2.1 Formation of Karstic Conduit Flow Systems

The formation of karst begins with flow in a network of connected fractures, such as open joints or along bedding plane openings (fractures) originating from rock stresses during crustal movements (for example, as shown in the middle-left photo on the cover of Glen Rose Limestone as seen along the channel of Canyon Gorge, Texas and notice fracture joints with vegetation). There are interconnected fractures with groundwater flow in other types of rock such as granite, but karst does not form because granite and most other igneous rock formations do not contain minerals soluble enough for substantial dissolution. After some time, the enlarged fractures in soluble rocks become connected to form solution channels and caves, which then provide the main paths for groundwater flow.

Before any solution enlargement has occurred in a fractured limestone, fresh groundwater originating from rainfall infiltrates through surficial soils then flows through a network of fractures, joints and bedding planes, as is the case in all fractured sedimentary rock. In order to dissolve rock, the water must be chemically aggressive, that is, thermodynamically undersaturated with respect to the mineral calcite (CaCO3). Carbon dioxide (CO2) is continually produced in the vadose zone from microbial decay of organic matter and from respiration of plant roots. That CO2 combines with water to form carbonic acid (H2CO3) which dissolves calcite. This process is well established in soil science.

Given the presence of chemically aggressive water, karst development proceeds by hydrogeochemical processes that involve complex interactions through time and space. Comprehensive discussions of the processes of fracture enlargement and cave genesis are provided by Holland and others (1964), Howard (1964a, 1964b), Thraikill (1968), Ford and Cullingford (1976), and Palmer (1999).

Long periods of time are required for solution channels to form a conduit flow system because, although recharge water is chemically aggressive, it quickly dissolves calcite and loses its chemical aggressivity, unless conditions change such that the water becomes undersaturated with respect to calcite. There are two ways that conditions may change: 1) chemically aggressive water may form at depth, or 2) the water table may decline allowing unsaturated zone processes to be active deeper in the system.


Introduction to Karst Aquifers Copyright © 2022 by Eve L. Kuniansky, Charles J. Taylor, and Frederick Paillet. All Rights Reserved.