Short Papers on Geologic Subjects. the Mt. Simon Sandstone In

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STATE OF ILLINOIS AI>LAI E. STEVENSON, Conmor DEPARTMENT OF REGISTRATION AND EDUCATION

NOBLE J. PUFFER, Director

DIVISION OF THE STATE GEOLOGICAL SURVEY

CIRCULAR NO. 170

SHORT PAPERS ON GEOLOGIC SUBJECTS

The Mt. Simon Sandstone in Northern Illinois

By J. S. Templeton

Petrology of Basal High-Purity Bed of the Burlington Limestone

By Donald L. Graf

Preglacial Gravels in Henry County, Illinois By Leland Horberg

The Neda Formation in Northeastern Illinois

By L. E. Workman

REPRINTED FROM THE TRANSACTIONS OF THE ILLINOIS STATE ACADEMY OF SCIENCE

VOL. 43, pp. 151-164, 171-182, 1950

PRINTED 1SY AUTHORITY OF THE STATE OF ILLINO:

URBANA, ILLINOIS

1951

ILLINOIS GEOLOGICAL SURVEY LIBR£* V

JUN 26 1951

Illinois Academy of Science Transactions, Vol. 43, 1950

THE MT. SIMON SANDSTONE IN NORTHERN ILLINOIS 1 JUSTUS STEVENS TEMPLETON, JR. Illinois Geological Surrey, Vrbana

Wells in northern Illinois pene- er studies the red elastics of north- trate from 1600 to 2100 feet of upper ern Illinois and similar strata in Cambrian Mt. Simon sandstone be- eastern Wisconsin were stated to be tween the Eau Claire formation and of unknown age but were not sepa- pre-Cambrian crystallines. In most rated from the Mt. Simon formation of the region the sandstone is pure (Thwaites, 1931, p. 742; Twenhofel, and light colored, but in the vicinity Raasch and Thwaites, 1935, p. 1693, of Boone County all but the upper- pi. 151). Possible correlation of the most beds are silty, argillaceous, and arkosic basal Mt. Simon strata of dark red. The light-colored sand- southern Lee County, Illinois, with stone long has been correlated with the supposedly Keweenawan Hinck- the Mt. Simon formation of north- ley sandstone of eastern Minnesota western Wisconsin, but recently the has been suggested (Payne, 1942, red elastics were assigned provision- p. 54). Recently the red elastics of ally to the pre-Cambrian and cor- northern Illinois were correlated ten- related with the Fond du Lac forma- tatively with the supposedly pre- tion of eastern Minnesota. The pres- Cambrian Fond du Lac formation ent study shows that the red elastics of eastern Minnesota (Bays and of northern Illinois are a local fa- others, 1945, p. 1146; Weller and des of the light-colored Mt. Simon others, 1945), and the regional lith- sandstone. On the basis of differ- ology and thickness of the "Mt. Si- " ences in grain size, the formation is mon-Fond du Lac ( ? ) sandstones divisible into seven regionally per- were summarized (Workman and sistent members. Evidence is pre- Bell, 1949, pp. 2041-2043). In the sented which suggests that the Mt. present study all available cuttings Simon sandstone of Minnesota and from wells in central northern Illi- the upper part of the Mt. Simon at nois which penetrated the Mt. Simon " the type locality belong to the Eau and "Fond du Lac ( ?) formations Claire formation. were carefully examined, as well as Previous and present studies.— samples from deep borings in part Early subsurface studies in northern of northeastern Illinois. Illinois referred all sandstone be- Name and definition.—At the type neath Eau Claire dolomitic strata locality the Mt. Simon formation, to the Mt. Simon, although a pos- named from exposures on and near sible Keweenawan age for the red Mt. Simon at Eau Claire, northwest- elastics at Dixon was suggested ern Wisconsin (Ulrich, 1914, p. (Thwaites, 1923, pp. 534, 553-555). 354), consists mainly of coarse- Thwaites' tentative Keweenawan grained, partly conglomeratic, thick- correlation at Dixon was rejected by bedded sandstone 234 feet thick, Knappen (1926, pp. 34-36). In lat- which overlies pre-Cambrian granite and underlies fine-grained thin-bedded Cedaria-be&ring Eau Claire Illinois Academy of Science Transactions

sandstone (Twenhofel, Raaseh and northeastern Wisconsin and south- Thwaites, 1935, pp. 1693, 1739-1740). western Ontario (Cohee, 1945, 1948). In this report the name Mt. Simon is Lithology.—In northern Illinois applied to all sandstone between pre- the Mt. Simon sandstone is divisible Cambrian crystallines and sediments into a light-colored facies, which con- which are classed as Eau Claire but stitutes the bulk of the formation in probably underlie the Cedaria zone. most of the region, and a red facies, Thickness.—In northern Illinois which is best developed at Belvidere, only four wells (fig-. 1, wells 5, 7, 10, Boone County. Much gradation and 11) have completely penetrated the interfingering characterize the trans- Mt. Simon sandstone, although two ition zone between the two facies, other wells (6, 9) are believed to which in places is less than 6 miles have passed almost through the for- wide (fig. 2, wells 4, 5). Both facies mation. The sandstone is thickest are nonfossiliferous. in a basin extending from Light-colored light- DeKalb facies.—The , Illi- to Cook counties in northeastern colored Mt. Simon facies is white, ; nois and attains a maximum known yellow, or pink to light brown. The thickness of 2120 feet in northeast- sandstone ranges from very fine to

ern DeKalb County. From this area very coarse grained and locally is

it thins rapidly to thicknesses of 400 silty. Much of it is conglomeratic, I feet or less in most of the states sur- containing quartz granules from 2 rounding Illinois, and lenses out in to 4 mm. in diameter and quartz peb- Mt. Simon Sandstone in Xoiilicrn Illinois

Mt. Simon formation from (Line A-A', fig.

bles from 4 to 8 mm. in diameter. As large amounts of disseminated dark- a whole the sandstone is coarser red hematitic clay and silt and by grained than any other in northern the presence of relatively thick in- Illinois. Sorting generally is poor, terbeds of dark-red, partly micac- but some well-sorted beds are pres- eous, partly sandy shale and silt- ent. Although the grains range from stone. Where weakly developed, as angular to well rounded, they are in southern Lee County, the facies chiefly subangular. The granules is distinguished mainly by hematitic and pebbles are well rounded. Most films on the sand grains. of the sandstone is incoherent, but Subdivision into members. — thin layers have been cemented by Throughout northern Illinois the iron oxide or silica. A zone varying Mt. Simon sandstone consists of a from 15 to 249 feet thick at the base cyclic alternation of relatively fine- of the formation is arkosic and local- grained units and coarse-grained, ly contains biotite and muscovite granule-bearing units (figs. 2, 3). flakes as well as granite grains and Although to the southeastward and pebbles. Occasional grains of feld- westward granules become scarcer spar and granite and very rare gab- and smaller, and the entire forma- bro and quartzite pebbles have been tion grows more silty and finer observed in higher beds. Thin layers grained, the sequence still is clearly of dark-red or variegated, partly distinguishable. Some lateral grada- micaceous shale and siltstone occur tion or interfingering may take place at different horizons. along the contacts between the units. Red units are ; fades.—Where best developed The seven major present •the red facies is characterized by herein regarded as members, and are j .

