just south of the Durham (which we mapped two years ago). The Kings River flows through Weathers, so this should be a good place to start while river levels are low (and it’s so hot!). I will update you as I can, but until then, I’ll see you in the field!

Richard Hutto

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Geo-pic of the week: Herringbone Cross-Bedding

 

Crossbedding

Pictured above is sandstone displaying classic herringbone cross-beds.  Cross-bedding results from either sediment transport by flowing water, such as in this example, or by wind flow, as in the case of dunes.

Cross-beds form by the migration of sediment, and tilt in the direction of flow.  As sediment grains are carried by the current, they migrate up the gentle ramp of previously deposited cross-beds.  When they reach the end, they tumble down the steeper face there and are deposited to become part of the next cross-bed.  In this way the sediment migrates in the downstream direction.

Each group of similarly tilted cross-beds is known as a set.  In herringbone cross-bedding, the sets are oriented contrarily, which gives the outcrop a fishbone appearance.  These differently oriented cross-bed sets indicate changing flow directions.    

Sandstone Paleokarst

If you have spent any time on Beaver Lake in northwestern Arkansas, then you have probably seen sandstone paleokarst features.  Some stand tall like towers while others appear to be irregular to rounded masses.  It is common to see only the tops of these features when the lake level is low to normal.

 ss paleokarst photo    Top of sandstone mass in Beaver Lake.  Photo taken in October, 2016.

ss paleo 2-01  Sandstone mass along Beaver Lake.  Photo taken in October, 2016.

These features have been in geology literature since 1858 when David Dale Owen made his first geological reconnaissance of the northern counties.  He described a mass of isolated sandstone within adjacent magnesian limestone (now called dolostone) which stands out forming a conspicuous feature in the landscape.  Purdue, 1907, called them cave-sandstone deposits and was the first to consider them paleokarst.  Purdue and Miser, 1916, noted many of these deposits and concluded several were ancient sinkholes that had been filled with sand.  Two theses that pre-date the construction of Beaver Lake (Arrington, 1962, and Staley, 1962) mention numerous sandstone bodies within the Powell.  One very large sandstone mass was seen in the White River (Arrington, 1962).  It is approximately 45 feet tall!  Unfortunately, it is now covered with water.

photo       Sandstone mass in Carroll County.  From Owen, 1858

photo2 Sandstone mass in the White River near Hwy 12 access to Beaver Lake.  From  Arrington, 1962.

So how did these features form?  First, let’s define paleokarst.  Paleokarst consists of karst features that formed in the geologic past and were preserved in the rock record.  Karst features include sinkholes, springs, and caves.  These features form when acidic rain and ground water dissolves carbonate rocks (mainly rocks that contain calcium carbonate – calcite, or calcium-magnesium carbonate – dolomite).

The majority of sandstone masses are surrounded by dolostone, composed of dolomite, in the Powell Formation.  The Powell is Lower Ordovician in age, meaning it formed around 470 million years ago (mya).  It is likely that this formation was exposed to weathering at that time.  Depressions of various size, called sinkholes, developed on the exposed land surface.  Later, sand filled the depressions and eventually became rock called sandstone.  The age of the sandstone masses ranges from Middle Ordovician (approx. 450 mya) to Middle Devonian (approx. 390 mya).  Therefore, there is a gap in the rock sequence, between dolostone in the Powell and the sandstone, called an unconformity, lasting from 20-80 million years.

ss mass 3-01Sandstone mass (center) surrounded by Powell dolostone along Beaver Lake.  Photo taken in September, 2016.

Why is paleokarst important, other than being interesting features to observe?  Paleokarst provides clues to former geologic conditions and changes in climate and sea level (Palmer and Palmer, 2011).  We know that sea level was high in the Lower Ordovician and shallow seas covered all of northern Arkansas.  But, in the Middle Ordovician, sea level lowered and the sandstone paleokarst features provide additional evidence supporting this change.

Many sandstone paleokarst features were located while mapping the War Eagle quadrangle.  Fifty-two sandstone masses were located around Beaver Lake.  This is not a complete list, however, since the main focus of mapping was not a paleokarst inventory.

paleokarst points    Sandstone masses (yellow) located from recent geologic mapping around Beaver Lake.

The War Eagle quadrangle was mapped in preparation for State Park Series 4 – Geology of Hobbs State Park.  Follow the link below to see the geologic map of the War Eagle quadrangle:  http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/War%20Eagle.pdf.

Until next time,

Angela Chandler

 

References:

Arrington, J., 1962, The geology of the Rogers quadrangle:  University of Arkansas M.S. Thesis, 61 p.

Palmer, A.N., and Palmer, M.V., 2011, Paleokarst of the USA:  A brief review:  in U.S. Geological Survey Karst Interest Group Proceedings, Fayetteville, Arkansas:  U.S. Geological Survey Scientific Investigations Report 2011-5031, p. 7-16.

