NUMERICAL MODELING AND FIELD MEASUREMENTS OF TOPSET AND FORESET-DOMINATED DELTAS (Invited Presentation)

Delta deposits are complex, in part because delta stratigraphy can vary significantly depending on conditions like the offshore bathymetry or the type of river feeding the deposit. Following recent work, we separate delta stratigraphy into two basic types depending on the relative thicknesses of the topset and foreset. If the ratio of the topset thickness to foreset thickness is greater than one, the delta stratigraphy is dominated by the character of the topset. If the ratio is less than one, then the character of the foreset dominates. 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. We test the predictions of the geometric model with simulations from a morphodynamic numerical model and stratigraphic sections from the Eocene Sobrabe Fm. The numerical model results confirm the geometric model predictions. Deltas forming over shallowly sloping shelves (< 0.01 degrees) are topset-dominated, their internal sand bodies have low connectivity, and the vertical coarsening upward sequence commonly associated with deltas is obscured. Deltas forming over steeply sloping shelves (> 0.5 degrees) are foreset-dominated, their internal sand bodies are connected, and they exhibit the classic coarsening upward signature. The character of deltaic stratigraphy in the Sobrabe Fm. is also consistent with the topset/foreset-dominated model. The stratigraphic character of delta deposition transitions from topset- to foreset-dominated moving across the low-sloped inner shelf to the steep-sloped shelf-edge. Furthermore, this stratigraphic transition is predicted by numerical models and is fundamentally related to the shelf slope. We suggest that the shelf slope may exert a first order control on delta stratigraphy.