Deepwater Channel-Lobe Transition Zones (CLTZs): Loss of Lateral Flow-Confinement Controls Patterns of Erosion and Deposition

Abstract

On the source to sink flow-trajectory, turbidity currents are initially laterally confined by canyon walls or channel levees. At some point they lose their confinement and begin to spread laterally, which reduces their capacity to keep sediment in suspension and promotes formation of sediment lobes. However, in some turbidite systems the channels are separated from lobes by a CLTZs, characterized by large fields (3 to >100 km) of erosive features such as scours and erosional lineations. Bed morphology suggests that the structure of the unconfined flow in the CLTZ is similar to that of the channel system, rather than the unconfined flow that developed further downstream. As such, there appears to be a lag between the loss of lateral confinement and response in the currents flow structure. This implies that the rate of confinement loss may be a significant factor of the resulting depositional morphology (e.g. splays versus lobes), yet there is a distinct lack of physical modelling and direct measurements of the flow to confirm this hypothesis.

Here we present the flow structure, along with the erosional and depositional patterns, from a series of experimental turbidity currents in a three-dimensional flume that is divided into a confined and an unconfined section. The rate of confinement loss was varied to study its impact on the systems morphodynamics. Sediment suspensions used to generate the currents were held constant, composed of 140 μm sand (d50) with a concentration of 17 %(vol.). Longitudinal and lateral variations in flow fields are mapped with an array of ultrasonic Doppler profilers and erosional/depositional patterns are mapped using a laser scanner. Results suggest that turbidity currents exhibit a rapid flow collapse upon loss of lateral confinement from the channel walls, and that this flow collapse is associated with a dynamic mechanism that enhances sediment scour and suspension. However, the degree of collapse and the depositional morphology is dependent on the rate in confinement loss.

The results are the first measurements of a flow mechanism that credibly explains the erosional features often observed in CLTZs. Improved understanding of the formation of CLTZs will improve the reconstructions and interpretations of CLTZs from the rock record as well as the verification of reservoir characterizations of turbiditic deposits close to areas of lateral confinement change.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018