Major Aquifers in Glacial Drift Near Mattoon, Illinois, by John W

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STATE OF ILLINOIS ADLAI E. STEVENSON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION C. HOBART ENGLE, Director

DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief

CIRCULAR NO. 179

SHORT PAPERS ON GEOLOGIC SUBJECTS

Major Aquifers in Glacial Drift near Mattoon, Illinois By John W. Foster

Notes on the Illinois "Lafayette" Gravel

By J. E. Lamar and R. R. Reynolds

Cambrian and Lower Ordovician Exposures in Northern Illinois

By H. B. Willman and J. S. Templeton

An Occurrence of Barite in the LaSalle Limestone of Illinois

By Raymond S. Shrode

Revision of Croixan Dikelocephalidae

By Gilbert O. Raasch

REPRINTED FROM THE TRANSACTIONS OF THE ILLINOIS STATE ACADEMY OF SCIENCE

VOL. 44, pp. S5-128, 137-151, 1951-

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS

URBANA, ILLINOIS

1952

JUJNQSS GEOLOGICAL SURVEY LIBRARY

JUL 16 1952

Illinois Academy of Science Transactions, Vol. 44, 1951

GEOLOGY MAJOR AQUIFERS IN GLACIAL DRIFT NEAR MATTOON, ILLINOIS* JOHN W. FOSTER State Geological Survey, Urbana

Subsurface Pleistocene studies location of the areas most favorable near Mattoon recently conducted by for well field construction. The pres- the Illinois State Geological Survey ent investigation, largely a subsur- have revealed the probable origin face study by the records of 114 drift and nature of occurrence of two borings, has been designed toward gravel aquifers of major importance. not only descriptive geology but an The report describes a veneer of insight into the physical develop- sand and gravel which rests on the ment of the Mattoon aquifers. It is Sangamon interglacial surface over hoped that this insight will lead to- a wide area north of the Shelbyville ward a better understanding of the moraine which here marks the maxi- groundwater available to Mattoon, mum stage of Wisconsin glaciation. its industries and hinterland farms, A second, more restricted but more and lead also toward the discovery favorable aquifer of Shelbyville age of other still unknown aquifer de- lies immediately south of the Shelby- posits of similar rank lying in areas ville terminus and extends from the of similar Pleistocene history. ground surface to the top of Illi- General setting.—The one hun- noian drift. The relationships of dred and ninety-two square miles these gravel aquifers to the Cerro of the Mattoon investigation lie en- Gordo and Shelbyville moraines, to tirely in southwestern Coles county the undulations of the Sangamon and include Townships 12 and 13 surface, and to the topography of north, Ranges 7 and 8 east, and the the top of the bedrock are described. north four section tiers of Township The investigation is expected to lead 11 north, Range 7 and 8 east. The to the discovery of unknown aquifer terrain is largely rolling to nearly deposits of similar genesis in areas level upland prairie, shedding drain- of similar geologic conditions. age west into the Kaskaskia, south Introduction. — Geological and into the Little Wabash, and east into geophysical studies of the gravel the Embarrass systems. Relief is aquifers near Mattoon have been only 165 feet, with elevations rang- conducted by the State Geological ing from about 620 feet on the Kas- Survey at intermittent periods for kaskia River in the northwest area many years. The older surveys have and the Little Wabash River in the been directed principally toward the southwest area to about 785 feet on the highest crest of the Shelbyville moraine (fig. 2) near the southwest Illinois Academy of Science Transactions corner of T.12 N., R.8 B. Mattoon, with a population estimated at more than 18,000, lies on the gentle north or backslope of the Shelbyville moraine in the south-central area.

As in most of east-central Illinois, the glacial drift near Mattoon rests on the shales, thin limestones, sandstones, and coals of the Pennsylvan- ian system. Bedrock here is exposed at very restricted locations along the Kaskaskia A^alley north of the Cerro Gordo moraine (figs. 2 and 3). Everywhere else drift, with estimated thicknesses up to approximately 220 feet in southwestern T.12 N., R. 7 E., buries the bedrock surface and completely obscures its topography. Drift of Illinoian age is capped with younger Wisconsin deposits associated with the Shelby- ville and Cerro Gordo ice lobes (Leighton, Ekblaw, Horberg, 1948, p. 22) of Tazewell age. Each of these drift mantles shows surface expres- sion, the Illinoian as a nearly level till plain beyond the influence of Fig. 1.—Index map showing location Wisconsin ice, the Shelbyville as a of the area of investigation near Mattoon, prominent moraine, and the Cerro Illinois. Gordo as a more gentle moraine and till plain in the far north portion appear to trend westward and north- of the Mattoon area. westward as part of the buried Mid- dletown valley which in turn is in The bedrock surface.—The buried confluence bedrock surface of the Mattoon area with the ancient course ;' of the Mississippi River in eastern is principally interfluve between the pre-glacial Embarrass (Horberg, Mason County. Although Illinoian drift to a great degree obscured the 1950, p. 84) and Middletown (Hor- subdued topography of the top of' berg, 1950, p. 73) drainage systems. the bedrock, the of the The shallow sags in the bedrock southernmost west-trending bedrock valleys ap- topography in the northeast portion of the area were apparently courses pears to have influenced the building of a Wisconsin gravel aquifer, and 1 tributary to the great Embarrass influences the hydrologie bedrock valley which trends south- probably characteristics of that aquifer. erly in the eastern part of Coles County. The more prominent bed- Illinoian drift.—For the most parf rock valleys of the Mattoon area the bedrock near Mattoon is mantled Aquifers in Glacial Drift

CONTOUR INTERVAL 50 FEET

MILES [9 | 3 ?

Fig. 2.—Surface contour and moraine configuration map. Illinois Academy of Science Transactions

CONTOUR INTERVAL 25 FEET ^-60<\ APPROXIMATE ELEVATION OF TOP OF BEDROCK

A = BEDROCK EXPOSURE IOI23 MILES Fig. 3.—Buried bedrock surface contour map. Aquifers in Glacial Drift

with Illinoian drift. Two exceptions this fact by no means precludes such are noted: (1) Reliable samples from a history. a 1944 Mattoon City test hole lo- Aquifer material in Illinoian cated NE. SE. NE. sec. 18, T.ll N., drift, though very uncommon near R. 7 E., indicate that the basal ma- Mattoon, may be extremely impor- terial is a fine to coarse silty sand tant in its influence on the hydrology with an abundance of humus and of Wisconsin gravels immediately most likely represents a Yarmouth south of the Shelbyville moraine. In interglacial alluvial deposit. No con- sec. 18, T.ll N., R.7 E., pre-Wiscon- vincing evidence of Kansan drift sin sand deposits are as thick as 45 has been noted in this subsurface feet and occupy there a well- investigation. (2) Three borings in developed valley of the bedrock sur- T.ll N., R. 7 E., penetrated to the face, drift boring number 4, fig. 4. bedrock without apparently cutting The nature of this pre-Wisconsin pre-Wisconsin material. These sug- sand is unknown, but the deposit is gest that Sangamon erosion denuded very likely restricted to the bedrock the bedrock at numerous though valley in which it lies, and probably probably restricted locations princi- trends westward in conformity with pally along Sangamon drainage sys- the valley configuration. Samples tems and in localities where Illinoian from boring number 4 suggest that drift was originally thin. Inasmuch this sand is silty to very silty and as the top of Illinoian drift appears may not in itself be an important to have been a gently undulating direct source of groundwater. plain, at an average elevation of Sangamon soil.—Well drillers in about 625 feet, the greater Illinoian the Mattoon area are familiar with drift thicknesses occur generally the "peat" which rests largely on over the depressions of the bedrock Illinoian till. It is this remarkably surface. The greatest Illinoian drift preserved soil section that has pro- thickness in the Mattoon area is esti- vided an excellent horizon marker mated to be about 90 feet where a here. low bedrock surface is overlain by a The excellence of the Sangamon relatively high Sangamon surface in interglacial soil development as a sec. 19, T.12 N., R.8 E. marker near Mattoon has enabled Illinoian drift over wide areas the use of driller's logs where they here is exclusively a grey or grey- might otherwise be of little value green calcareous clay till which lacks in geological Pleistocene interpreta- the sandy character typical of Illi- tion. Unusual preservation of Sanga- noian drift in most of Champaign mon loam here through the over- County. The upper 5 to 10 feet of running of the later Wisconsin Illinoian till shows clearly by repre- glacier is probably due to the equally sentative samples the weathering unusual nature of the advance of during the Sangamon interglacial the Shelbyville ice. period. The general textural homo- Shelbyville ice progression and geneity of the Illinoian drift lends the spreading of gravel veneer.—The

' no support to the possibility of mul- progression of Shelbyville ice into

1 tiple Illinoian glaciation, although the Mattoon area early in Tazewell Illinois Academy of Science Transactions

5 • V- ~v= Aquifers in Glacial Drift 91 time, with a southwesterly lobate first in the sag in the Illinoian drift axis, began the key period of aquifer surface (which is apparently the re- building. Subsurface data through- flection of a partially obscured bed- out the area show that a widespread rock valley). veneer of gravel and sand was laid The present valley of the Little directly on the Sangamon soil by Wabash River cuts deeply into the strong pro-glacial fluvial activity. Shelbyville moraine (fig. 2) directly Figure 4 illustrates the persistence behind the Shelbyville fan and out- with which these coarse Shelbyville wash complex and may represent elastics were spread, apparently the course of a subglacial or super- without much regard for the hills glacial torrent from which a portion and dales of the Illinoian (or Sanga- of the complex would have been de- mon) surface. The adaptability of rived. Periglacial deposits here orig- the gravel veneer to the topography inating from an ice-fed torrent indicates that the veneer was spread would likely have spread as an al- either subglacially under hydrostatic luvial fan close to the stream 's emer- pressure or as ice-contact accumula- gence from the ice mass. Such fan- tion along the advancing zone of type gravels would combine with strong periglacial fluvial activity. earlier and later outwash gravels Deposits of outwash origin distant to form an aquifer deposit similar from the glacier mass would con- to the complex found in sees. 17, 18, centrate in the existing sags of the 19, and 20, T.ll N., R.7 E.

Illinoian drift surface with very thin Borings 7, 8 and 9 of fig. 4 show deposition, if any, on the topo- the occurrence of a till bed five to graphic highs. The wide occurrence ten feet in thickness intercalated of the gravel veneer here apparently between gravels near the south edge eliminates the possibility of sub- of the Shelbyville moraine. This till glacial origin, leaving the likelihood zone may have been the result of a of ice-contact origin. minor northward retreat of the ice Fan and outwash complex.— front a distance of about two miles. Shelbyville ice appears to have The momentary ice front control by reached the north portion of T.ll ablation may then have been lost N., R.7 E., about 5 miles south of by reinvasion to a position nearly the present city of Mattoon, without equal to the former Shelbyville ter- great mechanical change. Here, how- minus, spreading as it progressed ever, ice-front progression ceased a second gravel veneer, shown on and deterioration of the front prob- fig. 4. ably paced the movement of ice into The second invasion renewed the the terminus area. During this part building of the fan and outwash of the Shelbyville time began the complex beyond the limit of Wis- building of a complex fan and out- consin glaciation. Subsequent till wash deposit immediately south of deposition resulted in the prominent the glacier terminus near sees. 17, Shelbyville moraine which is the 18, 19, and 20, T.ll N., R.7 E., landmark of the Mattoon region. south of the present Lake Mattoon. Fluvial deposits of sand and gravel Sands of this complex accumulated associated with the final recessional 92 Illinois Academy of Science Transactions

stage of the Shelbyville ice front are contain sample study records which very rare or absent within the till show that pre-Cerro Gordo soil has near Mattoon. Although the extent been identified at many well sites of the final retreat of the Shelbyville in Douglas and Champaign counties, ice front is not known, ice of this north of the Mattoon area. stage of Tazewell glaciation must Geo-hydrology of the aquifers.— have ablated at least 50 miles, as Despite the uniformity of thickness Shelbyville drift in central Cham- of the Shelbyville gravel veneer paign County exhibits a young soil north of the Shelbyville moraine, profile, but little or no leaching. the textural composition of the aqui- Cerro Gordo lobate ice. — Post- fer shows great variation from place Shelbyville glaciation in the Mattoon to place. The veneer exhibits over area is believed to include only the wide areas a three- to ten-foot bed development of the more restricted of very fine sand at the base of the Cerro Gordo lobe, with an axis simi- aquifer, directly above Sangamon lar in direction to that of the earlier soil. Where this fine sand composes Shelbyville lobe. Perhaps, because the entire thickness of the Shelby- of meager fluvial activity or because ville veneer, well construction even of lack of coarse clastic load, the for domestic groundwater is difficult. Cerro Gordo glacier failed to spread Borings in the veneer not restricted before it the gravel veneer which to any given locality commonly ex- Shelbyville ice spread with such uni- hibit a bed of clean, very coarse formity. Drilling above the Shelby- gravel with a thickness up to fifteen ville drift in and north of the Cerro feet intercalated between beds of less Gordo moraine has revealed no more well sorted silty sand and gravel. It than widely scattered thin sands of is the very coarse gravel deposit limited groundwater resources. Cer- which yields abundant ground- ro Gordo ice near Mattoon reached water at scattered sites through the its maximum in extreme northern Mattoon area, although even the T.12 N., R.7 and 8 E., about one poorly sorted sand and gravel, where mile north of the city (fig. 2). The the coarse material is missing, yields till sheet deposited by the Cerro more than adequate domestic Gordo ice rests on older Shelbyville groundwater supplies.

till. The horizon between the two Far more favorable than the tills should be recognizable from gravel veneer north of the Shelby- sample study on the basis of weath- ville moraine is the gravel aquifer ering on the top of the buried pf the fan and outwash complex in Shelbyville till and on the basis of the area of sees. 17, 18, 19, and 20, j pre-Cerro Gordo soil development. T.ll N., R.7 E., largely south of the | The subsurface distinction of the limit of "Wisconsin glaciation. The Cerro Gordo and Shelbyville tills, veneer and the complex gravel for- however, has not been made in the mations are distinct both geologi- area of this investigation for lack cally and by hydrologic characteris- of suitable sample cuttings in the tics and are being considered here J key area of Cerro Gordo overlap. as two aquifers. The fan and out-| Illinois State Geological Survey files wash complex has a number of, :

