Regressive Tidal Systems: Lessons Learned From Studying the Variability of Holocene Tide-Dominated and Tide-Influenced Deltas

Abstract

Tide-generated currents affect a large portion of the world’s intermediate to large deltaic systems. Most large deltas are, in fact, tide dominated. The effects of tides on depositional architecture, especially in regressive systems, remains poorly understood, which is a problem since many important hydrocarbon-bearing intervals were deposited under such conditions.

A recently compiled, global database of 60 mapped linear-to-lobate and 35 funnel-shaped shoreline systems, most of which are affected by tides, is used to identify key factors that impact tidal architecture in modern regressive systems. All studied systems have known tidal ranges, onshore and offshore bathymetric gradients, drainage basin areas, as well as well-understood geographical settings. Bathymetric maps allow observing the subaqueous expression of the deposits.

Tides can influence deposition on both a regional and a local scale. On a regional scale, tides can have an important effect on the nature of delta distributaries, which can in turn strongly influence shoreline architecture. The interaction between the position of (1) the tidal limit (the point where channels start feeling the effects of tides) and (2) the salinity limit (the point where channels become affected by salt water intrusion) relative to (3) the backwater limit (the point at which the bottoms of fluvial channels reach sea level) and (4) the point at which a system becomes unconfined (i.e., channels are not laterally restricted by a valley wall) can be especially predictive. Systems supplied by different river sizes, affected by different tidal ranges and controlled by different level of confinement can end up having very different distribution of sand depocenters. In some systems, most coarse-grained sediment will be stored as parts of channel belts on the delta plain, whereas in others the main sand depocenters can occur at the shoreline and beyond.

On a local scale, tides can have a strong influence on the nature of these sediment depocenters. They can directly influence variables such as channel width, channel depth and mode of migration of bar-scale features on the delta plain. They can also largely dominate deposition at the shoreline. An underappreciated effect of tides on depositional systems is their ability to scour deep channels both landward and seaward of the shoreline. Regional and local tidal effects will be illustrated with examples and the lessons learned will be applied to ancient case studies.

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