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Full text of "Early Pennsylvanian paleotopography and depositional environments, Rock Island County, Illinois"

iS Guidebook 18 



Early Pennsylvanian Paleotopography 
and Depositional Environments, 
Rock Island County, Illinois 



Richard L. Leary 
C. Brian Trask 




Nineteenth Annual Meeting, North-Central Section of the Geological Society of America 
Northern Illinois University, De Kalb, Illinois, April 25-27, 1985 

Sponsored by the Illinois State Geological Survey 



Early Pennsylvanian Paleotopography 
and Depositional Environments, 
Rock Island County, Illinois 



Richard L. Leary 
Illinois State Museum 
Springfield, Illinois 62706 



C. Brian Trask 

Illinois State Geological Survey 

Champaign, Illinois 61820 



ISGS Guidebook 18 



Illinois State Geological Survey 
Morris W. Leighton, Chief 

Natural Resources Building 
615 East Peabody Drive 
Champaign, Illinois 61820 

1985 



Digitized by the Internet Archive 

in 2012 with funding from 

University of Illinois Urbana-Champaign 



http://archive.org/details/earlypennsylvani18lear 



WARNING: ACCESS TO THESE QUARRIES IS RESTRICTED!! 



Persons entering without permission are subject to arrest and fines! 
Primarily due to OSHA (Occupational Safety and Health Administration) 
and MSHA (Mine Safety and Health Administration) federal regulations 
and insurance requirements, ALL persons entering property associated with 
quarries and mines MUST have WRITTEN PERMISSION and MUST WEAR 
hard hats, safety goggles, and steel-toed shoes and meet all other current 
federal safety regulations. 

WRITTEN permission MUST be obtained from the appropriate company 
PRIOR to entering the quarry property. Write well in advance of any planned 
visit to assure completion of necessary arrangements before the selected date. 

Write to: 

(Allied Stone Company quarry) (Collinson Stone Company quarry) 

Mr. James O. Ellis, President Mr. Richard DeShepper 

Moline Consumers 3115 23rd Avenue 

313 16th Street Moline, IL 61265 
Moline, IL 61265 




1,1 «»*■** 



EARLY PENNSYLVANIAN PALEOTOPOGRAPHY AND DEPOSITIONAL 
ENVIRONMENTS, ROCK ISLAND COUNTY, ILLINOIS 



INTRODUCTION 



The Geological Society of America, North- 
Central Section, presents this one-day field 
trip to Rock Island County, Illinois, where you 
will have the opportunity to examine the pre- 
Pennsylvanian unconformity, overlying sedi- 
ments, and an early Pennsylvanian upland 
compression flora. The itinerary repeats a 
trip originally offered during the Ninth Inter- 
national Congress of Carboniferous Stratig- 
raphy and Geology (IX-ICC) in 1979. 

In recent years the nature and significance 
of the Mississippian-Pennsylvanian boundary 
have received considerable attention. In Illi- 
nois, as in much of the continental United 
States, the boundary is marked by a major 
unconformity. Extensive topographic relief 
developed on underlying strata prior to initial 
Pennsylvanian deposition. In parts of Illinois, 
especially along the margins of the Illinois 
Basin, erosion removed great thicknesses of 
pre-Pennsylvanian rocks (although some forma- 
tions may have been thinner than equivalent 
strata in the deep part of the basin, or in fact, 
may never have been deposited). 

In the Rock Island County area, the 
Pennsylvanian System overlies rocks ranging 
in age from Silurian to Devonian. Pre-Pennsyl- 
vanian topography was characterized by deep 
valleys, high hills, and some karst features. 
Although much of this topography can be 
examined only by means of drilling records, 
some higher elevations are now exposed. 
Several of these features will be seen on this 
field trip. 

In recent years the senior author has col- 
lected a large number of well preserved early 
Pennsylvanian plant fossils in the Rock Island 
area. The fossils occur in rocks immediately 
overlying the erosional surface developed on 
Devonian carbonate strata (Cedar Valley and 
Wapsipinicon Limestones: Middle Devonian 
Series). The composition of the fossil plant 



assemblage is distinct from the more com- 
monly preserved "coal-swamp flora." 

The distinctive character of the flora has 
been considered the result of the distinctive 
environment that produced the flora. All flora 
from Rock Island County, and one from 
Brown County to the south, have been referred 
to as an "upland" flora. 

This field trip will give geologists and 
paleobotanists an opportunity to examine the 
stratigraphic and paleotopographic setting of 
this flora. It will also provide an opportunity 
to discuss the nature of the flora, its relation- 
ship to the setting, and the paleoecological 
and evolutionary implications. 

The guidebook itself is a reprint of Leary 
(1979). Some revisions have been made to up- 
date the guidebook with new exposures, new 
data, and new references. The geology of the 
field-trip area has been discussed by Leary 
(1981). Additional information concerning 
Labriscorpio alliedensis is also available (Leary, 
1980). Phillips and Peppers (1984) have sum- 
marized regional paleoenvironmental patterns 
of coal swamps and discussed the effect of cli- 
mate on coal occurrence in Euramerican coal 
fields. 

Figure 6 has been revised to reflect new 
data. Now this illustration better shows the 
pre-Pennsylvanian paleovalleys in the Rock 
Island County area. Figure 7 shows the full 
extent of the channel exposed in the Allied 
Stone Company quarry. It also indicates that 
most of the fossil-bearing strata have now been 
removed by quarrying. Figure 8 has been 
updated to show a current (1984) cross section 
of a channel. 

Russel A. Peppers at the Illinois State 
Geological Survey has examined the spore 
content of the coal from Stop 2. He believes 
(1984. personal communication) that this 
coal is lower Westphalian A in age. 



Guidebooklet: Illinois State Museum 



No. 4 




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Figure 1. Geologic map of the Rock Island County area. Circled numbers are stops. 
Base map from Edmund and Anderson, 1967. 



1979 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



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Figure 2. Generalized sub-Pennsylvanian geologic map of Illinois. 
(From Plate 2, Paleotectonic investigations of the Pennsylvanian 
System in the United States, McKee and Crosby, Coordinators, 
U.S.G.S. Prof. Paper 853.) 



Guidebooklet: Illinois State Museum 



No. 4 











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BACKGROUND 



Geological Background 

Stratigraphy. In the Illinois Basin, Penn- 
sylvanian strata overlie rocks ranging in age 
from Ordovician to Mississippian. The sub- 
Pennsylvanian strata are primarily carbonates 
and had been exposed to subaerial erosion. 
Along the western margin of the Basin, Penn- 
sylvanian rocks overlie older Mississippian 
rocks in the central part and Devonian rocks 
in the northwest (Reinertsen et a/., 1974). 
Silurian and Ordovician strata underlie the 
Pennsylvanian along the northern margin (see 
geologic maps, Figs. 2, 3). 

The relevant portion of the stratigraphic 
sequence in the Rock Island County area is 
illustrated in Figure 4. The major unconformity 
between the Devonian Cedar Valley Limestone 
and basal Pennsylvanian indicates a long period 
of erosion prior to earliest Pennsylvanian sedi- 
mentation. Because strata as young as the St. 
Louis Limestone (Valmeyeran = basal Visean) 
occur beneath the unconformity to the south, 
it is believed that the period of erosion came in 
Late Mississippian/Early Pennsylvanian (at the 
base of the European Namurian?). 

Pre-Pennsylvanian topography. Major valleys 
are known to have drained eastward and south- 
eastward across the pre-Pennsylvanian surface 
in the Illinois Basin (Smith, 1941; Horberg, 
1950). Similar valleys up to 140 m deep and up 
to 32 km wide are known from other parts of 
the Illinois Basin (Bristol and Howard, 1971). 

As can be seen in several quarries and natural 
exposures in this area, the unconformity surface 
in the Rock Island County area was relatively 
flat. A few years ago, one could walk on the 
unconformity, a rolling limestone surface with 
a maximum relief of 2 m that was exposed in 
the Allied Stone Company quarry; unfortun- 
ately, it was destroyed by quarrying operations 
between 1975 and 1977. The flat upper surface 
of Devonian limestone can still be seen along 
the north and south walls of the Allied Stone 
Company quarry (Stop 4). 

There are small knolls or "hills" on the 
limestone surface. One such knoll can be seen 
across the Rock River, north of the Allied 
quarry (Fig. 5). This one is only 3 to 4 m high. 
Another one, a higher limestone hill, is exposed 
northeast of the bridge where US 67 crosses 



the Rock River north of Vandruff Island and the 
Allied quarry. This hill is about 10 m above the 
river level which is the approximate elevation 
of the major portion of the unconformity here. 
This hill will be seen at Stop 2. 

