Ardith K. Hansel and W. Hilton Johnson
Robein Silt (Willman and Frye 1970; a direct replacement name for Farmdale silt, Frye and Willman 1960).
Robein, a village in Tazewell County.
Frye and Willman (1960, p. 6) defined the Farmdale silt as "... massive silt, noncalcareous, light brown to pale purple, that commonly contains wood fragments and locally is replaced by peat." They interpreted it to be primarily silt that was derived from the Roxana Silt and reworked by sheetwash and colluvial processes. The unit was renamed Robein Silt in 1970 by Willman and Frye to avoid confusion because the name Farmdale also had been applied to other sediment (Leighton 1960). They indicated the Robein Silt accumulated primarily during the Farmdalian Subage. Because the Robein Silt is leached and commonly contains organic remains, it is a distinctive stratigraphic marker unit (Horberg 1953, Kempton et al. 1991) and the most widely radiocarbon-dated unit in Illinois (Willman and Frye 1970).
Although distinct and traceable in the subsurface, the Robein Silt is thin (usually less than 2 m [6.6 ft] thick). In this report, we have lowered it in rank to a member and included it in the Roxana Silt (fig. 9a), to which it is lithologically related.
Farm Creek Section, along the south side of Farm Creek, 2 kilometers (1.3 mi) south of Sunnyland, Illinois; still exposed, but the Robein Member only locally present along the exposure.
Athens Quarry North, Danvers, and Charleston Quarry Sections; all good for lithology and contacts.
The Robein Member consists of stratified to massive silt, sandy silt, humic material, and peat. Where present, it occurs as the uppermost member of the Roxana Silt. It is overlain by the Peoria Silt, the Henry and Equality Formations, or the Wedron Group. The Farmdale Geosol is developed in it, and pedogenesis commonly obscures stratification within the unit (fig. 32).
The Robein Member of the Roxana Silt is differentiated within the formation by a combination of properties, mainly the presence of stratification or brown to black color and organic debris, as well as by its stratigraphic position at the top of the formation. It is distinct from organic silt that may occur in the overlying Peoria Silt in that it is leached of carbonate minerals and has pedogenic features; the undivided Peoria Silt and the Morton Member generally are dolomitic.
Extent and thickness
The Robein Member is discontinuous in the subsurface over an extensive area of Illinois. In northeastern Illinois, it occurs beneath the Tiskilwa Formation or tongues of the Peoria Silt and Henry and Equality Formations, whereas in central, western, and southern Illinois, it occurs beneath the Peoria Silt (Horberg 1953, Kempton et al. 1991). It commonly is less than 2 meters (6.6 ft) thick, but locally may be greater than 4 meters (13 ft) thick.
The Robein Member consists of light to dark brown to black, noncalcareous, stratified to massive silt loam that often contains organic debris. Locally, the uppermost layer of the unit is peat or muck (O horizon of the Farmdale Geosol).
Lower boundary: the contact with the undivided Roxana Silt or other formal members of Roxana Silt, the Glasford Formation, or other Illinoian or older units. Upper boundary: the contact with the Peoria Silt, the Henry and Equality Formations (fig. 32), or the Wedron Group. Contacts with the Roxana and Peoria Silts are usually gradational, whereas the contacts with the diamicton formations are abrupt.
Age and correlation
The Robein Member was deposited during the Athens Subepisode (fig. 10) from before 50,000 to about 20,000 radiocarbon years ago (appendix B2). Although originally thought to have accumulated mainly in the Farmdale Phase (Farmdalian Subage of Willman and Frye 1970), subsequent work has shown that locally it began to accumulate during the early part of the Alton Phase (fig. 10; Curry and Kempton 1985, Curry and Follmer 1992). The Robein Member correlates with similar unnamed silt units at this stratigraphic position in Wisconsin, Indiana, and Michigan.
Environments of deposition
The Robein Member is interpreted to represent silt and organic debris that accumulated in poorly drained, low- or flat-lying landscape positions and depressions prior to the beginning of the Michigan Subepisode. Commonly, it is in facies relationship with and was derived from Roxana loess, and it generally contains all or the upper part of the Farmdale Geosol in poorly drained situations.
Revised unit. Reduced in rank to member of the Roxana Silt. Formerly classified as a formation.
CURRY, B. B., and L. R. FOLLMER, 1992, The last interglacial-glacial transition in Illinois: 123-25 ka, in P. U. Clark and P. D. Lea, editors, The Last Interglacial Transition in North America: Geological Society of America Special Paper 270, p. 71-88.
CURRY, B. B., and J. P. KEMPTON, 1985, Reinterpretation of the Robein and Plano Silts: Geological Society of America Abstracts with Programs, v. 17, no. 7, p. 557.
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.
HORBERG, C. L., 1953, Pleistocene Deposits Below the Wisconsin Drift in Northeastern Illinois: Illinois State Geological Survey Report of Investigation 165, 61 p.
KEMPTON, J. P., W. H. JOHNSON, P. C. HEIGOLD, and K. CARTWRIGHT, 1991, Mahomet Bedrock Valley in east-central Illinois—Topography, glacial drift stratigraphy, and hydrogeology, in W. N. Melhorn and J. P. Kempton, editors, Geology and Hydrogeology of the Teays-Mahomet Bedrock Valley System: Geological Society of America Special Paper 258, p. 91-124.
LEIGHTON, M. M., 1960, The classification of the Wisconsin glacial stage of the north-central United States: Journal of Geology, v. 68, no. 5, p. 529-552.
WILLMAN, H. B., and J. C. FRYE, 1970, Pleistocene Stratigraphy of Illinois: Illinois State Geological Survey Bulletin 94, 204 p.
|Stratigraphic Code||Geo Unit Designation|