ABSTRACT The geology of Great Smoky Mountains National Park (GRSM) in Tennessee and North Carolina is dominated by siliciclastics and metamorphic strata. However, in the western portion of GRSM, a series of carbonate fensters (windows) expose the Lower Ordovician–age section of the Knox Group, a series of dolomite and limestone units that are partially marbleized as a result of contact metamorphism from the Great Smoky fault. The fensters create opportunities for allogenic recharge to occur at points along the contact of the surrounding insoluble strata with the underlying soluble carbonates. The combination of chemically aggressive surface recharge and vertical relief has resulted in the formation of deep caves, many of which have active streams and water resources. Though the karst is limited in extent and the number of caves is fairly small, the significance of the resources is substantial, with several of the caves in the area over 150 m in depth and at least two being major bat hibernacula. In 2017, the U.S. Geological Survey (USGS) began a study to better understand the hydrologic behavior of these karst systems through hydrologic and geochemical monitoring, groundwater tracing using fluorescent dyes, and seepage runs. Stage and water-quality instrumentation was installed in two caves in GRSM, the main stream of Bull Cave, and in a sump pool in Whiteoak Blowhole, at 173 m and 70 m below land surface, respectively. Following setup of the cave sites, dye injections were conducted to determine discharge points for four of the deep cave systems on Rich Mountain and Turkeypen ridge. Results show water in these systems has an extremely rapid travel time, with tracers detected from caves to springs in less than 24 h for each of the systems. This field guide describes the complex geology, regional hydrogeology, and unique landscape characterized by high-gradient subterranean streams, carbonate fensters, and deep caves of the GRSM karst.

ABSTRACT The southern Appalachian western Blue Ridge preserves a Mesoproterozoic and mid-Paleozoic basement and Neoproterozoic to Ordovician rift-to-drift sequence that is metamorphosed up to sillimanite grade and dissected by northwest-directed thrust faults resulting from several Paleozoic orogenic events. Despite a number of persistent controversies regarding the age of some western Blue Ridge units, and the nature and extent of multiple Paleozoic deformational/metamorphic events, synthesis of several multidisciplinary data sets (detailed geologic mapping, geochronology and thermochronology, stable-isotope chemostratigraphy) suggests that the western Blue Ridge likely records the effects of two discrete orogenic events. The earlier Taconic (470–440 Ma) event involved a progression from open folding and emplacement of the Greenbrier–Rabbit Creek and Dunn Creek thrust sheets as a foreland fold-and-thrust to low-grade hinterland system (D 1A ), followed by deep burial (>31 km), pervasive folding of the earlier-formed fault surfaces, and widespread Barrovian metamorphism (D 1B ). Because this high-grade (D 1B ) metamorphic event is recorded in Ordovician Mineral Bluff Group turbidites, this unit must have been deposited prior to peak orogenesis, possibly as a foreland basin or wedge-top unit in front of and/or above the developing fold-and-thrust belt. The later Alleghanian (325–265 Ma) event involved widespread northwest-directed brittle thrusting and folding related to emplacement of the Great Smoky thrust sheet (D 2 ; hanging wall of the Blue Ridge– Piedmont thrust). Mid-Paleozoic 40 Ar/ 39 Ar muscovite ages from western Blue Ridge samples likely record post-Taconic cooling (hornblende and some muscovite 40 Ar/ 39 Ar ages) and/or Alleghanian thrust-related exhumation and cooling (ca. 325 Ma muscovite 40 Ar/ 39 Ar and 300–270 Ma zircon fission-track ages), as opposed to resulting from a discrete Neoacadian thermal-deformational event. The lack of evidence for a discrete Neoacadian event further implies that all deformation recorded in the Silurian–Mississippian(?) Maggies Mill–Citico Formation must be Alleghanian. We interpret this structurally isolated sequence to have been derived from the footwall of the Great Smoky fault as an orphan slice that was subsequently breached through the Great Smoky hanging wall along the out-of-sequence Maggies Mill thrust.