Browsing by Subject "Cementation (Petrology)"

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The genetic association between brittle deformation and quartz cementation: examples from burial compaction and cataclasis
(2004) Makowitz, Astrid; McBride, Earle F.; Milliken, Kitty L.
Brittle deformation of quartz grains accompanied by quartz cementation is a porosity-reducing mechanism in sandstones. Brittle deformation has historically been overlooked as a mechanism of compaction because it has been poorly understood and techniques for observing it are not commonly used. I have used scanned cathodoluminescence (CL) to quantify brittle deformation of quartz grains, in sandstones undergoing burial and cataclasis. Sandstone samples of different ages and compositions, taken from two basins with contrasting burial histories, are used to examine the interaction viii between brittle deformation and quartz cementation in burial compaction. Trends of increasing deformation by microfracturing with maximum burial depth are observed in both the lithic-rich Frio Formation from the Gulf of Mexico basin and in the quartz-rich Mount Simon Formation of the Illinois basin. Combining information on the degree of brittle deformation and the amount of quartz cement localized within microfractures allows for the calculation of the amount that brittle deformation influences compaction (i.e. porosity loss). For the Frio, 0.12 to 8.37% of initial porosity is lost due to cementation related to brittle deformation, whereas the values for the Mount Simon lie between 0.25 and 2.16%. Diagenetic forward models are constructed for each formation using petrographic modal analysis and burial history information to determine the depth of quartz cement commencement as an influential factor affecting brittle grain deformation. Most fracturing probably occurred prior to the precipitation of > 2% quartz cement. Commencement of quartz cementation at shallow depths combined with slower burial rates resulted in less brittle deformation in the Mount Simon compared with the Frio, where sandstones underwent rapid burial and quartz cementation began at greater depths. Cataclastic sandstones within the Pine Mountain Overthrust, eastern Kentucky, show more extreme porosity reduction by fracturing and cementation than normally compacted sandstones. In contrast to normal burial compaction, cataclasis and cementation within the cataclasites occurred over several discrete episodes as evidenced by cross-cutting relationships of fractures and cement. Quantitative data on the distribution of inter- and intragranular quartz cement within cataclasized sandstones combined with CL observations show that the timing of deformation is in agreement with published dates of fault movement.
Geometry of calcite cemented concretions of the Arikaree group (Tertiary) : a clue to hydrodynamic processes of cementation
(1996) Gell, Charles Emmett; McBride, Earle F.
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.