MESOSCOPIC AND MICROSCOPIC DEFORMATION IN THE GLASFORD METEORITE IMPACT STRUCTURE, ILLINOIS: IMPACT FRACKING IN CARBONATE BEDROCK

A relogging of the 480 m-long Cowser #1 drill core through the central uplift of the Glasford meteorite impact structure near Peoria, Illinois, provides insight into impact-induced, meso- and microscopic deformation of a carbonate target rock. Units affected by the early Late Ordovician event include the Mt. Simon Sandstone up to the Galena Group of dolomite and limestone. Notable mesoscopic structures visible in the core include: breccias representing fracturing at a range of scales; steeply tilted beds; miniature extensional fault arrays that accommodated elongation parallel to the core axis; and asymmetric folds representing meso-scale ductile deformation during impact. At several depths, the core includes distinct 5 to 10 mm-thick “black bands” interlayered with light-colored dolomite. When the core was initially logged in 1961, these bands were interpreted as siltstone or shale. We found, however, that similar material also fills irregular lenses, and occurs in the matrix in dolomite breccias. Petrographic, SEM, and XRD analysis reveal that the black bands do not consist of siltstone or shale, but rather of cataclasite composed of angular fragments of quartz and feldspar. They do not contain significant quantities of clay minerals or glass, so we do not refer to the material as suevite. Locally, we observed mineral tails adjacent to clasts, similar to those found in shear zones. The composition of the cataclasite contrasts markedly with that of the wall rock, which consists of limestone and dolomite containing only minor quartz and only very rare feldspar. Hydrothermal iron-sulfide minerals have precipitated along the margins of cataclasite. We suggest that the black bands, and related occurrences of cataclasite, formed as a consequence of “impact fracking." During this process, heat and pressure generated during impact decomposed carbonate rocks, perhaps producing supercritical CO₂. This fluid transported more resistant silicates (residuals from the carbonate rocks as well as grains from siltstones and sandstones, and perhaps granitic basement) in suspension, injected them into tensile fractures, and then dissipated. The silicate-mineral fragments remained in the cracks as cataclasite, propping the cracks open so that syn-impact or post-impact hydrothermal fluids could circulate and precipitate sulfides.