Minerals and Geology

General Ozark National Forest Geology:

Much of the information presented here is taken from Arkansas Geological Commission sources and publications.

The Ozark National Forest is in the Ozark Mountains of the Ozark Plateau and the Arkansas Valley. This is also referred to as the Ozark Dome and the Arkoma Basin geologic provinces (Smith, 1989). The rocks of the Ozarks are in most cases nearly flat lying with many exposures that form bluffs and outcrops often hundreds of feet high in stream and road cuts. They are part of a regional uplifted dome with the central core of that dome located in the exposed granites of the St. Francis Mountains of Missouri (AGC). The Ozark Plateau is characterized by limestones and dolomites, with many caves, and cave and karst topography. It is subdivided into: the Salem Plateau of predominantly dolomites, limestones, and sandstones with elevations below 800 feet; the Springfield Plateau of predominantly cherty limestones with summit elevations from 1,000 to 1,500 feet; and the Boston Mountains of predominantly sandstones and shales with summits of more than 2,000 feet (Smith, 1989). Blanchard Springs Cavern, located on the Sylamore Ranger District near Mountain View, Arkansas and administered by the Forest Service, is the most noteworthy cave and cavern system in the Boone limestone Formation of the Ozark Plateau. The Arkansas Valley is characterized by sandstones and shales. Producing gas wells are located in the Atoka Formation in the southern portion of the Forest within the Arkoma Basin of the Arkansas Valley system. The St. Francis National Forest is a 20,000 acre unit located along the Mississippi River in the Mississippi River Delta in eastern Arkansas. It is characterized by relatively recent alluvial sediments and sands characterisitic of a major river system like the Mississippi River.

OZARK GEOBIBLEOGRAPHY:
A Bibliography of some of the Minerals and Geology References for the Ozark National Forest areas in Arkansas:

Arkansas Geological Commission

Oklahoma Geological Survey

Arkansas Energy Office (AEO), 1986, Minerals in Arkansas: Arkansas Energy office of the Arkansas Industrial Development Commission

Arkansas Geological Commission (AGC), unpublished geologic field maps of the quad sheets covering the subject area.

Arkansas Geological Commission (AGC), 1977, Geologic Map of the Ozark National Forest: unpublished.

Brewster, E.B., and Norman F. Williams, 1951 (reprinted 1956), Guide Book tothe Paleozoic Rocks of Northwest Arkansas: Arkansas Geological Commission (22 pages).

Bush, W.V., et.al., 1977, A Guidebook to the Geology of the Arkansas Paleozoic Area (Ozark Mountains, Arkansas Valley, and Ouachita Mountains): Arkansas Geological Commission, Guidebook GB 77-1

Caplin, William M., 1957, Subsurface Geology of Northwestern Arkansas: Arkansas Geological Commission Information Circular 19

Headrick, K.N. and O.A.Wise, 1975, Contributions to the Geology of the Arkansas Ozarks: Arkansas Geological Commission, Miscellaneous Publications MP-12

Imes, J.L. and L.F. Emmett, 1994, Geohydrology of the Ozark Plateaus Aquifer System in parts of Missouri, Arkansas, Oklahoma, and Kansas: USGS ProfessionalPaper 1414-D

McFarland, John David, circa 1995, The Rock Strata of the Arkansas Ozarks:Arkansas Geological Commission, unpublished paper

Smith, R.M., 1989, The Atlas of Arkansas: University of Arkansas Press, Fayetteville, Arkansas

Stroud, Raymond B., R.H.Arndt, et.al., 1969, Mineral Resources and Industries of Arkansas: U.S. Bureau of Mines, Bulletin 645

Geologic Descriptions for the Formations on the Ozark National Forest

Arkansas

Alluvium and Terrace Deposits, Qat, Quaternary:

Sand and Clay, Ksc, Cretaceous:

Cretaceous Rocks, Kr, Cretaceous:

Atoka Formation, Pal/Pam/Pau (lower/middle/upper), Pennsylvanian:
NOTE: This description of the Atoka Formation was originally written for work on the Ouachita National Forest. Aside from the references to the Ouachita National Forest, the description for the formation also applies to the Atoka on the Ozark National Forest.
Atoka Series - The Atoka Series is predominantly shale with considerable amounts of siltstone and sandstone. It is divided into three members -- Lower, Middle, and Upper. The Lower member is reported as 9000-13000 feet of thousands of alternating very thin to an occasional thick bedded silty, micaceous, fine grained, poorly sorted, brown/gray, sandstone (35%) -- massive sandstones are common in the lower half of the Lower member; micaceous, sandy, often sooty, gray siltstone (25%) -- abundant coalified plant fragments and coarse mica are also associated with the siltstone; and silty, gray/black shale(40%). The Middle member is about 6000 feet thick and typically has a thick (3000 feet) gray/black shale at the base generally overlain by three flaggy, fine grained, micaceous, gray sandstone units referred to as the "traceable three", which are separated by thick intervals of gray/black shale. The Uppermember is about 6500 feet thick and is primarily a gray/black shale with minor silty to occasionally quartzose sandstones. The Upper member contains coalbeds and plant fossils, and may contain ironstone concretions, pyrite, and thinbeds of siltstone, sandstone and limestone. At least six coal beds are present in the upper 3500 feet of the formation throughout much of west-central Arkansas. Most of the coal beds are less than 24 inches thick and are local in extent, with the exception of two beds in the Logan, Franklin, and Yell county areas north of the Forest that extend for 30 and 35 miles respectively. In Arkansas, the Atoka has a maximum thickness of about 25,000 feet in the frontal Ouachita Mountains near the Oklahoma-Arkansas border. In Oklahoma, the thickness of the formation is generally about 6800 feet in the Arkoma Basin, but an 18,500 foot section has been measured near the Oklahoma-Arkansas border,and it may exceed 25,000 feet in thickness at places in the Ouachita Mountains near the border. Exploration gas well drilling depths are depths of from 5,000 to 21,000 feet into the Atoka Formation. Gas wells in the Arkansas Valley just north of the Ouachita National Forest are producing mostly from the Atoka Formation.

