Solution Pipes and Focussed Vertical Water Flow in Carbonates with High Matrix Porosity

White, Susan1, Lipar, Matej 2, Szymczak, Piotr 3,  Webb, John 1

1Environmental Geoscience, Department of Ecology, Environment and Evolution, La Trobe University, Melbourne, Bundoora, Victoria, 3086, Australia, 2Anton Melik Geographical Institute, Research Centre of the Slovenian Academy of Sciences and Arts, Gosposka ulica 13, SI-1000 Ljubljana, Slovenia, 3 Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, 02-093 Warszawa, Poland

Focused vertical flow is one of the distinctive features of karst processes within porous calcareous media with matrix porosity, especially in calcarenites, and forms distinctive features termed solution pipes. These are vertical cylindrical voids of variable diameter and depth predominantly in the vadose (epikarstic) zone. Although dissolutional in origin, the initial trigger that focuses the water flow in the host rock matrix is poorly understood, although they appear to be related to vegetation and rock heterogeneities.

Measuring of the morphology of pipes e.g. pipe diameter, depth, density and shape,  is common but has not resolved the problem and neither has a 2D statistical data based on 5 x 5 m area in Australia which included inside and outside diameter of the pipes, their elongation, the thickness of cemented rim, and distance and direction to nearest neighbour. The stratigraphy, mineralogy, fabric and geochemistry of the surrounding rock and pipe rims and fills provide insights into pipe formation but do not address the issue of focussed flow. Thin section and X-ray diffraction has been used to distinguish between rim and host rock fabric. Stable isotope analysis partly correlated with host rock age indicates that the dissolutional process is under vadose conditions. Dating of the rim cement has not been successful due to the minute size of the carbonate cement samples. And the rock clasts of the host rock are obviously older.

Triggers to flow include lithological irregularities e.g. cracks in calcrete, surface geomorphology e.g. potholes and vegetation e.g. stem flow, buried tree trunks. If these are not present the instability of the wetting front infiltrating into the unsaturated zone breaks advances into unsaturated host rock as self organised fingers. This fingered flow, a non-uniform accelerated transport of sinking water, through the porous material due to an unstable wetting front, is particularly favoured if the media is water repellent, which causes uneven infiltration.

The roles of dynamic instabilities and self-organisation in the emergence of focusing patterns and the various challenges, especially in the vadose zone, can probably be only resolved with modelling, conjunction with field data. From the numerical point of view, modelling of solution pipe formation requires solving the coupled equations for the groundwater flow, chemical transport and porosity evolution. The ground-water system may be either saturated, or partly or completely unsaturated. Morphologically similar pipes seem to develop under both conditions. Pipes formed in deglaciated Miocene sediments in Poland  can be modelled using Darcian flow equations. However the pipes found in unglaciated areas e.g. Cape Bridgewater, Victoria, were clearly formed by dissolution in the unsaturated (vadose) so this model is inappropriate. However the Richards equation applies a  continuity requirement resulting in a general partial differential equation describing water movement in unsaturated non-swelling media. This appears to be the best option for modelling  water flow in the vadose zone

As most solution pipes form under vadose conditions and show potential as palaeoclimatic indicators, further understanding of their formation and behaviour is important.


Biography

A karst geomorphologist  who has worked on features and processes in high matrix porsity  Neogene and  Pleistocene carbonates for many years.

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