Aquifers in Glacial Drift 93

geologic features which encourage City of Mattoon well field con- favorable groundwater yields struction.—Mattoon wells are con- (1.) The sand and gravel com- centrated in two areas, the Dorans plex of Shelbyville age extends from field near sec. 30, T.12 N., R.8 E., and the ground surface to the top of the the Southwest field near sec. 18, T.ll Illinoian drift with thicknesses up N., R.7 E. City wells constructed to 65 feet. These thicknesses are prior to 1944 were located largely due to the well-defined sag in the in the Dorans area. These penetrate Illinoian drift surface previously de- to depths of 40 to 70 feet and scribed. No such thicknesses of tap groundwater in the Shelbyville gravel are known in the Shelbyville gravel veneer. The Dorans wells have veneer to the north. never shown capacities adequate for (2.) The fan and outwash com- the demands of a city the size of plex in a number of test holes in T.ll Mattoon. By 1935 Lake Mattoon Avas N., R.7 E., has two or more zones of supplying the city with the major clean, very coarse gravel with com- portion of its water needs. Limited bined thicknesses of twenty feet or surface inflow in the summer of more. 1944, caused by rainfall deficiencies (3.) The complex is in hydro- recorded by the United States logic contact with the buried Shelby- Weather Bureau in the months of ville gravel veneer on the north, May, June, and July of that year, thereby facilitating groundwater re- lowered surface storage to an alarm- charge. ing level. Geophysical exploration The complex is probably also in was conducted by the State Geologi- hydrologic contact with Illinoian cal Survey for the purpose of dis- gravels occupying the bedrock valley covering, if possible, groundwater which directly underlies part of the aquifers more prolific than that at complex area. Borings here have the old Dorans field. Electrical earth encountered an upper Illinoian till resistivity exploration delineated in deposit which, if continuous, might 1944 the very favorable gravels in effectively seal the deep gravels from the area of sees. 17, 18, 19, and 20, recharge of the shallower gravel T.ll N., R.9 E., described in this complex. Where erosion of the upper study as the Shelbyville fan and Illinoian till may have exposed deep outwash complex. Illinoian gravels during Sangamon Lake Mattoon still furnishes the time, Illinoian and Wisconsin major portion of water requirements gravels may be in direct contact. at Mattoon though limited pumping Inasmuch as the deeper Illinoian of the Dorans well field continues. gravels are believed to be generally However, the construction of wells silty and limited in permeability, in the Southwest area, since 1944, wells in the complex area will prob- which penetrate the most prolific ably be constructed in the clean deposit near Mattoon, safeguards the Shelbyville gravels, even though city's supplies against future low these are relatively shallow and water levels of Lake Mattoon. Fur- j

j: widely thermore, industries which can lo- 94 Illinois Academy of Science Transactions cate in the vicinity of the Shelbyville E. Ekblaw, Head of Engineering fan and outwash complex will be Geology and Topographic Mapping assured of a source of abundant Division, of the Illinois State Geo- groundwater available for indepen- logical Survey, for helpful comments dent development. This favorable during the assembling and interpre- situation exists at very few indus- tation of data. trial sites in this region. Acknowledgment. — The author REFERENCES gratefully recognizes the many con- Horberg, C. L. (1950), Bedrock Topo- tributors to the present concepts graphy of Illinois: Illinois State Geo- logical Survey Bull. 73, p. 73. of Illinois Pleistocene upon whose Horberg, C. L. idem. p. 84. groundwork this investigation has Leighton, M. M., Ekblaw, George E., and Horberg, C. L. (1948), Physiographic been made. The author is indebted Divisions of Illinois, Jour. Geol., vol. to M. M. Leighton, Chief, and George 56, no. 1, p. 22. Illinois Academy of Science Transactions, Vol. 44, 1951

NOTES ON THE ILLINOIS "LAFAYETTE" GRAVEL*

J. E. LAMAR and R. R. REYNOLDS State Geological Survey, Vrbana

Extreme southern Illinois contains many exposures of bro w n chert The six samples used in this in- gravel and red sand which are vestigation were taken from the loosely referred to as the "Lafay- following exposures (fig. 1) : ette" formation of Tertiary age. De- posits of Lafayette-type gravel occur in other parts of Illinois but are mostly thin. 1 One of the northernmost sizable deposits in Illinois which closely resembles the southern Illinois gravels is found near Hamil- ton in Hancock County, about 250 miles northwest of the main body of "Lafayette" sediments in southern Illinois. The "Lafayette" gravel is commonly regarded as stream deposited. The drainage patterns that must have been involved in most of this deposition have not been established, nor is it possible to say to what extent the gravels may have been re- worked. Solution of these problems will involve, among other things, a study of the sedimentology and lithology of many samples carefully

taken with regard to their topo- LAFAYETTE GRAVEL graphic occurrence and elevation. • Depot The preliminary work here described was undertaken to obtain better knowledge of some of the physical characteristics of the Lafayette Fig. 1.—Locations of deposits s and of other outcrops of "Lafayette" gravel which might be important to gravel as reported by Horberg, Leland, broader investigations. Preglacial gravels in Henry Co., Illinois, Trans. 111. Acad. Sci., vol. 43, p. 172, 1950.

Sample No. 1. SE% SW*4 sec. 6, T. 4 N., R. 3 W., near Hamilton,

I'ul .1 lr-.ll.-. I ],i |ienio>Moii .)(' 111.' Chirf. Illinois rll n „ „ , „ , . , State Geological Survey, Urbana. 111., 10 feet 01 "Tavel 111 gravel pit. 1 Horberg, Lelaml, I're-ghioial erosion surfaces in LievatlOllTJil„ TT „ + 4^^ „U„-,,+ fiOA 4? „4- Illinois: Jour. Ceo!., Vol. 1,1V, No. ;:, mi.;, ,,. 184. about 620 feet. 96 Illinois Academy of Science Transactions

Fig. 2.—Particle-size histograms. The vertical bars, reading from left to right, indicate percent retained on the following sieves: 2", 1.5", 1.05", .742", .525", .371", 3 mesh, 4, 8, 10, 14, 20, 28, 35, 48, 65, 100, 150, 200, and 270 mesh; passing 270 mesh plus 2 microns, and minus 2 microns.

Sample No. 2. NE% SEVi see. 4, 111., 3% feet of gravel in road cut. T. 44 N., E. 3 E., near Grover, Mo., Elevation about 400 feet. 30 feet of gravel. This deposit is Sample No. 5. SWV4 SWy4 SEVi not far from Calhoun County, 111., sec. 29, T. 14 S., R. 2 E. near Grand and was sampled instead of Calhoun Chain, 111., 8 feet of gravel in rail- County deposits because the expo- road cut. Elevation about 430 feet. sure was better and the thickness Sample No. 6. SW% SWVi sec. I of the gravel greater. Elevation 28, T. 14 S, R. 5 E. near Renshaw, about 820 feet. 111., 20 feet of gravel in road cut. Sample No. 3. SW% NEVi sec. Elevation about 520 feet. 34, T. 15 S, R. 3 W. near Fayville, Mechanical Analyses 111., 10 feet of gravel in small gravel pit. Elevation about 400 feet. Mechanical analyses of the six

Sample No. 4. NE% SWVi sec. samples were made by standard pro- :<

15, T. 16 S., R. 1 W. near Mounds, cedures. Results are shown in figure ;' ''Lafayette" Gravel 97

particle size analyses. The quartile and median data for the samples as a whole indicate that samples 2 and 3 are the coarsest and sample 6 is the finest. In the plus 10 mesh fraction, sample 1 is the coarsest and samples 4 and 6 the finest. In general a kinship is suggested between samples

1, 2, and 3 from Hamilton, Grover, and Fayville, respectively, and between samples 4, 5, and 6.

Pebble Counts

In order to determine the lithology of the "Lafayette" gravel, pebble counts were made on 100 pebbles selected at random from each of four size fractions of each sample. All chert pebbles were classified accord-

ing to color : brown, light buff, red, black. size fractions PLUS 10 MESH FRACTION and The counted and the results of the counts are shown in figure 4. The following characteristics are indicated for each of the size frac-

Samples 1, 2, and 3 contain black and red chert and sample 2 is relatively high in light brown chert. Sample 1 differs from the other samples in 12 3 4 5 6 having a high content MINUS 10 MESH FRACTION of red chert and vein Fig. 3.—Histograms showing first quartz. Samples 1, 2, quartile (left-hand bar), median (center and 4 contained more bar) and third quartile (right-hand bar) light brown chert than of "Lafayette" gravel samples. the other samples. Sample 1 differs from the other samples in 2. All the charts are bimoclal. Those having a high content for samples 2, 3, 4, 5, and 6 are of vein quartz and of roughly similar. The curve for sam- buff chert pebbles. Samples 1 and 3 are ple 1, however, differs in the shape high in vein quartz peb- and amplitude of its sand fraction bles. Sample 2 is high peak and in the particle size dis- in light brown chert pebbles and Sample 4 is tribution of the gravel fraction. high in red chert. In figure 3 are shown certain sta- Sample 1 is unlike all tistical values determined from the others because of its Illinois Academy of Science Transactions

1 1

! 1. 05x.742" L LL_ II .iii ; 'I 8 ^ 1 5" .742 x. 52 i—1—iRH— k m s^r&c I .

l ' - 355** 1

}\ .525x371

lL^lxLi_ ui 1

SAMPLE NUMBER

Fig. 4. —Results of pebble counts in percent by number of pebbles. Percent scale changes at 20 percent. —

" Lafayette" Gravel

low chert content and high compensating vein quartz content. Sam- ples 1, 2, and 4 contain the most light brown chert pebbles and also the most red chert pebbles.

To summarize the foregoing, sample 1 differs from all other samples in its high vein quartz content. Sam- ples 1 and 2 are akin in several respects, as are samples 5 and 6. Samples 3 and 4 are of intermediate character. This is probably to be expected because samples 1 and 2 are from the Mississippi Valley and samples 5 and 6 from the Ohio Val- ley, whereas samples 3 and 4, near the junction of the two valleys, show affiliations with both valleys. The Mississippi Valley gravel samples commonly differ from the Ohio Val- ley gravel in the following ways: they contain more vein quartz, light brown chert, red chert, and black chert pebbles. The Ohio Valley sam- SAMPLE NUMBER ples are characterized by a high content of chert pebbles. Fig. 5. —Degree of polish of pebbles. brown Left-hand bar indicates number of peb- Conclusions regarding the lith- having "good" polish, center bar bles ology of pebbles counted in relation "fair" polish, and right-hand bar "poor" polish. to size are shown in table 1.

Table 1. Prevalence of Pebbles by Kind

Kind of pebble

Brown chert 1 . 05 x .742 inches .371 x .19 inches

Light brown chert .371 x .19 .525 x .371

Red chert

Black chert

Vein quartz —

Illinois Academy of Science Transactions

Table 2. Shape of Pebbles (In percent by numbers of pebbles)

" ' 1. 05 "x.742 "fraction .742 "x. 525 "fraction 1 .05 x .525 ' fraction

Sample No. I* II III IV I II III IV I II III IV

1 64 16 4 16 64 24 12 64 19 8 9 2 32 40 12 16 28 24 32 16 30 33 21 16

3 32 36 12 20 44 16 20 20 37 27 16 ?0

4. . . . 32 16 16 36 60 20 12 8 47 18 14 21

5 52 28 4 16 56 12 16 16 54 20 10 16

6. . 36 24 8 32 40 28 8 24 38 26 8 28

39 55 6 37 47 16 38 52 10 38 30 16 16

* I—Flattened or disc shaped. Ill—Bladed or lath-like. II—Spheres or equidimensional pebbles. IV-—Rod shaped.

These data carry various impliea- pebbles from samples 2 and 3 were tions regarding source deposits. The measured to provide comparative black chert, and to a lesser extent data. the red chert, appears to have come Standard methods 2 were employed from deposits having a uniform par- in measuring the pebbles. Shape tide size distribution. In contrast designations were assigned by the 3 the vein quartz pebbles came from Zingg shape classification , which a source that yielded pebbles mostly provides for four classes. These are smaller than % inch. indicated in table 2 together with In addition to the kinds of pebbles results of the shape studies, mentioned, there were observed in Table 3 ._Average Sphericitt Valtjes the gravel samples, in amounts prob- == ably less than one percent, other Size of pebbles kinds of pebbles that may prove s valuable in determining the source 1.05 "by .742" .742" by .525 of the gravel. These were quartzite, agate, and jasper pebbles. No ig- neous rock pebbles were observed, 1 -

nor have they been seen in Illinois 2. outcrops.

Shape 4

Shape studies were made on two 5 sizes of chert pebbles, namely, 1.05" x .742" and .742" x .525". Pebbles 6 - of these sizes were selected because = 2 ,, . _ nn . Krumbein, W. C, Measurement and geological' and easily handled. significance of shape and roundness, Jour. Sed. Petr. they are Common ,

vol - e 9 In addition, the shapes of 25 quartz 3 Krumbein,' w a, op' c&'./p. 67. — .

'Lafayette" Gravel 101

The results indicate a similarity quartz pebbles are more round than of shape between pebbles in samples the chert pebbles. 2 and 3 in the 1.05 x .742 inch frac- The roundness values of the chert tion and a less striking similarity pebbles commonly result from a between samples 3 and 5 in the .742 rounding of the edges and corners x .525 inch fraction. Considering' of the pebbles rather than major both fractions together there is a alteration of the general pebble likeness between samples 4 and 5. shape. Well-rounded chert pebbles The shape data do not, therefore, are infrequent in the sizes studied; show any consistent relationship be- such pebbles as are found may have tween the samples. come from a distant source, although Flattened or disc-shaped pebbles some of them may have had a high are most common ; they comprise 45 initial roundness. percent of the six samples investigated. Twenty-four percent of the Table 4. Avkkaok Roundxess Values pebbles are spherical or equidimen- lath-like, sional, 13 percent bladed or Size of pebbles and 18 percent rod shaped. Sample No. Shape determination on two sizes 1.05" by .74 2" by of quartz pebbles from sample 2 and .742" 525" one size from sample 3 yielded differing results (table 2), but show a 1 —chert pebbles . . .44 40 dominance of flattened or disc- shaped and spherical or equidimen- 2 50 48 sional pebbles. Sample 2 contains 3 48 46 no bladed or lath-like pebbles. 4 48 48 Sphericity 5 50 51 Sphericity was determined direct- 6 52 48 ly from a modification of the Zingg diagram. 4 Table 3 gives the spheric- 2—quartz pebbles.. 73 ity data. No distinctive differences 3 73 are apparent between samples.

Roundness Since the quartz pebbles are more rounded the The roundness of the pebbles pre- than chert pebbles, and also are probably viously measured for shape was de- more resistant to termined by comparing them with abrasion, it seems likely that they 5 attained Krumbein's visual roundness chart. may have much of their present degree of rounding in a pre- The results are given in table 4 ; also " Lafayette" cycle comparable measurements of 25 vein of erosion. Con- ceivably quartz pebbles from samples 2 and most of them found in the "Lafayette" gravel in southern Illi- 3. The chert pebbles of sample 1 are nois have the least round. The other samples may been derived from the Caseyville (Pennsylvanian) show no outstanding differences. The conglomeratic sandstones which form prominent escarpments a few tens 102 Illinois Academy of Science Tr

of miles north of the gravel area. This is suggested by the appearance of the pebbles and by a petrographic and sedimentologic study made of them by Reynolds." The source of the quartz pebbles in the (1 rover and Hamilton samples is a problem for further study.

Polish

An outstanding characteristic of most "Lafayette" gravel deposits of Illinois is the polish shown by the pebbles. Some pebbles are complete- Fig. 7. —The natural surface of one ly or almost completely polished; side of a chert pebble which is silicified oolite. It suggests the surface of a freshly broken oolitic limestone. X 4.

others have rough corners and edges, with polish on only the larger flat or nearly flat surfaces; still others show polish only in re-entrants in the pebbles. The variations in the extent of the polish may be a func- tion of th« amount of wear to which a pebble has been subjected since the original development of the polish.

