Pressure Solution and Porosity

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Abstract
The lower Silurian Green Pond Conglomerate of northern New Jersey contains numerous sandy zones of highly pressolved quartz grains. These zones are characterized by sutured upper and lower grain contacts, clay coatings around grains, high ratios of horizontal/vertical grain dimensions, and lack of secondary quartz. These features in adjacent unpressolved zones are characteristically the opposite. The presence of clay seems to be necessary for thedevelopment of highly pressolved zones. Several thermal analyses of this clay indicate that it is a highly degraded illite. Similar analyses of clay from nearby shales also indicate the presence of illite, but in a lesser state of degradation. It may be inferred that a positive correlation exists between degree of pressure solution and loss of potassium from the illite structure. Solutions migrating through the sediment commonly are charged with CO2 and various cations capable of replacing potassium. Loss of potassium from the illite structure results in the formation of K2CO3, a strong alkali. It is suggested that a pH differential of decreasing alkalinity radiates away from the zone of clay. As pressure and solubility increase due to weight of overburden, a threshold is reached beyond which quartz is dissolved at grain contacts in regions of highest pH. Silica, probably in the form of potassium silicate, migrates to regeions which have a lower pH, and hence are below the threshold. There it is deposited as secondary overgrowths on unmodified quartz grains. Pressolving of grains results in loss of porosity in high pH zones, but in places of low pH where pressure solution is unfavorable porosity may be completely eliminated by cement derived from pressolved zones. Such complete cementation may limit remaining porosity to zones in which the degradation of illite prevents cementation, and in which, because, of insufficient pressure or other reasons, pressure solution has not been important.

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