UC Santa Barbara

Although sea-level highstands are typically associated with sediment-starved continental shelves, high sea level does not hinder major river floods. Hyperpycnal currents are turbidity currents generated by plunging of sediment-laden rivers at the fluvial-marine interface, and they allow for cross-shelf transport of suspended sand beyond the high-energy coastline. Hyperpycnal currents are an important mechanism operating within source-to-sink systems as they can form a link between terrestrial and marine environments during sea-level highstands. Because hyperpycnal currents are typically only generated during extreme river floods, their deposits may serve as paleoflood archives. Hyperpycnites also have the potential to serve as high quality hydrocarbon reservoirs; however, a broader understanding of where hyperpycnal currents are most likely to deposit significant volumes of sand and the morphology of those deposits is necessary for accurate reservoir modeling. The studies included here present results from field studies of Holocene fans on the continental shelf of Southern California and Jurassic hyperpycnites in the Neuquén Basin of Argentina, in combination with three-dimensional flume experiments, to demonstrate that hyperpycnal currents are capable of depositing well-sorted sand bodies on the continental shelf, and that their deposits may differ from “classic” sediment gravity flow deposits due to the effects of freshwater within the currents.

River-derived hyperpycnal currents and turbidity currents initiated in relatively shallow water that travel into deeper and colder water commonly contain interstitial fluid less dense than the surrounding ambient water. These currents are initially ground-hugging due to high suspended sediment concentrations. However, as sediment settles from suspension, bulk current density will decrease and may become less dense than the surrounding ambient water, at which point the current becomes buoyant and rises from the basin floor. This process of buoyancy reversal, or lofting, affects both the internal architecture of turbidites and their overall morphology. Cores collected from shelf hyperpycnites in the Santa Barbara Channel provide grain-size trends, radiocarbon dates, and overall stratigraphic architecture of lofted-current deposits (Chapter 2). The hyperpycnal currents deposited slightly graded, structureless fine- to medium-grained sand beds. These beds became well-sorted through the stripping of fine-grained material in suspension at the point of lift-off.

Lofting not only affects sorting within hyperpycnites, but also changes the lateral spreading rates of turbidity currents and therefore affects deposit morphology. Flume experiments show that lofting currents are width-limited and generate narrower, more elongate deposits than ground-hugging currents (Chapter 3). Factors such as steeper basin floor gradients and higher suspended-sediment concentrations push the lofting point farther basinward and ultimately result in wider deposits. Most importantly, the use of a 3-dimensional experimental tank allows for the first detailed analysis of the lofting process and its effects on length-to-width ratios of turbidite lobes.

Discoveries of modern shelf hyperpycnites and experimental work describing turbidity currents with light interstitial fluid provide a valuable framework for understanding and recognizing shelf hyperpycnites in the rock record. We compare our experimental results and findings from the Santa Barbara Channel with hyperpycnites of the Mid-Jurassic Lajas Formation of the Neuquén Basin, Western Argentina (Chapter 4). Hyperpycnites of the Jurassic Lajas Formation are characterized by dm thick beds of well-sorted medium-grained sandstones with parallel laminations. These beds are encased by organic-rich, thinly laminated sandstone and siltstone. These deposits represent obliquely-migrating sand lobes deposited on the continental shelf and were likely fed by small rivers. Recognition of shelf hyperpycnites in the Lajas Formation of the Neuquén Basin allows for a broader understanding of shelf processes and adds to the developing hyperpycnite facies models. Overall, recognizing and understanding the geometry and internal architecture of shelf hyperpycnites will improve understanding of sediment transfer from rivers to deeper water, paleoenvironmental interpretations of gravity-flow deposits, and has implications for modeling potentially high-quality hydrocarbon reservoirs.