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

Abstract

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 (<40°C), which is consistent with geological evidence that these rocks were only shallowly buried (<500m).Based on these δ18O(OQ) results and the modeled thermal history during burial of the basin, the earliest-formed quartz overgrowths were produced at low temperature from low δ18O(water) around 450Ma. The δ18O values in traverses of single overgrowths decrease by up to 9.1‰, showing continued cementation with increased burial, pressure solution, and heating until ~250Ma. In traverses of the outermost zone of some overgrowths, oxygen isotope values become constant or increase slightly, possibly due to clay mineral dehydration reactions or later fluid infiltration. We present a new cementation and basin evolution model, in which the δ18O of cement correlates to the age of formation and the late overgrowths formed between 270 and 250Ma, during and/or after the migration of brines that formed the Pb–Zn deposits of the Upper Mississippi Valley District (270Ma). Cementation around 270Ma would have reduced permeability, possibly ending the flow of ore forming brines.