Kansas City Group

Geological time period and characteristics

The Kansas City Group is part of the Missourian Series, approximately 307 to 303.7 Ma. This equates to the Kasimovian Stage of the ICS geologic timescale. The Missourian Series is the third of four stages of the Pennsylvanian period and is considered Late Pennsylvanian. The Pennsylvanian, also known as the Late Carboniferous, approximately covers the years 323.4 million years ago to 298.9 million years ago. At this time, the area that is now the Kansas City Group was periodically covered in low-lying seas, along the seashore, or in wet and swampy areas along the seashore. As the climate changed over hundreds of thousands to millions of years, the varying sea levels created a fairly predictable sequence of strata that became the modern-day limestone, shale, and occasional sandstone formations. These alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams, formed as a result of marine regressions and transgressions related to growth and decay of ice sheets and other climate phenomena, are known as cyclothems. As common for strata in the Carboniferous period, the Kansas City Group held commercially exploitable reserves of oil that have been tapped in both Missouri and Kansas; coal has been mined in some locations.

Upper Pennsylvanian cyclothems

The cyclothems in the Kansas City Group are fairly predictable, repeating layers of limestone-shale-limestone-shale created as the sea level rose and fell:

1. Outside Shale. At the lowest sea level, the area was a non-marine environment adjacent to the nearby sea. The area may have been low-lying and swampy, receiving outflows of detritus and, sometimes, sand from streams and downwash from adjacent land. The result is a gray to brown sandy shale with local coal and sandstone deposits. As the sea level begins to rise slightly, this turns to sandy shale with marine biota.
2. Middle Limestone. Sea levels begin to rise, typically due to melting of ice sheets. Because the melting process and resulting rise in sea level is typically quite fast-moving, this phase is shorter than the later regressive phase of the sea levels and thus creates a thinner, denser, and more uniform limestone layer. This phase begins when the sea level is deep enough that the layer was below the wave base, allowing calcium carbonate deposits to accumulate steadily, undisturbed by wave action. Invertebrate marine fossils are present as are preserved algae, the primary biological driver of the calcium carbonate deposits. Thin layers of shale between thicker section of limestone are often present, and may represent a layer of detritus from flooding or some other major outflow event or some other relatively brief change in conditions.
3. Core Shale. As the sea level continues to rise, the sea depth reaches a limit beyond which the algae can no longer create calcium carbonate. Accumulation of calcium carbonate, and thus limestone, ends. The deep-water conditions create a thin, gray layer of marine shale, typically 0.3 to 2 m (0.98 to 6.56 ft) thick, lacking sand and holding only fossils of organisms known to frequent deep water areas. These layers were created by very slow process of sedimentation in deep water under anoxic (very low oxygen) conditions. The deposits are rich in heavy metals and phosphate, which was created by the slow decay of immense blooms of plankton, which slowly settled to the sea floor.

1. Upper Limestone. As ice sheets slowly grow again, the sea levels slowly decline. As soon as sea depth in this area becomes shallow enough that algae can once again resume the production of calcium carbonate, the accumulation of limestone formations begins again. Because the decline in sea levels is far slower than the rise was, typically taking around three times as long, this layer of limestone is far thicker and also, typically, more varied than was the Middle Limestone. This layer can be 3–9 m (9.8–29.5 ft) in thickness. Again, preserved algae, marine invertebrates, and conodonts such as Ozarkodina, eel-like marine creatures lacking jaws but with numerous sharp teeth, are present. The lower part of this limestone typically consists of wavy-bedded limestone with a large number of fossils of many marine species. Thin layers of shale often separate the wavy-bedded limestone layers. The upper section of the Upper Limestone formation is often more varied from place to place, representing the disproportionate effect minor differences in topography can have in shallow waters, and in some cases reefs or shoals that accumulated atop the other layers. The limestone created under shallower seas reflects the greater agitation due to wave action and penetration of light expected at those shallower depths.
2. Outside Shale again. As sea levels continue to decrease, we finally reach the stage where shale accumulates in very shallow seas, along the seashore, and then in drier areas just above sea level, as in the beginning (Stage 1). Some areas were ancient stream or river deltas, with deposits of material weathered from rocks in present-day Oklahoma. In these areas we may see shale deposits interspersed with lenses of sandstone, siltstone, or other sedimentary rock. And, again, we may find local coal deposits created in more substantial swamps or marshy areas.