Concealed Deposits (Mainly in Bedrock Valleys)
In ancestral bedrock valleys, tributary to the Kaskaskia River valley, early Illinois Episode fine-grained deposits (Petersburg Silt) and/or pre-Illinois Episode deposits (classified as the Banner Formation) are preserved between the overlying Glasford Formation and bedrock below (see cross sections). Such areas include primarily the northeastern and southwestern portions of the quadrangle, which have experienced numerous alternating periods of fluvial incision and alluvial/lacustrine infilling from the pre-Illinois Episode to the present.
The Petersburg Silt, a stratified silty clay loam to silty clay with some fine sand beds, is 47 feet thick in stratigraphic test hole no. 30655 (cross section C–C9) and is interpreted as similar in thickness based on descriptions of “tacky gray or reddish brown clay” material in water-well boring no. 29644 (cross section B–B9). The Petersburg Silt occurs stratigraphically below the Glasford Formation by definition (Willman and Frye 1970). This unit, where mapped, is here mainly interpreted as slackwater lake sediment resulting from impoundment of the Kaskaskia River valley in response to Mississippi River valley aggradation during the advance of the Illinois Episode ice sheet. The lake would have been present prior to the area being buried by glacial ice and the deposition of Glasford till and ice marginal sediment.
Fossil gastropods in the Petersburg Silt (in boring 30655) include Fossaria sp., Valvata tricarinata, Gyraulus sp., and fragments of Stagnicola caperata. Other Petersburg Silt fossils include sporadic occurrences of small bivalves (~3 mm, Pisidium sp.) and the ostracode Candona rawsoni. The overall assemblage of fossils in the Petersburg Silt is very similar to that found in the Equality Formation, and thus the environment of deposition (shallow aquatic with fluctuating lake levels) and ecology are envisioned to have been similar in the Illinois Episode and Wisconsin Episode slackwater lakes. Some thin beds of loamy sand or fine sand in the Petersburg Silt probably reflect fluvial or deltaic deposits during periods of lake regression. In boring no. 30655, some of these sandy beds contain abundant coal fragments (up to 1 cm), suggesting a northeastern source for these deposits from the Herrin Coal subcrop.
Interglacial soil development (Yarmouth Geosol), where preserved within the uppermost Banner Formation (pre-Illinois Episode diamicton, sand, gravel, and silt), by definition helps distinguish this unit from Illinois Episode deposits in the Glasford Formation or Petersburg Silt (Willman and Frye 1970). The Banner Formation does not occur near-surface, but is found mainly in preglacial tributary bedrock valleys or lowlands (fig. 2 and cross sections) where it has been protected from erosion during later geologic events. In many areas, the Banner Formation was likely removed by stream incision and erosion during the succeeding interglacial (Yarmouth Episode) or the Illinois Episode glacial advance and associated meltwater streams. The unit’s distribution is thus sporadic and is absent from bedrock highlands in the central part of the quadrangle. The Banner Formation here is divided into four units (stratigraphically from top to bottom): (1) pedogenically altered sandy clay loam to clay loam accretionary or fluvial deposits (Lierle Clay Member), (2) a sandy clay loam to silty clay diamicton (Omphghent member), (3) calcareous, stratified sandy loam to silt loam to loamy fine sand (Harkness Silt Member), and (4) a brown to greenish gray, faintly laminated clay to loamy sand with basal beds of angular gravelly sand (Canteen member).
The uppermost unit of the Banner Formation is the Lierle Clay Member, which is typically an accretionary deposit in paleo-lowlands or depressions. The unit is clay-rich, leached of carbonates, and high in expandable clay minerals (Willman and Frye 1970). In this quadrangle, a sandier facies of the Lierle Clay is present in an ancestral lowland near Mud Creek (stratigraphic test no. 30654, Sec. 28, T2N, R6W). This unit is a clay loam to sandy clay loam, with faint stratification in lower zones, and probably represents a weathered fluvial deposit. Pedogenic alteration (including clay skins, gleying, iron staining, and soil structure) within this unit likely records interglacial soil development of the Yarmouth Geosol in a lowland environment. Grayish brown or greenish gray colors are the norm due to relatively poor drainage conditions where this unit is preserved below Illinois Episode deposits.
The Omphghent member of the Banner Formation is interpreted mainly as till and ice-marginal sediment. Few direct observations of Omphghent till were made in this quadrangle, and its physical characteristics appear to be variable, ranging from a sandy clay loam to clay loam to silty clay diamicton. Erratic pebbles (e.g., granite) up to 2 cm in diameter were noted. The upper portions of this unit are altered by Yarmouth Geosol interglacial soil development. Sand and gravel lenses may be present within the Omphghent member, but few were observed. The variable character and relative thinness of the unit, compared with its occurrence farther north, are probably related to being within several miles of the terminus of pre-Illinois Episode ice, suggesting relatively thin ice (of limited duration) and thus more local influence to till composition.
A relatively thick deposit (up to 20 feet) of calcareous, stratified coarse silt and fine sand (Harkness Silt Member) was observed below thin Banner Formation diamicton (possible glacial debris flows) in stratigraphic test no. 30654. The Harkness Silt Member in this boring is dark grayish brown and contains conifer wood fragments (probably Picea sp.) up to 5 cm long and horizontally oriented, implying the wood was washed from adjacent uplands into a stream or lakeshore. The environment of deposition of the Harkness Silt likely alternated between alluvial, lacustrine, and deltaic, and the unit probably accumulated in a proglacial environment.
The Canteen member, a basal unit of the Banner Formation, includes mainly noncalcareous to weakly calcareous,
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