Andrew L. Petter

Delta-front hyperpycnal bed geometry and implications for reservoir modeling: Cretaceous Panther Tongue delta, Book Cliffs, Utah

The Cretaceous Panther Tongue has an upward-coarsening and -thickening pattern and is well exposed in extensive large outcrops in the Book Cliffs area, west-central Utah. The deposits have been interpreted as having formed in a fluvial-dominated river delta environment that generated highly sediment-concentrated sustained (turbidity) flows during flooding, producing hyperpycnal-flow deposits on the delta front despite some resemblance to deep-water turbidites. The facies associations indicate terminal distributary channel, channel mouth, and proximal delta-front and distal delta-front depositional environments. The measured paleocurrents indicate a south-southwest transport of the sediments. The thickness of the hyperpycnal sandstone beds ranges from centimeters to meters. Sandstones are characteristically parallel laminated, sometimes structureless or rarely display inclined strata of cut-and-fill type. The sandstone hyperpycnal beds dominate the delta-front clinoforms and dip southward, consistent with the other paleocurrent indicators. Individual sandstone beds in the clinoforms have dips that range from 0.1 on the distal delta front (lower part of the outcrops) to 3 in the proximal parts (upper part of the outcrops). The hyperpycnal beds can be traced from a proximal mouth-bar environment to the distal delta front over a distance of hundreds of meters. As individual beds extend from mouth bar to distal delta-front environments, they become systematically finer grained and thinner. Over short distances (hundreds of meters), the beds thin with rates ranging between 0.0001 (i.e., dm/km) to 0.02 (i.e., tens of meters per kilometer). The sandstone beds thin to a greater degree in a dip direction than along strike, indicating a relatively strike-elongate (flow-normal) geometry of the hyperpycnal flows and of the delta lobes. The wider than longer geometry of the delta-front beds requires that reservoir development be more focused upon the downdip facies changes (heterogeneities) than the lateral (along strike) heterogeneities.

Hyperpycnal flow variability and slope organization on an Eocene shelf margin, Central Basin, Spitsbergen

Identification of bypass at the shelf margin is critical to deep-water exploration. We examine the shelf margin of an early Eocene fourth-order sequence with an attached basin-floor fan in the Spitsbergen Central Basin. Turbidity currents were fed mainly by hyperpycnal flow emerging from shelf-edge deltas. The life span of any turbidity current was determined primarily by the sediment concentration of the flow and the duration of the river flood. High-density hyperpycnal flows created sand-filled slope-channel complexes 10–15 m (33–49 ft) thick and 100–200 m (328–656 ft) wide that served as conduits for bypass to the basin floor. Low-density hyperpycnal flows were unconfined and deposited heterolithic lobes on the slope. Shelf-margin accretion of about 1.5 km (0.9 mi) during the falling stage gave way abruptly to bypass in the early lowstand. Most of the basin-floor fan growth was achieved after shelf-edge incision and before relative sea level rise. Coastal-plain aggradation in the late lowstand sequestered sediment from the shelf-edge distributaries, effectively diminishing high-density hyperpycnal flow output. The late lowstand was therefore marked by a second phase of shelf-margin accretion with only limited bypass to the basin floor, and a heterolithic, prograding complex downlapped the early lowstand channels. Transgression ultimately led to the abandonment of the shelf-edge delta complex and the accumulation of mainly mudstone on the margin. The shelf-margin architecture exhibited by this sequence should serve as a type example of a deep-water feeder system in which hyperpycnal flow is the primary initiator of turbidity currents for sand accumulation on the slope and basin floor.

Estimation of the paleoflux of terrestrial-derived solids across ancient basin margins using the stratigraphic record

A simple inversion scheme for estimating sediment flux from ancient shelf-margin successions is presented here by treating shelf-margin clinothems as the product of deposition associated with migration of a shelf-edge clinoform with constant shape at a rate equal to the shelf-margin progradation rate. Assuming sediment conservation, deposition can be broken into components of (1) response to subsidence and sea-level changes, and (2) basinward migration of the clinoform profile. Sediment flux can therefore be estimated with knowledge of progradation rate, subsidence/sea-level change rate, and clinoform dimensions. An advantage of this methodology is that it requires only two-dimensional data (i.e., dip-oriented cross sections) rather than three-dimensional volumes, making it ideal for use with sparse data sets as well as with outcrops. This methodology is also useful for analyzing areally limited data sets because it can predict the flux of sediment transported beyond the area of data coverage. The approach is able to accurately reproduce the sediment-flux estimates of previous workers from several margins (the Fox Hills–Lewis, Zambezi, New Jersey, and North Slope margins) using both volumetric and forward-modeling methods. Not only are the predicted distributions for sediment flux across ancient shelf-margins similar to distributions predicted by more data-intensive theoretical models, the estimated magnitudes for paleofluxes favorably compare with measured loads from modern rivers. Growth of continents is achieved in part by accretion of sediment on shelf margins. The rates and patterns of continental expansion are therefore partially dependent on the magnitude and distribution of mass transfer from eroding hinterlands to continental margins, fluxes which also play a critical role in global biogeochemical cycles. Flux estimates cast into a mass-balance framework suggest that approximately two-thirds of continental-margin sediments are exported past the shelf edge into deeper water at long-term geologic time scales. This finding implies that two-thirds of the terrestrial-derived, particulate organic carbon (POC) delivered from rivers to the ocean can be stored in deep water over geologic time scales. The observations presented here indicate that repetitive delivery of sediment to margins by shelf-edge deltas is fundamental to the long-term process of margin accretion.