Flow
Transformations in Slumps: a Case Study from the Waitemata
Strachan, Lorna J.1
(1) The
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 presentation aims to redress this
imbalance and investigates the process 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 a semi-continuous 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 (sensu Fisher, 1983). 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 was characterised 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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California