Although extensive areas of relatively low 
relief are present, some deep valleys were also 
developed prior to earliest Pennsylvanian 
sedimentation. One such valley occurs in south- 
eastern Rock Island County (see map, Fig. 6). 

A system of steep-sided, flat-bottomed 
channels was developed on Devonian limestone 
in the Rock Island County area. Although now 
largely destroyed by quarrying operations, these 
were observed in both the Allied Stone Company 
quarry and the Cleveland quarry. The primary 
trend of these channels was east-west (Fig. 7), 
but the locations were apparently controlled by 
jointing in the limestone. The exact nature of 
these channels has not been determined. 

These channels were 4 to 6 m deep and 8 
to 20 m wide (Fig. 8). They were filled with 
shale, mudstone, and occasional lenses or thin, 
discontinuous layers of sandstone. Occasional 
thin layers and masses of pyrite are also present 
with a few pyritized plant axes. The shale and 
mudstone are almost exclusively gray except 
for the upper portion which is sometimes thinly 
laminated black shale. 

Similar channels have been exposed in 
several other quarries in the Rock Island County 
area (Fig. 9). Plant fossils comparable to those 
of the Allied Stone Company quarry were 
observed in the quarry at Cleveland, Henry 
County, Illinois, but these were poorly preserved 
and recent quarrying operations have destroyed 
the fossil-bearing deposits. 

Pre-Pennsylvanian karst topography. A karst 
topography apparently developed at the close 
of the Mississippian when the region was up- 
lifted and exposed to subaerial erosion. At that 
time large areas of Silurian, Devonian, and 
Mississippian limestone strata were exposed 
on the western margin of the Illinois Basin. 
Caves, filled with Pennsylvanian mud, silt, 
and sand, are known in the area. Sinkholes 
have been reported from both surface and sub- 
surface data(McGinnis and Heigold, 1974). It is 
not known whether the channels exposed in the 
Allied Stone Company quarry drained into caves 
via sinkholes or into nearby deep valleys. 



PENNSYLVANIAN 



KEWANEE 



SPOON 



CARBONDALE 




System 



Group 



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SILURIAN 



DEVONIAN 



System 



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UPPER 



Series 



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WAPSIPINICON 



CEDAR VALLEY 



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ForiTiation 



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Figure 4. Composite section of Pennsylvanian, Devonian and Upper Silurian strata in 
the Rock Island County area. Pennsylvanian section from Searight and Smith, 1969. 



1979 



Leary: Pennsylvania!! Flora, Rock Island County, Illinois 



Paleobotanical background 

Historical perspective. Several occurrences 
of Early Pennsylvanian (Late Namurian- Early 
Westphalian) plant fossils in western Illinois 
have been known since the late 1800's (Worthen, 
1873). In 1907, David White (1908) examined 
fossils from several localities in this area and 
published a brief report with a list of 14 fossil 
plant genera. Since that time, only sporadic 
collecting has been done and no significant 
collections are known other than those at the 
Illinois State Museum. The above, and broader 
aspects of compression plant fossil studies in 
the area of the Illinois Basin, are reviewed by 
Phillips, Pfefferkorn, and Peppers (1973). 

Description of significant "upland" taxa. 
The term "upland" has been used to designate 
distinctive nonswamp floras, but the actual 
paleoecology of such floras is not known. 
The "upland" plants grew on soils derived from 
limestone bedrock in western Illinois, and 
many were near stream banks and ultimately 
were deposited in stream channels with mini- 
mal transport. 

Because the "upland" flora includes a 
number of genera which are unknown in coal 
swamps and are not well known, some general 
descriptions and illustrations are given. 



Several species of Megalopteris occur in the 
floras of western Illinois. These range from large 
trilobed M. dawsoni (Hartt) Andrews (PI. 1, 
Fig. 1) to smaller, pinnate (alethopteroid) forms 
such as M. ovata Andrews (PI. 2, Fig. 1). Other 
specimens (PI. 2, Fig. 2) probably belonging to 
the megalopteroid group, if not to the genus 
Megalopteris, are similar to published figures 
and descriptions of such genera as Neriopteris 
(Newberry, 1873), Orthogoniopteris (Andrews, 
1875) and Protoblechnum (Andrews, 1875). 
Differences between these genera and their 
relationships are not clear from the literature. 

Lesleya foliage is similar to Megalopteris 
pinnules but consists of simple leaves (PI. 1, 
Fig. 2). These leaves are greater than 30 cm 
long and 10 cm wide. The midvein is broad, 
though not as broad as that of Megalopteris; 
the lateral veins are curved and divide once or 
twice. Lesleya and Megalopteris are also 
separated on the basis of epidermal structures 
(Florin, 1933). 

Mesocalamites (PI. 3, Fig. 1) is distinguished 
from Calamites on the basis of the continuity 
of ribs across the nodes. Calamites ribs alternate 
whereas some ribs of Mesocalamites alternate 
and some are continuous. Mesocalamites is 
largely restricted to the Namurian, rarely ex- 
tending into the Westphalian. Archaeocalamites, 






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Figure 5. A limestone knoll exposed on the north side of the Rock River 
below the Watchtower, Black Hawk State Park. Sec. 14, T. 17 N., R. 2 W. 





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777/ Last remaining undistrubed shale, 1984 

Figure 7. Channel filled with fossil-bearing shale, siltstone, and sandstone in the Allied Stone Company quarry. 



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ROCK ISLAND COUNTY 

Major known localities: 

1. Wyoming Hill, Iowa 

2. farm road 

3. Allied Stone Company quarry 

4. Black Hawk State Park 

5. northeast jet. Rt. 67 and Rock River 

6. Collinson quarry 

7. Coal Valley 

8. Carbon Cliff 

9. Cleveland quarry 

10. Port Byron 

11. Slaymakers quarry 

12. Midway Stone Co. quarry 

13. Oakwood Golf Course Rd. 

14. Sec. 3,T. 16N..R.4W. 

sr?"^ Boundary of Pennsylvanian rocks 
(Generalized) 

• Accessible 

o Known from literature, 
unpublished records 



Figure 9. Map of Rock Island County showing locations of exposures of 
the Devonian-Pennsylvanian unconformity and lower Pennsylvanian strata. 



1979 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



11 



an older form, has all ribs continuous across the 
nodes. The correlation of various species of 
Mesocalamites/Calamites to species of foliage, 
Asterophyllites/Annularia, and fructifications, 
Calamostachys, is still not complete. 

Lacoea (PI. 3, Fig. 5) is a cone consisting of 
semicircular sporophylls which alternate on a 
thick axis. Lacoea and similar, perhaps synony- 
mous, forms are known from a few localities 
but in distant parts of the world: Holland 
(Hirmer, 1940, 1941); Belgium (Stockmansand 
Williere, 1962); Czechoslovakia (Feistmantle, 
1879); Germany (Hirmer, 1940); and China 
(Stockmans and Mathieu, 1957). Although 
Read (1946) suggested that Lacoea might be a 
pteridosperm, specimens from Brown County, 
Illinois, have permitted a reinterpretation; 
Lacoea is now considered a member of the 
Noeggerathiales (Leary, 1973). However, the 
broader natural affinities of the Noeggerathiales 
are not known. This order has been treated as 
a separate group or loosely allied with either 
the Pteridopsida or Sphenopsida (Boureau, 
1964). The Noeggerathiales are well represented 
in the "upland" floras of western Illinois. 

Gulpenia (PI. 3, Fig. 2), another member of 
the Noeggerathiales, is characterized by small, 
deeply lacerated leaves attached spirally to a 
thin axis. In compression, Gulpenia bears a 
superficial similarity to Sphenophyllum but the 
leaves clearly alternate. 

Megalopteris is not known to occur outside 
North America (Arnold, 1934)* where it has 
been reported from the following localities: 

St. John, New Brunswick 

Dawson (1871), Stopes (1914) 
Rushville, Ohio 

Andrews (1875), Cross (1962) 
Port Byron, Illinois 

Lesquereux (1880) 
Wyoming Hill, Iowa 

Noe(1925) 
Grand Ledge, Michigan 

Arnold (1934) 
Saginaw, Michigan 

Arnold (1934) 



* A single fragment, from Britain reported as Megalopteris 
(Arber, 1904) is now referred to Lesley a ( Leary, in press). 



