Mercury Content of Oklahoma and Ohio Shale Deposits

Supplying the Brick Industry

Report to the Brick MACT Project File

Prepared by: Susan Fairchild

September 23, 2015

   
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EXECUTIVE SUMMARY

This report supports development of the emission limits for mercury under the maximum achievable control

technology (MACT) standard for the Brick Manufacturing source category.   

Ten shale and clay deposits in Ohio and Oklahoma are currently used by U.S. brick manufacturing companies as

a source of raw material. Available information was reviewed to understand the source(s) of mercury in the

clays. The Oklahoma Geological Survey provided detailed historical and stratigraphic maps of rock sequences

across the state. The Ohio Geological Survey materials were less detailed, but provided similar information.

All clays used in these states are formed by the weathering of shale, a sedimentary rock. Clay deposits are

formed by the weathering of host rock, and are deposited through erosional processes to a local area, usually a

geologic basin. While clays can be formed from the weathering of all types of rock, the clays used by the brick

industry in Ohio and Oklahoma originate from shale sequences formed during the Devonian, Pennsylvanian, and

Permian periods. Note that all Ohio and Oklahoma clay deposits are extracted at or near the surface in ‘pits’.

Variability of mercury content is expected due to the geological events which occurred over the course of shale

sequence formation and the concentration of mercury by the weathering process. The severity of that variability

is dependent on several factors, including volcanic activity during shale formation, availability of associated

minerals with which to form stable bonds, the shales’ carbon content, and irregular rates of organic deposition.  

Given the above factors, we conclude a correlation is apparent between the clay’s organic content, general color

of the clay, and the relative mercury content.  

METHOD

Existing information was obtained to locate, identify, and characterize the geological history of the clay deposits

in this report. Analyses of the mercury content of each clay deposit used by brick manufacturers in Oklahoma1

and Ohio were supplied from existing company data collected in rock and soil assays. No new testing was

required. Companies identified each sample of the deposits by location (latitude, longitude), age, rock type, and

sample description. When locations were not identified by company, they were estimated using publicly

available data from Google Earth©.

Geologic maps from the State Geologic survey offices and reviews of the geological history were next located

online for both Ohio and Oklahoma. The geologic time scale was also reproduced for reference from the

Geological Society of America (see Appendix 1).

Google Earth © was used to locate deposits by latitude and longitude, or when these were withheld, by nearest

visible clay deposit. Once the deposit was located and a pit was identified, the clay deposit coordinates were

transferred to the respective geologic map. The historical geology of each area was reviewed and noted. State

Geological Survey maps, geologic history reports, cross‐sectional maps and related materials were used to

determine the age of surface deposits and near‐surface deposits, and to ascertain potential origins and

explanations of the variation of mercury content for the clay deposits, if possible. Other sources of information

were also retrieved and reviewed for relevancy to this study.

OHIO  

                                                            

1 One facility in this analysis, ACME Fort Smith, is actually located in Arkansas (see “OKLAHOMA” section).
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Brick companies extract clays from five locations in Ohio, as shown on page 6 in the geologic map (Figure 1.)

These are the Belden‐Sugarcreek fire clay/shale, site 1; Whitacre Greer‐Lower Kittaning clay, site 2; Whitacre

Greer‐Brookville clay, site 3; Whitacre Greer‐Red clay Pit, site 4; and Glen‐Gery, Caledonia and Iberia‐Green

shale and clay, site 5. While we did not obtain complete assays of all clays sampled, we note, in general, the red

color of clays indicates an oxidizing environment with a high‐iron content, and the green and gray colors of clays

indicates a reducing environment. These distinctions play a significant role in the geochemistry of the rock from

which clays are derived.

