Geologic Setting
Total Page：16
File Type：pdf, Size：1020Kb
Appendix B: KimballtonKimballton Geology DUSEL Appendix B: GEOLOGIC SETTING Introduction Since the beginning of the 2004 Fall semester an interdisciplinary team at Virginia Tech has been working to develop a comprehensive proposal for a national research laboratory to be based at the Chemical Limestone Corporation’s Kimballton mine in nearby Giles County. This Appendix summarizes geologic information about the Kimballton site. Geotechnical and rock mechanics information is provided in a separate appendix (Appendix C). For comparison with the other candidate sites, a general geologic map showing the locations of all eight potential DUSEL sites is shown below (Fig. 1). Figure 1: Map showing potential DUSEL locations. Among the eight candidate sites, Kimballton is unique in that it offers the capability for hosting the DUSEL facilities in sedimentary rock, at the full depth of 7,500 ft (WIPP is also in sedimentary rock, but is not proposing to develop laboratories at the full depth.) The geology at Kimballton is characterized by hard, strong sedimentary rocks, primarily limestone and dolostone, in ancient overlapping thrust sheets. The imbricate thrust faults give rise to repeated stratigraphy, and good surface exposure of the Kimballton rock units, allowing excellent stratigraphic control. Access tunnels and shafts at Kimballton will begin in limestone and dolostone within the Narrows thrust sheet, and then cross the Narrows fault into limestone and dolostone of the St. Clair thrust sheet. Access tunnels and underground laboratories will be within competent rock formations with strengths comparable to that of granite. Karst features are expected in the uppermost few hundred feet, but both karst and fracture intensity are expected to decrease significantly with 1 depth. Evidence from the existing Kimballton mine, which has large, unsupported chambers at depths over 2000 ft., suggests that the geomechanical properties of Kimballton rocks are well suited to the creation and maintenance of large scale, long term underground openings. Because of its sedimentary setting, Kimballton-DUSEL is particularly well-suited for significant research on hydrocarbon recovery and carbon sequestration. It also offers many advantages for bioscientists studying deep and ancient life. Kimballton-DUSEL will provide direct access to the core of the classic Appalachian foreland fold-thrust belt, the locus of ongoing studies on tectonics and orogenesis. Because of proximity to both east coast and mid-America seismic zones, it will provide valuable insight into earthquake hazard. Due to a wide range of scale of hydraulic pathways (from grain scale to major thrust faults), Kimballton will be a unique research facility for studying groundwater flow. Finally, Kimballton can serve as a primary center for studies on geomechanics of layered rock under high stress, with a major impact on future development of underground space in sedimentary host rocks. Additional geologic characterization, beyond that discussed in this appendix, will be conducted as part of the S-2 process to further reduce uncertainty and to plan for facility design and construction. These additional geologic characterization studies are described in a later section. Geological Setting of Kimballton The Kimballton Mine and Butt Mtn are located in the Allegheny Mountains of southwestern Virginia, near the western edge of the Appalachian Valley and Ridge Physiographic Province. This region is characterized by linear ridges held up by tilted strata of resistant sandstone with limestone and shale in fault line valleys. Geologically, Kimballton lies in the classic Appalachian foreland fold-thrust belt. The bedrock consists of folded sedimentary rock of Paleozoic age arranged in overlapping thrust sheets (Figure 2). Structurally, the proposed Kimballton DUSEL laboratory site lies within the St. Clair thrust sheet, beneath the Narrows thrust fault, and above the St. Clair thrust sheet. The Appalachian foreland fold-thrust belt has been studied for more than 150 years. Several fundamental concepts related to the evolution of continental crust and mountain chains were formulated here. Those concepts have been applied in other mountain chains, such as the Jura Mountains and the Canadian Rockies, but opportunities exist to expand and develop new concepts based both on current knowledge and the unique opportunities afforded by being able to conduct experiments at depth. Valley and Ridge topography (Figure 3) reveals some 300 million years of geological history. The Valley and Ridge overthrust structure reflects intense compression along the proto- Atlantic Continental margin during the Alleghenian collision event between North America and Africa. The collision