JRJ - Chesterian Plants
James R. Jennings


Aerial Distribution of Late Mississippian (Chesterian)

Plant Megafossils in the Eastern Interior Basin

by J. R. Jennings, 2015

The Eastern Interior Basin occupies large parts of Illinois, Indiana, and Kentucky with a smaller area along the southwestern margin extending into Missouri. Late Mississippian strata are referred to the Chester Series and crop out in an arcuate configuration around the southern margin of the basin, as shown on the map. The grid lines on the map define areas that are 7 1/2 minutes (0.125 degree) of latitude or longitude on a side, thus corresponding to USGS 1:24,000 Quadrangle maps. Displaying the unmodified latitude and longitude coordinates on a flat surface results in a simple cylindrical projection.

Extensive large-scale geologic mapping, done during the past century, is the primary source of information used for delineating the limits of Late Mississippian outcrop. If Late Mississippian strata are believed to crop out anywhere within the boundary of a 7 1/2 minute tract, the outline of the entire quadrangle is shown on the map. While a few quadrangles lie completely within the area of Late Mississippian outcrop, most have areas where older or younger strata crop out as well. Indeed, in some quadrangles surficial strata of the Late Mississippian consist of very small outliers or fault slices, while the surface of most of the quadrangle is occupied by other strata. In some cases the stratigraphic identification of strata in small fault blocks or slump structures is uncertain, and these are not generally shown. Proper assignment of strata can be particularly difficult wherever earliest Chesterian clastics are not present and the age-equivalent strata consist of carbonate rock that is essentially similar to that of the underlying Valmeyeran, or where Chesterian sandstone directly underlies Pennsylvanian sandstone. It is highly probable that some small areas of Late Mississippian outcrop are not represented, but their potential as a source of plant megafossils is distinctly limited.

Few, if any, individual quadrangles exhibit a complete Chesterian sequence in outcrop, and the depth of sub-Pennsylvanian erosion generally increases to the north. Thus, near the northern boundary of the outcrop, only the basal Chesterian beds are present and the potential to contain plant megafossils correspondingly less than for thicker sequences. In addition, the frequency and quality of exposures are quite different from one place to another. Indeed, one of the persistent problems facing an investigator of Chesterian fossil plants is that the best material is typically found in strata that have very low resistence to erosion and are therefore generally covered. This tends to give a false impression of rarity of fossil plant material.

Nevertheless, more than 70 confirmed sites with plant megafossils are presently known, though it seems virtually certain that additional sites exist. The maps show the aerial distribution of these plant megafossil occrrences, beginning with a composite overview of occurrences in all Chesterian formations, and proceeding to the occurrences in individual formations (or small bundles). No attempt has been made to distinguish on the maps between different types of occurrences. Thus, all of the allochthonous and all of the autochthonous occurrences are represented in the same fashion. Likewise, fossil plants that are preserved in shale, sandstone, coal, or limestone are all treated identically. Finally, fossils preserved as impressions, petrifactions, casts, or molds are also treated the identically. The crosshatching indicates a quadrangle with at least one occurrence of plant megafossils, but if multiple site are known, the representation is still the same. Click on any of the maps to see a larger (pdf) version.

Chester Outcrop

Composite of the Entire Chester

Aux Vases Fm. - Yankeetown Fm.

Bethel - Mooretown Fm.

Cypress Fm.

Big Clifty Fm.

Hardinsburg Fm.

Tar Springs Fm.

Waltersburg Fm.

Palestine Fm.

Clore Fm. (mostly Tygett Memb.)

Degonia Fm.

KInkaid Fm. (Cave Hill Memb.)

Grove Church Fm.

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