Geological Overview:

Overall, the bedrock geology of Ohio consists of late‐Precambrian age igneous and metamorphic rock at depth

overlaid by flat lying to gently dipping sedimentary rocks including carbonates, evaporite minerals, and shales

that are alternatingly high in silica and organic content. The shales were formed from sediments deposited

beginning 450 million years ago and continuing for another 150 million years thereafter (i.e., from Upper

Ordovician to Lower Permian). Surface‐layer Quaternary sediments irregularly overlie shale deposits north and

west of the glacial margin. Clays are found in two geologic basins: Appalachian (younger) basin and the Michigan

(older) basin, and the clays used by brick companies are located in Pennsylvanian and Devonian age shale

deposits in these basins. The Devonian period in Ohio was preceded by a period of widespread erosion, followed

by accumulation of sediments of marine and sub‐marine origin in the Michigan basin. Lithification of these

sediments formed sandstone, shale, and limestone. Another period of widespread erosion preceded the early

and mid‐Pennsylvanian period, characterized by a 16 million year accumulation of deltaic and marine sandstone,

siltstone, and shales in the Appalachian basin. High carbon content is a characteristic of all clays formed during

the Pennsylvanian system, as implied by the name of the period “Carboniferous Period”, particularly the shales,

which are deltaic in origin. Site 5 clays are of Devonian age2

, which predates the Carboniferous Period, and as

such would not have a high carbon content.  The Cincinnati and Findlay arches rise between the two basins and

several high‐angle normal faults, located north and central in the state, dislocate strata.

The cross sectional map at the base of Figure 1 depicts the underlying rock layers along the bent line from A to

A’. All five Ohio clay deposits are located in the Greenville Province, shown from the angle apex in the cross

section to A’. Due to regional uplift, rock units exposed at the surface increase in age from the east border of

Ohio going west to the center of the state, and then decrease in age mid‐state going west.  

One brick company reported a clay deposit (site 2 in Figure 1) of Precambrian age. However, this is an apparent

error. According to the information available from the Ohio Geological Survey, rocks of this age are not exposed

at or near the surface. Site 2 was located using its latitude and longitude coordinates with Google Maps© aerial

photography. The site 2 location then was transferred to the Ohio geologic map, and found to be in the

Northeastern quadrant of Ohio.  This site is not near known faulting that could explain Precambrian rocks at the

surface. The oldest exposed rocks in the state, consisting mainly of alternating shale and limestone sequences,

occur in strata of the Ordovician system located in the southwest corner of Ohio.3 We therefore conclude that

the site 2 clay deposit is most likely of Pennsylvanian age (potentially the clay from which this shale was initially

                                                            

2 Late Devonian age clastic rocks and clays from these deposits are found in both the Appalachian and the Michigan (older)

basins. However the Michigan basin is located in the Northwest quadrant of Ohio, and although clay deposits are available

in this area, brick companies are not currently using clay deposits from the Michigan basin.    3 The youngest Precambrian rocks are located at depth, and the Ohio Geological Survey reports that rocks of these ages are

igneous and metamorphic rocks over 900 million year old, and include granite, gneiss, schist, amphibolite, and marble.

From the Ohio Geological Survey stratigraphic map, we found that rocks of Precambrian age could only be reached by

drilling through 11 layers of rock accumulated in the Appalachian basin formation in eastern Ohio, which we believe would

be cost‐prohibitive.
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formed was Precambrian.) The remaining companies reported clay deposits of Devonian and Pennsylvanian

ages.   

Mercury Content of Ohio Clays

Ohio clay mercury content showed the concentration of mercury in clays varied widely between sites and even

within the same clay deposit. Sites 1, 3, and 5 were sampled multiple times, and one sample each was obtained

from sites 2 and 4. Eight samples were obtained from four distinct clay sources within site 1, ranging from 15.6

to 114 parts per billion (ppb) mercury (Hg).  Five samples were obtained from site 3, ranging from 32.8 to 132

ppb Hg. Site 3 has the overall highest average mercury content, highest single measurement, and the widest

range of mercury sample results. Four samples were obtained from site 5, ranging from 12.9 to 18.2 ppb Hg.

