Early Ordovician Strata Along Fox River in Northern Illinois

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CIRCULAR 100

EARLY ORDOVIC1AN STRATA ALONG FOX RIVER IN NORTHERN ILLINOIS

H. B. WILLMAN AND J. NORMAN PAYNE

Reprinted from the Journal of Geology

Vol. LI, No. 8, November-December, 1943

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS

URBANA, ILLINOIS

1943

EARLY ORDOVICIAN STRATA ALONG FOX RIVER IN NORTHERN ILLINOIS

H. B. WILLMAN AND J. NORMAN PAYNE Illinois State Geological Survey ABSTRACT

A nearly complete sequence of Ordovician strata along Fox River, southwest of Aurora, indicates the presence of the Kankakee arch—a major structure trending northwest-southeast across northeastern Illinois —and the probable location of the Sandwich fault, which parallels the arch for many miles and had been found previously by studies of well records. Outcrops of sandstone and dolomite which have long been correlated with the St. Peter and Galena-Platteville formations have been found to belong to the New Richmond sandstone and the Shakopee dolomite. Study of the outcrops and well records shows that the anticlinal structure underwent major movements both before and after deposition of the St. Peter sandstone.

INTRODUCTION subsurface data showed that this structure, termed the "Kankakee arch," In the earliest geological studies of this (1)

2 had its greatest deformation before, rather area the sandstone and the dolomite out- than since, St. Peter time; (2) extends at crops along the 15 miles of Fox Valley least from Oregon, Illinois, southeast between Millbrook and Sheridan were through Momence, Illinois; and in correlated with the St. Peter sandstone (3) Indiana is probably a continuation of the and the "Trenton" (Galena-Platteville) west branch of the Cincinnati arch. 6 formations, respectively. The alternate Because of this structure, Cambrian stra- occurrence of dolomite and sandstone at ta are exposed along the axis of the Kan- the level of Fox River was first inter- kakee arch at Oregon 7 and directly un- preted as the result of deposition of the derlie glacial drift at Sandwich, about dolomite in erosional troughs in the top 4 miles northwest of Fox Valley. 8 How- of the sandstone 3 and later as due to up- ever, because the occurrence of St. Peter lift of the St. Peter sandstone along sev- and Galena strata along Fox Valley was eral "anticlinals." 4 Some years ago it not necessarily inconsistent with these was noted that the structure of the St. new structural conceptions, correlation Peter sandstone indicated an anticline of the strata was not seriously questioned with a northwest-southeast axis crossing

5 Fox Valley in the vicinity of Millington. 6 George E. Ekblaw, "Kankakee Arch in Illi- More recently a study of outcrops and nois," Bull. Geol. Soc. Amer., Vol. XLIX (1938), pp. 1425-30. Reprinted as ///. Slate Geol. Surv. Circ. 40 (1938). 2 H. C. Freeman, "Geology of La Salle Coun- ty," in Geology and Paleontology, III. Geol. Surv., 7 A. C. Bevan, "Fault Block of Cambrian Strata Vol. Ill (1868), pp. 257-87; H. M. Bannister, "Geol- in Northern Illinois" (abst.), Bull. Geol. Soc. Amer., ogy of Kendall County," in Geology and Paleontol- Vol. XL (1929), p. 88; "Cambrian Inlier at Oregon, ogy, III. Geol. Surv., Vol. IV (1870), pp. 136-48. Illinois," Kan. Geol. Soc. Guidebook, gth. Ann. Field Conf. (1935), pp. 383-85; "Cambrian Inlier in 3 Freeman, p. 279 of ftn. 2 (1868). Northern Illinois," Bull. Amer. Assoc. Pet. Geol., Vol. XXIII (1939), pp. 1561-64; Ekblaw, p. 1426 of ftn. 6 (1938). * Bannister, pp. 145-46 of ftn. 2 (1870).