Bloyd Shale Formation and Prairie Grove Member of the Hale Formation, Pbh , Pennsylvanian: "The Hale formation is the lowermost of the Pennsylvanian formations of northern Arkansas. Its crop extends from eastern Oklahoma along the Boston Mountain escarpment to the vicinity of Batesville. Its thickness in northwestern Arkansas probably averages about 150 feet but approaches 300 feet in one locality.... The lowermost part of the formation is a basal conglomerate composed of ferruginous limestone matrix in which are pebbles of limestone...and sandstones and pieces of chert.... Worn fragments of fossil crinoids, cup corals, and Archimedes are present in some outcrops. Above the basal conglomerate is a black clay shale with thin ripple marked sandstones. This shale is normally followed by a sandstone section including thin and massive beds. The thin beds are gray, fine-grained and non-calcareous. The massive beds are gray to brown, cross-bedded, and in many places calcareous. Above these sandstones is a black fissile clay shale which in places contains much sandstone and sandy shale. The upper part of the formation is characteristically a series of massive, brown, cross-bedded, calcareous sandstones, which develop a cavernous surface on weathering. Lenses and beds of limestone occur throughout the sandstone and in places are as much as 50 feet thick. The limestones are usually massive, cross-bedded, coarsely crystalline, very fossiliferous, and gray to rusty in color but there are occasional beds of bluish-gray noncrystalline limestone". (Brewster, 1951).

The Bloyd Shale Formation: "The Bloyd Shale is best developed in the westernpart of the Boston Mountains in Arkansas where it is 200 feet thick. It thinsto the north, east, and west.... The formation consists of shale and twolimestone members. The lower part is a black fissile clay shale 5 to 20 feetthick. Above the shale is the Brentwood limestone member composed of one, two, or three limestone beds, each 3 to 10 feet thick. Above the shale is theBrentwood limestone member composed of one, two, or three limestone beds, each3 to 10 feet thick, separated by shales. The limestones are impure, veryfossiliferous, vary from finegrained to crystalline, and are locally crossbedded.... Above the Brentwood is a black carbonaceous shale in which athin coal bed occurs. A fossil flora of middle Pottsville age has been obtained from this unit. Above the coal-bearing shale is the Kessler limestone member. The limestone resembles the Brentwood in general appearance but is thinner, darker in color, locally conglomeratic and weathers into thin shaly plates. The shale above the Kessler resembles the Fayetteville shale but issomewhat more sandy". The Bloyd is underlain by the Hale formation, and overlain by the Atoka formation. (Brewster, 1951).

Cane Hill Member of the Hale Formation, Phc/Pch, Cretaceous:

Pitkin Limestone, Fayuetteville Shale (including Weddington Sandstone Member), and Batesville Sandstone (including Hindsville Limestone Member), Mpfb , Mississippian:

Ruddell Shale, Mr, Mississippian:

Moorefield Formation, Mm, Mississippian:

Boone Formation, Mb/Mbn, Mississippian:

Chattanooga Shale, Clifty Limestone, and Penters Chert, MDcp, Devonian (Chattanooga Shale is lower Mississippian/upper Devonian):

Lafferty, St. Clair, and Brasstown Limestones, Slsb, Silurian:

Cason Shale, Fernvale Limestone, Kimmswick Limestone, Plattin Limestone, and Joachim Dolomite, Ocj, Ordovician:

St. Peter Sandstone and Everton Formation, Ose/Ospe, Ordovician:
St.Peter Sandstone: "The friable to well cemented sandstones of this formation are made up of sub-angular to rounded, frosted grains. Occasional limestone or dolomite beds are present. The St. Peter is often difficult to separate from the underlying Everton formation, especially where the almost indistinguishable sandstones of both formations are in contact. Several wells in the extreme eastern parts of the Arkansas Valley, and several just inside the Coastal Plain, contain sandy limestones rather than sandstones in the St. Peter interval. These are thought to represent down dip St. Peter facies." Caplin,1957, Subsurface Geology of Northwestern Arkansas: Arkansas Geological Commission Information Circular 19.
Everton Formation: "The Everton is made up of sandy dolomite and friable to well-cemented sandstones containing sub-angular to rounded, frosted quartz grains. The Black Rock and Smithville limestones...[may be]...facies of the Everton formation." Caplin, 1957, Subsurface Geology of Northwestern Arkansas:Arkansas Geological Commission Information Circular 19.

