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Abstract

Sedimentation during the Upper Paleozoic in the Cordilleran geosyncline involved the introduction and incorporation of tremendous quantities of siliceous materials within the limestone, orthoquartzite, and shale sequence. The repository had the characteristics of a miogeosyncline, yet chert is abundantly represented among the siliceous sediments. This material was deposited copiously in strata of Mississippian, Pennsylvanian and Permian ages in thin beds, concretions, irregularly-shaped masses (blebs, “gobs,” and nodules), and as replacement of fossils and other structures. Thus, the chert and other forms of silica occur as epigenetic, syn-diagenetic, and syngenetic deposits and replacements.

Silica in the sediments at some localities is penecontemporaneous with carbonate, arenite, and lutite sedimentation. In other places all of the siliceous material was introduced during post-depositional stages. And at still other places penecontemporaneous and post-depositional silica occur within a stratigraphic succession in an interbedded sequence. Petrologic and petrographic studies reveal sharp contacts with subjacent and superjacent strata locally, and elsewhere blended contacts are characteristic. These relations are particularly well shown in the various carbonates. Within the stratigraphic section of interbedded orthoquartzite, calcarenaceous orthoquartzite, and arenaceous limestone there occur numerous interesting examples of the host of silica types. Many of the arenites show evidence of having been tightly cemented with silica during the diagenetic stage; 95 percent of some measured sections display this. Some of the finest examples of orthoquartzites occur in the thick geosynclinal pile. Authigenic quartz is commonly present as doubly-terminated prisms in many of the strata.

Regional studies suggest that the sources for the abundant siliceous material were not only peripheral to the Upper Paleozoic miogeosyncline, but within it as well. Volcanic archipelagoes to the west provided much siliceous material directly to the marine waters as ash and other effusives, and indirectly from weathering and erosion of the terrestrial accumulations of lava. Land masses within the geosyncline furnished silica attendant upon removal of sedimentary and crystalline material. Cratonic areas marginal to the geosyncline, particularly those which lay to the southeast, east and northeast undoubtedly comprised important provenances for additional silica. Investigations of the siliceous materials require combinations of field and laboratory techniques; the latter involve studies of thin sections, insoluble residues, chemical analyses, and cellulose peels. All are invaluable complements to correctly evaluate the conditions of sedimentation.

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