Cenozoic Silicification in the Niobrara Formation, Kansas-Nebraska: Significance of Regional Paleo-Weathering Phenomena

Astride the Kansas-Nebraska border, the Upper Cretaceous Niobrara Formation (Smoky Hill Chalk) is locally impregnated with gray to pale green opaline silica below its contact with the Ogallala Group (Upper Miocene). In northwestern Smith County, Kansas there is a distinct alteration profile (> 300 cm thick) consisting of, in descending order: (1) discontinuous laminar calcrete developed in basal Neogene sediments; (2) strongly silicified, medium-bedded Niobrara chalky limestone, with local intense brecciation; (3) strongly silicified chalky limestone with highly convoluted laminations, a single “teepee” structure, and widespread open vugs containing a terminal lining of milky opal; (4) medium to thick-bedded silicified chalky limestone with irregular pockets of intensely brecciated silicified chalky limestone and Liesegang-banded silica precipitates; (5) thin to medium bedded, strongly silicified chalky limestone; and (6) weakly silicified, irregular and thinly-bedded chalky limestone containing a few strongly-silicified beds. Solution pipes infilled with silty very fine sandstone extend from the top of horizon 2 to depths exceeding 200 cm. Below horizon 3, the predominant rock fabric is “blocky-brecciated”, meaning bedding-parallel or slightly inclined, blocks (“paleo-channery”) of silicified limestone with relict sedimentary lamination, surrounded by interstitial opaline silica (5-10% of the rock mass) containing common sand- to granule-sized limestone clasts.

The Smith County alteration profile records: (1) gentle post-Campanian tectonic deformation; (2) vadose dissolution and brecciation carbonate rock; (3) precipitation of at least two, and possibly more, phases of silica in the host rock, very likely in a phreatic environment; and (4) Neogene reburial by fluvial sediments and development of vadose calcrete. Silicification (absent in the sub-Neogene Pierre Shale to the west) clearly had a genetic relationship with the geochemical environment of carbonate host rocks of the Niobrara Formation, even though greenish silica cements appear within the Ogallala Group itself, and it was also a significant part of Cenozoic landscape evolution.