Fluvial Stratigraphy at the Shoreline Interface

Abstract

Conceptual models of the interface between fluvial and marine systems tend to focus on deltas associated with regression and estuaries associated with transgression. Furthermore, the assumption is that the rivers are tributary and that it is a trunk channel that reaches the shoreline. River systems with distributive patterns are now recognised as important sites of fluvial deposition, but there are few modern examples of these systems reaching the sea. This absence from present day coastlines may be attributed to the recent sea level fluctuations,namely recent sea level fall and rise creating incised estuaries. However, re-examination stratigraphic successions that include fluvial and paralic facies suggests cases that do not fit either trunk-river-delta model or a trunk-river-confined estuary model.

1. The Carboniferous of NW Ireland shows a fluvial to tidally-influenced marine transition with no evidence of lateral confinement.

2. In S England Lower Cretaceous Wealden facies show several transitions from fluvial to tidally-influenced conditions over a wide area in an overall retrogradational succession.

3. The Aptian-Albian strata on on the Antarctic Peninsula is predominantly fluvial, but a unit interpreted as a shallow marine bar deposit within the succession indicates a transgression across a coastal alluvial plain.

4. Palaeocene coal-bearing successions on the island of Spitsbergen have been interpreted as a mixture of fluvial and coastal facies in repeated cycles across a broad area in an overall transgressive succession.

5. The Miocene of the Barito Basin in S Borneo show a transition from fluvial to tidally-influenced facies over an area of over 60 km perpendicular to palaeoflow, interpreted as a distributive fluvial system undergoing transgression to form a wide unconfined ‘estuary’.

Models of fluvial stratigraphy at the marine interface tend to become pigeonholed into the conventional sequence stratigraphic frameworks that assume a trunk channel at the shoreline and that sea level falls will incise into the coastal plain to confine fluvial and estuarine facies. Alternative relationships involving distributive fluvial systems reaching a shoreline and relative sea level rise flooding the coastal alluvial plain will result in different stratigraphic signatures. These relationships are likely to occur in any setting where creation of accommodation due to tectonic subsidence is dominant and there is a distributive pattern of fluvial sedimentation.

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