Illinois Academy of Science Transactions

Illinois, to the vicinity

named and described in ascending derived from the Crane School, order. The members are made up of NWi/4 NEi/4 NW14 Sec. 22, T. 40 numerous subordinate units, many N., R. 5 E. DeKalb County, 4 miles of which extend practically through- south of the type well. The "type out the region. section" consists of samples 387 The type well for the four lower through 491 in sample set 1301, ex- members is Wyman No. 1 (well 7), tending from depths of 3105 to 3845 a cable-tool well in the NE}£ NEi/i feet. The member ranges from 620

SE*4 Sec. 35, T. 41 N., R. 5 B., De- to 740 feet thick (figs. 2, 3). The

Kalb County, Illinois, sample set grains range from very fine to very , 1301. 2 The type well for the three coarse, and a few granules are rarely upper members is McQueen No. 1 present, but the predominant grades • (well 6), a cable-tool well in the are fine and medium. The lower por- j

SW% NE% NE% Sec. 27, T. 42 N., tion of the member generally is j

R. 3 E., DeKalb County, Illinois, more or less shaly, silty, and arkosic. ' sample set 1466. Kenyon member.—Por the thin Crane member.—For the relative- conglomeratic sandstone overlying ; ly fine-grained basal member of the the Crane member the name Kenyon Mt. Simon formation the name Crane member is proposed. The name is . member is proposed. The name is derived from Kenyon school, NE}4 /

2 of set in subsurface files of NWy Sec. 17, T. 40 N., E. Number sample 4 NW% j; Illinois (ieolojrical Survey. Most samples in both 1 sets represent five- or ten-foot intervals. 6 E., Kane County, 3^ miles south- ! Mt. Simon Sandstone in Northern Illinois 155

east of the type well. The "type sec- made up of finer-grained nonconglo-

' tion ' consists of samples 367 meratic sandstone are from 10 to 90 through 386 in sample set 1301, ex- feet thick. In the McElroy well (11) tending from depths of 2975 to 3105 most of the sequence is conglomer- feet. The member ranges in thick- atic, but in the Taylor and Parish ness from 34 to 130 feet. Where less wells (5, 9) granule beds are very than 80 feet thick, it is composed subordinate. chiefly of coarse-grained sandstone Lacey member.—The fifth Mt. Si- containing quartz granules, but mon member consists principally of where thicker it consists of conglo- conglomeratic sandstone. It is here meratic sandstone interbedded with named the Lacey member from La- nonconglomeratic layers. cey School, NWy4 NW14 NW14 Sec. 31, T. 42 N., R. 4 E., DeKalb Lovell member.—The succeeding County, 2y miles east of the type relatively fine-grained unit is here 2 well. The "type section" consists of named the Lovell member for Lovell samples 257 through 278, in sample School, S-Wi/4 NWy4 NEi/4 Sec. 2, set 1466, extending from depths of T. 40 N., R. 5 E., DeKalb County, 1880 to 2070 feet. The thickness of three-quarters of a mile southward the unit ranges from 176 to 230 feet. from the type well. The "type sec- In most wells a bed or beds of com- tion" consists of samples 348 paratively fine-grained nonconglo- through 366 in sample set 1301, ex- meratic sandstone are present in the tending from depths of 2850 to 2975 middle of the member, and in the feet. In thickness the member varies Parish and McElroy wells (9, 11) from 65 to 190 feet. Although grain interbeds of fine-grained sandstone sizes range from very fine to very occur in the lower part of the unit. coarse, the predominant grade is fine Red shale layers commonly are more or medium. In the McElroy well abundant in the Lacey and higher (11) the member is partly coarse members than in the underlying grained to conglomeratic but is clear- strata. ly separable from the adjacent Gunn member.—The Lacey mem- coarser-grained units. ber is overlain by a relatively fine- May-field member.—Overlying the grained unit here named the Gunn Lovell member is a thick sequence member from Gunn School, NW^ of interbedded conglomeratic and NWi/4 NW14 Sec. 16, T. 42 N., R. nonconglomeratic sandstones for 3 E., DeKalb County, 2% miles which the name Mayfield member is northwest of the type well. The proposed. The name is obtained "type section" consists of samples from Mayfield Township, T. 41 N., 234 through 256 in sample set 1466, R. 4 E, DeKalb County, Illinois, 5 extending from depths of 1648 to miles west of the type well. The 1880 feet. In thickness the member "type section" consists of samples varies from 71 to 260 feet. The grain 300 through 347 in sample set 1301, size ranges from very fine to very extending from depths of 2495 to coarse, and a few thin beds of gran- 2850 feet. The thickness of the mem- ule conglomerate generally are pres- ber ranges from 145 to 390 feet. The ent. However, the grain size is con- units composed of very coarse- spicuously finer than in the adjacent grained to conglomeratic sandstone members, and in several wells the

| are from 5 to 125 feet thick; those dominant grade is fine. 156 Illinois Academy of Science Transactions

Charter member — The coarse- which is distinguished from the un- grained to conglomeratic uppermost derlying Mt. Simon sandstone by its member of the Mt. Simon formation finer grain-size and the absence of is here named Charter from Charter granules. Both the sooty and the Oak School, SB% SE*4 SE^4 Sec. yellow sandstone locally contain obo-

2, T. 42 N., R. 3 E., DeKalb County, loid brachiopod fragments. 3% miles north of the type well. The At the type section in Wisconsin "type section" consists of samples the Mt. Simon sandstone shows the 204 through 233 in sample set 1466, following sequence from the base extending from depths of 1381 to upward: (1) granule-bearing, 3 feet, 1648 feet. The member ranges from (2) without granules, 28 feet, (3) 145 feet to 315 feet thick. In some granule-bearing, 108 feet, and (4) wells it is composed almost entirely without granules and containing of very fine- to very coarse-grained oboloid brachiopod fragments, 95 sandstone which is mainly coarse feet (Twenhofel, Raasch and grained and has one or more thin Thwaites, 1935, pp. 1739-1740). It layers of granule conglomerate. In is thought most likely that units 1 other wells conglomeratic sandstone through 3 correspond, respectively, makes up most of the member. to the Lacey, Gunn, and Charter' Stratigraphic relations.— The Mt. members and that unit 4 is equiva- Simon sandstone rests unconformab- lent to the basal Eau Claire sand- ly on a pre-Cambrian basement com- stone of northern Illinois. Definite plex. Where it has been penetrated correlations must await detailed sur- by wells in northern Illinois, the face and subsurface tracing between complex consists of granite with lo- Beloit and Eau Claire, Wisconsin. cal felsite dikes (Grogan, 1949). However, if this interpretation is The Mt. Simon is overlain by the correct, it would appear desirable Eau Claire formation with apparent to include unit 4 of the Mt. Simon conformity in both Wisconsin and type section in the Eau Claire for- Illinois (Twenhofel, Raasch and mation because this unit (1) can be Thwaites, 1935, pp. 1693-1696, 1714- distinguished lithologically by the 1715). lack of granules, (2) is said to con- Correlation.-—In Kane County, tain marine fossils, in contrast to northeastern Illinois, the basal two- the barren, presumably fresh-water fifths of the Eau Claire formation beds beneath, and (3) grades east- is composed mainly of sandy dolo- ward into dolomite which is entirely mite and shale which contains obo- similar to the dolomite facies of the loid brachiopods in the lower half Eau Claire formation.