The pebbles of sample 6, which have dull, lusterless surfaces, are an exception. The sampled deposit was partly cemented, and the pebbles were coated with a film of limonite. Attempted removal of the limonitic coating by rubbing resulted in a moderate degree of polish, but this may have been caused by the rub- Fig. 6.—Polished section of a brown bing. Solution of the limonitic film "Lafayette" chert pebble. The marginal, by hydrochloric acid revealed a dull brown-stained zone which gives the pebble its exterior color is evident except unpolished surface on most pebbles, at the broken upper left corner. An area but in the re-entrants of a few peb- of brown discoloration produced by the polish evident. This infiltration of iron oxide along incipient bles some was fractures is shown by the dark-grey area suggests that the gravel of sample the left of the picture. Reflected at 6 was once normally polished. Its light. X 7. loss of polish could be attributed to processes related to cementation; which however, as the deposit from , sample 1 was obtained was also part- j "Lafayette" Gravel 103 ly cemented, though less so than buff, the characteristic brown is com- sample 6, and yet contained many monly restricted to a relatively thin highly polished pebbles, cementation zone at the surface and to bands or cannot be regarded as always ad- streaks along cracks and fractures versely affecting polish. in the pebble (fig. 6). Twelve peb- It was evident from an examina- bles from sample 1 were boiled in tion of 300 pebbles that in general hydrochloric acid. Six lost their

' most of the chert pebbles in the ' La- dark-brown color and changed to an

' fayette ' gravel have a higher degree orange brown or yellow brown with of polish than the other pebbles. The cream-colored blotches. Three peb- vein quartz pebbles and the few Jbles became light buff to cream. qnartzites as a rule have only a fair Three others were little changed. It degree of polish. is noteworthy that though there was In an attempt to determine varia- a marked color change, the polish of tions in degree of polish between the pebbles was not appreciably samples, the chert pebbles whose altered ; the gloss may have been in- shapes were measured were arbitra- creased, thus suggesting that the rily classified according to whether polish is not necessarily now linked their polish was deemed good, fair, with the brown color. This inference or poor. The results are shown in is further suggested by the fact that figure 5. Sample 1 from Hamilton, the "glazed" pebbles, subsequently

111., has the highest degree of polish. described, though white or cream Samples 2 and 3 have a moderate colored, are also polished. It seems degree of polish, whereas samples 4 possible, therefore, that the polish and 5 are the least well polished. of some pebbles is unrelated to color. The data at hand are not sufficient Oolitic Pebbles to permit an interpretation of these observations, but it is suggested that A considerable number of the peb- they probably relate to the history of bles in the samples studied are silici- the pebbles after polishing and per- fied oolite. The abundance of oolitic haps to the varying susceptibilities pebbles in the 50 pebbles studied for of different varieties of chert to shape is shown in table 5. Some of polishing. the pebbles are obviously chert; The means by which the "Lafay- others appear to be more coarsely ette" pebbles received their polish crystalline quartz, although they were not investigated. However, the may be varieties of chert. Many dense surfaces of some of the porous show pitted surfaces, but oolitic peb- chert pebbles and of some of the in- bles without pits are also common. frequent sandstone pebbles observed Numerous coarser-grained pebbles suggest that they may have been im- have on their flat or rounded sur- pregnated with silica. faces an intricate pattern composed The brown color and polish of the of roughly crescentic indentations "Lafayette" chert pebbles may be somewhat akin to chattermarks but in some way related. Thin sections not regularly distributed. These of chert pebbles show that, although markings also occur on pebbles the interiors of the pebbles may be which contain fossil detritus but only 104 Illinois Academy of Science Transactions

Fig. 8.—Thin sections of silicifled "Lafayette" pebbles. A. Originally probably a limestone composed of calcareous detritus and possibly some oolite grains. B. Oolite showing radial structure but little annular banding in grains; grains have dark centers. C. and D. Oolite showing well-developed annular and radial structure; the white centers are macrocrystalline quartz. X 18. scattered oolite grains. Many of the The oolitic pebbles are diverse oolitic pebbles showing the markings (fig. 8). Some consist largely of are maroon colored. A few oolitic roughly spherical oolite grains, gen- chert pebbles have one natural sur- erally showing an annular struc-

face that resembles a freshly broken ture ; others are composed of numer- piece of oolitic limestone. On these ous annular oolite grains, together surfaces the oolite grains resemble with an abundance of granular ma- rounded sand grains, and the pebbles terial, probably detrital, which does might be described as sandstone not show notable concentric struc- from casual examination (fig. 7). ture or spherical shape. Small fossils ''Lafayette" Gravel

Sample .742" x .525" x .525" x No. 1.05" .742" 1.05"

4 6 5 2 10 10 10 3 8 4 6 12 14 13 5 6 4 5 6

such as brachiopods and fragments of large fossils are common in some pebbles. The grain size of the oolites and other materials is variable, as

is the ease with which the textural characteristics of the pebbles can be determined without thin-sectioning. The source of the oolitic pebbles is not known. The textures and fayette" pebble showing: white "glaze" character of of the are many pebbles over part of exterior, and a light band approximated or essentially dupli- concentric with the glaze. Reflected cated in the oolite of the Ste. Gene- light. X 8. vieve and Salem formations, as now than those which normally produce exposed in Illinois, which may have chert in limestones. been the source of many of these pebbles. "Glazed" Pebbles It is of interest that no considerable amount of oolitic chert is known The samples studied, especially

in the Salem, Ste. Genevieve, or nos. 1 and 2, contain pebbles whose Chester formations as now exposed exterior consists partly or wholly of in southern or western Illinois. As white, siliceous, often porcelain-like previously noted, some of the oolite material resembling a "glaze" 7 or pebbles composed of relatively coating (fig. 9). On some pebbles coarse-grained quartz natural show the glaze is about % of an inch thick surfaces like those of a freshly and covers most of the surface. Other broken piece of limestone oolite, a pebbles show only small glazed areas. type of fracture which would prob- The glaze occurs alike on brown and ably not be expected from the silici- black chert pebbles. Both the un- fied pebbles as now found (fig. 8). glazed and glazed parts of many of This phenomenon can be accounted the pebbles are polished in normal for in various ways, but the question is raised whether these pebbles may

because the t i describes the appearance of t not be the result of some special phei „.„,. lis usage is not meant to eonnc

that the glaz as necessarily an addition t set of silicification conditions rather to .

Illinois Academy of Science Transactions

white, and brown. These pebbles are not numerous, but they are significant because they are rare or absent in the other samples. They were observed to be also relatively common in the "Lafayette" gravel in Mis- souri across the Mississippi River from the deposit in Alexander County, 111., from which sample 2 was obtained. The agate pebbles are rounded to irregular.

In sample 1, two brown chert pebbles were partly coated with banded quartz similar to parts of the agate pebbles. Fig. 10.—Chert from a bedrock outcrop showing the development of a white The agate pebbles are of different chalky material on the top of the specimen. Similar material occurs on the types. A common variety is of crys- bottom but is not visible in the photo- talline quartz, showing comb struc- graph. The white chert in other parts of ture in some specimens, which is the specimen is not chalky. About natural size. interspersed between irregular masses of banded chalcedony and within fashion. No glaze was observed on which round agates as much as y2 sandstone, vein quartz, or quartzite inch in diameter are distributed. In pebbles. In sample 1, 22 of the 50 one pebble studied in detail, the con- pebbles examined had more or less tact between the comb quartz and glaze ; 4 of the 50 pebbles in sample the banded chalcedony is transition- 2 had glaze. al and consists of a series of thin alternating bands of crystalline The origin of the glaze cannot be quartz and chalcedony. The crystal certainly determined from the mate- size of the macro-crystalline quartz rial at hand. It is suggested, how- bands decreases away from the comb ever, that a porous, white, weathered quartz and ultimately becomes zone, such as commonly develops on micro-crystalline chert (fig. 10), was later impregnat- ed with silica to produce the hard, The general structure and char- dense exterior material, or glaze. acter of the pebbles suggest that they

Different degrees of erosion of the were originally cavity linings or fill- weathered zone, possibly before im- ings. The chert coated with banded pregnation, would account for the silica presumably represents part of differences in the amount of glaze a cavity lining and cavity wall. now evident. No deposits from which the agate pebbles could have come are known Agate Pebbles in Illinois. Their presence along the

Samples 1, 2 and 3, from the Mis- Mississippi Valley suggests that they sissippi Valley, contain pebbles of may have come from Missouri or agate, mostly in shades of gray, other areas adjacent to the valley. "Lafayette" Gravel

Table 6- -Fauna List*

Sample No. Location Name Age

Grover Mo Mississippian

Fayville, 111.

Coliininaria alveolata? . . . . Upper Ordovician

Mississippian

Cya.lhophyllum Devonian and Mississippian

4 Mississippian Triplophyllum Mississippian to Permian Silurian to Pennsylvanian

5 Grand Chain, 111 Mississippian

6 Mississippian Fenestellid bryozoa Silurian to Permian

* Identification and age by W. H. s State Geological Survey, 1942.

Fossils that there is no need to assume a distant source for the fossiliferous Silicified fossils, commonly as par- chert in any of the southern Illinois tial specimens of individuals, are "Lafayette" deposits studied, as relatively abundant in some deposits southern Illinois or areas adjacent of "Lafayette" gravel. Corals are to it contain outcrops of limestone, most frequent. Some chert frag- some of it cherty, of Ordovician, ments contain a great many crinoid Silurian, and Mississippian age and stems, and molds or partial molds also extensive deposits of Devonian of brachiopods, bryozoa, and gastro- chert formations, especially near the pods. Table 6 lists the genera which localities from which samples 3 and were obtained as a result of limited 4 were taken. The Grover deposit collecting. is similarly situated with reference Undoubtedly a systematic collec- to outcrops of Ordovician, Silurian, tion yield from each locality would and Mississippian rocks. more detailed lists of fauna. The presence of fossil corals, crinoids, Conclusions and brachiopods of the Lower and It appears from the foregoing that 8 Middle Silurian is reported in a a study, including particle size dis- deposit of Lafayette-type gravel in tribution, particle shape, pebble Henry County in northwestern Illi- counts, and paleontology of fossil nois. The data in table molds, of a greater number of samples taken with cognizance of the relation of the deposits to topo- 108 Illinois Academy of Science Transactions

graphic levels might yield informa- special features, such as polish, tion which would add materially to glazed pebbles, agate pebbles, and the knowledge of the source or silicified oolite pebbles, are in them- sources of the "Lafayette" gravel selves problems that merit further in Illinois and adjacent states, and investigation and that may aid in which would also broaden the factual a better understanding of the broad- basis for interpreting the history of er problems of the "Lafayette" the formation of the gravel. Various gravel. Illinois Academy of Science Transactions, Vol. 44, 1951

CAMBRIAN AND LOWER ORDOVICIAN EXPOSURES IN NORTHERN ILLINOIS*

H. B. WILLMAN AND J. S. TEMPLETON Illinois Geological Surrey, TJroana

Drilling in central northern Illi- exposure at Oregon lies near the nois has revealed the presence of crest of the Oregon anticline, but Cambrian and lower Ordovician all of the remaining exposures are strata beneath glacial drift in a large near the crest of a major uplift on area south of the Sandwich fault the south side of the Sandwich fault zone, as shown on the state geologic zone, herein named the Ashton Arch map (Weller and others, 1945), and (figs. 1, 4). The exposures all are one exposure of Cambrian strata at located on preglacial bedrock up-

Oregon, Ogle County, 111., has long- lands (Horberg, 1950, plate 1, sheet been known (Bevan, 1929, 1935, 1 ) . The absence of outcrops between 1939). No other Cambrian expo- the Fox River and Rochelle is ex- sures and no exposures of lowermost plained by the presence of two Ordovician have been recognized major southward-trending pregla- previously in Illinois. cial valleys and deposition of a thick The present study has shown that Wisconsin morainic belt over the exposures west of the area mapped site of the valleys. as Cambrian, formerly believed to be of Shakopee, Platteville, or Gal- Stratigraphy ena age, actually belong to the Cam- The exposed pre-St. Peter forma- brian Trempealeau formation and to tions of central northern Illinois are the lower Ordovician Gunter and shown in columnar section in figure Oneota formations. The area under- 2, and are described below. lain by Cambrian and lower Ordo- Franconia formation. — The vician rocks has been extended west- Franconia formation, the oldest forward from Rochelle nearly to Eock mation exposed in Illinois, is known Eiver, and the Sandwich fault zone to crop out only in the lower part has been traced west of Rock River of an abandoned quarry 3,700 feet (fig. 1). Outcrops near Millbrook, from the north line and 1,100 feet Kendall County, previously assigned from the east line of sec. 3 (elon- to the Galena formation, now are gate), T. 23 N., R. 10 E., Ogle known to be Oneota. The exposures County (Oregon quadrangle), one- show that at least the lower part of half mile east of Oregon (fig. the Trempealeau formation in this 1). The exposure is poor and has a total region is a reef-type dolomite. thickness of about 30 feet. It con- Identification of the formations sists mainly of argillaceous, silty, has been based mainly upon lith- glauconitic and muscovite-bearing, ology as revealed by samples from partly dolomitic, greenish-gray sand- wells, but partly upon fauna. The stone which ranges from very fine grained to coarse grained, but is Illinois Academy of Science Transactions

Fig. 1.—Location of Cambrian, Gunter, and Oneota exposures in northern Illinois.

mostly fine grained. The grains are The following fossils from the clear and mainly angular, although sandstone, identified by G. 0. the medium and coarse grades are Raasch, show that the strata belong rounded. Abundant scattered fine to the Bad Axe or highest member

pellets of dark green glauconite give of the Franconia formation : Dikel- the rock a "salt-and-pepper" aspect. locephalns n. sp., Illaenurus n. sp., Green glauconitic clay occurs as Saukiella minor Ulrich and Resser, partings between the beds and as and Trilobita n. gen. and sp. In ad- streaks and mottlings within the dition, small fragments of oboloid beds. The heavy mineral suite con- brachiopod shells are common, and sists of abundant garnet, with sub- ropy excremental worm-castings cov- ordinate amounts of tourmaline and er many bedding surfaces. zircon. The sandstone is laminated Subsurface data indicate that the and friable. It is in lenticular to Franconia formation, exclusive of regular beds from a fraction of an Ironton strata, ranges from 65 to inch to 9 inches in thickness. A one- 120 feet thick in north-central Illi- foot bed of impure, buff, chalky nois. The middle portion of the for- dolomite in thin irregular layers is mation consists of sandy, glauconitic present 20 feet below the top of the dolomite and limestone, but the formation (fig. 3), and scattered lower part is composed of sandstone dolomite lenses occur elsewhere. and sandy dolomite like the higher, Partings of silty, partly dolomitic, exposed beds. A thin, dolomitic, olive-gray, laminated, friable shale coarsely glauconitic, coarse-grained are common. sandstone, formerly regarded as the Cambrian and Lower Ordovician Exposures

Sandstone, pure, white, fine- to St. Peter medium- grained, thick-bedded, wit h red to green clay at base. LOWER ORDOVICIAN Dolomite, variable, mostly argillaceous, finely cryslalline and medium -bedded, Shokopee with layers of sandstone, siltstone, shale, and oolitic chert.

/v,/y,/,. Sandstone, dolomitic, fine-grained, cross- l New Richmond l6-80' h /•;//•/ bedded, with layers of sandy ZT2Z litho graphic dolomite and chert. 7W Dolomite, coarsely crystalline, partly finely crystalline at top and bottom, 116-180' X77s sandy towards base, mostly thick- TM- bedded, with oolitic chert.

\!Jj.:./$ inter 17-55' UPPER CAMBRIAN

Dolomite, glauconitic, finely crystalline; Trempealeau 145-175' upper portion (unexposed) is cherty and sandy; lower portion is pure,

Fig. 2.—Pre-St. Peter formations exposed in northern Illir Illinois Academy of Science Transactions Cumbrian and Lower Ordovician Ex

'** 'Up— ri^-\. f

"' * h*m '

''*$$

' ''<& /: ^V .

basal member of the Franconia, now is considered the top unit of the Ironton formation. The Franconia formation is correlated tentatively with the Davis, Derby, and Doerun formations of Missouri (Workman and Bell, 1948, p. 2053). Trempealeau formation. — Twen- ty-three feet of basal Trempealeau dolomite overlies Franconia sandstone in the quarry at Oregon. An extension of this exposure is found in a low ridge littered with Trem- pealeau float % mile northwest of the quarry, 500 feet from the north line of sec. 3 and 1,700 feet from the west line. The major Trempealeau exposures found in this study are as follows (fig. 4) : (1) Quarry at Prairie Star School, 7 miles southeast of Oregon,

SW14 SW14 SE% sec. 5, T. 22N., R. 11 E., Ogle County (Dixon quad- Fig. 6. —Trempealeau and Gunter dolomites in quarry south of Rochelle, N. rangle), where 18 feet is exposed line NE14 SE1/4 NE1/4 Sec. 26, T. 39 N., (fig. 5). R. 1 E., Lee County, 111. Illinois Academy of Science Transactions

y ° n ' —A, Breccio \ Oon

SCALE IN FEET

Fig. 7.—Diagrammatic sketch of pre-St. Peter exposures in hill on north side of State Highway No. 2, NE% SE% SE14 Sec. 17, T.23 N., R.10 E., Ogle County, 111. Ct, Trempealeau; Oon, Oneota; Onr, New Richmond; Os, Shakopee; Osp, St. Peter, mainly filling pre-St. Peter channels.