8 J. N. Payne, in "Geology and Mineral Resour- 5 G. H. Cady, "The Structure of the La Salle ces of the Marseilles, Ottawa, and Streator Quad- Anticline," ///. State Geol. Surv. Bull. 36 (1920), rangles," ///. Stale Geol. Surv. Bull. 66 (1942), p. 127. P- 57-

S3 1 —

532 H. B. WILLMAN AND J. NORMAN PAYNE until a study of samples from a well STRATIGRAPHY drilled at the Illinois School for Boys, As the outcrops are widely scattered, mile south of Sheridan, revealed that \ the stratigraphic sequence is not com- Shakopee dolomite there lay immediately pletely exposed. Consequently, much of 9 below the surface. As a result of this dis- the information about the general charac- covery, a detailed examination of the ter, the thickness, and the stratigraphic area in was immediately undertaken relations of the formations, shown in December, as a project of the Illi- 1940, Table 1, is derived from study of samples nois State Geological Survey to supple- of cuttings from near-by wells." The ex- ment the authors' recent work on the Mar- posed formations are described below. seilles, Ottawa, and Streator quad- 10 RICHMOND rangles. This study determined that NEW SANDSTONE the sandstone exposed near Sheridan Outcrops of New Richmond sandstone was part of the New Richmond sand- occur along Fox Valley west of Sheridan stone and that the dolomite cropping out (Fig. 1). The sandstone is well exposed a short distance south of Sheridan and in the west bluff of the valley and along near Millington belonged to the Shako- the tributary ravines in Section 7 and the pee formation. south part of Sec. 6, T. 35 N., R. 5 E., especially near the highway bridge west of GEOGRAPHIC SITUATION Sheridan. On the east side of the valley the sandstone crops out in the bluffs and portion of the valley of That Fox River in a tributary ravine in the SW.j Sec. 8. considered in this study extends from Approximately the upper 40 feet of the miles north of south- about 1 1 Millbrook formation is exposed. The sandstone is west to a mile south of Sheridan (Fig. 1). mostly fine-grained (sample 4, Table 2) The valley is 75-100 feet deep, and the but contains thin lenticular streaks of valley floor \-1 mile wide—consists of coarse sand, especially along the bed- terraces in which the river is entrenched ing-planes. The coarse sand streaks in a narrow channel. The valley bluffs contain scattered but conspicuous grains and terraces are composed largely of of secondary white opaque silica which dis- glacial drift, which mantles a much is penetrated by some of the surround- sected bedrock surface. Fox River has ing quartz grains. The sandstone is eroded its valley to a level at which it thin-bedded, almost laminated in places, truncates some of the higher buried bed- but locally appears massive on weathered rock ridges, so that bedrock outcrops surfaces. Commonly the sandstone is locally extend as high as feet the 30 above much cross-bedded (Fig. 2), but at some river. Rapids occur in the river near places the beds are mostly parallel. Some most of the outcrops. Several long sec- beds are ripple-marked. tions of the valley in which there are no The heavy minerals in the New Rich- bedrock outcrops mark the position of mond sandstone are characterized by an major preglacial drainage channels. abundance of tourmaline, usually more than 50 per cent, and the presence of gar- 9 L. E. Workman, personal communication. net, especially a diagnostic brownish-

10 H. B. VVillman and J. N. Payne, "Geology pink variety, and of ferromagnesian min- and Mineral Resources of the Marseilles, Ottawa, erals. The heavy minerals in samples of and Streator Quadrangles," ///. State Geol. Snrv. Bull. 66 (1942). 11 Payne, pp. 53-69 of ftn. 8 (1942). EARLY ORDOVICIAN STRATA ALONG FOX RIVER, ILLINOIS 533 the upper part of the formation, omit- This sandstone is similar to the lower ting the upper 3-5 feet, collected from fine-grained part of the St. Peter sand- each foot of an outcrop at the mouth of stone (sample 1 , Table 2) but it is some- the ravine on the east side of Fox Valley what finer grained, more cemented, much R6E

ORDOVICIAN

SHAKOPEE rOofid ONEOTA

|>psp..| ST. PETER l

Fig. i.—Location of outcrops and areal distribution of formations along part of Fox Valley west of the Boys School, in the SW. \ thinner bedded, more poorly sorted, and Sec. 8, T. 35 N., R. 5 E., are as given in more cross-bedded, and the grains are 12 Table 3. more angular and less frosted than those

12 Mineral analyses by Paul Herbert, Jr. in the St. Peter sandstone. 534 H. B. WILLMAN AND J. NORMAN PAYNE

SHAKOPEE DOLOMITE On the east side of Fox Valley the Shak- The Shakopee dolomite crops out opee dolomite crops out along the south along Fox Valley near Sheridan and Mil- side of a ravine in the SW. \ SW. \ Sec. lington (Fig. i). Near Sheridan, 15-20 8, T. 35 N., R. 5 E., in washes on the sur- feet of the dolomite is exposed on the face of the terrace south of the ravine west side of Fox Valley in and near an and along the bank of the river south to