(fig. 2) . Westward these beds grade The sandstone termed "Mt. Si- laterally into gray, very fine- to mon" in eastern Minnesota directly coarse-grained sandstone which is underlies the Eau Claire Cedaria given a sooty aspect by enerusting zone and in places contains much particles of black pyrite (Workman glauconite and glauconitic shale and and Bell, 1949, pp. 2043-2049). The siltstone (Atwater and Clement, sooty sandstone extends into west- 1935, pp. 1674-1676), which in Illi- ern Illinois and northward to Win- nois is entirely confined to the Eau nebago County. However, before Claire formation. Most of this sand- reaching Beloit, Wisconsin, it passes stone probably corresponds to the laterally into light yellow sandstone basal Eau Claire sandstone of north-' .

Mt. Simon Sandstone in Northern Illinois 157

Table 1. Typical Heavy Mineral Analyses in Percentages

I | | § 1 J < < 3 1 P3 'f- z

6 4 31 59 1 3 27 68 1 Tr. 2x 62 9a 11 11 69 20" Tr. 40 20 20 b 5 1 10 76 Hinckley sandstone at Sandstone 7 2 3 C 18 Tr. d 2 12 63 Tr 8 c d Hinckley sandstone at Net River 1 l "7 12 44 1 13 27 Tr. 50 A 23 13 4 ? 15 5 65 15 c C R C P P P C

iWell 7, 1710'-1720' (Herbert, 1944); cept in the basal arkosic zone, and aCharter member, Well 7, 1730'-1750' present only in very low percentages (Herbert, 1944) ; sQrane member, Well 7,

379C-3805' (Herbert, 1944) ; *Crane mem- in the Mt. Simon of the type area 3835'-3840' (Herbert, 1944); ber, Well 7, (Stauffer and Thiel, 1941, p. 18, 5Crane member, Well 5, 2900'-2905' (Marsden, 1933); eQunn member, Well table 1). 5, 1645'-1650' (Herbert, 1944); ^Tyler The contact between the Hinckley sample P438B; and others, 1940, p. 1512, and the underlying red Fond du Lac sTyler and others, 1940, p. 1512, sample P423B; sTyler and others, 1940, p. 1508, formation is gradational (Stauffer sample P405; loWell 5, 2955'-2960' and Thiel, 1941, p. 17). The chief (Marsden, 1933); "Well 11, 3760'-3772' differences between the two sand- (Winchell, 1932; Grogan, 1949, pp. 98-99); aangular; bunzoned; concludes stones are (1) the higher iron-oxide, zoisite; dincludes ilmenite; ezoned; shale, and feldspar content of the A = abundant but not included in anal- du Lac, and the presence ysis; C:= common; P = present; R = Fond (2) rare; Tr = trace. of apatite and garnet and a higher percentage of leucoxene in the Fond em Illinois, although true Mt. Simon du Lac heavy mineral suite. It strata locally may be included. seems most likely that the Fond du Except that it is indurated, the Lac is an arkosic, impure, basal type Hinckley sandstone is exceed- facies of the Hinckley formation, the ingly similar to the light-colored Mt. heavy minerals of which were de- Simon sandstone of northern Illinois rived from sources that later became (Atwater and Clement, 1935, pp. buried. An analogous situation oc-

1669-1672, 1680 ; Stauffer and Thiel, curs in northern Illinois where the 1941, pp. 15-23). The heavy mineral heavy mineral suite of basal Mt. suites are fairly similar (table 1), Simon strata locally differs from although the Hinckley generally has that of the higher beds (table 1) higher proportions of magnetite, il- Raasch (1950) has presented evi- menite and leucoxene and lower pro- dence for the Cambrian age of the

1 portions of tourmaline and zircon. Hinckley and Fond du Lac forma- Feldspar is rare in the Hinckley, tions. The writer tentatively corre- ,rare in the Mt. Simon of Illinois, ex- lates the entire Hinckley-Fond du :

Illinois Academy of Science Transactions

Lac sequence with the Mt. Simon north and was transported to the sandstone of northern Illinois. Fur- sea by streams. The general lack of ther study is needed to determine rounding, frosting, and sorting indi- whether it has the same succession cates that the sand was not greatly of conglomeratic and finer-grained reworked before deposition. The members. Since no lithologic or differences between the heavy miner- physical evidence has been found for al suites of the Mt. Simon and the a formational break in the northern underlying granite in northern Illi- Illinois sequence, the name Mt. Si- nois (table 1) and the partial round- mon is applied throughout. ing of most Mt. Simon heavy min- The Mt. Simon sandstone of north- erals show that the bulk of the basal ern Illinois also is considered equiv- sand was not derived from the gran- alent to the Bayfield and Jacobsville ite of that region. However, ero- sandstones of the south shore of sion of the granite contributed angu- Lake Superior (Raasch, 1950) and lar feldspar fragments to the ini- to the Lamotte sandstone of eastern tial sediments. The arkosic zone Missouri (Workman and Bell, 1948, probably is thickest in the areas of pp. 2041-2043, figs. 2, 3). greatest basement relief, as adjacent to monadnocks. That a minor part Historical Interpretations of the basement in northern Illinois or nearby areas consisted of schist In Wisconsin the pre-Cambrian is demonstrated by the presence of surface appears to have been a pene- angular garnets in the heavy min- plain interrupted by monadnocks. eral suite of basal Mt. Simon strata

The sparse data provided by wells in the Taylor and Wyman wells (5, in northern Illinois also suggest that 7) and by the occurrence of small the basement surface had slight re- schist fragments in the Mt. Simon of lief. the Parish well (9). It has been suggested that the Mt. In an area northward from Illi- Simon sandstone was rapidly depos- nois a red clayey mantle probably ited in a shallow, fresh- or brackish- developed on granitic rocks under water inland sea ( Twenhofel, Raasch conditions of low relief and a warm, and Thwaites, 1935, pp. 1714-1715). humid climate, such as prevail at The fact that the sandstone seems to present in the southern Appalach- be thickest in northern Illinois in- ian piedmont. It is believed that the stead of thickening southward like Mt. Simon red facies was deposited most other Paleozoic formations sup- off the mouth of a river which ports this concept. Rapid sinking drained this area and discharged of the basement must have accom- into the sea north of Belvidere, panied deposition to produce by far Boone County. The similarities in ! the thickest formation in Illinois. the heavy minerals of the red and ,

Periodic uplifts of the surrounding light-colored facies suggest that both < borderlands sent influxes of pebbly were derived from the same gran- ; sand into the sea. Nonconglomeratic itie terrane. Enlargement of the ;

sand was laid down during intervals sea just prior to the marine invasion . of crustal quiescence. of Eau Claire time (Twenhof el, Probably the Mt. Simon quartz Raasch, and Thwaites, 1935, pp. f] sand was derived mainly from weath- 1714-1715) is reflected in abrupt !J ered granites and gneisses to the overlap of the red sediments by a'. Mt. Simon Sandstone in Northern Illinois 159 relatively small thickness of white Acknowledgments. — The writer sandstone. Persistence of a drainage expresses sincere appreciation to system which furnished red elastics H. B. Willman, L. E. Workman, and is suggested by the recurrence of a G-. 0. Raaseh, all members of the similar red facies in the Franconia Illinois Geological Survey staff, for and uppermost Eau Claire strata of critical review of the manuscript Boone County. and for much helpful advice.