(2) Three quarries 1% miles A few small outcrops of Trempea- northeast of Ashton, NWy4 NWy4 leau dolomite occur in the vicinity NE*4 sec. 22, SE% NW*4 NW% of the principal exposures (fig. 4). sec. 23, and NE corner sec. 23, T. 22 The area has not been mapped in N., R. 11 E., Lee County (Eochelle detail, and other outcrops may be quadrangle), where 14 to 22 feet of present. dolomite is exposed. The Trempealeau formation is composed of light buff, finely crystal- (3) Quarry 6 miles south of line to medium crystalline, reef-type Rochelle, N. line NB% SE^ NE%* dolomite, which is finely porous to sec. 26, T. 39 N., R. 1 E., Lee County vuggy, and occurs mainly in irregu- (Rochelle quadrangle), where 6 feet lar beds from 1 to 4 feet thick. The of dolomite is exposed, with 5 addi- basal beds at Oregon are glauconitic tional feet below water level (fig. 6). and silty, but the dolomite in the (4) Bank on north side of State remaining and stratigraphieally Highway 2, 2% miles southwest of higher exposures is pure. Many vugs Oregon, NE% SE^ SB 14 sec. 17, are lined with white to pink crys- T. 23 N, R. 10 E., Ogle County talline quartz which readily differ- (Dixon quadrangle), where 4 feet entiates the Trempealeau from other of dolomite is exposed at road level formations in this region. In places and 3 feet is exposed 15 feet above the bedding disappears into or is road level (fig. 7). draped over structureless columnar Cambrian and Lower Ordovician Exposures 115

masses of dolomite more than 10 feet correlated with the Eminence forma- thick, which are interpreted as reef- tion of Missouri (Workman and cores of algal origin. Weathered Bell, 1948, p. 2053) and with the bedding surfaces commonly are cov- Jordan and Madison members of the ered with a network of low, narrow, Trempealeau formation in Wiscon- vertically-sided ridges, and weath- sin. Continuous subsurface tracing ered faces are marked by narrow, of the Illinois Trempealeau units irregular, vertical flutings; these into the Wisconsin outcrop area is phenomena also may be of algal prevented by the presence of a ma- origin. The rock contains algal jor westward-trending pre-St. Peter domes and rare trilobite and gastro- valley just north of the Cambrian pod remains. G. 0. Raasch has iden- exposures, along which much or all tified the following forms: Crypto- of the Trempealeau formation has zoon sp. Hypseloconus sp. nov., and been removed (Meyer, 1948).

Saukiella sp., cf. indenta Ulrich and In Wisconsin the Franconia-Trem- pealeau contact is marked by a In the subsurface of central north- diastem (Twenhofel, Raasch and ern Illinois the Trempealeau forma- Thwaites, 1935, pp. 1705-1706, tion ranges from 145 to 175 feet 1717), but in the Illinois subsurface, thick where not reduced by pre-St. Pranconia-Trempealeau relations Peter erosion. It consists of (1) a are conformable and nearly grada- lower, partly glauconitic dolomite tional. The contact at Oregon is containing quartz-lined vugs, with- concealed by a short covered inter- out disseminated sand except in val. The Trempealeau-Oneota 1 con- places at the base, and (2) an upper, tact in Wisconsin seems unconform- cherty, mostly sandy dolomite which able (Twenhofel, Raasch and lacks quartz-lined vugs and com- Thwaites, 1935, pp. 1712, 1718). In monly is non-glauconitic. All of the the Illinois subsurface the Trem- exposed Trempealeau belongs to the pealeau-Gunter contact is abrupt, lower unit, which is correlated with and in the quarry at Rochelle it is the Potosi formation of Missouri sharp and undulatory, with a relief (Workman and Bell, 1948, p. 2053) of six inches. These features, to- and with the St. Lawrence and Lodi gether with the absence of upper members of the Trempealeau forma- Trempealeau (Eminence) strata in tion in Wisconsin. Ogle County, suggest that a diastem South of the area of Trempealeau or unconformity also separates the exposures glauconite in the lower Trempealeau and Gunter formations unit occurs in three discontinuous in northern Illinois. zones of irregular thickness (Will- Except at Rochelle the exposed man and Payne, 1942, 56-57). pp. Trempealeau is overlain unconform- The upper unit apparently is not ably by St. Peter sandstone or by exposed. Although it is encountered glacial drift. At Oregon the upper in wells at Ashton and Dixon, Lee Trempealeau beds in places have County, it lenses out northward and is absent at Rochelle and elsewhere ^Although 1he Gunter formation is present in in Ogle County. The upper unit is ;

116 Illinois Academy of Science Transactions been leached and altered to siltstone to a depth of at least 4 feet by pre- St. Peter weathering, and just south of the quarry the Cambrian strata abut against a channel-filling of St. Peter sandstone. Post-St. Peter thrust-faulting has altered the sandstone overlying the Trempealeau formation to a quartzite-breeeia (Bevan, 1935, p. 384).

Gunter formation.—In wells in central northern Illinois, 17 to 55 feet of impure dolomite and sandstone lies between the Trempealeau and Oneota dolomites. In the past these beds have been variously assigned to the Jordan, Gunter-Jor- dan, or Oneota formations. Recent stirdies have shown that they are very similar lil hologically to the type Fig. 8. Gunter and Oneota Gunter formation of Missouri, and — dolomites in quarry south of Rochelle, N. line can be traced into that formation NE14 SE% NE14 Sec. 26, T. 39 N., R. they also can be traced to southern 1 E., Lee County, 111. Wisconsin, where they overlie the latest Cambrian Madison sandstone of argillaceous, silty, finely glau- and are included in the Oneota for- conitic, greenish-gray to cream, mation. Differentiation of Gunter chalky dolomite which contains very strata thus involves a slight redefini- fine muscovite flakes, and is slightly tion of the term Oneota as used in sandy in the lower 2 feet 9 inches. Iowa and Wisconsin. In Minnesota The dolomite is in irregular, lami- the Kasota and Blue Earth forma- nated beds from 1 to 12 inches thick, tions (Powell, 1935) probably are separated by green clay partings. equivalent to all or part of the Some layers are algal and have high- Gunter formation. ly undulatory bedding surfaces. Ir- Known Gunter exposures in Illi- regular masses of oolitic chert are nois are confined to the quarry 6 common near the base and middle miles south of Rochelle, described of the formation. Five feet below above under Trempealeau formation, the top of the Gunter is a 30-inch and to a small exposure in a pros- bed of buff, medium crystalline, 1 1 pect pit and ravine, NW /! NW ^ vuggy, cherty dolomite which closely SW14 sec. 31, T. 23 N., R. 11 E., resembles the overlying Oneota for- Ogle County (Dixon quadrangle), mation. The exposure 5 miles south- 5 miles southeast of Oregon (fig. 4). east of Oregon is very poor, and is In the Rochelle quarry (figs. 6, 8) composed of eight feet of impure, the Gunter formation is 10 feet 3 thin-bedded dolomite like that at inches thick, and consists principally Rochelle, but partly maroon. The Cambrian and Lower Ordovician Exposures 117

Gunter here is overlain by two feet quarry, SE corner NE 1/^ NE^i 1 of Oneota dolomite, but no Trem- SE /^ sec. 8, and 2 feet is exposed pealeau strata are exposed. in an abandoned quarry, SW corner In subsurface (Workman and SWy4 SWy4 NWy4 sec. 9. Bell, 1948, pp. 2054-2055) the Gun- (2) In the quarry 6 miles south ter formation consists of (1) very of Rochelle, where 31 feet is exposed fine- to coarse-grained, poorly sorted (fig. 8), and in a shallow quarry to locally well sorted, incoherent Y^ mile eastward. sandstone, (2) varicolored dolomite (3) At Ashton, Lee County, in which is chiefly chalky to litho- three quarries in the NW14 sec. 27, graphic and dense, and is partly in two quarries on opposite sides of argillaceous, silty or sandy, (3) vari- the road along the north line of the oolitic to locally sandy chert colored, Ey2 sec. 27, just east of State High- which is principally white to orange, way 330, and in a shallow quarry in and (4) small amounts of slightly the center of the NE14 sec. 27, T. 22 sandy, blue-green and dark-red, N., R. 11 E. (Rochelle quadrangle). laminated, hard, smooth shale. Here from 10 to 31 feet of rock is The sandstone ordinarily is clean exposed in the individual quarries, and white, but locally is argillaceous and the total stratigraphic interval and gray-green. The fine sand is represented in the quarries exceeds angular; the medium and coarse 50 feet. sand is mostly rounded, although (4) Shallow quarries 2 and 3% some grains have been made angular miles northwest of Ashton, W. line crystalline quartz. by secondary The NEi/4 NEi/4 NWy4 sec. 20 and normally poorly sorted sandstone NWy4 NWi/4 SWy4 sec. 18, T. 22 contains free, medium to coarse, sili- N., R. 11 E., Lee and Ogle counties, ceous oolites. A five-foot bed of ar- exposing 8 and 16 feet of dolomite, gillaceous, fine- and coarse-grained respectively. sandstone marks the top of the Gun- (5) Quarry 5 miles southeast of ter in wells between Dixon and Ore- Oregon, S. line SEy4 NWy4 NEy4 gon, but lenses out eastward before sec. 31, T. 23 N., R. 11 E., Ogle reaching Ashton. County, exposing 12 feet of deeply Small Cryptozoon domes in the weathered rock. quarry south of Rochelle are the only (6) Quarry and ravine 4 miles fossils observed in Gunter strata. south of Oregon, SW^4 SW% sec. Oneota formation.—The principal 26, T. 23 N., R. 10 E., Ogle County, exposures of the Oneota formation exposing 45 feet of dolomite. in northern Illinois are as follows Cut on north side of State (figs. 1, 4) : (7) Highway 2, two miles south of Ore- (1) On the west side of Fox SEi/4 sec. T. 23 River opposite Millbrook, Kendall gon, NE14 SEy4 17, N., R. 10 E., Ogle County. County, where 12 feet crops out in a stream channel at a breached earth (8) Ravine 6 miles south of Ore- dam, SE14 NWiy4 SEy4 sec. 8, T. gon, SE14 SEi/4 SEi/4 sec. 4, T. 22 36 N., R. 6 E. (Sandwich quad- N„ R. 10 E., Ogle County, where rangle) ; 10 feet is exposed in a small 35 feet of dolomite crops out. 118 Illinois Academy of Science Transactions

Localities 4 through 8 are located this basal zone greenish-gray ; to buff in the Dixon quadrangle. A few shale partings are common, and the other small, scattered exposures less upper 6 inches of the zone are silty than 5 feet thick also occur near the and laminated. Here the basal zone principal localities listed above (fig. represents a transition from Gunter 4). to typical Oneota sedimentation.

Most of the Oneota formation con- Gray, white, and light yellow to sists of light-gray to blue-gray, uni- pink or red chert, most of which is formly coarsely crystalline, dense, sandy, oolitic, conglomeratic, or cherty dolomite in massive beds as strongly banded, occurs throughout much as 15 feet thick. Most indi- the Oneota formation. In places the vidual crystals range from 1 to 2 chert forms layers, lenses, or spher- mm. in diameter, and make the rock oidal masses as much as 1 foot thick, the most coarsely crystalline dolo- but in others it consists of numerous mite in the Illinois Paleozoic se- small, irregularly shaped and orient- quence. Upon weathering the dolo- ed, angular fragments which locally mite breaks down into regular, fri- form a disconnected meshwork with- able, 6- to 24-inch beds with dull in the beds. The lower 12 to 60 feet brown-gray, coarsely "sandy" sur- of the formation ordinarily are fine- faces, and the interior turns light ly glauconitic and somewhat sandy; buff with white flecks representing a few sandy streaks are found at decayed cement between the dolo- higher horizons. mite crystals. Vertical faces gener- Cryptozoon is present to common ally are regular, although occasional in nearly all Oneota exposures and beds have a brecciated or pseudo- forms conspicuous small domes on fucoidal structure and a rough face. the floor of the quarry at Ashton, Bedding surfaces are rough but have 1 center NE /^ sec. 27 (part of Lo- low relief. Except for occasional cality 3). Gastropod molds pre- green clay films within the beds the served mainly in chert from the rock is pure. At one locality (No. quarry at Ashton, SW*4 SW% 8) thirty-five feet of upper Oneota SE14 sec. 22 (part of Locality 3) dolomite is mainly finely crystalline have been identified as : Gasconadia and weathers to a coarsely pitted putilla (Sardeson), Ophileta sp., face; these features have not been Bhachopea sp., and Sinuopea sp. noted elsewhere and apparently re- Identifiable fossils other than Cryp- flect local facies-change. . tozoon have not been seen elsewhere. Basal Oneota strata exhibit con- The fauna is typical of the Oneota siderable differences from the main formation of Wisconsin and the Gas- body of the formation. Much of the conade formation of Missouri (Ul- lowermost 9 to 10 feet is lithographic rich, Foerste and Bridge, 1930, pp. to finely crystalline, is in regular 186-222; Branson, 1944, pp. 35-47). 2- to 8-ineh layers with smooth bed- Correlation of this exposure with ding surfaces, and weathers deep cuttings from two Chicago and buff. In the quarry south of Rochelle North Western Railway wells at some boulder-like, algal masses of Ashton (Illinois Geological Survey, lithographic limestone are present in sample sets 289 and 402), y2 mile Cambrian and Lower Ordovician Exposures 119 south, show that the quarry floor is (Willman and Payne, 1943, pp. 532- not less than 60 feet above the 533; Workman and Bell, 1948, pp.

Gunter-Oneota contact. Part or all 2057-2058) . In central northern Illi- of this concealed interval is believed nois its thickness ranges from 16 to equivalent to the Van Buren forma- 80 feet, and increases rapidly to a tion of Missouri (Workman and known maximum of 190 feet at Bell, 1948, p. 2056). Starved Rock State Park, LaSalle The total thickness of the Oneota County. It is correlated with the dolomite in central northern Illinois Roubidoux formation of Missouri, ranges from 116 to 180 feet. The which carries the distinctive Lecano- formation shows pronounced thin- spira fauna. along the crest of the western ning Exposures of New Richmond in part of the Ashton arch. Illinois previously had been known The contacts of the Oneota dolo- only along Franklin Creek, Lee mite with the adjacent Gunter and County, and Fox River, LaSalle New Richmond formations seem con- County (Cady, 1920, p. 107; Knap- formable. Although Payne (Will- pen, 1926, pp. 40-43; Willman and man and Payne, 1942, pp. 58-59) Payne, 1943, pp. 532-533). The reports an unconformity between the present study revealed poor expo- Gunter ("Jordan") and Oneota sures in (1) a small quarry 3% formations to the southward, the miles east of Grand Detour,' SE14 presence of such an unconformity SE14 NWy4 sec. 9, T. 22 N., R. 10 has not been confirmed by recent E., Ogle County (Dixon quadran- studies. gle), where 3 feet of sandstone is ex- New Richmond formation.—The posed beneath Shakopee dolomite, New Richmond formation superfi- and (2) a cut on the north side of cially resembles the St. Peter sand- State Highway 2, two miles south of stone. However, it generally (1) Oregon, NE*4 SE*4 SE% sec. 17, T. has finer, more angular, less frosted 23 N., R. 10 E., Ogle County (Dixon grains, which more frequently are quadrangle), where 11 feet of sand- enlarged by secondary crystalline stone, with covered intervals above quartz, (2) is less well sorted, (3) and below, crops out between Oneota contains free siliceous oolites and and Shakopee strata. Samples from grains or nodules of chert, (4) has both outcrops are slightly dolomitic a much higher proportion of heavy and show the typical New Richmond minerals, with abundant accessory heavy mineral suites (table 1). It tourmaline, considerable ilmenite is very probable that other outcrops and leucoxene, and some garnet, of New Richmond exist in the north- (5) is much thinner-bedded and east quarter of the Dixon quad- more strongly cross-bedded, and (6) rangle, where they have not been is better cemented. It contains beds differentiated from St. Peter sand- of lithographic dolomite and dolo- stone. Because the New Richmond mite-cemented sandstone, has green offers little resistance to erosion, ex- argillaceous streaks in the upper posures not on major stream chan- part, and has gray, blue, or red nels are mostly veneered by soil or shale layers in the lower portion Shakopee float. —

