One way in which low-temperature, near-surface excess silica was introduced into the Newark sedimentary system was by extensive chemical weathering in the source area, when tectonic quiescence allowed it. Further concentration of silica within the basins was accomplished in the following ways: (1) eogenesis, by fluctuating pH, with values greater than 9.5 causing silica to be extracted from silicates within playa lakes or within soil zones; (2) eogenesis, by stands of silica-concentrating plants such as Equisetites, within ponds, playas, or fringes of larger lakes; and (3) mesogenesis, by pressure solution among quartz grains and possibly other silicates, possibly associated with high pH, causing release of silica.

In all situations, the silica, either from solution or gel, probably initially formed as opal-A and proceeded to the opal-CT stage, and then to the stable quartz phases of chalcedony, microquartz, and the megaquartz that we find today. The stable quartz phases are associated with primary silica precipitation in playa lakes, replacement of carbonate in playa lakes, possible replacement of carbonate oolite in playa lakes, vadoids in silcrete paleosols, possible replacement of vadoids in caliche paleosols, replacement of wood cell walls, syntaxial overgrowths on quartz grains, and nonaffiliated pore filling.

The various processes of quartz formation, coupled with associated microfacies and paleoenvironments of deposition, may be used for finer delineation of fluctuating paleoclimates during Newark deposition.

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