Morphometric Analysis of Polygonal Karst in New Guinea

Abstract

An attempt is made to analyse the planimetric morphometry of humid tropical karst in eight districts in east New Guinea. The geology, process environments, and morphology of sample localities provide a background for judging the morphometric results. The morphometry section includes information on procedure, spatial dispersion and association analysis, and terrain differentiation; it concludes with a growth model for polygonal karst. Closed depressions completely pit the landscapes and, when delimited on the basis of their topographic divides, form a cellular network termed polygonal karst. These cells tend more toward pentagons than hexagons. The stream-sinks associated with the depressions have dispersion patterns that tend toward uniformity. This probably comes about through competition. Each polygonal cell is a solution depression that drains centripetally to an internal stream-sink, and some striking morphometric similarities to “normal” drainage basins are observed. Correlation and regression techniques demonstrate that various morphologic parameters have a well-defined and generally similar form of association in each of the sample districts. There is no sign of the landscape chaos often ascribed to karst. Yet despite organizational similarities, multiple range tests indicate significant differences in terrain statistic values. The range tests permit the identification of three polygonal karsts: pinnacle, conic, and linear styles or subsets. The disjoint pinnacle and conic subsets are intersected by linear subsets, which thus possess intermediate landscape types. The conic and linear styles correspond roughly to “cone karst” and “directed karst,” respectively. A growth model for polygonal karst is proposed that envisages scattered small depressions on an uplifted surface expanding and capturing smaller neighbors until the entire surface is occupied by adjoining polygonal depressions. In the majority of cases, depression growth leads to no significant change in external shape, although it is often accompanied by increasing internal asymmetry about the stream-sink. Parallel retreat of slopes is suspected in one locality only. The boundaries of the depressions are largely stabilized by competition, and growth of the depression is also limited by the permeability of karstified rock, with large depressions breaking into smaller units if a certain size is exceeded. Once a cellular polygonal karst is established, its geometry undergoes only minor modification through time, unless altered by an important environmental factor.