Structural analysis and mechanical origins of gilgai at Boorook, Victoria, Australia

Abstract

Gilgai microrelief at an undisturbed site is described. Surface and subsurface structural patterns in a duplex soil (solodic planosol) are analysed using principles and techniques adapted from structural geology. The microrelief is situated on a gently dipping regional slope and some mounds show evidence of surface erosion. Large cracks on the ground surface have an orientational relationship with the strike and dip of the regional slope. Some of the surface cracks may post-date the original time of gilgai formation. Beneath the microrelief surface a lenticular gravel layer of sedimentary origin is folded into a series of anticlines and synclines. Compositional layers also have fold-like geometries. Two-dimensional crack pattern analysis on a vertical section through several mounds, revealed upwards fan-like and concentric-convex patterns, where upwards movement had occurred. Three-dimensional measurements were made on monoliths and on an undisturbed block sample. The mean slickensided shear plane dip below a mound is 43° (standard deviation 9°). There is a statistical decrease in shear plane dip with depth, which is in partial agreement with soil mechanics theory. Shear planes are distributed from 0.3 m below ground surface down to a projected limit of 1.3 m to 1.8 m which is shallower than the limits reported for slickensided cracks in non-gilgai soils of Israel. The mechanism proposed to explain the surface and subsurface structures involves moisture concentrations that focus near, and below pre-gilgai surface cracks and a gravel lens. The moisture interacts with a swelling component and a triaxial state of stress is achieved. Differences between lateral and vertical stresses due to swelling pressures and overburden loads are sufficient to cause small, inclined shear displacements in definable depth zones. Accumulations of vertical movement components arising from the shear displacements, and vertical sliding of blocky non-sheared units nearer the ground surface, cause the gilgai microrelief and fold-like deformations in the soil profile. Zones of possible downward movement are located at the margins of mounds. Mechanical models proposed in the past are assessed in the light of the Boorook findings. Some proposals appear to be mechanically unlikely for the Boorook gilgai.