Category Archives: Notes From The Field

2019 STATEMAP Field Calendar now available for download

Download a commemorative 25th anniversary STATEMAP Field Calendar here:

https://www.geology.arkansas.gov/publication/other-publications/statemap-field-calendar-2019.html

We are celebrating the 25th year of detailed geologic mapping in Arkansas made possible by the passage of the National Geologic Mapping Act of 1992. It established STATEMAP which distributes funds to the states, typically geological surveys, in the form of cooperative grants which are used to partially fund various geologic mapping projects. The first grant received by the Arkansas Geological Survey, then known as the Arkansas Geological Commission, was for a proposal in fiscal year 1994.  Since that time, seventy-eight 1:24,000-scale geologic maps have been completed, with two more on the way this year.  Two maps at the 1:100,000-scale have also been published.  This marks an unprecedented commitment to gathering data about the surface of the earth in our state. Following is a factsheet summarizing the STATEMAP projects in Arkansas since 1994.

Statemap Factsheet-front-2019

Here is the law establishing STATEMAP:

National Geologic Mapping Act of 1992

PUBLIC LAW 102-285

102d Congress

signed May 18, 1992

 

An Act

To enhance geologic mapping of the United States, and for other purposes.

 

Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled,

 

43 USC section 31a. Findings and purpose

(a) Findings

 The Congress finds and declares that–

(1) during the past 2 decades, the production of geologic maps has been drastically curtailed;

(2) geologic maps are the primary data base for virtually all applied and basic earth-science applications, including–

(A) exploration for and development of mineral, energy, and water resources:

(B) screening and characterizing sites for toxic and nuclear waste disposal;

(C) land use evaluation and planning for environmental protection;

(D) earthquake hazards reduction;

(E) predicting volcanic hazards;

(F) design and construction of infrastructure requirements such as utility lifelines, transportation corridors, and surface-water impoundments;

(G) reducing losses from landslides and other ground failures;

(H) mitigating effects of coastal and stream erosion;

(I) siting of critical facilities; and

(J) basic earth-science research;

(3) Federal agencies, State and local governments, private industry, and the general public depend on the information provided by geologic maps to determine the extent of potential environmental damage before embarking on projects that could lead to preventable, costly environmental problems or litigation;

(4) the combined capabilities of State, Federal, and academic groups to provide geologic mapping are not sufficient to meet the present and future needs of the United States for national security, environmental protection, and energy self-sufficiency of the Nation;

(5) States are willing to contribute 50 percent of the funding necessary to complete the mapping of the geology within the State;

(6) the lack of proper geologic maps has led to the poor design of such structures as dams and waste-disposal facilities;

(7) geologic maps have proven indispensable in the search for needed fossil-fuel and mineral resources; and

(8) a comprehensive nationwide program of geologic mapping is required in order to systematically build the Nation’s geologic-map data base at a pace that responds to increasing demand.

 

(b) Purpose

The purpose of sections 31a to 31h of this title is to expedite the production of a geologic-map data base for the Nation, to be located within the United States Geological Survey, which can be applied to land-use management, assessment, and utilization, conservation of natural resources, groundwater management, and environmental protection.

 

section 31c. Geologic mapping program

 

(c) Program objectives

The objectives of the geologic mapping program shall include–

(1) determining the Nation’s geologic framework through systematic development of geologic maps at scales appropriate to the geologic setting and the perceived applications, such maps to be contributed to the national geologic map data base;

(2) development of a complementary national geophysical-map data base, geochemical-map data base, and a geochronologic and paleontologic data base that provide value-added descriptive and interpretive information to the geologic-map data base;

(3) application of cost-effective mapping techniques that assemble, produce, translate and disseminate geologic-map information and that render such information of greater application and benefit to the public; and

(4) development of public awareness for the role and application of geologic-map information to the resolution of national issues of land use management.

(d) Program components

(3) A State geologic mapping component, whose objective shall be determining the geologic framework of areas that the State geological surveys determine to be vital to the economic, social, or scientific welfare of individual States. Mapping priorities shall be determined by multirepresentational State panels and shall be integrated with national priorities. Federal funding for the State component shall be matched on a one-to-one basis with non-Federal funds.

