The fluvial-tidal transition and its stratigraphic significance

Introduction

The transition from fluvial to tidal environments includes a relatively wide zone characterised by the interaction of unidirectional fluvial currents, bidirectional tidal currents and other shallow-marine processes. This zone can range from few kilometres (e.g. Fly River delta is 80-100 km) to hundreds of kilometres (e.g. Gambia River estuary is 200-250 km) and occurs in both estuaries and deltas, albeit associated with different features in the two depositional settings. The fluvial-tidal transitional zone can be monitored in modern depositional environments; however, the zone remains poorly understood in ancient successions particularly in non-channelized settings.

A greater understanding of the fluvial-tidal transition zone will provide insight into palaeogeographies and basin filling history in ancient successions.  The work may lead to more effective modelling of oil and gas reserves within fluvial-tidal transitional deposits through a better understanding of the distribution of sedimentary facies and their associated reservoir properties.

Illustration of the fluvial-tidal transition zone (from Martinius et al., 2011).

Objectives:

Understanding the characteristics of the fluvial-tidal transition zone, its geometry, variability and any stratigraphic controls on its nature are the main aims of this research project. This study is focused in the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) that consists of tidal-dominated/-influenced deposits intercalated with fluvial deposits and overlain by the continental Challaco Formation, which thus offers an opportunity to investigate the fluvial-tidal transition zone and its character over several million years.  This project will examine the nature of the facies change through the fluvial to tidal transition, in order to develop a better set of criteria to classify this complex area.


Specific points to be addressed include:

  • How does grain size change through the transition?
  • In what order do the various marine indicators begin to be expressed seaward?
  • What impact do tidal currents have on reservoir character?
  • How does the transition change as a function of parasequence stacking pattern, etc?
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