--> Abstract: Ichnological Signature of Wave-Dominated Deltas Fed by Suspended-Load Rivers: The Jurassic (Oxfordian) Nuiqsut Formation, North Slope of Alaska, by Beverly A. Burns and James A. MacEachern; #90082 (2008)

Datapages, Inc.Print this page

Ichnological Signature of Wave-Dominated Deltas Fed by Suspended-Load Rivers: The Jurassic (Oxfordian) Nuiqsut Formation, North Slope of Alaska

Beverly A. Burns1 and James A. MacEachern2
1ConocoPhillips Alaska, Anchorage, AK
2Earth Sciences, Simon Fraser University, Burnaby, BC, Canada

Integrated ichnological-sedimentologic analysis of the Nuiqsut Fm indicates south-southeast progradation of wave-dominated deltas across a broad, low-relief shelf. Prodeltaic units are mud-prone and markedly heterolithic, with oscillatory generated sandstone laminae draped by dark, fissile, silt-poor carbonaceous mudstones. Bioturbation ranges from BI 1-3, with diminutive ichnogenera comprising low-diversity and low-abundance suites dominated by opportunistic deposit-feeding and grazing structures. Mudstones accumulated rapidly under conditions of marked physico-chemical stress.

Heterolithic prodeltaic units pass upwards into pervasively bioturbated (BI 5) muddy to silty, distal to proximal delta-front sandstones possessing few preserved primary structures. Ichnological suites show elevated ichnogenera diversities, recording predominantly deposit-feeding behaviors attributable to archetypal and proximal expressions of the Cruziana Ichnofacies.

Sandstones reflect slow, continuous, fully marine accumulation at and above fair-weather wave base affected by low-energy wave agitation. Turbid water, probably owing to hypopycnal (buoyant) mud plumes, inhibited suspension-feeding behaviors.

Conventional wisdom would suggest such relationship to be paradoxical - proximal facies tend to record more stressful conditions and higher deposition rates compared to their distal facies counterparts. We suggest, however, that this juxtaposition is actually a distinctive characteristic of deltas fed by suspended-load dominated rivers. In such conditions, bedload is volumetrically minor and supplied to the shoreline slowly, whereas emplacement of hypopycnal plume-derived mud to the prodelta is comparatively rapid. Modern analogues suitable for comparison are probably small tropical rivers feeding low-gradient, dissipative shorelines with low tidal ranges and weak storm climates.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery