Development of a Confined Turbidite System Prone to Hybrid Event Beds, Namurian (Late Mississippian-Early Pennsylvanian), U.K

Sedimentary gravity current deposits exhibiting both turbiditic and debritic characteristics (collectively known as hybrid event beds (HEBs)) have been recognized in Deepwater fan systems from a variety of basins over the last decade. Features of such deposits imply that at least part of the parental flows were not fully turbulent, at the time of deposition. Bathymetry is often considered to play an important role in triggering downstream transformations from turbulent to transitional or laminar flow, through bulking and clay enrichment following incision and/or through forced flow decelerations due to gradient reduction or flow interaction with obstacles.

Understanding the mechanisms driving the onset of transitional and hybrid behaviour is important as this may help better predict the distribution and extent of associated clay-rich sandy facies that typically have poor reservoir properties. Furthermore the distribution of such deposits can provide insight into the wider system response such as the presence and persistence of upstream slope disequilibrium related to tectonism or sediment supply changes that may link with sea-level or climatic changes.

New research in the bathymetrically complex Namurian Southern Pennine Basin, Northern England, documents the character and distribution of HEBs in a basin floor to base-of-slope fan system that banked against a downstream confining slope. Comparative field outcrop studies of basin margin and centre settings indicate the following: 1) flow bulking and clay enrichment over above-grade slopes is considered to have been the principal driving mechanism of flow transformation; 2) forced bathymetry-driven flow decelerations had no apparent further effect on flow transformations; 3) length scales (flow run-out distances) in the basin were sufficient to allow autogenic flow transformations prior to any additional influences of confining bathymetry - which would otherwise have resulted in forced flow transformations and differing HEB characteristics and distributions; 4) HEB character may be influenced by, and thus record, changing substrate type and grade; 5) HEBs can also be sources of heterogeneity in relatively more proximal sand-rich settings; 6) HEB characteristics may provide a means by which models of basin infill can be assessed (e.g. progradation vs. aggradation; progressive vs. out-of-sequence deposition).

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