Historical:Hunton Limestone Megagroup

Lithostratigraphy: Hunton Limestone Megagroup
Chronostratigraphy: Paleozoic Erathem >>Silurian System

Authors

H. B. Willman and Elwood Atherton

Name Origin

The Hunton Limestone Megagroup (Taff, 1902 , p. 3; Swann and Willman, 1961, p. 478), is named for the former hamlet of Hunton, Coal County, Oklahoma.

Extent and Thickness

Hunton strata occur throughout most of Illinois, but are absent in the central northern region and in small areas in western and southern Illinois where Ordovician and older rocks are exposed. The megagroup thickens from those areas to over 1800 feet in the southeastern part of the state (fig. S-13).

Fig. S-13 -- Thickness of the Hunton Limestone Megagroup (modified from Bond et al., 1971).

Stratigraphic Position

The Hunton Limestone Megagroup comprises dominantly carbonate rocks of Silurian and Devonian age that in Illinois lie between the late Ordovician clastic rocks (Maquoketa Shale Group) and the late Devonian clastic rocks (New Albany Shale Group) at the base of the overlying Knobs Megagroup (fig. 14). In local areas, siltstone and shale in the base of the Silurian System are not included in the megagroup, whereas limestone in the uppermost Ordovician strata is included. At the top of the Hunton, shale of Middle Devonian age, locally present, is excluded, but basal Upper Devonian limestone is included.

Fig. 14 -- Pre-Pennsylvanian megagroups and sequences of Illinois (Swann and Willman, 1961).

Description

The Hunton Limestone Megagroup varies from being almost entirely dolomite in northern Illinois to limestone in southern Illinois. It contains some thin or local noncarbonate units-- particularly the Devonian chert units in the extreme southwestern part of Illinois, some shaly strata in the Silurian Moccasin Springs Formation, and the patchy Dutch Creek Sandstone at the base of the Middle Devonian Series.

The Hunton Megagroup includes within it the sub-Kaskaskia unconformity, along which in some areas Middle Devonian strata completely truncate Lower Devonian and Silurian strata, leaving only Middle Devonian strata in the Hunton Megagroup. In other areas, the sub-Absaroka unconformity, at the base of the Pennsylvanian System, truncates all the Devonian strata, so that the megagroup consists entirely of Silurian carbonates. In a few places the sub-Absaroka unconformity cuts out the Hunton entirely. In a small area in northwestern Schuyler County, the Upper Devonian New Albany Shale Group truncates the Hunton Megagroup and rests on the Ordovician Maquokota Shale Group.

References

SWANN, D. H., and H. B. WILLMAN, 1961, Megagroups in Illinois: American Association of Petroleum Geologists Bulletin, v. 45, p. 471-483; Illinois State Geological Survey Reprint 1961-N.
TAFF, J. A., 1902, Atoka Quadrangle: USGS Geological Atlas Folio 79, 8 p.

ISGS Codes

Stratigraphic Code	Geo Unit Designation
5890	--