TABLE 1 Geologic Formations in Area along Fox Valley from Aurora to Wedron

Approximate Normal System Series Formation Material Thickness (Feet)

Pleistocene Glacial drift

Niagaran Joliet Dolomite

Silurian Unconformity Alexandrian Kankakee 60 Dolomite Edgewood Dolomite

Major unconformity

Cincinnatian 1 Maquoketa IS© Shale, limestone, dolomite

Unconformity Galena Dolomite, limestone *Decorah 35° Dolomite, limestone, shale Mohawkian *Platteville Dolomite, limestone Ordovician Unconformity

Chazyan 1 St. Peter 175 Sandstone

Major unconformity Shakopee 5° Dolomite Prairie du Chien New Richmond 60 Sandstone *Oneota I4S Dolomite (cherty)

Unconi *Jordan 3° Dolomite, sandstone, shale *Trempealeau 200 Dolomite *Franconia 140 Sandstone, dolomite, shale (glauco- Cambrian St. Croixan nitic) *Galesville 185 Sandstone *Eau Claire 400 Sandstone, shale, dolomite

*Mount Simon 1 Sandstone , 700

Major un<;onformity Pre- Cambrian Igneous and metamorphic rocks

*Unexposed. abandoned quarry in the NW. \ NE. \ near the center of Section 18, where Sec. 18, T. 35 N., R. 5 E., and along a ra- about 25 feet of the dolomite is exposed vine about 100 yards north of the quarry. in a recently abandoned quarry. About Small outcrops of the lower few feet of I mile southwest of Millington, 3-5 feet the formation occur farther north along of dolomite is exposed on a railroad cut

ravines in the north part of Section 7. near the center of the west line of the .. 62 1 1

EARLY ORDOVICIAN STRATA ALONG FOX RIVER, ILLINOIS 535

NE. \ Sec. 36, T. 36 N., R. 5 E., in a abandoned quarry, known as "Brodie's small quarry southeast of the railroad quarry," and in outcrops near the quarry

cut, and also in the south bank of Fox (Fig. 3)- River extending west of the railroad cut The exposed Shakopee consists prin-

for about \ mile. About | mile northeast cipally of dolomite, much of which is

of Millington, 9 feet of Shakopee dolo- sandy and argillaceous. It is highly variable in bedding, grain size, and porosity. TABLE 2 Many of the more massive beds are brec- Sieve Analyses of Sandstones* ciated or conglomeratic. The dolomite is mostly gray and greenish-gray, but the Sample No. coarse-grained beds weather brown and the dense argillaceous beds weather I 2 3 4 Dening Mesh ° gray or light buff. Some beds contain IE Mm. thin partings of green clay, and the 20. O 833 O. I 0. Tr. 1 highly argillaceous dolomite contains 28.. S89 0.2 2. 2 2.0 4 shale partings as much as inch thick. 35 417 32 26.6 30.0 1 .2 \ 48.. 29S 17-4 37-8 37 4 6.4 Beds of medium to coarse-grained sand-

65 • 208 21.8 16.0 14.4 22 .0 stone as much as 1 foot thick are com- 100. . 147 38.6 12 .0 10.4 58.8 150 104 11. 3" 2.6 10 mon.

200 . 074 3-6 i-4 14 1 270. °S3 0.8 0.4 0. 2 0. Discontinuous beds of white oolitic

— . 270. 02 1 0.4 0. 2 Tr. chert as thick as 8 inches occur in nearly — c >.02 2. 2 0.4 1-4 0.4 all outcrops of the Shakopee dolomite.