Only one mercury measurement for Site 4 was provided, and is the lowest single measurement of mercury of

any site of Ohio brick clay deposits. Site 2 mercury content is comparable to that for site 1, a result that is not

apparent from a review of the geological map alone.  

We note that pre‐anthropogenic mercury is deposited geologically through volcanic, igneous intrusive,

hydrogeologic, and other geothermal processes.  While faulting in Ohio is noted, the fault zones are not located

proximate to, or within the Ohio clay deposits. Orogenous processes west (Rocky Mountain range) and east

(Appalachian Mountain range) of the Ohio sediment deposition basins are the likeliest sources of mercury

during the Pennsylvanian period.  

Mercury is readily absorbed by organic matter, and is also known to bond readily with minerals containing

manganese, sulfur, and iron. Chemical assays of the sample were not obtained, but as noted earlier, the Ohio

geological survey reports interspersing of high‐organic content shale with other sedimentary deposits.  

Neither igneous intrusives nor active geothermal vents are noted locally, but volcanic activity, which is known to

be a source of mercury, may be responsible for deposition during this period. Remote volcanic activity during

the Pennsylvanian period could be a potentially significant source of mercury deposition to the deltaic muds,

especially those high in organic matter, a characteristic of the thick deltaic shale sequences of the Pennsylvanian

system.  Proximity to the area of volcanic activity is not necessary for this mechanism to occur. Mercury, emitted

to the atmosphere from volcanic activity outside the area, may remain airborne, carried by air currents, and be

deposited locally.  Erosional processes would also be expected to contribute to migration and accumulation of

mercury in the Appalachian basin sediments, and mercury would be expected to become concentrated

manganese, sulfur, and iron‐bearing minerals, and in areas with accumulated organic matter. Figure 2 is an

aerial photograph of site 3, with dark piles indicating high carbon content at the same site with clays of lighter
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color indicating lower carbon content. The formation of high carbon content sediments during a time of active

volcanism indicates the conditions are right for concentration of mercury in the sediment.  

Figure 1. Geologic Map of Ohio showing the locations of active clay pits used by the US Brick Industry (Ohio GS, 2015.)

5

1

3

4

2
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The diagenesis4 of the rocks could account for a wide range of mercury content of samples within the same

deposit and age. Studies5 of regional geochemistry, particularly of mercury deposition to sedimentary rocks,

explain this process. “The main factors controlling the concentrations of mercury in sedimentary and

metasedimentary rocks are: the presence or absence of volcanism in the source areas and areas of deposition;

the amount of manganese oxides in oxidizing environments or the amount of organic material and sulfur in

reducing environments; the thermal history of the rock; the intensity of regional hydrothermal activity; and the

effect of diagenesis.”

McNeal also suggests a mechanism of mercury enrichment of soils during the weathering process. Mercury

compounds which are stable in the environment may be concentrated by the weathering of a large thickness of

shale into a smaller volume of clays.   

A significant variability of mercury in the clays would be expected as a result of the diagenesis of the shale

sequences and the concentration of mercury by weathering as each layer of shale is exposed and weathered,

forming the clay deposit. Moreover, because

the shale that is a source of the clay was also

previously formed from clay deposits, the

accumulation of mercury from volcanic origin

likely includes earlier geologic periods

potentially in the Cambrian and Precambrian

series. Mercury content in the clay is not

evenly distributed or invariable across a single

clay layer because deposition of organic

matter, to which mercury forms a stable bond,

is not evenly distributed. Minerals associated

with mercury in the oxidizing environment

such as manganese are typically heterogenous

in the host rock, and as a result the mercury

accumulation in the clay from this source is

likely to be variable across the clay deposit.  

   

                                                            

4 Diagenesis in this context refers to processes including alteration by deposition of mercury of volcanic origins, erosional

processes and transmittal of mercury through water borne movement, and geochemical processes in both reducing and

oxidizing environments. 5 “The Geochemistry of Mercury in Sedimentary Rocks and Soils in Pennsylvania”. McNeal, James M. and Rose, Arthur W.