Powell Dolomite, Op, Ordovician: "This unit is considered to be the youngest Arbuckle correlative in northern Arkansas. It consists of silty, shaly dolomite and subordinate sands and sandy dolomite." Caplin, 1957, Subsurface Geology of Northwestern Arkansas: Arkansas Geological Commission Information Circular 19.

Cotter and Jefferson City Dolomites, Ocjc, Ordovician:

The Following Geology and Formation descriptions are borrowed from the Arkansas Geological Commission, Senior Geologist John David McFarland, III.
Arkansas Geological Commission

McFarland, John David, III, 1979, Geologic Float Guide on the Upper Buffalo National River (Ponca Low-water Bridge to Camp Orr Boy Scout Camp): Arkansas Geological Commission Guidebook GB-79-2.

GEOLOGY
The bed of the river along this geo-float trip is incised in Ordovician age rocks of the Everton or subjacent Powell Formation except when passing through the Jim Bluff Graben where the Lower Mississippian age St. Joe Limestone occurs at river level for short stretches.
The oldest unit to be found along the upper Buffalo River is the lower Ordovician Powell Dolomite. The Powell is a light gray, fine grained, argillaceous dolomite with some layers of concentrically banded nodular chert. The best exposed outcrops of Powell are found in Hemmed-in-Hollow.

Majestic bluffs cut from alternating beds of sandstone, limestone, and dolomite of the middle Ordovician Everton Formation dominate an upper Buffalo River float trip experience. The Everton is about 350 feet thick in this area and has been divided into several members by various workers. The most prominent of these embers along the upper Buffalo is the 100 foot thick Newton Sandstone of the middle Everton. This sandstone, like other sandstones in the Everton, is a fine to coarse grain d, well rounded, frosted, often friable, occasionally well sorted quartz arenite. Cementation of the Everton sandstones is accomplished by dolomitic or calcitic carbonates or interlocking overgrowths of silica. In some beds the calcitic cement forms a "poikiloblastic" texture.

The Everton above and below the Newton Sandstone Member consists of alternating beds of dolomite, limestone, and sandstone, none of which persist for any great distance laterally. The limestones consist of calcilutites and calcarenites, with variable amounts of dolomite. Quartz sand of the type found in the Everton sandstones often occurs in many limestone beds as scattered grains, lenses, pods, and thin sometimes disrupted stringers. Allochemical constituents of the calcarenites include pellets, intraclasts, oolites, ostracods, and rarely other invertebrate fossils, all of which may occur together or separately within any distinct bed. Everton dolomites are very fine to coarsely crystalline and include variable amounts of pellets, intraclasts, oolites, ostracods, and gastropods as allochems. Quartz sand is commonly found as scattered grains gradational to very sandy dolomite.

A few conglomerates, breccias, and cherts occur at various discontinuous horizons in the Everton throughout the area. The nature of the carbonates and sands of the Everton in this area allows a veneer of travertine or sandy travertine to form across much outcrop area obscuring both the bedding and lithology.

The top of the Everton along the upper Buffalo River is represented by the Jasper Member. The Jasper can be divided into two lithologic units, a lower sandstone or limy sandstone and an upper limestone or sandy limestone with subordinate dolomite. Along the route of this geo-float trip the upper limestone facies is missing. Whether this is due to post Everton erosion, nondeposition, or facies change is unknown at this time.
At one place on Big Bluff, just below the "Goat Trail", a limestone lens of uncertain stratigraphic assignment is exposed overlying the Jasper sandstone. The lower beds of this limestone are similar to the younger middle Ordovician age Plattin Limestone, a sublithographic to finely crystalline limestone with a few scattered quartz grains or thin sandstone stringers. This is in contrast with the upper limestone beds which are coarsely crystalline, fossiliferous calcarenites and calcirudites and resemble the upper Ordovician Fernvale Limestone.

Unconformably overlying the Everton Formation is a 4 to 6 foot thick, clean, fine to medium grained, rounded, friable quartz arenite of Devonian or Basal Mississippian age. The name Sylamore Sandstone Member of the Chattanooga Shale, a Devonian unit, has often bee. applied to this sandstone but some workers have demonstrated a basal Mississippian sandstone of similar characteristics. A conglomerate occurs at the base of this unit and phosphatic nodules can be found at the top. Above this sandstone the unquestioned Mississippian age (Kinderhookian and Osagean) St. Joe Limestone is developed. The St. Joe in this area is a 50 to 60 foot thick interval of very fine to coarse grained, fossiliferous, tabular bedded, argillaceous calcarenite. and calcareous shale. Occasional bedded and/or nodular chert has been observed in some outcrops of the St. Joe. Beds of brick red limestone and shale distinguish the St. Joe throughout this region.

