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Keywords:

  • Debris flow;
  • flow transformation;
  • slump;
  • turbidity current;
  • Waitemata

Abstract

The process by which slumps transform into other flow types is an understudied phenomenon; generally, sedimentologists have assumed that this is how many debris flows and turbidity currents form, yet there is a paucity of information relating to the specific processes involved. This paper aims to redress this imbalance and investigates the processes of slump flow transformation using a well-exposed example, where the precursor slump and flows to which it was transforming have been preserved in the outcrop. A detailed field investigation of the Lower Miocene, Little Manly Slump, located within the Waitemata Basin, New Zealand, reveals a complex bi-partite deposit, comprised of a lower slump-debrite unit and an upper turbidite unit, separated from one another by an impersistent contact. Reconstruction of slump evolution, from preserved strain indicators, shows that slump motion was unsteady and non-uniform, and that the slump arrested rapidly. Flow transformation is directly linked to slump evolution and progressed through the multiple processes of body transformation, fluidization transformation and surface transformation. The resultant flow comprised a very dense lower unit with slump and debris flow phases overlain by a turbidity current. This study shows that flow transformation did not result in en masse transformation to a debris flow by a single process, but rather it was characterized by partial transformation of the slump to generate a three-phase flow. Density is thought to be the key parameter in controlling flow transformation efficiency, which appears to have varied rapidly as a function of slump unsteadiness and non-uniformity.