REFERENCES

Atwater, G. I., and Clement, G. M., Streator quadrangles: Illinois Geol. 1935, Pre-Cambrian and Cambrian re- Survey, Bull. 66. lations in the upper Mississippi val- Raasch, G. O., 1950, Current evaluation ley: Geol. Soc. Amer. Bull., vol 46, no. of the Cambrian-Keweenawan prob- 11, pp. 1659-1686. lem: Trans. 111. State Acad. Sci., vol. Bays, C. A., and others, 1945, Strati- 43, pp. 131-143. graphy and structure of northeastern Stauffer, C. R., and Thiel, G. A., 1941,

Illinois (abstract) : Geol. Soc. America The Paleozoic and related rocks of Bull., vol. 56, no. 12, pt. 2, pp. 1146- southeastern Minnesota: Minnesota 1147. Geol. Survey, Bull. 29. Cohee, G. V., 1945, Sections and maps of Thwaites, F. T., 1923, The Paleozoic lower Ordovician and Cambrian rocks rocks found in deep wells in Wiscon- in the Michigan basin, Michigan and sin and northern Illinois: Jour. Geol. adjoining areas: U. S. Geol. Survey, vol. 31, no. 7, pp. 529-555.

Oil and Gas Inv., Prelim. Chart 9. , 1931, Buried pre-Cambrian of

, 1948, Cambrian and Ordovician Wisconsin: Geol. Soc. Amer. Bull., rocks in Michigan basin and adjoin- vol. 42, no. 3, pp. 719-750. ing areas: Amer. Assoc. Petrol. Geo- TWENHOFEL, W. H., RAASCH, G. O., AND logists Bull., vol. 32, no. 8, pp. 1417- Thwaites, F. T., 1935, Cambrian strata 1448. of Wisconsin: Geol. Soc. Amer. Bull., Grogan, R. M., 1949, Present state of vol. 46, no. 11, pp. 1687-1744. knowledge regarding the pre-Cam- Tyler, S. A., and others, 1940, Studies brian crystallines of Illinois: 111. of the Lake Superior pre-Cambrian by Acad. Sci. Trans., vol. 42, pp. 97-102; accessory mineral methods: Geol. Soc. 97- Illinois Geol. Survey, Circ. 157, pp. Amer. Bull., vol. 51, no. 10, pp. 1429- 102. 1537. Herbert, Paul, 1944, in Bays, C. A., and Ulrich, E. O., 1914, in Walcott, C. D., others, Stratigraphy and structure of Cambrian geology and paleontology, northeastern Illinois with special ref- pt. II, no. 13, Uikchxi'iihttlus and other erence to groundwater: unpublished genera of the Dikelocephalinae: manuscript in technical files of Illinois Smithsonian Misc. Coll., vol. 57, p. 354. Geological Survey. Wellee, J. M., and others, 1945, Geo- Knappen, R. S., 1926, Geology and min- logic Map of Illinois: Illinois Geol. eral resources of the Dixon quad- Survey. rangle: Illinois Geol. Survey, Bull. 49. Marsden, R. W., 1933, A study of the Winchell, A. N., 1932, letter to L. E. accessory minerals of a drill record Workman. from northern Illinois: unpublished Workman, L. E., and Bell, A. H., 1948, report in technical files of Illinois Deep drilling and deeper oil possi- Geol. Survey. bilities in Illinois: Amer. Assoc. Payne, J. N., 1942, in Willman, H.B. ,and Petrol. Geologists Bull., vol. 32, no. 11, Payne, J. N., Geology and mineral re- pp. 2041-2062; Illinois Geol. Survey, sources of the Marseilles, Ottawa and Rept. Inv. 139. ;

160 Illinois Academy of Science Transactions, Vol. 43, 1950

PETROLOGY OF THE BASAL HIGH-PURITY BED OF THE BURLINGTON LIMESTONE*

DONALD L. GRAF State Geological Survey, Urbana

A high-purity, virtually chert-free Dolomite rhombs occur mostly in limestone occurring near the base of the fine-grained interstitial carbon- the Burlington limestone in western ate component, and are thus com- Illinois is being mined or quarried monest in fine-grained varieties of at Quincy, Gladstone, and Pearl. the rock. This article presents the results of History of Events an initial study of the petrology of this 10 to 25-foot unit. Twenty thin The history suggested by the thin- sections, perpendicular to the bed- section study is indicated in table specimens ding,.were prepared from I. In the first stage crinoid frag- collected at the localities mentioned, ments were deposited with a mud especially the Quincy area. containing abundant fossil frag- The specimens are mixtures, in ments. The interstitial material be- varying proportions, of five princi- tween the crinoid fragments con- large frag- pal components: (1) tains far too much debris and is too ments of crinoid plates and stem fine grained to have crystallized segments, which are inclusion-rich, unit-extinguishing sheets of calcite; (2) secondary white calcite, deposited in optical continuity upon the detrital fragments; (3) moderate to fine-grained fossil detritus, includ- ing fragments of bryozoa, pelecy- pods, brachiopods, and smaller bits of crinoid plates and stem segments (4) very fine-grained dark gray in-

terstitial calcite ) unit- ; and ( 5 small extinguishing rhombs of dolomite. Coarsely crystalline specimens of the limestone consist chiefly of the secondarily enlarged pieces of cri- noidal calcite. Finely crystalline varieties show pronounced banding in thin section, with different bands varying both in components and in particle size. The banding, in general, parallels the bedding planes of the rock, but in detail it may be con- torted or lenticular at some places. Fig. 1.—Subparallel alignment of smal- * ler detrital fragments. Parallel nicols, Published h\ ) ..-i in issi. u i uf the Chief, Illinois State Geological Survey. 20 X. ;

Petrology of the Burlington Limestone 161

from circulating' water. A streak- that are coarsely crystalline aggre- iness in the fine-grained portion of gates (fig. 3). The deposition of the interstitial material arises in secondary white calcite has been so part from the presence of elongate, extensive in some pla'ces as to cut laminated pieces of pelecypod shells, across and isolate areas of streaked and, almost certainly, in part from interstitial material. In these areas the original deposition of elongate rearrangement of particles by flow- pieces of fossil detritus in planes age would have been unlikely with- essentially parallel with the ocean out some disruption of the second- floor. However, a "streamline" ef- ary calcite, which has not been seen fect seen in some places (fig. 1), in which sub-parallel particles diverge to go around larger particles, suggests local flowage during compaction which accentuated the other two features. Secondary white calcite, free of

Fig. 3.—Secondary calcite rims on adjacent grains grown together to form a coarse-grained rock, with small rem- nant areas of dark fine-grained interstitial calcite. Parallel nicols, 20 X.

