486 STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION The Late Woodfordian Jules Soil and Associated Molluscan Faunas John C. Frye A. Byron Leonard H. B. Willman H. D. Glass Leon R. Follmer ILLINOIS STATE John C. Frye, Chief CIRCULAR 486 GEOLOGICAL SURVEY Urbana, IL 61801 1974 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/latewoodfordianj486illi THE LATE WOODFORDIAN JULES SOIL AND ASSOCIATED MOLLUSCAN FAUNAS John C. Frye, A. Byron Leonard, H. B. Willman, H. D. Glass, and Leon R. Follmer ABSTRACT The Jules Soil formed during the late Woodfordian episode of maximum glacial retreat in the Lake Michigan Glacial Lobe during the interval between 15,500 and 16,500 radiocarbon years B.P. The soil is directly radiocarbon dated and stratigraphically framed by a group of dates. The molluscan faunas above and below the soil indicate a cool, moist climate. The significantly higher content of expandable clay minerals (montmorillo- nite) in the Jules Soil than in the loess above and below indicates that there was a decrease in Lake Michigan outwash discharged to the Illinois Valley during the episode of soil formation. GENERAL STATEMENT The Jules Soil was named in 1970 (Willman and Frye, 1970) from its occurrence in the Peoria Loess at the Jules Section (Frye, Glass, and Willman, 1968), Cass County, Illinois. The soil had earlier been recognized as an un- named soil in the Peoria Loess in the Frederick South Section (Leonard and Frye, 1960), and in other loess sections in the lower Illinois Valley and the East St. Louis area (Frye, Glass, and Willman, 1962; Frye and Willman, 1963). The stratigraphic significance of this weakly developed soil was discussed by Frye and Willman (1973), who suggested an age of slightly more than 17,000 radio- carbon years B.P. The presence of this minor soil in the Peoria Loess is evi- dence of a pause in loess deposition that reflects a significant glacial retreat during late Woodfordian time in the region of the type Wisconsinan. In the steepest valley bluffs at the margin of the lower Illinois Valley and in the Mississippi Valley bluffs in Madison and St. Clair Counties, the Jules Soil is split into a complex of two or more A-horizons separated by a foot or more of typical loess. However, where the exposures are extensive enough to trace the soil as much as 100 yards away from the bluff line, the multiple I ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 486 A-horizons merge into a single A-horizon only 1 to 2 feet thick, the thickness of loess above the soil thins sharply, and the texture of the loess becomes finer grained. The type section of the Jules Soil is in such a situation. Exposures in roadcuts at the Cottonwood School South Section, 1 mile west of the Jules Section in Cass County, Illinois, afforded an especially good opportunity to study this buried soil, including its age by radiocarbon dating; the molluscan faunas above and below the soil; and the clay mineral composition of the deposits. The Cottonwood School Section had been studied in some detail (Willman and Frye, 1970); the Cottonwood School South Section occurs in a road- cut immediately south of this classic section. A description of the Cottonwood School South Section follows. Cottonwood School South Section Cottonwood School South Section, measured in roadcuts in NW^NW^SW^; sec. 12, T. 18 N., R. 11 W. , Cass County, Illinois (1972). (Sample numbers preceded by "P" are the numbers used in the Illinois State Geological Survey collection of lithologic samples.) Thickness (feet) Pleistocene Series Wisconsinan Stage Woodfordian Substage Peoria Loess 6. Loess, coarse; contains Modern Soil in top; calcareous in lower part; the gray to gray-tan A-horizon of the surface solum is granular and friable; the light tan-brown B- horizon has weak textural contrast and indistinct structure . 6.0 5. Loess, coarse, calcareous, massive, fossilif erous, gray to gray-tan; radiocarbon date on snail shells 10,410 ± 650 (ISGS-138). Samples: P-7586, h foot below top; P-7585, middle; P-7584, h foot above base 2.0 4. Loess, massive, calcareous, gray, highly fossilif erous; radiocarbon date on snail shells 15,640+580 (ISGS-137) . Samples: P-7583, top; P-7582, h foot above base; P-7581, base 1.0 3. Jules Soil (A-horizon), silt with some clay; dark gray- brown in upper part becoming lighter tan-brown in lower part; granular to massive, weakly calcareous, less friable than bed above; generally lacks snail shells but contains rare examples of Suaoinea sp. and some small nodules of CaC03. Radiocarbon