The Mississippian age Boone Formation conformably overlies the St.- Joe. The Boone is composed of cherty limestones with chert dominating much of the sequence. Along this stretch of the Buffalo River the Boone is around 400 feet thick but it thins to less than 250 feet atop the Sneeds Creek Dome.

Capping the hills surrounding the Buffalo River and generally unseen from the River are strata of Mississippian and Pennsylvanian age. Overlying the Boone, these strata include (in ascending order): the Batesville Sandstone, the Fayetteville Shale, and the Pitkin 1imestone of Mississippian age; and, the Hale, Bloyd, and Atoka Formations of Pennsylvanian age.

Structure and Mineralization

Most float trips on the upper Buffalo begin near the town of Ponca, Arkansas. Lead and zinc have been sporadically mined around the Ponca area from Civil War days into the 1950's. Over 4000 tons of galena, zinc carbonate, and zinc silicate concentrates have been produced from deposits in the Boone and Batesville Formations. The mineralized deposits in the Ponca District occur along the Ponca lineament which is thought to be the surface expression of a Pennsylvanian age shear fracture related to the Ouachita orogeny effecting nearly all of the Paleozoic rocks. The Ponca lineament strikes N. 30o E. and can be traced on high altitude photography and Landsat satellite imagery for over 40 miles -- from the Arkansas River Valley to near Harrison, Arkansas. Small faults associated with this lineament have been noted in and around many of the mines of this area.

Centered about 3.5 miles north northeast of Ponca (Sec. 6, T. 16 N., R. 22 W.) is the Sneeds Creek Dome, a feature of unknown origin covering 12 to 15 square miles. Field data supporting its existence are differences in elevation of stratigraphic horizons, thinning of recognizable intervals, and sparse strike and dip data across the structure.

A pair of grabens with their associated faults strike east southeast from the Sneeds Creek Dome. The displacement varies along the grabens, but in general is between 200 and 300 feet. The traces of the bordering faults cross the river at various places between mile 6 and 10.

Above Quoted From: McFarland, John David, III, 1979, Geologic Float Guide on the Upper Buffalo National River (Ponca Low-water Bridge to Camp Orr Boy Scout Camp): Arkansas Geological Commission Guidebook GB-79-2.

The Rock Strata of the Arkansas Ozarks
By John David McFarland, Senior Geologist,Arkansas Geological Commission - July 1993
Arkansas Geological Commission

Seventy-five percent of the earth's land surface is covered by sedimentary rocks. In Arkansas almost all of the surface is made up of sedimentary rocks. To deal with these rock layers is a systematic manner a system of organization and nomenclature has been developed. This organizational and nomenclatural system is derived from the principles of stratigraphy. The geologic study of the form, arrangement, geographic distribution, chronologic succession, classification, correlation, and mutual relationships of the rock strata in a normal sequence of rock layers is called stratigraphy.

Stratigraphy has these goals: 1) the discrimination of recognizable "units", 2) a physical description of the unit by indicating the geographic distribution of the sequence, the lithic and paleontologic composition of the unit, and the geometry (thickness and overall shape) of the unit; 3) a definition of the upper, lower, and possibility the lateral boundaries; and, 4) the age and relative position of the unit within the overall column.

The basic unit of stratigraphy is the Formation. Formations are lumped together to form Groups and subdivided into Members. A formal Formation is mappable at a 1:24,000 scale, has a definite lithic composition or a definite succession of lithic types, has definable boundaries, and has been assigned a "type" locality. Formation names are usually two part: a geographic name followed by either a lithic name or the word "Formation". We name formations for some geographic feature in the "type" region, usually a town, county, stream, or mountain.

Practically, mappability means that the unit is wide-spread enough to need more than one standard USGS 7.5'topo map to indicate its distribution. This also implies that the unit is thick enough to be drawn on one of these maps. In some cases a Formation that is thick enough on one quad sheet pinches out on another and has to be shown either exaggerated or lumped with other units.

Usually a Formation will made up a limited number of rock types. The normal attempt is to define a Formation on the basis of an environment of deposition. When you see a bed or a sequence of rock what you are usually seeing is the remains of some environmental circumstance of that ancient time. (This is not to mean that a Formation defines the same specific portion of geologic history in all places, but the depositional history of the sequence does strongly influence our selection of boundaries.) This frequently means that the Formation will be composed of one rock type with a few minor variations. However, some depositional situations will lead to the deposit of a variety of lithic types yielding a mixed section. As nature is never perfect and often changing and our man-made classification schemes are artificial and may have little to do with the real world a lot of slop has crept into the system.

