Abstract The karst of the central Shenandoah Valley has characteristics of both shallow and deep phreatic formation. This field guide focuses on the region around Harrisonburg, Virginia, where a number of these karst features and their associated geologic context can be examined. Ancient, widespread alluvial deposits cover much of the carbonate bedrock on the western side of the valley, where shallow karstification has resulted in classical fluviokarst development. However, in upland exposures of carbonate rock, isolated caves exist atop hills not affected by surface processes other than exposure during denudation. The upland caves contain phreatic deposits of calcite and fine-grained sediments. They lack any evidence of having been invaded by surface streams. Recent geologic mapping and LIDAR (light detection and ranging) elevation data have enabled interpretive association between bedrock structure, igneous intrusions, silicification and brecciation of host carbonate bedrock, and the location of several caves and karst springs. Geochemistry, water quality, and water temperature data support the broad categorization of springs into those affected primarily by shallow near-surface recharge, and those sourced deeper in the karst aquifer. The deep-seated karst formation occurred in the distant past where subvertical fracture and fault zones intersect thrust faults and/or cross-strike faults, enabling upwelling of deep-circulating meteoric groundwater. Most caves formed in such settings have been overprinted by later circulation of shallow groundwater, thus removing evidence of the history of earliest inception; however, several caves do preserve evidence of an earlier formation.

Abstract During the past 25 yr, several different tight-gas sandstone reservoirs have been brought into the nation’s productive natural-gas inventory. These include reservoirs of many different ages in many different basinal settings. In this chapter, reservoir discovery and management efforts at select fields in the Silurian Tuscarora, Devonian Oriskany, Pennsylvanian Pottsville and Jackfork, Jurassic Cotton Valley, Cretaceous Frontier and Almond, and Eocene Wilcox sandstones are reviewed, compared, and contrasted. Each of these target reservoirs is unique and both simple and complex. However, from a general understanding of the characteristics and variety of tight-gas reservoirs, a set of common generalities can be developed that may even be developed into rules for discovery. Although many tight-gas sandstone reservoirs may be classified as continuous-type reservoirs, (i.e., unconventional gas accumulations lacking well-defined field boundaries), tight-gas sandstone reservoirs are complexly subtle, with reservoir properties that are anything but continuous across their extent. Intentional discovery and development of tight-gas sandstone reservoirs requires knowledge, planning, careful execution, flexibility, and patience. A discovery model for the exploration and development of tight-gas sandstone reservoirs is proposed: (1) locate wells within a dry, gas-prone basin or part of the basin to avoid liquid (water, crude oil, or condensate) production, which will hurt gas-production rates; (2) select as intended targets depositionally heterogeneous reservoirs (i.e., channel systems), which are close to organic-rich intervals; (3) target slightly higher-shale-content sandstones instead of lower-shale-content sandstones (quartz arenites) to avoid loss of reservoir storage volume caused by cementation; (4) take advantage of whatever structure there is, and drill as high up on that structure as possible; (5) consider how you plan to manage a fractured, tight-gas reservoir (if fractures are anticipated to be present); (6) try to avoid sandstones with the potential for high water flow and low gas flow; (7) develop a clear petrophysical understanding of the reservoir early in the life of the field; and (8) plan on infill drilling once the initial spacing unit design is approved and implemented.

Study of slope movements triggered by the storm of November 3–5, 1985, in the central Appalachian Mountains, U.S.A., has helped to define the meteorologic conditions leading to slope movements and the relative importance of land cover, bedrock, surficial geology, and geomorphology in slope movement location. This long-duration rainfall at moderate intensities triggered more than 1,000 slope movements in a 1,040-km 2 study area. Most were shallow slips and slip-flows in thin colluvium and residuum on shale slopes. Locations of these failures were sensitive to land cover and slope aspect but were relatively insensitive to topographic setting. A few shallow slope movements were triggered by the same rainfall on interbedded limestone, shale, and sandstone. Several large debris slide-avalanches were triggered in sandstone regolith high on ridges in areas of the highest measured rainfall. Most of these sites were on slopes that dip 30 to 35° and lie parallel to bedding planes, presumably the sites of least stability.

Rock block slides as large as 20,000,000 m 3 occur in northeastern Pennsylvania where dip-slopes are undercut by rivers or by man. Slippage occurs along bedding in mudstone units where bedding dips out of the slope. The planar bedrock slabs are bounded by joints or the ground surface. The slab’s rectangular, arcuate, or triangular plan-view shape is controlled by joint and outcrop orientation on the slope. A 10 4 variation in slide-block volume is controlled primarily by differences in slope length and block surface area. Some blocks slide off the slope and onto the flood plain, while others only open up fissures and remain on the slope. Blocks move straight downslope or pivot toward an unbounded or more undercut side. The slides are part of an on-going process dating from post-late Wisconsinan glaciation (18,000 yr B.P.) to present. The region is seismically inactive; three historic slides are associated with high moisture conditions, so prehistoric slides were also probably triggered by high cleft-water pressure.