Genesis of Microfracture Evolution in Epikarst

Abstract:

Evolution of the epikarst begins with the enlargement of micro-fractures. As hydraulic channels become connected to an exit system, the hydraulic gradient will drive continued development of primary karst channels. But, what is the genesis of micro-fracture evolution into channelized paths? We investigate two mechanisms and their role in the physical and chemical evolution of the microfracture. During liquid drainage, air-water interfacial instability leads to development of capillary droplets embedded in rock surface films. Pressure gradient between capillary droplets and rock surface films drives liquid into droplets via the matrix skin. Droplet paths may predispose the rock phase to greater dissolution relative to paths that experience only film flow. Successive droplets down the same path, as seen in experiments, could lead to linear erosional features that evolve toward pipe development. The rate of geochemical dissolution by these liquid elements depends upon fluid carrying capacity and dissolution kinetics. One of the mechanisms important to calcite dissolution is diffusion of CO₂ from fracture air into the liquid elements. Exchange of fracture and atmospheric air can reset gas composition within the fracture system. A mechanism for regular flushing of microfracture air is suggested where nightly inverted thermal gradients in the upper 30 cm of rock near the rock-atmosphere interface, can trigger gas density instabilities within the microfracture that travel deeper into the epikarst, potentially flushing the entire microfracture system with atmospheric air. Such diurnal gas flushing would serve as a reset switch for microfracture-gas-chemistry towards atmospheric values on a daily basis. The net dissolution rate of microfractures, and the rate and spatial distribution of dissolved substances delivered to subsurface caverns, would depend on these two mechanisms of liquid motion and gas venting.