A Rational View of Alluvial Sequence Stratigraphy
Dave G. Quirk
The application of sequence stratigraphic principles to alluvial strata has proved to be far from simple. At present there is little consensus as to the influence that changes In relative sea-level have on fluvial deposition and erosion. Furthermore, current models rarely describe how continental processes, such as climate and hinterland tectonics, affect accommodation space in the alluvial environment. This paper addresses these issues and provides some simple solutions for how alluvial strata can best be correlated and interpreted in a sequence stratigraphic framework.
One of the main points that will be made is that accommodation space is created for alluvial sediment by one or more of the following processes:
1 subsidence within the drainage basin;
2 decreased river discharge;
3 increased coarse-grained sediment input;
4 eustatic sea-level rise;
5 coastal progradation.
Accommodation space is removed when the converse, such as uplift, happens. The creation and removal of accommodation space leads to fluvial aggradation and degradation, respectively. These in turn affect the amount of coarse-grained sediment that reaches the coast.
It is fairly straightforward to assess the importance of 1-5 in the deposition of any particular alluvial system tract. For example, an alluvial systems tract deposited because of an increase in the amount of coarse-grained sediment (3) will be characterised by a) erosional surfaces or hiatuses at the base and top; b) pinch-out towards the coast; c) downlap of strata; d) upwards-thickening and upwards-coarsening of strata; e) braided channel deposits towards the top; f) little evidence of marine influence.
Present-day and ancient geological examples will be used to illustrate the main points and a new model for alluvial sequence stratigraphy will be presented.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California