Non-Equilibrium Source-to-Sink Systems: Controls and Examples

Martinsen, Ole J.; Sømme, Tor

Source-to-sink analysis is a method for understanding recent and ancient erosional-depositional systems. An assumption in the analysis is that systems reach equilibrium in terms of sediments their catchments deliver, and which are deposited within their sinks. Equilibrium source-to-sink systems require substantial time to develop, preferentially occur in certain climatic and tectonic terranes, and provide bias for confident analysis of ancient systems because many systems may be non-equilibrium.

Various non-equilibrium situations occur, and their controls must be understood in order to avoid flaws in the analysis of ancient source-to-sink systems. One control is where there exist significant sediment sinks en route to the ultimate sink, commonly a function of tectonics and the margin type the source-to-sink system occurs on. Another situation is where climate controls the catchment in such a way that there is significant under-delivery of sediments. The third situation may be where there has been insignificant time to produce and transport a substantial volume of sediments to the sink, and this situation may be particularly common where there also has been a climatic shift.

The Tana River catchment and Tana Delta sink in Arctic Norway occur in an area with ongoing isostatic uplift and relative sea-level fall with the river incising. The catchment is approximately 16400 km2 while the approximate volume of sediments deposited in the delta front is 4 km3. The Tana system is non-equilibrium for the size of the catchment and undersupplied in the sink, despite the relative sea-level fall. The controls are: (i) undersupply in the short time since the last glacial maximum at 18000 B.P., (ii) significant storage of sediments in the river valley, (iii) an Arctic climate with low erosion rates where the fluvial system freezes up in the winter, a very high peak/average discharge ratio making yearly sediment delivery to the delta front inefficient, and (iv) a delta front dominated by very slow and inefficient gravitational processes such as creep and gliding. The result is that the incising valley and forced regressive delta has no submarine fan in front, contrary to conventional models.

In conclusion, source-to-sink system may be non-equilibrium for one or a combination of reasons. Understanding these controls is imperative for confident application to ancient systems and consequently for prediction in hydrocarbon exploration.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013