Slump folds as estimators of palaeoslope: a case study from the Fisherstreet Slump of County Clare, Ireland

ABSTRACT

Folds and faults preserved within slump horizons are thought to be the only sedimentary structures that primarily reflect palaeoslope direction. By evaluating methods for the analysis of slump folds, the robustness of many palaeoslope and palaeogeographical reconstructions can be tested. Five methods for estimating palaeoslope direction from slump fold orientation data are tested with reference to the Upper Carboniferous Fisherstreet Slump horizon of County Clare, Ireland. These methods are the Mean Axis Method, the Separation Arc Method, the Downslope Average axis Method, the Axial–planar Intersection Method and the Fold Hinge Azimuth and Interlimb Angle Method. Transport determination techniques have mixed success when applied to this example and reveal a mean downslope transport direction of 067°. This result reflects the varied boundary conditions that are possible during slumping, resulting from variations in initial fold geometries, progressive fold formation, spatial and temporal changes in velocity, material properties, pore fluid pressures and the local slope. Without a comprehensive analysis of slump data – using all available techniques – large errors in interpreted transport direction can exist and may invalidate palaeogeographic reconstructions. It is essential to use as many transport determination techniques as possible to determine slip direction accurately. With this in mind, a general procedure for determining palaeoslope directions is suggested and used to examine existing palaeogeographic models for the Clare Basin.