created the Pangean super continent some 300 million years ago. Our present mountains are much younger, having formed by erosion in the post-Cretaceous Era. 2 Figure 2: Map showing overlapping thrust sheets and location of DUSEL. Figure 3: Hillshade showing Valley and Ridge Topography. The break up of Pangea began in the Triassic Period about 245 million years ago and by the Jurassic Period continental rifts forming the Atlantic and Gulf of Mexico Ocean basins were well established (Figure 4). The modern Appalachian Mountains are forming by gentle uplift, rejuvenation and entrenchment of rivers along the eastern flanks of the Atlantic Basin (Figures 4 and 5). Butt Mountain is located along a passive continental margin. Moderate seismic activity has been localized in basement rocks several kilometers below the site (Figure 6). 3 Figure 4: Jurassic Period continental rifts forming Atlantic and Gulf of Mexico ocean basins Figure 5: Location of Butt Mountain along passive continental margin. Source: National Geographic 4 Figure 6: Regional seismic activity (moderate). Source: M. C. Chapman, Virginia Tech Previous Work The Kimballton area was studied first by Charles Butts in a landmark synthesis of regional stratigraphy and structural geology (1932, and 1941). Wartime needs for strategic minerals including high calcium limestone provided the impetus for VPI professor of geology Byron Cooper (1944) to map and describe details of the valuable Middle Ordovician Limestone sections in the Kimballton area. Photographs of the plant and original entrance to the underground mine workings beneath Butt Mountain published in 1944 (Virginia Geologic Survey Bulletin 62, Plate 6) give testimony to the long- term geologic stability in this area. The well-exposed fold and thrust structures along the Appalachian Structural Front (Figure 1) were major exploration targets during the post-World War Two oil and gas exploration boom. A wildcat exploration well 1,443 deep that was drilled by the California Company on the Bane Dome southwest of the Kimballton Mine in 1948 was the first attempt to explore the Appalachian Thrust Belt of SW VA (Perry et al., 1979). The Straler well penetrated the nearly flat lying Narrows thrust fault on the crest of the Bane dome but was abandoned in massive Knox dolomite that was misidentified as the older Cambrian Shady Dolomite. This wild cat well helped to trigger a 30-year controversy on the nature of thin-skinned deformation in the Appalachians. In preparing the geological model for the NSF proposal, the team has revisited the previous 60 years of geological and geophysical research by Virginia Tech faculty and graduate students. Surface and subsurface mapping by Thomas Gathright II, William Ekroede, Art Schultz, Chuck Stanley, Robert McDowell, and Jerry Bartholomew have proven invaluable in fleshing out Cooper’s original stratigraphic studies of the Middle Ordovician limestone formations in the New River- Roanoke River District published in Virginia Geological Survey Bulletin 62, in 1944. 5 Geologic Map A geologic map of the Kimballton area is shown as Fig 7. Lines AA’, BB’, CC’, DD’, EE’ HH’ and I-I’ are cross sections developed as part of the initial geologic investigations for Kimballton-DUSEL which will be presented in Figs. 8 and 9. The proposed deep laboratories are to be located beneath Butt Mountain., approximately under the intersection of cross section lines I-I’ and H-H’. Following Fig 7 are detailed lithologic descriptions, which also serve as the legend for the geologic map. Figure 7: Geologic map of Kimballton area. The geologic cross sections are shown in Figs. 8and9 6 Lithologic Descriptions MISSISSIPPIAN Hinton Formation – Mh Dark-gray, greenish-gray, and reddish-gray mudstone, gray siltstone, and dark- to light-gray, fine- to medium-grained sandstone; basal Stony Gap sandstone Mh member is a resistant, 10-30-m thick, pebbly quartzite (Cooper, 1961). About 200 m of the lower Hinton is exposed in the northwestern part of this quadrangle within the Allegheny Plateau. Bluefield Formation – Mb The lower 200 m of the Bluefield Formation consists of interbedded medium- to dark-gray, argillaceous calcilutite and medium-gray calcareous mudstone. Total thickness of the Bluefield is about 600 m in the Allegheny Plateau; 420 m was Mb measured by Humphreville (1981) near Glen Lyn. The upper 400 m of the Bluefield Formation consists of interbedded gray to greenish-gray to reddish-gray mudstone, medium- to light-gray, fine- to medium- grained sandstone, and light-gray siltstone with minor dark-gray to black coal beds. Greenbrier Group (undivided) – Mg The basal 40 m consists of interbedded medium- to dark-gray, fine-grained, argillaceous limestone and medium- to dark-gray mudstone. The basal unit is overlain by about 10 m of medium-