Putnam County, Indiana 

Arnold (collected 1936, oral comm.) 
Pictou, Canada 

Bell (1940) 
West Virginia 

White (1913) 
Greene County, Indiana 

Canright (1959), Wood (1963) 
Brown County, Illinois 

Leary and Pfefferkorn (1977) 
Rock Island County, Illinois 

Leary (1974b, 1976, this guidebooklet) 

In almost half of these localities, Megalop- 
teris occurs with a flora which is distinct from 
the common coal swamp flora. The Megalopteris 
floras appear to contain greater percentages of 
pteridosperms and Cordaites and smaller per- 
centages of ferns and lycopods than do Early 
Pennsylvanian floras associated with coal 
seams ("swamp floras") (Phillips et al., 1974). 
White (1931, p. 275-276) suggested that 
Megalopteris and Lesley a are unique to uplands 
developed on carbonates. Recent studies by 
Cross supports this concept (personal communi- 
cation). The flora associated with Megalopteris 
apparently grew on drier uplands underlain 
by limestone. 

Environmental changes indicated in the 
Allied flora. Studies of the paleoecological 
differences in Mississippian-Pennsylvanian floras 
have shown that several paleoenvironments can 
be recognized (Peppers and Pfefferkorn, 1970, 
for summary, also Remy and Remy, 1977). At 
the top of the sequence in several channel fills 
in the Allied quarry, the flora is characterized 
by an overwhelming abundance of Meso- 
calamites with Lepidodendron and Psaronius; 
little else is present. This change in composition 
apparently represents a change from drier 
"upland" to wet lowland, or swamp, conditions 
as stream channels were filled with sediment 
and the water table correspondingly rose to 
near ground level. Tenchov (1976) described 
floral changes within a Carboniferous floral 
sequence of western Bulgaria and related these 
to continuous uplift of the basin. Changes in 
the elevation of the western margin of the 
Illinois Basin, relative to the central area of the 
Basin, may have brought about changes in floral 
composition in this area. 



12 



Guidebooklet: Illinois State Museum 



No. 4 



Comparisons of "upland" floras. Recent 
studies of an Early Pennsylvanian flora from 
Brown County, Illinois (Fig. 10) (Leary, 1973, 
1974a, 1974b, 1976; Leary and Pfefferkorn, 
1977) have identified twenty -one genera of 
plants (Table 1). Among the most common are 
Lacoea, Sphenopteris, Cordaites, and Alethop- 
teris. Megalopteris and Lesleya are also well 
represented. Thirty-two genera of foliage, 
fructifications, and stems have so far been 
identified from the Allied Stone Company 
quarry (Table 1). The most abundant taxa at 
this site are Megalopteris, Lesleya, Samaropsis, 
Cordaites, and Cardiocarpus. 

The two floras (Table 1) have many genera 
in common; differences are in the relative abun- 
dance of certain genera and presence or absence 
of certain others. Arborescent lycopods are 
common in the nonswamp Rock Island flora 
but very rare in the Brown County flora; only 
one determinable specimen and two fragmen- 
tary specimens of Lepidodendron have been 
found at the latter site; Lepidophloios is absent 
from the Brown County locality and rare in the 
Allied quarry site. Lycopods are the dominant 
plants in most Lower and Middle Pennsylvanian 
coal swamp floras of the Illinois Basin (Phillips 
et al., 1974). Sphenopsids and Cordaites are 
more abundant in parts of the Rock Island 
flora than in the Brown County flora; ferns are 
more common in the the Brown County flora. 

Although the ages of the two floras are 



probably not exactly the same, the differences 
in generic composition probably reflect environ- 
mental differences rather than age differences. 
The presence of a permanent body of water in 
the channels apparently permitted the growth 
on the drier sites in Rock Island of genera (e.g., 
Lepidodendron and Mesocalamites) common 
to coal swamps. 

Comparison with European Floras. The 
flora exposed in the Allied Stone Company 
quarry is one of very few Namurian-age floras 
known in North America and provides a rare 
opportunity for comparison with floras of 
similar age in Europe. Analysis of the Illinois 
"upland" floras enables us to make comparisons 
with the "flozformend" (seam-forming) and 
"flozfern" (distant from the seam) floras of the 
European Namurian (Havlena, 1961). Numerous 
similarities exist between these as well as signi- 
ficant differences. 

The greatest similarity appears to be with 
the flora of the Ostrava-Karvina coal district of 
Czechoslovakia (Sustra, 1928; Purkynova, 
1970). Here, Havlena (1970) recognized several 
microenvironments within the Namurian hygro- 
phile environment in addition to a major 
separation of the hygrophile and mesophile 
environments. The mesophile ("upland") flora 
described by Havlena from the Ostrava-Karvina 
coal district was present only as fragments 
(Havlena, 1971, p. 245) whereas preservation 
of the plants in the Allied quarry is excellent. 



TABLE 1 

LIST OF GENERA PRESENT AT THE LOCALITIES IN 

BROWN AND ROCK ISLAND COUNTIES, ILLINOIS 



13 



ABUNDANCE 



BROWN ROCK ISLAND 

GENERA COUNTY COUNTY 

Lycophytina 

Lepidodendron vr c 

Lepidophloios vr 

Lepidophylloides f c 

Lepidostrobus c 

Lepidostrobophyllum c 

Lepidocarpon c 

Stigmaria vr 

Sphenophytina 

Mesocalamites c c 

Asterophyllites c c 

Annularia vr 

Calamostachys c c 

Sphenophyllum r 

Filicophytina (Ferns) 

Alloiopteris c vr 

Dactylotheca r 

Pteriodospermales 

Alethopteris c r 

Sphenopteris a c 

Lagenospermum vr ? 

Telangium c r 

Megalopteris c a 

Lesleya c a 

Samaropsis c a 

PNeuropteris vr 

PMariopteris vr vr 

Whittlesey a vr 

Aulacotheca vr 

Rhodea r vr 

Rhodeopteridium vr ? 

Noeggerathiales 

Lacoea a c 

Palaeopteridium c vr 

Gulpenia r vr 

Cordaitales 

Cordaites c a 

Cardiocarpus r c 

Cordaianthus r 

Artisia vr 

Approximate abundance indicated by the following: 

vr = very rare (1,2) 
r = rare (3-5) 
c = common (5-25 
a = abundant (> 25) 

Based upon 350 specimens from each locality. 



14 



Guidebooklet: Illinois State Museum 



No. 4 



1 St. Louis, Mo. 



2 Golden Eagle 



3 Brown County 



4 Monmouth 



5 Wyoming Hill, Iowa 



6 Cleveland quarry 



7 Midway quarry 



8 Port Byron 



9 La Salle County 



10 Channahon 



11 Kankakee 



12 Putnam County, Ind. 



13 Green County, Ind. 



ALLIED STONE 
CO. QUARRY 




Figure 10. Map of reported localities of Early Pennsylvanian plant 
fossils on the margins of the Illinois (Eastern Interior) Basin. 



12 



Margin of Pennsylvanian strata 
Locality no longer accessible 



Existing locality 
Spores only 



DESCRIPTIONS OF STOPS 



15 



Stop 1: Collinson Stone Company Quarry. 

SE SE NE NW Sec. 25, T. 17 No., R. 2 W., 
Milan 7.5' Quadrangle. 

Caution: Do not approach the edge of the main 
quarry. Look where you step and watch what 
is above you on the upper level. 

Exposures of Pleistocene, Pennsylvanian, 
and Devonian strata can be seen in this quarry. 

Strata exposed by quarrying operations 
here include slightly more than 30 m of middle 
Devonian limestones that are divided into about 
10 m of Wapsipinicon Limestone in the deeper 
part of the quarry and about 18 m of over- 
lying Cedar Valley Limestone (Fig. 11). The 
limestones composing these two formations 
accumulated in warm shallow seas that covered 
the midcontinent region about 370 to 395 
million years ago. Although initially it would 
appear that conditions of deposition were 
fairly consistent for long periods so that 
thick limestones could accumulate, a closer 
examination of the Wapsipinicon shows that it 
contains a considerable amount of brecciation. 
The brecciation has been interpreted as being 
the result of the removal of soluble beds, such 
as gypsum or salt. Their removal then caused 
collapse of overlying limestone layers. Signifi- 
cantly, gypsum is currently being mined from 
the Spring Grove Dolomite Member of the 
Wapsipinicon at Mediapolis, Iowa, less than 80 
km to the southwest. If removal of an evaporite 
bed were the actual cause of brecciation, it 
would appear that the formation accumulated 
in a restricted basin of deposition, such as 
a large lagoon. The water temperature and 
salinity may have been high because there was 
limited open circulation with the deeper oceans 
nearby and conditions were right for the 
deposition of evaporite materials. The scarcity 
of fossil forms indicates that the water did 
not support abundant life. Since brecciation 
and contorted bedding is restricted to the 
Wapsipinicon, removal of the soluble evaporite 
materials must have preceded the deposition of 
the overlying Cedar Valley. 