Illinois Academy of Science Transactions

Table 1. Heavy Mineral Analyses of common. Bedding surfaces generally New Richmond Sandstone in Northern Illinois, in are ripple-marked and mud-cracked, Percent and show occasional molds of cubic salt crystals. Beds of relatively pure, medium crystalline dolomite, Locality 1 2 3 as much as 18 inches thick, are common in the lower part of the forma- Garnet 3.0 17.0 3.0 tion. Although such beds locally re- Ilmenite 17.0 17.0 20.0 semble Oneota dolomite they lack the distinctive chert layers, chert Leucoxene 22.0 10.0 18.0 fragments, and coarsely "sandy" Magnetite 1.0 6.0 2.0 weathered surfaces of the latter formation, are thinner-bedded, and con- Tourmaline 42 . 35.0 36.0 tain thin interbeds of typical Shako- Zircon 12.0 10.0 15.0 pee argillaceous dolomite. The chert is mostly oolitic, but partly sandy Miscellaneous. . 3.0 5.0 6.0 or conglomeratic. The sandstone Percent of heavy heavy mineral suites minerals by have a higher weight 0.006 0.003 0.006 percentage of garnet and ferromag- nesian minerals than do those of the New Richmond sandstone (Willman Locality 1: 5' below top of sandstone in bluff on north side of Franklin Creek, and Payne, 1943, p. 535). Several N. line SE^ NW% SW% sec. 34, T. 22 layers of bentonite are present in a N., R. 10 E., Lee County. Locality 2: 2' below top of sandstone in quarry, quarry in the SW^ SE^ NW% SEy4 SE14 NW14 sec. 9, T. 22 N, R. 10 sec. 6, T. 23 N., R. 10 E., Ogle E., Ogle County. Locality 3: 15' below County (Oregon quadrangle), 2y top of sandstone in bluff on west side 2 of State Highway 2, NE% SE% SE14 miles west of Oregon. sec. 17, T. 23 N., R. 10 E., Ogle County. The thickness of the Shakopee in locality ]t : Average of 19 samples from upper 23 feet of sandstone at ravine central northern Illinois ranges from mouth, sec. 8, T. 35 NW% SW14 SWy4 zero to about 165 feet, but in- N., R. 5 E., LaSalle County. Analyses for localities 1, 2 and 3 by T. C. Busch- creases rapidly to more than 600 feet bach; analyses for locality 4 by Paul in south-central Illinois. The Shako- Herbert, Jr. pee is correlated with the Jefferson City and Cotter formations of Mis- Shakopee formation.—The Shako- souri (Workman and Bell, 1948, pee formation consists of irregular, pp. 2058-2060), although equivalents alternating, varicolored beds of dolo- of younger Missouri formations may mite, shale, siltstone, and sandstone, be included locally. with dolomite greatly predominat- ing. The dolomite shows great varia- The Shakopee dolomite rests con- tion in purity, color, crystal size, formably on New Richmond sand- is over- porosity, and bedding. Most of it stone, but separated from the is argillaceous, silty, sandy, cherty, lying St. Peter sandstone by a major locally glauconitic, light gray to yel- unconformity. low-buff, chalky to finely crystalline, Known exposures of the Shakopee dense, and thin-bedded. Brecciated formation in Illinois are found (1) and conglomeratic beds are very along the Illinois River and its tribu- Cambrian and Lower Ordovician Exposures 121

taries from LaSalle east to Utica, east corner of Illinois (Bays and LaSalle County (Cady, 1919, pp. others, 1945; Cohee, 1945, fig. 4; 35-36), (2) along- Fox River near Meyer, 1948) and (2) that the post- Sheridan and Millington, in LaSalle St. Peter axis (Cohee, 1945, fig. 5; and Kendall counties (Willman and 1948, p. 1441, fig. 1) merges into Payne, 1943, pp. 534-538), and (3) the east flank of the Hersher anti- along Rock River and its tributaries cline which is separated from the between Franklin Grove, Lee Ashton arch by a northward-trend- County, and a point 2y2 miles west ing syncline (fig. 9). Accordingly of Oregon, Ogle County. Its dis- it appears desirable to introduce a tribution in the latter area is shown separate name, Ashton arch, for the in fig. 4. relatively local uplift between Will and Ogle counties. Structural Geology The Ashton arch is bounded on Ashton arch.—Most of the Cam- the north throughout most or all of brian and lower Ordovician expo- its length by the Sandwich fault zone. a syncline sepa- sures described above lie on the A graben and rates the western crest or flanks of a major anticline part of the arch which trends N. 60° W. across north- from the smaller, parallel Oregon anticline the (fig. ern Illinois from central western on north 4). A Will County to central Ogle County, syncline also separates the eastern portion of the arch from the LaSalle and which is here named the Ashton anticline (fig. 9). The LaSalle anti- arch (figs. 1, 4). The name is derived from the town of Ashton, Lee cline merges into the arch along the LaSalle-Lee County boundary, and County, which is located near the cannot distinguished crest of the western portion of the be farther north. arch, and near which Cambrian However, a southward prong and lower Ordovician formations, of the arch, which may represent a continuation of the LaSalle anti- brought to the surface by the arch, are well exposed. clinal trend, extends northwestward from Dixon to the Savanna-Sabula The Ashton arch first was rec- anticline. The southwestward flank ognized by Cady under the name of the Ashton arch dips steeply into Ogle, Lee, and LaSalle counties anti- the Illinois basin. The western end cline (Cady, 1920, pp. 90, 127-128, plunges into the small but deep Polo fig. 8), although later studies modi- basin, named herein from the town fied the original concept of its loca- of Polo, Ogle County, which lies on tion and extent. It formerly was the west side of the basin. considered to mark the crest of the Kankakee arch, a broad uplift which The Ashton arch has a length of connects the Cincinnati and Wis- 80 miles and a width of 17 to 25 consin arches and separates the Illi- miles. The structural relief on the nois and Michigan basins (Pirtle, southwestern side is about 1,900 feet, 1932, p. 149, fig. 1; Ekblaw, 1938). and the maximum relief on the However, it has been found (1) that northern side at least 900 feet. The the pre-St. Peter axis of the Kanka- axis of the arch is at, or a short dis- kee arch passes through the north- tance south of, the Sandwich fault Illinois Academy of Science Transactions Cambrian and Lower Ordovician Exposures zone. The arch has a broad, nearly branch uplifts from the main body flat summit area, most of which dips of the Wisconsin arch on the north. gently southward. The steepest very A narrow graben and a tightly on the structure, found on the dips compressed syncline, bounded on the southwest flank, are only from 1° south by the Sandwich fault zone, C to 3 . separate the Oregon anticline from

Sandwich fault zone and Oregon the Ashton arch (fig. 4) . The graben anticline.—The Sandwich fault zone is well exposed in the cut on the parallels the strike of the Ashton Chicago, Burlington, and Quincy arch and bounds the northeastern Railroad, just south of the center side of the arch for most or all of of see, 7, T. 23 N., R. 10 E., where its length. It has been traced from a dropped block of Galena dolomite southern Will County to a point in is bounded by faults which bring northeastern Lee County 6 miles the top of the St. Peter sandstone on south of Rochelle, a distance of 68 the southwest and lower Platteville miles, and probably continues at beds on the northeast to the same least 20 miles farther, to a point 2% elevation as the Galena strata. The miles southwest of Oregon, in central throw on the south side of the block

Ogle County (figs. 1, 4, 9). The is more than 165 feet and that on the fault zone has a maximum down- north side is over 130 feet, The throw of at least 900 feet on the block is about one-half mile wide and northeast side. Because the fracture is cut by numerous minor step- appears to be compound rather than faults. Sharp synclinal folding with- single the term fault zone is applied. in the graben is well developed in Platteville strata exposed at inter- Between Rochelle and Oregon the vals from the NW% SW^ SE*4 structure on the northeastern or see, 26, T. 23 N., R. 10 E., to the downthrown side of the fault is com- NEi/4 NWy NEi/4 sec. 31, T. 23 plicated by a sharp uplift called the 4 N., R. 11 E., Ogle County (Dixon Oregon anticline (Bevan, 1935), the quadrangle), where northward dips crest of which approximately paral- range from 2° to 10° and southward lels the Ashton arch and Sandwich dips from 14° to 30°. fault zone and lies from 2 to 3 miles northward from the latter. The Cam- Structural history. — The major brian exposure at Oregon, a small movement along the LaSalle anti- Oneota exposure in the SE14 SE14 cline in northe r n Illinois was NBi/4 sec. 30, T. 22 N., R. 11 E., post-Mississippian, pre-Pennsylvan- Ogle County (Dixon quadrangle), ian, followed by lesser uplift in post- and a Shakopee exposure in the Pennsylvanian time (Payne, 1939). SWi/4 SE% NW14 sec. 6, T. 23 N., On the Ashton arch, Sandwich fault R. 10 E., Ogle County (Oregon zone and Savanna-Sabula anticline, quadrangle), are located on the anti- and probably on the Oregon anticline. The Oregon anticline contin- cline as well, the principal movement ues northwestward to join the Sa- was at least post-Silurian, and may vanna-Sabula anticline. A synclinal have taken place at about the same belt separates the Oregon-Savanna- time as uplift on the LaSalle anti- Sabula anticlines and associated cline. It is uncertain whether the 124 Illinois Academy of Science Transactions

Ashton arch owes its present form structure in amount or direction solely to downthrow along the north- (Willman and Payne, 1943, fig. 1). ern side of the Sandwich fault zone, At Oregon, Ogle County, a post- or whether the arch formed prior Shakopee, pre-St. Peter fault cut- to the faulting. ting Cambrian strata has a downthrow of 285 feet on the western The general northwestward to side. Where adequate subsurface westward structural grain of north- control is available the interval be- ern Illinois (fig. 9) is affected by tween the of subordinate northeastward-trending top the Glenwood formation and pre-St. Peter horizons cross folds of uncertain age. It is in central northern Illinois shows possible that the post-St. Peter Kan- sharp irregularities which cannot be kakee arch of northwestern Indiana, explained by variation in the thick- uplifted mainly in post-Pennsyl- ness of the formations, and which vanian time, once was continuous are attributed to post-Shakopee, pre- with the Ashton arch, but was sepa- St. Peter deformation. Unfortunate- rated from it by later cross folding. ly the amount or trend of such de- There is considerable evidence for formation along most of the Ashton pre-St. Peter deformation in the arch cannot be estimated because area occupied by the Ashton arch, (1) the deeply incised pre-St. Peter although generally it cannot be drainage system makes the base of proved that the trend of the defor- the St. Peter valueless for struc- mation paralleled that of the present tural control, and (2) erosion has structure. Upper Trempealeau stripped away the post-St. Peter (Eminence) strata thin or wedge formations. out northward over the arch, but reappear in northernmost Illinois REFERENCES and southern Wisconsin, across a major pre-St. Peter valley. The Bays. C. A. and Others (1945), Ground- Oneota formation thins conspicuous- water geology of northeastern Illinois: ly over at least the western part of Illinois Geol. Survey, unpublished manuscript. the arch, parallel to its general Bevan, A. C. (1929), Fault block of trend. The earlier movements cul- Cambrian strata in northern Illinois minated in uplift, folding, and ero- (abstract): Geol. Soc. Amer. Bull. 40. p. 88. sion in post-Shakopee, pre-St. Peter (1935), Cambrian inlier at Ore- time. At places in Lee and Ogle gon, Illinois: Kansas Geol. Soc. Guide- counties Shakopee strata are thrown book 9th Ann. Field Conf., pp. 383-385. into close folds which do not affect (1939), Cambrian inlier in northern Illinois: Am. Assoc. Petr. Geol. the overlying St. Peter sandstone Bull. 23, pp. 1561-1564. 83-84, al- (Knappen, 1926, pp. 111) ; Branson, E. B. (1944), Geology of Mis- though formerly attributed to pre- souri: Univ. of Missouri Studies, vol. 19, no. 3. St. Peter slumping, the folding is Cady, G. H. (1919), Geology and min- considered diastrophic by the writ- eral resources of the Hennepin and ers. Along Fox River in LaSalle LaSalle quadrangles: Illinois Geol. and Kendall counties the Shakopee Survey Bull. 37. (1920), Structure of the LaSalle shows steep dips which do not ac- anticline: Illinois Geol. Survey Bull. cord with the gentler post-St. Peter 36, pp. 85-180. Cambrian and Lower Ordovician Exposures

Cohee, G. V. (1945), Sections and maps Powell, L. H. (1935), A study of the of Cambrian and Lower Ordovician Ozarkian faunas of southeastern Min- rocks in the Michigan basin, Michigan nesota: St. Paul Inst. Sci. Mus. Bull. 1. and adjoining areas: U. S. Geol. Sur- Twenhofel, W. H., Raasch, G. O., and vey, Oil and Gas Inv., Prelim. Chart 9. Thwaites, F. T. (1935), Cambrian (1948), Cambrian and Ordovician strata of Wisconsin: Geol. Soc. Amer. rocks in Michigan basin and adjoin- Bull. 46, pp. 1687-1744. ing areas: Am. Assoc. Petr. Geol. Bull. Ulrich, E. O., Foerste, A. F., and Bridge. 32, pp. 1417-1448. Josiah (1930), Systematic paleontol- Ekblaw, G. E. (1938), Kankakee arch in ogy of late Cambrian and early Or- Illinois: Geol. Soc. Am. Bull. 49, pp. dovician formations of Ozark region, 1425-1430; Illinois Geol. Survey Cir. 40. Missouri: Missouri Bur. Geol. and Horberg, C. L., (1946), Pre-glacial ero- Mines, 2d ser. vol. 24, pp. 186-222. sion surfaces in Illinois: Illinois Geol. Weller, J. M. and Others (1945), Geo- Survey Rept. Inv. 118, fig. 2. logic map of Illinois: Illinois Geol. Knappen, R. S. (1926), Geology and Survey. mineral resources of the Dixon quad- Willman, H. B. and Payne, J. N. (1942), rangle: Illinois Geol. Survey Bull. 49. Geology and mineral resources of the Meyer, M. P. (1948), Pre-St. Peter struc- Marseilles, Ottawa, and Streator quad- ture and topography of northern Illi- rangles: Illinois Geol. Survey Bull. 66. nois: Illinois Geol. Survey, unpub- (1943), Early Ordovician strata lished manuscript. along Fox River in northern Illinois: Payne, J. N. (1939), The age of the Jour. Geol., vol. 51, pp. 531-541; Illi- LaSalle anticline: Illinois Acad. Sci. nois Geol. Survey Circular 100. Trans., vol. 31, pp. 182-183. Workman, L. E. and Bell, A. H. (1948), Piktle, G. W., (1932), Michigan struc- Deep drilling and deeper oil possibili- tural basin and its relationship to sur- ties in Illinois: Am. Assoc. Petr. Geol. rounding areas: Am. Assoc. Petr. Geol. Bull. 32, pp. 2041-2062; Illinois Geol. Bull. 16, pp. 145-152. Survey Rept. Inv. 139. Illinois Academy of Science Transactions, Vol. 44, 1951

BARITE IN THE LASALLE LIMESTONE OF ILLINOIS*

RAYMOND S. SHRODE Illinois State Geological Surrey, Urbana

Many strata in the LaSalle lime- which owes its color to a very thin stone contain quartz, pyrite, and coating, believed to be ferruginous. vugs of crystalline calcite in relative The triangular shape of many of the abundance. To date, however, the grains results from the exposure of presence of barite has not been re- only three faces of rhombic crystals. ported. This paper discusses and de- Some dark calcite grains are also scribes an occurrence of this mineral. evident adhering to the main body The barite was found in the upper of the barite. part of the LaSalle Limestone near Figure 2 is another cavity which the top of the limestone exposed in has been sawed, smoothed and etched the abandoned LaSalle Stone Com- with hydrochloric acid to bring the pany quarry near LaSalle, in the barite into relief. The white-appear- sec. 13, T.33 N. SW14 SW14 NE% ing barite in the center of the figure R.l E. and also in another quarry is partially surrounded by dark, adjacent on the west. The mineral coarsely crystalline calcite which is easily distinguished in hand speci- blends into more finely crystalline mens of the limestone by its light clear calcite. The outline of the pink color. It occurs in small masses barite, however, is well defined and of thin blades, or sheets, which are conforms to the outline of the coarse easily separated with the fingernail. calcite crystals. The mottled area in The barite was identified by blow the lower right-hand portion of the pipe and optical mineralogy tech- photograph is limestone matrix. niques which were confirmed by Figure 3 is a thin section of a por- x-ray analyses. 1 Indices of refrac- tion of a filled cavity. The cavity tion check well with those commonly wall shown in the figure is part of reported for barite. series of spe- A a fossil or fossils. Just inside the cific gravity determinations with a cavity wall in the upper right a very Jolly balance averaged 4.151. This thin zone of fine calcite is indis- value is lower than the commonly re- tinctly visible. Radiating from the ported gravity, but may be due to finely crystalline calcite is a layer porosity resulting from the bladed of coarse calcite. A large calcite character of the mineral. crystal is visible in the lower left of Figure 1 shows a broken surface the photograph. The barite fills the through the interior of a calcite- and space within the coarse calcite zone, barite-filled cavity. The light area and does not show crystal bounda- in the center is the barite. Sur- ries, but is in part characterized by rounding the barite is a zone of a lack of evident crystallinity and coarsely crystalline dark calcite faint striations running from upper

Published with the perm: left to lower right which result from Illinois State Geological Surve; 1 its bladed structure. The conforming Bradley, W. F., personal c LaSaUc Limestone r iW^S^f^

* £ n'fc* *2£

and lined wit.li dark calcite. X 10.

