Diagenetic Replacement of Feldspar by Quartz in the Uinta Mountain Group, Utah and its Geochemical Implications

Abstract

ABSTRACT Sandstone of the Precambrian Y Uinta Mountain Group has undergone a diagenesis in which quartz replaced feldspar during lithification. Generally, lithification of sandstone was characterized by sequential precipitation of iron oxide iron oxide + quartz quartz quartz + illite illite. Microgranular hematite bands mantle detrital quartz and feldspar. Authigenic quartz encloses hematite bands on quartz nuclei and protrudes into adjacent feldspar grains across preserved feldspar outlines. Illite is present as inclusions in authigenic quartz and illite fills interstices in quartz cement Feldspar dissolution and its replacement by quartz occurred simultaneously. Illite formed only after replacement of feldspar by quartz was well advanced. Diagenetic replacement of feldspar by quartz has not been noted as a common authigenic feature, even though the geochemical conditions of diagenesis do not seem unusual. A geochemical model developed here to illustrate the processes of diagenesis in sandstone includes the following elements, 1) the water initially trapped with the sediment was probably brackish or similar to normal seawater in composition, 2) probable maximum burial depth was about 4,800 m, 3) the estimated maximum paleotemperature was 200°C at the depth (using Gulf Coast geothermal gradients as an analogue), and 4) membrane-filtering and intracrystalline water expulsion from clay during progressive burial resulted in profound changes in pore-water composition that governed the sequential stability of authigenic minerals. Published data on compaction experiments and chemical analyses of modern sandstone-porewaters, together with textural evidence from the Uinta Mountain Group, sug est the possibility that adjacent interbedded shale was a major source of dissolved SiO2 for quartz precipitation in sandstone beds. Dissolved SiO2 that was expelled during early compaction equilibrated at saturation and supersaturation values for quartz, and quartz precipitated on detrital grains in sandstone units. K+ and H+ increased in proportion to other ions in sandstone beds during early lithification, and feldspar became a stable solid phase. As burial continued, K+ in sandstone pore water decreased and microcline dissolved while quartz continued to precipitate, resulting in quartz replacement of feldspar and development of extensive quartz overgrowths. Illite joined quartz as a stable solid phase in the final stages of lithi ication diagenesis. Because Mg++, Ca++, and Cl- were trapped in shale beds, the chemical-equilibrium conditions favored crystallization of chlorite in shale units, but pore-water composition caused illite to be the dominant authigenic clay mineral in sandstone units. Lithification was probably complete by the time the sediment had reached a burial depth of about 4,800 m and a temperature of about 200°C.