Morton Tongue

Lithostratigraphy: Mason Group >>Peoria Silt >>Morton Tongue
Chronostratigraphy: Cenozoic Erathem >>Quaternary System >>Pleistocene Series

Primary source

Hansel, Ardith K., and W. Hilton Johnson, 1996, Wedron and Mason Groups: Lithostratigraphic Reclassification of Deposits of the Wisconsin Episode, Lake Michigan Lobe Area: Illinois State Geological Survey Bulletin 104, 116 p.

Contributing author(s)

Ardith K. Hansel and W. Hilton Johnson

Name

Original description

Morton Loess (Frye and Willman 1960).

Derivation

Morton, a village in Tazewell County, central Illinois.

History/background

In 1960, Frye and Willman applied the term Morton to the loess that occurs stratigraphically above the "Farmdale silt," which they later (Willman and Frye 1970) named the Robein Silt, and below the "Shelbyville till" (Wedron Formation of Willman and Frye 1970). Before 1960, the Morton Tongue was called the "Iowan loess" (Leighton 1933, Leighton and Willman 1950), but Frye and Willman (1960) maintained it required a separate lithostratigraphic name because radiocarbon evidence indicated it was at least 15,000 years younger than the type "Iowan" of Iowa. In this report, the Morton Loess is classified as the Morton Tongue of the Peoria Silt; it is the lower tongue of the Peoria Silt that extends below the Tiskilwa Formation of the Wedron Group.

• 
  Figure 9 -- History of the litho-stratigraphic classification of the Mason Group deposits with respect to the Wedron Group deposits: (a) Peoria and Roxana Silts, (b) Henry Formation, and (c) Equality Formation. Vertical lines indicate part of section missing due to erosion.

Type section

Type location

Farm Creek Railroad Cut Section, located 9.6 kilometers (6 mi) northwest of Morton.

Type status

No longer exposed.

Reference section

Reference location

Farm Creek Section, Gardena Section, Glendale School Section, Danvers Section; all good for lithology and contacts.

• 
  Figure 16 -- The Morton Tongue of the Peoria Silt beneath the Delavan Member of the Tiskilwa Formation at the Gardena Section.

Stratigraphic relationships

The Morton Tongue is the Peoria Silt that extends beneath the Tiskilwa Formation of the Wedron Group.

The underlying Robein Member of the Roxana Silt is usually leached of carbonates, stratified, and more organic rich than the Morton Tongue. The silt of the Morton Tongue is readily distinguishable from the overlying coarse grained sediment of the Ashmore Tongue and the diamicton units of the Delavan Member and undivided Tiskilwa Formation. In contrast to the fine grained sediment of the Peddicord Tongue, silt of the Morton Tongue generally lacks bedding structures.

Extent and thickness

The Morton Tongue is a subsurface, wedge-shaped unit that occurs in a belt about 50 kilometers wide in the up-ice direction from the western and southern margin of the Wedron Group. It is generally less than 2 meters thick and pinches out beneath the Tiskilwa Formation to the north and east.

Lithology

The Morton Tongue consists of gray to yellow tan, dolomitic, fossiliferous silt that is generally unbedded but is locally organic-rich. At some sections, it contains a thin organic layer including moss beds. It may also contain stratified peat, muck, and wood.

Contacts

Lower boundary: contact with the Robein Member of the Roxana Silt or older units. Upper boundary: contact with Ashmore Tongue, Peddicord Tongue, Delavan Member, or undivided Tiskilwa Formation.

Fossils

Contains dispersed pulmonate gastropod shells, often fragmented.

Age and correlation

Deposited during the early Michigan Subepisode between about 26,000 and 20,000 radiocarbon years ago. Correlates with lower Peoria Silt beyond the Wedron Group and equivalent deposits in Indiana.

Environments of deposition

Proglacial loess deposited beyond and later overridden by advancing glacial ice, derived primarily from the Illinois River valley.

Remarks

Revised unit. Downgraded in rank to a tongue of the Peoria Silt.

References

FRYE, J. C., and H. B. WILLMAN, 1960, Classification of the Wisconsinan Stage in the Lake Michigan Glacial Lobe: Illinois State Geological Survey Circular 285, 16 p.
KEMPTON, J. P., and D. L. GROSS, 1971, Rate of advance of the Woodfordian (Late Wisconsinan) glacial margin in Illinois— Stratigraphic and radiocarbon evidence: Geological Society of America Bulletin, v. 82, no. 11, p. 3245-3250.
LEIGHTON, M. M., 1933, The naming of the subdivisions of the Wisconsin glacial age: Science, v. 77, no. 1989, p. 168.
LEIGHTON, M. M., and H. B. WILLMAN, 1950, Loess formations of the Mississippi Valley: Journal of Geology, v. 58, no. 6, p. 599-623. (Reprinted as Illinois State Geological Survey Report of Investigations 149)
WAYNE, W. J., 1963, Pleistocene Formations of Indiana: Indiana Geological Survey Bulletin 25, 85 p.
WILLMAN, H. B., and J. C. FRYE, 1970, Pleistocene Stratigraphy of Illinois: Illinois State Geological Survey Bulletin 94, 204 p.

ISGS Codes

ISGS Code and Symbol

Stratigraphic Code	Geo Unit Designation
0560	p-m