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Episodic incision punctuated by periods of base level stability during the Plio-Pleistocene left the Upper Cumberland River in Tennessee and Kentucky deeply entrenched into the unglaciated Appalachian Plateaus. The relative chronology of episodic river incision and base level stability is well documented thanks to over a century of careful mapping of upland surfaces, inset straths, and terrace gravels. Constraining the timing of these incision events has been difficult, however, primarily due to a lack of suitable dating methods for terrace materials ranging from several hundred thousand to several million years of age, and reworking of upland gravels onto lower terraces. These problems are solved by dating the burial age of undisturbed cave sediments in place of terrace deposits, using the differential decay of cosmogenic 26Al and 10Be in quartz exposed to cosmic radiation at the surface. This study offers a new chronology of river incision beginning with initial incision into the Highland Rim after ~3.5 Ma; development of the Parker strath between ~3.5 and ~2 Ma; incision of the Parker strath at ~2 Ma; development of a major terrace beneath the Parker strath between ~2 and ~1.5 Ma; incision into this terrace at ~1.3 Ma; and the development of several discontinuous terraces above the modern flood plain between ~1.3 Ma and the present.
Large caves on tributaries of the Upper Cumberland River record a headward wave of incision in the Pliocene and Early Pleistocene. The passage of a knickpoint in the system is modeled as a perturbation to steady-state incision according to the stream power law, which is tested against the abandonment dates in seven caves. Model results for m/n = 0.68 are within previously published theoretical and empirical values of 0.5 to 1.0, but suggest that values for the drainage-area exponent m are several times higher
than previous studies. This may be caused by a stronger variance of discharge to drainage area in fluviokarst reaches compared with non-karst watersheds. Knickpoint migration rates in limestone bedrock channels of fluviokarst tributaries to the Cumberland River are calculated between 10-18 cm/year during the Plio-Pleistocene, with m = 1.91 and m/n = 0.79.
Identifying cave levels provides insight into cave development and climatic changes that have affected a karst system over time. Cosmogenic dating has been used to interpret levels in Mammoth Cave and the Cumberland Plateau. This absolute dating technique has proven successful in determining cave paleoclimates and regional geomorphic history, but is expensive. The study presented here is a preliminary method to cosmogenic dating that can outline a region’s speleogenesis using a Geographic Information System (GIS) and published denudation rates. The Carter Cave system in northeastern Kentucky is within the karst landscape found along the western edge of the Appalachians and contains multiple daylighted caves at various elevations along valley walls. These characteristics make the Carter Caves an ideal location to apply GIS to cave level identification and evolution as described by Jacoby et al. (in review), who identified the cave levels within the area. The authors concluded that an argument can be made for either four or five cave levels in the Carter Cave system; however, studies identified four levels in both Mammoth Cave and the Cumberland Plateau. Further analysis indicated that the fifth level formed as a result of a change in lithology rather than an event that influenced the local base level. This research is an extension of the conclusions presented by Jacoby et al. (in review). The GIS was used to calculate the volume of surficial material lost within each level as a result of degradational geomorphic processes. Then, level thickness lost and published denudation rates were used to calculate the relative time required to form each level. There was not one denudation rate applicable to each level within the cave system, but the rates varied between 12 m/Ma and 40 m/Ma. This study concludes that the cave system took between 3.4 and 5.7 Ma to form. This study did not perform an absolute dating of cave sediments or assess any detailed stratigraphic influence.
