Abstract This trip seeks to illustrate the succession of Cambrian and Ordovician facies deposited within the Pennsylvania and Maryland portion of the Great American Carbonate Bank. From the Early Cambrian (Dyeran) through Late Ordovician (Turinan), the Laurentian paleocontinent was rimmed by an extensive carbonate platform. During this protracted period of time, a succession of carbonate rock, more than two miles thick, was deposited in Maryland and Pennsylvania. These strata are now exposed in the Nittany arch of central Pennsylvania; the Great Valley of Pennsylvania, Maryland, and Virginia; and the Conestoga and Frederick Valleys of eastern Pennsylvania and Maryland. This field trip will visit key outcrops that illustrate the varied depositional styles and environmental settings that prevailed at different times within the Pennsylvania reentrant portion of the Great American Carbonate Bank. In particular, we will contrast the timing and pattern of sedimentation in off-shelf (Frederick Valley), outer-shelf (Great Valley), and inner-shelf (Nittany arch) deposits. The deposition was controlled primarily by eustasy through the Cambrian and Early Ordovician (within the Sauk megasequence), but was strongly influenced later by the onset of Taconic orogenesis during deposition of the Tippecanoe megasequence.

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.

Soil-gas radon and ground radioactivity surveys across a portion of the Great Valley of West Virginia indicate that residuum and soils formed above some carbonate rocks have sufficient levels of radon gas to cause high indoor radon values. Data indicate no correlation of soil-gas radon concentration with faults, cleavage, joints, or calcite veins. Instead, soil-gas radon distribution appears to be controlled by the solution of carbonate bedrock and the subsequent development of thick, red, clay-rich residuum, which may contain as much as 4 times the concentration of radium, 10 times the concentration of uranium, and 5 times the concentration of thorium as the underlying bedrock. Such residuum and associated soil develops over some parts of the Elbrook, Conococheague, and Beekmantown Formations, and can have concentrations of radon in soil-gas exceeding 4,000 pCi/L. In areas of the Great Valley underlain by siltstone, fine-grained sandstone, and shale of the Martinsburg Formation, soil-gas radon values can exceed 4,000 pCi/L. In these areas, bedrock alone appears to have sufficient thorium, radium and uranium concentrations to generate the soil-gas radon measured. Previous work by others and our own preliminary evaluations indicate that soil-gas radon levels are high enough to cause indoor air in homes to exceed 4 pCi/L, the U.S. Environmental Protection Agency’s (EPA) action level for radon. Aeroradiometric maps and National Uranium Resource Evaluation (NURE) Program data do indicate anomalously high radioactivity in some areas where radon soil-gas concentrations were high. These data, used with available geologic maps, soil maps, and maps showing thickness of residuum, are useful in predicting areas of radon soil-gas hazards.