hence deposition of secondary calcite is placed after compaction and

Fig. 2.—Portion of crinoid stem seg- The margins of many of the sec- ment (a), secondary calcite rim (b), ondarily enlarged calcite sheets in vein-like extension of rim (c), "island" (d), and fine-grained aggregate inter- contact with interstitial carbonate preted as result of late partial destruc- are ragged, and the question arises tion of vein-like extension (e). Parallel nicols, 135 X. as to which calcite is replacing the other. The fact that there are out- continuity upon the large, inclusion- lying islands of secondary white cal- rich, unit-extinguishing sheets of calcite in optical continuity with larger cite in crinoid plates. Less common- areas (fig. 2) suggests that the finely (fig. 2) it forms vein-like exten- grained interstitial calcite is the ac- sions into fine-grained interstitial tive replacing agent, but the "is- .calcite. Secondary calcite from two lands" may actually be continuous or more nearby fragments common- in the third dimension Avith the main ly meets along a curving contact, mass. Further evidence is furnished giving rise to local areas in the rock by a few fine-grained aggregates of Illinois Academy of Science Transactions

constant composition. They are later than the postulated compaction, for they cut across "streamline" textures. They are earlier than at least part of the period of replacement of coarsely crystalline calcite, for many thin sections contain rhombs that have been partially dissolved (fig. 5). The prominence of this solution varies considerably within limited stratigraphic thicknesses. The age of the rhombs with regard to the deposition of secondary calcite is uncertain. A few rhombs project into the secondary

[ of small calcite, but it is not yet known two unit- whether the rhombs have replaced extinguishing sheets of inclusion-rich Crossed nicols, the coarsely crystalline material, or the latter has replaced the material around the rhombs but left them clear white calcite adjoining the untouched. secondary calcite projections, which are best explained as partially re- Minor Components sorbed marginal portions of the pro- small irregular jections. Uncommon areas of low birefringence, moderate in- "Ghosts" in the interstitial cal- dex, and non-uniform extinction are cite show outlines of larger calcite found within large unit-extinguish- particles that are now represented ing crinoid plates (fig. They are by an aggregate of small grains (fig. 6). not potash feldspar, since a sodium 4). Although the "ghosts" were not cobaltinitrite staining necessarily formed at the same time test gave neg- as the partial solution of the secondary carbonate rims, their presence does indicate that coarsely crystalline calcite has at one time during the history of the rock undergone alteration to a fine-grained aggregate. Both these solution effects can be placed, with considerable assur- ance, later than deposition of secondary calcite because cores of organic calcite lying within the secondary calcite rims are smooth and uncorroded. The dolomite rhombs have been identified by Debye powder X-ray

diagrams ; in the absence of evidence Fig. 5.—Dolomite rhomb partially dis- to the contrary, they are assumed to solved along left-hand margin. Parallel be of a single age and of essentially nicols, 280 X. Petrology of the Burlington Linuston.

Table I. History of Events

1. Deposit ion of det ritus from crinoid s; inter sti ces between larger crinoid fr agments filled w ith mixtu e of lime mud, gl auconite ranules, and fos sil detritb

2. Compaction locally accen- tuates parallelism of elongate particles in interstitial material.

3. Deposition of secondary cal- Formation of cite in optical continuity upon dolomite rhombs.

Some crinoid fragments in inter- 4. Secondary calcite partially stices reduced to replaced by fine-grained fine-grained aggre- aggregate, perhaps contem- gates. poraneously with solution of dolomite rhombs and with re- Partial solu- placement of small detrital tion of dolomite fragments.

1 1

-.^v/.- :-

1 Fig. 6. —Aggregate of grains which Fig. 7.—Glauconite grain in dolomite- nay be secondary silica, lower center rich Burlington limestone. Parallel ;iear margin of crinoid stem segment. nicols, 65 X. grossed nicols, 65 X. 164 Illinois Academy of Science Transactions

ative results, but they may be sec- critical X-ray reflections were ob- ondary silica. scured by lines of quartz, which oc- curred even in the 1-micron fraction greenish ma- Rounded areas of a ' ' of the ' glauconite. ' These rounded terial (fig. 7) give an X-ray pattern greenish areas do not cut across and a differential thermal curve other structures, and are most likely correspond in general with which detrital in origin. Many dolomite for glauconite, but those expectable crystals end abruptly against the this identification will not be mean- "glauconite," without completing ingful until glauconite and related the rhomb shape. In some cases they layer-lattice minerals have under- penetrate into* cracks, but no actual gone much more study as mineral replacement of "glauconite" by types. A basal spacing of 10 Ang- either calcite or dolomite has been strom units was noted, but other noted.

i Illinois Academy of Science Transactions, Vol. 43, 1950

PREGLACIAL GRAVELS IN HENRY COUNTY, ILLINOIS LELAND HORBERG University of Chicago, Chicago

Preglaeial deposits of iron-stained Kewanee, Henry County, Illinois chert gravels underlie the glacial (400 feet N. of S. "W. cor. sec. 3, T. drift at a number of localities in the 15 N., R. 4 E.), in the section de- Mississippi Valley region (fig. 1). scribed below. These gravels, which generally have The Tertiary gravels differ from been referred to the late Tertiary all known glacial deposits in the area Lafayette formation, are significant in their degree of surface alteration, in recording geologic events during polish, and uniform siliceous compo- closing stages of the lost interval sition. Their lithology, based on a between Pennsylvanian bedrock de- count of 120 pebbles, is 79% chert, position and Pleistocene giaciation. 18% vein quartz, and 3% quartzite. It is the purpose of the present Crystalline rocks and local bedrock

Wisconsin drift Thickness Depth Peorian loess Ft. In. Ft. In. Soil, gray 6 6 Loess, tan, mottled gray, non-calcareous 3 3 6 Loess, tan to gray, calcareous, concretions 2 5 6 Farmdale loess Silt, carbonaceous, peaty, gray with black humus streaks, abundant gastropods in pockets, non-calcareous 4 5 10 Loess, maroon-brown, compact, non-calcareous, gastropods 2 7 10

Illinoian drift Gumbotil, gray-brown, siliceous residuals, non-calcareous except for some secondary enrichment at top 2 6 10 4 Till, yellow-brown, abundant chert mixed with other

rock types, non-calcareous . 1 11 4 Sand, brown, in local channel 1 12 4 Tertiary gravel Gravel, 80% brown fossiliferous chert, iron oxide patina- tion, rounded to angular, average 1 to 2 inches, irregular basal contact 3 15 4 Pennsylvanian (Canton) shale Shale, yellow-buff, micaceous, clayey, broken down by weathering 3 6 18 10 Shale, buff-gray, stratified 5 23 10 paper to describe gravels from a new appear to be completely absent, locality in western Illinois and to in- Nearly all the cherts have a yellow- dicate their bearing on late Tertiary brown iron oxide patina 1-3 mm. geomorphic events. thick which is as hard and dense as the interior of the pebbles. A few Description cherts haye a whitish surface altera- The gravels are exposed in a high- tion and the quartz and quartzite kvay cut (fig. 2) 7 miles northwest of pebbles show surficial iron-staining. '

172 Illinois Academy of Science Transactions

Fig. 1.—Occurrences of preglacial Lafayette-type gravels in the Mississippi Valley region. The Henry County locality is circled. The eastern boundary of Cretaceous deposits in Iowa and Minnesota is shown by a dashed line. Based on references cited and field notes on south-central Missouri by J H. Bretz.