Upper and lower boundaries of Formations are placed at changes in the rock type. If a distinct boundary is available, such as at the contact between a sandstone and a limestone or between two limestones of different texture, the contact may be fixed as the boundary. (Unconformities usually make ideal boundaries.) If the contact is gradational a somewhat arbitrary boundary is indicated. This latter situation may result in a definition such as "the first shale sequence greater than 2 feet thick" in a sequence that grades from a sandstone to a shale for example. Lateral boundaries are treated much the same as the upper and lower boundaries, but have their own special problems. In either case the guideline is utility and practicality.

Every modern formal Formation has a "type" locality. A type locality is usually a physically accessible outcrop or sequence of outcrops in a specific area that typifies the unit under consideration. The definitions used to describe the Formation must be based on the lithic criteria of the actual rock material at the type location. For units that embody a lot of variations additional reference sections may be designated. The type section is the name bearer of the Formation and cannot be changed. If a type location is destroyed a principle reference section can be assigned but not a new type. Formations defined long ago may not have type locations, but rather type regions. Several of the traditional Formation names used in Arkansas fall into this latter category.

The name given to a Formation is normally taken from a named geographic feature in the type region. This first part of the binomen is coupled with a descriptive lithic term such as "Limestone", "Dolostone" and the like. If the unit is composed of several rock types the word "Formation" is preferred. Stratigraphic unit lithic names may not hold over the entire outcrop area. The "XYZ Sandstone" may grade laterally into a sand/silt/shale sequence. In such cases the phrase "XYZ Formation" is more appropriate for those areas of mixed lithologies. One problem many people have with stratigraphic nomenclature is the "cultural faults" that exist at the state's boundaries (with all other states on the downthrown side). For example, the sequence of strata we call the Boone Formation (including the St Joe) here in Arkansas is known by seven Formation names in Missouri and none of them are "Boone" or "St Joe".

The following list is the Formations of north Arkansas with reference to their type localities:

JEFFERSON CITY DOLOMITE/FORMATION
Age & Distribution: Lower Ordovician; Missouri and northern Arkansas
Geology: The Jefferson City Dolomite underlies the Cotter Formation and unconformably overlies Roubidoux Formation. It consists of fine grained dolostone and chert with some rare thin beds of sandstone, shale, and oolite. Few fossils are known from the "Jeff City". No attempt is made to differentiate the Cotter Formation from the Jefferson City Formation in Arkansas.
Original reference: A. Winslow, 1894, Missouri Geological Survey, v. 6, p. 331, 373, 375.
Type: Named for exposures at Jefferson City, Cole County, Missouri.

COTTER DOLOMITE/FORMATION
Age & Distribution: Lower Ordovician; Northern Arkansas, southern Missouri, and Oklahoma.
Geology: Predominantly cherty dolostone varying from gray to buff; fine-grained to coarse-grained; thin to massive bedded. Some minor beds of shale and sandstone do occur. The chert nodules associated with the Cotter frequently have concentric light and dark bands. The thickness is 340 feet in vicinity of Cotter. Lower contact disconformable. Some fossils are present.
Original reference: A. H. Purdue and H. D. Miser, 1916, U.S. Geological Survey Geological Atlas, Folio 202.
Type: Starting at valley bottom just upstream from west end of White River bridge at Cotter and extending to top of hill westward along U.S. Highway 62, Baxter County, Arkansas

POWELL DOLOMITE/FORMATION
Age & Distribution: Lower Ordovician; northern Arkansas and southeastern Missouri.
Geology: Fine-grained gray argillaceous dolostone with thin beds of shale, sandstone and sandy dolostone at places; rare chert beds. Thickness is as much as 215 feet. Overlies Cotter dolomite; underlies Everton Formation. Lower contact disconformable. Few fossils are present.
Original reference: A. H. Purdue and H. D. Miser, 1916, U. S. Geological Survey Geological Atlas, Folio 202.
Type: Name derived from Powell Station, now abandoned, on Missouri-Pacific Railroad about 2 miles down Crooked Creek from present village of Pyatt, Marion County, Arkansas

EVERTON FORMATION
Age & Distribution: Middle Ordovician; northern Arkansas and southern Missouri.
Geology: The Everton shows considerable differences in character from one place to another. It is composed of various mixtures of dolostone, sandstone, and limestone with few "pure" lithologies. The formation may have traces of shale and chert in limited areas. The sands tend to be white, well rounded, medium sized grains. Bedding is thin to massive. Lower contact unconformable. Other disconformities occur within the formation. Named Members include Newton SS, Calico Rock SS, and Kings River SS.
Original reference: A. H. Purdue, 1907, Geological Society of America Bulletin, v. 18, p. 251-256.
Type: Named from Everton, and Boone County, Arkansas.

ST. PETER SANDSTONE/FORMATION
Age& Distribution: Middle Ordovician; Minnesota, Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Missouri, Ohio, Oklahoma, and Wisconsin.
Geology: Massive bedded, medium grained, well rounded, friable, white sandstone. Base unconformable often with several feet of relief. Fossils are rare.
Original reference: D. D. Owen, 1847, Preliminary Report of the Geological Survey of Wisconsin and Iowa: U. S. General Land Office Report 1847 (U. S. 30th Congress, lst session S. Ex. Doc. 2), p.169 and 170.
Type: At Fort Snelling, Hennepin County. Named for exposures on St. Peter River, now called Minnesota River, southern Minnesota.