Fig. 2. —Smoothed and etched surface of a barite- Illinois Academy of Science Transactions

' T i - - • of a cavity lined with coarse calcite

and fl. contact of the barite and the coarse thought to be ferruginous. In some calcite crystals is marked by rela- cavities this stage is absent, and the tively dark lines caused by the coat- phenomenon is a local one. The ing on the calcite previously men- barite postdates all other stages. tioned. This is indicated by the well-defined contact between the barite and the The figures show two and possibly dark coarse calcite shown in figures three stages of cavity filling. Dur- 2 and 3, and is further borne out by ing the first the clear calcite of rel- the fact that some cavities contain atively small crystal size was formed the fine and coarsely crystalline cal- on the cavity walls, followed by cite in varying amounts, but are coarser and more euhedral crystal- devoid of any barite. The presence line calcite which appears dark. of a few calcite crystals attached to Lack of a clearly defined line be- or within the barite, figures 1 and 3, tween the two calcites suggests con- might be interpreted to mean continuous deposition with conditions temporaneous or penecontemporan- in the latter stage favoring the eous deposition of the two minerals, growth of larger and more perfect but it is believed more likely that crystals. Subsequent to their com- the calcite crystals projected from plete growth the coarse calcite crys- a portion of the cavity wall de- tals have been coated with a thin stroyed during preparation of the brown to light-brown coating specimens for study. Illinois Academy of Science Transactions, Vol. 44, 1951

REVISION OF CROIXAN DIKELOCEPHALIDS*

The trilobites under consideration Dikelocephalidae to a useful status. are classic in that before 1914 all This has involved a reduction of species then known were assigned to names from Ulrich and Resser 's 123 the genus Dikelocephalus, Owen, to 41, or by exactly two-thirds, thus 1852, which genus was regarded as eliminating 82 names. Such a re- the principal index fossil for the vision is of course tentative, but is Upper Cambrian of the Pacific quite representative of the degree of realm. species designation that the situa- Today these same trilobites are tion merits. consciously ignored by biostratig- raphers. This change dates from the Consideration op Stratigraphic 1930 early 's, when Ulrich and Resser Assignments published their monographic revision of the Upper 'Mississippi Valley A secondary objective is assign- Dikelocephalidae (Milwaukee Public ment of the species to their proper Museum Bull., vol. 12, nos. 1 and 2, places in the fauni-stratigraphic 1930 and 1933). succession. This is necessitated be- Before 1914, less than a dozen cause Ulrich 's conception of the re- species of Dikelocephalus were rec- gional stratigraphic relations was at ognized as occurring in Upper Mis- serious variance with the facts. sissippi Valley strata. That year Since these erroneous stratigraphic Walcott's paper reduced the scope considerations seriously affected Ul- of the genus by adding several new rich and Resser 's taxonomic conclu- genera, as well as several new species sions, the results were unfortunate. to his "Dikelocephalinae." For example, what is now con- By 1933, Ulrich and Resser had sidered a single sequence, the Franc- multiplied this conservative figure onia formation, Ulrich formerly to the astounding total of 123 species interpreted as three successive for- and varieties. They simultaneously mations. He applied the succeeded in rendering the Dikelo- name Franconia to the greensand facies cephalidae useless for purposes occurring in the region of the Missis- either of biostratigraphy or phyl- sippi. To the equivalent non-glau- ogeny, and this important fossil conitic sandstone facies in central group has subsequently been Wisconsin he applied the name shunned by paleontologists and stra- Mazomanie. To a shore facies of the tigraphers. same, he applied the term Devils The primary intent of the present Lake formation. He interpreted the paper is the restoration of the Mazomanie as successive to the [ulilHn'i] wilh permit Q of the Chief, Illin State Geological Survey, Urbana. Franconia and the Devils Lake as ' ;

138 Illinois Academy of Science Transactions

successive to both, in fact post-Cam- Consideration of Taxonomic brian. Thus the phylogenetic suc- Criteria cession of Franconian dikeloeepha- A third objective of the present lids is thrice repeated. paper is The Trempealeau formation, which to present a concrete in- stance of the effect, succeeds the Franconia, he conceived on taxonomic results, of differing species as a simple lithologic succession of concepts among different paleontologists. basal dolomite (his St. Lawrence), Once we leave the realm of living siltstone (his Lodi), fine sandstone forms, the species concept is no (his Norwalk) . All fossils, therefore, longer subject to the restraint that occurred in siltstone he con- of ' ' genetic criteria. Inevitably, individ- sidered to be ' Lodi ' in age, and all ' ual paleontologists resort to their in sandstone to be " Norwalk. own methodology in grouping organ- The fact is that Lodi siltstone lith- isms into species, genera, and higher ology occurs ;i1 eight different time- categories. The only test stratigraphie horizons and Norwalk against which the quality of sandstone Lithology at six different any worker's results may be applied is the horizons. Not only does the same prag- matic one of applicability of his lithology occur at a number of differ- bio- logic classification in the fields ent stratigraphie horizons, but the of biostratigraphy and of phylogeny. same stratigraphie horizon is com- If the classification contributes to monly represented by different lith- order it may be assumed to be valid ologies in different areas. In these if it contributes to confusion it may cases there is commonly a double with equal justice be assumed to be set of names for each species, one invalid. The responsibility for the supposed "Lodi" and one demand- ed of the paleontologist is for the supposed "Norwalk." accordingly great. Some of the authors' stratigraphie misconceptions have interesting re- From this viewpoint it follows sults. For example, the rich faunas logically that paleontologic studies from leached calcareous fine sand- must go hand in hand with strati- stones at Osceola, Wis., are con- graphic studies. But erroneous stra- sidered to be the latest of Trem- tigraphy renders confusion twice pealeau faunas; they are in fact, confounded. the earliest. In the case of Ulrich and Resser's Another unfortunate result of the work in the Dikelocephalinae, the stratigraphie confusion was the ap- following practices and concepts plication of some singularly inap- have contributed to confused tax- propriate specific names. Dikelo- onomy : cephalus norwalkensis, Saukiella Basing norwalkensis, and Tellerina norwalk- (1) of species on type individuals rather than popula- ensis do not occur in the Norwalk, on tions. nor does Osceolia lodensis occur in the Lodi. Fortunately, all drop into (2) Seeking for minute differ- the synonymy. ences rather than for likenesses. : : :

Croixan Dikelocephalids 139

(3) Misassoeiation of the vari- about a foot below the bare rock ous, separated component parts of summit of the mesa, the area of the skeleton, commonly combining a which is less than an acre. From this cranidium from one locality, a pyg- layer, Ulrich and Resser describe idium from a second, and free cheeks eight species of Prosaukia, all but from a third. one of which appear to constitute a

single species. eighth, P. ? (4) Failure to consider that The variations may be a residt of succes- anomala, belongs to another genus, sive growth stages. not of the Dikelocephalidae. Most of" the "species" the authors credit to (5) Rejection of any concept of individual variation. the type locality only. Quoting from 146: (6) The influence of strati- p. graphic misconceptions. All of the . . . species found at the Table Rock locality, except P.? anomala, trinomial clas- (7) Rejection of seem very closely related. Their cranidia, sification for varieties or subspecies. in particular, though distinguishable by features that may seem of little import- (8) Establishment of species on ance, are much alike. The associated inadequate material. pygidia, on the contrary, indicate six readily discriminated forms, and on this species individ- Basing of on type account the species are based mainly uals rather than on populations.— on these. Cranidia, and in most cases Under discussion of Calvinella spini- also free cheeks, are assigned to them according to our best judgment. ger, on page 222, the authors state However, we will admit at once that Actually none of their pygidia in studying the many slightly differing are complete, most are highly frag- cranidia of Calvinella that we possess mentary, and in the writer 's opinion from the same bed and place [italics the writer's] on Trempealeau Mountain that show no significant differences. provided Hall's original type of the In connection with one of Ulrich species, we found it no easy task to determine precisely which particular and Resser 's species, "P. alternata," kind is best entitled to the distinction these authors frankly state of being recognized as the typical form Again the material, which consists of of C. spiniger. Though our final selec- a single cranidium, a good free cheek, tion is not entirely satisfactory to us we and three imperfect but supplementing are so nearly correct that the margin pygidia, leaves much to be desired. But of error is practically negligible. . . . as none of these specimens quite fits any The remainder of the specimens of of the previously described species and, Calvinella that were found with C. despite their imperfections, permits ac- spiniger, as here restricted, is divided quiring . . . probably a true conception into seven varieties and species. of the complete animal, we feel war- ranted ... in describing these and other The series is clearly intergrading and in this writer's opinion, consti- tuted a single interbreeding species Misassoeiation of the various com- population of distinct zonal value. ponent parts of the skeleton.—Many Seeking for minute differences examples might be cited, but the fol- rather than for likes.—An example lowing should suffice of proliferation of species by Ulrich Dikelocephalus inaequalis is based and Resser is provided by the on three types from the basal Jordan Prosaukia occurrence at Table Rock at Trempealeau, Wis., but the fotTrth in Adams County, Wis. The fauna type (an hypostoma) comes from here comes from a six-inch layer the basal Trempealeau (five zonal 140 Illinois Academy of Science Transactions units lower in the section) at a dis- The degree of stratigraphic error tant locality. Its proper association might best be summed up by citing should have been with quite a differ- the fact that, of the 473 figures used ent species, D. thwaitesi. The to illustrate the Saukinae, at least writers, of course, were of the im- 290 are referred to the wrong strati- pression that the enclosing strata graphic horizon. at the two localities were of the same Rejection of trinomial classifica- age. tion.—The authors' proliferation of Failure to consider that variations species is in part a result of the fact be a res nil successive growth may of that they discriminated, according stages. Ulrich and Resser's multi- — to their criteria, down to the variety plication of species of Dikelocepha- or subspecies level and then usually lus from the Locli Member is a con- omitted the middle or species name. sequence largely of failure to recog- This is borne out by such printed nize morphological changes which statements as the following: accompany growth stages. Ulrich However, in general we are opposed and Eesser cite (p. 31), but do not to trinomial designations, [p. 206.] figure, evidence to refute this, but It is of little concern to us whether these forms are regarded such evidence this writer has never as "species" or "varieties," or "mutations" or "hybrids." observed although he collected most [p. 170.] of the specimens used by them. He Establishment of species on inade- feels that at least four of their Lodi quate material. That many species species are successive growth stages — were set up by the writers on a single of a single species. Since most speci- fragmentary cranidium, pygidium, mens are molts, it was quite possible or free cheek is readily evident from for a single individual to produce even a casual inspection of the four of Ulrich and Resser's species. plates. Rejection of any concept of individual variation.—This point is Revision of the Family best illustrated by an oral communi- DlKELOCEPHALIDAE cation of the junior author to the writer. When the latter raised the Inasmuch as the family Dikelo- question as to whether some of the cephalidae, as conceived by Miller differences among specimens of (1889), Walcott (1914), and Ulrich Dikelocephalus not represent might and Resser (1930), is revealed by individual variation, Resser cate- unpublished studies of the writer to gorically "trilobites stated that do be polyphyletic, it is proposed that not vary. If two specimens are Ulrich and Resser 's (1930) Saukinae different, they represent different be raised to a separate family, the Saukidae. Biostratigraphic studies The influence of si rati graphic mis- show that the Saukinae arise from conceptions. — Instances are innu- the mid-Franconian Ptychaspidae, merable and lead the authors which are descended in turn from \ repeatedly into frustrating phylo- early Franconian conaspid trilobites. : genetic discussions, in an attempt The latter appear to arise directly • to rationalize seeming anomalies. from Old World Parabolina, present ji : '

Croixan Dikcloct phalids 141

in the Upper Mississippi Valley sec- U & R, 0. praecipta U & R Dikelocephalus thwaitesi U & R in the base of the Conaspis zone. tion Synonyms: D. weidmani U & R, The Dikelocephalinae (including the D. halli U & R, D. inaequalis U & R, Osceolinae), on the other hand, ap- pars (pi. 19, fig. 6) Note: D. thwaitesi of pi. 21, figs. have descended through the pear to 7-10 is D. oweni U & R. late mid-Franconian Briscoia, from Lodi member-.s'«»?L-m suhrecta faunal the earlier Franconian genus Wil- unit bernia. The writer also questions Dikelocephalus (oweni var.?) barretti U & R the need for the subfamily Osceo- Synonyms: D. brevis U & R (except linae, comprising the genera Wal- pi. 14, fig. 2), D. edwardsi U & R, D. subplanus pars (pi. figs. cottaspis and Osceolia. Walcottaspis 14, 3, 5), "D. cf. gracilis and retrorus" (pi. 14, appears to be merely a descendant of fig. 9) Dikelocephalus, whereas Osceolia is D. norwalkensis U & R (except pi. 21, figs. 19, 20) an early Trempealeauan, probably Lodi member-Saukia suhlonga faunal terminal offshoot of Briscoia. unit Dikelocephalus oweni U & R Revision of the Synonyms: D. gracilis U & R, D. ovatus U & R, D. ivisconsinensis ' ' U ' Dikelocephalinae & R, D. subplanus U & R, D. retror- sus U & R, D. bcani U & R, D. Ulrich and Resser's, 1930, Dikelo- raaschi U & R (pi. 10, fig. 1 may be cephalinae, comprising 36 species of a distinct variety), D. brevis pars (pi. 9, fig. 5?), D. marginatus pars the genera Dikelocephalus, Briscoia, (pi. 15, figs. 6, 7), D. thwaitesi pars Osceolia, and Walcottaspis, are re- (pi. 21, figs. 7 to 10). D. norwalk- duced by the writer to a status of ensis pars (pi. 21, figs. 19, 20) There is some doubt as to the 12 species, as indicated in the follow- proper reference of the specimen, ing presentation, in ascending bio- fig. 6, pi. 9, designated as D. gracilis stratigraphic order by Ulrich and Resser. Note: There is admittedly consid- Fraxconia erable variation evident among the Hudson member-Prosaukia curvico- various parts of the dorsal shields stata faunal unit of Dikelocephalus occurring in the Briscoia schucherti U & R Lodi member. Much of this seems to Hudson member-Briscoia sinclairensis be a matter of the age or stage of faunal unit the molt, which doubtless most of Briscoia sinclairensis Walcott the specimens represent. On this Note: referred by Ulrich and Resser basis the Saukia sublonga zonal (1930—p. 59) to the "Lower Ozark- unit material seems to group itself ian, Devils Lake Formation," but as follows: actually occurring late in the Pro- 1. Small pustulose forms, with saukia subzone of the Franconia, palpebral lobes close to parallel with Hudson member. longitudinal axis, and relatively pos- Bad Axe member-Saukiella minor terior in position. Pygidia with five faunal unit pairs of equal pleural ribs and long Dikelocephalus postrectus U & R slender postlateral spines. 2. Medium-sized forms, generally •' Trempealeatt not pustulose, with palpebral lobes Arcadia member-Osceoh'a osceola intermediate between those of faunal unit groups 1 and 3 in orientation and Osceolia osceola (Hall) position. Pygidia averaging four Synonyms: 0. obsoleta U & R, 0. pairs of subequal pleural ribs, with arguta U & R, 0. lodensis reflexa more triangular spines. —

