Author Archives: argeology

Geo-pic of the week: Solution sinkholes

Sinkhole from angela

A sinkhole is an area of ground that has no external surface drainage.  Water that enters a sinkhole exits by draining into the subsurface.  Many people are leery of sinkholes because of the damage they sometimes cause.  Every now and then, a catastrophic sinkhole-collapse makes headlines, typically by swallowing someone’s house, or even draining an entire lake. 

Not every kind of sinkhole is the dangerous kind though.  The picture above shows a solution sinkhole.  Unlike the feared collapse sinkhole, the solution sinkhole forms by chemical weathering of rock at the ground surface resulting in gradual lowering of the surface to form a depression.  Solution sinkholes form in areas where fractures and joints in the bedrock create pathways through which rainwater can infiltrate the ground. 

In, Arkansas, sinkholes are common in the northern part of the Ozark Plateaus where much of the bedrock is limestone or dolostone.  These types of rocks are notorious for sinkhole development because they are soluble in weakly acidic rain water.

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Geo-pic of the week: Chalk

 

Annona Fm (2)

Like the White Cliffs of Dover, England, the “White Cliffs of Arkansas” (pictured above) are composed of chalk.  Chalk is a marine sedimentary rock that forms of calcite-rich mud that accumulates in semi-deep marine environments.  The mud is composed of the accumulated skeletal remains of algal microorganisms called coccolithophores.  These algae grow and shed skeletal parts called coccoliths which they arrange around them, in life, in a structure called a coccosphere.  Below is a scanning electron microscopic image of some coccospheres (borrowed from news.algaeworld.org).

coccolithophores

Chalk in Arkansas is found in the Annona Formation, which formed in the late Cretaceous Period, and crops out in southwest Arkansas as well as parts of Texas.  In addition to being mined to make blackboard chalk, this resource is also used in brick, and cement manufacture. 

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

Geo-pic of the week: Igneous Dike

igneous dike

 

100 million years ago, during the Late Cretaceous Period, a preponderance of igneous activity occurred in the continental region now known as Arkansas.  In fact, all of the igneous rocks discovered in the state were emplaced around that time.  Some of them are well known, such as Magnet Cove, located east of Hot Springs, or the diamond-bearing intrusion near Murfreesboro.    There are also lots of smaller igneous intrusions like the one shown in the picture above. 

Small igneous intrusions are found throughout the Ouachita Mountains.  There are so many small intrusions that new ones are regularly discovered.  Weathering at the earth’s surface has typically destroyed the original rock’s characteristics and what remains is mostly soft clay because the minerals that make up the intrusion are unstable under surface conditions. 

If you happen to notice an unusual-looking body of rock that cuts across the strata of a road cut or other rock outcrop when you’re exploring the Ouachita Mountains, it’s likely that you have seen a Cretaceous igneous dike.

Differential Weathering

 

The STATEMAP geologic mapping team came across an unusual outcrop in the Atoka Formation while working in northwest Arkansas earlier this year.  Luckily they took lots of photos to document the odd features that formed in this bluff during weathering.

We refer to the process that created these features as differential weathering, meaning the rock does not weather uniformly.  In sedimentary rock it is typically caused by variations in the degree or type of cementation binding the grains of the rock together.

One example of differential weathering is the gray masses of rock that seem to cling to the bluff shelter in several of the photos.  For reasons not entirely clear, those masses are cemented with calcite.  The surrounding rock has been mostly leached of cement.  The picture at top right shows sub-concentric bands of calcite that precipitated in the rock at some point in its history.  Perhaps that phenomenon explains the clinging calcite-cemented rocks.  Where this secondary calcite cement has formed the rock is harder.  The softer rock surrounding it weathers away faster and those calcite-cemented areas progressively stand in relief.

At any rate, it’s a visually stunning outcrop and well worth a look.  Enjoy the pictures, courtesy of the STATEMAP crew!

 

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Geo-pic of the week: Syringodendron

Syringodendron

The above picture is of a syringodendron, the fossilized trunk of a giant tree-like fern that lived in Arkansas in the Pennsylvanian Period (318 to 299 Million years ago).  The Pennsylvanian Period is a sub-division of the Carboniferous Period of geologic history which was so named due to the preponderance of lush vegetation that existed at the time.  Because there was so much vegetation , the geologic record for the Pennsylvanian period is coal-rich and also preserved a lot of plant fossils.  This syringodendron was rescued from an abandoned coal mine in Scott County Arkansas by the land’s owner prior to the land being reclaimed.

The syringodendron is a somewhat rare as a fossil.  It is actually from the same plant that produces another more common plant fossil called a Sigillaria.  Here is an artists representation of what that plant may have looked like:

sigillaria_by_unlobogris

Syringodendron is a Sigillaria trunk that has lost its bark prior to fossilization.  The double impressions that run vertically at regular intervals are called Parichnos scars, or “hare’s trails” colloquially.  Between the vertical impressions are slight linear protrusions called ribs.  If you examine the top edge of the fossil, the ribs give a wavy or undulating shape to the margin of the trunk.

Geo-pic of the week: Siliceous Oolite

Silicified oolite

Ooids are tiny grains that are typically composed of CaCO3 either as calcite or aragonite.  They precipitate from seawater in concentric bands around a nucleus (for instance a fragment of rock or fossil) in turbulent shallow conditions. 

Once ooids form, they can accumulate and be cemented to form a sedimentary type of limestone called oolite.  The above picture is a magnified and tumbled piece of oolitic chert collected fromgravel on Crowley’s Ridge in northeast Arkansas.  The difference between this and typical oolite is that it came into contact with silica(SiO2)-rich ground water after it formed.  The SiO2 then replaced the CaCO3 the rock was initially composed of.   The polished surface provides an ideal view of the internal structure of the spherical ooids.