Stratigraphy, Depositional Environments and Coalbed Methane Resources of Cherokee Group Coals (Middle Pennsylvanian)--Southeastern Kansas
Kansas Geological Survey
Open-file Report 2003-82

2.1.2 Blocky Mudstone Facies

The blocky mudstone facies consists of micaceous shale that is light gray to medium gray. Other features that characterize the blocky mudstone facies include slickensides, blocky and mottled texture, and plant fragments (Figure 2.04). Clay ironstone nodules (siderite) are usually present in the lower portion and decrease in abundance upwards. When the blocky mudstone facies is associated with an underlying limestone, caliche nodules may be present that are 0.25 inches in diameter (8 mm). Plant fragments and rhizoliths increase in abundance upwards toward the top of the blocky mudstone facies. No trace or body fossils were observed in the blocky mudstone facies. Thickness of the blocky mudstone ranges from 0.25 to 10 feet (.1 to 3 m). The lower contact is typically gradational with the underlying facies, while the upper contact is sharp.

Paleoenvironmental Interpretation
Blocky or mottled texture with soil slickensides, and plant fragments are evidence that the blocky mudstone lithofacies is a paleosol. Marine fossils were not observed. Development of slickensides indicates frequent shrink-swell cycles. These cycles are directly tied to paleoclimate. A higher frequency of cycles is expected in a subhumid temperate climate (Gustavson, 1991). Factors such as parent rock mineralogy, paleoclimate, duration of exposure, and rainfall played a major roll in soil development (Gustavson, 1991). The facies was probably formed in either an overbank area within an interfluve setting, or in swampy conditions of a mire, with the sediment of the facies being derived from pedogenic process acting on the underlying sediment, and wind derived sediment. The gradational contact with the underlying facies is interpreted as a C zone of a paleosol (Retallack, 1990). Historically, the blocky mudstone facies would be interpreted as part of the “outside shale” in the cyclothem model (Heckel, 1977).

Figure 2.04 - Polished core section showing blocky mudstone facies. Note pedogenic nature and weathered appearance of the mudstone. Soil slickensides are also observed within the blocky mudstone. Sample 1 from 780' and sample 2 from 870' in the Hinthorn CW#1 well, 14-T32S-R16E, Montgomery County, Kansas

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Last updated January 2004