Small Lowstand Deltas as a Consequence of Sustained Alluvial Aggradation During Relative Sea-Level Fall

Andrew L. Petter¹, Tetsuji Muto², and Ronald J. Steel^1
1Department of Geological Sciences, The University of Texas at Austin, Austin, TX
²Faculty of Environmental Studies, Nagasaki University, Nagasaki, Japan

Physical and numerical (geometric) modeling of fluviodeltaic systems, conducted under constant rates of relative sea-level fall, sediment discharge, and upstream water discharge, demonstrates that sustained alluvial aggradation is the inherent stratigraphic response when the gradient of the alluvial plain exceeds the gradient of the shelf. Expansion of the alluvial plain during relative sea-level fall causes the sediment budget to be increasingly partitioned subaerially prior to reaching the delta. Therefore, the resulting lowstand deltas will be smaller than preceding highstand deltas. 2-D geometric modeling (based on geometrical parameters of the fluviodeltaic wedge, rate of relative sea-level fall, and sediment discharge) shows that lowstand-delta foreset heights are 37% and 45% smaller (following 50 km and 100 km of forced regression, respectively) when rivers are 10% steeper than the shelf as opposed to 10% gentler than the shelf. Given sufficient shelf width, some deltas will disappear entirely so that further regression causes the shoreline to become detached from the alluvial river. High alluvial gradients are primarily a consequence of high sediment supply relative to available water discharge (i.e., low rates of terrestrial diffusion). The Canterbury Plains of New Zealand is a natural example of such a system. Extensive alluvial aggradation has been documented in the Canterbury Plains from Quaternary forced-regressive strata. Our model suggests that standard sequence-stratigraphic models, which predict increased sediment supply to lowstand deltas, may fail to accurately represent fluviodeltaic systems with extreme sediment supply. Rather, deltas with these geometric parameters become poorly supplied during forced regression, and large lowstand deltas should not be expected for these sequences.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas