..,

J

~ I---'-' :< s--7 U,PEft ~]ENNSYLVANIAN SUCCESSION

. _y' IN

ILLIN.IS COAL BASI~

1 Publisned with the permission of the Chief, Illr ~ois

Survey.

MINERAL RESOURCE l

RECORDS DIVISION We// er 'fl Alew1o,,

~s- '1 [LT lNO!C: ~: A ~E

By J. Marvin Weller and illiam A. Newton

Studies of Pennsylvanian stratigraphy conducted by the Ill1no1s

State &eolog1cal Survey 1n southeastern Illinois in recent years have

enabled the identification and separation of at least thirteen cyclo­

thems tne1·e. Beginning with the olaest t11ese are~ 1n succession, the

Shoal Creek, Flannigan, Macoupin, La Salle, Cohn, Lower Bogota, Upper

Bogota, Newton, Greenup, G1la, oodbury, Shumway, and an unnamed one

at the top. (Fig. 1) Many of these occur only in this part of Illinois

Upper _Jj . Figure 1. General1ze a section of the Pennsylvanian strata 1n

" 'I i 1,f aoutneaatern Illinoi a. ~t .,_~-and so ~ave been given the names of local communities near which their

type sections occur.. They represent :J;he youngest Pen-1sylv-anian strri,ta

in Illino1s . The cyclical repetition of similar strata constituting

the cyclothems is obvious, but practically eveyy cyclothem possesses

at least one peculiarity- a faunal or physical characteristic of one c,,- - ta as 3o C.l'-rt 'f,'o~ of' u n.'f, - ·

or more un1tsft which by itself or in conjunction with others determines

the identity of the cyclothem.

The Shoal Creek cyclothem is characterizeu by a thin fossiliferous

shale containing black pebbles and lying below .a massive fossiliferous

limestone and above a thin black "slate". The "slate" 1s underla1n "#fl

~ecally by a coal never more than 2 inches thick. The lii estones

belonging to the Shoal Creek and to the La Salle cyclothems are often

very similar in appearance, but in southeastern Ill1no1s the La Salle

, .

j

( 21

cyclothem lacks a black slate.

The Flannigan cyclothem u1ffers from most 1n tnat it has no lime­

stone above the coal. The coal 1s persistent and relatively tn1ck,

and is overlain by la non-silty shale with rrar1ne fossils and ironstones

at the base.

The Macoupin cyclothem contains two.marine limestones above the coal.

They are separated by fossiliferous shale and a #black "slate" containing

hard, massive, oval concretions. The lower limestone is very argillaceous

and contains many crushed and broken fossils. The upper limestone ia

overlain by a shale locally containing a thin, lenticular fossiliferous

sandstone or a ferrug1nous nodular horizon.

The La Salle cyclothem includes two massive limeGtones separated by

a thin shale with coal a,t some localities in Clark, !Joles, and Fayette

counties. At some places they attain a combined thickness of. 15 feet.

The lowe~ limestone is the more persistent and on the eastern side of

the basin is underlain by coal 1½ feet thick.

Both the Gohn and Lower Bogota cyclothems include dark, thinly

laminated or flaky, Estheria-bearing shales and ironstones. In the

Cohn cyclothem the ironstone is often in flat lenses half an inch

thick and cro~ded with Estheria, whereas in the Lower Bogota cyclo­

them it is usually nodular. A fresh-water limestone containing

Spirorbis is often present below the Cohn coa:ib. Infrequently a thin

lenticular fossiliferous limestone occurs in the Lower Bogota snale,

and on the east side of the basin the Lower Bogota coal becomes as

much as 2 feet thick.

The Upper Bogota cyclot11em almost always includes a black "slate 11

containing massive calcareous concretions. The 11 slate"is overlain

by a fossiliferous shale and limestone as much as 7 feet tn1ck and

' 4

> (3)

underlain by a very argillaceous limestone with aoundant crushed and

broken fossils, and by a very platy lenticular limestone containing

fossil Spirorbis, ostraoodes, and plant stems. Underclay and fresh-also

water limestone,iflf#1always occur in the Upper Bogota cyclothem. The

coal is thin and only locally developed on the east side f#f but thickens

to 2 feet on the west side of the basin.