Illinois Academy of Science Transactions

3. Large forms with palpebral Dikelocephalid fragments occur- lobes relatively far forward and ori- ring higher, in the Madison, may ented to anterior convergence. Py- represent Walcottaspis rather than gidia averaging 3% pairs of pleural Dikelocephalus, but generically iden- ribs, alternatingly wide and narrow; tifiable material has not yet been pygidial spines short and basally obtained. broad. The Dikelocephalus of the under- lying Sa)ikiu subrecta unit is not Revision of the Saukinae greatly different from the 8. sub- lon'ga species except that the more Simple synonymic cross references juvenile characters of slender spines and of pustulation seem to persist do not suffice, in the case of the in individuals of medium size. Saukinae revision, to indicate pro- Lodi raemlwv- Sun kin lodensis faunal posed revised nomenclature. This is unit Dikelocephalus minnesotensis Owen because many of Ulrich and Resser's Synonyms: D. Hotchkissi U & R, D. (1933) species not only appear to be intermedins U & R, D. granosus U synonyms, but the illustrated ma- & R, D. wiltonensis U & R, "D. cf. orbicularis" U & R (pi. 17, fig. 1), terial which they figure as one spe- D. brevis pars (pi. 14, fig. 2). Prob- cies from one stratigraphic horizon ably also 1). orbiculatus U & R, and Briscoia? sp. (pi. 17, fig. 10) commonly must be distributed Note: As in the immediately an- among several species and several forms, cestral D. oweni group of stratigraphic horizons. Accordingly, young individuals are granulose (cf. D. granosus U & R, above). The D. in many cases it has been necessary minnesotensis group differs from the to make reassignments, both tax- D. oweni group particularly in pos- stratigraphic, on the sessing a more expanded frontal onomic and limb and a narrower and more ellip- basis of individual plate figures. tical pygidium with subequal ribs Stratigraphic references cited below even in the largest specimens. are those of the writer, not of Ulrich Jordan member-0 alvinella wisconsinensis faunal unit and Resser. Dikelocephalus margi iiatus U & R (except pi. 15, figs. 6 and 7) Plate 24. Synonym: D. declivis U & R Figs. 1-9. Prosaukia misa (Hall) Jordan member-Calvinella pustulosa =P. misa (Hall) faunal unit Franconia formation, Prosaukia sub- Dikelocephalus marginatus U & R zone, misa—longicornis zonal unit (as above) (misa facies). Dikelocephalus inaequalis U & R 10-13. resupinata TJ (except pi. 19, fig. 6) Figs. Prosaukia & Note: D. juvenalis U & R of this R=p. misa (Hall) zone is not recognized, being most Same occurrence as above. probably the very young of the above. Plate 25 Jordan member Saukiella pepinensis Figs. 1-7. Prosaukia faunal unit curvicostata U & R Dikelocephalus marginatus U & R Figs. 8-12. Prosaukia curv Jordan membev-Walcottaspis van- alternata U & R =p - costata hornei faunal unit Figs. 13-16. Prosaukia \- Walcottaspis vanhornei (Walcott). transversa U & R U & R Note: The true position of this spe- Figs. IT-l&.Prosaukia cies in the Trempealeau faunal suc- demissa U & R cession was pointed out to the writer Fig. 19. Prosaukia by W. C. Bell and R. Berg in con- subconica U & R nection with its occurrence near Franconia formation, Prosaukia sub- Reno, Minnesota. zone, curvicostata zonal unit. —

Croixan Dil-cloccph til ids

Fig. 20. Saukiella transitu U & R= Franconia formation, Prosaukia sub- 8. conica U & R zone, misa — longicornis zonal unit Franconia formation, Saukiella Min- (longicornis facies). or zonal unit, Prairie du Sac, Wis. Figs. 6-7. Prosaukia longula U & R= (Not "Gibraltar Rock, Wis.") P. tuberculata U & R Franconia formation, Prosaukia subzone, misa—longicornis zonal unit Plate 26. (longicornis facies). Fig. 1. Prosaukia concava U & R= Fig. 8. Prosaukia valida U & R=P. P. misa U & R valida U & R Franconia formation, Prosaukia sub- Franconia formation, Prosaukia zone, misa—longicornis zonal unit zone, zonal unit undetermined. facies). (misa Fig. 9. Prosaukia lodensis U & R= Saukia cf. curvata U & R Figs. 2-8. Prosaukia Trempealeau formation, Lodi mem- subrecta U & R ber, $. subrecta zonal unit. Figs. 9-12. Prosaukia Figs. 10-11. Prosaukia sp. undet.= subaequalis U & R ' Saukia subrecta U & R Franconia formation, Prosaukia sub- Trempealeau formation, Lodi mem- zone, curvicostata zonal unit. ber, S. subrecta zonal unit. Figs. 12-17. Prosaukia incerta U & R= 13-17. Prosaukia clelecostata Figs. U Saukiella (?) incerta (U & R) & R=P. delecostata U & R Trempealeau formation, Lodi mem- Franconia formation, Prosaukia sub- — ber, 8. lodensis zonal unit (sand- zone, misa longicornis zonal unit. stone facies). Fig. 18. Prosaukia granosa U & R= Plate 27. P. beani U & R Franconia formation, Hudson mem- Figs. 1-2. Prosaukia longa U & R= ber. P. longa U & R Fig. 19. Prosaukia Franconia formation, Prosaukia sub- berlinensis U & R: validity undetermined zone, Briscoia sinclairensis zonal Exact stratigraphic position unde- unit. termined. Figs. 3-9. Prosaukia halli & U R= Fig. 20. Prosaukia dubia U & R=P. P. halli U & R beani U & R Franconia formation, Prosaukia sub- — Franconia formation, Prosaukia subzone, P. misa longicornis zonal zone, misa- -longicornis zonal unit unit (P. misa facies). (longicornis facies). Figs. 10-11. Prosaukia brevisulcata U Fig. 21. Prosaukia beani U & R=P. & R=P. longicornis var. brevisul- beani U & R cata (U & R) Occurrence same as preceding. Franconia formation, Prosaukia subzone, P. misa longicornis zonal Plate 29 unit (longicornis facies) Figs. 1-3. Prosaukia (?) anomala U & Figs. 12-21. longicornis Prosaukia U & R: not a Prosaukia, but belongs to R=P. longicornis U & R an undescribed, non-saukid genus Occurrence same as preceding. Franconia formation, Prosaukia sub- Figs. 22-25. Prosaukia am.pla U & R= zone, curvicostata zonal unit. P. ampla U & R Figs. 4-6. Saukia obtusa U & TL=8. Franconia formation, Saukiella acuta U & R minor zonal unit. Trempealeau formation, Lodi member, 8. lodensis zonal unit. Plate 28 Fig. 7. Saukia sublonga U & R=8. sublonga U & R Figs. 1-2,4. Prosaukia magnicornuta U Trempealeau formation, Lodi mem- & R=P. longicornis & R U ber, 8. sublonga zonal unit. Franconia formation, Prosaukia sub- Fig. 8. Saukia angusta U & H=S. zone, misa—longicornis zonal unit lodensis (Whitfield) (longicornis facies) Trempealeau formation, Lodi mem- Fig. 3. Prosaukia magnicornuta U &R ber, 8. lodensis zonal unit. =P. Z. var. brevisulcata (U & R) Figs. 9-10. Saukia modesta U & R= Occurrence same as preceding. S. lodensis (Whitfield) Fig. 5. Prosaukia tiibercitlata U & R= Trempealeau formation, Lodi mem- P. tuberculata U & R ber, 8. lodensis zonal unit. Illinois Academy of Science Transactions

Fig. 11. Saukia rudis hybrida U & R Figs. 17-25. Saukia rudis U & R=S. cf. 8. acuta U & R lodensis (Whitfield) Trempealeau formation, Lodi mem- Occurrence same as preceding. ber, S. lodensis zonal unit (sand- Fig. 26. Saukia parva U & R=S. acuta U & R Figs. 12-13. Saukia whitfleldi U & R= Trempealeau formation; exact zonal 8. lodensis (Whitfield) occurrence not known. Trempealeau formation, Lodi mem- 31. ber, 8. lodensis zonal unit. Plate Fig. 14. 8a itl- hi whit Heidi U & R=S. Figs. 1-5. Prosaukia acclivis U & R= acuta U & R P. halli var. acclivis (U & R) Occurrence same as preceding. Franconia formation, Prosaukia sub- Figs. 16-17. Saukia acuta U & R=S. zone, misa—longicornis zonal unit acuta U & R (misa facies). Occurrence same as preceding. Figs. 6-8. Prosaukia (?) ambigua U & Fig. 18. San kid cf. irhitfleldi U & R= R=undescribed genus aff. Taenice- 8. subrecta U & R plialus Trempealeau formation, Lodi mem- Franconia formation, Prosaukia sub- ber, 8. subrecta zonal unit. zone, misa—longicornis zonal unit Fig. 19. Saukia subrecta U & R=£f. (misa facies, late stage). subrecta U & R Figs. 9-10a. Saukia prima U & R= Occurrence same as preceding. Prosaukia (?) dilata U & R Fig. 20. Saukia subrecta U & R=un- Stratigraphic occurrence uncertain. determined trilobite, cf. Eurekia Fig. 11. Saukia gran.ilineata U & R: Occurrence same as preceding. not recognized Trempealeau formation, Lodi mem- Plate 30 ber, Saukia subrecta zonal unit. Figs. 1-2. Saukia nitida U & R=8. Figs. 12-13. Prosaukia minuscula U & subrecta U & R R=P. halli var. acclivis (U & R) Trempealeau formation, Lodi mem- Franconia formation, Prosaukia sub- ber, 8. subrecta zonal unit. zone, misa—longicornis zonal unit Fig. 3. Saukia ornata U & R (misa facies, late stage). lodensis (Whitfield) Figs. 14-20. Saukia separata IT & R= Trempealeau formation, Lodi m S. lodensis (Whitfield) ber, S. lodensis zonal unit. Trempealeau formation, Lodi mem- Fig. 4. Saukia ornata U & R=unde- ber, 8. lodensis zonal unit (sand- termined trilobite cf. Eurekia. stone facies). Trempealeau formation, Lodi mem- Figs. 21-25. Saukia imperatrix U & R ber. =8. imperatrix U & R Fig. 5. Saukia curvata U & R=S'. cur- Trempealeau formation, exact strati- vata U & R graphic occurrence undetermined. Trempealeau formation, Lodi mem- Fig. 26. Saukia lodensis (Whitfield): ber, 8. subrecta zonal unit. relation undetermined; preservation Fig. 6. Saukia laerigevata U & R= inadequate S. lodensis (Whitfield) Trempealeau formation, Lodi mem- Trempealeau formation, Lodi member, S. sublonga zonal unit.

ber, 8. lodensis zonal unit. Fig. 27. Saukia lodensis (Whitfield ) = Fig. 7. Saukia subgranosa U & R=S. 8. lodensis (Whitfield) lodensis (Whitfield) Trempealeau formation, Lodi mem- Trempealeau formation, Lodi member, 8. lodensis zonal unit. ber, S. lodensis zonal unit. Figs. 28-31. Prosaukia dilata U & R== Figs. 8-10. Saukia separatoidea U & R P. dilata U & R =8. subrecta U & R Stratigraphic occurrence uncertain. Trempealeau formation, Lodi member, 8. subrecta zonal unit. Plates 32 and 33. Figs. 11-12. Sdiikia tumida U & R: not All specimens figured are conspecific recognized; material inadequate. with Saukiella pepinensis Owen and Trempealeau formation, Lodi mem- accordingly the following names may ber, S. lodensis zonal unit (sand- be considered synonyms: stone facies). Saukiella typicalis Figs. 13-16. Saukia retusa U & R=£. Saukiella typicalis convexa acuta U & R Saukiella typicalis subrecta Occurrence same as preceding. Saukiella subgracilis Croixan Dikelocephalids

Saukiella subgraeilis hybrida Fig. 4. Saukiella indenta intermedia Saukiella subgraeilis parallela U & R=S. indenta U & R Saukiella ampla Trempealeau formation, Lodi mem- The occurrence is not from the Lodi ber, sandstone facies. shale, as the authors state, but from Figs. 5-11. Saukiella noriralkensis U & the Saukiella pepinensis zonal unit of R=Saukiella pyrene (Walcott) the overlying Jordan member of the Trempealeau formation, Arcadia Trempealeau formation. member, Osceolia osceola zonal unit. Figs. 12-14. Saukiella norivalkensis Plate 34 U & R=S. indenta U & R Saukiella pyrene ( Walcott )=&att- Trempealeau formation, Lodi mem- kiella pyrene (Walcott) ber, S. sublonga zonal unit (sand- Trempealeau formation, Arcadia stone facies). member, Osceolia osceola zonal unit. Figs. 15-25. Saukiella norivalkensis U & R=S. pyrene (Walcott) Plate 35. Trempealeau formation, Arcadia Figs. 1-8. Saukiella pyrene (Walcott) member, Osceolia osceola zonal unit. =S. pyrene (Walcott) Figs. 26-27. Saukiella norivalkensis Trempealeau formation, Arcadia U & R—8. indenta U & R member, Osceolia osceola zonal unit. Trempealeau formation, Lodi mem- Fig. 9. Saukiella cf. pyrene=S. pyrene ber, S. sublonga zonal unit (sand- (Walcott) stone facies). Trempealeau formation, Lodi mem- Figs. 28-30. Saukiella simplex U & R= ber, sandstone facies. S. minor U & R Fig. 10. Saukiella cf. pyrene=S. minor Franconia formation, Bad Axe mem- U& R ber, Saukiella mi nor zonal unit. Franconia formation, Bad Axe mem- Figs. 31-32. Saukiella (?) weidmani ber, Saukiella minor zonal unit. U & R—Tellerinn .' leucosia (Wal- Fig. 11. Saukiella cf. pyrene=S. in- cott) Trempealeau formation, Ar* denta U & R cadia member, Osceolia osceola Trempealeau formation, Lodi mem- zonal unit. ber, Sdukia lodensis zonal unit (sandstone facies). Plate 37. Figs. 12-14. Saukiella pyrene limbata Figs. 1-5. Saukiella conica U & R= U & R=S. indenta U & R S. minor U & R Trempealeau formation, Lodi mem- Franconia formation, Bad Axe member, Saukia sublonga zonal unit ber, Saukiella minor zonal unit. (sandstone facies). Figs. 6-17. Saukiella minor U & R= Figs. 15-21. Saukiella signata U & R= 8. minor U & R S. pyrene (Walcott) Same occurrence as preceding. Trempealeau formation, Arcadia Figs. 18-29. Calvinella spiniger (Hall) member, Osceolia osceola zonal unit. =C. spiniger (Hall) Fig. 22. Saukiella frontalis U & R= Trempealeau formation, Jordan Saukiella frontalis U & R member, C. spiniger zonal unit. Trempealeau formation, Jordan Figs. 30-35. Tellerina granistriata U & member, Norwalk sandstone, Sau- R=T. granistriata U & R kiella frontalis zonal unit. Trempealeau formation, Lodi mem- Figs. 23-25. Saukiella indenta U & R ber, exact zonal position undeter- =S. indenta U & R mined. Trempealeau formation, Lodi member, Saukia sublonga zonal unit Plate 38. (sandstone facies). All specimens figured are conspecific Figs. 26-30. Saukiella indenta U & R with Calvinella spiniger (Hall), and, =Saukiella frontalis U & R coming all from the same stratum at Trempealeau formation, Jordan the same locality, represent the range member, Norwalk sandstone, Sau- of variation of a single species popula- kiella frontalis zonal unit. tion. Accordingly, the following names, appearing on the explanation of plate Plate 36. 38, may be considered as synonyms: Figs. 1-3. Saukiella indenta U & R= Calvinella spiniger altimuralis S. indenta U & R U & R Trempealeau formation, Lodi mem- Calvinella spiniger communis U & R ber, Saukia lodensis zonal unit Calicinella spiniger communis muta- (sandstone facies). tion U & R )

Illinois Academy of Science Transactions

Calvinella spiniger postlevata U & R Trempealeau formation, Jordan Calvinella clivula U & R member, exact zonal position unde- Calvinella spiniger (Hall) ? (of termined. U & R) Occurrence: Trempealeau forma- Plate 41 tion, Jordan member, Calvinella Figs. 1-9. Tellerina crassimarginata spiniger zonal unit, which the writer (Whitfield) =T. crassimarginata believes to lie at the base of the (W.) Jordan, below siltstone bearing the Trempealeau formation, Lodi mem- Calvinella wisconsinensis fauna. ber, Saukia sublonga zonal unit.

