Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Evidence of Dynamic Marine Processes in a "Black Shale" Dominated Succession Formed by Suspension Settling from Non-Glacial, Sediment-Laden Sea-Ice
(1) Western Geology & Geophysics, U.S. Geological Survey, Menlo park, CA.
(2) Earth Sciences, Memorial University of Newfoundland, St. John's, NF, Canada.
New studies of sedimentary processes in modern oceans, recent flume experiments, and ancient rocks indicate that fine-grained siliciclastic sediment contains floccule aggregates, low angle cross lamination, and erosional features. These textural attributes indicate that marine environments where mud was being deposited were commonly subjected to much greater energy than considered “normal” for deposition of shale/mudstone. In this study, our aim was to investigate a Lower Cretaceous, organic-carbon rich mudstone-dominated succession (pebble shale unit and lower part of Hue Shale) of the North Slope of Alaska to evaluate the occurrence and abundance of these features. To this end we examined 150+ thin-sections from 3 widely separated locations -- Canning River exposures and cores from the Mikkelsen Bay State #1 and Orion #1 wells.
Results from our lithofacies, textural, and micro-fabric studies -- particularly the presence of sand to granule and larger sized drop stones throughout these rock sections -- show the importance of suspension settling from melting sediment-laden seasonal sea-ice and related processes to the deposition of this succession at all studied locations. In the Orion well however, the presence of rare to common sand laminae <1-2 mm thick, scour and erosional surfaces, low-angle cross and parallel lamina geometries indicate that lateral transport (advection) and dynamic processes of deposition occurred more commonly at this location. In all 3 sections, lamina-sets are present that fine upwards from silty or sandy basal laminae suggesting deposition from waning flows. In addition, silt and sand filled burrows that commonly occur in clay-dominated parts of the succession having no obvious silt/sand source probably indicate subsequent erosion of coarser laminae.
These observations show that this mudstone dominated succession formed in an Early Cretaceous high latitude sea in which dynamic advective marine processes interacted with suspension settling from seasonal sea-ice in a variable manner through time and space along the margin, and are more common at the location of the Orion well.