date on humus 12,740+210 (ISGS-195) ; date on humus from Jules Soil at the type section, 1 mile to the east of this section, 15,020 ±300 (ISGS-179) . Samples: P-7580, top; P-7579, h foot below top; P-7578, 1 foot below top 1.2 2. Loess (C-horizon of Jules Soil), medium, massive, calcar- eous, fossilif erous, gray to tan-gray; contains nodules of CaC03 up to h. inch in diameter. Radiocarbon date on outer one-third of nodules, 5,030 ±100 (ISGS-139A) ; on middle one-third of nodules, 5,867 ± 80 (ISGS-139B) ; on inner one- third of nodules, 7,370±140 (ISGS-139C). Samples: P-7577, top h foot; P-7576, \ foot below top 0.8 1. Loess, medium, massive, calcareous, friable; contains sparsely distributed snail shells; gray, with some streaks of light brown; slumped in lower part. Samples: P-7575, h foot below top; P-7574, 1 foot below top; P-7573, lh feet below top; P-7572, 2 feet below top; P-7571, 2% feet below top. To level of road grade 6.0 Total 17.0 THE JULES SOIL AND ASSOCIATED MOLLUSCAN FAUNAS 3 The Peoria Loess includes all of the eolian silts and fine sands above the Robein Silt of Farmdalian age and beyond the limit of Woodfordian glacial tills. It is typically developed south of Peoria along the Illinois Valley and in the Mississippi Valley south of the southern terminus of the Illinois River. The Peoria Loess in the Frederick South Section and the Cottonwood School Section were suggested as reference sections for the type Woodfordian (Frye, Willman, Rubin, and Black, 1968). More than 30 radiocarbon dates determined in central and northern Illi- nois place the Robein Silt, of Farmdalian age, within the time span of 28,000 to 22,000 radiocarbon years B.P. (Willman and Frye, 1970). Several additional dates from the Morton Loess (stratigraphically equivalent to the lowermost part of the Peoria Loess) and from the basal part of the Wedron Formation tills fall within the range of 19,000 to 22,000 radiocarbon years B.P. Within the Peoria Loess are two radiocarbon dates, 17,950 ±550 (W-1055) from the Collinsville Section and 17, 100 ±300 (W-73 0) from the Burdick Branch Section (Frye, Glass, and Willman, 1962), that were determined on materials that are stratigraphically below the position of the Jules Soil. Radiocarbon dates determined from the Cottonwood School South and Jules Sections are placed stratigraphically in the described section in this report. The date of 10,410 ±650 (ISGS-138), determined on snail shells from 1 1/2 to 2 1/2 feet above the top of the Jules Soil, is the youngest date so far determined from the Peoria Loess in Illinois. The youngest date previously determined is 13,700 ±230 (1-1720) on snail shells from the uppermost part of the Peoria at the Bald Bluff Section (Frye, Glass, and Willman, 1968) in the bluffs of the Mississippi Valley in Henderson County. Immediately below the youngest date at Cottonwood School South, however, a date of 15,640 ±580 (ISGS-137) was determined on snail shells from the one-foot interval immediately above the top of the Jules Soil. In an effort to date the soil directly, dates were run on organic carbon extracted from the sediments. At Cottonwood School South the organic carbon content of the one foot of Jules Soil from which humus was extracted ranges from 0.20 percent to 0.37 percent. The dates from organic carbon from the Jules Soil were 15,020 ±300 (ISGS-179) at the Jules Section, and 12,740 ±210 (ISGS-195) at the Cottonwood School South Section. As the probability of some contamination by modern hair-roots in such shallowly buried soils is high, these soil dates are probably somewhat too young. A reasonable evaluation of the available dates indicates that the Jules Soil developed during part of the interval between 15,500 and 16,500 B.P. rather than at about 16,500 B.P. (Frye, Glass, and Willman, 1968) or 17,000 B.P. (Frye and Willman, 1973) as has been suggested. In an attempt to relate the nodules of CaCO} that occur in the one-foot zone below the base of the soil to the soil-forming process, they were radio- carbon dated. One-third, by weight, of a group of nodules was dissolved and dated as 5,030 ±100 (ISGS-139A), another one-third was dissolved and dated as 5,867 ±80 (ISGS-139B), and the remaining one-third was dissolved and dated as 7,370 ±140 (ISGS-139C). From the dates obtained, it is concluded that most of the concretionary growth of the nodules took place after the development of the Jules Soil and, as some concretions occur within the Jules Soil itself, they may be largely related to the development of the modern surface