--> Abstract: Wave-Dominated Deltaic Facies of Suspended-Load Rivers: The Jurassic (Oxfordian) Nuiqsut Formation, North Slope of Alaska, by James A. MacEachern and Beverly A. Burns; #90078 (2008)

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Wave-Dominated Deltaic Facies of Suspended-Load Rivers: The Jurassic (Oxfordian) Nuiqsut Formation, North Slope of Alaska

James A. MacEachern1 and Beverly A. Burns2
1Earth Sciences, Simon Fraser University, Burnaby, BC, Canada
2ConocoPhillips Alaska, Anchorage, AK

The Nuiqsut Fm comprises stacked, shoaling-upward successions, reflecting south-southeast progradation of wave-dominated deltas across a broad, low relief shelf. Integrated ichnological-sedimentologic analysis identifies prodeltaic facies as mud-prone and markedly heterolithic, with oscillatory generated 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 attributable to impoverished expressions of the Cruziana Ichnofacies. Facies record high rates of mudstone deposition under conditions of marked physico-chemical stress.

These heterolithic units pass upwards into pervasively bioturbated (BI 5) muddy to silty distal to proximal delta-front sandstones with few preserved physical sedimentary structures. Ichnological suites are characterized by high diversities of ichnogenera recording predominantly deposit-feeding behaviours, attributable to archetypal and proximal expressions of the Cruziana Ichnofacies. Sandstones reflect slow, continuous, fully marine sedimentation at and above fair-weather wave base, in the presence of low-energy wave agitation. Turbid water, due to hypopycnal mud plumes, inhibited suspension-feeding behaviours.

Conventional wisdom would suggest a relationship such as this 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 distinctive of deltas fed by suspended-load dominated rivers, wherein bedload is volumetrically minor and supplied to the shoreline slowly, while emplacement of hypopycnal plume-derived mud to the prodelta is rapid. Modern analogues suitable for comparison are probably small tropical rivers feeding to low-gradient, dissipative shorelines with low tidal ranges.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas