Within the cyclical Pennsylvanian-age succession of the U.S. Mid-continent, a number of Fossil Lagerstatten occur within incised valley fills (IVFs). These Lagerstatten contain a variety of well-preserved terrestrial plants, articulated fish (particularly acanthodians and xenacanths), and reptiles. Although the fossil evidence has traditionally been used to infer freshwater deposition, the IVFs were formed predominantly within fluvio-estuarine to brackish marine facies of the lowstand systems tract (LST). That is supported by widespread occurrence of acanthodian scales and xenacanth teeth in marine sediments.
The IVFs are related to the high degree of eustatic flux that characterizes cyclothems of this period. During sealevel lowstands, rivers incised drainage networks that were subsequently filled during base-level rise. Depth and lateral extent of paleovalley incision of the IVFs is defined from outcrops and wireline well logs. Core analysis indicates that estuarine facies, including cyclic tidal rhythmites, tidal bedding, and mud-draped bedforms, are common in the IVFs.
In Kansas there are both small- and large-scale IVFs. The small IVFs, such as the Hamilton and Garnett localities, have paleovalleys that are less than 1 km in width and incised less than 10 m deep. These contain a significant amount (>30%) of carbonate sediments. Their fossil flora and lack of regionally derived siliciclastics suggest that they were formed during relatively drier paleoclimates. Conversely, large IVFs, such as those that characterize the Douglas Group of Kansas, are about 10 km in width and exhibit 10s of m of incision into older strata. The fill is dominated by distal-source siliciclastics, contains thin coals, and has estuarine lithofacies containing sideritic concretions with well-preserved fossils. The extrabasinal siliciclastics and associated coals suggest that they were formed during relatively wetter paleoclimates.
Kansas Geological Survey
Updated April 17, 1997
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