AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Defining the First Order Controls on Delta Stratigraphy
(1) Earth and Environmental Sciences, Boston College, Chestnut Hill, MA.
(2) Geological Sciences, University of Texas, Austin, TX.
Deltas are among the most important sedimentary deposits because they record changes across marine and terrestrial environments, and they commonly are the site of oil and natural gas accumulation. However, interpreting delta deposits is not straightforward, in part because delta stratigraphy can vary significantly depending on environmental conditions like the offshore bathymetry or the type of river feeding the deposit. Here we propose that there are two basic types of delta stratigraphy based on the relative thicknesses of the topset and foreset. If the ratio of the thickness of the topset to foreset is greater than one, the delta stratigraphy is dominated by the character of the topset. If the ratio is less than one, then the stratigraphy is dominated by the character of the foreset. Based on this definition we use a geometrical model and field data to derive the environmental conditions that lead to the formation of topset versus foreset-dominated deltas. Our model and field data show that topset-dominated deltas contain deep distributary channels that commonly incise into pro-delta mud obscuring their foreset. Model results predict that topset-dominated deltas will form in shallow receiving basins with gently sloping bottom topography. Foreset-dominated deltas have foresets with large relief relative to the topsets and are commonly created by shallow rivers debouching into basins with steep bottom slopes. Based on the character of the world’s coastlines, our results suggest that topset-dominated deltas should be the dominant type, whereas foreset-dominated deltas will likely form in receiving basins with steep bottom slopes. These results have implications for stratigraphic interpretation because the presence of incision of distributary channels is often invoked as evidence of changes in climate or relative sea level, whereas if the delta is topset-dominated, the incision could be the natural result of delta growth.