Tag Archives: Ozark Plateaus

Geo-pic of the week: Rock Beds

Bluff above Buffalo River edited

Why do rocks have beds?  Are rock beds where geologists sleep?  Sometimes, but that’s not the point of this article.  The picture above, taken on the Goat Trail at Big Bluff, overlooking the Buffalo National River, is a great example of a sedimentary rock composed of many individual beds (layers).  The reason that rocks are bedded is due to either gaps in deposition or abrupt changes in the grain size of sediment being deposited in an environment.

Here’s an example;  when a storm causes a river to flood its valley, the water deposits sediment as the flood recedes.  Typically, there’s a period of non-deposition before the next flood event deposits a new layer of sediment over that one. This time between floods allows weathering to alter the character of the first flood deposit.  That weathered surface will eventually differentiate the flood deposits into distinct beds of rock. 

Bedding can also form as a result of flowing water gaining or losing velocity.  The size of sediment that water carries (and eventually deposits) is directly related to flow rate.   A sudden change in flow rate creates bedding distinguished by differences in grain size.

Everyone in the photo above was eventually air-lifted to safety… Just kidding!  They’re still up there clip_image001

Advertisements

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.

image

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

Japton_final-red.jpg

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:

Alluvium in War Eagle CreekDSCN1957[14]

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

DSCN0110

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

Herringbone Crossbeds in AtokaDSCN1357DSCN1353

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

DSCN0602

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

DSCN2294DSCN2258DSCN2279

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.

DSCN1057

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

Top of the Parthenon sandstone in Lollar's CreekDSCN0491

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).

DSCN0319DSCN0773

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

DSCN0065

Biohermal mounds in the Brentwood Limestone in Jackson Creek.

DSCN3028

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

DSCN3307DSCN2742

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

DSCN1710DSCN1753

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.

DSCN1799

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

Geo-pic of the week: Fluvial Erosion

Buffalo River Valley

Recently, we posted a blog explaining that the Ozark Mountains are actually incised plateaus and that the hills are remnants standing between the incised river valleys. If you missed that one you can see it here.  Now, we will talk about how a river is able to erode solid rock.

The picture above is of the Buffalo National River in its valley. As you can see, an impressive volume of rock has been excavated by this little river. A common misconception is that the water is carving the rock. Water is soft and softer things generally do not abrade harder things. Slightly acidic water can dissolve rock very slowly, particularly carbonate rock like limestone, However, the majority of the erosion in a river is due to the sediment suspended in the flowing water. As the sediment – which can range from tiny grains of silt to boulders– is carried downstream by the current, it skips along the channel, colliding with the bedrock. The repeated collisions break down the sediment, chipping off edges and rounding it. By the same process, new sediment is ground away from the bedrock and the valley is slowly enlarged.

The same thing is true of wind erosion such as in a desert setting.  The wind itself really can’t erode the rock.   The erosion is due to strong winds lifting loose sand and blasting it against the solid rock, slowly wearing it away.

Geo-pic of the week: Ozark Plateaus

Ozark Plateau

If you live in Arkansas, chances are you’ve heard of the Ozark Mountains.  Actually, the correct geologic term is Ozark Plateaus.  Unlike typical mountains in which the bedrock has been squashed and folded, the Ozarks are one broad dome-like structure made up of flat-lying sedimentary bedrock.  The hills and valleys of the Ozark topography are the result of rivers carving into this dome, rather than compression or deformation.  

The picture above was taken overlooking the Buffalo River.  The various hills, from the foreground to the distance, are roughly the same height.  Of course they are!  If not for this and other rivers, the landscape pictured here would be one solid flat surface, as tall as the highest peaks in the picture, stretching to the horizon.  

Geo-pic of the week: Calcite-filled Tabulate Coral

tabulate coral

Continuing with our previous theme “Sharkansas”,  this week’s geo-pic is on Arkansas corals.  Of course, corals don’t live in Arkansas today, but from about 480 million years ago, up until roughly 40 million years ago, coral would have been a fairly common sight in the natural state.

The picture above is of a tabulate coral: a now-extinct variety of colonial coral.  Each hexagonal corallite chamber housed a simple, individual animal, called a polyp, that could protrude and retract to filter food from the water.  The chambers in this fossil are in-filled with the mineral calcite, but that occurred after the coral died and was incorporated into the rock.  It was photographed in the Ozark Plateaus, in the Prairie Grove Member of the Hale Formation.

Other varieties of coral are found in the rocks of Arkansas.  For more views of Arkansas corals click here

Archimedes in Pitkin Limestone

Notes from the Field: Pitkin Limestone

 

The Pitkin Limestone

One of the most fossiliferous formations in the state is the Pitkin Limestone. It was referred to as the Archimedes Limestone in the late 1890s because it contains an abundance of the screw-shaped bryozoan fossil Archimedes. It was formally named the Pitkin Limestone in 1904 for exposures near Pitkin Post Office in Washington County, Arkansas. If you can’t find the town of Pitkin on a map, don’t worry–it’s now known as Woolsey.

The Pitkin began as carbonate sediments deposited in the Mississippian Period around 320 million years ago.  At that time, northern Arkansas was covered by a shallow sea that was fairly close to the equator.  Warm, shallow seawater is a prime environment for the build-up of carbonates.  Marine organisms extracted calcium carbonate out of the seawater to form shells or other hard parts.  This material accumulated and eventually turned into limestone.  Some of those secreted structures are preserved as fossils in the rock and are clues to the environmental conditions that existed at the time.

