Paragenesis of Diagenetic Minerals in the St. Peter Sandstone (Ordovician), Wisconsin and Illinois
I. Edgar Odom, Timothy N. Willand, Richard J. Lassin, 1979. "Paragenesis of Diagenetic Minerals in the St. Peter Sandstone (Ordovician), Wisconsin and Illinois", Aspects of Diagenesis, Peter A. Scholle, Paul R. Schluger
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Mineralogical analyses of the silt- and clay-size fractions and detailed SEM studies of whole rock samples show that diagenetic minerals in the St. Peter Sandstone formed in essentially five separate stages. The first stage, beginning after compaction, involved the precipitation of K-feldspar in selected pore spaces, and the precipitation of quartz as a thin, conformable envelope on quartz grain surfaces not in contact with other grains. During the second stage illite and locally smectite and chlorite were precipitated, sometimes with dolomite and calcite. A mixed layer clay, which is occasionally present, is believed to be an alteration product of illite. The third stage is marked by development of terminated (euhedral to subhedral) quartz overgrowths. Faces of terminated overgrowths are nearly always free of clay minerals, dolomite and calcite. The fourth stage is the formation of pyrite. The fifth stage is the development of kaolinite which does not appear to have formed directly from the alteration of K-feldspar.
The authigenic mineral suite and paragenetic sequence record a pronounced change in the chemistry of the pore fluids through time. The early precipitation of K-feldspar and quartz indicates that the pore fluids had a high K/H ratio and silica content. The K/H ratio decreased through time to bring the pore fluid chemistry in equilibrium with illite, smectite, and chlorite. A high silica concentration in the pore fluids, not attributable to pressure solution, existed after the illite stage. The ionic content of these pore fluids was higher than that of fresh water because terminated quartz overgrowths and pyrite, both of which were precipitated from these fluids, occur where the St. Peter presently contains saline pore fluids. Diagenetic kaolinite is observed only where fresh water has invaded the St. Peter.
Most, if not all, of the authigenic minerals in the St. Peter were precipitated from migrating pore fluids. The source of cations may be related to high salinity conditions during the deposition of overlying and underlying carbonates, the dissolution of silica during carbonate diagenesis, and the later in situ alteration of potassium silicates.
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Aspects of Diagenesis
There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.