Pennsylvanian-Early Permian Reefal Shelf-Margin and Adjacent Slopes, Sverdrup Basin, Arctic Canada
The Sverdrup Basin, Canadian Arctic, displays one of the world’s largest (up to 2.0 km thick) and longest (400 kms long) reefal shelf margin of the Pennsylvanian to Early Permian interval. A prolific carbonate factory developed along the northwestern margin of Pangea at a time when an influx of warm waters derived from the Tethys favored the growth of phylloid algal and Palaeoaplysina reef complexes.
A first phase of shelf/slope progradation occurred during the Late Moscovian-Kasimovian interval, as steep reliefs inherited from the underlying keep-up reef and shelf morphology in concert with high carbonate sedimentation rates led to very steep (30 to 45 degrees) slopes that were stabilized early by submarine cementation. Reefs developed at the shelf margin and reef facies pass directly and abruptly into slope deposits within a few hundreds of metres. Sediment gravity flows include debris flows with shelf margin-derived, early-cemented clasts ranging from a few cm to tens of metre across. Some of these deposits are channelized; others accumulated as chaotic aprons of debris at the toe of the slope. Turbidites display partial to complete Bouma sequences. Hemipelagic fine-grained silt- to mud-sized carbonates sediments derived from the adjacent shelf complete the sedimentological spectrum. The slope gradient became gentler with time, and accordingly, large clasts and debris become increasingly rare upward.
A second phase of extensive carbonate shelf/slope progradation occurred during the Asselian (Early Permian) following keep-up reef and shelf development in the Gzhelian. Early Permian slopes were considerably gentler than before, in part the result of a different shelf to slope configuration whereby algal mud mounds were located in a mid shelf position several kms inboard of the shelf break. In that setting, the mid shelf reef divides the shelf into a variably restricted and at times evaporative inner shelf and an open marine outer shelf. Much of the sediments produced by this carbonate factory remained trapped onto the shelf, especially during glacio-eustatic low stands, and as a result, the adjacent slopes were starved most of the time and dominated by lime mud. Debris flows are rare to non-existent and their clasts are rarely more than 2-3 cm in diameter. Turbidites are poorly developed and scant. The bulk of the slope sediment consists of hemipelagic material deposited from suspension probably following storms on the adjacent shelf.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California