Organic Carbon-Rich Sedimentation in from Early Cretaceous Arctic Sea: Nutrients Supplied by Sediment-Laden Seasonal Sea-Ice and Volcanic Ash
On the eastern North Slope of Alaska, the Lower Cretaceous pebble shale unit (informal) and lower part of the Hue Shale comprise a marine succession of predominantly mudstone with subordinate interbedded volcanic ash. Rare mud-supported, sand to pebble-sized clasts occur throughout the studied succession, but abundant clasts commonly occur in the basal pebble shale unit overlying a regional Lower Cretaceous unconformity. Over the past decade, our high-resolution petrography, SEM, and geochemical studies of this succession have identified previously unknown facies and microscopic fabrics of these rock formations, providing new evidence of the processes that formed them and led to their organic-carbon richness -- important to the petroleum source potential in this region of the Arctic.
Bulk and molecular organic geochemical analyses (Rock-Eval, elemental, δ13C isotopes and biomarker composition) of the succession in the Mikkelsen Bay State #1 well indicate highly variable and overall high organic richness (TOC 2.1-5.9%; average ≈ 3.5%; 36 samples over 57 ft of section) and dominantly marine Type II oil-prone kerogen. Organic carbon-rich facies of the succession occur within pelleted laminae and lenses of clay-rich and clay-dominated mudstones and associated with ash-fall tuffs that are rare to unknown in the pebble shale unit but common in the Hue Shale. Sedimentary fabrics include relict bedding, burrow mottling, and pelleting; mud-supported, and commonly penetrative fabrics of clasts as well as poorly sorted, discontinuous, bedding-parallel clusters of clasts; and rare wavy-based and discordant bedding.
These results indicate that the pebble shale unit is locally more oil prone than previously determined regionally; and given the fabrics present, organic-carbon accumulation has occurred primarily in association with organic productivity and deposition from melting sediment-laden seasonal sea-ice—likely the main source of mud and silt plus sand and pebbles. Peaks of higher organic-carbon content (up to 5.9% TOC) are associated with rare thin ash units in the pebble shale unit. In the Hue Shale, nutrient supply and organic-carbon accumulation are attributed primarily to the same processes but with relatively more influence from volcanism and less from sea-ice. Analogous Holocene processes have been shown to supply nutrients such as Fe that otherwise limit productivity at high latitudes today.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009