I ;

Preglacial Gravels in Henry County, Illinois 173

The brown chert pebbles, which 186-188.) Farther south in western make up 70% of the deposit, con- and southern Illinois nearly all oc- tain fossil corals, crinoids, and bra- currences of preglacial gravel are on chiopods of Lower and Middle Silur- this surface or equivalent surfaces ian age (Heinz LoWenstam, personal correlated with the Ozark and Cal- communication, 1950). houn peneplains. The pebbles range in diameter from a fraction of an inch to 7 in- Correlation and Age ches, the average being between 1 and 2 inches. The brown chert peb- Although it is probable that simi- bles are larger than the other types lar deposits of different ages are rep- and there is a distinct variation in resented and that some gravels are average size from one part of the ex- reworked, the deposits in the Missis- posure to another. All the cherts sippi Valley region can be correlated are angular. Many pebbles are in a general way by lithology and

broken rounds, , some are chipped, physiographic position. On this and a few have percussion mark- basis the gravels in Henry County ings. The gravels are unconsolidated, appear to be equivalent with the and the matrix, if it was ever pres- Windrow formation of Wisconsin, ent, has been removed by weather- Iowa, and Minnesota (Thwaites and ing. Twenhofel, 1921); the ''Tertiary" The basal contact of the gravel is gravels of LaSalle County (Will- irregular and slopes to the south and man, 1942, pp. 140-141), Peoria west. Since the low ridge crest on County (Udden, 1912, p. 50) and which the gravels occur slopes north- other counties in western Illinois east, it is clear that the gravels were (Worthen, 1866, p. 330; 1870, p. 37 laid down on a land surface unre- Bannister, 1870, p. 179; Salisbury, lated to the present topography. 1891, pp. 252-253) ; the Grover The upper part of the Canton gravel of the St. Louis region (Salis- shale underlying the gravels is oxi- bury, 1892, pp. 183-186; Rubey,

dized and mechanically weathered to 1931) ; and the Lafayette gravel of a depth of about 3 feet. Under the southern Illinois and southeastern microscope the weathered shale is Missouri (Lamar and Sutton, 1930, seen to differ from the unaltered pp. 857-859). Correlation with the shale in its lack of stratification, Rockville conglomerate at Dyers- disaggregation into silt and clay-size ville, Iowa (McGee, 1891, p. 304) particles, oxidation, and content of and the Pine Creek conglomerate limonitic aggregates. near Muscatine, Iowa (Udden, 1899) The gravels occur at an elevation is questionable because of the high of 730 feet near the northern margin proportion of crystalline rocks in of a buried bedrock upland, which these deposits. Also it is uncertain locally has an average elevation of whether the two occurrences at lower [about 750 feet and lies 450 feet elevations within the area of the (above the buried bedrock valley of Central Illinois peneplain (fig. 1) the ancient Mississippi river some are primary or reworked. '15 miles to the north. The upland A Tertiary age for the gravels surface has been correlated with the farther south is evidenced by their Lancaster peneplain of the Driftless presence on truncated Eocene form- Area (fig. 1) (Horberg, 1946, pp. ations at the head of the Gulf em- Illinois Academy of Science Transactions

Pig. 2.—Highway cut 7 miles northwest of Kewanee, Illinois. Wp—Peorian loess; Wf—Farmdale loess; It—Illinoian gumbotil and till; Tg—Tertiary gravel; Pc—Pennsylvanian (Canton) shale. bayment and by the occurrence of further suggests that they represent Tertiary wood in the gravels near a widespread aggradational deposit Grover, St. Louis County, Missouri rather than a channel fill and also (Rubey, 1931). Because the Eocene that they are primary rather than formations were beveled by the locally re-worked from older gravels. Ozark peneplain before deposition of The physiographic position and the gravels, a late Tertiary age is character of the gravel in Henry indicated. A similar age for the County are in full accord with other gravels in western Illinois is evi- occurrences in the Mississippi Val- denced by their occurrence on a ley region and suggest that the fol- buried bedrock surface which ap- lowing major geomorphic events pears to be the northward continua- were contemporaneous over a wide tion of the Ozark peneplain (Hor- area: berg, 1946, pp. 186-188). 1. Development of the Lancaster- Calhoun-Ozark peneplain in middle to late Tertiary time. Weathering Interpretation of the Canton shale in Henry County. The gravels in Henry County 2. of probably were derived largely from Uplift gravel-source areas ; residual accumulations on Silurian to the north and initiation of a new I formations, which occur less than 20 cycle marked by gravel deposition. miles to the north, and were depos- Removal of the upper part of the;* ited, along with pre-Cambrian mate- weathered zone on the Canton shale rials from farther north, by a south- and deposition of gravel in the local .' flowing stream. This is evidenced area by a south-flowing stream.

by the composition of the gravels 3. Development of the Central i and their angularity and poor sort- Illinois peneplain and Havana ing. The absence of locally-derived strath in central Illinois (Horberg, Pennsylvanian rocks in the gravels 1946). Weathering of gravels and' Preglacial Gravels in Henry County, Illinois 175 probably some subsequent erosion ing in further stripping of gravels and redeposition in western Illinois. and their final burial. Henry 4. Erosion of deep bedrock val- County probably was glaciated dur- leys and removal of gravels except ing the Nebraskan and Kansan for patches on remnants of the Lan- stages, as well as the Illinoian, al- caster surface. though the record at the gravel local- 5. Pleistocene giaciation result- ity is incomplete.

REFERENCES

Bannister, H. M. (1870), Geology of basin: Geol. Soc. Am. Bull., vol. 3, Tazewell, McLean, Logan and Mason pp. 183-186. counties: Geol. Survey of Illinois, vol. Thwaites, F. T. and Twenhofel, W. H.

4, pp. 176-189. (1921), Windrow formation, an upland HoRBERG, Leland (1946), Preglacial ero- gravel formation of the Driftless and sion surfaces in Illinois: Jour. Geol- adjacent areas of the upper Missis- ogy, vol. 54, pp. 179-192. sippi Valley: Geol. Soc. Am. Bull., vol. Lamar, J. E. and Sutton, A. H. (1930), 32, pp. 293-314. Cretaceous and Tertiary sediments of Udden, J. A. (1899), The Pine Creek con- Kentucky, Illinois, and Missouri: Am. glomerate: Iowa Acad. Sci. Proa, vol. Assoc. Petroleum Geologists Bull., vol. 6, pp. 54-56. 14, pp. 845-866. (1912), The geology and mineral McGee, W J (1891), The Pleistocene resources of the Peoria quadrangle, history of northeastern Iowa: U. S. Illinois: U. S. Geol. Survey Bull. 506, Geol. Survey Eleventh Ann. Rept., pp. pp. 1-103. 189-577. Willman, H. B. and Payne, J. N. (1942), Rubey, W. W. (1931), Geology and min- Geology and mineral resources of the eral resources of the Hardin-Brussels Marseilles, Ottawa, and Streator quad- quadrangles, Illinois: unpublished rangles: Illinois Geol. Survey Bull. 66, manuscript. pp. 1-388. Salisbury, R. D. (1891), A further note Worthen, A. H. (1866), Geology of Han- on the age of the Orange sands: Am. cock County: Geol. Survey of Illinois; Jour. Sci., vol. 42, pp. 252-253. vol. 1, pp. 327-349. (1892), On the northward and (1870), Geology of Pike County: eastward extension of the pre-Pleis- Ibid, vol. 4, pp. 24-52; Geology of tocene gravels of the Mississippi Fulton County: Ibid, pp. 90-110. 176 Illinois Academy of Science Transactions, Vol. 43, 1950

