The Behavior of Silica in Diagenesis
Chemical reactions between detrital minerals, the incorporated biota and their tests, and interstitial fluids are considered to tend toward equilibria under three different stages ot diagenesis of sediments. Such stages are: initial or depositional, intermediate or early burial, and late burial or pre-metamorphic. Collectively the effect upon quartzose sandstones is: (1) During deposition, there is distinct pitting and rounding of quartz grains by solution, and instability of detrital chert, i.e., minor but persistent solution of silica. (2) Early burial is characterized by precipitation of quartz as overgrowths, i.e., minor but characteristic precipitation of quartz. (3) Late burial is characterized by addition of carbonate cement and as a replacement of quartz grains; or, where carbonate is absent, interpenetration of quartz grains increasing in degree with depth of burial or application of pressure as indicated by folding. Among subgraywacke sandstones chert tends to be metastable during the early burial stage and reacts with clay to form authigenic clay minerals and micas. Deep burial is characterized by introduction of carbonates corroding quartz and replacing the clay matrix, and authigenesis of mica from interstitial clay.
Occurrence of silica in Silurian carbonates with coral reef development shows interrelationships between carbonate minerals and chert which indicate chert is introduced during lithification of the reef flank sediments and precedes dolomitization.
Analyses of saline formation waters from Paleozoic carbonate strata buried to depths of several thousands of feet within the Illinois Basin provide data which suggest that solubility of silica is independent of pH below values of 8, and that temperature may be the important control of silica solubility.
Figures & Tables
Silica in Sediments
The Symposium on Silica in Sediments was presented in March, 1958 at the meeting in Los Angeles. The subject was selected by the Research Committee and approval of the Council then proceeded to develop the symposium and organized the papers, and later was authorized to edit and prepare the papers for publication. Cards for written questions directed at the authors of the papers were available during the presentation of the papers. The authors had an opportunity to examine the questions and later to answer them as a panel before those attending the discussion. Additional questions and replies developed between panelists and members of the audience and such were admitted to the discussion for as long a period as seemed feasible within the time available. Written answers to most of the questions were prepared later by the panelists and appear in this publication after respective articles.