Miss January

https://www.geology.arkansas.gov/publication/other-publications/statemap-field-calendar-2019.html

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The St. Peter Sandstone

Recent mapping adventures have reminded me just how much I enjoy studying the St. Peter Sandstone.  This sandstone was named by Arkansas’ first State Geologist, David Dale Owen, for exposures on the St. Peter River, now called the Minnesota River, in southern Minnesota.  The sandstone is Middle Ordovician in age (around 460 million years old) and during that time it extended all the way from Minnesota into Texas.

It is easy to recognize the St. Peter Sandstone whether you are in Minnesota or Arkansas – clean, sugary, white sandstone.  In fact, here is a photo taken from the place where it was first described, known as the type section, under a bridge near Fort Snelling, in St. Paul, Minnesota.

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St. Peter Sandstone in its type area. 

In Minnesota the sandstone easily falls apart.  In Arkansas, the surface of the outcrop is case- hardened meaning there is a hard rind on the rock that forms due to iron-rich water percolating through it and depositing iron on the surface as the water evaporates.  Where this rind is broken, the sand grains fall apart more easily, as at the type area.

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The St. Peter Sandstone cropping out in the parking area for the spring at Blanchard Springs Recreation Area.

The contact of the St. Peter Sandstone with the underlying Everton Formation is particularly interesting.  It is unconformable which means there was a period of non-deposition and erosion before the St. Peter was deposited.  It is also undulatory with as much as 20 feet of relief in Arkansas.  The relief is the difference between the top and bottom of an undulation.  Flint, 1956, reports that these undulations can reach up to 200 feet in Wisconsin.  Pretty amazing!

st pete everton unconformity 3

In the photo above, the relief at the contact is probably around 5-6 feet.  Note the rock hammer for scale.  The rock above the hammer is the St. Peter Sandstone while the rock the hammer is resting on is the Everton Formation.  Also notice the curvature of the contact.  The reason for the unconformable undulating contact is that the sand in the St. Peter was deposited upon the karsted Everton surface.  Karst forms when rock such as limestone is exposed to slightly acidic rainwater or groundwater and develops sinkholes, caves, and enlarged fractures. Since this karst surface has been buried by the St. Peter Sandstone, it is considered paleokarst.

The geologic story goes something like this.  After the sea that deposited the Everton Formation retreated, the limestone at the top of the formation was exposed.  Weathering and erosion lasted for up to tens of millions of years, during which time an extensive karst surface developed (Palmer and Palmer, 2011).   Sand was brought into the area from the source area to the north (Great Lakes region) by rivers and wind. Later, as the sea advanced again, it spread the sand over the area filling in the depressions and forming a thick deposit covering a large portion of the mid-continent.

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In this photo the relief is approximately 18 feet.  Note the 6-foot-tall geologist for scale.  The red line indicates the contact between the St. Peter above and the Everton below.

The St. Peter Sandstone is relatively resistant to erosion compared to the surrounding rocks; therefore, it is usually a bluff-former.  The tallest bluff I have seen crops out at Blanchard Springs Recreation Area near the group camp and the amphitheater.

Enjoy these photos of the St. Peter Sandstone and hope to see you in the field!

Angela Chandler

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Tall (approximately 70 feet tall) St. Peter bluff behind the amphitheater at Blanchard Springs Recreation Area.

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    St. Peter Sandstone bluff near Blanchard Springs Recreation Area. 

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The St. Peter Sandstone dipping to creek level at the swimming area in Blanchard Springs Campground.     

References and other sources on the St. Peter Sandstone:

Flint, A.E., 1956, Stratigraphic relations of the Shakopee Dolomite and the St. Peter Sandstone   in southwestern Wisconsin: Journal of Geology, vol. 64, no. 4, pp. 396-421.

Giles, A.W., 1930, St. Peter and older Ordovician sandstones of northern Arkansas:  Arkansas Geological Survey Bulletin 4, 187 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, pp. 7-16.

