ABSTRACTOxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co‐precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non‐ferroan calcite cements indicates that both are original precipitates that formed during pre‐Pennsylvanian time at shallow burial depths (<215m) implying precipitation temperatures less than 30°C. Under these constraints the δ18Os of megaquartz (mean =+27.00/00SMOW; range =+ 24.8 to + 28.90/00) and calcite (mean =+ 28.00/00SMOW; range =+ 27.3 to + 28.40/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre‐Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre‐Pennsylvanian opal‐CT precursors, and therefore probably re‐equilibrated during recrystallization in late or post‐Mississippian time.We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ18Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground‐water system, the seaward terminus of which moved southward during lowering of pre‐Pennsylvanian sea level. The calcite cements and some of the opal‐CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ18Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturate