By Brian Scheidt
This virtual field trip will take the user through the geologic features of Rocky Branch Nature Conservation Area. This site is located in east-central Illinois just south of the Shelbyville terminal moraines. This area is located in the Central Lowland Province approximately equal distant from the Atlantic Ocean and the Gulf of Mexico in the center of the Illinois Basin. In this preserve, there is a stream that is eroding the glacial drift deep enough to expose Pennsylvanian bedrock. Part of a classic cyclothem can be seen here due to the downcutting of the stream. Finally one can see evidence of a fault and associated joints in the bedrock.
From the parking lot you can walk north and see a bluff and a stream eroding through it. The bluff exposes a good cross section of the area.
Rocky Branch is located in the SE 1/4 of section 30, and the SW 1/4 of section 29, T. 12 N.; R. 12 W. This area can be viewed on the Clarksville 7.5 minute and Casey 15 minute quadangles.
Geomorphology, Weathering and Depositional Features
Click here for a map of where the glaciers in the area advanced to (Willman and Frye, 1970)
Chick here for a map of the glacial deposits in Illinois (Willman and Frye, 1970)
The long term geomorphic record of this part of Illinois indicates that the upland till has been relatively stable for the past 200,000 years (IDNS 1991). During this time, the landscape has been modified by sheetwash deposition of a thin sand layer. The soil that devolops on the till is used near Rocky Branch for the farming of corn and soybeans.
As ground water percolates down through the till and permeable sandstone then it migrates horizontally along the upper contact of a shale which acts as an aquatard.
Subsequent freezing and thawing produce small pits or notches where the sand grains are individually removed by gravity.
This can be seen throughout the preserve but the notches are most predominant on the north facing slopes.
As you walk to the south to the west you can see youthful v-shaped valleys formed by erosion into the till.
As you continue on the path to the south you can see evidence that the stream was higher than it currently is. The proof of this from the outwash on the banks of the stream.
A person can go upstream to the west and see stream sorted pebbles, gravels and sands.
Near the same spot that you see the sorting you can see iron staning on the sandstone and shales near the bank of the stream.
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Click here for a stratigraphic column of Rocky Branch
The stratigraphy of Rocky Branch includes Quaternary Vandalia Till Member of the Glasford Formation which overlies the Pennsylvanian Mattoon Formation in the Mcleansboro Group. Click Here for General Lithologic sequence of this area. There is an unconformity which represents approximately 290,000,000 years. This unconformity is located at the contact between the Pennsylvanian rocks and the overlying less well underrated Illinoian till.
For general statigraphic information about Illinois consult Willman and others (1975), Willman and Frye (1970), or Jorstad Illinois Stratigraphic stack.
The Glasford Formation is composed of glacial till disconformably overlying the Paleozoic Strata. The Vandalia Till Member is divided into three sub units: Vandalia Till, Vandalia Sand and Fractured Vandalia Till. The Vandalia till was present at Rocky Branch and consists of a hard, silty, sandy clay that is relatively massive with scattered sand lenses and fractures. The Vandalia Till Member consists of brown clay-rich till which contains some small pebbles of various composition. Overlying this unit is modern soil with a gradational contact. The Vandalia Sand consists of discontinuous, narrow channel deposits within the Vandalia Till. These deposits are typically silty, fine to medium sand. The fractured Vandalia Till is continuous across the upland at the top of the Vandalia Till Member. Fractures, including sand-filled fractures, and sand lenses a few common within the unit. The frequency and sand lenses typically decreases with depth. The sand and fractured facies above have not been reconized at Rocky Branch but have be described by IDNS at nearby Martinsville. A change in slope can be seen at the contact between the till and the very thick sandstone.
Near the sandstone bluff which were seen in the overview you can also see an exposure of a discontinuous coal bed which is probably the Cohn coal
The undifferentiated Missourian and Virgillian stages of the Pennsylvanian System are represented here by the Mattoon Formation. Here, the Mattoon Formation consists of an unnamed sandstones, shales, clays and siltstones and correlates with the upper part of the Sturgis Formation of western Kentucky; the lower 150 feet of the formation extends into Indiana, (Bandy, 1981). At Rocky Branch, the light brown sandstone is fine grained, and is mica rich, contains cross bedding, and it forms steep bluffs. In some areas the sandstone includes a thin, discontinuous clay layer. There is a general coarsening as up section with grain sizes reaching coarse grained sand at which point there are only patches of mica. The siltstone in this formation is gray and contains abundant amounts of mica; the shale is typically black and the bedding in the shale is thin to laminated. There is a thin, discontinuous layer of coal which is believed to be the Cohn Coal Member of the Mattoon Formation. The nomenclature of the coal is proposed by the position in the lithologic rock sequence, approxamently 15 meters above the Livingston Limestone (Odum and others 1961). In the black shale, fossil bivalves and a possible gastropod have been found (Jorstad, 1991).
The conglomerate which is exposed by the stream contains rounded pebbles, is poorly indurated and contains a large amount of sand matrix.
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The Pennsylvanian rock sequence at Rocky Branch provides great example of a classic Illinois cyclothem. The term cyclothem is used to designate a series of beds deposited during a single sedimentary cycle of the type that prevailed during the Pennsylvanian period (Wanless and Weller, 1932). In the Midwest, the cyclothems are the result of non-marine and marine deposition associated with the building of an ancient river such as the Michigan River (Jorstad, 1991). In Illinois facies migration during delta genesis was extensive as shown here.