Total . 100. 7 100 .3 100. 100 4 Many of the oolites consist of alternating bands of white opaque and translucent chert. Some consist of rounded grains of Gradest clear quartz sand surrounded by white chert. The chert breaks across the oolite Coarse 1 10 11 1 Medium 29 66 66 15 grains, including those with centers of Fine 58 21 18 80 quartz sand. Some of the chert is not Very fine 9 2 3 4

Silt and clay. . . . 3 1 2 Tr. oolitic and has a brecciated or conglomeratic structure. It consists of somewhat * Location of samples: rounded masses of dense white chert 1. St. Peter sandstone, lower 10 feet exposed above water- level in abandoned silica-sand pit north of Millington, in SW. i locally containing nests or streaks of NW. } SE. i Sec. iq, T. 36 N., R. 6 E. 2. St. Peter sandstone, 15 feet exposed above sample I. round quartz sand grains. 3. St. Peter sandstone, 15 feet exposed above sample 2, to top of pit. 4. New Richmond sandstone, 10 feet exposed along ravine The sandstone beds in the Shakopee south of Sheridan, in SW. SW. Sec. 8, T. N., R. E. J } 35 5 dolomite contain more garnet and ferro- t Wentworth scale; amounts approximate, estimated from cumulative curves of analyses. magnesian minerals than do the New Richmond and St. Peter sandstones. A mite is exposed in an abandoned quarry sample from the railroad cut southwest of on the northwest side of the Fox River, Millington contained the heavy min- in the NE. | NW. | Sec. 30, T. 36 N., 13 erals listed in Table 4. R. 6. E. One and a half miles northeast In Table 5 are listed the heavy min- of Millington, on the northwest side of erals found in a sandstone bed in an the Fox River, near the center of the abandoned quarry north of Millington, N.E. \ Sec. 19, T. 36 N., R. 6. E., about

25 feet of dolomite is well exposed in an 'J Mineral analyses by Paul Herbert, Jr. 71 81 8 1

536 H. B. WILLMAN AND J. NORMAN PAYNE

2. Fig. —Cross-bedded New Richmond sandstone near mouth of ravine on east side of Fox Valley, i mile south of Sheridan, in the SW. \ SW. } Sec. 8, T. 35 N., R. 5 E. in the NE. \ NW. \ Sec. 30, T. 36 N., Fossils are extremely rare in these 14 R. 6 E. strata. The only identifiable fossil found TABLE 3 was a high-spired gastropod of Canadian Heavy Minerals in Upper Part of age. 15 New Richmond Sandstone The character of the basal 18 feet of the Shakopee dolomite is revealed by an Tourma- Leu- outcrop at Zircon Ilmenite Garnet the sewage-disposal plant of Sample line coxene (Percent- (Percent- (Percent- the Boys' the No. (Percent- (Percent- School on east side of Fox age) age) age) age) age) Valley, about 1 mile south of Sheridan. The bottom 2 feet is a conglomeratic go (top) 19.6 62.6 4 3 12.9 0.6 91 26. 56.3 6 1 10.9 0.6 bed, above which the dolomite is argilla-

92 29.O 47 6 17 . 2 03 5 ceous, dense, fine-grained, and thin- 93 30.0 51-5 4 4 14. I 94 33 3 47.2 5 6 13.2 0.7 bedded and contains several thin beds of 95 33-2 49-7 IO.9 05 5 7 sandstone. Slightly higher strata are ex- 96 14.8 62 . 4 4 17.6 1 .

97 22 . 60.I 3 7 13 5 6.0 posed about 100 yards south along a 98 27-3 44-5 2 250 Tr. 3 small ravine east of an old lime kiln. The gg ... 28.6 53-9 4 2 13 03 100 31-5 48.4 3 1 17 .0 Tr. strata consist of 2 feet of green dolomitic 101 29.7 51-7 6 2 11. 0.6 shale, overlain by 4-6 inches of oolitic 102 23 9 61.7 2 1 12.3 103 26.4 54-7 4 6 13-9 0.4 chert and 1 foot of laminated dolomite, 104 27.7 52. 3 8 15 1 06 probably cryptozoan, and 3 feet of 105 24.7 60.8 3 7 10.8 Tr. 106 32.6 51-5 3 1 12.8 brown, porous dolomite in 1- to 4-inch 107 n. 765 63 5 5 beds. Higher strata are exposed in the 108 25.6 61.6 3 8 go Tr. recently abandoned quarry about 200

