Introduction

Anastomoses are solutional features that are common in the low dip strata of Central Kentucky. Their abundance has raised questions as to their importance in the growth of caverns and passages in the region. This paper will examine anastomoses from three different viewpoints. The first section examines how anastomoses initially develop in unweathered limestone. The second examines how anastomoses effect the development of individual cave passages while the third shows their impact on cave systems. These sections are preceded by a definition of the term.

Definition

The name anastomoses is originally derived from anatomical terms (Cullingford 97). In the geological meaning, anastomoses are small undulating tubes that interconnect to form a maze-like pattern (Palmer 1981 117). They are planar solution features that tend to closely follow their parent bedding plane. In rocks of high primary porosity, however, anastomoses are often free to form in any direction relative to the plane of initial growth (Jennings 141). They can also occur along joints, but this form of genesis is uncommon (Cullingford 97).

The Formation of Anastomoses

Anastomoses are phreatic features (White 93) that form in areas of slow laminar flow (Jennings 141) and gentle dips of less than 5% (Dreybrodt 207). In areas with dips greater than 5%, the anastomoses are replaced with singular down dip tubes as flow velocity increases (Dreybrodt 207). Anastomoses initially begin as a network of tiny tubes with minimal flow and solution. As time progresses, the anastomoses undergo a period of dissolution and enlargement through the chemical reaction of limestone and carbonic acid. This process of enlargement is very important in the featurešs evolution. As an anastomotic tube enlarges it is able to move greater amounts of water, decreasing its hydraulic gradient. The amount of decrease is inversely proportional to the fifth power of the passage radius. This can be represented by the formula:

Hydraulic Gradient =   ___1____    
		       Radius 5			(Palmer 1975 117)

As can be seen, a minor increase in passage radius has a drastic effect on the hydraulic head. As the anastomotic tubes enlarge, they do so at slightly different rates dependent upon many factors, such as the initial size of the tube, the local changes in dip or the sedimentary properties of the rock. Growth can also be increased where anastomotic beds are bisected by joints (Cullingford 98). Eventually, various passages gain a size advantage over their neighbors by forming low spots in the potentiometric surface as the hydraulic head drops. Water flows from areas of high to low head, thus increasing the water input into these larger passages. This further increases the tubes discharge, allowing more contact with the undersaturated water that causes dissolution. In addition, a pirating effect acts upon competing passages, further inhibiting their growth as their source of water is removed. These processes eventually feed upon themselves until the dominant passages effectively stop all growth around them. Ultimately, a preferred path of flow evolves, forming a main passage in the center of the anastomotic zone that conducts most of the flow (Ewers 1788). The surrounding tubes decrease in size with distance from the central conduit, recording the legacy of the passages that lost the competition for flow (Palmer 1981 117).

Anastomoses and the Formation of Cave Passages

Anastomoses can play an influential role in the development of individual cave passages. As shown above, they may be the precursor to the processes that form the passage as flow becomes concentrated into single conduits. When this occurs, the passages are shown to be controlled by the bedding plane that initiated the anastomotic growth. Joints are also a common controller of cave passages. Solution features that enlarge these joints in the ceiling or floor are indications of joint control of a passage. Therefore, if there is an anastomoses layer along the highest bedding plane of a cave passage, the passagešs initial formation was controlled by bedding planes.(Dreybrodt 192-193).

Anastomoses are also important in the breakdown of a passage (Cullingford 98). If a series of successive bedding planes initiate anastomotic growth, a zone of weakness is established in the rocks due to the high concentration of voids. As the water down cuts from the upper anastomotic zones, growth is concentrated in the lower band of anastomoses. As the lower tubes enlarge, they place larger and larger stress zones on the ceiling. When the stress zones intersect the upper anastomoses beds they come upon the zone of weakness and breakdown is initiated. This will often leave a series of half tubes in the ceiling. These are the upper halves of the anastomoses in the plane of failure (Kozloski, Palmer 1981 142).

Anastomoses are also responsible for the formation of vertical rills in cave passages. During a flood event, water is forcefully injected into the anastomoses when the main conduit fills to its maximum capacity of discharge. As the flood abates, water drains from the anastomoses back into the main channel. As the water flows from the anastomotic mouths, it dissolves rills into the limestone just beneath the openings due to the aggressivness of the undersaturated flood water (White 93-94).

Anastomoses and the Development of Cave Systems

Anastomoses play several roles in the development of a cave system, the first being the role of flood storage. When the cave floods, water is stored in the anastomoses, a process analgalous to bank storage in a surface stream environment. During this time, several things happen to the anastomoses. Due to the high discharges during the flood, the water in the cave system is extremely aggressive. This water stagnates with little to no flow when injected into the anastomoses. As a result, the undersaturated water dissolves the entire anastomotic system at a uniform rate (Palmer 1991 15) and the anastomoses layer grows, extending itself further and further away from its parent passage. As the flood water loses velocity in the anastomotic zone, its capacity to hold sediment decreases. The anastomoses then preform the role of a sediment trap as the suspended clays and silts drop out of the ponded flow (Cullingford 98-99). These clays often fill the anastomoses entirely, leaving a deposit of block-like compacted clay (Kozloski).

Anastomoses are also important in bypassing constrictions or blockages in stream passages. As water backs up it is injected into the anastomoses and once inside it often finds one or more bypasses to the constriction. If no path exists, however, the aggressive floodwater dissolves deeper into the bedrock until a through-flow conduit forms around the constriction. The bypass routes have a lower head than the rest of the surrounding anastomoses and will pirate any smaller flow tubes in the same fashion as when the anastomoses formed. If the head difference is equal to or higher than the flow through the blockage or constriction, these routes will accept the discharge of the passage. Through this process an anastomotic maze will form. Eventually, one of the bypass routes will take on all the flow and will become the main passage, leaving the others abandoned except during floods.

The final way in which anastomoses can effect the development of cave systems is through their role in the change of flow routes. If two cave streams flow near each other, their anastomoses may grow during floods to interconnect with one another. During subsequent flood events, the water from one cave stream will overflow into the other as the anastomoses are filled with the flood pulse. This through flow will encourage the enlargement of the connection route until it takes on the flow of the stream with the higher head. The old stream passage will become an overflow route itself and eventually will be abandoned.

Summary

Anastomoses are phreatic features that form in areas of slow laminar flow and shallow dips. They are found mainly on bedding planes and form a maze of interconnected tubes. Anastomoses enlarge at nearly uniform rate until slight differences in individual tube characteristics allow preferential enlargement. Eventually anastomoses form cave passages as the larger tubes pirate flow from their neighbors.

Anastomoses are indicative of bedding plane control of a cave passagešs initial flow route. They form zones of weakness in the beds where they form, often speeding the processes of breakdown in the cavern. Anastomoses are important factors in cavern enlargement during floods, forming rills in the passages or anastomotic mazes and flood bypass routes. They act as sediment traps and play a large role in the changing of underground stream flow routes. They are important features in the development of subterranean karst in areas of shallowly dipping rocks.

Bibliography:

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Dreybrodt, W., Processes in Karst Systems, Berlin Heidelberg, New York, NY, 1988.

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