A modified Waldron test based on sliding skewness for determining asymmetric cycle in a turbidite section

Abstract

The non-parametric ‘sign-of-difference’ Waldron test, conventionally used for identifying asymmetric cyclicity (thinning-or thickening-up) in turbidite successions, often fails to offer convincing results either when bed thickness distribution is lognormal or where positive and negative cycles are present in equal abundance. A modified Waldron test based on sliding skewness method (taking a number of bed thickness at a time) is proposed here for determining asymmetric cycles in turbidite successions, which appears to be random in their bed thickness distribution based on conventional Waldron test. The method has been tested on two turbidite section, which belong to two different settings of a single submarine fan system (Andaman Flysch Fan) that is known to have a lognormal bed thickness distribution. Application of the sliding skewness methodology reveals thinning-upward asymmetric cycles in both of these sections, which corroborate with the field observations and paleoenvironmental interpretations. Statistical significance for the proposed methodology was determined on the studied sections both through the Z-value and appropriate two-tailed null hypothesis. It is proposed that the result (thickening or thinning upward) is invariant with the interval (number of bed thickness taken for calculation) chosen for the sliding skewness calculation. This new method can be used on bed thickness measurement data from any turbidite succession in addition to other tests for identifying asymmetric cyclicity where non-parametric Waldron Test fails.