OPAL IN THE OGALLALA FORMATION IN KANSAS' ADA SWINEFORD and PAUL C. FRANKS State Geological Survey, University of Kansas, Lawrence

ABSTRACT Two distinct mineralogic types of opal are found in the various members of the Neogene Ogallala Formation of western Kansas, where the opal occurs in abundant diatom tests, has replaced wood, has partly replaced siliceous roots and seeds, cements feldspathic sandstone, occurs in bentonite beds, and is present as discontinuous massive beds below a surface of late Pliocene and early Pleistocene weathering.

Four modes of origin are recognized: (1) biogenetic precipitation of opal as diatom tests; (2) addition of opal to silica-containing plants; (3) desilication of volcanic glass and precipitation of the silica in underlying rocks as opaline cement and replacement of wood; and (4) desilication in conjunction with calcium carbonate calichification below a late Plioceneearly Pleistocene weathering surface and precipitation of the silica as discontinuous opaline beds in the upper part of the Og-allala Formation. INTRODUCTION

Pleistocene

One of the most abundant forms of nondetrital (secondary, authigenic) silica in Cenozoic sediments is opal. A reconnaissance study of the distribution of opal in Cenozoic rocks in Kansas (the Neogene Ogallala Formation) shows

that opal is, by far, the most common form of nondetrital silica. Opal occurs as two varieties and has several modes

Ogallala Formation

o

of origin. The Ogallala Formation consists predominantly, of fluvial sand, silt, and gravel derived from the Rocky Mountains.

.

Valentine Member

underlies the surface of the High Plains in the western third of Kansas It

and was deposited by eastward-flowing streams in broad valleys cut in Cretaceous

bedrock, eventually producing a coalescent alluvial plain -(Frye, Leonard,

and Swineford, 1956). While Ogallala sediments were being deposited the climate became increasingly arid, starting with a mild humid climate in earliest Neogene and culminating in aridity at least as severe as that in western Kansas today (Frye and Leonard, 1957).

Cretaceous FIG. i

Pictorial generalized stratigraphic

section or Ogallala Formation in western Kansas. Approximate vertical scale: 1 inch = 130 feet. Adapted from Jewett and others (in preparation).

(Moore and others, 1951, p. 20). The Valentine Member, which is reball

tricted to valleys cut in Cretaceous

the Valentine, Ash Hollow, and Kim-

shale and limestone, locally attains a thickness greater than 100 feet. It consists predominantly of gray to greenishgray feldspathic sand, in part poorly indurated by calcium carbonate, and in-

1 Publication authorized by the Director, State Geological Survey of Kansas.

cludes some silt, volcanic ash, bentonitic clay, and opal-cemented sandstone lenses.

The Ogallala Formation in Kansas (fig. 1) is subdivided into three members

111

ADA SWINEFORD AND PAUL C. FRANKS

112

The Ash Hollow Member ranges in thickness from a knife edge to about 200 feet, because of its overlap onto

Cretaceous rocks. Where the entire member is present, its thickness is of the

order of 75 to 200 feet. It is characteristically gray to broWn feldspathic sand,

silt, and gravel, in part irregularly cemented with calcite;

thin lacustrine

limestone; and many lentils of volcanic ash.

The Kimball Member, which. has an average thickness of about 30 feet, is predominantly calcareous feldspathic sandy silt capped by nodular pisolitic caliche.

The presence of deposits of opaline diatoms in the Ogallala Formation of Kansas has been known for many years

(Elias, .1931), but other types of opal were not recognized as such until 1946 (Frye and Swineford, 1946). The purpose of this report is to describe the occurrence, character, and genesis (insofar as possible) of the various opal deposits of the Ogallala Formation.

Aiggt"49-

FIG. 2. Dense opaline sandstone in quarry face, NW% sec. 10, T. 6 S., R. 19 W., Rooks County, Kansas. Note semiconchoidal fracture.

OCCURRENCE AND STRATIGRAPHIC DISTRIBUTION

Opal in the Ogallala Formation occurs chiefly as cement in dense, green to gray, feldspathic sandstone.; massive beds or lentils in association with chal-

cedony and calcite; tests of diatoms; fossil seeds, roots, and stems; multi-colored siliceous wood; and nodules and joint-fillings in bentonite.