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

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.

 

 

 

Cannon Creek Waterfall at Parthenon/Brentwood Contact

Notes from the Field-Durham Quadrangle

 

Geologic Map of the Durham Quadrangle, Madison and Washington Counties, Arkansas

Geologic mapping of the Durham 7.5-minute quadrangle in northwest Arkansas was recently completed by the STATEMAP field team.  STATEMAP in Arkansas is currently focused on detailed 1:24,000-scale mapping in the Ozark Plateaus Region in north Arkansas.  It is accomplished through a cooperative matching-funds grant program administered by the US Geological Survey.   Field work was performed between July and February, and included hiking/wading/swimming the entire 12-mile stretch of the upper White River located on the quad.  Previous mapping delineated five stratigraphic units for the 1:500,000-scale Geologic Map of Arkansas, but at the 1:24,000 scale, we were able to draw ten. Further division is possible, but several units were considered too thin to map on the available 40-foot contour interval.

You can download your own copy of the map at this link:

http://www.geology.arkansas.gov/maps_pdf/geologic/24k_maps/Durham.pdf

 

DurhamStratColumn

Generalized Stratigraphic Column of Durham Quadrangle

The Drakes Creek Fault, which runs diagonally from the southwest corner to the northeast corner, is the most striking feature on the map.  It is part of a major structural feature in northwest Arkansas, forming a lineament that can be traced at the surface for over 45 miles.  The Drakes Creek displays normal movement, is downthrown to the southeast, and offsets strata an average of 230 feet.  Associated with the fault on the northwest side is a large drag fold. There, rocks parallel to the fault are deformed such that units typically present at higher elevations away from the fault bend down to a much lower elevation next to the fault.  Erosion along this side of the fault has exposed the core of the fold along Fritts Creek, Cannon Creek, and other places.

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Detail of Cross-section of Durham Quadrangle

The Durham quad is far-removed from areas of previous STATEMAP projects in north Arkansas.  We completed work on the Mountain View 1:100,000-scale quad last year, ending on the Brownsville quad near Heber Springs.  Focus has now turned to the Fly Gap Mountain 1:100K quad as the next high-priority area.  When completed, we will have continuous 1:24K coverage for a large portion of the central Ozark Plateaus Region.  The Durham quad was an appropriate choice to begin mapping in this area due to its proximity to designated type sections for many of the formations in north Arkansas.  This facilitated easy comparisons between our field observations on Durham with the classic outcrops where these formations were first described.  Initial field investigations included locating, describing, and sampling these historic outcrops near Fayetteville. We visited many places the names from which the stratigraphic nomenclature we still employ was derived.  These places have such names as: Bloyd Mountain, Kessler Mountain, Lake Wedington, Cane Hill, Prairie Grove, Brentwood, Winslow, and Woolsey.  Having seen the stratigraphy in these areas firsthand better prepares us to track changes in lithology and sedimentation as we continue to map to the east and south of Durham in the coming years.

The following images were taken during this year’s field season and are arranged in stratigraphic order from youngest to oldest:

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Liesegang boxworks–Greenland Sandstone.  Mapped into the Atoka Formation

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Asterosoma trace fossils–Trace Creek Shale of the Atoka Formation

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Kessler Limestone just below the Morrowan/Atokan Boundary–mapped into the Dye Shale of the Bloyd Formation

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Parthenon sandstone resting on the Brentwood Limestone, both of the Bloyd Formation.  The Parthenon was also mapped into the Dye

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Mounded bioherms in the Brentwood Limestone

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Tabulate coral colony in the Brentwood Limestone

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Herringbone cross-bedding in calcareous sandstone–Prairie Grove Member of the Hale Formation

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Goniatitic Ammonoids in calcareous sandstone–Prairie Grove

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South-dipping sandstone in the White River south of the Drakes Creek Fault–Cane Hill Member of the Hale Formation

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Soft-sediment deformation–Cane Hill

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Pitkin Limestone, below the Cane Hill near West Fork—Mississippian/Pennsylvanian Boundary

