Deepwater Slope Channels and Their Levees: New Insight from a Revisit to the Namurian (Early Pennsylvanian) Gull Island Formation, Western Ireland

Deepwater slope channels range in their expression from simple incisional forms through high aspect ratio (W/D) forms with wings and poorly developed levees to low aspect forms with very prominent levees. Incisional channels generally represent erosion and sediment bypass, while the low- and high-aspect ratio examples with co-eval deposits on their margins show overbank deposition which resulting morphologies are dependent on grain size and flow nature.

While the morphology and general stratigraphy of slope channels are many times superbly imaged in seismic data, it is more difficult to predict their detailed actual fill. Thus, outcrop studies are a necessity to understand the processes, bed geometries and sand/mud ratio. Outcrops however, a generally limited in extent, and thus the linkage between planform morphology, complete stratigraphy (gained from seismic) and sedimentological details (gained from outcrops) is challenging.

The Namurian Gull Island Formation of western Ireland exposes both vertical stratigraphic sections through a complete slope succession from basin floor to shelf break, as well as laterally, seismic scale extensive exposures of high-aspect ratio, sandy channel-levee systems. Due to the global ice-house conditions in the Namurian, a cyclic pattern is developed where the channel systems occupy a distinct position in terms of sediment supply and external controls, which is a probable cause for their sandy nature in an overall low sand-mud ratio setting.

Studies reveal that both internal and external low-relief levees exist with an unusually high sand-mud ratio which over several kilometres passes out into thinner-bedded sandstones and mudstones. In addition, the channels were repeatedly occupied by larger, bypassing flows and mass-transport complexes which overtopped the levees. The exposures afford a close description and interpretation of distinguishing criteria between low-relief sandy levees and frontal splay deposits, and set a norm and predictor for identification of such facies in vertical sections, including cores and well logs. In addition, the seismic scale provides a close link to subsurface seismic data and provides an additional detailed norm for the expected sedimentary facies and fill of such systems in analogue subsurface settings.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California