ON THE THEORY OF GROWTH FAULTING*: A GEOMECHANICAL DELTA MODEL BASED ON GRAVITY SLIDING

Abstract

A geomechanical delta model is presented that explains and permits quantitative reproduction of the main features associated with growth faulting. The model is based on a soil‐plasticity analysis of gravity sliding of overpressured clays and silts on very gentle delta slopes. In this analysis, the packets involved in gravity sliding —called ‘units’—are well quantified. The delta body may behave as a stack of such units (‘multi‐unit’ delta model), which can behave differently depending on such parameters as sedimentation rates, changes in lithology and compactional behaviour. Two prominent and essentially different structural expressions of the model are discussed: (a) the regularly spaced growth‐fault pattern, without recognisable toe regions; (b) the complete slide structure with a well‐developed toe region. The shapes of the roll‐over structures (particularly the positions of the crests), the thickening of the layers near the growth faults, antithetic faulting and horsetailing of growth faults can be derived from the model without the need to invoke any deeper seated mechanism in the substratum. The basic assumptions of this geomechanical model are supported by observations in Ireland and examples from literature. It may contribute to the reconstruction of the hydrocarbon migration history in a delta by accounting for the synsedimentary development of stresses and fault structures that may control hydrocarbon migration. Recently, geomechanical aspects came in focus again for the assessment of seismic amplitude anomalies in the search for hydrocarbons (Crans and Berkhout, 1979).