Geochimica et. Coamochimica. Acta. Issue 12, Vol.38, pp.1759 To 1784. December 1974. Pergamon Press.

Figure 2. Light and dark‐colored piles are indicative of a range of organic

content, and of associated mercury content of the clays. Courtesy Google

Earth©, 2015.
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Table 1. Ohio Clay Deposit Mercury Analysis

Hg Content

(ppb)

Geologic

Description

Name Site # Average Hg

content

Age

15.6 Clarion (#4

Shale)

Belden‐ Plants

2,3,4,6,8,9

1 49.4

Pennsylvanian

23

23.5

31.7 Lower

Kittanning (#5

Fireclay)

56.9 50% Clarion

(#4 Shale) &

50%

Brookville

60.4 Brookville (#4

70 Fireclay)

114

39 Lower

Kittanning #5

Whitacre

Greer

2 39.0 Pre Cambrian

(reported)

32.8 Brookville Whitacre

Greer

3 81.3

Pennsylvanian

60.2

67.7

114

132

5.43 Buffalo Shale

#7

Whitacre

Greer

4 5.4

12.9 Shale Glen‐Gery

Caledonia

5 15.3

Devonian

13.1 Glen‐Gery

Iberia

17 Glen‐Gery

Iberia

18.2 Glen‐Gery

Caledonia

   
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OKLAHOMA  

Brick companies extract clays from four locations in Oklahoma, as shown in the geologic map below (Figure 3.)

Three clay deposits accessed by the brick industry (Boral‐Muskogee, site 1; Acme‐Fort Smith, site 2 (located in

Arkansas); and Acme‐Tulsa, site 3) are from Pennsylvanian‐age shales. One deposit (Boral–Union City, site 4) is

from younger Permian‐age shales. As with the clays in Ohio, complete metals assays of the Oklahoma clay

deposits were not obtained.  

Figure 3. Geologic Map of Oklahoma. (Oklahoma GS, 2015.)  

Geological Overview:

The Pennsylvanian Period (300‐324 million years ago) in Oklahoma was characterized by crustal activity marked

by both mountain building and subsidence across the area. Uplifts to areas west and northwest established the

Ancestral Rockies mountain chain. Previously deposited sediments in the Wichita, Arbuckle, and Ouachita Uplifts

became lithified6

, deformed and raised, forming significant mountains, while rapid nearby subsidence created

the Arkoma basin, which received sediments as they eroded from the highlands. 7

  

The cross‐sectional diagrams (Figure 4) along B to B’, and (Figure 5) along E to E’, show the age and configuration

of the strata.

The Arkoma basin, in which sites 1‐3 (Figure 4) are located, is on the outer rim of the Ozark Uplift. These clay

deposits originate from marine shale of mid‐Pennsylvanian age, and the deposits are up to 18,000 feet thick. The

                                                            

6 The term lithified refers to the process by which sediments ‐‐ in this case clay, calcareous deposits, sand, and organic

matter‐‐ become rock, i.e., shale, limestone, sandstone, and coal, respectively. 7 Kenneth S. Johnson, Oklahoma Geological Survey, Educational Publication 9, 2008.  

4

3

1

2
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shale occurs with interbedded sandstone, limestone, conglomerate, and coal. Sites 1 and 2 are bordering

quaternary sand, silt, clay and gravel flood plain. Site 3 does not border this gravel deposit. Sites 1 and 3 are

both located in a structure known as the Claremore Cuesta Plains, and Site 2 is capped by Pennsylvanian age

sandstone. While we generally group sites 1‐3 together geologically, we note that variations within the sediment

sources (mountains) would be expected to yield similar variations in the accumulated sediment. We also note

site 2 is not in Oklahoma. While the brick company may operate out of Oklahoma, the latitude and longitude

coordinates of the clay pit is about two miles across the border into Arkansas.8

  

A shallow inland sea covered most of Western Oklahoma during the Permian period. Layers of shallow‐marine

limestones and gray shales in this area intersperse and gradually give way to high iron content deposits of red

shales and limestones.  Evaporite minerals such as salts and gypsum are also common in the area marked by the

last vestiges of the ancient sea.  Site 4 is located in this area, known as the Anadarko Basin.   