The Cedar Valley Limestone must have been 
deposited under much different conditions. 



The absence of brecciation and contorted bed- 
ding and the presence of abundant fossil forms 
indicate that the seas were shallow, warm, and 
open to free circulation. (Fossil collecting is 
best from the bench upward.) 

At the close of the Devonian Period, when 
seas withdrew from the midcontinent region, 
strata in this area were subjected to erosion 
before Mississippian seas advanced across the 
region. Although strata from this latter marine 
advance have not been identified from this 
vicinity, chert fragments from the overlying 
basal conglomerate of the Pennsylvanian Period 
contain fossil fragments indicative of the early 
and middle portions of the Mississippian time, 
indicating that Mississippian sedimentation did 
occur in this area. After Mississippian sedi- 
mentation ceased in this region, the land surface 
again stood above sea level and was so exten- 
sively eroded that all Mississippian rocks were 
removed, except for the relatively insoluble 
chert rubble. An uneven topography was devel- 
oped; when shallow Early Pennsylvanian seas 
encroached upon the region, new sediments did 
not blanket all earlier strata. The higher hills 
and knobs of the older topography must have 
been low islands in the sea. Stream channels, 
sinkholes, and caves which developed on and in 
the old eroded surface were the first to receive 
earliest Pennsylvanian sediments consisting of 
sands, silts, and muds that later formed sand- 
stone, siltstone, and shale. A description of 
the topography of this surface can be found 
under BACKGROUND, page 5. 

Pennsylvanian deposits noted at this quarry 
seem to be only sinkhole fillings, although as 
the quarry is expanded farther south and west, 
more extensive Pennsylvanian strata should be 
encountered. Because of slumping, the exact 
relations are not always visible; so it is possible 
that the Pennsylvanian strata in the south- 
western part of the quarry highwall could be 
located in a shallow stream channel on the 
old surface. 

Overburden exposed along the south- 
western part of the quarry reveals possible 
Kansan and Yarmouthian deposits beneath 
Illinois, Sangamonian, Wisconsinan, and Holo- 
cene materials.* 



♦Modified from the I.S.G.S. Milan Guidebook, 1974. 



16 



Guidebooklet: Illinois State Museum 



No. 4 



HELM 




15.8m 




Sandstone, sandy greenish shale in solution cavities. 

Limestone, gray to buff. Cystodictya , Stropheodonta , 
Spirifer . Bioherm with Hexagonaria , Cystphyl lum , 
Favosites , Cladopora , and Atrypa common 

Limestone, gray to buff, dolomitic, with abundant 
crinoids and some micro-conglomerate. 

Many stromatoperoids and bryozoa with fossils 
1 isted above. 

Limestone, greenish-gray to buffish-gray, fine grained, 
argillaceous, massive, weathers readily, oblique 
joints prominent. 

Relatively few fossils. 



Fossils present but not abundant. 



Limestone, greenish-gray to grayish-brown, fine grained, 
argillaceous to shaley partings, weathers easily, 
fossi 1 iferous. Atrypa , Strophodonta , Leptostrophia , 
Chonetes , Spirifer , Schizophoria , Pentamerel lo , 
He! iophyllum , Cystiphyllum , Favosites , Productel la , 
also crinoidal . 

Limestone, grayish-brown, subl ithographic to fine grained, 
weathers with brown splotches, very fossil iferous. 
Astreospongia , Stromatoporoids, Hel iophyllum , Favosites . 

Limestone, very light gray, lithographic to subl itho- 
graphic, brecciated, pyritic sandy streaks. Grades 
down to brownish-gray lithographic limestone. 

Limestone, very light gray to light gray, lithographic 
to subl ithographic, hard, brecciated to bedded. 

Limestone, brown to brownish-gray, subl ithographic 
with thin light-colored laminae. 

Limestone, brownish-gray, subl ithographic, fractures 
often calcite filled, some pyrite. 

Limestone, gray, lithographic, contorted bedding, 
brecciated. 



Limestone, light buff, very fine grained, dolomitic, 
wavy bedded, mostly thin bedded and slabby. 

Limestone, blue-gray to brownish-gray, medium to 
coarse grained, thin dark wavy laminae at top, 
locally brecciated; dolomite in lower part, 
cavities numerous with crystals of pyrite, calcite, 
and quartz. 



Figure 11. Stop 1. Collinson Stone Company quarry stratigraphic section. NW 
Sec. 24, T. 17 N., R. 2W., Rock Island County, Illinois. From 31st Tri-State 
guidebook, Edmund and Anderson, 1967. 



1979 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



17 



Stop 2: Borrow pit east side of US 67. 

SW NE SW Sec. 14, T. 17 N., R. 2 W., 
Milan 7.5' Quadrangle. 

Devonian, Pennsylvanian, and Pleistocene 
strata are exposed in this locality. 

The borrow pit exposes strata of Pennsyl- 
vanian age that are older than those seen at Stop 
1. Above a gray underclay in the background is 
a 20 to 26 cm coal bed that is one of the oldest 
coals known in this part of the state. A gray 
silty shale lies above the coal. The shale is either 
locally nearly cut out by a sandstone channel 
that crosses the area or is replaced by a sand- 
stone facies to the south (see Fig. 12). The large 
covered area near the center of the exposure 
obscures an important part of the section. The 
southern half of the east wall of the pit and the 
floor of the pit reveal Pennsylvanian strata dip- 
ping to the north and truncated to an even, 
nearly horizontal surface upon which Illinoian 
till was deposited. The till is overlain by 
Wisconsinan silts and thick loess. Along the 
southern part of the exposure, a Pleistocene 
channel was cut through Illinoian till to the 
bedrock surface and was later filled with 
Wisconsinan silts. Beneath this channel, bed- 
rock strata have been considerably disturbed. 
Although a slight possibility exists that the 
Pleistocene channel had something to do with 
the disturbance, the location of the two features 
is coincidental.* 

The Devonian -Pennsylvanian unconformity 
is exposed at several points along the north side 
of the Rock River for nearly 1 km east of Stop 
2 (Section 14, T. 17 N., R. 2 W.). In addition 
to the exposure at this point, a second good 
exposure of basal Pennsylvanian sediments now 
exists on the bluff below the lodge in Black 
Hawk State Park (Stop 3, Fig. 13). 

Although the Pennsylvanian -Devonian con- 
tact is approximately at river level, small knobs 
or "hills" of Devonian limestone are exposed in 
cross section along the Rock River. One of 
these is shown in Figure 5 and can be seen from 
Vandruff Island. It can be reached (though not 



♦Modified from the I.S.G.S. Milan Guidebook, 1974. 



easily) from Black Hawk State Park. The largest 
hill is exposed at Stop 2. The easternmost "hill" 
is approximately 3 m high and slightly irregular 
in form. The upper portion of the limestone is 
iron stained, with a sideridic top. Because the 
"hill" is essentially exposed only in cross 
section, the north-south extent is unknown. 

The "hill" is overlain by 14 m of shale with 
some thin sandstone beds. Above the shale is a 
thick, cross-bedded sandstone. Both the shale 
and sandstone are unfossiliferous at this point. 

About 100 m to the west is possibly another 
limestone "hill." This "'hill" appears from 
the distance to be lower than the previously 
described one because the upper portion is 
weathered to a thin-bedded state resembling 
shale and thin-bedded sandstone. Less iron 
appears to be present here than at the eastern 
"hill." This second "hill" is also overlain by 
shale; but weathering, slumping, and vegetation 
obscure details of the section. 

The top of the westernmost hill, 100 m 
east of US 67 (Stop 2), is approximately 10 m 
above the Rock River. It is heavily iron stained 
and in part replaced by iron. Because excavation 
has only exposed the top of this hill, its extent 
is not known. 



Stop 3: (Lunch) Black Hawk State Park. 

NE NW SE Sec. 14, T. 17 N., R. 2 W., 
Milan 7.5' Quadrangle. 

Devonian and Pennsylvanian strata are ex- 
posed along the Rock River below and to the 
east of Watch Tower Inn. 

(The following discussion is taken from the 
31st Tri-State Geological Guidebook: The Mis- 
sissippi River Arch, by R.W. Edmund and R.C. 
Anderson, Augustana College, 1967). 

