The genetic association between brittle deformation and quartz cementation: examples from burial compaction and cataclasis



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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.