Pennsylvanian Period

Geologic Map of Missouri, 1990, Missouri Department of Natural Resources, Division of Geology and Land Survey (now Geological Survey and Resource Assessment Division).

Pennsylvanian Period (320 to 286 million year old rocks)

Coal and the Pennsylvanian Period are synonymous. Missouri is not normally thought of as a coal producing state, but the coalfields of northern and southwestern Missouri are large enough to produce local supplies, though not nearly as extensive as the the coals of nearby Illinois. Unfortunately, (and like the coals of Illinois), Missouri coal is bituminous, and high in sulfur, making it much less desirable in these days of acid rain consciousness, and the industry has lain dormant for years.

The fourth, and final transgressive sea, which covered much of the state, although much more thickly in the relative lowlands of north and southwest Missouri was the Absaroka. The Absaroka sea came and went 5 times in the state, leaving as evidence the Morrowan, the Atokan, the Desmoinsian, the Missourian and the Virgilian series of rocks. Most of these rocks are shales, sandstones and clays, although thin limestones and coals are also common. Pennsylvanian strata are the dominant rock north of the Missouri River and in the Osage Plains region; they exist as hilltop outliers even over much of the Ozark uplift, often resting unconformably on Ordovician rocks.

Even though the Pennsylvanian deposits are quite extensive, they form usually thin to mediumly bedded layers of distinctive composition, called cyclothems. A cyclothem results when a sea transgresses and regresses very rapidly along a coastal area, and in a repeating pattern. Often, this pattern consists of a sandstone (beach); silty shale or siltstone (tidal); freshwater limestone (lagoon); underclay (terrestrial); coal (terrestrial swampy forest); shale (nearshore tidal); limestone (shallow marine); and black shale (deep marine). This sequence can then repeat itself as the sea first regresses from the land, and then transgresses again. According to Levin, in The Earth Through Time, at least 50 cyclothems are recognized within a vertical section only 750 meters thick.

Although one would think of the coal as being of great economic importance, the underclay probably has brought more actual dollars to the state. This underclay is often refractory grade, suitable for brick-making, and ceramics. The clay pits of Cheltenham in St. Louis furnished brick for a growing city in the 19th century, and the fireclay works in Audrain County and midstate have created great wealth in those regions.

Geologically, the Pennsylvanian was quite important to Missouri for another reason. During this period, the Ouachita Mountain range to our south was raised as South America crashed into the southern edge of the continent. What exact effect this had on Missouri is still being determined, but two geologists with the University of Missouri have developed an hypothesis that this collision had the effect of forcing hot basinal fluids generated from the mountain building though the lower layers of existing Missouri limestones, converting them to dolomites, and chemically releasing, then concentrating and precipitating galena in the older and deeper reef and vug-filled rocks of the state. (Dr. Kevin L. Shelton, personal comm., 2000). Since these galena deposits are some of the richest in the world, still producing after 300 years of mining, knowing just how they were formed can help find new deposits in similar geological settings.

The up and down warping of continental crust is rather subtle, not as dramatic as mountain building, so exact periods of regional uplift are hard to determine. Because of the Ouachita orogeny, some evidence exists that southern Missouri rose in response during this time. This evidence is in the form of faulting of existing rock.

When the Absarokan sea retreated at the end of the Pennsylvanian, the great days of Missouri sedimentary deposition came to a close. Most of our geology since then has been a matter of shaping what exists, not laying down new landscape.

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