Black Hawk State Park preserves the 
most scenic portion of the lands once 
occupied by Saukenuk, the largest 
Indian village in the Upper Mississippi 
Valley. It was here that Chief Black 
Hawk and his band of Sauk and Fox 
came in conflict with the territorial 
claims of the white man in the years 



18 



Guidebooklet: Illinois State Museum 



No. 4 




& 



tS' 



o 

a 

o 

0/ 

si 



a> 
•a 



8 
w 

S3 

i-H 

OJD 



CO 



Fig. 12. Stop 2. Borrow pit, east side of U.S. 67. SW NE SW Sec. 14, 
T. 17 N., R. 2 W., Milan quadrangle, Rock Island County, Illinois. 



1979 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



19 




Loess, cream to light buff, 
silt and clay with calcareous 
concretions. 



Till and glacial drift. 

Sandstone, light brown to tan, 
medium to fine grained with 
irregular streaks of carbon- 
aceous shale. 

Shale, gray, fissile, silty. 
Coal, impure, shale partings. 

Shale, dark gray, with silty 
lenses and thin streaks of 
sand. Grades from fissile 
shale in thin beds to blocky 
claystone in rather massive 
beds. 



Mery few fragments of 
plant fossils. 



Figure 13. Stop 3. Stratigraphic section at Black Hawk State Park, Rock Island, Illinois. 



20 



Guidebooklet: Illinois State Museum 



No. 4 



preceding the Black Hawk War of 1832. 
The area is rich in Indian lore, and some 
of it can be savored in the Black Hawk 
Museum on the "Watch Tower." 

From the Watch Tower, a high prom- 
ontory on the north bluffs of the Rock 
River, many of the features of the land- 
scape in the Rock Island area can be seen. 
This area is characterized by flat upland 
prairies greatly dissected along the major 
streams and by broad valleys bounded 
by steep bluffs and occupied by sizable 
streams such as the Rock River. 

Chief Black Hawk is reported to have 
said, "At the Watch Tower, which was 
frequently visited by me alone I could 
sit and smoke my pipe and look with 
wonder and pleasure at the grand scenes 
that were presented by the sun's rays, 
even across the mighty water." The 
quarry on Vandruffs Island makes it 
impossible for us to view this scene with 
the same serenity as did Black Hawk, 
but it has provided some very fine 
exposures of the Devonian Cedar Valley 
limestone (and fossil-bearing basal 
Pennsylvanian, ed.). This will be our 
first stop upon leaving the park. 

Within the confines of Black Hawk 
State Park the fossiliferous Cedar 
Valley limestone, Devonian, crops out 
along the north bank of the Rock River, 
exposing approximately ten feet of 
section above water level. The Devonian 
beds here are overlain by sandy carbon- 
aceous shale and gray shale totaling 
approximately 40 feet of strata, Pennsyl- 
vanian in age. Fragments of plant fossils 
have been observed directly above the 
Devonian in fine silty sandstones and 
dark gray shales. The Pennsylvanian 
shales are overlain by about 50 feet of 
Pleistocene loess (Fig. 13). 

The Pennsylvanian rocks at Black 
Hawk State Park are representative of 
the exposures found on the south side 
of Rock Island and Moline, adjacent to 
the Rock River. 

Dr. Russel Peppers, paleobotanist at the 
Illinois State Geological Survey, has examined 



the spore flora in the coal in this section and 
determined that the coal is a unit in the Casey- 
ville Formation of the McCormick Group, being 
older than the Reynoldsburg Coal Member and 
younger than the Gentry Coal Member. This 
particular coal has a number of spores not found 
in other Illinois coals. It seems likely that this 
coal is of limited extent and probably was 
deposited in a small, discontinous basin, as 
along an upper delta plain or abandoned stream 
channel. The overlying Rock Island (No. 1) 
Coal is likewise discontinuous and is thought to 
have been deposited in an estuary (Moody, 
unpublished thesis 1959; Wanless, 1975). 



Stop 4. Allied Stone Company Quarry. 

NW SW SE Sec. 14, T. 17 N., R. 2 W., 
Milan 7.5' Quadrangle. 

Devonian, Pennsylvanian, and Pleistocene 
strata are exposed in this quarry (Fig. 14). This 
stop affords a better opportunity to study the 
lower part of the Wapsipinicon Limestone 
(Middle Devonian) than Stop 1. As noted previ- 
ously, the Wapsipinicon contains only scattered 
fossils. The Solon Limestone Member is the only 
portion of the overlying Cedar Valley Limestone 
present here. 

Here the Allied Stone Company is quarrying 
Cedar Valley and Wapsipinicon limestones of 
Middle Devonian age (Fig. 14) and crushing the 
stone for road and general construction stone. 
As we enter we will pass the field office, scales 
for weighing trucks loaded with crushed stone, 
piles of crushed stone of various sizes, the 
crushing plant, and maintenance shops. The 
quarry itself is nearly 1.6 km long, 0.5 km wide, 
and up to to 12 m deep. Because this is an active 
quarry, and the plant fossils occur within the 
overburden, locations of fossil -bearing strata 
change from year to year. Over large areas of the 
quarry the Devonian limestone was immediately 
overlain by outwash as a result of erosion and 
deposition during the Pleistocene. Few occur- 
rences of Pennsylvanian strata can be observed. 

Probably the most striking feature of this 
quarry is the presence of a number of channels 
and caves that developed in Devonian strata. 



1979 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



21 




Loess and very thin glacial drift. 

Shale, black to dark gray, fissile to blocky 
with interbedded thin sandstones. Grades 
down to greenish gray sandy shales and light 
colored pyritic sandstones. 



Limestone, buff, fine to medium grained, 
numerous brachiopods. 



Limestone, very light gray, subl i thographic 
to lithographic, very brecciated, green clay 
and shale streaks, graded downward to 
greenish gray and light brownish gray 
limestone, hard and brittle, numerous wavy 
bands and irregular thin beds. 



Cave fillings common with fine to 
medium angular sand interbedded with 
greenish clay and shale. Fragments 
of plants and much pyrite is common. 



Limestone, very light gray to very light buff, 
dolomitic, lithographic to subl i thographic, 
styolitic, weathers to thin slabs, some brecciation 

Limestone, brownish to bluish gray and dark 
colored, fine to coarsely crystalline with 
thin dark shale laminae, brecciated with 
shale and limestone matrix. 



LTJ\ 



Dolomite, brownish-gray, fine grained 



Figure 14. Stop 4. Allied Stone Company quarry, stratigraphic section. 
SE'/4, Sec. 14, T. 17 N., R. 2 W, Rock Island County, Illinois. 



22 



Guidebooklet: Illinois State Museum 



No. 4 



These features must have formed after the end 
of Mississippian deposition because they contain 
no sediments of Mississippian age. On the other 
hand, they must have existed before widespread 
Pennsylvanian sedimentation began in this area, 
for the sediments filling them are among some 
of the oldest-known Pennsylvanian strata in the 
state. These Pennsylvanian rocks occur strati- 
graphically lower than the Pennsylvanian layers 
at a neighboring borrow pit (Stop 2) and across 
the Rock River at Black Hawk State Park. 

The solution features strongly suggest that 
this area was topographically high— part of the 
Mississippi River Arch— when they were formed. 
Four conditions contribute to the development 
of such large numbers of sinkholes: First, sol- 
uble rock, preferably limestone, must occur at 
or near the surface and should be flat-lying, 
or nearly so. Second, and one of the most 
important factors, the limestone should be 
dense, abundantly jointed, and preferably 
thinly bedded. The limestone should not be 
porous; if it is, rainwater will be absorbed, 
moving through the whole body of the rock 
rather than being concentrated along joints and 
bedding planes. Third, major valleys must be 
entrenched below the uplands, serving as 
outlets toward which the ground water can 
move in the subsurface. Fourth, there must be 
ample rainfall. 

Sinkholes form in two ways. Collapse sinks, 
known as ponors, are caused by the collapse of 
the roofs of caves that lie near the surface. 
Caves were initially formed above the zone of 
saturation by ground water percolating down- 
ward (such water is called vadose water). Roof 
collapse followed uplift of the area and subse- 
quent entrenchment of major drainage channels. 



Collapse sinks are generally deep and steep 
walled. A second type of sinkhole is called a 
doline, in which solution takes place along 
joints beneath the soil mantle so that the 
surface is gradually lowered without collapsing 
the rock mass. They do not require the presence 
of large subterranean cavities, most of them 
being shallow, saucer-shaped depressions. Their 
depth is controlled by the depth of the water 
table at the time of their formation. Both types 
of sinks are generally present in a sinkhole area, 
but dolines generally are more common.* 

The basal Pennsylvanian deposits in this 
quarry have been the major source of plant fos- 
sils representing Early Pennsylvanian "upland" 
floras in Illinois. Although now largely removed 
by quarrying operations, some fossil-bearing 
channel deposits can be seen and plant fossils 
can be collected. 