Cave level delineation often yields important insight into the speleogenetic history of a karst system. Various workers in the Mammoth Cave System (MCS) and in the caves of the Cumberland Plateau Karst (CPK) have linked cave level development in those karst systems with the Pleistocene evolution of the Ohio River. This research has shown that speleogenesis was closely related to the base level changes driven by changes in global climate. The Carter Caves Karst (CCK) in northeastern Kentucky has been poorly studied relative to the MCS to the west and the CPK karst to the east. Previously, no attempt had been made to delineate speleogenetic levels in the CCK and relate them to the evolution of the Ohio River. In an attempt to understand cave level development in CCK we compiled cave entrance elevations and locations. The CCK system is a fluviokarst typical of many karst systems formed in the Paleozoic carbonates of the temperate mid-continent of North America. The CCK discharges into Tygarts Creek, which ultimately flows north to join the Ohio River. The lithostratigraphic context of the karst is the Mississippian Age carbonates of the Slade Formation. Karst development is influenced by both bedding and structural controls. We hypothesize that cave level development is controlled by base level changes in the Ohio River, similar to the relationships documented in MCS and the karst of the Cumberland Plateau The location and elevation of cave entrances in the CCK was analyzed using a GIS and digital elevation models (DEMs). Our analysis segregated the cave entrances into four distinct elevation bands that we are interpreting as distinct cave levels. The four cave levels have mean elevations (relative to sea level) of 228 m (L1), 242 m (L2), 261 m (L3), and 276 m (L4). The highest level—L4—has an average elevation 72 m above the modern surface stream channel. The lowest level—L1—is an average of 24 m above the modern base level stream, Tygarts Creek. The simplest model for interpreting the cave levels is as a response to an incremental incision of the surface streams in the area and concomitant adjustment of the water table elevation. The number of levels we have identified in the CCK area is consistent with the number delineated in the MCS and CPK. We suggest that this points toward the climatically-driven evolution of the Ohio River drainage as controlling the speleogenesis of the CCK area
This manuscript offers preliminary geochemical evidence that investigates the potential for hypogene speleogenesis in the Cumberland Plateau of southeastern Kentucky, U.S.A. The region was traditionally considered a classic example of epigenic karst, but new insights have uncovered tantalizing observations that suggest alternatives to simple carbonic acid speleogenesis. Such first-order observations have included natural petroleum seeps at the surface and in caves, occasional cave morphologies consistent with action of hypogene fluids, and prolific gypsum within cave passages. To this point, geochemical data from caves and springs verify carbonic acid as the primary dissolutional agent; however, these same analyses cannot rule out sulfuric acid as a secondary source of dissolution. In this paper, Principal Component Analysis of ionic data reveals two components that coordinate with parameters associated with “karst water” and shallow brine. In contrast, molar ratios of Ca+ and Mg+ as compared to HCO3 - and SO4 2- closely follow the reaction pathway stipulated by the carbonate equilibria reactions. Despite these data, the role, if any, of hypogene speleogenesis in the karst of the Cumberland Plateau remains inconclusive. It is very likely that carbonic acid dominates speleogenesis; however, contributions from sulfuric acid may influence our understanding of “inception” and carbon flux within these aquifers.
In this study, the concentration and isotopic composition of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) are measured in the karst groundwater of the Otter Creek watershed of the Cumberland Plateau of Kentucky, USA. Comparisons among these data and with the geochemistry of carbonate and gypsum equilibrium reactions reveal that DOC concentration is inversely related to discharge, multiple reaction pathways provide DIC with isotopic enrichment that may be directly related to mineral saturation, and oxidation of reduced sulfur is possible for dissolution. DOC is derived from C3 vegetation with an average δ13C DOC of ‒27‰. DIC in groundwater is derived from both pedogenic CO2 and HCO3 - from dissolved carbonate. At input sites to the karst aquifers DIC concentrations are expectedly low, less than 1 mmol/L, in waters that are undersaturated with respect to calcite. At the output of these karst aquifers DIC concentrations reach 3 mmol/L in waters that are at or above calcite saturation. Values of δ13C DIC range between ‒6.3 and ‒12.4‰ with CO2 degassing and calcite precipitation at some sites obfuscating a simple relationship between δ13C DIC, discharge, and mineral saturation. In addition, concentrations of DIC in sulfur seeps within the watershed range between 2–7 mmol/L with δ13C DIC values in some samples skewed more toward the anticipated value of carbonate bedrock than would be expected from reactions with carbonic acid alone. This suggests that the oxidation of reduced sulfur from shallow oilfield brines liberates bedrock DIC through reactions with sulfuric acid.