JOACHIM DOLOMITE/FORMATION
Age & Distribution: Middle Ordovician; Eastern Missouri, northern Arkansas, and southwestern Illinois.
Geology: Chiefly fine grained dolostone and dolomitic limestone with thin beds of shale. Supratidal deposits are frequent and display mud-cracks, calcite pseudomorphic salt crystals, and laminated horizons.
Original reference: A Winslow, 1894, Missouri Geological Survey, v. 6, p. 331, 352.
Type: Named for exposures along Joachim Creek, Jefferson County, Missouri.

PLATTIN LIMESTONE/FORMATION
Age & Distribution: Middle Ordovician; eastern Missouri, central northern Arkansas, and western Illinois.
Geology: A very fine-grained limestone in mostly thin beds. Limestone often has small blebs of calcite. Fossils are common to some upper beds. Lower contact disconformable.
Original reference: E. 0. Ulrich, 1904, Missouri Bur. Geology and Mines, v. 2, 2d ser., p.111.
Type: Named for exposures near mouth of Plattin Creek, Jefferson County, Missouri.

KIMMSWICK LIMESTONE/FORMATION
Age & Distribution: Middle Ordovician; eastern Missouri, northern Arkansas, and southwestern Illinois.
Geology: The formation is chiefly an even-bedded, thin to massive, light-gray, medium- to coarse-grained limestone with a characteristic sugary texture. Some beds of dolostone have been noted. The boundaries above and below display "welded," disconformable contacts.
Original reference: E. 0. Ulrich, 1904, Missouri Bur. Geology and Mines, V. 2,2d ser., p. ill.
Type: Named for exposures at Kimmswick, Jefferson County, Missouri.

FERNVALE LIMESTONE/FORMATION
Age & Distribution: Upper Ordovician; western Tennessee, northwestern Alabama, northern Arkansas, southwestern Illinois, southeastern Missouri, and central-eastern and northeastern Oklahoma.
Geology: Massive, sometimes cross-bedded, coarsely crystalline, fossiliferous, crinoidal limestone with an occasional trace of chert. Light gray to pink and weathers lead gray. Fossils found throughout but most at top and base of unit. Top bed in the eastern outcrops frequently displays nautiloid fossils of large size. Basal contact unconformable.
Original reference: C. W. Hayes and E. 0. Ulrich, 1903, U. S. Geological Survey Geological Atlas, Folio 95, p. 2.
Type: Named for Fernvale, Williamson County, Tennessee.

CASON SHALE/FORMATION
Age & Distribution: Upper Ordovician, (lower Silurian); northern Arkansas
Geology: The Cason consists of several rock types and contains at least two unconformities. The rock types are phosphatic sandstone and shale, oolitic limestone, pelmatozoan limestone, and sandy, calcareous shale containing flattened algal "buttons". Some modern workers place the Brassfield Limestone within the Cason as a Member below the "button" shale.
Original reference: H. S. Williams, 1894, American Journal of Science, 3d, v. 48, p. 325-331.
Type: Named for Cason tract, near Batesville, Independence County.

BRASSFIELD LIMESTONE/(MEMBER) FORMATION
Age & Distribution: Lower Silurian; central Kentucky, central northern Arkansas, southern Indiana, southwestern Ohio, and southern Tennessee.
Geology: This unit is a thin, light gray, fossiliferous limestone with traces of glauconite. Its distribution in Arkansas is somewhat limited. Considered a Member of the Cason by some modern workers.
Original references: A.F. Foerste, 1905, Kentucky Geological Survey Bulletin 6, p. 156; 1906, Kentucky Geological Survey Bulletin 7, p. 10, 27.
Type: Named for exposures along Louisville and Atlantic Railroad between Brassfield and Panola, Madison County, Kentucky.

ST. CLAIR LIMESTONE/FORMATION
Age & Distribution: Silurian; northern Arkansas, southern Illinois, and central eastern Oklahoma.
Geology: The St. Clair is coarse-grained, highly fossiliferous, generally light gray to pinkish-gray limestone. Exposures are somewhat limited to small areas in Independence, Izard, and Stone counties. Trilobite and brachiopod fossils are common.
Original reference: R.A.F. Penrose, Jr., 1891, Arkansas Geological Survey Annual Report1890, v. 1, p.102-103, 112-114, 124-128, 166-174.
Type: Named for St. Clair Springs, 8 miles northeast of Batesville, Independence County, Arkansas.

LAFFERTY LIMESTONE/FORMATION
Age & Distribution: Silurian; central northern Arkansas.
Geology: Even bedded, earthy limestone. Usually distinctly red, but upper beds are gray. Limited extent. Few fossils.
Original reference: H. D. Miser, 1920, U. S. Geological Survey Bulletin 715-G.
Type: Well exposed at Tate Spring, 1 1/4 miles north of Penters Bluff Station, Izard County. Named for West Lafferty Creek, which is half a mile east of exposure.

