Geometry of calcite cemented concretions of the Arikaree group (Tertiary) : a clue to hydrodynamic processes of cementation
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Eolian and fluvial deposits of the Arikaree Group (Tertiary) of western Nebraska, eastern Wyoming, and southwestern South Dakota contain abundant calcite-cemented concretions and paleosols with a variety of morphologies and distribution patterns. The morphologies include: laterally extensive horizons, highly elongate forms in the plane of bedding, rhizocretions, spheres, and sand crystals occurring as both single crystals and clusters. Calcite was precipitated from groundwater during ionic diffusion, during fluid flow in the phreatic zone, and during vadose processes. The distribution of concretions in outcrop was found to be selective to zones of coarser grain size and higher permeability for elongate forms, to areas of lower fluid flow velocity for diffusion-generated forms, and to areas which were once pedogenic horizons for concretions created by vadose processes. Elongate "pipy" concretions, which are found as isolated, regularly spaced and laterally merging concretions, generally show a preference for particular stratigraphic horizons, and were found to favor zones of coarser grain size and higher permeability. The orientation of the elongate concretions is remarkably uniform (the standard deviation of the axial trends are less than 14 degrees at any one locality) and parallel with the inferred regional fluid flow, which has remained unchanged since the Miocene. Rhizocretions were observed in the process of forming around modern roots. Massive laterally extensive cemented horizons are interpreted to be ancient pedogenic horizons by their abundance of burrows and bioturbation. Sand crystal rosettes are interpreted to be replacement features after gypsum rosettes. Spherical concretions and scalenohedral calcite sand crystals formed by ionic diffusion. Electron microprobe analyses indicate that the calcite cement is close to stoichiometrically pure calcite. Oxygen and carbon isotope ratios show that the diagenetic fluid was meteoric water. Cementation occurred before significant burial. Some calcite samples were found to have a complex cement stratigraphy in cathodoluminescence, indicating episodic fluctuation in groundwater composition or temperature. The calcite in the pipy concretions has been imported from limestone units which crop out up hydrogeologic gradient from the Arikaree. Dissolution features are present in pedogenic horizons and in some pipy concretions. Local dissolution of calcite is thought to supply calcite to vertical concretions. Fluvial channels contain clasts of pipy concretions indicating that calcite cementation occurred during deposition.