solum. The Jules Soil is an immature A-C profile. Carbonate minerals persist through the solum; but with the exception of a rare Succinea, snail shells are not present in bed 3, the A- horizon of the soil. The A- horizon is darker colored and less friable than the loess above and below; it is more clayey and is streaked with some secondary carbonate, but it lacks distinctive soil structure. The 1 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 486 C-horizon contains abundant nodules of CaC03 and fossil snail shells; it is massive and lighter colored than the A-horizon. A B-horizon is not recognizable in the profile. The percent of less-than-2 -micron clay through the profile, shown in figure 1 and listed in table 1, shows a progressive upward trend of -| 1 1 1 1 I- 10 20 30 40 50 60 Percent clay and percent clay minerals Fig. 1 - Cottonwood School South Section, NW^NW^SWi sec. 12, T. 18 N. , R. 11 W. , Cass County, showing percent of less-than-2-micron clay in Peoria Loess and percent of illite and expandable clay minerals in the clay mineral assemblage. Sample numbers at left are ISGS file numbers, and bed numbers at right refer to numbered beds in the described section in this report. THE JULES SOIL AND ASSOCIATED MOLLUSCAN FAUNAS TABLE 1 — GRAIN SIZE AND CLAY-MINERAL COMPOSITION OF PEORIA LOESS AT COTTONWOOD SCHOOL SOUTH SECTION Sample no. Gravel* (%) Matrix particle size (%) Clay minerals (%) Sand* Silt Clay Expandable clay minerals Illite Kaolinite and chlorite P-7586 2.2 1.7 90.8 7.5 29 52 19 P-7585 3.3 0.6 92.0 7.4 35 47 18 P-7584 0.7 1.2 91.1 7.7 32 50 18 P-7583 0.8 0.4 93.4 6.2 38 45 17 P-7582 0.0 0.8 93.5 5.7 34 51 15 P-7581 0.4 1.3 92.2 6.5 36 46 18 P-7580 3.1 2.0 83.2 14.8 60 29 11 P-7579 1.1 1.5 85.3 13.2 58 30 12 P-7578 0.4 0.9 89.2 9.9 56 32 12 P-7577 3.6 1.0 88.8 10.2 50 36 14 P-7576 1.5 1.7 87.6 10.7 53 33 14 P-7575 0.9 1.0 88.7 10.3 38 47 15 P-7574 0.5 0.8 91.2 8.0 31 51 18 P-7573 0.6 0.3 93.4 6.3 29 53 18 P-7572 0.6 1.1 90.7 8.2 33 49 18 P-7571 0.8 1.3 91.4 7.3 44 39 17 'Mostly CaCO. concretions. increase in percentage of clay that is sharply terminated at the top of the Jules Soil. The clay mineral composition (fig. land table 1) shows a sharp change at both the top and bottom of the soil. Clay mineral data for the nearby Jules Section have been published previously (Frye, Glass, and Willman, 1968) and show similar relationships. In order to show the extreme situation that locally occurs at the bluff line in exceptionally thick, coarse-textured Peoria Loess, we sampled the Jules Soil interval at the Bunkum South Section, SEiNWiNEi sec . 24, T. 2 N. , R. 9 W. , St. Clair County. The clay mineral data and the positions of A-horizons are shown in figure 2. At this locality four A-horizons occur within a vertical dis- tance of 9 feet and are separated by 1 to 2 1/2 feet of loess that superficially resembles the deposit above and below the soil zone. Although the sequence is much thicker and the deposits are coarser textured, the relationship of the clay mineral assemblages strikingly parallels that shown at the Cottonwood School South Section (figs. 1 and 2). ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 486 P-7626 P-7643 Percent 10 Fig. 2 - Bunkum South Section, measured in cuts of Interstate Highway 64 in SE^NWiNEi sec. 24, T. 2 N., R. 9 W. , St. Clair County, showing percent of illite, expandable clay minerals, and kaolinite plus chlorite in the clay mineral assemblage in the Peoria Loess. The complex A-horizon of the Jules Soil is shown in relation to clay mineral percent- ages. Sample numbers at left are ISGS file numbers. PALEONTOLOGY Molluscan faunas occur in beds 5 and 4 above the Jules Soil and in 1-2 below the Jules Soil in Peoria Loess at the Cottonwood School South Section; in two faunal zones above the Jules Soil at the Bunkum South Section in St. Clair County near the village of Bunkum; and above the position of the Jules Soil in Peoria Loess in the Bald Bluff Section in Henderson County, where radiocarbon dating demonstrates the age of the fauna to be equivalent to that of faunas above the Jules Soil at the Cottonwood School South locality (fig. 3). While shells are sufficiently abundant in the deposits in the Bunkum South Section to permit dating them by the radiocarbon method, this has not yet been done. The two uppermost molluscan faunas at the Cottonwood School South Section have been THE JULES SOIL AND ASSOCIATED MOLLUSCAN FAUNAS ^^^^^^ LOCALITIES MOLLUSCAN SPECIES ^^^-^^^ Cottonwood School South Bunkum South Bald Bluff x) PQ T3 T3 CN