THE NEDA FORMATION IN NORTHEASTERN ILLINOIS 1

L. E. WORKMAN^ Illinois State Geological Survey, Urbana

In the vicinity of Mayville, Wis- sented a map (fig. 2) showing the consin, an "oolitic hematite" called known occurrences of iron ore and ' " " ' seed ore, " flaxseed ore, " or shot reel rock. He reported also that in ore," was originally designated by the Green Bay region the ore is in- Chamberlin 3 as the "Clinton iron ore terbedded with shale, and in an area deposit." He described the grains of iron ore as "little lens-shaped concretions" composed of hydrated hematite averaging 1/25 inch (1 mm.) in diameter, but varying from 1/10 inch (2.5 mm.) to those that are very minute. Cross sections indicated that the ore occurs between the Cincinnati shale and the "Niagara limerstone. " He stated that, although a few fossils of Cincinnatian age were reported to have been found in ore enclosed in a mass of glacial drift, an obvious unconformity at its base in outcrop and a less appar- about 15 miles southeast of Mayville ent break at the top indicate the de- it appears from well cuttings to be posit to be Silurian. He suggested broken up and mixed, or perhaps that it was made in detached shallow interstratified, with limestone. 5 basins over which the succeeding Sil- Savage and Ross pointed out that urian sea spread more widely. Fig- the relations of the iron ore to for- ure 1 is a photograph of ore recently mations both above and below are obtained at Mayville. unconformable, but that fossils col- Thwaites 4 further described the lected from the ore in place indicate deposits of "Clinton" ore as occur- it to be of Maquoketa (Cincinna- ring in eastern Wisconsin in broad tian) age. They described the < pebbles of shale, lenses varying in thickness up to a posit as containing , known maximum of 55 feet. He iron ore, and iron-oxide-reph fossil fragments and suggested that pointed out that at many places j where the iron ore is not present the deposit is a formation laid down ; there are nevertheless beds of red in local basins that, because of the,-: rock at the same horizon, and pre- presence of marine fossils, probably;' were connected and remained after"

the main portion of the normal i rine Maquoketa sea had withdrawn;

:: Chainl.eiiin, T. C, Geologj from the greater part of the region^ ?onsin (Jeol. Sur., vol. 2, 1878, 1 Thwaites, F. T., Recent dis. , The age of the: Neda Formation in Northeastern Illinois 111

(Fe ,H 0), calcite, and halloysite / c 2 3 2 (Al 2 3 .2Si0 2 .nH 2 0). Halloysite is a clay that was found to * rf compose most of the insoluble residue after / * 4 / treatment with acid and that looked under the binocular microscope like 1 % J ^ finely granular unglazed porcelain, lb / ° making up the fragile spheroidal \ u / ^ shells of the spheroids. There are ten varieties of phosphates, and 50% of all phosphorus is contained in the outer shells of the spheroids. The non-oolitic portion of the ore is f largely pore space. Crystalline hematite has grown from the spheroids into part of the space, and there are KEY a number of sedimentary minerals ron ore covered by such as quartz and materials from i,,e * w ) Silurian dolomite the weathering of igneous rock. The o v* 1 i Red beds at or near most abundant transported material horizon of Neda consists of angular grains of scoria- iron ore ceous lava largely altered to iron Western border of oxide. There are rounded fragments Ll £ Silurian dolomite \ of reworked ore and nodules of calcite and dolomite. The authors state that the source of the predominant ^WISCONSIN \ ""iLLINO.S / ore minerals is a baffling problem. The Neda iron-bearing formation is reported in the same stratigraphic 7 position in eastern Iowa , eastern Kansas 8 and northwestern Mis- farther south in the Mississippi val- , 9 ley. They named it the "Neda Iron souri . 10 Ore" formation. Athy very fully described the Hawley and Beavan 8 made a de- Neda deposit in Illinois but called tailed analysis of the minerals of the it the Noix Oolite of basal Alexand- Neda Iron Ore formation. They de- rian (Silurian) age. The outcrops termined that nuclei observed in the which he described, situated in sec- oolites consist of: (a) fragments of tions 26, 27, and 35, T.32 N., R.10 E., reworked ore, (b) fossil fragments, Kankakee County, are the only ones (c) mineral or rock fragments, and known in Illinois. He reports the in a l(d) cross-shaped objects. They em- iron ore spheroids as occurring phasized, however, that most of the 7 Howell, J. V., An outlier of the so-ca oolites show no central nuclei. They Clinton formation in l>ul>u<|ue Count.v, Iowa. ]'< showed that the spheroids are com- Iowa Acail. Sri., Vol. XXIII, 11116, pp. 121-124 s Lee, Wallace, The stratigraphy and f' posed of at least 26 varieties of min- development of the Forest City Basin ir Kansas, Crol. Sin. Kan.. Hull. .'.1. 11)43, p. 42. erals, chief of which are goethite "Crane, G. W.. The iron ores of Missouri, Mis- souri Bur. Geol. and Mines. '2nd Series, Vol. X, 1912, pp. 148-149. " 10 Hawley. J. E., and I P., Mineralogy Athy, L. F., Geoiom and i e of Wisconsin, t, pp. 493-514. Illinois Academy of Science Transactions

Fig. 3.—Graph of insoluble residues from Neda formation and associated strata in outcrop above spring in NW. % SE. % NE. % sec. 27, T. 32 N., R. 10 E., Kankakee County, Illinois.

matrix of reddish brown, purplish, 10y2 feet of Neda ore-bearing mater- and green ferruginous shales and ial on top of the Maquoketa shale shaly dolomite reaching a thickness and under the Edgewood formation. of 8 feet, 2 inches, and resting un- The insoluble material of the Neda conformably on olive-green Rich- deposit consists of: (a) silty shales mond (Maquoketa) shale. The to siltstones variously colored brown, spheroids are generally the size of yellow, greenish, and purplish by fine to coarse sand averaging .5 mm. iron oxide cement; (b) some very across. The shale was reported to fine sand that grades in the middle

contain much silt and fine quartz of the deposit to very fine sandstone ; sand (up to .25 mm. diam.) and (c) iron-ore spheroids scattered in

some medium sand (.25 to .50 mm.). variable proportions but nowhere :]

writer ' The obtained insoluble res- composing most of the deposit as in i idues from samples, taken at one- Wisconsin. The total residue varies, j foot intervals or less from several of from 54 to 93 percent. These pro-" the outcrops along Kankakee River, portions are not essentially different; by treating the rock samples with from the proportions in the Maquo-; weak hydrochloric acid. The graph keta shale below, which in the single, of the most complete section (fig. sample taken amounted to 85 per-.'I