The Newton cyclothem contains the cmal locally m1ned 1n Shelby

county and cowJnonly known as the Shelbyville or Trowbridge coal, but

east of Jasper county the coal is absent and an argillaceous lime-

stone and Gark shale overlie the underclay. The upper part of the

Newton cyclothem in Shelby county consists of strata that may actually

constitute another cyclothem. They include a ferruginous limestone con­

taining an abundance of fossil Myalina subguadrata and overlying a thick

underclay and sandstone. Farther south, in Fayette and Marion counties,

the Myalina-bearing limestone lies only a few feet above the Newton either

coal. A shale as much as 70 feet thick occurs i~Athe Newton cyclothem

proper or with the Upper Newton strata. At least one, and usually two,

fresh water limestones occur in the Newton cyclotnem.

The marine li~estone of the Greenup cyclothem 1s an outstanding

stratum. It is relatively widespread, has a normal tnicKness of about

5 feet, 1s massive and hard, and contains fossils of Wffff~dffffl/.

Triticites sp., at some places abundantly, and fragments of crino1d

stems up to half an inch in diameter. In Shelby county, west and

southwest of Shelbyville, it is locally 25 feet thick, is very sili­

ceous, and is fossiliferous only in the lower part. In northwestaEn

Marion county it is as much as 14 feet thick, and is also very

impure but less siliceous than in Shelby county. The Greenup coal is

as much as one foot thick on the west s1de,i but is #IHfJ#fW absent

t

'

l4J

on the east side of the basin.

The Gila is a lent1C1.1.lar cyclothem characterized by a red, non-silty

shale above the basal sandstone and below a thick #underclay. The

underclay has a loess-like structure at some localities. A thin

lenticular coal and a thin, impure, crinoidal, and often ferrug1nous

limestone o~iElies the underclay.

The °' oodbury cyclothem. usually contains 5 or 6 inches of coal

overlain by a thin, Estheria-bear1ng,black 11 slate 11 and thin limestone

and shale. The limestone at the type locality is very ferrug1nous

and hard, and contains abundant gastropod fossils.

The Shumway cyclothem is the youngest complete cyclothem 1n the

Ill1no1s basin. It is characterized by a persistent black "slate"

underlain by a lenticular argillaceous lime~tone that often contains and

fusulinids,~overlain by a hard, dark bluish-gray, fossiliferous lime-

stone with Productids, weathering yellowish.

The highest bed in the basin is apparently a massive friable

sandstone t~at blankets a large part of the area, and localiy fills

old erosion channels in the younger beds. It is correlated with the

Merom sandstone found at .LIIIerom, Indiana. It may not be of the same

age at all localities where sandstone has been referred to the 1terom.

As iibo1·r.R eB tll.e provi ei eFml a.real gee legs# RiB.1:, ef BGel:tl::1.eestern

Illinois (f1g. ~h the youngest Pennsylvanian strata crop out in

Effingham and Clay counties, whereas the deepest part of the basin ac­

cor.ding to a contour map of the base of the Pennsylvanian system is 1n

tjkn4; eastern Wayne;and western Edwards comt1es/ ~tis thepcfo1e obvious

'

(5)

that during the Pennsslvanian period the locus of maximum depression

s:P,ifted northwesterly and that the ala.er strata are thinnmr in that

direction. This shift may be the consequence of uplift along an axis

marked by the local structural highs !E'eve·aled by recent 011-well

drilling in Wayne, Richland, and Clay counties.

The foregoing data on Pennsylvanian strata 1n southeastern Illinois

have been presented with the idea that they may be of some assistance to

the various geologists who are currently atterpting to locate subsurface

structures favorable for 011 and gas accumulat1n, as such structures

are often revealed by the distribution, relation, and attitudes of

exposed beds. However, correlation o~ surf1c1al with subsurface struc­

tures is complicated by the va,riat1on in lithology o.f any one stratum

or group of strata, by variation 1n thickness, and by irregular or

original djps. Many slight dips are due only to the local var1at1ons

in thickness, whereas in other cases thinning may reflect structures

not shown surficially. Strata exposed in surfic1al outcrops can often

be satisfactorily correlated with those recorded 1n ca~eful logs or

revealed by samples and cores, coal tests and other borings.