14 Mineral analysis by Paul Herbert, Jr. 15 Carl A. Bays, personal communication. :

EARLY ORDOVICIAN STRATA ALONG FOX RIVER, ILLINOIS 537 yards south of the lime kiln and consist Identification of these scattered out- of the strata listed in Table 6. crops as Shakopee is based principally on lithologic characteristics. Oolitic TABLE 4 chert, thought to be diagnostic of the Heavy Minerals in Shakopee Dolomite Prairie du Chien dolomites in this region, Percentage Zircon 11.2 Tourmaline 32.8 Leucoxene 34.4 Ilmenite 7.2 Garnet White 7.2

Light pink ' 0.8 Brownish pink 0.8 Pyroxene 5.6

TABLE 5

Heavy Minerals ln Sandstone Bed ln Shakopee Formation

Percentage Zircon 10.4 Tourmaline 19.2 Leucoxene 8.4 Ilmenite 0.4 Garnet 62.6

TABLE 6 Strata Exposed in Abandoned Quarry Southwest of Boys' School Near Sheridan Thickness

Feet Inches Dolomite, argillaceous, brownish-gray, dense, 5- to 2-inch beds, shale partings 4 Fig. 3.—Shakopee dolomite in Brodie's quarry Covered 5 1 \ miles north of Millington, in the center of the Dolomite, silty, brown, porous NE. I Sec. 19, T. 36 N., R. 6 E. to dense, 2- to 4-inch beds. 1

Chert, white, oolitic 1 although it is also found in conglomerates Dolomite, sandy, brown, por- at the base of the St. Peter sandstone ous, 1- to 4-inch beds, green 16 clay partings 2 and the Platteville dolomite, occurs in Dolomite, silty, mottled brown practically all the outcrops. On this and gray 1 10 basis some of the outcrops might belong Dolomite, shaly 2 to the Oneota dolomite. However, both Dolomite, gray, dense, argilla- the outcrop relations and the well data ceous 1 8 at Sheridan indicate correlation Dolomite, brown, porous... 1 6 with Dolomite, greenish-gray, the Shakopee dolomite, and the fact slightly sandy 4 that all the other outcrops in the region Dolomite, brown, coarse- are lithologically similar suggests that grained, massive 9 Base concealed 16 Bays, personal communication. 538 H. B. WILLMAN AND J. NORMAN PAYNE they are not only Shakopee but even exposed above water-level in a sand pit represent the same part of the formation. (Fig. 4) on the northwest side of Fox Valley in the SW. \ NW. \ SE. \ Sec. ST. PETER SANDSTONE 19, T. 36 N., R. 6 E. The lower 10 feet

The St. Peter sandstone is exposed along of sandstone is fine-grained (sample 1, Fox River at the south edge of the area Table 2) and finely laminated. The fine- and also north of Millington. Two small grained sandstone has a sharp contact outcrops at the south edge of the area with the overlying sandstone, which is represent only the lower few feet of the medium-grained (samples 2, 3, Table 2)

Fig. 4. —St. Peter sandstone in silica-sand pit 1 mile north of Millington, in the SW. i NW. 1 SE. j Sec. 19, T. 36 N., R. 6 E. formation, but a high bluff of the sand- and mostly thin-bedded, contains some stone occurs on the west side of Fox cross-bedded layers and is largely white River in the central part of the NE. \ with a few limonite-stained beds. Sec. 19, T. 35 N., R. 5 E., just south of The sandstone extends to depths of the area. The sandstone in these out- at least 25 feet, possibly 50 feet, below crops is lithologically similar to the fine- the top of outcrops of Shakopee dolo- grained sandstone which comprises ap- mite which occur near by both north- proximately the lower half of the forma- east and southwest of the sandstone tion in the vicinity of Ottawa. outcrops. Although the sandstone ap-

The St. Peter sandstone is well exposed pears to fill a channel eroded in the lower in abandoned silica-sand pits on both strata, the contacts are not exposed, and sides of Fox Valley from 1 to i| miles the relations may be explained by local north of Millington. About 40 feet of folding, although the observable dips white, loosely consolidated sandstone is do not support such an explanation. EARLY ORDOVICIAN STRATA ALONG FOX RIVER, ILLINOIS 539