The plant fossils occur in the shale and 
mudstone which fill elongate depressions 
(channels) eroded in the Middle Devonian Cedar 
Valley Limestone. The channels are 4 to 6 m 
deep and 8 to 20 m wide. One channel can be 
seen in cross section at the top of the north wall 
at about mid-quarry. As of late 1978 they can 
best be seen at the west end of the quarry where 
several channels are visible in both cross section 
and horizontally (see diagram, Fig. 8). During 
the autumn of 1978 the best plant fossil col- 
lecting was in the channel nearest the large gravel 
pile (arrow on Fig. 7). The only Pennsylvanian 
animal fossils yet discovered here, a scorpion 
(Leary, in press) and a single fish scale, were 
found at this site in October, 1977. 



♦Modified from the I.S.G.S. Milan Guidebook, 1974. 



CARBONIFEROUS DEPOSITION IN THE ILLINOIS BASIN 



23 



The following section on Mississippian de- 
position is taken from Illinois State Geological 
Survey Report of Investigations 216: Classifi- 
cation of Geneuievian and Chesterian . . . Rocks 
of Illinois (1963) by D.H. Swann, pp. 11-16. 
The section on Pennsylvanian deposition is 
taken from Illinois State Geological Survey 
publications. 



MISSISSIPPIAN DEPOSITION 

During the Mississippian Period, the Illinois 
Basin was a slowly subsiding region with a 
vague north-south structural axis. It was 
flanked by structurally neutral regions to the 
east and west, corresponding to the present 
Cincinnati and Ozark arches. These neighboring 
elements contributed insignificant amounts of 
sediment to the basin. Instead, the basin was 
filled by locally precipitated carbonate and by 
mud and sand eroded from highland areas far 
to the northeast in the eastern part of the 
Canadian Shield and perhaps the northeastward 
extension of the Appalachians. This sediment 
was brought to the Illinois region by a major 
river system, which it will be convenient to call 
the Michigan River (Fig. 15) because it crossed 
the present state of Michigan from north to 
south or northeast to southwest .... 

The Michigan River delivered much sediment 
to the Illinois region during early Mississippian 
time. However, an advance of the sea midway 
in the Mississippian Period prevented sand and 
mud from reaching the area during deposition 
of the St. Louis Limestone. Genevievian time 
began with the lowering of sea level and the 
alternating deposition of shallow-water carbon- 
ate and clastic units in a pattern that persisted 
throughout the rest of the Mississippian. About 
a fourth of the fill of the basin during the late 
Mississippian was carbonate, another fourth 
was sand, and the remainder was mud carried 
down by the Michigan River. 

Thickness, facies, and crossbedding ... in- 
dicate the existence of a regional slope to the 
southwest, perpendicular to the prevailing north 
65° west trend of the shorelines. The Illinois 
Basin, although developing structurally during 
this time, was not an embayment of the interior 
sea. Indeed, the mouth of the Michigan River 



generally extended out into the sea as a bird-foot 
delta, and the shoreline across the basin area 
may have been convex more often than concave. 

. . . The shoreline was not static. Its position 
oscillated through a range of perhaps 600 to 
1,000 or more miles (970-1,600 km). At times 
it was so far south that land conditions existed 
throughout the present area of the Illinois 
Basin. At other times it was so far north that 
there is no suggestion of near-shore environment 
in the sediments still preserved. This migration 
of the shoreline and of the accompanying sedi- 
mentation belts determined the composition 
and position of Genevievian and Chesterian 
rock bodies. 

Lateral shifts in the course of the Michigan 
River also influenced the placement of the rock 
bodies. At times the river brought its load 
of sediment to the eastern edge of the basin, 
at times to the center, and at times to the west- 
ern edge. This lateral shifting occurred within a 
range of about 200 miles (320 km). The Cincin- 
nati and Ozark areas did not themselves provide 
sediments, but, rather, the Michigan River 
tended to avoid those relatively positive areas 
in favor of the down-warped basin axis. 

Sedimentation belts during this time were 
not symmetrical with respect to the mouth of 
the Michigan River. They were distorted by the 
position of the river relative to the Ozark and 
Cincinnati shoal areas, but of greater importance 
was sea current or drift to the northwest. This 
carried off most of the mud contributed by the 
river, narrowing the shale belt east of the river 
mouth and broadening it west of the mouth. 
Facies and isopach maps of individual units 
show several times as much shale west of the 
locus of sand deposition as east of it. The facies 
maps of the entire Chesterian . . . show max- 
imum sandstone deposition in a northeast- 
southwest belt that bisects the basin. The total 
thickness of limestone is greatest along the 
southern border of the basin and is relatively 
constant along that entire border. The pro- 
portion of limestone, however, is much higher 
at the eastern end than along the rest of the 
southern border, because little mud was carried 
southeastward against the prevailing sea current. 
Instead, the mud was carried to the northwest 
and the highest proportion of shale is found in 
the northwestern part of the basin. 



24 



Guidebooklet: Illinois State Museum 



No. 4 



\ WISCONSIN 
ILLINOIS 




O 



OZARK 
SHOAL 



y 



J 



^> 



^ 
A 



\) 



i 



j 



~d- 



i 



W 



J 



( 

-J 



KENTUCKY 



50 Miles 



CINCINNATI 
SHOAL 



\ 



Figure 15. Paleogeography at an intermediate stage during Chesterian (Mississippian) sedimentation. 
From Swann, 1963. 



1979 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



25 



Genevievian and Chesterian seas generally 
extended from the Illinois Basin eastward across 
the Cincinnati Shoal area and the Appalachian 
Basin. Little terrigenous sediment reached the 
Cincinnati Shoal area from either the west or 
the east, and the section consists of thin lime- 
stone units representing all or most of the 
major cycles. The proportion of inorganically 
precipitated limestone is relatively high and the 
waters over the shoal area were commonly 
hypersaline .... Erosion of the shoal area at 
times is indicated by the presence of conodonts 
erorded from the St. Louis Limestone and 
redeposited in the lower part of the Gasper 
Limestone at the southeast corner of the 
Illinois Basin .... 

The shoal area included regions somewhat 
east of the present Cincinnati axis and extended 
from Ohio, and probably southeastern Indiana, 
through central and east-central Kentucky and 
Tennessee into Alabama .... 

Toward the west, the seaway was commonly 
continuous between the Illinois Basin and 
Central Iowa, although only the record of 
Genevievian and earliest Chesterian is still 
preserved. The seas generally extended from the 
Illinois and Black Warrior regions into the 
Arkansas Valley region, and the presence of 
Chesterian outliers high in the Ozarks indicates 
that at times the Ozark area was covered. Al- 
though the sea was continuous into the Ouachita 
region, detailed correlation of the Illinois sedi- 
ments with the geosynclinal deposits of this 
area is difficult. 



Pennsylvanian Deposition 

At the close of the Mississippian Period, 
about 310 million years ago, the Mississippian 
sea withdrew from the Midcontinent region. A 
long interval of erosion took place early in 
Pennsylvanian time and removed hundreds of 
feet of the pre-Pennsylvanian strata, completely 
stripping them away and cutting into older rocks 
over large areas of the Midwest. An ancient 
river system cut deep channels into the bedrock 
surface. Erosion was interrupted by the invasion 
of the Morrowan (early Pennsylvanian) sea. 

Depositional conditions in the Illinois Basin 
during the Pennsylvanian Period were somewhat 



similar to those that existed during Chesterian 
(late Mississippian) time. A river system flowed 
south westward across a swampy lowland, 
carrying mud and sand from highlands in the 
northeast. A great delta was built out into the 
shallow sea. As the lowland stood only a few 
feet above sea level, only slight changes in rela- 
tive sea level caused great shifts in the position 
of the shoreline. 

Throughout Pennsylvanian time the Illinois 
Basin continued to subside while the delta front 
shifted owing to worldwide sea level changes, 
intermittent subsidence of the basin, and varia- 
tions in the amounts of sediment carried seaward 
from the land. These alternations between 
marine and nonmarine conditions were more 
frequent than those during pre-Pennsylvanian 
time, and they produced striking lithologic 
variations in the Pennsylvanian rocks. 