Figure 4.  This cross‐section of the Anadarko basin is the location of site 4 . (Oklahoma GS.)

Mercury content of Oklahoma Shale and Clay Deposits

Thus the clays and shale deposits found in the Arkoma Basin (sites 1‐3) and those found in the Anadarko Basin

(site 4) are of different ages, formed under different conditions, and of different origins.  It is therefore not

surprising to find that the associated mineral assemblages vary widely between these deposits, and may also be

expected to vary within each deposit due to the sediment source and alluvial contribution.

                                                            

8 This political boundary does not affect the geology.

Site 4  
‐ 9 ‐ 

Figure 5. The Arkoma Basin is the location of sites 1, 2, and 3, as shown in this cross section E to E’. (Oklahoma GS)

Mercury samples were fairly consistent within each site, but varied across the Arkoma Basin deposits. Sites 2

and 3 were each sampled and analyzed for mercury twice, but site 1 was sampled and analyzed once. Mercury

content of the deposits in the Pennsylvanian shales runs from 14.7 to 34.4 parts per billion, and are generally in

the same range.   

Among both the Anadarko and Arkoma basins, Site 4, Permian age shale used by the Union City Boral pit

measured the lowest mercury content and the Fort Smith Acme pit at site 2 averaged the highest.  As with the

Ohio clay deposits, contribution to the concentration of mercury in the clays is largely attributed to distant

volcanic activity (in this case, areas to the east and northwest) and to accumulation of stable mercury

compounds in organic material and minerals in the clays as a result of weathering processes. Figure 5 illustrates

visual differences in clays as an indicator of mercury content.  

Table 2. Oklahoma Clay Deposit Mercury Analysis

Hg Content

[ppb]

Geologic

Description Name Site # Average Hgcontent Age

21.7 Shale Mix

Boral‐

Muskogee Plant 1 21.7

Pennsylvanian

34.4

Shale

Acme‐Fort

Smith Plant 2 33.9 33.4

14.7 Acme‐Tulsa

Plant 3 16.7 18.7

1 Shale Mix

Boral‐ Union

City Plant 4 1 Permian

Figure 6 below shows aerial photographs of two different clay deposits in Oklahoma. The Boral plant in Union

City uses a clay/ shale mix that is weathered from Permian age shales high in iron, but lower in organic content,

as evidenced by the red color. The Union City clay averaged 1 parts per billion (ppb) mercury. The clay pit used

by Acme in Fort Smith, Oklahoma is probably higher in organic content (indicated by the dark gray color), and

has a higher mercury content. The Fort Smith deposit averaged about 34 ppb mercury.

Sites 1, 2, and 3
‐ 10 ‐ 

Figure 6. Two clay deposits used by the Brick industry in Oklahoma illustrate how color can be an indicator of geochemistry, organic

content, and mercury content. (Google Earth, 2015.)

   
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References:

1. Oklahoma Geological Survey. “Geologic Map of Oklahoma”.   Kenneth S. Johnson, Educational

Publication 9:2008.

2. Oklahoma Geological Survey.  “Geomorphic Provinces of Oklahoma” Kenneth S. Johnson, Neville M.

Curtis, Jr., William E. Ham. Educational Publication 9:2008.

3. Ohio Geological Survey. “Geologic Map of Ohio”. Slucher and others. Ohio Department of Natural

Resources, Geologic Records Center, Division of Geological Survey.

4. Google Earth, 2015.

5. “The Geochemistry of Mercury in Sedimentary Rocks and Soils in Pennsylvania”. McNeal, James M. and

Rose, Arthur W. Geochimica et. Coamochimica. Acta. Issue 12, Vol.38, pp.1759 To 1784. December

1974. Pergamon Press.  
Appendix 1. Geologic Time Chart