On the southeast side of Fox Valley beds are crowded with small intercon- about 20 feet of St. Peter sandstone is ex- necting masses of gray, white, and brown posed in several sand pits along the chert. Some of the chert is banded, and Chicago, Burlington, and Quincy Rail- some contains small white round grains road, in Sec. 19, T. 36 N., R. 6 E. The similar to, but smaller than, oolites and sandstone is similar in lithology to the without concentric rings. The chert upper medium-grained sandstone in the masses are extremely irregular in shape. pit described above; but, because of the Most of them have rounded outlines, but southerly dip, it is probably stratigraph- some have angular edges, as if they were ically higher. broken fragments. A few poorly pre- 18 The heavy minerals in the St. Peter served fossils are Galena in age. sandstone consist almost entirely of zir- Other outcrops occur along a small con and tourmaline, zircon usually pre- valley tributary to Fox Valley about \ dominating. A sample from the pit for- mile southwest of the quarry described merly operated by the Ballou White above, in the center SE. \ Sec. 8, Sand Company, north of Millington, con- T. 36 N., R. 6 E. The dolomite is similar tained 70 per cent zircon and 30 per cent to that in the quarry but is generally tourmaline. 17 more massive, with some beds 2 feet thick, and contains less chert. Most of GALENA-PLATTEVIXLE FORMATIONS these outcrops have been recently covered by a small dam and flooded by a The Galena, Decorah, and Platteville lake. Because of the local easterly dip, formations are present in the northeast these strata probably underlie those part of the area, but only the Galena exposed in the quarry. dolomite is exposed. About i| miles north of Millbrook the The Galena dolomite crops out along Galena crops out (1) in the banks of Fox Fox Valley near illbrook and also from M River and in several small abandoned 1 to 2 miles north of Millbrook. Near quarries near the old mill i\ miles north Millbrook 6 feet of Galena dolomite is of Millbrook, in the E. \ Sec. 4, T. 36 N., exposed in a small abandoned quarry R. 6 E. (Fig. 1); (2) in an abandoned west of the road in the NE. \ NE. \ SE. quarry and outcrops near the mouth of I Sec. 8, T. 36 N., R. 6 E. The dolomite Little Rock Creek; and (3) along the is coarse-grained and porous, so that southeast side of Fox River opposite the weathered surfaces appear sandy. The mouth of Little Rock Creek. Only 25-30 dolomite is gray in some relatively fresh feet of the formation is exposed. beds, but it is mostly weathered brown. The dolomite is medium-grained, por- It contains streaks of green clay along ous, chert-free, mottled light and dark the bedding-planes and in some of the brown, and mostly massive but weathers massive beds. The thickness of the beds to beds 2-4 inches thick. It is fossilifer- is variable, but generally on weathered ous, and Receptaculites is common. As a faces they are shown to be less than 2 well near the old mill penetrated 234 feet inches thick. Some bedding-planes have of the combined Galena-Platteville for- conspicuous fucoidal markings. Many mations, the outcropping strata must belong in the upper part of the Galena for- 17 E. Lamar, "The St. Peter Sandstone of J. mation. Illinois," 77/. State Geol. Sum. Bull. 53 (1928), p. 60. 18 Bays, personal communication. 54° H. B. WILLMAN AND J. NORMAN PAYNE

