MUSCOVITE, ILLITE AND ORGANIC MATERIAL DRIVE QUARTZ PRESSURE SOLUTION AND STYLOLITE DEVELOPMENT IN PLIO-PLEISTOCENE, JURASSIC AND ORDOVICIAN SANDSTONES

Muscovite and illite are integral drivers of pressure solution in Ordovician (Bromide), Jurassic (Rhum Field, North Sea) and Plio-Pleistocene (Tulare Formation) sandstones. In all cases, pressure solution was not found between clean quartz grains, but was found at mica-quartz interfaces and between illite-coated quartz grains.

Preliminary cathodoluminescence observations of the Ordovician Bromide Sandstone of Oklahoma revealed highly sutured quartz grain contacts in millimeter-scale zones of illite-coated quartz grains. Away from the illite-rich zones where quartz grains lack illite coatings, no interpenetration of quartz grains was observed. This variation in quartz pressure solution demonstrates that a) illite is necessary to promote pressure solution and b) the localization of pressure solution is due to primary differences in sandstone mineralogy.

A cathodoluminescence study of pressure solution and stylolite development in Jurassic sandstones from a North Sea core also revealed that no pressure solution occurred between clean quartz grains. However, quartz dissolution, including dissolution of overgrowths, did occur along mica-, organic- and clay-rich stylolites, and between quartz grains with thin clay coatings. Microprobe and XRD analysis confirmed that illite and muscovite are the clays within both the stylolite and bulk rock, thus no preferential chemical alteration of the clay occurs in the stylolite seam. Therefore, in these samples, muscovite or illite and/or organic material are necessary for dissolution of quartz and stylolitization. Furthermore, observations show that stylolites begin as originally flat clay/organic-rich laminae (~3% organic) and acquire stylolite morphology as varied dissolution of quartz grains occurs along the stylolite interface.

Shallow pressure solution is observed in core samples of the Plio-Pleistocene Tulare Formation from less than ~600 meters depth. Point-count analysis shows that mica-rich areas have 2.5 times more flattened grain contacts relative to mica-poor areas of the sandstones. These relationships hold in the presence of biotite or muscovite. Interpenetration of illite-coated quartz grains was also observed. Clay and mica enhance quartz dissolution at all depths and should be considered when modeling quartz sandstone compaction.