Abrasive (sand) Blasting as a Means of Cleaning Weakly Lithified Impact- Related Drill Core from the Wetumpka Impact Structure, Elmore County, Alabama

Introduction: Impact structures are unlike most geologic features because there commonly is little or no surface manifestation of their complex crater-filling stratigraphy. Further, many impact structures lie completely buried by as much as several hundred meters of sediment. Frequently, the only means of accessing tangible material from within such structures is by drilling. Of the 172 proven impact structures on Earth, 99 have been drilled [1], and 12 to 15 still have core in existence. In 1998, two core holes were drilled within the Late Cretaceous, 7.6 km diameter Wetumpka shallow-marine impact structure in Elmore Co., AL. (32° 31.2’N, 86° 10.4’W) [2]. Drilling recovered NX core in 10-foot (3-meter) legs from depth ranges of 31.9 m to 191.6 m for Well 1, and 73.0 m to 179.5 m for Well 2. Well locations are 32° 31.368’N, 86° 10.369’W and 32° 31.303’N, 86° 10.379’W, respectively [2]. These two core sets would later show a bimodal crater-filling stratigraphy with a basal unit of clastand matrix-supported polymict breccias having a poorly consolidated coarse to fine sandy matrix; this unit is overlain by a chaotically deformed unit of friable Cretaceous target blocks (sandstones and mudstones) also in a poorly consolidated coarse to fine sandy matrix [2]. However, before these discoveries, extensive portions of the core were boxed immediately after drilling while still covered with ~2 mm of drilling mud. This procedure was contrary to that typically practiced wherein drill mud is washed from the core with a water hose upon removal from the drill hole resulting in significant erosion of the core and removal of fines. Because most of the Wetumpka core was not water washed, it remained largely unaltered by artificial erosion, but almost fully encased in mud that later dried to a hard crust. Removal of the mud crust had to be achieved before the core could be thoroughly studied. Problematically, the core was too weakly lithified and friable to be cleaned with liquid solvents such as water or alcohol, and too clay-rich for cleaning by brushing or scraping, as this only polished it and obliterated structural details at every scale. Cleaning by abrasive (sand) blasting: Recently, a successful means of removing the undesired mud crust has been achieved through a process of abrasive (sand) blasting using commercially produced 20/30 sieve-size crystalline silica sand blown through a common sand blaster. This effective method of mud removal proved to be a relatively quick, easy, and inexpensive oneor two-person job utilizing readily available equipment. Nonetheless, blasting with crystalline silica posed a serious health hazard, which had to be addressed in order to do this work. Health hazard mitigation. Freshly broken, aerosolized silica dust is considered a carcinogen and represents a Class-3 (severe) pulmonary health hazard [3]. It was essential that all blasting operations take place in a “fully enclosed” blasting cabinet under a partial vacuum (Fig. 1). By fully enclosed we mean with exception to an ambient-air intake port which allows for proper air flow between periods of actual blasting. The vacuum unit is a commercially available shoptype model that incorporates a 95% efficient, 0.1micron collection bag surrounding a 99.7% efficient, 0.3-micron HEPA filter (purchased separately as manufacturer-approved add-on accessories).