Evidence for Dynamic Marine Processes in the Lower Cretaceous Pebble Shale Unit and GRZ of the Hue Shale, Petroleum Source Rocks, North Slope, Alaska
Keller, Margaret A. and Macquaker, Joe H. S.
Sedimentary process studies in modern oceans, flume experiments, and ancient rocks indicate that fine-grained siliciclastic sediment contains floccule aggregates, low-angle cross lamination, and erosional features. These textural attributes show that the environments where mud was deposited were commonly subjected to greater energy than considered usual for deposition of shale/mudstone. In this study, our objective was to investigate the occurrence and abundance of these features in the Lower Cretaceous, organic-rich mudstone-dominated succession of the North Slope of Alaska. We examined 150+ thin sections from three widely separated locations–Canning River exposures and cores from the Mikkelsen Bay State 1 and Orion 1 wells.
Results from lithofacies and micro-fabric studies–particularly the occurrence of sand to granule and larger sized dropstones throughout the lower part of these rock sections–show the importance of suspension settling from melting seasonal sea ice and related processes to the deposition of this succession at all studied locations. In the Orion well, however, rare to common continuous sand laminae <1-2 mm thick, erosional surfaces, and low-angle cross and parallel laminae geometries indicate that deposition by lateral transport (advection) and dynamic processes also occurred here. In all three sections, lamina sets are present that fine upward 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 with no apparent silt or sand source probably indicate subsequent erosion of coarser laminae.
These observations indicate that this mudstone-dominated succession formed at high-latitude in an Early Cretaceous sea in which suspension settling of all grain sizes from seasonal sea ice was probably the main depositional process. Periodically, this process was interrupted by deposition/erosion from dynamic advective marine processes that varied over time and space along the margin, and operated more commonly at the location of the Orion well.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013