Here is a diagram of a classic cyclothem (Willman and Payne, 1942)
The fluvial depositional facies can also help explain the abundance of micas found in some of the sands. The platy micas traveled from great distances due to their flat nature. At this site the conglomerate, sandstone, shale, clay, siltstone, and coal lithofacies characteristic of a cyclothem are exposed. The clastic portion of the lithologic sequence grades upward from conglomerate on top of a scour surface through the sandy shale and to the clays and coal. This clearly illistrates an energy reduction through time as the stream built it's delta seaward.
There is also a great example of cross bedding is the sandstone this can be seen throughout the area these pictures were taken from the southern portion of Rocky Branch near the stream.
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Click here for regional structure (Buschbach and Kolata 1991)
The Illinois Basin is a spoon-shaped depositional and structural depression occupying the southern two-thirds of the Illinois and adjoining parts of Indiana, Kentucky, and Missouri. It is bounded by a series of uplifts, arches, and domes, including the Ozark uplift to the southwest, the Cincinnati Arch to the southeast, the Kankakee and Wisconsin Arches to the north, the Mississippi River Arch to the northwest, and the Pascola Arch to the south (IDNS, 1991).
The LaSalle Anticlinal Belt is the most prominent structural zone in the state and underlies Rocky Branch. It is constituted of a complex series of minor north-northwesterly trending en echelon flexures that bound the central, deepest part of the Illinois Basin (Jorstad, 1991). The deformation of the LaSalle Anticlinal belt began at least as early as Ordovician time and continued to develop slowly until the close of the Mississippian (Clegg, 1965). The faults exposed at Rocky Branch maybe related to the genesis of the LaSalle Anticlinal Belt and the Marshall-Sidell Syncline or the normal faults of the Wabash Valley Fault System which in turn in may be related to the New Madrid Fault System (Jorstad,1991).
Rocky Branch is located near the axis of the Marshall-Sidell Syncline on the east flank of the LaSalle Anticlinal Belt. Older rock units, specifically the Livingston Limestone Member of the Bond Formation exposed about 3 miles to the west also close the city of Marshall (Odum and others, 1961). There is also a fault with associated joints that can be observed in a tributary stream west of the parking area. At this location there are laterally adjacent sandstone and shale units exposeing a north dipping fault plane in the creek bed.
One set of joints has a trend of N10°E the other set of joints has an orientation of N80°E. There is no apparent correlation with the two sets of joints. Notice that the offset does not extend through the till. This is because the till was deposited after the faulting occurred. The fault must have been active before the Illinois glaciation had occurred. Because the exposures of the faults are small and they lack slickensildes, the dip slip motion can only be assumed and not proven. We can see some offset of the beds probably due to some faulting that occurred. This faulting could be seen in the stream bed as seen in the pictures as well as near the entrance to the cunservation area.
These pictures shows where the exposed shale shows both sets of joints which can be seen at Rocky Branch. These pictures were taken throughout the preserve along the trend of the joints in areas which the stream has eroded away.
There are also joint sets in the sandstone. These were found along the same trend of the joints in the shale however the spacing of the joints is greater. This may be due to the sandstone being a greater distance from the fault, lithologic differences, or a comboation of factors.
Bandy, William F.; 1981; Paleontology and Stratigraphy of the Mattoon Formation (Pennsylvanian) Coles County, Illinois, Department of Geology Indiana University p 23-27.
Buschbach and Kolata; 1991; Regional setting of the Illinois Basin, in M.W. Leighton, D.R. Kolata, D. F. Oltz, and J.J. Eidel, editors, Interior Cratonic Basins: American Association of Petroleum Geologists, Memoir 51, p. 29-55.
Clegg, Kenneth E.; 1965; "The LaSalle Anticlinal Belt and Adjacent Structures in East-Central Illinois" Transactions of the Illinois State Academy of Science, Vol. 58. p 16.
Illinois Department of Nuclear Safety; 1990; Alternative Site Investigation Studies Clark County, Illinois; vol 2 p2.1-3.3.
Frankie,W.T.; 1998; Guide to the Geology of the Lake Shelbyville Area, Shelby and Moultrie Counties, Illinois; Illinois State Geological Survey; p 6-10.
Jorstad, R.; 1991; 55th Annual Tri-State Geological Field Conference, Department of Geology Eastern Illinois University; p13-15.
Jorstad, R. B.; Illinois Stratigraphy; http://lincweb.com/jorstad/ILstrat.hqx
Odum, E., Thompson, M.L., Thut, H., Simon, J., Ekblaw, G., Evers, R. and Clegg, K; 1961; Charleston Area Geological Science Field Trip Guide Leaflet: Illinois State Geologic Survey; p10.
Wanless, H. R. and Weller, J. M.; 1932; Correlation and extent of Pennsylvanian cyclothems. Bull. Geol. Soc. Am., Vol 43. p1004-1005.
Willman, H.B. and others; 1975; Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261p.
Willman, H.B. and Frye, J.C.; 1970; Pleistocene Stratigraphy of Illinois: Illinois State Geological Survey Bulletin 94, 204p.
Willman, and Payne, 1942; Cyclothem units: Illinois State Geological Survey Bulletin 28, 162p.
For more information about the geology of this area and other Illinois locations contact the Illinois State Geological Survey
Web page by Brian Scheidt
Under the supervision of Dr. Robert Jorstad, Eastern Illinois University
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