A Quantitative Assesment of the Effects of Base Level Fall and basin Depth on River-dominated Deltas

Open Access
Author:
Cederberg, James Andrew
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 24, 2014
Committee Members:
  • Rudy Slingerland, Thesis Advisor
Keywords:
  • deltas
  • rivers
  • base level fall
  • sea level fall
  • sequence stratigraphy
  • sedimentology
  • modeling
  • Delft3D
  • morphodynamics
Abstract:
A better understanding of how deltas form and their resulting morphologic and stratigraphic characteristics is needed to improve geoscientist’s abilities to manage deltas and their wetland systems as well as explore and develop hydrocarbon resources. Inherently, deltas form as shoreline regressions, making sea level cycle interpretation difficult. Here we seek to quantify the effects of relative base level fall (BLF) and basin depth on the morphology and internal geometry of river-dominated deltas in order create a model that can be applied to distinguish between deltas experiencing forced regressions and normal regressions. Doing so will allow us to more accurately interpret the sequence stratigraphic record. We propose measuring the relative influence of BLF and basin depth on delta formation through the shoreline trajectory. The shoreline trajectory is defined as the locus of points defined by the shoreline in the vertical plane. We find that as basin depth increases, the number of active distributaries decreases because river mouth bars take longer to aggrade leading to fewer bifurcations. Increased basin depth increases the avulsion period because it takes longer for enough sediment to be deposited such that a distributary channel becomes super-elevated and can avulse. Fewer active distributaries and longer avulsion periods lead to deposition being focused in one region for greater periods of time which results in more rugose shorelines and more variability in foreset dip directions. Deeper basin depths are associated with smaller average lobe areas because more sediment is required to form a lobe of equal areal extent in a deep basin than in a shallow basin. We find that the greater volume of sediment required in deeper basins outweighs the more focused deposition also associated with a deeper basin, thereby forming smaller delta lobes on average. We find that the thickness of topset iv deposits varies little with basin depth while the foreset thickness varies greatly. This leads to decreased volumetric topset/foreset ratios in deeper basins. Deeper basins have delta fronts that are less affected by tractional sediment transport resulting in larger clinoform average dip magnitudes. Higher rates of BLF results in elongate a deltas with greater topset roughness caused by down-stepping lobes. Higher rates or BLF are also associated with larger total topset areas. Twelve deltas simulated deltas are formed under different rates of BLF and basin depths using Delft3D, an engineering-grade, 2D vertically integrated hydrodynamic and morphodynamic model. The model is an improvement over earlier models because it accounts for multiple grain size fractions, cohesive sediment fractions, and bed stratigraphy. The model findings are validated with data collected from the Goose River Delta, sandy, fjord-style delta prograding into the 30m deep basin of Goose Bay, Labrador, Canada and experiencing 5 mm of BLF per year for the last 8000 years. A re-interpretation of the Cretaceous Panther Tongue Member of the Starr Point Formation in the Book Cliffs of Utah, USA is based on clinoform dip and dip direction variability data. We propose that the southern lobe of the Panther Tongue Delta, near Crandall Canyon, has higher clinoform dips due to it prograding into deeper water as opposed to BLF because clinoform heights increase from proximal (north) to distal (south) indicating deeper water depths in the south.