Opaline Cement in Sandstone Lentils of dense, opal-cemented, wellsorted coarse-grained feldspathic sandstone

(figs. 2 and 3), ranging from a

few feet to a half mile in lateral extent and from about 1 to 15 feet in thickness, are abundant in the Valentine Member, particularly in north-central and north-

western Kansas. The rock is gray to green and is nearly as hard as true quartzite. It has been described in detail

FIG. 3. Thin section of opal-cemented sandstone from NW1/4 sec. 29, T. 2 S., R.

opaline sandstone lentils commonly are associated with unconsolidated, noncal-

32

by Frye and Swineford (1946). The

W.,

Rawlins

County,

Kansas; plane

polarized light; X 10. 0, opal; Q, quartz; and F, feldspar,

OPAL IN THE OGALLALA FORMATION, KANSAS

113

careous sand and with bentonite beds. Some of the opaline sandstone near the base of the Valentine Member contains angular fragments of silica-soaked chalk

or shale derived from the underlying Upper Cretaceous rocks.

Opal in Silicified Wood Many varicolored silicified logs are

closely associated with and partly imbedded in the Valentine opaline sandstone at a locality in Smith County (SEA sec. 18, T. 2 S., R. 13 W.). Some logs are several feet long and a foot in diameter.

Colors

range

from

white

through yellow, green, and purple, to black. Mineralogically the logs consist predominantly of opal. Chalcedony and montmorillonite are the major impurities. Thin sections of the logs show that most of the cell structure has been de-

stroyed; in a few small areas of some thin sections it is preserved. In one thin section some of the cell walls have been replaced by montmorillonite. Montmoril-. lonite is particularly abundant in the

green parts of the wood; chalcedony is abundant in the white parts. The brown and black shades seemingly are produced by organic matter.

Opal in Bentonite Irregular nodules and joint fillings of

white to gray translucent opal locally are found in beds of bentonite near the base of the Valentine Member. Massive Opal In the Kimball Member light-brown-

ish-gray to light-gray opal forms discontinuous beds (some as thick as 12 feet) in sandy silt a few feet below the top of the Ogallala Formation (fig. 4). Some of the opal is minutely fractured but some is so massive that it has been used locally as building stone. Impurities in this opal include chalcedony, calcite,

scattered detrital grains of quartz and feldspar, and manganese oxide dendrites

(fig. 5). The hardness ranges from 4.5 to 6.0 (Franks and Swineford, in preparation).

FIG. 4. Massive opal in Kimball Member ; ;

SE% SE'/4 sec. 9, T. 4 S., R. 36 W., Rawlins County, Kansas.

Diatoms

Diatoms are known from all three members of the Ogallala Formation. They seemingly are invariably present to a minor extent in gastropod-bearing lacustrine limestones, and they constitute about 20 to 25 percent of certain lentils of diatomaceous marl in the Ash Hollow

Member in northwestern Kansas. The porous character of the opal in the diatom tests is shown in an electron micrograph

of diatoms from the Ash Hollow Member (fig. 6). Seeds, Roots, and Stems

For many years stratigraphers have used opaline fossil seeds in zoning the Ogallala. Hulls of grass seeds and fruits of several forbs and hackberries (fig. 7)

are present in Ogallala rocks, and at least some seeds may be found in almost any exposure (Frye, Leonard, and Swineford, 1956, p..38).

114

ADA SWINEFORD AND PAUL C. FRANKS

-1

A FIG. S. Thin section of impure massive opal from Kimball Member, NE'/4 sec. 16, T. 4 S., R. 36 W., Rawlins County ; shows fine-grained calcium carbonate (appears black), opal, and chalcedony ; x 35. A, plane polarized light. B, crossed nicols.

FIG. 6.

Electron micrograph of diatoms

from diatomaceous marl pit, sec. 11, T. 11 S., R. 38 W., Wallace County, Kansas ; x4500.

Fossil hackberry endocarps (Celtis willistoni) from Ogallala Formation ; X 3.