Conditions at various places on the shallow 
sea floor favored the deposition of sandstone, 
limestone, or shale. Sandstone was deposited 
near the mouths of distributary channels. These 
sands were reworked by waves and spread as 
thin sheets near the shore. The shales were 
deposited in quiet-water areas— in delta bays 
between distributaries, in lagoons behind barrier 
bars, and in deeper water beyond the near-shore 
zone of sand deposition. Most sediments now 
recognized as limestones, which are formed from 
the accumulation of limey parts of plants and 
animals, were laid down in areas where only 
minor amounts of sand and mud were being de- 
posited. Therefore, the areas of sandstone, shale, 
and limestone deposition continually changed 
as the position of the shoreline changed and as 
the delta distributaries extended seaward or 
shifted their positions laterally along the shore. 

Nonmarine sandstones, shales, and lime- 
stones were deposited on the deltaic lowland 
bordering the sea. The nonmarine sandstones 
were deposited in distributary channels, in 
river channels, and on the broad floodplains of 
the rivers. Some sand bodies, 100 or more feet 
thick (30 m), were deposited in channels that cut 
through many of the underlying rock units. The 
shales were deposited mainly on floodplains. 
Freshwater limestones and some shales were de- 
posited locally in freshwater lakes and swamps. 
The coals were formed by the accumulation of 
plant material, usually where it grew, beneath 



26 



Guidebooklet: Illinois State Museum 



No. 4 



the quiet waters of extensive swamps that pre- 
vailed for long intervals on the emergent delta 
lowland. Lush forest vegetation, which thrived 
in the warm, moist Pennsylvanian climate, 
covered the region. The origin of the under- 
clays beneath the coals is not precisely known, 
but they were probably deposited in the swamps 
as slackwater muds before the formation of the 



coals. Many underclays contain plant roots 
and rootlets that appear to be in their original 
places. The formation of coal marked the end 
of the nonmarine portion of the depositional 
cycle, for resubmergence of the borderlands 
by the sea interrupted nonmarine deposition, 
and marine sediments were then laid down 
over the coal. 



REFERENCES CITED 



Andrews, H.N. 

1875 Description 
Ohio). Rept. 
p. 413-426. 



of fossil plants (Rushville, 
of Geol. Surv. of Ohio. 2(2), 



Arber, E.A. 

1904 Culm Measures of northwest Devon and the 
age of the coal beds. Phil. Trans. Royal Soc. 
London, B. 197, p. 307. 

Arnold, C.A. 

1934 A preliminary study of the fossil flora of 
the Michigan Basin. Museum of Paleon- 
tology, University of Michigan, 4(11), 
p. 177-204. 

Bell, W.A. 

1940 The Pictou coalfield, Nova Scotia. Geol. 
Surv. Canada, Mem. 225, 160 p. 

Boureau, Edward (Editor) 

1964 Tome 3: Sphenophyta, Noeggerathiophyta. 
Traite de Paleobotonique, Paris. 

Bristol, H.M. and R.H. Howard 

1971 Paleogeologic map of the sub-Pennsylvanian 
Chesterian (Upper Mississippian) surface 
in the Illinois Basin. 111. State Geol. Surv. 
Cir. 458, 16 p. 

Canwright, J.E. 

1959 Fossil Plants of Indiana. Indiana Geol. Surv. 
Report of Progress, 14, 45 p. 

Cross, A.T. 

1962 The Rushville (Ohio) Pennsylvanian Flora 
(abs.). Amer. Jour. Botany 49(6), Pt. 2, 
p. 669. 

Dawson, J.W. 

1871 The fossil plants of the Devonian and Upper 
Silurian formations of Canada. Canada 
Geol. Surv., 92 p., 20 pi. 

Edmund, R.W., and R.C. Anderson 

1967 The Mississippi River Arch, 31st Annual Tri- 
State field conference, Augustana College, 
Rock Island, 111., 64 p. 

Feistmantel, K. 

1879 Eine naue Pflanzengattung aus bohmischen 
Steinkohlen-schichten. Sitzber. Ges. Wiss., 
p. 298-303. 

Florin, Rudolf 

1933 Zur Kenntnis der palaozoischen Pflanzen- 
gattung Lesleya Lesquereux und Megalop- 



teris Dawson. Arkiv for Botanik, (K. 
Svenska Vetensk. Akad.), 25A(19), 23 p. 

Havlena, V. 

1961 Die flonznahe und flozfremde Flora des 
oberschlesischen Namurs A und B. Palaeon- 
tographica Abt. B, Vol. 108, p. 22-38. 

1970 Ecology and mode of deposition of the 
macroflora in cyclothems of the Ostrava 
Formation (Namurian A). Vestnik Ustred- 
niho ustavu geologickeno, 45, p. 291-294. 

1971 Die zeitgleichen Floren des Europaischen 
Oberkarbons und die mesophile Flora des 
Ostrau-Karwiner Steinkohlenreviers. Rev. 
Palaeobot. Palynol., 12(4), p. 245-270. 

Hirmer, M. 

1940 Die Karbon-Flora des Saargebietes, 3. Fili- 
cales und Vermandte, 1. Noeggerathiineae. 
Paleontographica, 9, p. 3-44. 

1941 Noeggerathia, neuentdeckte Formen und 
ihre Stellung im System der Fame, Biol. 
Generalis, 15(1, 2), p. 134-171. 

Horberg, C.L. 

1950 Bedrock topography of Illinois. 111. State 
Geol. Surv. Bull. 73, 111 p. 

Leary, R.L. 

1973 Lacoea, A Lower Pennsylvanian Fructifi- 
cation of Noeggerathialian Affinity from 
Brown County, Illinois. Review of Palaeo- 
bot. Palynol. 15(1), p. 43-50. 

1974a Stratigraphy and floral characteristics of the 
basal Pennsylvanian strata in west-central 
Illinois. Compte Rendu, 7th International 
Congress of Stratigraphy and Geology of 
the Carboniferous, Krefeld, Germany, 
1971, Vol. 3, p. 341-350. 

1974b Two early Pennsylvanian floras of western 
Illinois. Illinois State Academy of Science 
Transactions, vol. 67, no. 4, p. 430-440, 
1974. 

1976 Early Pennsylvanian paleogeography of an 
upland area, western Illinois. Bulletin de la 
Societe beige de Geologie, vol. 84, no. 1, 
p. 19-31, 1975 (1976). 

(In press) Namurian paleogeography of the western 
margin of the Eastern Interior (Illinois) 
Basin. Compte Rendu, Eighth International 



28 



Guidebooklet: Illinois State Museum 



No. 4 



Congress on Stratigraphy and Geology of 
the Carboniferous, Moscow, USSR, 1975. 

(In press) Labriscorpio alliedensis, gen. et sp. nov: 
a new Carboniferous scorpion from Rock Is- 
land County, Illinois. Jour, of Paleontology. 

Leary, R.L., and H.W. Pfefferkorn 

1977 An Early Pennsylvanian flora with Mega- 
lopteris and Noeggerathiales from west- 
central Illinois. 111. State Geol. Surv., Cir. 
500, 77 p. 

Lesquereux, Leo 

1880 Description of the Coal Flora of the 
Carboniferous Formation in Pennsylvania 
and throughout the U.S. Second Geol. 
Surv. Penn. Rept. Progress, Harrisburg. 
Vol. 1 & 2. 

McGinnis, L.D., and P.C. Heigold 

1974 A seismic refraction survey of the Meredosia 
Channel area of northwestern Illinois. 111. 
State Geol. Surv. Cir. 488, 19 p. 

Moody, D.M. 

1959 The geology of the Rock Island (No. 1) 
Coal in northwestern Illinois; Rock Island, 
Henry, Mercer and Warren counties. Un- 
published M.S. thesis, Univ. of Illinois. 38 p. 

Newberry, J.S. 

1873 Descriptions of fossil plants. Palaeontology, 
vol. I, pt. II Geological Surv. of Ohio, 
p. 359-385. 

Noe, A.C. 

1925 Pennsylvanian Floras of Illinois. Illinois 
State Geol. Surv. Bull. 52, 113 p. 

Peppers, R.A., and H.W. Pfefferkorn 

1970 A comparison of the floras of the Colchester 
(No. 2) Coal of the Francis Creek Shale. In 
Depositional environments in parts of the 
Carbondale Formation, western and north- 
ern Illinois, 111. State Geol. Surv. Guidebook 
series no. 8, p. 61-74. 



Purkynova, Eva 

1970 Die Unternamurflora des Beckens von 
Horni Slezsko (CSSR). (The lower Namur- 
ian flora of the Upper Silesian Basin, Czech- 
oslovakia.) Palaontologische Abhundlunge 
Palaobotanik, Abt. B, 3(2), p. 129-268. 



Read, C.B. 
1946 



A Pennsylvanian florule from the Forkston 
Coal in the Dutch Mountain Outlier 
Northeastern Penn. USGS Prof. Paper 
210B, p. 17-27. 