STRUCTURE ing Fox Valley at the west end of the KANKAKEE ARCH Galena outcrops. To the southwest the next bedrock exposures are about a mile The character of the Kankakee arch down the valley and also consist of the where it is crossed by the Fox River is Galena dolomite. However, a well in the shown both by the general distribution valley-flat only a short distance south- of the formations and by the dips of the west of the escarpment entered sand- strata in the individual outcrops (Fig. i). stone, presumably St. Peter, at a depth The varying directions of dip indicate of 50 feet below glacial drift. As the well the complexity of the structure, which on the Galena outcrop near the old mill, consists of a major arch with many which is practically on the escarpment, smaller structures on its flanks. The re- encountered the top of the St. Peter gional subsurface data indicate that the sandstone at a depth of 234 feet, it in- Kankakee arch is asymmetrical with a dicates a displacement of at least 175 southwest dip so much steeper than the feet, plus whatever has been eroded from northeast dip that it is essentially a the top of the St. Peter sandstone on the monocline. 19 southwest side of the escarpment. If, as Because of the scarcity of outcrops believed, the escarpment is the result of and their divergent dips, as well as the erosion of the less resistant St. Peter flatness of the crest of the arch, the exact sandstone, leaving the more resistant position of the axis of the structure can- Galena dolomite, the St. Peter sandstone not be determined. However, as the once probably extended to the top of the study of wells has shown that, because escarpment, which indicates a minimum of the major pre-St. Peter folding of the displacement of 234 feet. arch, the St. Peter sandstone overlies bas- Although the relations of the strata al Ordovician or Cambrian strata along could be accounted for by a very sharp the crest of the arch, the principal axis of monocline and the escarpment could be the structure is northeast of Brodie's the margin of a preglacial valley, the quarry, i| miles northeast of Millington, postulation of a fault is favored because where the Shakopee dolomite is present. (1) there is no evidence that lower Galena It probably occurs in the vicinity of the and Platteville strata occur west of the Sandwich fault, about 4 miles northeast escarpment, where, because they are as of Brodie's quarry, along which the St. resistant as the upper Galena strata, they Peter sandstone generally overlies the would be expected to crop out if they Oneota dolomite or older strata. were present; (2) the apparent steepness and height (approximately feet, in- SANDWICH FAULT 75 cluding the buried portion) of the escarp- The stratigraphic relations revealed by ment are unusual, even for preglacial val- certain wells in Kendall and DeKalb ley walls, and imply an abrupt change counties are best explained by the pres- in lithology, such as would occur if less ence of a fault which has been named resistant St. Peter sandstone and more 20 the "Sandwich fault" (Fig. 1). This resistant Galena dolomite were adjacent; fault is believed to be partly recorded by (3) there is a sharp increase in dip from an escarpment about 20 feet high cross- nearly horizontal to 4 at the escarpment, 's Ekblaw, p. 1428 of ftn. 6 (1938). which may be the result of drag on the

20 Payne, p. 186 of ftn. 8 (1942). downthrow side of the fault; (4) the loca- EARLY 0RD0VIC1AN STRATA ALONG FOX RIVER, ILLINOIS 541

tion and the trend of the escarpment co- St. Peter sandstone, and this was in turn incide with the line of the fault as deter- covered by Platteville, Decorah, Galena,

mined from well data; and (5) samples and Maquoketa formations.

from a well in the NE. \ Sec. 9, T. 36 N., Just when the later uplift of the arch R. 6 E., along the line of the fault \ occurred cannot be definitely determined. mile southeast of Fox Valley, show a It may have occurred during any one or brecciated zone containing galena and more of the several post-Maquoketa sphalerite in the Galena dolomite, indi- periods of folding that are known to have cating fracturing and mineralization. 21 affected northern Illinois. One was at the close of the Ordovician period; another STRUCTURAL HISTORY 22 at the close of the Mississippian period; and a third after the deposition of Penn- The Kankakee arch was formed dur- sylvanian strata, probably at the close of ing at least two periods of folding, the the Permian period. Others may have more important of which followed the occurred also at the close of the Silurian deposition of the Shakopee formation. and of the Devonian periods. There is This structure was truncated by erosion, some evidence that the principal part of which in the first stage essentially pene- the later deformation occurred at some planed northeastern Illinois and in a post-Silurian time and that the Sandwich later rejuvenated stage dissected the fault was developed at the same time. peneplain. This erosion removed all of the strata down to the Franconia forma- Acknowledgments.—This study was made under tion along the axis of the arch and down the direction of George E. Ekblaw, geologist and head of the Areal and Engineering to and locally through the Oneota dol- Geology Division of the Illinois State Geological axis. 23 omite northeast of the The dissect- Survey. His counsel has been especially helpful ed peneplain was then covered by the because of his previous work on the Kankakee arch. Other members of the Survey staff also 21 Payne, "Discovery of Sphalerite and Galena gave valuable assistance, particularly L. E. near Millbrook, Kendall County," III. State Acad. Workman, geologist and head of the Subsurface Set. Trans., Vol. 182-83. XXXI (1938), pp. Division, who contributed to the study his

" Ekblaw, ftn. 6 (1938); Payne, pp. 193, 195 of knowledge of the subsurface characteristics of ftn. 8 (1942). the formations discussed, and Carl A. Bays,

2 special geologist, whose studies of the insoluble -> The area of Shakopee dolomite underlying residues and heavy minerals were of great value glacial drift in Sees. 13 and 14, T. 35 N., R. 5 E. (Fig. 1), as determined by well data, is presumably in the identification of the formations. Carl a hill created by this erosion and re-exhumed by Bays and J. S. Templeton, of the University of preglacial erosion. Illinois, identified the fossils.