Owen, D.D, 1858, First report of a geological reconnaissance of the northern counties of Arkansas made during the years 1857 and 1858:  Little Rock, 256 p.

Purdue, A.H., 1907, Cave-sandstone deposits of the southern Ozarks:  Geological Society of America Bulletin, vol. 17, pp. 251-256.

Purdue, A.H., and Miser, H.D., 1916, Geologic Atlas of the United States, Eureka Spring-Harrison Folio, Arkansas-Missouri:  U.S. Geological Survey Folio No. 202, 82 p.

Staley, G.G., 1962, The geology of the War Eagle quadrangle, Benton County, Arkansas:   University of Arkansas M.T. Thesis, 56 p.

 

 

 

Geo-pic of the week: Zebra Weathering

Zebra weathering enhanced

Pictured above is an exposure of Prairie Grove Sandstone near Durham, Arkansas, southeast of Fayetteville.  The ribbed, planar faces that are central in the photo resulted from a weathering phenomenon called zebra weathering.

Zebra weathering occurs in sandstones cemented with calcite – a soluble mineral.  Calcite is common in marine sediment and, in the tidal environment where this rock was deposited, marine sediment mixed with insoluble sand from the continent.   The ratio of marine sediment to sand changed continuously in that environment due to seasonal and climatic cycles.  Today, the beds of sandstone weather at different rates depending on their calcite content.  As the rock weathers, the sandier beds stand out in relief since they wear away more slowly than the soluble beds between them.  Hence, the banded zebra pattern.

Geo-pic of the week: Frankenstein scar on stylolite

Frankenstein boxwork on stylolitic surfaceBoris-Karloff-Frankenstein

I was working near Lake Fort Smith State Park this last week when I came across a peculiar mineral deposit resembling Frankenstein’s Scars (Fig. 2).  It was just in time for Halloween!  The resemblance is uncanny.  Despite the horror, there is a lot of geology illustrated in this rock. 

The mineral that forms the “scars” seen in the photo is called limonite, and it was deposited within a cavity in a stylolite.  A stylolite is a surface, typically a bedding plane, that has recrystallized due to pressure from the weight of overlying rock material.  Stylolites can be recognized by their rough, jagged appearance (it’s difficult to see in this photo, but trust me – it’s there).  The limonite “scars” formed in a pattern called boxwork and, surrounding the boxwork, limonite is also present in botryoidal form: a crystal shape resembling small round globs (the orange goosebumps around the scars).

At this time, rocks are not thought to celebrate Halloween, although more work needs to be done to verify that.

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

Notes from the Field

 

On a recent fieldtrip I realized how many great geologic features exist in the Everton Formation of northern Arkansas. Here’s a little background on the Everton Formation. The Everton Formation is named for exposures near the town of Everton in Boone County, Arkansas. All geologic formations are named for nearby geographic locations. This formation was deposited during the Middle Ordovician Period which means it formed around 470 million years ago. It crops out across northern Arkansas from Beaver Lake in Benton County to Sharp County. Depending on where you are in that portion of the state you might see sandstone, limestone, dolostone, or all three rock types.

Now let’s look at some neat features in the Everton Formation. We’ll start with stromatolites. Stromatolites are laminated structures built by blue-green algae, also called cyanobacteria, one of the simplest and earliest known life forms. Notice the mounded laminations in the photo below. These are stromatolites. The rock is a fine-grained limestone. Also notice the bumpy, weathered surface mid-photo. This is where individual stromatolites are weathering out of the rock.

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The next photo shows a better look at the top of this weathered surface. Finding these fossilized accretionary structures in outcrop helps geologists determine the environment in which this rock formed – in this case, a tidal flat.

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The next photo shows that modern stromatalites are still forming in similar environments today.

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Next, let’s look at travertine. Travertine is a chemically-precipitated, continental limestone composed of calcite or aragonite that forms around seepages, springs, and along rivers and streams (Pentecost, 2010). Precipitation results primarily through the transfer of carbon dioxide to or from a groundwater source, which leads to supersaturation and crystal growth on surfaces. Travertine cascades and dams are present on many of the small streams that are sourced by springs issuing from the limestone and dolostone of the Everton Formation.

The first photo shows a travertine cascade over a dolostone ledge.

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The second photo shows a geologist standing beside a tall travertine dam across a small creek.

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Finally, have a look at these fossilized mud cracks. These formed in a similar way to modern mud cracks. These rocks were originally mud that dried out and formed polygonal cracks. These were later filled with additional mud and over time all of it lithified into dolostone. Mud cracks preserved in this manner are another clue that helps geologists determine the environment in which the sediment was deposited. Again, this would indicate a tidal flat.

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Till next time. Get out in the field!!

Angela Chandler