PENTERS CHERT/FORMATION
Age & Distribution: Lower or Middle Devonian; central northern Arkansas.
Geology: A dense, massive, mottled gray chert. The Penters is unfossiliferous at its type locality.
Original reference: H. D. Miser, 1920, U. S. Geological Survey Bulletin 715-G.
Type: Named for exposures at Penters Bluff Station, Izard County.

CLIFTY LIMESTONE/FORMATION
Age & Distribution: Middle Devonian; northwestern Arkansas.
Original reference: H.D. Miser, 1916, U. S. Geological Survey Geological Atlas, Folio 202.
Geology: At its type locality the Clifty is a thin, very sandy limestone with a few fossils. Outside the type area the unit is mostly a sandstone often confused with the Sylamore Sandstone. Disconformable at base.
Type: Named for East Fork of Little Clifty Creek, Eureka Springs quadrangle, Carroll County.

CHATTANOOGA SHALE/FORMATION
Age & Distribution: Upper Devonian (and lower Mississippian); Tennessee, Alabama, Arkansas, Kentucky, Mississippi, and Oklahoma.
Geology: Typically a black, fissile clay shale that weathers into thin flakes. The beds are usually cut by prominent joints. Chattanooga is all Devonian in Arkansas. Lower sandstone Member called Sylamore. Basal contact unconformable.
Original reference: C. W. Haynes, 189 1, Geological Soc. America Bulletin, v. 2, p. 143.
Type: Hillside exposure at north end of Cameron Hill, Chattanooga, Hamilton County, Tennessee. Standard section: Cut on Tennessee Highway 26, at east approach to Silgo Bridge over Caney Fork, 7.1 miles east of courthouse at Smithville, DeKalb County, Tennessee.

ST. JOE LIMESTONE/(FORMATION) MEMBER
Age & Distribution: Lower Misssissippian; northern Arkansas, Oklahoma, southern Missouri.
Geology: A crinoidal limestone that may contain some smooth bedded chert in limited places. Limestones (and cherts) may be red, pink, gray, purple, brown, or amber. It is a frequent bluff former. Phosphatic basal greenish shale or conglomerate is common. Thin shaly intervals common in lower beds. Basal contact disconformable. Officially considered a member of the Boone in Arkansas, a group in Oklahoma, and four other formations in Missouri.
Original Reference: T. C. Hopkins, 1893, Annual Report of the Geological Survey of Arkansas for 1890, v. 4.
Type: Named for exposures near St Joe, AR. Principal reference section in a quarry two miles northwest of St. Joe (NENW, Sec.18,Tl6N,Rl7W).

BOONE FORMATION/(LIMESTONE/CHERT)
Age & Distribution: Lower and Middle Mississippian; northern Arkansas, southwestern Missouri, and eastern Oklahoma.
Geology: Fine to coarse grained fossiliferous limestone interbedded with chert. Some sections may be predominantly limestone or chert. The cherts tend to be dark in color in the lower part of the sequence and light in color in the upper part of the section. Includes oolite member near top in western exposures. Thickness is 300 to 350 feet in most of northern Arkansas.
Original reference: J.C. Branner and F.W. Simonds, 1891, Arkansas Geological Survey Annual Report1888, v.4, p.xiii, 27-37.
Type: Named for extensive development in Boone County, Arkansas.

MOOREFIELD FORMATION/SHALE
Age & Distribution: Upper Mississippian; northern Arkansas and central eastern Oklahoma.
Geology: The Moorefield in Batesville district consists of a lower member of black calcareous shale and siliceous limestone and upper member of dark fissile clay shale. The unit listed as the Ruddell Shale on the state geologic map is usually considered part of the Moorefield and is equivalent to the upper member listed here. Fossiliferous.
Original reference: G.I.Adams, 1904, U.S. Geological Survey Professional Paper 24, p. 26.
Type: Named for Moorefield, Independence County, Arkansas

BATESVILLE SANDSTONE/FORMATION
Age & Dist: Upper Mississippian; northern - Arkansas, southem Missouri, and northeastern Oklahoma.
Geology: The Batesville is a flaggy, medium-grained, cream-colored to brown sandstone. The Hindsville Member is a limestone that, when present, usually occurs at the base of the formation. In western Arkansas the Hindsville may be the only representative of this unit. The basal contact is unconformable and often marked by a chert conglomerate. Fossiliferous.
Original reference: J.C.Branner, F.W.Simonds, 1891, Arkansas Geological Survey Annual Report 1888, v.4, p.xiii, 26, 49-53.
Type: Named for Batesville, Independence County, Arkansas.

FAYETTEVILLE SHALE/FORMATION
Age & Distribution: Upper Mississippian; northern Arkansas, southern Missouri, northeastern, central, and eastern Oklahoma.
Geology: A black, fissile, concretionary, clay shale. Dark gray, fine-grained, limestones interbed with the formation in north central Arkansas. A sandstone member, the Wedington Sandstone, interrupts the section in western Arkansas. Fossiliferous.
Original reference: F. W. Simonds, 1891, Arkansas Geological Survey Annual Report 1888, v. 4, p. 26, 42-49.
Type: Named for Fayetteville, in valley of West Fork of White River, Washington County, Arkansas.