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A cluster of solitary Rugose corals–Pitkin Limestone

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Wedington Sandstone of the Fayetteville Shale at West Fork

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Base of the Wedington–mapped into the upper Fayetteville Shale

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Large septarian concretion–lower Fayetteville Shale

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Pyritized Holcospermum (seed fern seed-left) and goniatitic ammonoid (right)–lower Fayetteville Shale

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Boone Formation, along the White River in the northwest corner of the Durham quadrangle

This year, we’re moving east to map the Japton and Witter quads. Wish us luck as we begin a new field season.  We’ll try to keep you apprised, so until next time, we’ll see you in the field!

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Richard Hutto and Garry Hatzell

Buttress on Little Buffalo River near Parthenon

Notes From the Field: Boone Buttresses

Buttress on Little Buffalo River near Parthenon

Buttress on Little Buffalo River near Parthenon, Newton County

The photo above shows an unusual rock column located near Parthenon in Newton County.  Judging from the man standing at the base, it is probably over 100 feet tall.  Recently, I was asked what to call these impressive features.  The term we’ve used at the Survey is buttress, which is defined by the Glossary of Geology as a protruding rock mass on, or a projecting part of, a mountain or hill resembling the buttress of a building; a spur running down from a steep slope.  Example: a prominent salient produced in the wall of a gorge by differential weathering.  We’ve used the term buttress, instead of other terms like pinnacle or rock pillar, because these terms refer to a free-standing column of rock, whereas a buttress is, at least nominally, attached to the bluff line.  The term also differentiates these particular features from others that are similar in shape, such as pedestals or hoodoos, which typically form in clastic rocks like sandstone and siltstone.  Their development is controlled by joints, which are planar fractures with no displacement, and by the presence of a resistant caprock, which acts to protect the underlying, less-resistant rock from weathering as quickly.  This process leads to a characteristic shape that is wide at the top and narrower below.

Sandstone pedestal at Pedestal Rocks, Pope County

Sandstone pedestal at Pedestal Rocks, Pope County

A buttress, on the other hand, is typically either uniform in diameter or may taper slightly towards the top, probably because they develop in fairly homogeneous rock.

Buttresses are known to be present in two locations in Arkansas: along the Little Buffalo River near Parthenon in Newton County and along Bear Creek near Silver Hill in Searcy County.

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Buttresses on Bear Creek, Searcy County

They are all developed in the Mississippian Boone Formation which averages about 320 feet in thickness, and is composed of interbedded limestone and chert.  Limestone is dissolved by slightly acidic surface and groundwater, and over time this process leads to many unusual surface and subsurface features known as karst.  Buttresses are one such feature.

The exact mechanism for their development is poorly understood, but some of the factors that contribute to their formation are known.  First, dissolution of limestone can produce similar shapes on a small scale, as seen in this photo of coarsely crystalline Fernvale Limestone in a creek bed.

Dissolutioned limestone in creek bed, Stone County

Dissolutioned limestone in creek bed, Stone County

This process may be all that is needed to produce the buttresses at a larger scale.  Second, all rock units have planes of weakness due to the regional history of tectonic stress.  This stress is usually expressed as a joint system, and is one of the most commonly observed structural features in an outcrop.  Observations at these two sites have shown that jointing is poorly exposed, but as you can see from the aerial photograph on Bear Creek, weathering of the buttresses roughly aligns with the most prominent joint trends in the area (N/S and NE/SW) as indicated by the joint diagram from the Geologic Map of the Marshall Quadrangle.

Aerial view of buttresses on Bear Creek showing prominent regional joint orientationsMarshall Rose

Aerial photo of buttresses on Bear Creek showing prominent regional joint orientations

So even though the joints are poorly developed, one can interpret that pathways for water preferred these orientations, enlarged them over time, and left the buttresses as erosional remnants. 

However they occur, they are certainly beautiful rock formations and worthy of further study.

Buttresses on Bear Creek, Searcy County

Buttresses on Bear Creek, Searcy County

Many thanks to Angela Chandler for the featured image!

Richard Hutto

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