Plate 42. Plate 39. Fig. 1. Teller 'ma Figs. 1-10. Calvinella pustulosa U & R crassimarginata =Calvinella pustulosa U & R ( Whitfield ) =T. crassimarginata Trempealeau formation, Jordan (W.) Occurrence as member, C. pustulosa zonal unit. same above. Figs. 2-3. Figs. 11-16. Calvinella sparsinodata Tellerina curta U & R=T. U & R=cf. Calvinella spiniger crassimarginata ( W. (Hall) Occurrence same as preceding. Trempealeau formation, Jordan Figs. 4-5. Tellerina gothamensis U & R gothamensis member, C. spiniger zonal unit. =T. U & R Trempealeau Figs. 17-18. Calvinella pustulosa ver- formation, Lodi mem- nonensis U & R=Calvinella pustu- ber, Saukia subreeta zonal unit. Fig. 6. Tellerina bigeneris losa U & R U & R: cf. Trempealeau formation, Jordan Saukia acuta U & R Trempealeau fromation, Lodi member, C. pustulosa zonal unit. member, Saukia lodensis zonal unit. Figs. 19-24. Calvinella notata U & R= Calvinella walcotti U & R Fig. 7. Tellerina strigosa U & R=T. Trempealeau formation, Jordan crassimarginata (W.) member, zonal unit undetermined. Trempealeau formation, Lodi member, Saukia sublonga zonal unit. Figs. 25-30. Calvinella lata U & R= Calvinella walcotti U & R Plate 43. Occurrence same as preceding. Figs. 31-34. Calvinella walcotti nor- Figs. 1-5. Tellerina strigosa U & R= walkensis U & H=Calvinella spin- T. crassimarginata (W.) iger (Hall) Occurrence same as preceding. Trempealeau formation, Jordan Fig. 6. Tellerina lata U & R: cf. T, member, C. spiniger zonal unit. crassimarginata (W.) Figs. 35-36. Calvinella wisconsinensis Occurrence undetermined. junior U & R= Calvinella spiniger Fig. 7. Tellerina recurva U & R=T. (Hall) crassimarginata (W.) Occurrence same as preceding. Trempealeau formation, Lodi member, Saukia sublonga zonal unit. Plate 40. Plate 44. Figs. 1-14. Calvinella walcotti U & R= C. walcotti U & R All specimens figured are conspecific Trempealeau formation, Jordan with Tellerina ? leucosia (Walcott), member, exact zonal position unde- and the following names may be re- termined. garded as synonyms: Figs. 15-22. Calvinella iralcotti planu- Tellerina leucosia (Walcott) lata U & R=C. walcotti U & R Tellerina leucosia variety Occurrence same as above. Tellerina leucosia parallela U & R Figs. 23-33. Calvinella wisconsinensis The occurrence is not the Norwalk U & R=C. wisconsinensis U & R sandstone (a member of the Jordan Trempealeau formation, Jordan formation), but the basal Trem- member, Calvinella wisconsinensis pealeau, Arcadia member. zonal unit. Fig. 34. Calvinella walcotti ? U & R: Plate 45. cf. C. walcotti U & R Figs. 1-10. Tellerina extrema U & R= Fig. 35. Calvinella notata ? U & R: cf. Tellerina ? leucosia (Walcott)

C. wisconsinensis U & R Occurrence same as preceding plate. ;

Figs. 36-39. Calvinella walcotti ? U & Figs. 11-13. Tellerina norivalkensis i R=C walcotti U & R U & R=T. crassimarginata (W.) Croixan Dikelocephalids 147

Trempealeau formation, Lodi mem- Hall and Dikelocephalus thwaitesi Saukia sublonga zonal unit ber, were collected. (sandstone facies). U & R Fig. 14. Telleriiia nonralkensis U & R: The Arcadia succession is over- not identifiable. lain by the basal St. Lawrence con- Figs. 15-16. Tellerina norwalkensis U & R=r. crassimarginata (W.) glomerate (with dolomitic matrix Trempealeau formation, Lodi mem- and Arcadia sandstone pebbles), ber, Saukia sublonga zonal unit (sandstone facies). succeeded by 11.2 feet of sandy dolomite. Stbatigraphic Note St. Lawrence Member arcadia member The fauna of the St. Lawrence Since the latest published account member continues to be designated of Trempealeau stratigraphy by the term Platycolpus, to which (Raasch, 1939), an additional mem- the specific term eatoni has been ber of that formation has been dis- added, since the range of the genus criminated. This member, for which greatly exceeds the span of the mem- the term Arcadia is proposed, lies ber. However, as the St. Lawrence beneath the basal conglomerate of member, which seldom exceeds 20 the St. Lawrence member, and rests feet in the Wisconsin-Minnesota re- with a strong basal conglomerate gion, attains a thickness of more than upon the Franconian, Bad Axe 100 feet in parts of northeastern greensands. Areally the member is Illinois, it may represent a complex evidently continuously extensive of faunal units, in addition to the north of the Black River valley, in substantive one to which the term Wisconsin and Minnesota, but to Platycolpus eatoni is properly ap- the southward is only locally present plied. small remnants have escaped where Lodi Member pre-St. Lawrence erosion. The member bears the Osceolia osceola faunal The Lodi member is characterized faunally by a succession of at least

The type locality selected is a road three closely related faunal units. cut and small quarry on State High- Of these, the middle or Saukia sub- way No. 93, 1 mile south of its junc- longa faunal unit yielded most of tion with Highway No. 95, east of the classic collections from the

' ' Arcadia, Trempealeau County, Wis- ' Dikelocephalus beds. ' The earlier consin. Here the persistent green- Saukia subrecta faunal unit occurs sands of the Franconia, Bad Axe in a lentil of limited thickness, as member, are unconformably over- yet known only from the lower Wis- lain by a spectacular 3-foot bed of consin Valley. The Saukia lodensis edgewise conglomerate, flat pebbles faunal unit occurs in siltstone in the of buff siltstone in a greensand upper part of the Lodi member, matrix. This bed grades upward largely in Sauk, Columbia, and Dane into 17.1 feet of somewhat lenticular counties in southern Wisconsin, but strata of varied lithology, dominant- also in similar lithology at Still- | ly dolomitic siltstone and fine, green- water, Minnesota. In the interven-

, ish gray to light brown, somewhat ing area (in a direct line), the Lodi dolomitic sandstone. Osceolia osceola member passes wholly to sandstone, —

148 Illinois Academy of Science Transactions

Chart No. 1. Zonal Succession of Dikelocephalid

Zonal Unit Dikelocephalidae Associated Genera

14. "Madison Fauna" undescribed Tellerina, / 1:-iht urn ii>i>us, K/ilimins/iis, Cnlri in Hit, Dikelocephalus^) Stenopilus, Plethopeltis unconformity base of Madison formation

13. "Van Oser Fauna" undescribed Saukia, Stenopilus, Saukiella, Tellerina Euptychaspis

12. Saukiella frontalis So itkirlla frontalis U.& R

11. Walcottaspis vanhornei Walcottaspis vanhornei (Wal- Stenopilus cott)

10. Saukiella pepinensis Sankiiila pepinensis (Owen) Plethometopus Dikelocephalus marginatusU&R

9. Calvinella pustulosa Calvinella pustulosa U&R Eurekia, Corbinia, Dikelocephalus inaequalis Triarthropis, Entomaspis U&R Sii iiii/>ilu:\, "Agnosias ilis- Dikelocephalus marginatusU&R parilis" Hall unconformity 8. Calvinella wisconsi- ( 'nli'inillii irisn in sinensisU&R Eurekia, Corbinia O Dikelocephalus marginatusU&R < Tellerina sp. nov. Prosaukia sp. nov.

p 7. Calvinella spiniger 1 Calvinella spiniger (Hall) Eurekia, Corbinia 02 unconformity base of Jordan member

6. Saukia lodensis2 Sun l:ia lodensis (Whitfield) Illaenurus, Eurekia, Saukia acuta U&R Corbinia, Plethometopus, Tellerina crassimarginata Triarthropsis, Entomaspis, (Whitfield) '•'ii/ii .'

5. Saukia sublonga Saukia sublonga U&R > Illaenurus, Acheilops, SuuLiiila indenta U&R Euptychaspis, Tellerina crassimarginata "Agnostus disparilis" Hall, (Whitfield) Plethometopus, Corbinia, Dikelocephalus oweni U&R Entomaspis, Eurekia, Triarthropsis, Stenopilus

4. Saukia subrecta U&R Saukia subrecta U&R Illaenurus Saukia curvata U&R Tellerina gothamensis U&R Dikelocephalus oweni var. barretti (U & R) base of Lodi member

3. Platycolpus eatoni undescribed Tellerina, Illaenurus, Eurekia, (Whitfield) Dikelocephalus Plethometopus, Stenopilus, Corbinia, Platycopus unconformity base of St. Lawrence member Croixan Dikelocephalids

2. Osceolia osceola Osceolia osceola (Hall) Illaenurus, Eurekia,

/')il;eloei phalus Hi innli si 1 A K < 'orlniiia, Triarllrropsis, Saukiella pi/rene (Walcott) Euptychaspis Tellerina ? leiicosm (Walcott) base of Arcadia member, o unconformity Trempealeau formation

< 1. Saukiella minorV&H San kidla minor U & R Illaenurus, Mimocheilus M Prosaukia ampla U & R P Dikelorephatus pos/rcel us UifcR <»j Dikelocephalus sp. nov. Briscoia sp. nov. Calvinella sp. nov. base of Bad Axe member, unconformity? Franconia formation

4. Briscoia sinclairensis Briscoia sinclairensis Platycolpus Prosaukia longa U & R

3. Briscoia (unnamed) Briscoia sp. nov. Platycolpus, Prosaukia near Zonf/a U & R Monocheilus

2. Prosaukia curvicostata Prosaukia curi'icostata U & R Briscoia schucherti U & R

3 1. Prosaukia longicornis Prosaukia misa (Hall) 3 Dartonaspis* p Prosaukia halli U & R3 Cliarioeephalus3 Prosaukia halli a eel iris V it k' Idahoia o Prosaukia delecostata U & R 4 Ellipsncephaloides3 « Prosaukia longicornis U & R Ptychaspis* 3 Pmsaukia I. brevisulcata* Wilbernia Prosaukia lubereulata U & R 4 Litagnostus 3 Briscoia sp. nov. 4

Ptychaspis subzone Wilbernia, ancestral to Dikelocephalus Moiiorheilus, PsalasptS, Ptychaspis, ancestral to lila hoi a, Kill psoce phuloitles, Prosaukia Lilai/iiosius, Pseuilagnostus base of Hudson member, local unconformity Franconia formation

A. central and soi

Greensand facies ;

Non-greensand fac s faunal facies. 150 Illinois Academy of Science Transactions in which somewhat different faunal fossiliferous and best characterized facies of both the Saukia suolonga by Saukiella frontalis U & R. and Saukia lodensis faunal units The Jordan member was deeply have tentatively been identified. eroded previous to the deposition of the Madison formation in central Jordan Member and southern Wisconsin; but along the Mississippi and in the lower In western Wisconsin, conglome- Minnesota Valley the member is ratic and dolomitic strata marking essentially its original thickness, the base of the Jordan member over- with terminal sands ("Van Oser lie the Lodi siltstone. The limited Beds," Stauffer, 1940) interpreted thickness of basal Jordan strata, in- as a regressive phase of Jordan cluding intercalated dolomitic deposition (Raasch, 1939). Stauffer shales, contains a closely packed suc- (1940) figures, but does not de- cession of faunal units, each char- scribe, a fauna from these beds. acterized by a distinct species of Calvinella. Thus there appears to Sunset Point Formation have prevailed here, as in the ease of the lithologically rather similar At most places, south of a line St. Lawrence member, a condition through central Trempealeau where factors favoring deposition County, an independent depositional were closely balanced by those favor- cycle, dominantly sand with varied ing limited submarine erosion proportions of dolomite, intervenes and/or nondeposition. Faunal units between the Jordan member of the thus tend to be closely packed ver- Trempealeau and the Ordovician tically and in lenses in areal dis- Oneota formation. To this unit, the tribution. term Madison has been applied (ref. Along the Mississippi, the basal Raasch, 1935). Because pre-Madi- Jordan Calvinella beds are succeed- son erosion deeply truncated the ed by siltstone strata similar litho- Jordan member in central and logically to those of the Lodi mem- southern Wisconsin, the Madison ber. These strata enclose fossils merits the status of an independent belonging to the Saukiella pepin- formation. However, the applica- ensis faunal unit, and where the beds tion of the term Madison to these attain their greatest thickness, in strata has created much confusion, Dakota County, Minnesota, the suc- owing especially to the widespread ceeding Walcottaspis vanhornei use of the same name for a Missis- fauna as well. The siltstone grades sippian limestone unit in Montana eastward into unfossiliferous sand- and Wyoming. The writer therefore stone. The main body of the Jordan proposes that, for the Cambrian member above and lateral to the unit, the term Sunset Point be sub- siltstones and conglomeratic dolo- stituted, after the bluff of that name mites just described consists of fine, at the Madison sandstone type lo- friable, more or less dolomitic sand- cality. stones which Ulrich (1924) em- The unit is locally fossiliferous, braced in his term Norwalk. The but the fauna has not been specifi- higher Norwalk beds are sparingly cally described. Croixan Dikelocephalids 151

The Saukia Zone except for the genus Prosaukia, which is ancestral to the other mem- Because many trilobite genera bers of the subfamily and charac- range through much of the Trem- terizes the Prosaukia subzone of pealeau and in fact back into the the preceding Ptychaspis-Prosaukia Bad Axe member of the Franconia zone. and into the Madison (or Sunset REFERENCES Point) formation, this inclusive suc- Miller, S. A., North American geology cession of faunal units is considered and paleontology, 1889. to represent a single faunal zone. Owen, D. D., Report of a geological sur- The saukid genera, Saukia, Sauki- vey of Wisconsin, Iowa, and Minne-

sota . . ., 1852. Tellerina, Calvinella, i.e. ella, and Raasch, G. O., Stratigraphy of the Cam- Saukia in the sense of Walcott's brian system of the Upper Mississippi Valley, Guidebook Ninth Annual Field (1914) use of the term, together Conference, Kansas Geol. Soc, 1935, characterize this succession, but the pp. 302-315. later subdivision and restriction of , Cambrian Merostomata, Geol. Soc. Amer. Special Paper 19, 1939. the term Saukia by Ulrich and Ees- Stattffer, C. R., Fauna of the Van Oser ser leaves no single genus whose beds, Jour. Paleontology, vol. 14, no. 1, range coincides completely with the 1940, pp. 54-56. ulrich, E. 0., and C. E. Ressek, The full time and rock span. Accordingly Cambrian of the Upper Mississippi the writer proposes to apply the Valley; Part I, Trilobita, Dikeloce- phalinae Osceolinae, term Saukia to the succession in- and Milwaukee Public Museum Bull., vol. 12, no. 1, volved, using that term in the broad- 1930. er sense of Walcott. Thus the bio- Ulrich, E. 0., and C. E. Resseb, idem.; Part II, Trilobita, Saukinae. Milwau- chron of the Saukinae coincides with kee Public Museum Bull., vol. 12, no. the time span of the Saukia zone, 2, 1933.