FIG. 7.

The seeds are nearly 100 percent opal.

OPAL IN THE OGALLALA FORMATION, KANSAS

115

A thin section of part of a hackberry (CcItis willistoni) endocarp is shown in figure 8. The cell walls, which are com-

posed of opal, are clearly visible, but opal also fills some of the space within the cell. Some seeds are lined with minute quantities of calcium carbonate.

Opaline roots or stems, or both, are present locally in all three members of the Ogallala Formation. They generally occur in place in unconsolidated sand and silt. Figure 9 shows some roots from the Kimball Member, and figure 10, a

thin section of a root (or stem) from

the Valentine Member. There are large areas of plant cell walls made of opal, and opal fills the cells. Some thin sections show many regions of clear structireless opal that seemingly has replaced the cell walls. The index of refraction of cell-wall opal, in both roots and seeds, is lower than that of the secondary opal.

FIG. 9. Opaline roots from Kimball Mem-

ber, Ogallala Formation, NEl4 sec. 12, T. 14 S., R. 39 W., Wallace County, Kansas; X TA.

MINERALOGIC VARIETIES OF OPAL

X-ray diffraction studies of opal samples from the Ogallala Formation indicate that the opal comprises two distinct varieties. Patterns A to D inclusive (fig. 11) have similar opaline X-ray reflections that yield a broad peak between 4.8 and 3.7A (18.4 to 24.0° 20) and a smaller peak at about 2.51A (35.7° 20). In addition to the opal reflections, the

FIG.

stem)

10. Thin section of fossil root (or from Valentine Member, Ogallala

Formation; X 35. Cells are walled and filled by opal.

patterns show various quartz and feldspar reflections at 6.51, 4.27, 3.78, 3.35, 3.25, 3.21, 2.46 ( ?), and 2.28A (13.6 20.8, 23.5, 26.6, 27.4, 27.8, 36.5 ( ?), and 39.5° FIG. 8. Thin section of part of fossil hackberry endocarp, showing cell structure preserved by opal; X 35.

20). Patterns E to G inclusive show some of the same quartz and feldspar reflections, but the character of the opal

ADA SWINEFORD AND PAUL C. FRANKS

116 40

30

20

2

10

1 26

1

A

E

40

_L

30

1

20

10

1

2

29

FIG. 11. Diffractograms of samples of opaline rocks and fossils from the Ogallala Formation, Kansas. A, Massive opal, Kimball Member, NE% NW% sec. 5, T. 16 S., R. 34 W., Scott County. B, Opal-cemented feldspathic sandstone, Valentine Member, NE% sec. 21, T. 3 S., R. 12 W., Smith County. C, White opalized wood, Valentine Member, SE% sec. 18, T. 2 S., R. 13 W., Smith County. D, Green opalized wood, showing montmorillonite; same sample as C. E, Diatoms, Ash Hollow Member, sec. 11, T. 11 S., R. 38 W., Wallace County. F, Fossil hackberry seeds (Celtis willistoni). G, Fossil roots, NE'/4 sec. 12, T. 14 S., R. 39 W., Wallace County.

OPAL IN THE OGALLALA FORMATION, KANSAS

117

reflections between 4.8 and 3.7A is significantly different and the reflection at 2.51A is absent. For purposes of this report, the opal shown in patterns A to D is referred to as "low-cristobalite-tridymite type" and that shown in patterns E to G is referred to as "diatom type."

flections, a sharp reflection at 14.7A

Low-Cristobalite-Tridymite Type Pattern A, figure 11, which typifies the low-cristobalite-tridymite type of opal, is

