BURIAL HISTORY OF THE CAMBRIAN EAU CLAIRE FORMATION BASED ON IN SITU OXYGEN ISOTOPE ANALYSIS OF QUARTZ OVERGROWTHS AND HEALED FRACTURES

In situ analyses of oxygen isotope ratios of detrital quartz grains, quartz overgrowths, and quartz filling factures from the Eau Claire Formation in the Illinois Basin and Wisconsin Arch record growth history, and changes in fluid compositions and temperatures during burial and diagenesis. Detrital grains (10-500μm dia.) average δ¹⁸O = 10.2 ±3.6‰ (2SD), consistent with an igneous source for fine- and coarse-grained quartz in mudstones and sandstones in the Eau Claire Fm, identical to the Mt. Simon and St. Peter Sandstones. The δ¹⁸O values of early-formed quartz cements in the Eau Claire samples from the Illinois Basin are relatively uniform in all samples (27-28‰, 3-10μm spots). In overgrowth traverses, early-formed overgrowths show consistently higher δ¹⁸O values than late-formed overgrowths (Δ¹⁸O_(early-late)=1.5-7.5‰). This trend is also reported from overgrowths in the Mt. Simon Sandstone. These findings suggest that all overgrowth formation in the Eau Claire from the Illinois Basin started shallowly. Overgrowths grew with increasing temperatures during burial, and continued growth possibly at constant temperature and pore fluid composition at the maximum depths, until impinging on other overgrowths and loss of porosity. The δ¹⁸O values in samples from the WI Arch, on the other hand, show no difference between early- and late-formed overgrowths, and are much higher (ave. 30‰) than overgrowths from the Illinois Basin. The WI Arch overgrowths are interpreted as quartz cements formed at near-surface temperature (<50ºC). The difference between the samples from Wisconsin and Illinois likely resulted from proximity to the shoreline. We also investigated intragranular quartz cement in fractures (healed cracks) and observed healed cracks in 20-50% of detrital grains in coarse silt to medium sand samples in the Eau Claire Fm. The δ¹⁸O values of healed cracks (n=23) from the Illinois Basin and the WI Arch samples are identical to those of the detrital quartz grains (4-14‰), except two healed cracks that are similar to overgrowths (24‰). Most of fractures observed in the Eau Claire samples were thus filled by quartz at high temperatures before sedimentary transport and burial, and thus the use of healed fractures from the Eau Claire Fm as an indicator of compaction and depth of burial would be in error.