Reinertsen, D.L., D.J. Berggren, R.C. Anderson, 
and M. Killey 
1974 Milan area, guide leaflet, 111. State Geol. 
Surv. guide leaflet 1974-C, 1975-B, 19 p. 



Remy, W., and R. Remy 

1977 Die Floren des Erdaltertums. 
Gluckauf Gmbh. Essen. 468 p. 



Verlag 



Searight, T.K., and W.H. Smith 

1969 Strippable coal reserves of Illinois, pt. 
5B— Mercer, Rock Island, Warren, and 
parts of Henderson and Henry counties. 
111. State Geol. Surv. Cir. 439, 22 p. 

Smith, M.H. 

1941 Structure contour map of the pre-Pennsyl- 
vanian surface of Illinois. 111. State Acad. 
Sci. Trans. 34(2), p. 160-163. 

Stockmans, F. and F.F. Mathieu 

1957 La Flore Paleozoique du Bassin Houilles 
de Kaiping. Pub. 32 Assoc, pour L'etude 
de la Paleoutologie et de la Stratigraphic 
Houilles Bruxelles, 89 p. 

Stockmans, F. and Y. Williere 

1962 Heninia, Discinites et Tongshania. Inst. R. 
Sci. Nat. Belg., 38(3), p. 1-8. 

Stopes, M.C. 

1914 The "Fern Ledges" Carboniferous flora of 
St. John, New Brunswick, Geol. Survey 
Canada Mem. 41, 167 p. 



Phillips, T.L., H.W. Pfefferkorn, and R.A. Peppers 

1973 Development of paleobotany in the Illinois 
Basin. 111. State Geol. Surv. Cir. 480, 86 p. 

Phillips, T.L., A.B. Kunz, and D.J. Mickish 

1974 Paleobotany of petrified peat (coal balls) 
from the Herrin (No. 6) Coal Member of 
the Illinois Basin. Geol. Soc. Amer., Abst. 
Prog., vol. 6, no. 7, p. 912. 



Sustra, V. 
1928 



Stratigraphie des Ostrau-Karviner Stein- 
kohlenreviers im lichte der Palaontologie. 
Der Kohlenbergbau des Ostrau-Karviner 
Steinkohlenreviers, I, p. 385-484, 75 pi. 
Ostrava. 



Swann, D.H. 

1963 Classification of Genevievian and Chesterian 



1979 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



29 



(Late Mississippian) rocks of Illinois. 111. State 
Geol. Surv. Rept. Investigations 216, 91 p. 

Tenchov, Y.G. 

1976 Composition peculiarities of the Carbonifer- 
ous flora of the Svoge Basin, west Bulgaria. 
Geologica Balcaniea, vol. 6, no. 1, p. 3-11. 

Wanless, H.R. 

1975 Distribution of Pennsylvanian coal in the 
United States. Paleotectonic investigations 
of the Pennsylvanian System in the United 
States, part II, p. 33-47. 

White, David 

1908 Report on Field Work Done in 1907. 111. 
State Geol. Surv. Bull. 8, p. 268-272. 



1913 Fossil flora of West Virginia. West Virginia 
Geol. Surv. 5(A), pt. II, p. 390-453. 

1931 Climatic Implications of Penn. Flora. Papers 
presented at the Quarter Centennial Cele- 
bration of the ISGS. 111. State Geol. Surv. 
Bull. 60, p. 271-281. 

Wood, J.M. 

1963 The Stanley Cemetery Flora (Early Pennsyl- 
vanian) of Green County, Indiana. Ind. 
Geol. Surv. Bull. 29, 73 p. 



Worthen, A.H. 

1873 Geological 
p. 217-234 



Survey of Illinois, Vol. 5, 



ADDITIONAL REFERENCES 1985 



Leary, R.L. 

1979 Early Pennsylvanian upland compression 
flora, Rock Island County, Illinois: Illi- 
nois State Museum Guidebooklet Series 
4. 42 p. 

Leary, R.L. 

1980 Labhscurpio a/liedensis, sen. et sp. nov: 
a new Carboniferous scorpion from Rock 
Island County, Illinois: Journal of Paleon- 
tology, v. 54, no. 6, p. 1255-1257. 

Leary, R.L. 

1981 Early Pennsylvanian geology and paleo- 
botany of the Rock Island County, 
Illinois, area. Part 1: Geology: Illinois 
State Museum Reports of Investigation 
37, 88 p. 

Phillips, T.L., and R.A. Peppers 

1984 Changing patterns of Pennsylvanian coal- 
swamp vegetation and implications of 
climatic control on coal occurrence: 
International Journal of Coal Geology, 
v. 3, p. 205-255. 



30 



PLATES 



Plate 1 




Fig. 1. Megalopteris dawsoni x V2 



Fig. 2. Lesleya cheimarosa x V2 



31 



Plate 2 




Fig. 1. Megalopteris ovata xl 



Fig. 2. Megalopteris ovata ? xl 



32 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



Plate 3 




Fig. 1. Mesocalamites sp. xl 

Fig. 2. Gulpenia sp. x2 

Fig. 3. Samaropsis new berry i xl 

Fig. 4. Sphenophyllum tennerrimum xl 



Fig. 5. Lacoea seriata xl 
Fig. 6. Sphenopteris sp. xl 
Fig. 7. Mariopteris sp. xl 



Guidebooklet: Illinois State Museum 



33 



Plate 4 




Fig. 1. Palaeopteridium reussii x2 
Fig. 2. Sphenopteris sp. xl 
Fig. 3. Sphenopteris sp. xl 



Fig. 4. Sphenophyllum cuneifolium ? x2 
Fig. 5. Samaropsis newberryi xl 



34 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



Plate 5 







Fig. 1. Lepidostrobus sp. x% 
Fig. 2. Lepidodendron sp. xl 
Fig. 3. Lepidocarpon sp. x2 
Fig. 4. Lepidocarpon sp. x2 



Fig. 5. Sphenophy Hum sp. xl 
Fig. 6. Triletes auritus tetrad x5 
Fig. 7. Triletes auritus tetrad xl5 



NOTES 35 



36 



SKETCHES OF 

LOWER PENNSYLVANIAN PLANT FOSSILS 

OF WESTERN ILLINOIS 




X 1 /2"1 



Lesleya 




Megalopteris 



Guidebooklet: Illinois State Museum 



37 





38 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 



■'<{ 




i\ 



^3 



^ x1 



Cordaianthus 







base 



Cordaites 
x1 - x.5 



ttttut 




*W 




^ 



Calamostachys 



x2 



x1 



Mesocalamites 
( Catamites) 
x1 -x.25 




tl 



'"•70 



Wtt 



I 



I 



<. 



^VWW 



x1 



Lacoea 






Guidebooklet: Dlinois State Museum 



39 





Cordaicarpus 





Whittlesey a 





Rhodeop teridium 




Samaropsis 





Telangium 

W0 & 



x1 



Aulacotheca 



x1 -2 




Gulpenia 




"seeds" 





x1 






? Aphlebia 



M 



H 




? Rhodeopteridium 



40 



Leary: Pennsylvanian Flora, Rock Island County, Illinois 









Lepidodendron 




x5 




Lepidostrobus 



A 





Lepidocarpon 



Lepidophyllum 





Sphenopteris 





Aleth op ten's 



Neurop ten's ? 




< 




m 



x2 



Sphenophyllum 





x2 




? Megasporangium 



Guidebooklet: Illinois State Museum 



41 







x1 






Cyclopteris 



Alloiopteris 



V 



mm 



mmr, 

mi 




xl 



42 



ACKNOWLEDGEMENTS 



Many people have helped in the preparation of this field trip. The organizers 
of the International Carboniferous Congress have cooperated with the author in 
planning and publicizing the trip. Dr. Tom L. Phillips, as chairman in charge of 
one-day field trips, has helped with arrangements. Roy Nelson, supervisor of the 
Allied Stone Company quarry, Richard DeSchepper, supervisor of the Collinson 
Stone Company quarry, and Howard Dixon, geologist with Moline Consumers, 
made access to the quarries possible. Special thanks go to them for their continued 
cooperation without which research at these sites would be impossible. 

Parts of the text and some of the illustrations used in this guide were taken 
from guidebooks of the Illinois State Geological Survey and the guidebook 
for the 31st Tri-State Field Conference (1967). The author appreciates permission 
to use this information. The author also gratefully acknowledges the assistance, 
comments, and suggestions given by staff members of the Illinois State Geological 
Survey and faculty at the University of Illinois and at Augustana College.