Diatom Type Pattern E, figure 11, is from a sample of diatomaceous marl from the middle of the Ash Hollow Member of the Ogallala Formation. Calcium carbonate was removed from the sample by reaction with cold dilute hydrochloric acid. The diffractogram shows quartz reflections at 4.27A (very weak) and at 3.35A (20.8 and 26.6° 20 respectively) and a weak feldspar reflection at 3.2A (27.7° 20). The opal reflections, however, are distinctly different from those of the disordered low-cristobalite type and appear only as broad swells or diffraction bands at about 9.8A (9° 20) and between 4.9

a diffractogram of a sample of massive opal from the Kimball Member of the Og-allala Formation. A broad swell is

present at about 14A (6.3° 20). The broad peak that extends from 4.8 to 3.7A (18.4 to 24.00 20) has lesser reflections at 4.31 and 4.11A (20.6 and 21.6° 20) also present in the broad peak is a reflection at 4.27A (20.8° 20). In addition, the

diffractogram shows a sharp peak at

3.35A (26.6° 20) and a smaller peak at 2.51A (35.7° 20). Superimposed on the 2.51A peak is a small shoulder at 2.47A (36.3° 20). The reflections at 4.11 and 3.35A are attributed to quartz. Franks and Swineford (in preparation), following work by Flörke (1955a, 1955b), have indicated that the other reflections can be attributed to low-cristobalite that is disordered by cations (e.g., K-1-, Na+,

Ca2+,

and A13-9 that have produced periodically interstratified low-tridymite layers in the low-cristobalite lattice.

Pattern B, from a sample of opal-

cemented sandstone in the Valentine Member of the Ogallala Formation, generally is similar to Pattern A except that numerous quartz and feldspar reflections are superimposed on a weaker opal dif-

fraction pattern. Pattern C, from the

white part of a sample of opaline petrified wood in the Valentine Member, also

is similar to pattern A. The cristobalite reflections at 4.11 and 2.51A and the tridymite reflections at 4.31 and 2.47A, however,

are

much

better

defined

than they are in pattern A. Pattern D, which is from a g-reen part of the same

petrified wood sample, shows, in addition to low-cristobalite and low-tridymite re-

(6.0° 20) and a weak reflection at 4.5A (19.7° 20). These are the 001 and the nonbasal 11 and 02 montmorillonite reflections respectively (MacEwan, 1951, Table IV, 1). On treatment with glycerol, the 001 peak shifted to 18A.

and 3.4A (18 and 26° 20). Pattern F, from a sample of fossil hackberry seed endocarp (Catis willistoni), and pattern G, from a sample of fossil root, exhibit essentially similar opal reflections. The weak quartz reflections in the patterns obtained from the fossil seeds and roots probably can be attributed to incomplete cleaning of the seeds and to grains of quartz that are incorporated in the fossil roots.

Iler (1955, p. 157) has classified diatoms under the heading of amorphous silica, as has Siever (1957, p. 824). Eitel (1957, p. 146) indicates that diatom tests lack the cations necessary to formation of low-cristobalite structure that

is disordered by incorporated tridymite. On cooling from high temperatures, tridymite does not appear unless cations have been added to diatomaceous earth.

Seemingly, therefore, the structure of diatom-type opal is a direct response to

the lack of cations or to the chemical purity of the silica. Further, the shape of the X-ray reflections can be attributed directly to a very low degree of crystallinity.

ADA SWINEFORD AND PAUL C. FRANKS

1 18

Desilication of Volcanic Glass

GENESIS

Massive Opal in Kimball Member Recent work by Franks and Swineford (in preparation) proposed that leaching of the Kimball Member, which is everywhere rich in feldspar and quartz, under conditions of extreme aridity, freed SiO2,

Opal-ccincntcd sandstonc.-Wherc erosion has not removed overlying Valentine rocks the opal-cemented sandstone

of the Valentine Member is overlain either directly by bentonite, or at some

places, as much as 10 feet of coarseK, Na, Ca, and Al, which were rede- grained permeable sandstone separates

posited in the Kimball Member as opal having

a

disordered

low-cristobalite

structure like that described by Flörk., (1955a, 1955b). Ample opportunity for inclusion of K, Na, Ca, and Al in the opal is afforded. Swineford, Leonard and

Frye (1958) have evidence for destruction of both feldspar and quartz by soilforming processes that produced the overlying pisolitic limestone, in which quartz and feldspar, both sodic and potassic, are extensively replaced by calcium carbonate (fig. 12). In addition, deposition of the opal in a "mortar bed" host at the expense of the host probably liberated additional cations that could have been incorporated in the opal.

bentonite from the opal-cemented sandstone. Seemingly there is a direct correlation between bentonite and the presence of opal-cemented sandstone. Alteration of volcanic ash to bentonite probably liberated silica that was transported

downward and redeposited as opaline cement (Frye and Swineford, 1946, p. 57). Sufficient cations to produce a lowcristobalite structure disordered by low-

tridymite could have been derived directly from leaching of the volcanic ash. Further deposition of minor amounts of disordered low-cristobalite-type opal has taken place in shrinkage cracks ( ?) in the bentonite at some localities.