3), taken at the locality figured by cent. Neither is the silty shale to(

Athy on page 32 and described on siltstone matrix greatly different, ex-; page 33 (see footnote 10), shows cept for some sand content, from the

i Neda Formation in Northeastern Illinois

NEDA IRON ORE KEY - OUTCROP FLAXSEED ORE WELL PENETRATING FLAXSEED ORE ^ UPPER MAQUOKETA RED SHALE WELL PENETRATING RED SHALE WESTERN BORDER SILURIAN DOLOMITE

SCALE IN Ml. 180 Illinois Academy of Science Transactions

Maquoketa silty shale. On the other hand, the residue proportions are quite different from the 25 percent residue of the overlying Edgewood and there is an abrupt change from the silty sandy shale below to a sandy conglomeratic dolomite above. The pebbles of the conglomerate consist of grains of underlying shale and a few weathered iron ore spheroids. Were it not for the pebbles the proportion of insoluble residue of the lowest sample of Edgewood would be similar to that of the next sample above, that is, less than 10 percent. In the subsurface of the Chicago region (fig. 4) the spheroids of iron ore are found in a variety of sediments. Like those along Kankakee River, they occur in silty shales having various iron-oxide colors. As in the Wisconsin outcrops, they occur as spheroids loosely cemented with fio. 5.—Thickness of Maquoketa forma- crystalline hematite, though the beds tion in the Chicago region; isopach are so thin that only a relatively interval 25 feet (after DuBois). small part of a 5-foot well sample consists of such material. They are Bois' 11 isopach map of the Maquo- commonly associated with red to keta (fig. 5). green weak silty clay that contains Elsewhere in Illinois Neda spher- small pebbles and grains of weath- oids have noted in two wells ered Maquoketa dolomite. In some been in eastern Whiteside County and in samples they appear to occur in another in eastern Peoria County a yellowish brown silty clay which the Maquoketa thickness is has a starch-like fracture and con- where near a maximum for the region of tains brown flakes, suggesting a soil little 200 feet. All these zone. In one sample the spheroids a more than conditions are interpreted as indfc i occur in a very fine sandstone con- deposited taining grains of dolomite, hematite, eating that the Neda was

relatively flat surface of the l< and a hematite-replaced spicule. All on a eroded widely occurrences are in the midst of more Maquoketa and was

along with the formation t extensive areas where the top of the Maquoketa in an interval previous to Edgewood Maquoketa is represented by red clay shale containing weathered deposition. dolomite fragments. Such red shale The iron-oxide spheroids in the. areas are present only where the matrix of silty shale of the Neda de-' Maquoketa reaches its maximum thickness of 190 to 250 feet, as may be noted by comparison with Du- 5 Geol. Survey Kept. . 105, fig. 1, p. 8.,« :

Neda Formation in Northeastern Illinois

g. 6.— (Left) Thin section of Neda Fig. 8.— (Right) Thin section of Neda iron oxide spheroids in matrix of deposit showing spheroids replaced by silty shale from Illinois (X18). calcite (X18). Fig. 7.— (Middle) Broken surface of Neda silty shale showing spheroids altered to clay (X18). posit in Illinois are generally small- deposition lost their iron-oxide con- er than those in Wisconsin, ranging tent, leaving a residuum of clay up to 1 mm. in larger diameters, (fig. 7). This clay has been identi- 12 only a few reaching 1.25 mm. dia- fied by Grim as illite. Usually its meter, and averaging .75 mm. or less. texture is dense and porcelaneous, They are similar in appearance to showing only faintly or not_at all those in Wisconsin, being generally the former concentric rings of the spheroidal and having smooth sur- oolites. The dull olive-green color faces not only on the outsicles of disappears on treatment of the grains but on secondary surfaces sample with acid, leaving the clay made by breaking off the oolitic almost white. These clay masses ap- shells. Many of the spheroids in thin pear slightly smaller than the aver- section show no nuclei, but the most age iron-ore spheroid, and the common nuclei observed are frag- shapes of the cavities in which they ments of other spheroids, especially occur, though roughly spheroidal, fragments of the outer shells (fig. are somewhat distorted, indicating 6). A few were observed that were that the sediments have been some- fragments of the matrix of silty what compacted to fill partly or en- shale, and a very few contained tirely the former oolite spaces. How- single silt grains or other material ever, it becomes evident that, be- at the center. Most of them show a cause of the large amount of resi- slightly darker brown outer hull. dual clay, some original spheroids

; Some observations of interest in were not highly iron-bearing. Some considering the composition and geo- clay masses are soft and porous, logic history of the oolites are as and occupy proportionately less follows space than the dense variety in the cavity left by solution of the iron 1 . In the upper portions of out- oxide. Some consist only of flat '.ropping Neda beds, varying in ob- round blebs of green clay in sizes served thickness from a few inches typical of the oolites. Evidently o as much as eight feet, the former these last formerly contained high jiolites which were subjected to

yeathering previous to Edgewood 12 Grim, Ralph E., personal 182 Illinois Academy of Science Transactions

proportions of iron oxide before than the materials making up the the iron was dissolved and the sedi- remainder of the oolite. ments collapsed. The considerable variety of sedi- 2. Some of the former oolites mentary materials in which the contain partial replacements of iron-ore spheroids are found, the oc- gray crystalline calcite (fig. 8). The currence of the spheroids in north- cleavage faces of the calcite are so eastern Illinois and elsewhere only oriented as to indicate that each where the Maquoketa is thickest, oolite replacement developed as a and recognition of the same type of single crystal. Surrounding the iron-ore spheroids at about the crystal mass is the residual clay. same geologic horizon in widely Some of the calcite masses .occupy separated areas from Kansas to Wis- practically all the former oolites consin and Illinois, suggest that the except the outer hulls, whereas Neda formation was a widespread others are smaller and the residual deposit of variable character lying clay masses seem to be larger ac- upon known Maquoketa (Cincin- cordingly. It is suggested that cal- natian) shale, and was to a great cite formation took place only after extent eroded away during pre- the cavities had been left by iron- Edgewood (pre-Lower Alexand- oxide solution and that the sizes of rian) uplift. It appears significant the crystals were governed by the for considerations of correlation available space. Thus it would ap- that the Neda type of oolite occurs pear that the calcite deposited after elsewhere in the eastern half of the the beginning of Edgewood time, United States in Silurian strata possibly long after that time. only, that is, the Red Mountain for- 3. Some of the residual material mation of Upper Alexandrian and enclosing the calcite grains has the Lower Niagaran ages in the Birm- appearance of perfect fragments of ingham District, and the Clinton outer hulls of iron-ore spheroids group of Lower Niagaran age in ' except that they are dark brown to New York. black, suggesting the presence of There is no difficulty in assuming

' organic material. Kosanke 1 1 ex- that conditions of deposition favor- amined some of these and reported able to the formation of Neda oolite that they show no cellular structure recurred at intervals from late Cin- but appear rather to be composed cinnatian to early Niagaran times, of amorphous material. Upon be- but in order to be sure that these ing heated in a test tube the black conditions prevailed across the color disappears, leaving light- Ordovician-Silurian boundary the brown amorphous clay suggestive possibility should be examined that

of that described by Hawley and the Maquoketa fossils found in the I Beavan as halloysite in the hulls of Neda in both Wisconsin and Iowa oolites in Wisconsin and of a some- may have attained their position by j what different chemical character being reworked by an early Silur-

13 Kosanke, R. M., personal ian sea.