PITKIN LIMESTONE/FORMATION
Age & Distribution: Upper Mississippian; northern Arkansas and eastern Oklahoma
Geology: Fine to coarse grained, oolitic, bioclastic limestone. May have black shales interbedded with limestones. Gets more shaly to the south. Fossils are common, mostly crinoids. The bryozoan Archimedes is a good marker fossil but it does occur in other formations. Basal contact conformable.
Original reference: G.I.Adams and E.0.Ulrich, 1904, U. S. Geological Survey Professional Paper 24, p. 27, 109.
Type: Named for exposures near Pitkin post office in Washington County, Arkansas.

HALE FORMATION
Age & Distribution: Lower Pennsylvanian (Morrow Series); northern Arkansas, southern Missouri, and northeastern Oklahoma.
Geology: Lower silty shale, the Cane Hill Member, and an upper calcareous sandstone, the Prairie Grove Member. Rests with pronounced unconformity upon Pitkin and older strata; grades upward into Bloyd Formation; contact is typically placed at base of the first two foot shale above the Prairie Grove.
Original reference: G.I. Adams and E. 0. Ulrich, 1905, U. S. Geological Survey Geological Atlas, Folio 119.
Type: Named for Hale Mountain, Washington County, Arkansas.

CANE HILL MEMBER (of Hale Formation)
Age & Distribution: Pennsylvanian (Morrow Series); northwestern Arkansas and northeastern Oklahoma.
Geology: Cane Hill name applied to lower member of Hale Formation. Composed of silt, silty sandstone, and fine-grained sandstone; locally calcareous. Bedding is generally shaly or thin and sheety, or consists of shaly siltstone alternating with thin hard beds of sandstone.
Original reference: L.G.Henbest, 1953, American Association of Petroleum Geologists Bulletin, v.37, no.8, p.1938-1939,1948 (fig. 2).
Type: Typically exposed in southern part of Washington County, Arkansas, in vicinity of Cane Hill. Well exposed beside Arkansas State Highway 59 for 3 miles along the road south of Evansville in SE 1/4 sec. 35, T.13 N., R.33W.

PRAIRIE GROVE MEMBER (of Hale Formation)
Age & Distribution: Pennsylvanian (Morrow Series);: northwestern Arkansas and northeastern Oklahoma.
Geology: Name applied to upper member of Hale Formation. Described as a massive, variously sandy, crossbedded, pockmarked limestone or limy sandstone with lenses of relatively pure, crinoidal, highly fossiliferous limestone and oolitic limestone. Thickness is 60 to 200 feet.
Original reference: L. G. Henbest, 1953, American Association of Petroleum Geologists Bulletin, v. 37, no. 8, p. 1935, 1940-1942.
Type: Named for exposures in mountains east and south of Prairie Grove, Washington County, Arkansas.

BLOYD FORMATION/SHALE
Age & Distribution: Lower Pennsylvanian (Morrow Series); northwestern Arkansas and eastern Oklahoma.
Geology: Formation consists of (ascending) Brentwood Limestone Member, Woolsey Member composed of terrestrial sediments, Dye Shale Member, Kessler Limestone Member, and the Trace Creek Shale Member (now considered part of the Atoka Formation). The Brentwood becomes a sandstone east of the type region. Thin coal beds are known from this unit. One of the most prominent bluff-formers in north central Arkansas is due to a thickening of the basal sandstone of the Dye Shale (and often contains quartz pebbles).
Original reference: A. H. Purdue, 1907, U. S. Geological Survey Geological Atlas, Folio 154.
Type: Named from Bloyd Mountain, 9 miles southwest of Fayetteville, Washington County, Arkansas.

ATOKA FORMATION
Age & Distribution: Middle Pennsylvanian; eastern Oklahoma, western and southwestern Arkansas, eastern New Mexico, and central and western Texas.
Geology: A sequence of marine sandstones, siltstones, and shales. Some calcareous beds are known. This unit has the largest areal extent of any of the Paleozoic formations in the state. It is the surface rock of the Boston Mountains. The unit locally contains discontinuous streaks of coal and coaly shale to the south.
Original reference: J.A. Taff and G.I.Adams, 1900, U. S. Geological Survey 21st Annual Report, pt. 2, p. 273.
Type: Named for Atoka, Atoka County, Oklahoma.

USDA Forest Service

Ouachita National Forest

Box 1270

Federal Building

Hot Springs, AR 71902

Phone: 501-321-5202

FAX: 501-321-5353

For any additional information, contact John C. Nichols
Forest Geologist for the Ouachita National Forest, at 501-321-5285.

USDA Forest Service - Ouachita National Forest
Last Modified: Monday, 12 May 2008 at 16:19:09 EDT