Petrified wood.Petrified wood that

is preserved in the opal-cemented sand-

FIG. 12. A, Photomicrograph shom ing partial replacement of feldspar grain by fine-grained calcium carbonate in caliche, top of Ogallala Formation, sec. 3, T. 14 S., R. 11 W., Russell County, Kansas ; crossed nicols ; x320. B, Photomicrograph showing partial replacement of quartz grain by calcium carbonate in caliche, top of Ogallala Formation, sec. 7, T. 25 S., R. 42 W., Hamilton County, Kansas ; crossed nicols ; X16.

OPAL IN 7'HE OGALLALA FORMATION, KANSAS stone of the Valentine Member shows structural similarities to the low-cristobalite-tridymite type of opal that cements the sandstone, but commonly the X-ray reflections at 4.31 and 4.11A and at 2.51A are even more pronounced than in diffraction patterns of massive Kimball opal (fig. 11, pattern C). Some parts of petrified wood samples also contain montmorillonite in addition to opal (fig. 11, pattern D). Delicate preservation of cell structure in the wood by montmorillonite suggests that the montmorillonite is itself a chem-

ical precipitate or a replacement. Perhaps excessive addition of impurities (possibly under

local

conditions

of

slightly lower pH) to the layered structure of disordered low-cristobalite produced the layered montmorillonite structure.

Biogenetic Silica

Abundant diatom tests are preserved in fresh-water liMestones throughout the Ogallala Formation as a direct result of

diatom metabolic processes. The presence of diatom-type opal in cell walls of

119

endocarps of fossil seeds (Celtis willistoni and Biorbia fossilia) and roots may he interpreted as further evidence of accumulation of opal by living plants.

Chemical analysis (tables- 1 and 2) of Recent (1957) hackberry seeds (Celtis occidentalis) shows that the dried fruit contains 5.51 percent silica by -weight. The silica (along with about 46 percent CaCO3 is localized in the endocarp and silica approximates 9 percent, by weight, of the endocarp. As Der (1955, p. 277) indicates, numerous plants commonly contain silica, and in some the silica

forms a past of the plant structure, as in Equisetum.

Although about 9 percent silica is present in the endocarp of present-day hackberry seeds, the only substance detected in diffraction patterns was aragonite. Perhaps 9 percent amorphous silica would not be detectable in the patterns,

or it may occur as part of an organic complex.

If, as previously suggested, the diatom

type of silica structure is a function of the absence of cations, it may he pos-

TABLE 1.Chemical analysis of 10 hackberry seeds (Celtis occidentalis) from Manhattan, Kansas. Weight of 100 seeds= 15.3 grams (Analysis by Russell T. Runnels of the Geochemistry and Industrial Minerals Division of the Kansas Geological Survey) Raw Seeds (wt. %) Si 02

NI-140H ppt,

Total CaO

Calculated Compounds (wt. %)

5.51

0.86 26.64

5.51

CaCO3

excess CaO MgO

0.32

P205

0.47 0.17 0.05 4.31 41.76

K20 Na20 LOI 105°C (H20) LOI 105-600°C (org) LO1 600-1000°C (CO2)

Cl-

Total 1

is not included in total.

20.01 0.331

100.10

Ca phosphate MgO MgC12

See calcium KC1 NaC1

H20 organic See calcium See Mg, K, Na

0.86

45.51 0.52 1.09 0.23

0.20

0.27 0.09 4.31 41.76

100.32

ADA SWINEFORD AND PAUL C. FRANKS

120

TABLE 2.Qualitative spectrographic analysis of ash of 100 hackberry seeds (Celtis occidentalis) from Manhattan, Kansas (Analysis by W. E. Hill of the Geochemistry and Industrial Minerals Division of the Kansas Geological Survey) Minor

Trace

Small Trace

Si

Al

Mg Ca

Mn

As V

Zn

Major

Fe

Ti

Cu

Ni Cr W.

SUMMARY

Two distinct mineralogical varieties of opal occur in the Ogallala Formation of

western Kansas. The amorphous silica type is produced (directly or indirectly) by the growth processes of certain plants

and diatoms. There are two modes of

origin for the disordered low-cristobalite variety : (1) solution and reprecipitation of silica by soil-forming processes near the close of Ogallala time ; and (2) desilication of volcanic ash (vitric tuff) to bentonite in Valentine time and precipi-

tation of the silica as opal cement and replacement of wood.

tulated that any available cations were used or rejected by the plants in their metabolism, and that the silica of the cell walls is a residual accumulation. The fossil hackberry endocarps con-

ACKNOWLEDGEMENTS

We are indebted to Donald E. Wil-

tain about two and one-half times as

liams, Long Island, Kansas, for providing a sample of opal in bentonite, and to Mrs. Arno Windscheffel, Smith Cen-

much silica as do the present-day endocarps. Furthermore, some of the silica in the fossil seeds (and roots) fills space within the cell. This secondary silica also is of the diatom type.

opalized-wood locality in Smith County. Thanks are expressed to A. Byron Leonard for photomicrographs of a .root (or stem) and Celtis willistoni.

ter, Kansas, for directing us to the

REFERENCES EITEL, WILHELM, 1957, Structural anomalies in tridymite and cristobalite: Am. Ceramic Soc. Bull., v. 36, pp. 142-148.

ELIAS, M. K., 1931, the geology of Wallace County, Kansas: Kansas Geol. Survey Bull. 18, 254 pp.

FLÖRKE, O. W., 1955a, Zur Frage des Hochcristobalit in Opalen, Bentoniten und Gläsern: Neues Jahrbuch für Mineralogie, Monatshefte, Heft 10, pp. 217-223 . , 1955b, Strukturanomalien bei Tridymit und Cristobalit: Berichte der Deutschen Keramischen Gesellschafte, v. 32, pp. 369-381.

FRANKS, P. C., and SWINEFORD, ADA, Character and genesis of massive opal in Kimball Member, Ogallala Formation, Scott County, Kansas: In preparation.

FRYE, J. C., and LEONARD, A. B., 1957, Ecological interpretations of Pliocene and Pleistocene stratigraphy in the Great Plains region: Am. Jour. Sci., v. 255, pp. 1-11.

LEONARD, A. B., and SWINEFORD, ADA, 1956, Stratigraphy of the FRYE, J. Ogallala C.'Formation (Neogene) of northern Kansas.. Kansas Geol. Survey Bull. 118, 92 pp.

FRYE, J. C. and SWINEFORD, ADA, 1946, Silicified rock in the Ogallala Formation: Kansas Geol. Survey Bull. 64, pt. 2, pp. 33-76.

ILER, R. K., 1955, The colloid chemistry of silica and silicates: Cornell University Press, Ithaca, New York, 324 pp.

JEWETT, J. M., and OTHERS, Graphic column of Kansas rocks: Kansas Geol. Survey, chart, in preparation.

MAcEWAN, D. M. C., 1951, The montmorillonite minerals: in X-ray identification and crystal structures of clay minerals. The Mineralogical Society, London, pp. 86-137. MOORE, R. C., FRYE, J. C., JEWETT, J. M., LEE, WALLACE, and O'CONNOR, H. G., 1951, The Kansas Rock column: Kansas Geol. Survey Bull. 89. 132 pp. SIEVER, RAYMOND, 1957, The silica budget in the sedimentary cycle: Am. Mineralogist, v. 42, 821-841.

SWINEFORD, ADA, LEONARD, A. B., and FRYE, J. C., 1958, Petrology of the Plio-

cene pisolitic limestone in the Great Plains : Kansas Geol. Survey Bull. 130, pt. 2, pp. 96116.