Named as upper member (of 2) of Dakota formation; probably named for Janssen Station, Ellsworth Co, KS on the Central Kansas uplift. Type area designated near Janssen Station in center S line sec 3, T16S, R9W, Ellsworth Co. Occurs in Ellsworth, Ford, and Hodgeman Cos, KS. In generalized measured section of the Dakota in north-central KS, Janssen consists largely of gray to dark-gray clay and silt, some beds of fissile shale, and commonly a bed of lignite or highly lignitic clay; ranges in overall thickness here from 0.1 to 83 ft total. Dominant clay mineral is kaolinite. In some areas, forms well-defined bench on the hills. Most characteristic feature is presence of vertical holes or channels, some of which are branched. Overlies Terra Cotta clay member (new) of Dakota formation; underlies Graneros shale. Measured sections. Cross section. Of Cretaceous age.

Source: GNU records (USGS DDS-6; Denver GNULEX).