Outcrop versus core and geophysical log interpretation of mid-Cretaceous paleosols from the Dakota Formation of Kansas

Paleosols of the mid-Cretaceous (late Albian to early Cenomanian) Dakota Formation in Kansas and Nebraska are well exposed in road, river and clay pit exposures, but show systematic overprinting by modern weathering. Examination of deep drill cores reveals that modern weathering effects include (1) oxidation of pyrite to jarosite, (2) reaction of jarosite and calcite to clear selenite, and (3) oxidation of sphaerosiderite and siderite nodules to hematite. Surface oxidation in this region extends no deeper than 2 m from the surface. Hematite nodules and coatings of leaves are found in core only at one stratigraphic horizon, which was not as appears in outcrop, a laterite. Transformed matrix density and photoelectric absorption logs of cores reveal that kaolinite clays are most abundant within deeply weathered paleosols (Sayela pedotype) at this stratigraphic level, near the Albian–Cenomanian boundary, when angiosperm forests were especially diverse and lived in a subtropical humid climate. Other paleosols of the early Cenomanian were less deeply weathered, and fossil plants of this age included temperate humid angiosperm mangal and swamp woodland, but conifer-dominated floodplain forests. This short-lived mid-Cretaceous warm–wet spike has also been recognized in other paleosols of Minnesota, Utah, Nevada, and New Mexico, and coincides with marine black shale of the Breitstroffer event, midcontinental marine transgression, a sequence-stratigraphic boundary, and a marked increase in early angiosperm diversity.