Diagenesis of sandstones and carbonates of the Ignacio Quartzite and McCracken Sandstone Member of the Elbert Formation, southwestern Colorado, U.S.A.

Syndepositional events that affected fluvial, estuarine, and shallow-marine sediments of the units studied in southwest Colorado—the McCracken Sandstone Member of the Elbert Formation and the Ignacio Quartzite—included bioturbation, local formation of glaucony grains and phosphate crusts, local carbonate pedogenesis, and possibly silcrete pedogenesis. Although the sediments were ultimately buried up to approximately 4 km, shallow burial was the site of: (1) reddening of feldspathic sand and shale of the Ignacio Quartzite; (2) initial loss of porosity in both units by compaction of locally abundant clay clasts and clay drapes; and (3) initial partial cementation by overgrowths of feldspar and microquartz crystals. During deeper burial, the quartzarenite sands of the McCracken Sandstone Member of the Elbert Formation underwent compaction to porosity values below that typical of stable packing patterns of rigid grains followed by complete cementation by megaquartz overgrowths to produce highly indurated sandstones. Silica derived from intergranular pressure dissolution in local sandstone beds and during stylolite formation was inadequate to provide the amount of silica present in the cement. Much silica apparently was imported in fluids expelled from adjacent sedimentary basins. Such fluids were also responsible for bleaching red beds in the Ignacio Quartzite, dolomitizing limestone in the McCracken Member, and introducing small amounts of oil, some of which survives as pyrobitumen. Values of stable carbon and oxygen isotopes in dolostones in the McCracken indicate that dolomitization was accomplished by hot fluids. Kaolinite—an alteration product of K-feldspar, illite, and trace amounts of barite and sphene—formed during intermediate to deep burial.