The Mississippian in Arkansas

The area of what is now Arkansas during the Mississippian

The Pitkin Limestone is a bluff-former that crops out in the southern portion of the Ozark Plateaus from just south of Fayetteville eastward to Batesville, typically along the Boston Mountains Plateau Escarpment.  It is mostly limestone, however, there is some nodular black chert present locally.  Black shale intervals are common in the eastern portion.  Because limestone is a soluble rock, karst features such as caves, sinkholes, springs, and disappearing streams are common in this Formation.  About 9% of the known caves in Arkansas are in the Pitkin.  Its thickness varies from an average of about 50 feet on the west side of the state to about 200 feet in the eastern part with a maximum of about 400 feet in the central portion.  It typically rests on the Fayetteville Shale and is overlain by the Cane Hill Member of the Hale Formation in western Arkansas and by the Imo interval from the area of western Searcy County eastward.

Geologic Map of Arkansas-detail

The Pitkin outcrop belt is within the light-brown area in this Ozark Plateaus detail of the Geologic Map of Arkansas

To download the entire Geologic Map of Arkansas, click here: http://www.geology.ar.gov/ark_state_maps/geologic.htm

Cane Hill/Pitkin Contact near West Fork

The Cane Hill overlying the Pitkin near West Fork, Washington County

Pitkin/Fayetteville Contact at Hwy 65 Roadcut

The Fayetteville underlying the Pitkin near Marshall, Searcy County

Pitkin top in Little Red Creek

Top of Pitkin in Little Red Creek near Canaan, Searcy County

Now, let’s look at fossils commonly found in the Pitkin.

Archimedes in Pitkin-Batesville Archimedes in Pitkin-Fayetteville

The photos above contain fossils of Archimedes.  The fossil is named for the ancient Greek engineer who invented a device that incorporated a large screw to lift water for irrigation.  The left photo was taken south of Batesville and the right photo was taken south of Fayetteville.  It’s remarkable that these fossils are so persistent along this great extent.  Although this fossil is characteristic of the Pitkin, it can also be present in adjacent formations.  The illustration below is a sketch of a fenestrate Bryzoan of which Archimedes is a type.

Fenestrate Bryzoan

Archimedes as it may have appeared in life

Crinoid stems and Columnals-Batesville Crinoid Stems-Batesville

Pieces of fossilized Crinoids are also abundant in the Pitkin.  Most commonly, small button-shaped pieces of the stem and arms, known as columnals, are preserved in the limestone.  That is a columnal in the center of the left photo.  The larger crinoid fossils above were preserved in shale and were most probably washed onto a mud flat during a storm event.  These photos were taken south of Batesville, but crinoid detritus is abundant throughout the Pitkin and most other limestone in Arkansas.

Crinoid

Crinoid as it may have appeared in life

A great location to see the Pitkin is along Richland Creek at its confluence with Falling Water Creek.  When the creek level is low, you can hike upstream from the campground and see many fine exposures of Pitkin Limestone in the creekbed.  Locally, colonies of tabulate and rugose coral were preserved in the Pitkin and can be discovered upon close inspection of the outcrop.

Moore Quadrangle-detail

Detail of Geologic Map of the Moore Quadrangle showing Pitkin along Richland Creek (Mp=Pitkin)

To download the entire Geologic Map of the Moore Quadrangle, click here: https://ngmdb.usgs.gov/Prodesc/proddesc_76560.htm

Tabulate Coral in Pitkin Limestone

Tabulate or colonial coral in the Pitkin Limestone along Richland Creek.

Rugose Coral Colony in Pitkin Limestone

Rugose coral in Pitkin

Locally, the Pitkin consists of oolite, a type of sedimentary rock composed of ooliths.  Ooliths are small, spherical structures (<2 mm) that form by accretion of numerous concentric layers of calcite on a central nucleus such as a shell fragment or sand grain.  The environment of deposition would have been areas where strong bottom currents or wave action rolled the fragment around in carbonate-rich sea water.  This would include environments like beaches and tidal flats.

Oncolites and stromatolites are also preserved in the Pitkin.  They have a similar structure to ooliths, but are much larger (up to 10 cm), can be round or irregular-shaped, and are formed by a different mechanism.  Like ooliths, they nucleate on a shell or other fragment, but are built up by encrusting layers of blue-green algae or cyanobacteria.  Stromatolites form in much the same way,  but create columns, mats, or large heads.  Stromatolites and oncolites typically indicate a paleoenvironment of warm, shallow water in a calm sea, lagoon, or bay.

Oolitic Pitkin

Oolitic Pitkin

Oncolitic Pitkin

Oncolitic Pitkin

Stromatolitic Pitkin

Stromatolitic Pitkin

During fieldwork for our geologic mapping, finding Pitkin Limestone is always exciting because there is something new and interesting to discover every time.  We hope this brief introduction to one of Arkansas’ most intriguing formations has convinced you to seek out the Pitkin and have a closer look.

Until next time, we’ll see you on the outcrop!

Richard Hutto, Angela Chandler

Geo-pic of the week: Conostichus trace fossils

Conostichus topConostichus bottom

Above are two pictures of a trace fossil, Conostichus, from the Ozark Plateaus region of Arkansas.  Like other trace fossils, Conostichus are structures found in sedimentary rock that represent the spot where an animal lived, fed, or travelled.  Despite their abundance, especially in rocks of the Carboniferous period (299 to359 million years ago), it’s not certain what kind of animal made Conostichus, because the animal’s body wasn’t preserved. 

The upper picture is the top of the Conostichus and shows the hole through which the animal entered or exited the structure.  The lower picture is the same Conostichus with the top facing down.  As you can see, they taper and come to a rounded point at the base, vaguely resembling a badminton birdie.  

At present, the most widely accepted theory for their origin is that Conostichus are burrow traces left by Sea Anemone.