Evolution of quartz cementation and burial history of the Eau Claire Formation based on in situ oxygen isotope analysis of quartz overgrowths

Individual quartz overgrowths in siltstone of the late Cambrian Eau Claire Formation (Fm.) are systematically zoned in oxygen isotope ratio (δ18O). In situ analysis of δ18O was performed with 3 and 15μm beam spots by secondary ion mass spectrometer (SIMS) on detrital quartz grains and quartz overgrowths. These results from thin lenses within impermeable mudstones reflect samples that were sealed from basin-wide fluid flow and compliment previous studies of more permeable sandstones. Individual grains of detrital quartz (DQ) are homogeneous in δ18O. The average δ18O values in fine-grained detrital quartz in mudstones and siltstones and in coarser-grained quartz in the Eau Claire Fm., Mt. Simon and St. Peter Sandstones (Ss.) are essentially identical at δ18O=10‰ VSMOW, suggesting that detrital quartz is dominantly igneous in origin. The δ18O values of overgrowth quartz (OQ) of buried samples from the Illinois Basin are higher and quartz overgrowths are systematically zoned outward from the detrital cores. These gradients are similar to those from the underlying Mt. Simon Ss., and are best explained by increasing temperatures during burial. Pressure solution is evident in thin section and may have supplied significant silica for overgrowths. In contrast to the deeply buried samples from the Illinois Basin, quartz overgrowths in samples from the Wisconsin Arch are homogeneous and higher in δ18O. Those overgrowths are interpreted as quartz cements formed in a near-surface environment (